Colloids and Surfaces, 38 (1989) 305-311 Elsevier Science Publishers B. V., Amsterdam Printed in The Netherlands 305 Tris(2,2'-bipyridyl)ruthenium(lI) Chloride as a Probe of Adsorption Characteristic of Sodium Dodecyl Sulfate on Alumina JOY T. KUNJAPPU. and P. SOMASUNDARAN Langmuir Center for Colloids and Interfaces. School of Engineering and Applied Science. Colwnbia University, New York, NY 10027 (U.S.A.) (Received 30 March 1988; accepted 11 November 1988) ABSTRACT Analysis of steady-state luminescence emission of tris(2.2' -bipyridyl)ruthenium(li) chloride incorporated in the alumina-sodium dodecylsulfate hemimicelles (more appropriately called as "solloids") substantiates the reverse orientation model at high surface coverage suggested earlier to explain the surface aggregation process. The inadequacy of the bilayer model to account for the above process, especially in region II of the adsorption isotherm, is brought forth. INTRODUCTION Aggregation of ionic surfactants on charged oxide solid minerals from aqueous solutions has been extensively studied by one of the authors [1-3] and others [4,5] due to their well-recognized importance in many industrial processes ranging from classical froth flotation [6] to many new and emerging areas like enhanced oil recovery and microelectronics [7]. Insight into the nature of the adsorbed layers of sodium dodecyl sulfate (SDS) on alumina, a representative system, has been gained by both conventional (adsorption density, zeta potential, etc. [8]) and newer (fluorescence, electron spin resonance, etc. [3,9-11]) techniques. Surfactant aggregation on alumina is described to occur above a critical concentration called the hemimicellar concentration (HMC) subsequento which the adsorption proceeds by building up more aggregates of similar size and then by forming bigger aggregates. This continues until mono- or bilayer surface coverage is complete or the critical micellar concentration is reached. A consequence of this adsorption process is the changes in the zeta potential of the solid-liquid interface which inverts from positive to negative values..post-doctoral research scientist from Chemistry Division, BARC, India-400 085. 0166-6622/89/$03.50 @ 1989 Elsevier Science Publishers B. V.
306 This communication aims at following the aggregation process of SDS on alumina with a positively charged probe [tris(2,2'-bipyridyl)ruthenium(ii) chloride (RUBP)] which is expected to bind the alumina-surfactant adsorbed layer when a favorable negative charge is developed at the interface. RUBP being a good luminescent material [12] can report, in small concentrations, the changes occurring on the alumina surface as changes in intensity and shifts in the wavelength maximum of luminescence emission. Here, RUBP's role is more elaborate than pyrene or other polyaromatic hydrocarbons [9] employed earlier since RUBP's inclusion in the adsorbed layer is largely dependent on the charge at the solid-liquid interface. RUBP in the low concentration range used in this study did not perturb the hemimicelle formation as observed from the invariance of adsorption isotherm of SDS on alumina in the absence and presence of RUBP. We had earlier shown that probes like pyrene and doxyl stearic acids [13] also did not perturb the hemimicelle formation in the alumina-sds system. Quantitative estimation of RUBP incorporated in the hemimicelles was also performed by analyzing the supernatant of the adsorption samples from absorption measurements at 450 nm. Furthermore, an attempt is made to evaluate the existing adsorption models for the alumina-sds system. t i. EXPERIMENTAL RUBP from Alfa Products was used without further purification. The adsorption samples were prepared as described elsewhere [3] by shaking alumina (Linde A from Union Carbide; particle size: 0.3 JJ.m; surface area: 15 m2 g-l) with the appropriate SDS solution and RUBP (5.3.10-5 M) solution in 0.1 M NaCI at a ph of 6.5 for 13 h. The ph of the adsorption samples was adjusted with 1 N hydrochloric acid. The slurry was used for luminescence studies. Samples for the analysis of RUBP content were prepared by centrifugation of the adsorption samples to collect the clear supernatant. Luminescence spectra were recorded on a Spex Fluorolog fluorescence spectrophotometer in the range of 500-750 nm using solid/liquid slurry taken in a 2 mm quartz cell and excited at 450 nm under front-face geometry. The spectrt\. were corrected by a built-in program by normalizing the emission intensity for the uneven detector response at various wavelengths. A Beckman UV /VIS spectrophotometer was used for the determination of RUBP in the supernatant. The amount of RUBP incorporated at the S/L interface was calculated from the difference in the absorbance of RUBP at 450 nm before and after adsorption. RESULTS AND DISCUSSION Figures 1, 2 and 3 respectively show the intensity of luminescence of RUBP at its emission maximum, wavelengths corresponding to the emission maxi-
307 \&I U Z \&I U U) \&I 2 3 I&- 0 >- t: U) z Fig.i. IntensityofluminescenceemissionofRUBP at emission maximum inalumina-sds hemimicelles corresponding to selected concentrations on the adsorption isotherm. The upper curve represents the adsorption isotherm of alumina-sds system. The adsorption isotherm was invariant even in the presence of the probe (data points not shown). Fig. 2. Luminescence emission maximum of RUBP in alumina-sds hemimicelles corresponding to selected concentrations on the adsorption isotherm. mum, and the amount of RUBP adsorbed, at concentrations corresponding to different points of the adsorption isotherm. Each figure also depicts the adsorption isotherm for alumina-sds under identical conditions of the present
308 Ne ".. '0 e % Q - D- o: 0 '" a <... I- <... -J ::I '" -J >- V... a 0 a, Fig. 3. Amount of RUBP incorporated in alumina-sds hemimicelles. experiment as reported from our laboratory [3], for ready comparison. The luminescence intensity curve in Fig. 1 runs parallel to the adsorption isotherm above the HMC. Luminescence intensity values actually remain steady until the onset of hemimicellization and then increase with further adsorption. It may be noted that the intensities of luminescence emission do not have any absolute significance. In the sub-hemimicellar region, RUBP does not have any interaction with alumina since the luminescence intensity value agreed with that in pure aqueous solution of the same concentration. This inference was further supported by the observation that there was no direct adsorption of RUBP on alumina in the absence of SDS under the experimental conditions. The data in Fig. 3 show that above the HMC, RUBP is incorporated in the alumina-sds interface which also indicates maximum solubilization of RUBP in region III. The point of maximum solubilization lies near and above the isoelectric point of the mineral as can be seen from Fig. 4 (from Ref. [3]). The luminescence maximum corresponding to each point of the adsorption isotherm in Fig. 2 also shows a variation similar to the intensity variation (Fig. 1). The luminescence maximum increases from 632 to 647 nm and then goes down. The steady increase in the luminescence maximum values is indicative of the overall polarity felt by the RUBP probe. It may be noted that the amount of RUBP solubilized comes to a maximum much earlier than the maximum in the wavelength curve in Fig. 2. This indicates that the shifts in the luminescence maximum in region II and above are not controlled by the amount of RUBP incorporated in the hemimicelles alone, but also by the environmental effect brought by enhanced surface coverage. The luminescence maximum of RUBP is known to depend strongly on the
309 polarity of the medium. One study [14] indicates that the luminescence maximum shifts to longer wavelength in non-polar media. Thus, it can be concluded that RUBP is subjected to non -polar environments at higher adsorption densities. The luminescence maximum in the sub- hemimicellar region is somewhat higher (632 nm) than in pure water (626 nm). In micellar solutions of SDS, this value goes upto 638 nm whereas in hemimicelles, at the plateau region of the adsorption isotherm, it reaches upto 647 nm. It may be noted that the luminescence maximum ofrubp in micellar solution remained a constant at 638 nm even in 10-1 M NaCI solution. The increase in the luminescence maximum in the sub-hemimicellar region may be attributed to the species formed by the complex formation between RUBP and free SDS [15]. The high values observed in hemimicelles and micelles show that RUBP is exposed to more non-polar environments. Quenching studies on luminescence emission of RUBP by ferrocyanide ion in micelles and hemimicelles in the beginning of region IV (Kunjappu and Somasundaran, unpublished results) show that the excited state of RUBP is not accessible to ferrocyanide in these cases. This means that RUBP molecules (or more precisely the excitation center, since the excitation energy in RUBP is localized in one of the bipyridyl rings [16] ) are not dangling out into the aqueous region at the solid-liquid interface to be readily quenched by ferrocyanide ion. At this point no RUBP could be detected in the supernatant. The possibility of surface precipitation of the probe-surfactant complex on alumina surface exists since RUBP may form insoluble complex with SDS and deposit on the solid surface. To test this possibility, the probe concentration was varied to a lower value (1.0'10-5 M) and the luminescence parameters were determined. The observation that these parameters essentially exhibited the same variation as those registered at higher probe concentration rules out any possible artefact of the probe. However, the absolute luminescence intensities were affected at low probe levels. One interesting feature of this study is that in region IV of Figs 1, 2 and 3, the property measured suddenly changes to the initial values. This is because RUBP tends to partition between the micelles and hemimicelles as indicated in Fig. 3. At higher SDS concentrations, all of the RUBP were found to remain in the supernatant (not shown in the figure). An inference of equal importance which may be made on the basis of the above results is with respect to the development of the charge on the adsorbed layer. The primary forces responsible for the adsorption of SDS on alumina are coulombic and hydrophobic in nature: coulombic because of the electrostatic interaction between ionic head and charged mineral surface, and hydrophobic due to the tail-tail interaction of surfactant molecules. Generation of the negative charge on the alumina surface even in the beginning of region II as concluded from the binding of the positively charged RUBP when an overall positive potential is indicated from the zeta potential studies (Fig. 4) suggests that some of the surfactant molecules in the hemimicelle may be oriented with
310 60.i 40 :! 20 I- 0 I- 0-20 Fig.. (from "'.3) 4.-40 12 0( "'"-.I. n "-60... T... 0 X)"5 10-4 10-3 10-2 RESIDUAL OOOECYLSULFATE. moles/ilte, :_.;;.;,. m :, Fig. 4. Zeta potential at aiumina/sds-water interface as a function of equilibrium concentration of SDS (from Ref. [3]). their charged head groups towards the aqueous phase (reverse orientation), though most have their head groups oriented down toward the charged surface of the alumina, with their tail groups forming a hydrophobic layer at the interface (see Ref. [3] for a pictorial representation of the adsorption model). This can leave some of the negatively charged surfactant molecules pointing towards the aqueous phase. However, other possibilities cannot be totally ruled out since the hydrophobic interaction between the bipyridyl rings of RUBP and the hemimicellar aggregates may also playa role in the solubilization of the probe. We are aware of the bilayer theory postulated by one group [5] to explain this aggregation process according to which aggregation on the surface occurs by the formation ofbilayers of surfactant aggregates right from the onset of hemimiceuization. This results in a negatively charged surfactant layer facing the aqueous phase in all the regions of the isotherm. But, such a model would necessitate the same luminescence emission behavior of RUBP in all the regions since one would expect the same environment for RUBP irrespective of adsorption density. However, shifts and intensity changes of the luminescence maximum for various points of the isotherm (Figs 1 and 2) coupled with our earlier results of hydrophobicity studies on similar systems [17), which indicated considerable hydrophobicity even at the end of region II, suggest monolayer adsorption at least in region II. It may be noted that our reverse orientation model [3] does not exclude bilayer formation at higher surface coverages. But, the same bilayer model cannot be acceptable in the early hemimicellar region where total hydrophobicity is often achieved. Reorientation of the surfactant can occur either in a single layer or in a bilayer once the charge of the solid is neutralized. For a solid with a low surface charge density (near the p.z.c. as in the present case) adsorption with a reverse orientation can be expected to begin under sub-monolayer conditions whereas, for a highly charged solid, reverse orientation may occur only at higher surface coverages including bilayer coverages. In conclusion, luminescence emission studies using RUBP as a probe gives one insight into the structure of the adsorbed layer of SDS on alumina espe-
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