Aggregation Behavior of Mixed Fluorocarbon and Hydrocarbon Molecules in Aqueous Organic Solvents. Nonideality and Ideality of Mixing

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1 J. Phys. Chem. 1995, 99, Aggregation Behavior of Mixed Fluorocarbon and Hydrocarbon Molecules in Aqueous Organic Solvents. Nonideality and Ideality of Mixing Chen-Ho Tung* and Hai-Feng Ji Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing , China Received: December 2, 1994; In Final Form: February 15, The emission spectra of naphthalene labeled hydrocarbons (NpHCn) and fluorocarbons (NpFCn) and anthracene labeled hydrocarbons (AnHCn) and fluorocarbons (AnFCn) in aqueous organic mixed solvents were investigated. The fluorescence spectrum of NpHC 16 in dimethyl sulfoxide-water (DMSO-H20) is dominated by excimer emission due to formation of aggregates. Addition of a fluorocarbon compound with chain length of seven perfluoromethylenes results in a reduction in the excimer emission and an enhancement of monomer emission, suggesting formation of coaggregates. In contrast, addition of a perfluorocarbon compound with chain length of 12 perfluoromethylenes results in no change in the fluorescence spectrum of NpHC16. Similarly, NpFC7 and NpFC12 in DMSO-H20 also exhibit excimer emission exclusively. Fluorescence studies show that hydrocarbon compounds coaggregate with NpFC7 but do not coaggregate with NpFC12. Selective excitation of the naphthalene moiety in the mixture solution of NpHC16 with AnFC7 or in the mixture solution of NpFC7 with AnHC12 exclusively leads to the emission of the anthracene moiety, suggesting that efficient energy transfer between the naphthalene and the anthracene chromophores occurs. On the other hand, for the mixture of NpHC16 with AnFC12 or NpFC12 with AnHC12, no energy transfer was detected. All these observations indicate that fluorocarbon compounds with short chains can mix with hydrocarbon compounds ideally, while those with long chains behave nonideally. The mixed critical aggregation concentrations of NpHC16 with NpFC7 and with NpFC12 are measured as a function of overall composition of the mixtures, which give insight into the magnitude of the nonideality of the mixings. Introduction Fluorocarbons and hydrocarbons exhibit pronounced mutual phobicity.',2 The miscibility of fluorocarbon and hydrocarbon surfactants has been the subject of numerous investigation^.^-'^ It has been reported that in many cases the critical micelle concentrations (cmc's) of the mixed systems are larger than the cmc values of the isolated surfactant^.'^,'^ Furthermore, many works indicate the existence of two cmc's in such systems.'~~ All these observations suggest the coexistence of two types of micelles, one rich in the fluorocarbon surfactants and the other rich in the hydrocarbon surfactants. The results of nuclear magnetic resonance self-diffusion measurements are also consistent with the above suggestion.'6 However, evidence for the existence of one type of mixed micelles was also rep~rted.~.~ Thus, the possible coexistence of two types of micelles vs formation of one type of mixed micelles has been subject to debate. It is well established that in aqueous or aqueous organic binary solvents hydrophobic interactions force electrically neutral hydrocarbon molecules to form aggregates at low concentration^.'^,'^ Recently, we have demonstrated that fluorocarbon analogues tend to form aggregates more easily in aqueous organic solvent^.'^ The aggregates thus formed may serve as simple models for molecular assemblies and may have valuable applications in synthetic chemistry and other disciplines Hence, the miscibility of electrically neutral hydrocarbons and fluorocarbons in aggregates is an interesting problem to study. A variety of approaches such as surface tension6 and condu~tivity~.'~~'~ measurements, fluorescence probe methand 'H NMR, I3C NMR, and I9F NMR spectros~opies'~ have been employed to explore and examine the miscibility of hydrocarbon and fluorocarbon surfactants in micellar Abstract published in Advance ACS Abstracts, April 1, and the structural and dynamic features of their mixed micelles. Most of these approaches give information on the cmc data. In the present work we use chromophore end-labeled hydrocarbon and fluorocarbon molecules as fluorescence probes to investigate their excimer formation and energy transfer in aqueous organic mixed solvents. We obtained straightforward evidence for ideal and nonideal mixing of fluorocarbon and hydrocarbon molecules in aggregate systems, depending on the chain lengthes of the molecules. The probes we used have the following structures: NpCOOCH,C,F,,+, NpCOOC,H,,+, /95/ $09.00/ American Chemical Society NpFCn (n = 4,7, 12) NpHCn (n = 6, 12, 16) C,F,,+,COOCH,An AnFCn (n = 7, 12) C,H,,+,COOCH,An AnHCn (n = 12) where Np = 2-naphthyl and An = 9-anthryl. Results and Discussion Excimer Formation of NpHCn, AnHCn, NpFCn, and AnFCn. The fluorescence spectra of NpHC16 in dimethyl sulfoxide-water (DMSO-H20) mixture with the volume ratio of 5050 are shown in Figure 1. At low concentration, this compound exhibits structured fluorescence characteristic of the naphthoate monomer with maxima at 342, 358, and 370 nm. As the concentration of NpHC 16 increases, the monomer fluorescence intensity decreases, and a structureless excimer emission centered at ca. 420 nm is observed. At concentration greater than 7 x lo-' M, only excimer fluorescence is detected. The plot of the ratio of fluorescence intensities of excimer to monomer, ID/IM, as a function of NpHC16 concentration displays a break, which is a typical behavior of aggregation

2 8312 J. Phys. Chem., Vol. 99, No. 20, 1995 Tung and Ji ( I. 5 '.' x I O - ~ M Figure 1. Fluorescence spectra of NpHC16 in DMSO-H20 with the ratio of 5050, A,, = 280 nm; 1 x lo-' M NpHC16 (-); 6 x IO-' M NpHC16 (- - -); 5 x M NpHC16 (e e). Ii I 'x10+ M [NpHC16] Figure 2. Plot of the ratio of fluorescence intensities of excimer (at 420 nm) to monomer (at 340 nm), ID/IM, for NpHC16 in DMSO- H20 (5050) versus concentration (Aex = 280 nm). (Figure 2). Thus, we attribute the excimer formation at great concentration to aggregation, which induces the enhancement of local chromophore concentration. The concentration corresponding to the break point in Figure 2 is defined as the critical aggregation concentration (cac). The anthracene labeled hydrocarbons AnHCn show similar behavior in aqueous organic solvents. [NpFClZ] Figure 3. Plot of the ratio of fluorescence intensities of excimer (at 420 nm) to monomer (at 360 nm), ID/IM, for NpFC12 in DMSO- H20 mixed soivents with the ratio of 40:60 as a function of the concentration (A,, = 280 nm). The behavior of NpFCn and AnFCn in aqueous organic mixed solvents is analogous to that of NpHCn and AnHCn. Figure 3 shows the IDAM as a function of concentration for NpFC12 in DMSO-H20 with the volume ratio of 40:60. The plot also displays a break, suggesting formation of aggregates. The driving force for aggregate formation for both hydrocarbons and fluorocarbons in aqueous organic solvents would be hydrophobic interactions. Each molecule of the probes used in the present work contains a huge and planar building moiety, either naphthyl or anthryl. These hydrophobic moieties undoubtedly make contribution to the formation of aggragates. To estimate the extent of the importance of these moieties in the formation of aggregates, we examined the fluorescence spectra of NpHCn and NpFCn with short chains. Both NpHC6 and NpFC4 only exhibit monomer fluorescence even in aqueous organic solvents with a large ratio of water to organic solvent, suggesting that the hydrophobic force underwent by these molecules is not large enough for aggregate formation. Thus, we believe that the naphthyl and anthryl groups in the molecules studied in the present work play an important role in the formation of aggregates, but themselves are not sufficient to induce aggregation. Furthermore, it has been established that the expression of hydrophobicity involves water-structure effects and the substrate molecule-molecule interaction^.^.^ Within the aggregates the chain-chain interactions are expected to differ from the interactions between the aromatic moieties, particularly for the NpFCn or AnFCn systems. This fact might affect the orientation of the component chromophores within the aggregates. To ascertain this suggestion, we performed the photodimerization of AnHCn and AnFCn in aqueous organic solvents. Photoirradiation of 9-substituted anthrancene results in the dimerization of the aromatic rings at 9,lO-positions to yield head-to-head (h-h) and head-to-tail (h-t) photodimer~**~~~ as shown in Scheme 1. For the photodimerization of AnHC12 or AnFC12 in organic solvents the main product is the h-t dimer. However, in DMSO-HzO photoirradiation of AnHC12 or AnFC12 yields exclusively h-h rather than h-t dimers. Thus, we demonstrated that the hydrophobic interactions underwent by the anthryl groups in aggregates affect their orientation and enhance the regioselectivity in the photodimerization.

3 Mixed Fluoro- and Hydrocarbons in Organic Solvents J. Phys. Chem., Vol. 99, No. 20, Figure 4. Fluorescence spectra of NpHC16 in DMSO-H20 with the ratio of 50:50, I,, = 280 nm: 5 x M NpHC16 (-); 5 x M NpHC16 in the presence of 5 x M C7Fl&OOCH3 (- - -); 5 x M NpHC16 in the presence of 6 x M C12F25COOCH3 (.*a). SCHEME 1 Figure 5. Fluorescence spectra of NpFC7 in DMSO-H20 with the ratio of 40:60, I,, = 280 nm: 5 x M NpFC7 (-); 5 x M NpFC7 in the presence of 1 x M CISH~X e..). R I hv h- h h--1 Effects of Unlabeled Fluorocarbons on Excimer Formation of NpHCn and Unlabeled Hydrocarbons on Excimer Formation of NpFCn. To obtain information on the miscibility of fluorocarbons and hydrocarbons in aggregate systems, we examined the effects of unlabeled fluorocarbon compounds on the excimer formation of NpHC16. As shown in Figure 4, at a concentration of 5 x M NpHC16 only emits excimer fluorescence due to the formation of aggregates. Addition of methyl perfluorooctanoate (C~FI 5COOCH3) to the solution results in an enhancement of the monomer emission and in a reduction of the excimer fluorescence. In the presence of sufficient methyl perfluorooctanoate, the fluorescence spectrum of NpHC16 is dominated by the structured monomer emission. Obviously, this is because methyl peffluorooctanoate coaggregates with NpHC16, and the chromophores are diluted in the coaggregates. In contrast, addition of a fluorocarbon compound with a long chain, such as methyl perfluorotridecanoate (C12F25- COOCH3), to the solution results in no change in the fluorescence spectrum of NpHC16, even when the concentration of L fnn) Raveleng th Figure 6. Fluorescence spectra of NpFC12 in DMSO-H?O with the ratio of 40:60, lex = 280 nm: 5 x M NpFC12 (-); 5 x M NpFC12 in the presence of 1 x M C18H38 e*-). methyl perfluorotridecanoate is greater than 6 x lop4 M. These observations suggest that the fluorocarbon compound with short chain length can mix with NpHC16 in a large range of compositions, but that with a long chain is not miscible with NpHC16 in aggregate systems. This is consistent with the observation that, to cause the phase separation in liquid-liquid mixtures and in mixed micelles of fluorocarbon and hydrocarbon surfactants, a chain longer than a certain length is necessary.'o The effects of unlabeled hydrocarbons on the emission spectra of NpFCn in aqueous organic mixed solvents were also studied. As shown in Figures 5 and 6, NpFC7 and NpFC12 in DMSO- H20 with the ratio of 40:60 exhibit exclusively excimer fluorescence at high concentration due to formation of aggregates.

4 8314 J. Phys. Chem., Vol. 99, No. 20, 1995 Tung and Ji J a Y (nn) Wave 1 ength Figure 7. Fluorescence spectra of mixtures of NpHC16 with AnFC7 and NpHC16 with AnFC12 in DMSO-H20 (50:50), A,, = 280 nm: 2 x M NpHC16 (-); 2 x M NpHC x M AnFC7 (- - -); 2 x M NpHC x M AnFC12 (.e-). Addition of a hydrocarbon compound such as octadecane to the solution of NpFC7 leads to a reduction in the excimer emission and an increase in monomer fluorescence, indicating that octadecane molecules coaggregate with NpFC7 (Figure 5). However, addition of octadecane to the solution of NpFC12 results in no change in the emission spectrum of NpFCl2 (Figure 6). These observations support the suggestion that fluorocarbons with short chains can mix with hydrocarbons, but those with long chains cannot coaggregate with hydrocarbon compounds. Energy Transfer between Naphthalene Labeled and Anthracene Labeled Molecules. The evidence for the miscibility between the hydrocarbons and the fluorocarbons with short chains and the immiscibility between the hydrocarbons and fluorocarbons with long chains in aggregate systems based on excimer formation is further strengthened by the study of energy transfer between naphthalene labeled and anthracene labeled compounds. The fluorescence spectrum of the equimolar (2 x M) mixture of the naphthalene labeled hydrocarbon (NpHC16) and the anthracene labeled fluorocarbon (AnFC7 or AnFC12) in DMSO-H20 with the ratio of 5050 is shown in Figure 7 by using 280 nm excitation. As mentioned above, NpHC16 in DMSO-H20 at concentration above its cac only exhibits excimer emission due to formation of aggregates. However, in the presence of AnFC7 the structured fluorescence characteristic of anthracene monomer with maxima at 396 and 418 nm and a shoulder at 440 nm is observed in addition to the weak fluorescence of the naphthalene chromophore. The absorption spectra indicate that for the equimolar mixture of NpHC16 and AnFC7 more than 95% light at 280 nm is absorbed by NpHC 16. Evidently, efficient energy transfer from NpHC 16 to AnFC7 occurs. This suggests that NpHC16 coaggregates with AnFC7, and within the coaggregates the mutual chromophore separation favors energy transfer. In contrast, for the equimolar mixture of NpHC16 and AnFC12 in DMSO-H20 excitation at 280 nm only leads to the excimer emission of NpHC16, suggesting the absence of energy transfer. Thus, NpHC16 molecules do not coaggregate with AnFC12. J Figure 8. Fluorescence spectra of mixtures of NpFC7 with AnHC12 and NpFC12 with AnHC12 in DMSO-H20 (50:50), A,, = 280 nm: 2 x M NpFC7 (-); 2 x M NpFC7 + 2 x M AnHC12 (- - -); 2 x M NpFC12 (-e-); 2 x M NpFC x M AnHC12 (.*e). The energy transfer between the naphthalene labeled fluorocarbon (NpFC7 or NpFC12) and anthracene labeled hydrocarbon (AnHC12) in aqueous organic solvents was also examined. Figure 8 shows the fluorescence spectra of the mixed solution of NpFC7 with AnHC12 and NpFC12 with AnHC12 in DMSO-H20 with the ratio of 5050 by using 280 nm excitation. Efficient energy transfer from NpFC7 to AnHC12 is observed, while no evidence for energy transfer from NpFC12 to AnHC12 is obtained. The above study provides additional evidence for the proposal that the fluorocarbons with short chains are miscible with hydrocarbons, but those with long chains cannot mix with hydrocarbons in aggregate systems. Magnitude of the Nonideality of Mixing. To examine the magnitude of the nonideality of mixing of fluorocarbon compounds with hydrocarbon compounds in aggregate systems, we measured the cac s of the mixtures of the naphthalene labeled hydrocarbons and fluorocarbons in various compositions in aqueous organic solvents by using the excimer formation method described as in Figure 2. Figure 9 shows the cac s of the mixtures of NpHC16 with NpFC7 in DMSO-H20 with the ratio 5050 as a function of mole fraction of NpHC16, and Figure 10 shows those for NpHC16 with NpFC12 in 1P-dioxanewater (DX-H20) with the ratio 25:75 as a function of mole fraction of NpFC12. In the treatment of mixed micellization in binary surfactant mixtures, the following equations have been derived to calculate the mixed cmc as a function of the overall composition of the mi~ture.~~.~~ where C*, Cl, and C2 are the cmc s of mixed system and pure surfactants 1 and 2, respectively. al and a2 are the mole fractions of surfactants 1 and 2 in total mixed solute, respectively, and5 andf2 are the activity coefficients of surfactants 1

5 Mixed Fluoro- and Hydrocarbons in Organic Solvents J. Phys. Chem., Vol. 99, No. 20, CAC x ~ O - M ~ /3 is the net interaction parameter between surfactants 1 and 2 and can be readily obtained when C* is known. This requires solving XI by using a relationship such as Then,4 is given directly by NpFC? NpHClb Figure 9. Cac s (0) of the mixtures of NpHC16 with NpFC7 in DMSO-H20 (50:50) as a function of the mole fraction of NpHC16. The dashed line presents the prediction for ideal mixing, and the solid line is that for ideal mixing with /3 = CAC ~10-7~ NpHClb NpFCl2 Figure 10. Cac s (0) of mixture of NpHC16 with NpFC12 in DX- HzO (25:75) as a function of the mole fraction of NpFC12. The dashed line presents the prediction for ideal mixing, and the solid line is that for the nonideal mixing with /3 = and 2 in mixed micelles, respectively. For ideal mixed micelles fi andf2 are unity, and eq 1 is reduced to eq 2.26, a1 a, c* c, c2 In the case of nonideal mixed micellesfi andfi are given by fl = e xpw - X O~I (3) f2 = e xpl~x,~~ (4) where XI is the mole fraction of surfactant 1 in mixed micelles. We applied the above equations to the calculation of the mixed cac s of NpHC16 with NpFC7 in DMSO-H20 and NpHC16 with NpFCl2 in DX-H20. Results are shown in Figures 9 and 10, where the experimental points are compared with calculated results from the nonideal mixing model (solid line) and the ideal mixing model (dashed line). Evidently, the NpHC16-NpFC7 system behaves almost = 0.13), while NpHC 16-NpFC12 deviates significantly from ideality with a,4 value of Experimental Section Materials. Perfluoroalkanoic acids C,F~,+ICOOH (n = 4, 7, and 12) were donated by Shanghai Organic Institute, Chinese Academy of Sciences. Methyl perfluorooctanoate and methyl perfluorotridecanoate were synthesized by the reaction of the corresponding peffluoroalkanoic acids with methan01.i~ Perfluoroalkanols CnF2,+1CH20H (n = 4, 7, and 12) were synthesized by reduction of the corresponding methyl perfluoroalkanoates with LiAlH4. 2-Naphthoyl chloride was prepared by reaction of 2-naphthoic acid with thionyl chloride and distilled at reduced pressure. Fluorinated alkyl 2-naphthoates (NpFCn) were synthesized by esterification of 2-naphthoyl chloride with CnF2,+1CH20H. I9 9-Anthrylmethyl perfluoroalkanoates (AnFCn) were obtained by esterification of perfluoroalkanoyl chlorides with 9-anthrylmethanol. n-octadecane was of chromatographically pure grade. Various alkyl 2-naphthoates were synthesized as described previously.28 All of the obtained compounds were identified by IR, MS, and IH NMR as well as I9F NMR spectra. Spectral grade dimethyl sulfoxide and 1,4- dioxane were used for fluorescence measurements without further purification. The water was doubly distilled. Instrumentation. H NMR spectra were determined with a lt80 (80 MHz) or a Varian XL-400 (400 MHz) instrument in chloroform-d using TMS as the internal standard. I9F NMR spectra were recorded on a Varian XL-400 (400 MHz) instrument using TFA as the internal standard. Mass spectra were determined with a Finnigan 4021C mass spectrometer with an electron impact source at 25 or 70 ev. Fluorescence spectra were run on a Hitachi-850 or MPF-4 spectrofluorimeter. Fluorescence Measurements. The samples were purged with nitrogen for at least 30 min before measurements. The excitation wavelength was 280 nm for both the excimer formation and energy transfer studies. The spectra were fully corrected for instrument response. In the measurement of cac s based on the excimer formation, of particular interest is the excimer-to-monomer intensity ratio, ID/IM, which was calculated from the peak heights at 420 and 360 nm for excimer (ID) and monomer (IM), respectively. The peak heights and peak areas were proportional. References and Notes (1) Mukerjee, P. J. Am. Oil. Chem. SOC. 1982, 59, 573.

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