Physical Chemistry Chemical Physics

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1 PAPER Physical Chemistry Chemical Physics Water penetration/accommodation and phase behaviour of the neutral langmuir monolayer at the air/water interface probed with sum frequency generation vibrational spectroscopy (SFG-VS) Zhen Zhang,w De-sheng Zheng,w Yuan Guo and Hong-fei Wang* Received 1st August 2008, Accepted 29th September 2008 First published as an Advance Article on the web 29th October 2008 DOI: /b813187b A strong and broad hydrogen bonded O H band around 3520 cm 1 is observed in the insoluble monolayer of the neutral liquid crystal molecules of n-pentyl-4-p-cyanobiphenyl (5CB) and n-octyl-4-p-cyanobiphenyl (8CB) throughout the whole surface density range, but not in the 4-pentyl-4 0 -cyanoterphenyl (5CT) monolayer, at the air/water interface. This novel spectral feature suggests the existence of an oriented water cluster species which has penetrated or accommodated into the Langmuir monolayer of the 8CB and 5CB molecules. This finding provided a molecular level mechanism for the stark difference in the phase behaviour between the CB and CT insoluble Langmuir monolayers at the air/water interface. It also calls for attention to the details of the specific water surface interaction in mediating the structure and the phase behaviour of the molecular assemblies at the heterogeneous aqueous interfaces. I. Introduction Hydrogen bond structure of the liquid water molecules plays an important and complex role in chemical and biological processes. 1 6 Besides the complex hydrogen bond interactions, the detailed balance between the dipolar repulsive forces and the van der Walls attractive forces of the molecular systems in the condensed phase determines the structure, the phase transition and the dynamic behaviour of the complex molecular assemblies, as well as that of the solvent molecules surrounding them For example, recent theoretical simulations showed that the wetting and dewetting of the hydrophobic surfaces, or the hydrophobic collapse, in the process of protein folding were controlled by detailed molecular interaction forces Because of the important role of the water molecule in controlling and mediating the protein folding structure and dynamics, some even called the water molecule the 21st amino acid. 12 The water molecule also exhibits interesting properties and dynamics behaviour in confined environment, such as in the nanoporous cavities, the nanosize inversed micelle droplets, etc In application, the ability to understand and to control the specific and the non-specific protein adsorption to materials surface, which is mediated by the hydrophilic and hydrophobic interactions with water molecules, is also of tremendous importance in surface bioengineering for designing biological sensors and medical devices with biological compatibility The heterogeneous interaction and transport between the water solution phase with another phase, either gaseous, liquid Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China. hongfei@iccas.ac.cn; Fax: ; Tel: w Also Graduate University of the Chinese Academy of Sciences. or solid (crystalline or amorphous), is located at the interface region. In recent years, because of the importance to understand and control the molecule surface interaction in the heterogeneous chemical processes in environmental, material and biological processes, the structure and reactivity of interfacial water molecules have attracted much attention. 23 One of the fundamental scientific problems in such heterogeneous interactions is how the interfacial molecules are solvated, and particularly, whether the water molecule penetrates into the interfacial molecules to solvate them. Even though there have been some indirect experimental evidence and suggestions in the studies of the molecular surfaces, such as in the Langmuir monolayer and the Langmuir Blodgett (LB) films, that water penetration or accommodation did occur, 7,24 26 such phenomena and the role in the structure and the phase behaviour of the Langmuir monolayer and the LB films have not been well explored, due to difficulties in dealing with the phenomena using the available experimental techniques and theoretical methods. 27 Spectroscopic methods are basic and important tools that have tremendously facilitated the understanding of the molecular structure and interactions. However, the spectroscopic features of the water molecules are yet to be fully understood. Particularly, spectroscopic studies on the dynamic hydrogen bonding structure of the water molecules are yet to reach consensus on the interpretation of some basic spectral features. For example, the interpretation of the broad O H stretching vibrational Raman and infrared bands in liquid water in the range of cm 1 is still controversial People are still trying to understand this broad spectral feature: whether there are several overlapping bands belonging to the different hydrogen bonded water species or it is just a continuous spectral fluctuation of the same water structure The development of interface specific nonlinear optical spectroscopy has vastly contributed to the understanding of This journal is c the Owner Societies 2009 Phys.Chem.Chem.Phys., 2009, 11,

2 the structure and bonding as well as the dynamics and reactivities of water molecules at various interfaces In the sum frequency generation vibrational spectroscopy (SFG-VS) studies of the air/water interface, besides the characteristic hydrogen bonded O H vibrational spectral features of the liquid water in the range of cm 1, some new spectral features, unique to the interfacial water molecules, were identified. 33,34,36 40 One of the new features is the narrow peak in the surface SFG-VS spectra at about 3700 cm 1, attributed to the single non-hydrogen bonded O H stretching of the oriented water molecules pointing out from the bulk liquid phase at the topmost layer of the interfacial region. Another new broad feature at about 3550 cm 1 was attributed to the singly hydrogen-bonded O H of the same oriented water molecules at the topmost layer. 37,41,42 Using polarization analysis in SFG-VS, both new spectral features were determined as with the C Nv symmetry, a confirmation of the above attributions. 37,43 Shen and Bonn s group have recently studied ultrafast O H vibrational dynamics of the interfacial water molecules using SFG-VS. 44,45 However, similar to the bulk water vibrational spectra, although much has been learned, the disagreement on assignments and interpretations of the SFG vibrational spectral features in the cm 1 region of the hydrogen bonded species at the air/water interface are still to be resolved. 33,46 Besides the neat air/water interface, the SFG vibrational spectra of the hydrogen bonded water molecules at the various aqueous interfaces have been studied. When the interface is covered with a charged molecular layer or layers, the 3700 cm 1 narrow band of the free O H vibration is usually not observable. The SFG spectral intensity in the ssp polarization combination in the range of cm 1 was usually one order or more of magnitude stronger than that at the neat air/water interface, with the broad 3200 cm 1 band being the strongest. 47,48 This enhancement of the hydrogen bonded water spectral intensity was a result of the surface electric field induced sum frequency generation (EFISFG) of the bulk water molecules polarized in the vicinity of the charged interface the same mechanism as the electric field induced second harmonic generation (EFISHG) of the bulk water molecules in the vicinity of the charged interface which was established by Eisenthal and co-workers more than a decade ago. 49,50 Because of the strong EFISFG signal being from the bulk water molecules in the vicinity of the interfacial region, the spectral features of the particular water species at the interfacial layer could consequently be overlooked. When the interface is covered with a neutral molecular layer or layers, the EFISFG is absent and the SFG spectra intensity in the same spectra region is about the same level or smaller as that of the neat air/water interface. 48,51,52 Recently, Tyrode et al. studied the SFG vibrational spectra of the water molecules at the interface adsorbed with several different neutral surfactants. In these cases, the 3700 cm 1 band was suppressed, and a new broad spectra feature was observed in the ppp and sps polarization peaked at about 3550 cm 1. Even though this feature overlapped with the 3550 cm 1 band in the neat air/water interface SFG spectra, the polarization dependence was completely different. 37,51 For the latter, the ppp intensity is evidently stronger than the sps intensity, and the 3550 cm 1 band was attributed to the hydrogen bond with C Nv symmetry. While in the Tyrode et al. s case, the sps intensity is significantly stronger than the ppp and ssp intensities. Thus, using the symmetry argument, this 3550 cm 1 band was attributed to the hydrogen bonded water molecules with asymmetric C 2v modes. 51,52 The ability to distinguish the different origins of the overlapping spectral features nevertheless suggested the importance of polarization and symmetry analysis in the SFG-VS studies. 43,51,53 56 Furthermore, they also suggested that the complexities of the different and overlapping vibrational spectral features of the hydrogen bonded water species at the different interfaces and in different environments are yet to be fully understood and explored. Recently, Allen et al. reported the observation of a broad O D stretching vibration spectral feature around 2700 cm 1 in the ssp polarization combination at a DPPC (chain-perdeuterated dipalmitoylphosphatidylcholine or d62-dppc) zwitterion lipid monolayer at the air/d 2 O interface. 57 The intensity of this broad band is about half or less than that of the much narrower free O D stretching vibration around 2750 cm 1 observed at the neat air/d 2 O interface. This feature was attributed to the van der Waals interactions between the hydrocarbon chain and the dangling O D bonds of the interfacial D 2 O molecules. Here we report the observation of an intense and broad vibrational spectral feature of the hydrogen bonded water species around 3520 cm 1 in the SFG spectra from a certain kind of neutral Langmuir monolayer at the air/water interface. This feature is about 6 to 10 times more intense in the ssp polarization combination than the intensity of the ssp SFG spectral features of the hydrogen bonded water species for the neat air/water or air/d 2 O interface. In contrast, the broad peaks in the same spectral range for the neutral Langmuir monolayers mentioned in the paragraph above were all smaller than the intensity of the narrow free O H band of the neat air/water interface. The detailed assignment of this new spectral feature is yet to be worked out. Nevertheless, because this spectral feature was associated with the Langmuir monolayer with a particular kind of expanded phase behaviour, we concluded that this new spectral feature was from the hydrogen bonded water molecular species more penetrated or accommodated in-between those aligned Langmuir molecules. People have speculated the role of the penetrated or accommodated water molecules in the peculiar phase behaviour of the same series Langmuir monolayers. 7 However, experimental and theoretical understanding of such phenomena is still lacking. 27 To our knowledge, this new spectral feature is a direct observation of the water penetration or accommodation and the structured water molecules in the Langmuir monolayer. It can offer a microscopic mechanism to understand the structure and phase behaviour of a different kind of Langmuir monolayers, as well as the properties and behaviour of various molecular films and membranes. II. A Experimental Chemicals 4-Pentyl-4 0 -cyanoterphenyl (5CT) (498%) and n-octyl- 4-p-cyanobiphenyl (8CB) (498%) was purchased from 992 Phys. Chem. Chem. Phys., 2009, 11, This journal is c the Owner Societies 2009

3 diagram measured at a compression speed of ca 0.2 mm s 1. The room temperature is kept at C, and the humidity in the room is kept at ca 40%. Fig. 1 Structures of the 8CB, 5CB and 5CT molecules: (a) npentyl-4-cyano-p-terphenyl (5CT); (b) n-pentyl-4-cyano-p-biphenyl (5CB); (c) n-octyl-4-cyano-p-biphenyl (8CB). Sigma-Aldrich and used as received n-pentyl-4-p-cyanobiphenyl (5CB) (498%) was purchased from the Center of Liquid Crystal of the Tsinghua University at Beijing, China and used as received. The chemical structures of the 8CB, 5CB and 5CT molecules are shown in Fig. 1. Chloroform solvent (analytical grade 499.0% with % ethanol) was purchased from Beijing Chemical Reagent Inc. ( bjhgch.com.cn). Liquid water used was the double distilled water purified and deionized with a Millipore Simplicity 185 (18.2 MO o cm). B Phase diagram measurement 5CT, 5CB and 8CB were dissolved in chloroform at a concentration of ca 0.1 mmol L 1 for spreading. The solution was spread on the air/water interface in a Langmuir trough ( mm) using a microsyringe. The surface pressure surface density phase diagram was measured with a paper Wilhelmy plate by slowly compressing the horizontal bar at various compressing speeds controlled by a PC computer. The Teflon Langmuir trough was home-build and the surface pressure was monitored with a commercial electromagnetic sensor (PS4, Nima). 58 The data presented in Fig. 2 are the p-a phase C SFG spectra The details of the SFG spectrometer laser system were described previously. 53,54,59,60 Briefly, the 10 Hz and 23 ps SFG spectrometer laser system (EKSPLA) is in a co-propagating configuration. The efficiency of the detection system has been improved for the weak SFG signal of the air/water interface. A high-gain low-noise photomultiplier (Hamamatsu, PMT-R585) and a two-channel Boxcar average system (Stanford Research Systems) are integrated into the EKSPLA system. The voltage of R585 was 1300 V in the measurement of the air/water interface because of the low signal level of hydrogen bonded water spectra, and 900 V for the Z-cut quartz surface. The incident angles are b 1 =63 11 for the visible beam, and b 2 =55 11 for the IR beam. The wavelength of the visible is fixed at 532 nm and the full range of the IR tunability is 1000 to 4300 cm 1. The SFG signal is collected at around (b) at the reflection geometry, within a small range (about 0.31) which depends on the corresponding IR wavelength tuning range (from cm 1 in the experiment). The specified spectral resolution of this SFG spectrometer is o6 cm 1 in the whole IR range, and about 2cm 1 at ca 3000 cm 1. Each scan whad a 5 cm 1 increment and was averaged over 300 laser pulses per point. Each spectrum was an average of several runs. The energy of the visible beam was typically less than 300 mj and that of IR beam was less than 150 mj atca 3000 and 3700 cm 1, and in some parts in-between it was less than 100 mj. These results are comparable to literature reported values for measurement of the air/water interface spectra. 36 The Langmuir film in the SFG measurement was spread on a static flat cylindrical Teflon cell (f 90 5 mm) All measurements were carried out at controlled room temperature ( C) and humidity (40%). The whole experimental setup on the optical table was covered with a plastic housing to reduce air flow. No detectable evaporation effect was observed for the SFG spectrum during each scan. The spectra were normalized to the signal from a thick (5 mm) z-cut crystalline quartz. The details of the normalization procedure followed the descriptions in the literature. 37,61,62 Fig. 2 p-a phase diagrams of the 5CT, 8CB and 5CB monolayers at the air/water interface at a room temperature of 22 1C. The large filled circles indicate the surface density where the steep rise starts. The arrows indicate where the SFG spectra were measured. The second plateau on the 8CB and 5CB curves is the single layer triple layer coexistence region. 7 The part with the slope on the 8CB and the 5CB curves is in a homogeneous single-layer condensed phase. 7,63,64 III. Results and discussion A Phase behaviour of the 8CB, 5CB and 5CT Langmuir monolayer In order to understand the SFG-VS data from the 8CB, 5CB and 5CT Langmuir monolayers at the air/water interface, the difference between their phase diagrams and related interface structures need to be understood. There have been many well established studies on the peculiar single triple-multi layer transitions of the 8CB, as well as ncb Langmuir monolayers, in the literature. 7,63 71 Here n represents the number of the carbon atoms of the alkyl chain, i.e. the tail, in each molecule. The short chain ncb and nct molecules are thermotropic liquid crystal (LC) molecules insoluble in water. They can This journal is c the Owner Societies 2009 Phys.Chem.Chem.Phys., 2009, 11,

4 form insoluble monolayers at the air/water interface. 7,63 71 The ncb molecules are particularly interesting. They can form anti-parallel layers on the solid substrate or in the free standing film. 72 The linear and nonlinear optical properties of the ncb films have been extensively studied as model systems for understanding of the structure and phase transitions of the LCs with theoretical importance and potential technological use Studies on the structure and film stability of the ncb and nct monolayers, and the binary monolayers with a mixture of the ncb or the nct with other surface active molecules at the air/water interface have also been pursued using surface second harmonic generation (SHG) techniques 7,77,78 and surface pressure and Brewster angle microscopy (BAM). 7,63 71 The phase diagram and phase behaviour of the 8CB or 5CB and the 5CT Langmuir monolayes are significantly different. Fig. 2 shows the phase diagrams, i.e. the surface pressure vs. surface density diagrams or the p A curves, for the 8CB, 5CB and 5CT Langmuir monolayers. Even though the molecular cross-sections are perpendicular to the CRN axis, i.e. the molecular dipole axis, of the cyano-biphenyl (CB) and the cyano-terphenyl (CT) groups are the same, their p A curve began to rise, or undergo a phase transition, under a lateral compression at very different surface densities. The phase diagram and phase behaviour of the normal insoluble Langmuir monolayer were understood quite well. 79 Generally speaking, from low to high surface pressure, there is a gaseous-condensed phase coexistence region at nearly zero surface pressure, and it is followed by a condensed phase region with a steep rise of the surface pressure before the monolayer collapses at very high surface pressure. Some of the film may undergo more complex structural changes and the phase diagram in the condensed phase may not be monotonic. However, the single monolayer is maintained before the film collapses. The phase diagram of the 5CT Langmuir monolayer in Fig. 2 indicates that it is one of the normal Langmuir monolayers. Xue et al. first discovered the peculiar and distinct single triple-multi layer phase transition behaviour for the 8CB Langmuir monolayer at the air/water interface. 7 Immediately after this discovery, de Mul and co-workers and Friedenberg and co-workers confirmed and visualized the phase transition behaviour of the 8CB Langmuir monolayer using the BAM technique. 63,64 According to Xue et al., the phase diagram of the 8CB monolayer was divided into five consecutive regions according to their different surface pressure values. 7,63,64 From low to high surface pressure, Region I represents a submonolayer coverage, or the gas-condensed phase co-existence phase, at near-zero surface pressure; Region II corresponds to the rise in the surface pressure as the monolayer becomes a complete homogeneous monolayer. Region III is the plateau region with a surface pressure around 5mNm 1, where an inhomogeneous anti-parallel double layer is formed on top of the lower full homogeneous monolayer; Region IV represents the second upturn in surface pressure after compression through the plateau, corresponding to a fully packed triple layer phase. Region V corresponds to the second plateau after further compression, corresponding to the multilayered phase. These phases were clearly characterized in the BAM studies. 63,64 Friedenberg and co-workers also showed that Region II was fully homogeneous both in the compression and the expansion processes of the 8CB Langmuir monolayer. 64 In Fig. 2, the part of the phase diagram with Regions I, II and III is shown for the 8CB Langmuir monolayer. The molecular detail of the orientational order of the 8CB Langmuir monolayer before and after the transition between Regions I and II was quantitatively analyzed with a surface second harmonic generation (SHG) measurement in different polarization combinations. 58,77 It was found that a continuous orientational distribution narrowing (ODIN) process started before Region II and ended at just the transition point of the surface pressure, where a closely packed homogeneous monolayer was finally formed to give a steep rise of the surface pressure under further compression. 58 Using BAM, Inglot and co-workers studied the ncb Langmuir monolayers and found that for 4 r n r 12 their phase diagram and phase behaviour were both similar to those of the 8CB monolayer. 69 Martyn ski and the same group of co-workers also used BAM and compared the detailed phase behaviour of the 8CB and 5CT, as well as the mixture of 8CB/5CT, Langmuir monolayers. 69 The phase diagram of the 5CT Langmuir monolayer only had a single transition around A 2 before the monolayer collapsed around 24 A 2. It did not show anything similar to the single triple-multi layer transitions as the ncb monolayers. The phase diagrams for the 8CB, 5CB and 5CT Langmuir monolayers in this report as shown in Fig. 2 are consistent with those in the above-mentioned literature sources. Region II of the 8CB and 5CB monolayers started the steep rise of surface pressure at about 51 and 46 A 2, respectively; while the 5CT started the rise at much higher surface density about A 2, and the 5CT monolayer collapsed at about 25 A 2. Summing up the previous studies on the ncb and 5CT Langmuir monolayers, for these monolayers the following conclusions can be reached. 7,63,64,69 The differences on the dipolar repulsion between the 8CB and between the 5CB molecules, and that between the 5CT molecules are responsible for their different phase behaviour. The dipolar repulsion between the 8CB and between the 5CB molecules was much stronger than that of the 5CT molecules. This is because the terphenyl groups in the 5CT molecules have stronger attractive forces to each other than the biphenyl groups in the ncb molecules to compensate for the dipolar repulsive forces between the end CRN groups when the molecules are aligned parallel at the air/water interface. Various experiment results also showed that not only the CB groups were more tilted, but also the space between the CB groups was more separated, than that of the CT groups. 66 The Langmuir film of the binary mixture of the 8CB and the 5CT molecules showed that segregation was throughout the whole range of mole fractions. 66 This immiscibility of 8CB and 5CT molecules indicated that the adhesion forces between 8CB and 5CT molecules were much weaker than the self-adhesion forces of either the 8CB or the 5CT molecules, or both. The stability and segregation behaviour of the Langmuir films with the binary mixture of the 8CB or 5CT with other common surfactants were also studied. 68 It was found that the 8CB 994 Phys. Chem. Chem. Phys., 2009, 11, This journal is c the Owner Societies 2009

5 molecule can form stable miscible Langmuir monolayers; while the 5CT cannot and would segregate. One conclusion from these results is that the self-adhesion forces between the 5CT molecules are much stronger than those of the 8CB molecules. A similar conclusion is also true for the 5CB molecules. The second phase transition of the 8CB and 5CB monolayers was identified as the single layer to the triple layer transition using surface density dependent measurements with ellipsometry thickness, the SHG and BAM surface 7,63,64 This second phase transition is unique for the ncb molecules, and a plausible mechanism was also proposed. One possibility was that water molecules might have penetrated up to the hydrophobic alkyl chain of the CB molecules in the Langmuir monolayer, and this might have attracted the ncb bilayers to be formed on top of the first 8CB monolayer. 7 Even though the ncb monolayers with a different alkyl chain length had a similar second phase transition in their p A curves, the stability or the formation ability of their Langmuir monolayers was found to be dependent on the alkyl chain length. 69 In general, when n r 3orn Z 13, ncb cannot form stable compressible monolayers at the air/water interface. For those ncb molecules with the chain length in-between, the rigidity and stability as well as the molecular packing in the Langmuir monolayer also vary with the alkyl chain length. 69 The difference of the p-a curves for the 8CB and the 5CB monolayers in Fig. 2 clearly illustrates that the 5CB monolayer is less rigid and more compressible than the 8CB monolayer. Even though the experiments have constantly shown stark differences in the phase behaviour for the 5CT and ncb Langmuir monolayers, theoretical simulation so far has not been successful in reproducing these differences. A recent Monte Carlo (MC) simulation study on the 5CB and 5CT molecules at the air/water interface concluded that the properties of the adsorption layers of these two different molecules were rather similar to each other. 80 In summary, the difference of the surface density at which the gaseous-liquid condensed co-existence phase to the condensed phase transition for the 8CB, 5CB (between Regions I and II, as defined above) and 5CT monolayers can be understood through the difference of the stronger attractive forces of the terphenyl in the nct molecules than that of the biphenyl groups in the ncb molecules. The molecular origin of the second phase transition, which is unique for the ncb molecules, has not been fully understood, even though it was thought to be related to possible water penetration into the ncb monolayers. 7 The better miscibility of the 8CB molecule with other surfactants suggested stronger repulsive forces between the aligned ncb chromophores. Thus, it is also understandable that the water molecules may have a chance to penetrate or to be accommodated in-between the ncb molecules. Whether the water penetration or accommodation also influences the first phase transition of the ncb Langmuir monolayers, has not been discussed in literature. 1993, and have been investigated by quite a few other groups afterwards. 33,35 37,46 Comparison of the SFG spectra of the water species in this spectral region at other interfaces can provide information on the change of the hydrogen bonding and other interactions with the interfacial water molecules. 33,35,38,48,51,52,57,81,82 The SFG vibrational spectra of the water species at the charged interfaces or the neutral electrolyte solution interfaces have been explored extensively. 33,35,38,48,51,57,81,82 The study on the water species at a neutral interface covered with organic molecules has not been well explored. 52,57,83 The SFG-VS spectra of the water species of the air/water interface covered with the 8CB and 5CB monolayers were significantly different from those of the neat air/water interface. However, the SFG-VS spectra of the 5CT monolayer were only slightly perturbed from those of the neat air/water interface. Fig. 3 shows the SFG vibrational spectra B SFG vibrational spectra of the interface water species in the Langmuir monolayers SFG vibrational spectra of the neat air/water interface in the 3000 to 3800 cm 1 region were first reported by Shen et al. in Fig. 3 SFG vibrational spectra in the cm 1 region of the 8CB, 5CB and 5CT monolayers at typical surface densities in the ssp polarization combination (s-sf, s-vis, p-ir). The spectra of the neat air/water interface (K) in all three panels are identical. This journal is c the Owner Societies 2009 Phys.Chem.Chem.Phys., 2009, 11,

6 in the cm 1 of the ssp polarization combination at the 5CT, 5CB and 8CB monolayers with different surface densities at the air/water interface, respectively. The ssp SFG spectrum of the neat air/water interface is also present for comparison. 37,42 The neat air/water spectra are identical but on different scales, as shown in the 8CB/5CB and 5CT spectra. The spectra for the 8CB and 5CB monolayers are similar to each other, and they are significantly different from those of the 5CT monolayer. There is an intense and broad band around 3520 cm 1 in the SFG spectra for the 8CB and 5CB monolayers at the surface densities in the whole phase diagram region. This band has not been experimentally observed or theoretically predicted before in the SFG spectra of the interfacial water molecules The intensity of this band is several times stronger than that of the free O H band around 3700 cm 1, and remained almost the same below and above the surface density at the single layer to the single layer/triple layer co-existence phase transition point for both the 8CB and the 5CB monolayers. From Fig. 3 one can see that because this band is so broad and strong in the 8CB and 5CB SFG spectra, the SFG spectral features of the neat air/water interface which appeared in the 5CT SFG spectra are almost unidentifiable in the 8CB and 5CB spectra. For the 8CB monolayer, the intensity of this broad 3520 cm 1 band at the 44.4 A 2 surface density is about 70% higher than that of the lower density of 65.2 A 2. This indicates that the SFG signal of this 3520 cm 1 band for the unspecified water species is roughly proportional to the square of the surface density of the monolayer. Here the calculation involves using of the Level Law of the phase diagram in the co-existence region since the surface density of 65.2 A 2 is in the co-existence region, while the one of 44.4 A 2 is in the condensed phase. 79,91 According to the Level Law, since the surface density of the condensed phase starts at around 52 A 2, the surface coverage of the condensed phase domain for 65.2 A 2 is about 52/65.2. Since the SFG, as well as the second harmonic generation (SHG), is a coherent process, the surface SFG intensity is proportional to the square of the number density of the interfacial species which generate the SFG signal. The average of the SHG signal from a two phase co-existence surface was given in literature, 91 and the SFG signal also follows the same formula. Since the SFG signal from the free air/water surface (the gas phase in the 8CB monolayer) is much smaller than that of the condensed phase, the ratio between the SFG signals for the 44.4 and 65.2 A 2 is (1/44.4) 2 /((1/52) 2 (52/65.2)) = 1.72 : 1. This quantitative agreement further suggests that the interfacial water molecules contributing to this new spectral feature are indeed associated with the condensed phase of the monolayer. Because the 3520 cm 1 band intensities for the 8CB monolayer at 44.4 A 2 (inregionii)and37.0a 2 (inregioniii) are almost the same, and the 3520 cm 1 band intensities for the 5CB monolayer at 33.5 A 2 2 (in Region II) and 27.9 A (in Region III) are also the same, one can also conclude that the formation of the triple layer does not increase the number of the same water species in the 8CB film. This suggests that the water molecules do not penetrate up to the upper layers in the triple layer phase. Though the 3520 cm 1 band intensity for the 5CB is about 25% smaller than that of the 8CB monolayer spectra, the same conclusions can be made for the 5CB monolayer as well. The same spectral features which were present in the 8CB and the 5CB spectra throughout the whole surface density range were clearly absent in the SFG spectra of the 5CT Langmuir monolayer, as shown in Fig. 3. The SFG spectra of the 8CB and the 5CB in different polarization combinations were also apparently different from those of the 5CT monolayer. We observed that the spectral features for the 8CB and the 5CB monolayers were similar to each other, and all the spectra features for the 8CB, 5CB and 5CT were mostly independent of their respective surface densities. The SFG spectra for the 8CB at 37.0 A 2 and the 5CT at 24.8 A 2 in the ssp, ppp, and sps polarization combinations are shown in Fig. 4. As in Fig. 4, the broad 3520 cm 1 band in the SFG spectra for the 8CB monolayer in the ssp polarization was also presented in the sps spectra, but apparently disappeared in the ppp spectra. This band is so broad that it may contain contributions from different interfacial hydrogen bonded water species at the 8CB monolayer interface. According to the polarization dependence of the whole 3520 cm 1 band, it may come from the combination of hydrogen bonded modes with the C Nv and the C ss 2v (ss denotes symmetric stretch mode) symmetries. 37,42,43,51 Because of its weak ppp intensity, it can not belong to the C as 2v (as denotes asymmetric mode) symmetry mode. Therefore, this 3520 cm 1 band is clearly different from the broad 3550 cm 1 band appearing in the SFG spectra of the neutral tetra(ethylene oxide) n-dodecyl ether (C 12 E 4 ) and Fig. 4 SFG vibrational spectra in the cm 1 region of the 8CB and 5CT monolayers at surface densities in the liquid condensed (LC) phase in the ssp, ppp and sps polarization combination. In the 5CT spectra (lower panel), the 3580, 3610 and 3670 cm 1 bands, as pointed with the three arrows, can be identified on the ppp, sps and ssp spectra, respectively. 996 Phys. Chem. Chem. Phys., 2009, 11, This journal is c the Owner Societies 2009

7 octa(ethylene oxide) n-dodecyl ether (C 12 E 8 ) monolayer interfaces, which was the strongest in the ppp and sps polarization combinations and was assigned to the C as 2v symmetry using the polarization selection rules. 37,43,51 This 3520 cm 1 band is not only much more intense but also much more red-shifted and broader than the 2700 cm 1 O D band (about 3630 cm 1 for the O H band if considering the isotope shift from the O H) as reported by Allen et al. for the fully deuterated DPPC monolayer at the air/d 2 O interface. 57 Therefore, this 3520 cm 1 band is not likely generated from the contribution of some kind of dangling O H bond in the confined environment. Considering the fact that the Raman and infrared (IR) vibrational spectral features of the hydrogen bonded water species in the bulk water solution are broad bands peaked around 3250 and 3450 cm 1, 92 the water species contributing to this broad 3520 cm 1 band are certainly less hydrogen bonded than those major hydrogen bonded species in the bulk water. Therefore, we attribute them to some small water clustering species in the 8CB and 5CB monolayers. There were experimental and theoretical reports that the small water clusters, especially the hydrogen bonded O H stretches in the water trimer in the gaseous as well as inert gas matrix, exhibited O H stretching vibration around 3530 and 3515 cm Hence, we surmise that the broad 3520 cm 1 spectral feature observed here may belong to the hydrogen bonded O H stretching vibrations of a series of small water clusters formed in the 8CB and the 5CB Langmuir monolayer. There is also a broad spectral feature above the 3700 cm 1 in the sps polarization of the 8CB and 5CB spectra. The characterization of this feature is not clear. The C H bands in the range of cm 1 are strong in the ssp and sps polarizations, and weak in the ppp polarization. Since no single symmetry can satisfy such polarization dependence, they most likely belong to some combination of different C Nv and the C as 2v symmetry modes. The SFG spectra of the 5CT monolayer in the ssp polarization combination are similar to the spectrum of the neat air/ water interface, not only according to the spectra peak positions, but also to peak intensities, as shown in the Fig. 3, except that for the neat air/water interface the narrow peak is at around 3700 cm 1, while for the 5CT monolayer, there is a slightly red-shifted and broader peak around the 3670 cm 1. All these spectral features are much weaker than the major spectral features in the 8CB and 5CB SFG spectra. Therefore, they are almost unidentifiable in the 8CB and 5CB SFG spectra. Close inspection of the 5CT spectra in the ssp, ppp and sps polarization combinations in Fig. 4 indicated that there are three new bands in the cm 1 region. The 3580 cm 1 band is most apparent in the ppp spectra, the 3610 cm 1 band is apparent in both the sps and ssp spectra, and the 3670 cm 1 band is apparent both in the ssp and sps spectra. According to their polarization dependence, 37,42,43,51 these three bands can be tentatively attributed to the C as 2v, C Nv and the C Nv modes of the hydrogen bonded water species, respectively. In support to the assignment of C Nv symmetry to the 3670 cm 1, a band slightly above 3650 cm 1, which was broader than the free O H band, was attributed to the hydrogen bonding O H stretching between the CH 3 CN and H 2 O molecules both in the gaseous and in the liquid mixture phase Thus, this 3670 cm 1 band in the 5CT spectra can be attributed to the hydrogen bonded O H between the topmost water molecule to the CN group of the 5CT molecule monolayer sitting above. In support to this assignment, recently Richmond and co-workers observed a similarly red-shifted 3675 cm 1 band attributed to the SO 2 : water complex formed at the air/water interface. 99 Also in the SFG spectra of the 5CT monolayer, the polarization dependence of the broad band in the cm 1 range is similar to that of the neat air/water interface, 37 with the much stronger feature in the ssp polarization and the weaker and almost shapeless feature in the ppp and the sps polarization. In the cm 1 spectral region for the C-H modes, there is a very small to essentially no spectral feature in the 5CT SFG spectra. According to the susceptibility tensor elements worked out by Richter and co-workers, 100 SFG intensity of the C H modes in the phenyl groups decreases significantly when the phenyl chromophore becomes more upright. This fact indicates that the phenyl groups of the CT chromophore oriented significantly more upright than those phenyl groups of the CB chromophores in the 8CB and the 5CB monolayers. According to recent studies, 58,68 in the 8CB monolayer, the CB chromophore is tilted around 601 from the surface normal, while in the 5CT monolayer, the CT chromophore is tilted around 20 to 301 from the surface normal. Tilting of the aligned dipoles at the interface can lower the repulsive forces between the hydrophilic end group in the Langmuir monolayer. 101 Therefore, these facts suggested that the net repulsion in the 8CB monolayer is indeed much stronger than that in the 5CT monolayer. In short, the unusually broad and strong 3520 cm 1 feature in the 8CB and 5CB monolayers has not been observed in SFG-VS previously. The broad 3520 cm 1 feature at the 5CB and 8CB surfaces and the broad 3670 cm 1 feature at the 5CT surface can be clearly identified at the neutral Langmuir monolayer surfaces. At the interfaces with negatively or positively charged surfactants, the SFG spectra are dominated with the electric field induced SFG responses from the underneath bulk water molecules. 81,82 Furthermore, in the SFG spectra of the CB and the CT monolayers, both the O H stretching vibrations of the water species in the interface region and the C H stretching vibrations of the phenyl groups were remarkably different from each other. C Water penetration/accommodation and the phase behaviour of the Langmuir monolayer The significantly different SFG spectra for the CB and CT monolayers can be well correlated to the phase behaviour of these two monolayers. The SFG data showed that the water molecules were completely excluded from the 5CT monolayer, while remaining penetrating/accomodated into the 8CB and the 5CB monolayers. For the 5CT monolayer, the emergence of the 3670 cm 1 band for the hydrogen bonded O H of the water molecule with the CRN end group of the 5CT monolayer at the top of the air/water interface along with the similarity of the This journal is c the Owner Societies 2009 Phys.Chem.Chem.Phys., 2009, 11,

8 cm 1 spectral features of the 5CT monolayer to those of the neat air/water interface, shown in the Fig. 3, indicated that the 5CT monolayer at the top and the water bulk subphase below may mutually exclude each other. Because these spectral features persisted in the whole surface density range of the 5CT monolayer, such mutual exclusion is not unique for the condensed 5CT monolayer film, and can not be a result of the lateral compression. Such mutual exclusion can keep most of the structure of the water sublayers as that of the free air/water interface. Therefore, the water hydrogen bonding structure, as probed with the SFG spectral in the cm 1 region, remained the same. In order to maintain such a water surface structure the best arrangement may be that the topmost water layer is linked with the 5CT monolayer through a hydrogen bonding between the free O H bond and the CRN end group of the 5CT monolayer. The end CRN group is nevertheless very hydrophilic and the strong CRN dipoles exposed to the water sub-phase still tend to be solvated by the water molecules. Such solvation is nevertheless much reduced or even not present in the 5CT monolayer, because the surface density of the 5CT monolayer in the liquid condensed phase is higher than 29 A 2, which is a much higher surface density than that of the straight chain octadecanenitrile (C 17 CN) Langmuir monolayer, where the solvation of the end CRN group may still be possible. 79,102 Therefore, the exclusion of the water molecule from the 5CT monolayer is rather complete and the end CRN group may not be much solvated. In addition, the SFG spectra in the cm 1 for the nonadecanenitrile (C 18 CN) Langmuir monolayer were closely similar to those of the 5CT monolayer. 103 This indicates that with or without the solvated end CRN group, the hydrogen bonding structure underneath such Langmuir monolayer remains the same. Consistent results from the SFG studies on the CRN group in the 8CB, 5CT and other surfactant molecules have also been obtained in our laboratory, and they will be reported elsewhere. Recently, Allen et al. studied the O D spectra in the monolayer of the DPPC at the air/d 2 O interface. They concluded that hydrophobicity induced dry transition caused the exclusion of the water molecules from the hydrophobic DPPC film, and the underneath hydrogen bonded water structure remained almost undisturbed. 57,104 Here, our results for the 5CT monolayer are quite similar. Allen et al. further concluded that the red-shifted and broadened O D SFG spectral feature from that of the neat air/d 2 O interface belonged to the dangling O D group confined in the DPPC Langmuir monolayer. In our case, the SFG data suggested the existence of hydrogen bonding between the end CRN group and the water free O H bond at the topmost layer beneath the 5CT monolayer. The 5CT Langmuir monolayer with the mutual exclusion of the 5CT monolayer and the water sub-phase fits well the standard physical picture of the insoluble Langmuir monolayer. 27,79 However, the 8CB and the 5CB monolayers are clearly not the case. Both the phase diagrams and their SFG spectra of the 8CB and 5CB monolayers were completely different from those of the 5CT monolayer. Obviously, the existence of the single-layer to triple-layer phase transition, i.e. the second plateau in Fig. 2, indicated that the 8CB and 5CB monolayers are certainly unconventional Langmuir monolayers. 7,58 The strong and broad 3520 cm 1 band in the ssp and sps SFG spectra indicated significantly different water species existing in the two monolayers. As discussed in section IIIB, this 3520 cm 1 spectral feature is certainly not from the known water species either in the liquid water phase or at the neat air/water interface. Since the more the water molecule is hydrogen bonded, the more red-shift of the O H vibrational spectral feature, the 3520 cm 1 feature nevertheless indicates an intermediately hydrogen bonded water species between the singly hydrogen bonded and the liquid like species. 34 The intense 3520 cm 1 band for the 8CB monolayer in the ssp SFG spectra was blue shifted from the broad hydrogen bonded band in the cm 1 range of the neat air/water interface. This suggested that the corresponding water species are less hydrogen bonded than the water species below the topmost water layer at the neat air/water interface or underneath the 5CT monolayer. This 3520 cm 1 band for the 8CB monolayer is about ten times more intense than that of the broad hydrogen bonded band in the cm 1 range. There are three factors can affect the SFG intensity, namely, surface number density, molecular orientation and orientational distribution, and strength of the nonlinear susceptibility tensors. The last two factors can not cause such a ten-fold increase of the SFG signal. Firstly, the strength of the nonlinear susceptibility tensors is fairly constant for the different hydrogen bonded species whose vibrational spectra spreads across the whole cm 1 spectral region. Secondly, the orientation distribution of the hydrogen bonded species at the interface is not extremely broad and is with a certain distribution width. 37 Our simulation showed that the ten fold increase of the intense 3520 cm 1 band SFG signal can not be explained by simply assuming the intense 3520 cm 1 band in the 8CB and 5CB monolayers comes from the same or a subset of the water species in the 5CT monolayer only with significantly different orientation. Therefore, this indicates that there are much more water molecules contributing to the SFG signal in the 8CB monolayer case than in the 5CT monolayer. If we assume that the strength of the differently hydrogen bonded water species differs not so significantly, the ten times SFG signal indicates a number of IIIB times for that of the hydrogen bonded water molecules contributing in the 8CB monolayer than that in the 5CT monolayer. In the case of 5CB, the number is about 2.5, according to the data in Fig. 3. Increasing the number of the interfacial water molecules at the air/water interface with the bulk salt concentration was recently studied with both SFG-VS and non-resonant second harmonic generation (SHG). 92,105 However, in these cases, the increase of the number of interfacial water species contributing to the SFG-VS or SHG signal was less than 30%, and the corresponding SFG or SHG signal increased by less than 100%. Therefore, the significant increase of the number of water molecular species in the 8CB and 5CB monolayers here can not be explained with the perturbations of the interface water structure as with the 5CT monolayer. There has to be some structural mechanisms responsible for such a big 998 Phys. Chem. Chem. Phys., 2009, 11, This journal is c the Owner Societies 2009

9 difference. As discussed in section 3.2, this 3520 cm 1 band bears the signature of the O H stretching vibrational spectra of the hydrogen bonded small size water clusters. For example, the water trimer in the gaseous phase and in the inert gas matrices Furthermore, in order to be observed in the SFG-VS spectra, such water species has to be ordered at the surface. Therefore, such small size clusters can only be formed in a directionally confined environment. Hence, we surmise that this intense hydrogen bonded water O H band comes from water species penetrated or accommodated into the gaps between the aligned 8CB or the 5CB molecules in the 8CB or 5CB monolayers. The 3520 cm 1 band existed throughout the whole surface density range of the 8CB and the 5CB monolayers. The forming of this water cluster species is therefore not a result of high surface pressure. According to the 8CB and 5CB monolayer phase diagram, the single 8CB and 5CB Langmuir monolayer films were broken at less than 6 mn m 1 to start forming a triple-layer structure. The SFG spectra showed that the new 3520 cm 1 band remained the same when the film underwent this second phase transition. Thus, it is unlikely that the water molecules existed beyond the first 8CB or 5CB layer immediately above the water phase. According to the proposed structure by Xue et al., 7 the two 8CB or 5CB layers above had an anti-parallel structure of the CB chromophores. Such anti-parallel structure also existed in the freely suspended films in the smectic A phase with the pure 8CB liquid crystal. 72 From these facts we conclude that the lateral compression was not able to reduce the number of these water species in the bottom layer of the 8CB and 5CB monolayer. An interesting fact in the phase transition of the 8CB and the 5CB monolayer film is that they were willing to undergo a single-layer to triple-layer phase transition (region II to region III) under lateral compression and still keep these water species inside the 8CB monolayer. This indicates that the existence of these water molecules in the 8CB and the 5CB monolayers kept the aligned 8CB and the 5CB monolayer stable. The only structural difference between the CB and the CT chromophores is that the CT molecule has one more phenyl group. Therefore, the persistence of the water molecules inside the CB monolayers instead of the CT monolayers suggests a delicate balance between the repulsive forces between the parallelly aligned CRN group and the attractive van der Waals forces between the phenyl groups in these molecules. Since the CT chromophore in the 5CT monolayer can be closely packed, as shown in its phase diagram in Fig. 2, the attraction between the terphenyl groups in the 5CT chromophore must be strong enough to counterbalance the strong repulsive force between the more closely aligned and less solvated CRN groups. If this picture is true, the role of the water molecules inside the 8CB and 5CB monolayers is to reduce the strong repulsive force between the parallelly aligned neighbouring CB chromophores. On the other hand, because of such repulsive force, the distance between the neighbouring CB chromophores has to be larger and thus leaves space for the water molecules to penetrate or to be accommodated into the monolayer film, and to provide additional dielectric shielding between the repulsive dipoles. Thus, the water penetration or accommodation is more a result of the repulsion in the monolayer film, rather than a cause for the separation. This distinction is meaningful because one would expect that the monolayer fluctuation might allow some structural mixing of the different chemical species in the monolayer. In fact, the total exclusion of the water molecules in the 5CT monolayer even with the closely packed and aligned CRN group barely solvated suggested that the water molecules acted rather passively than actively in penetrating into the 8CB and 5CB monolayers. This is why we find that the term penetration/accommodation might be a better description of the real physical picture than simply use the term penetration. However, this does not diminish the importance of the water molecules in mediating the structural and phase behaviour of the molecular films as well as the molecular aggregates. The difference in the phase diagrams and the SFG spectra of the 8CB and the 5CB monolayers is also interesting. Obviously, the 8 member alkyl chain in the 8CB molecule would have a larger attractive force between each other than the 5 member chain in the 5CB molecule. This should have resulted in more compressibility for the 8CB monolayer than that for the 5CB monolayer. However, their phase diagrams in Fig. 2 demonstrate that the 5CB monolayer is more compressible than the 8CB monolayer. 69 The stronger 3520 cm 1 band in the 8CB SFG spectra also shows that more water penetration or accommodation exists in the 8CB monolayer than that in the 5CB monolayer. These facts indicate that the attractive forces between the alkyl chains might actually act in the opposite direction as the phenyl groups. How the longer chain could result in larger repulsion and more water penetration or accommodation into the 8CB monolayer than the 5CB monolayer is not clear at this moment. One scenario we can offer here is that because of the different cross-sections of the alkyl chain and the biphenyl groups, the alignment of the chain may be spatially mismatched to the alignment of the biphenyl group. Therefore, the stronger attraction and alignment between the longer alkyl chain might actually reduce the attraction and alignment between the biphenyl groups, leaving more room for the water penetration or accommodation and less lateral compressibility. In summary, the proposed mechanism for the water penetration or accommodation into the 8CB and 5CB monolayers, and the water exclusion from the 5CT monolayer is illustrated in Fig. 5. The upper horizontal arrows represent the attractive force between the phenyl groups, while the lower horizontal arrows represent the repulsive force between the aligned CRN groups. The 8CB molecule is more tilted and more spaced to reduce the repulsion, allowing water penetration or accommodation into the monolayer. The 5CT molecule is less tilted and more closely packed because of the stronger attractive forces between the terphenyl groups, allowing no water penetration or accommodation. The differences in the SFG spectra and the phase diagrams of the 8CB/5CB and the 5CT monolayers provided an intriguing opportunity to understand the detailed balance of the attractive/repulsive forces in the molecular films and aggregates. The penetration or accommodation of the water molecules into the 8CB and 5CB monolayers but not the 5CT monolayers also suggests that solvation of the hydrophilic This journal is c the Owner Societies 2009 Phys.Chem.Chem.Phys., 2009, 11,

10 Fig. 5 Illustration of the proposed mechanism for the water penetration or accommodation into the 8CB monolayer and the water exclusion from the 5CT monolayer. The upper horizontal arrows represent the attractive force between the phenyl groups, while the lower horizontal arrows represent the repulsive force between the aligned CRN groups. The 8CB molecule is more tilted and more spaced in order to reduce the repulsion, allowing water penetration or accommodation into the monolayer, while the 5CT molecule is less tilted and more packed because of the stronger attractive forces between the terphenyl groups, allowing no water penetration or accommodation. groups can be neglected when there is strong attraction between the fragments directly connected to the hydrophilic groups. It is also interesting that the role of water is rather passive than active in this physical picture. IV. Conclusions SFG vibrational spectra of the water species in the Langmuir monolayers of the neutral liquid crystal molecules of 8CB, 5CB and 5CT showed that in the more expanded 8CB and 5CB monolayers there is a strong and broad hydrogen bonded O H band around 3520 cm 1. This novel spectral feature has not been observed for the aqueous interfaces before and can be attributed to the water species penetrated or accommodated into the 8CB and 5CB monolayers. Such water species was clearly absent from the closely packed 5CT monolayer, where the interfacial water structure was only slightly perturbed, as indicated from the SFG spectra. The surface pressure vs. surface density phase diagrams of the 8CB, 5CB and 5CT monolayers were also remarkably different. Because of the water penetration or accommodation into the 8CB and 5CB monolayers, the films became much more expanded. This was a result of the repulsive forces between the aligned neighbouring end CRN groups that can not be balanced by the attractive forces of the biphenyl groups in the CB chromophores. As a result, the repulsive forces created a space for the water penetration or accommodation, and the water penetration or accommodation further provides screening of the electrostatic repulsion between these dipoles of the CRN groups. These findings provided a molecular level mechanism for the stark difference of the phase behaviour between the CB and CT insoluble Langmuir monolayers at the air/water interface. It also calls for attention to the specific water surface interaction in controlling the structure and phase behaviour of the molecular assemblies at heterogeneous aqueous interfaces. The importance of the detailed balance between the dipolar repulsive forces and the van der Walls attractive forces in the determination of the structure of complex molecular assemblies, as well as that of the solvent molecules that solvate the polar groups, has been well recognized in the literature. Recent theoretical studies showed that the wetting and dewetting of the hydrophobic surfaces in the process of protein folding are controlled by detailed molecular interaction forces Therefore, the possibility as demonstrated here to experimentally understand the role of the water species and the detailed mechanism of such interactions in the interface region and in confined environments can be important and revealing. This physical picture may contribute more to the understanding of the dynamic nature and functions of the molecular films as well as the biological membranes, because the expanded phases generally exist in the Langmuir films and other molecular assemblies. Therefore, water penetration/ accommodation into these films and molecular aggregates can be a general phenomenon. These water molecules can form structured water species inside the confined environments and further mediate and influence their structures and functions. Importantly, such water species can be probed with interface specific SFG vibrational spectroscopy (SFG-VS) as demonstrated in this study. With the penetration/accommodation of the water molecules, such molecular film or membrane possesses greater miscibility with other molecules, and the whole system is more dynamic and more susceptible to slight changes in the balance of the involving repulsive/attractive forces. In such a way, its flexibility can be chemically mediated by the interacting molecules, which may lead to complexity and specificity of the transmembrane functions. Acknowledgements Z.Z. appreciates the helpful discussions with Wei Gan and Ran-ran Feng. 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13 PCCP Articles Page 1 of Publishing An international journal for the fastest publication of high-quality original work in physical chemistry, chemical physics and biophysical chemistry. Subscribers PDF HTML article Non-subscribers Purchase article PDF [ 27 + taxes] Purchase article PDF member offer [ 5 + taxes] Free access Paper Search for citing articles Phys. Chem. Chem. Phys., 2009, 11, , DOI: /b813187b Water penetration/accommodation and phase behaviour of the neutral langmuir monolayer at the air/water interface probed with sum frequency generation vibrational spectroscopy (SFG-VS) Zhen Zhang, De-sheng Zheng, Yuan Guo and Hong-fei Wang A strong and broad hydrogen bonded O H band around 3520 cm -1 is observed in the insoluble monolayer of the neutral liquid crystal molecules of 4 -n-pentyl-4-p-cyanobiphenyl (5CB) and 4 -n-octyl-4-p-cyanobiphenyl (8CB) throughout the whole surface density range, but not in the 4-pentyl-4 -cyanoterphenyl (5CT) monolayer, at the air/water interface. This novel spectral feature suggests the existence of an oriented water cluster species which has penetrated or accommodated into the Langmuir monolayer of the 8CB and 5CB molecules. This finding provided a molecular level mechanism for the stark difference in the phase behaviour between the CB and CT insoluble Langmuir monolayers at the air/water interface. It also calls for attention to the details of the specific water surface interaction in mediating the structure and the phase behaviour of the molecular assemblies at the heterogeneous aqueous interfaces. Royal Society of Chemistry 2009