Absolubilization of Styrene and Isoprene in Cetyltrimethylammonium

The adsolubilization of styrene, isoprene, and mixtures of styrene and isoprene ... palisade layer and the core of the admicelle, while isoprene adsol...
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Absolubilization of Styrene and Isoprene in Cetyltrimethylammonium Bromide Admicelle on Precipitated Silica Boonyarach Kitiyanan,† John H. O’Haver,*,‡ Jeffrey H. Harwell,‡ and Somchai Osuwan† The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand, and School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019 Received September 20, 1995. In Final Form: January 25, 1996X The adsolubilization of styrene, isoprene, and mixtures of styrene and isoprene into cetyltrimethylammonium bromide (CTAB) bilayers (admicelles) on precipitated silica is investigated. The results show that the styrene adsolubilization constant is nearly unchanged with increasing styrene equilibrium concentration in the aqueous phase, while the isoprene adsolubilization constant increases with increasing isoprene partial pressure. The adsolubilization constants suggest that styrene adsolubilizes into both the palisade layer and the core of the admicelle, while isoprene adsolubilizes into the palisade layer. In the co-monomer system styrene adsolubilization is slightly increased in the presence of isoprene but isoprene adsolubilization is significantly enhanced in the presence of styrene; however, the adsolubilization isotherms of both monomers have increasing slopes in the co-monomer system and show that there is a synergistic effect, possibly due to swelling of the admicelle due to the addition of adsolubilizate.

Introduction Layers of adsorbed surfactants on a solid surface can be considered as a two-dimensional solvent. This concept opens up a variety of applications, including both separation and reaction processes on the surface of the solid. A previously explored separation process, known as admicellar chromatography, has been examined recently for environmental applications.1,2 Previously studied reaction processes include both admicellar catalysis3 and admicellar polymerization.4,5 Utilizing the latter process, hydrophobic molecules concentrate and react within a surfactant layer on a hydrophilic surface to form a polymermodified or copolymer-modified solid oxide surface.4-6 One of the substrates on which polymer films are thus formed is precipitated, amorphous silica.5,6 This type of silica is extensively used as a filler and reinforcer in shoe soles and in tractor and off-road tire treads.7 The presence of silica in rubber compounds provides a number of advantages, such as improvement in tear strength, reduction in heat buildup, and increase in compound adhesion in multcomponent products, e.g. tires.7 Recently, so-called “all-silica” tires have been touted as “green tires” because their decreased rolling resistance results in substantial reductions in fuel consumption and air pollution. Styrene-butadiene and styrene-isoprene copolymers are the first copolymers used to modify the silica surface.6 When compared to unmodified silica, both modified silicas * Author to whom correspondence should be addressed. † Chulalongkorn University. ‡ University of Oklahoma. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Barton, J. W.; Fitzgerald, T. P.; Lee, C.; O’Rear, E. A.; Harwell, J. H. Sep. Sci. Technol. 1988, 23, 637. (2) Nayyar, S. P.; Sabatini, D. A.; Harwell, J. H. Environ. Sci. Technol. 1994, 28, 1874. (3) Yu, C.; Wong, D. W.; Lobban, L. L. Langmuir 1992, 8, 2582. (4) Wu, J.; Harwell, J. H.; O’Rear, E. A. Langmuir 1987, 3, 531. (5) O’Haver, J. H.; Harwell, J. H.; O’Rear, E. A.; Snodgrass, L. J.; Waddell, W. H. Langmuir 1994, 10, 2588. (6) Waddell, W. H.; O’Haver, J. H.; Evans, L. R.; Harwell, J. H. J. Appl. Polym. Sci., in press. (7) Barbin, W. W.; Rodgers, M. B. In Science and Technology of Rubber, 2nd ed.; Mark, J. E., Erman, B., Eirich, F. R., Eds.; Academic Press: San Diego, 1994.

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show improved rubber physical properties, including decreased cure time, increased tear strength, improved elongation to break, and improved cut growth resistance.6 These results show the potential for economically modifying silica by admicellar polymerization for use in elastomers. Previous studies show that styrene-butadienemodified silicas perform the best, followed closely by styrene-isoprene-modified silicas. For these reasons, it is both scientifically interesting and industrially important to more fully understand and quantify the amount of each monomer that is available for polymerization on the silica surface. The co-adsolubilization of styrene-butadiene is investigated by O’Haver.8 His results show that butadiene both adsolubilizes into admicelles on the silica surface and condenses in the pores of the silica. The total uptake of butadiene is greatly increased in the presence of styrene, probably due to increased pore condensation. Unlike butadiene, isoprene is a liquid at room temperature and pressure. If this process of silica modification by admicellar polymerization were taken to the production stage, it would be advantagenous to use liquid monomers, e.g. styrene and isoprene, and eliminate the use of butadiene. Background Adsorption of Surfactant on the Solid Oxide Surface. The adsorption isotherm of ionic surfactants on oxide surfaces is typically an elongated ‘S’-shaped curve when one plots the log of the adsorbed surfactant density versus the log of the equilibrium concentration of surfactant.10,11 Normally, this ‘S’-shaped isotherm can be separated into four regions, as shown in Figure 1. Region I corresponds to both very low concentration and low adsorption of surfactant. This region is commonly referred to as the Henry’s law region because the adsorbed surfactant is considered to be in infinite dilution in the (8) O’Haver, J. H. Ph.D. Dissertation, University of Oklahoma, 1995. (9) Iler, R. K. The Chemistry of Silica; John Wiley & Sons Inc.: New York, 1979. (10) Somasundarun, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. (11) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463.

© 1996 American Chemical Society

Adsolubilization of Styrene and Isoprene

Figure 1. Typical adsorption isotherm of surfactants on solid oxide surfaces.

surface phase and, thus, the interaction between molecules of surfactants is negligible. Adsorbed surfactants in this region are viewed as being adsorbed alone and not forming any aggregates. Region II is distinguished by a sharply increased isotherm slope relative to the slope in region I. This increase in slope indicates the beginning of lateral interactions between surfactant molecules, which result in the formation of surfactant aggregates on the most energetic surface patches.11 These adsorbed surfactant aggregates are called admicelles12 or hemimicelles,10 depending upon whether the aggregates are viewed as bilayers or monolayers. The admicelle is considered as a local bilayer structure with a lower layer of head groups adsorbed on the substrate surface and an upper layer of head groups in contact with solution. The hemimicelle is a monolayer structure having the head group adsorbed on the surface while the tail group is in contact with the aqueous phase. The transition point from region I to region II, representing the first formation of adsorbed surfactant aggregates, is called the critical admicelle concentration (cac)12 or the hemimicelle concentration (hmc).10 The slope of the isotherm decreases in region III. This is thought to be caused by either repulsion between the like-charged head groups on the surface or the beginning of admicelle formation on lower energy surface patches. Region IV is the plateau region, having almost constant surfactant adsorption with increasing surfactant concentration. Typically, the equilibrium surfactant concentration at the transition point from region III to region IV is approximately at the critical micelle concentration (cmc). The adsorption of surfactants on solid substrates is controlled by several parameters, including the electrochemical nature of the substrate, the pH of the solution, and the type of surfactant molecule. The net charge on a solid oxide surface can be manipulated to be either positive or negative by adjusting the pH of the contacting aqueous solution because both hydrogen and hydroxyl ions are potential determining ions for metal oxides. The pH at which the net charge on the surface is zero is called the point of zero charge (PZC). When the pH of the contacting aqueous solution is below the PZC of the solid oxide surface, the surface will be protonated and positively charged. On the other hand, the oxide surface will be negatively charged at a pH above the PZC. For instance, silica, having 2 e PZC8 e 3, will be negatively charged when the pH of the aqueous solution exceeds 3. Therefore, cationic surfactants such as cetyltrimethylammonium bromide (CTAB) adsorb readily on the surface of silica when the pH of the contacting aqueous phase is greater than 3. Adsolubilization. The partitioning of organic solutes from aqueous solution into the interior of adsorbed (12) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251.

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Figure 2. Phenomena of solubilization and adsolubilization.

surfactant aggregates is termed adsolubilization. This phenomenon is the surface analog of solubilization, with adsorbed surfactant bilayers playing the role of micelles, as shown in Figure 2. The suggested definition of adsolubilization is “the incorporation of compounds into surfactant surface aggregates, which compounds would not be in excess at the interface without surfactant.”13 In 1982, Nunn et al.14 presented visual evidence of pinacyanol chloride, a dye of the cyanide class, adsolubilized into the organic environment of adsorbed surfactants. When pinacyanol chloride was dissolved in aqueous solutions in which the surfactant was below the cmc, the solution was red in color, indicating that the dye was in an aqueous environment. When the dye was dissolved in organic solvents or in aqueous solutions of surfactants above the cmc, a blue color occurred, indicating that the dye was in an organic environment. The blue color also appeared on an alumina surface on which anionic surfactants had been adsorbed and the surfactant concentration in the aqueous phase was below the cmc. When the bulk surfactant concentration was above the cmc, the blue color was observed both on the surface of the alumina and in the aqueous phase. This indicated the partitioning of pinacyanol between micelles and admicelles and showed that the interior of the admicelle is similar in nature to the interior of micelles. To examine the structure of the adsorbed bilayer of Triton X-100 (octylphenoxyethanol with an average of 9-10 oxyethylene units), Levitz et al.15,16 used fluorescence decay spectroscopy and pyrene as a probe. This work also supported the hydrophobic environment of the admicelle’s core and confirmed the adsolubilization of hydrophobic molecules. At low levels of adsorbed surfactant, the probe behaved similarly to patchwise-adsorbed micelles on the surface. At higher surfactant concentrations, the result showed that the adsorbed surfactant phase seems to be continuous on the silica surface. Subsequently, O’Haver et al. systematically studied the adsolubilization of both a series of alcohols and a series of alkanes into admicelles on alumina.17 For the adsolubilization of alcohols, the ratios of alcohol to SDS molecules in the admicelles were very high at low levels of surfactant adsorption. As the surfactant adsorption was increased, these ratios decreased to a value which is similar to the (13) Scamehorn, J. F.; Harwell, J. H. In Surfactants in Chemical/ Process Engineering; Wasan, D. T., Ginn, M. E., Shah, D. O., Eds.; Marcel Dekker, Inc.: New York, 1988. (14) Nunn, C. C.; Schechter, R. S.; Wade, W. H. J. Phys. Chem. 1982, 86, 3271. (15) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1984, 88, 2228. (16) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 1302. (17) O’Haver, J. H.; Yeskie, M. A.; Harwell, J. H. ACS Symp. Ser., in press.

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ratios of alcohol to surfactant molecules in SDS micelles. Below the cmc (in regions I, II, and III) the amount of surfactant adsorption was not only considerably increased in the presence of alcohol but also increased with increasing alcohol molecular weight. Although surfactant adsorptions were increased below the plateau region, adsorptions in the plateau region (region IV) were slightly decreased. O’Haver et al. also observed that alkanes adsolubilized to a high degree in SDS admicelles. Moreover, alkane adsolubilization increased with increased surfactant adsorption. It was observed, as predicted, that the standard-state free energy of alkanes adsolubilized into the SDS admicelles is approximately the same as for the alkanes solubilized into micelles. This further indicated that admicelles have an interior similar to the micellar interior. To explain the results of alcohol adsolubilization, a “patchy bilayer” of adsorbed surfactant was proposed by Lee et al.18 They further proposed that the adsorbed surfactant in regions II and III of the isotherm is present at disk-shaped aggregates. Because of an alcohol’s polar end group, the disklike admicelles model explained how alcohols could be adsolubilized into two sites of these admicelles. One site is between the head groups of the surfactant; this site is also present in micelles. The other site, which is not present in micelles, is the hydrophobic perimeter arising from the formation of the patchy, disklike admicelles. The fraction of alcohols adsolubilized at the perimeter can only be significant when the patchy aggregates are small, so the ratios of adsolubilized alcohols to adsorbed surfactant are very high at low surfactant adsorption. Furthermore, the adsolubilized alcohols at the perimeter increased the hydrophobicity of the surface phase, so the surfactant adsorption is higher in the presence of alcohols. As the chain length of the alcohol increased, this hydrophobic contribution became greater and, therefore, resulted in enhanced surfactant adsorption. Admicelles have also been used as two-dimensional templates for reactions. Yu et al.3 studied sodium dodecyl sulfate (SDS) adsorbed on high surface area alumina used to catalyze the hydrolysis of trimethyl orthobenzoate in a process called admicellar catalysis, the surface analog to micellar catalysis. Their results show that the highest specific activity of the admicelles is less than the maximum specific activity of the corresponding micelles but that the admicelle activity increases with increasing surface coverage above a certain value. Another successful application of admicelles is admicellar polymerization. Polymerization of adsolubilized styrene monomer in SDS admicelles on the surface of alumina was investigated by Wu et al.4 The process for creating these ultrathin polymer films was considered to consist of four steps. Step one is the forming of admicelles, providing both the high surface coverage and a template for dissolving the reactants. Step two in this process involves the adsolubilization of monomer into the admicelle. Because organic monomers have limited solubility in water, they preferentially partition into the hydrophobic interior of admicelles. Step three is the in-situ polymerization of the adsolubilized monomer. The final step is the removal of excess surfactant in order to expose the polymer films. Gaseous and polar (more water soluble) monomers can also be successfully used in this process. The investigation of tetrafluoroethylene (TFE) adsolubilized and polymerized in perfluorinated surfactant admicelles on alumina was carried out by Lai.19 Subsequently, more polar (18) Lee, C.; Yeskie, M. A.; Harwell, J. H.; O’Rear, E. A. Langmuir 1990, 6, 1758.

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monomers and co-monomers were used to form polymer films on precipitated silica with CTAB as the surfactant.5,8 Experimental Section Materials. All Chemicals were obtained commercially and used as received. Cetyltrimethylammonium bromide (CTAB), at a purity of 99%, styrene (99%), and isoprene (99%) were obtained from Aldrich Chemical Company, Inc. (Milwaukee, WI). Hi-Sil 255, a porous, amorphous, precipitated silica was received from PPG-Siam Silica Co., Ltd. (Rayong, Thailand) with a reported surface area of 170 m2/g, a mean particle size of 18.95 µm, and 100% surface-silanol group coverage. Methods. The quantity of adsorbed CTAB on Hi-Sil 255 was calculated by the concentration difference method. Using a mass balance and the concentration of CTAB in the aqueous feed solution and the equilibrium supernatant, the amount of adsorbed surfactant can be calculated. The initial aqueous solution was adjusted to pH 8 using sodium hydroxide solution. Although a more basic solution, (e.g. higher than pH 8) causes a higher driving force for CTAB adsorption on silica, the solubility of silica in alkaline solution reaches a minimum close to pH 8 and increases rapidly above the value.9 The CTAB concentration was determined via titration against standard sodium dodecyl sulfate (SDS) solution in a two-phase titration.20 In order to determine the adsolubilization of styrene, feed solutions with known CTAB and styrene concentrations in pH 8 water were brought into contact with samples of amorphous precipitated silica of known mass in sealed glass vials (42 mL empty volume borosilicate glass vials with screw caps and PTFEfaced silicone septa obtained from Baxter Diagnostics, Inc., McGaw Park, IL). The system was allowed to equilibrate at 30 °C for at least 48 h. After equilibration, the supernatant was decanted and centrifuged. The styrene concentration was then measured by UV/vis spectrophotometry at 280 nm. A Bausch & Lomb Spectronic 1001 spectrophotometer was used to determine UV absorbance of styrene at 280 nm. The amount of adsolubilized styrene was calculated from the concentration difference of styrene in the solution from the feed and after equilibration. The solute vapor pressure apparatus used to find isoprene solubility in water and isoprene adsolubilization was similar to one developed by Christian and co-workers.21,23 In this experiment, this apparatus consisted of a glass vessel having a working space of 850 mL, a screw-on nylon cap, a pressure transducer, an HPLC injector, a vacuum pump, a water temperature controller, and a computer interface recording the pressure of the system and controlling injection of isoprene into the system. Figure 3 shows schematically the significant parts of the apparatus. The vessel and cap were custom-ordered from Ace Glass Inc. (Vineland, NJ). The HPX811-020AV pressure transducer and WB-AASC-GP high-speed interface card were obtained from Omega Engineering, Inc. (Stamford, CT). The ISIS Auto inject actuator and Model 7010 Rheodyne valve were obtained from Altech (Deerfield, IL). The Dyna-Sense Model 2149 temperature controller was obtained from Cole Parmer, and the heating unit was obtained from Fisher Scientific. The PTFE diaphragm pump and pump motor used to circulate the bulk liquid through the UV cell were obtained from Cole Parmer (Niles, IL). The UV flow cell, model 71-Q-10-T, was purchased from Starna Cells, Inc. (Atascadero, CA). An HPLC solvent filter from Rainin (Emeryville, CA) was used to exclude silica from getting into the pump/UV cell circuit. The partial pressure of isoprene was calculated from the difference between the total pressure and the vapor pressure of pure water at the experimental conditions. The solubility of isoprene in water at 30 °C was determined by recording the pressure of the sealed reaction vessel containing 500 mL of degassed water. The system was first allowed to reach a steady temperature of 30 °C and was then evacuated to the vapor (19) Lai, C.-L.; Harwell, J. H.; O’Rear, E. A.; Komatsuzaki, S.; Arai, J.; Nakakawaji, T.; Ito, Y. Langmuir, in press. (20) Barr, T.; Oliver, J.; Stubbings, W. V. J. Soc. Chem. Ind., London 1948, 67, 45. (21) Tucker, E. E.; Christian, S. D. J. Chem. Thermodyn. 1979, 11, 1137. (22) Smith, L. S.; Tucker, E. E.; Christian, S. D. J. Phys. Chem. 1981, 85, 1120. (23) Tucker, E. E.; Christian, S. D. J. Phys. Chem. 1977, 81, 1295.

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Figure 4. CTAB adsorption isotherm on Hi-Sil 255.

Figure 3. Schematic of the solute vapor pressure apparatus. pressure of water (0.615 psia). Then, 200 µL of isoprene was injected into the system and left allowed to equilibrate; the injections were continued until the system’s pressure reached the vapor pressure of isoprene. In order to see the interaction between isoprene and unmodified silica, the procedure was repeated, except that this time 40 g of Hi-Sil 255 was added into the sealed vessel containing 500 mL of distilled water. For the adsolubilization of isoprene, a feed solution consisting of 500 mL of pH 8 water, 7.2 g of CTAB, and 40 g of silica was loaded into the vessel, stirred, and allowed to equilibrate for at least 12 h. The CTAB concentration was selected so that no micelles would be present in the bulk phase. The system was then evacuated repeatedly to the vapor pressure of the system until no increase in pressure was noted when the system was allowed to sit. Water losses due to evacuation were estimated on the basis of the volume of the vapor phase and assuming saturated vapor. These losses represent less than 0.01% and were therefore ignored. Isoprene was then injected into the system via the HPLC injector in 200 µL increments, until the system equilibrated at the vapor pressure of the isoprene. The system was allowed to reach equilibrium between injections. The co-adsolubilization of isoprene and styrene was determined by dissolving styrene and 7.2 g of CTAB in 500 mL of pH 8 water and contacting it with 40 g of silica in the vessel. The system was allowed to equilibrate for 12 h at 30 °C before it was evacuated. Styrene losses, based upon losses in the vapor phases, were estimated and found to be negligible, because the styrene vapor pressure is very low. Isoprene was then injected into the system as previously described. The isoprene adsolubilization was calculated from a mass balance and the total pressure recorded from the transducer. The styrene concentration in the supernatant was determined by measuring the UV absorbance at 280 nm for the liquid phase circulated through the flow cell. Styrene adsolubilization was then determined from a mass balance. The UV adsorbance readings were done manually after each isoprene injection had equilibrated.

Results and Discussion CTAB Adsorption on Hi-Sil 255. The CTAB adsorption isotherm on Hi-Sil 255 is shown in Figure 4. The isotherm illustrates the characteristics of regions II, III, and IV. The slope of the isotherm is greater than 1 from a concentration of CTAB in the aqueous solution of 70 µM to a concentration of 120 µM. This very high slope indicates the onset of CTAB aggregation on the surface of the silica, which occurs either at or below 70 µM. From the plateau region data, the maximum adsorption of CTAB on Hi-Sil 255 is approximately 550 µmol/g. The reported specific surface area of Hi-Sil 255 from the manufacturer is 170 m2/g. On the basis of monolayer coverage, this silica gives a coverage of 1.95 molecule/nm2 or 51 Å2/ molecule. These values can be compared to those for

Figure 5. Comparison of CTAB adsorption on Hi-Sil 255 and Hi-Sil 233.

tetradecyltrimethylammonium bromide and octadecyltrimethylammonium bromide, which respectively have 61 and 64 Å2/molecule for the area per molecule at surface saturation of surfactants at the water-air interface.24 We estimate the area/head group of CTAB to be 62 Å2. If the admicelles were present over all of the surface, the area per CTAB molecule should be =30 Å2. However, the CTAB molecule is too large to adsorb in the very narrow pores of this silica, so the observed area per head group on the silica surface is greater than the area per head group estimated for complete bilayer coverage. We believe, therefore, that the aggregation of CTAB on the silica surface is most likely in the form of a local bilayer (admicelles) but that all the BET surface area is not accessible to the CTAB. Figure 5 presents a comparison of CTAB adsorption (molecules per nm2) on Hi-Sil 255 and Hi-Sil 233 (another precipitated silica), having specific surface areas of 170 and 145 m2/g, respectively.5 It should be noted that the increase in the maximum CTAB adsorption is not linear with respect to specific surface area. The adsorption density of CTAB is 51 Å2/molecule on Hi-Sil 255 and 89 Å2/molecule on Hi-Sil 233. Styrene Adsolubilization. The adsorption of styrene on silica in water is also investigated. The result shows that there is no significant adsorption of styrene on the wet silica. The relation between the amount of adsolubilized styrene and the equilibrium styrene concentration, as well as the adsolubilization of styrene versus reduced concentration, is shown in Figure 6. The reduced concentration is dimensionless, calculated by dividing the equilibrium styrene concentration by the solubility of styrene in water. As expected, when the equilibrium concentration of styrene increases, the amount of adsolubilized styrene increases. (24) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley Interscience: New York, 1989.

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Figure 8. Solubility of isoprene in water.

Figure 6. Styrene adsolubilization versus styrene equilibrium concentration and reduced styrene concentration.

Figure 7. Normalized adsolubilization equilibrium constant versus styrene mole fraction.

The micellar solubilization equilibrium constant has been defined as the ratio of the solute mole fraction in the micelle to the equilibrium concentration of solute in bulk aqueous solution.25,26 Analogously, this work defines the adsolubilization equilibrium constant as Ki ) Xi/Cib, where Ki is the adsolubilization equilibrium constant of the adsolubilizate (i), Xi is the mole fraction of adsolubilizate (i) in the admicelle, and Ci,b is the concentration of solute (i) in the bulk aqueous phase (M). In a work which studied the solubilization of decahydronaphthalene (naphthane), naphthalene, and 1-naphthol in micelles,27 it was concluded that, with increasing solute mole fraction in the bulk aqueous phase, the partition coefficient, which is virtually the same as the solubilization equilibrium constant, decreased for solutes which primarily solubilize in the palisade layer and increased for solutes which primarily solubilize in the micelle core. For solutes believed to partition into both the core and the palisade layer, the partition coefficient remained relatively constant. Figure 7 shows the relation between the normalized adsolubilization equilibrium constant and the styrene mole fraction in the admicelle, assuming a constant surfactant adsorption of 500 µmol/g. Lee et al. have shown that the presence of an adsolubilizate does not significantly affect (25) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. J. Colloid Interface Sci. 1989, 130, 254. (26) Mahmoud, F. A.; Higazy, W. S.; Christian, S. D.; Tucker, E. E.; Taha, A. A. J. Colloid Interface Sci. 1989, 131, 96. (27) Rouse, J.; Sabatini, D.; Deeds, N.; Brown, E.; Harwell, J. H. Environ. Sci. Technol. 1995, 29, 2484.

the plateau adsorption of surfactant.18 The normalized adsolubilization equilibrium constant is the adsolubilization equilibrium constant divided by the value obtained by extrapolation to a zero mole fraction, analogous to the infinite dilution partition coefficient. The styrene adsolubilization equilibrium constant does not change significantly. This implies that styrene is adsolubilized into both the palisade layer and the core of the admicelle, as would be expected from the known behavior of aromatics in micelles.27 The maximum observed adsolubilization of styrene is approximately 850 µmol/g, which corresponds to a ratio of adsolubilized styrene to adsorbed surfactant of about 1.7:1. Isoprene Adsolubilization. The virial equation truncated after the second virial coefficient is used to determine the isoprene density as a function of partial pressure.28 Therefore, the amount of isoprene in the gaseous phase can be determined from the recorded pressure. The solubility of isoprene in water is found by following the pressure drop of each injection in the system containing water, as shown in Figure 8. The solubility data can be fit by the equation M ) 0.0000304(P2) + 0.000425(P) where M and P, respectively, are the molar concentration of isoprene in the water and the partial pressure of isoprene in the vapor (solid line in Figure 8). The adsorption of isoprene on silica in water is also investigated. The result shows that there is no significant adsorption of isoprene on the wet silica. In the system also containing surfactant, the adsolubilization of isoprene is calculated utilizing a mass balance after the system is equilibrated. Figure 9 shows the relationships between the amount of adsolubilized isoprene and the equilibrium partial pressure and the reduced pressure (the partial pressure divided by the vapor pressure of pure isoprene at 30 °C). It should be noted that the time required to reach equilibrium in the system containing silica and water or water only is about 90 min, compared to about 2 h in the system that also contains surfactant. The longer time required to reach equilibrium in the presence of surfactant may imply increased mass transfer resistance for the isoprene that is transferring from the aqueous phase into the admicelles in the silica’s pore. It may be due, however, to transfer through the aqueous phase occurring at the same rate but a larger amount being transferred when surfactant is present. This longer equilibrium time in the presence of surfactant is consistent qualitatively with observations made for butadiene adsolubilization, but the actual equilibrium time in the isoprene system is shorter than that in the butadiene system.8 Similarly, gas phase solutes take longer times to reach equilibrium than liquid solutes for solubilization into micelles. (28) Daubert, T. E.; Danner, R. P. Physical and Thermodynamic Properties of Pure Chemicals: Data Compilation; Hemisphere Publishing Corp.: New York, 1989.

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Figure 12. Isoprene adsolubilization in the presence of styrene. Figure 9. Isoprene adsolubilization versus isoprene partial pressure and reduced pressure.

Figure 13. Styrene adsolubilization equilibrium constant in the co-monomer system. Figure 10. Normalized adsolubilization equilibrium constant versus isoprene mole fraction.

Figure 11. Styrene adsolubilization in the presence of isoprene.

Using the estimated value of 500 µmol/g CTAB adsorption, the adsolubilization equilibrium constant is isoprene is calculated and normalized. The normalized adsolubilization equilibrium constant of isoprene is plotted against the isoprene mole fraction in the admicelle, as shown in Figure 10. The decreasing value with increasing mole fraction implies that isoprene is preferentially adsolubilized into the palisade layer of the admicelle.27 The maximum observed adsolubilization of isoprene is approximately 550 µmol/g, which corresponds to a ratio of adsolubilized isoprene to adsorbed surfactant of about 1:1. Styrene-Isoprene Co-Adsolubilization. Figure 11 is the plot of styrene adsolubilization versus the equilibrium concentration of styrene in the aqueous phase at various isoprene partial pressures. The adsolubilization of styrene was slightly increased as isoprene was incrementally injected into the system. However, the equi-

librium concentration of styrene decreases approximately by half as the equilibrium partial pressure of isoprene nears its vapor pressure. Because most of the added styrene (about 95%) was already adsolubilized, with only about 5% of the initial added styrene remaining in the bulk aqueous phase, the amount of styrene adsolubilization is relatively unchanged. The styrene adsolubilization is actually higher in the presence of isoprene, for a given bulk styrene concentration, than it would be in the absence of isoprene. The adsolubilization of isoprene for three different feed concentrations of styrene is presented in Figure 12. It is evident that the presence of styrene significantly enhances the adsolubilization of isoprene: at the same isoprene partial pressure, the amount of adsolubilized isoprene increases with increasing styrene loading. Moreover, the difference in isoprene adsolubilization between those systems containing styrene and systems with isoprene alone increases with increasing isoprene partial pressure. The adsolubilization equilibrium constants of styrene and isoprene at various styrene feed concentrations are plotted versus the styrene and isoprene mole fractions in Figures 13 and 14, respectively. The adsolubilization equilibrium constant of styrene (KS) is approximately constant at styrene feed concentrations of 0.005 18 and 0.0205 M, while KS increases with increasing styrene mole fraction at a 0.0409 M styrene feed. For isoprene, the adsolubilization equilibrium constant of isoprene (KI) decreases with increasing mole fraction of isoprene at styrene loadings of 0.005 18 and 0.0241 M, while KI is almost constant at a styrene feed concentration of 0.0409 M. The above results suggest that, at low styrene loading, styrene is adsolubilized into both the palisade layer and the core of the admicelle, while isoprene is adsolubilized primarily into the palisade layer. The results at high styrene loading imply that styrene is adsolubilizing primarily into the core of the admicelle, while isoprene is

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Figure 14. Isoprene adsolubilization equilibrium constant in the co-monomer system. Table 1. Ratios of Adsolubilized Styrene and Adsolubilized Isoprene to Adsorbed Surfactant, the Ratio of Adsolubilized Styrene to Adsolubilized Isoprene, and the Trends of both KS and KI amount ratio of ratio of ratio of of adsolubilized adsolubilized adsolubilized loaded styrene to isoprene to styrene to styrene adsorbed adsorbed adsolubilized (mole) CTAB CTAB isoprene 0.0044 0.0174 0.0348

0.21 0.85 1.7

1.11 2.03 3.69

0.187 0.42 0.46

KS

KI

constant decrease constant decrease increase constant

Table 2. Some Physical Properties of Styrene and Isoprene monomer styrene isoprene a

MW

molecular formula

104.15 C6H5CHCH2 68.11 CH2CCH3CHCH2

solubility in water

dipole momenta (C‚m)

3.07 mM at 4.3363 × 10-31 25 °C 8.0 mM at 8.3391 × 10-31 20 °C

From ref 28.

adsolubilizing into both the palisade layer and the core of the admicelle. From Figures 11 and 12, the adsolubilization isotherms of both monomers have an increasing slope in the co-monomer system. This significant increase in slope indicates that the monomer adsolubilization dramatically increases in the co-monomer system. One might reasonably expect that, with two adsolubilizates in the palisade layer, they would compete for adsorption sites, resulting in decreased adsolubilization equilibrium constants for both. This is clearly not the case. Because the silica is composed of overlapping spheres, the silica surface is everywhere convex, like the outer layer of a lipid bilayer. One possible explanation of the synergism is that as the bilayer becomes swollen by solubilized monomer, the area per surfactant head group in the admicelle increases, thereby increasing the volume of the palisade layer. A CTAB admicelle, with extended 16 carbon atom chains, has sufficient room for two monomers of styrene or isoprene to be adsolubilized by one surfactant molecule. From the trends shown by the styrene and isoprene equilibrium adsolubilization constants, KS and KI (Table 1), as isoprene adsolubilization increases, the styrene may begin to preferentially partition into the core. Furthermore, by comparing the solubility in water and the dipole moment of both monomers (Table 2), isoprene can be seen to have a higher polarity than styrene; this suggests that adsolubilized isoprene would stay closer to the CTAB head group than adsolubilized styrene. A suggested model is shown in Figure 15.

Figure 15. Structure model of adsolubilized monomers at styrene feed concentrations of (a) 0.005 18 M, (b) 0.0205 M, and (c) 0.0409 M.

One possibility is that, while styrene would compete with isoprene to stay in the palisade layer, styrene in the core would increase the solubility of isoprene in the core. Because of their very low solubilities in water, both adsolubilizates should have hydrophobic interactions with each other, resulting in higher adsolubilizations. Another possibility is that though pore condensation appears to be nearly absent in the isoprene alone system, it might be present in the mixed-monomer system. Additional work needs to be done to answer this question more satisfactorily. Conclusions CTAB adsorption at a feed pH of 8 on Hi-Sil 255, an amorphous, precipitated silica, is found to have a plateau value of 550 µmol/g of silica or 1.95 molecules/nm2. Though the CTAB adsorption is believed to exist in the form of fully formed admicellar aggregates, some of the surface area is in inaccessible pores resulting in lower CTAB adsorption. The adsolubilization equilibrium constant of styrene in the CTAB bilayer is almost constant with increasing styrene concentration, which suggests that styrene is adsolubilized into both the palisade layer and the core of the admicelle. The ratio of adsolubilized styrene to adsorbed CTAB at near saturation conditions is found to be approximately 1.7 at bulk concentrations near the solubility of styrene in water. The adsolubilization equilibrium constant of isoprene decreases with increasing isoprene mole fraction in the admicelle; in analogy to the behavior of alkanes in micelles, the isoprene is therefore believed to be adsolubilized into the palisade layer of the admicelle. The ratio of adsolubilized isoprene to adsorbed CTAB at near saturation conditions is about 1:1. In the co-monomer system, while the total amount of styrene adsolubilization does not increase significantly with increased isoprene adsolubilization, the styrene equilibrium concentration decreases significantly. In contrast, in the presence of styrene, isoprene adsolubilization significantly increases; possible mechanisms for this are proposed. LA950783F