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Langmuir 1999, 15, 6226-6232
Effect of Water-Soluble Alcohols on Surfactant Aggregation in the C12EO8 System Kenji Aramaki,† Ulf Olsson,‡ Yoko Yamaguchi,† and Hironobu Kunieda*,† Graduate School of Engineering, Yokohama National University, Yokohama, Japan, and Department of Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, Lund, Sweden Received January 21, 1999. In Final Form: May 10, 1999 We investigated the effect of water-soluble alcohols (glycerol, propylene glycol, and 1-propanol) on the surfactant aggregation and its structure by phase study, surface tension measurement, small-angle X-ray scattering (SAXS) measurement, and pulsed field gradient NMR self-diffusion measurement. The phase behavior in the (water + glycerol)/C12EO8 system as a function of the glycerol content in water at constant temperature is similar to that in the water/C12EO8 system as a function of temperature, and a phase separation (clouding phenomenon) takes place at a high glycerol content. The phase separation was not observed in the other alcohol systems. However, a hexagonal liquid crystal (H1) changes to an isotropic solution (Wm) in all the systems with increasing the alcohol content. The SAXS measurement for the H1 and the Wm phases, and the self-diffusion measurement for the Wm phase suggest that the dehydration of the ethylene oxide chain takes place and the aggregates tend to grow with increasing glycerol content. On the other hand, propylene glycol or 1-propanol molecules tend to penetrate into the palisade layer of the aggregates and the micelles are downsized, and eventually they are broken into monomers with increasing the alcohol content. The critical micelle concentration measurement also supports the difference in the alcohol effects on the phase behavior in the aqueous nonionic surfactant system.
Introduction Surfactant forms micelles in aqueous solutions due to the reduction in the free energy when transferring the hydrophobic part (or hydrocarbon chain) from the polar solvent to the micellar interior, the so-called “hydrophobic effect”.1 This energy gain correlates with the solvent property. A measure of the solvent property is proposed as cohesive energy density, γ/V1/3, where γ is the surface tension and V is the molar volume of the solvent.2 For example, the values for water and ethylene glycol are 27 and 12 dyn‚cm-2‚mol1/3, respectively. Experimental results3,4 show that the critical micelle concentration (cmc) increases with decreasing this parameter, suggesting that the free energy gain of micellization, which decreases proportionally to the logarithm of the mole fraction of surfactant at cmc,5 decreases with decreasing the parameter, γ/V1/3. It was reported that the γ/V1/3 value should be higher than 10-11 dyn‚cm-2‚mol1/3 for micellization.6 It was also reported that the formation of liquid crystals is interrupted upon addition of a polar nonaqueous solvent.7-10 However it has not been studied how the liquid crystals are decomposed by the polar nonaqueous solvent. * To whom correspondence should be addressed. † Yokohama National University. ‡ Lund University. (1) Tanford, C. The Hydrophobic Effects, 2nd ed.; Wiley: New York, 1980. (2) Gordon, J. The Organic Chemistry of Electrolyte Solutions; Wiley: New York, 1975; pp 158-162. (3) Ray, A.; Nemethy, G. J. Phys. Chem. 1971, 75, 809. (4) Blokhus, A. M.; Høiland, H.; Gjerde, M. I.; Backlund, S.; Ruths, M. Douhe´ret, G. Prog. Colloid Polym. Sci. 1990, 82, 243. (5) Rosen, M. J. Surfactant and Interfacial Phenomena; Wiley: New York, 1989; p 151. (6) Ramadan, M.; Evans, D. F.; Lumry, R.; Philson, S. J. Phys. Chem. 1985, 89, 3405. (7) Sagitani, H.; Hirai, Y.; Nabeta, K.; Nagai, M. Yukagaku 1986, 35, 102. (8) Auvray, X.; Petipas, C.; Anthore, R.; Rico, I.; Lattes, A. J. Phys. Chem. 1989, 93, 7458. (9) Auvray, X.; Perche, T.; Anthore, R.; Petipas, C.; Rico, I.; Lattes, A. Langmuir 1991, 7, 2385.
It is well-known that the cloud temperature of aqueous solution of a polyoxyethylene-type surfactant lowers or raises depending on the types of the added inorganic salts, which is called as the salting-out or -in effect, respectively.11 The salt effects are related to the adsorption/ depletion of the salt at the surfactant film.12 The similar effects are also observed when alcohols are used instead of a salt. Polyols such as glycerol, poly(ethylene glycol), D-sorbitol etc. show a “salting-out” effect whereas nalcohols or glycols show the opposite effect.13,14 In a water/CmEOn-type surfactant/hydrocarbon system, a structure of microemulsions varies from oil-in-water (O/W) to water-in-oil (W/O) type with increasing temperature at low surfactant concentration.15 When a mixture of water and glycerol,16 or propylene glycol and glycerol,17 for instance, is employed instead of pure water, the same structural transition of microemulsions from O/W to W/O is observed with changing the composition of the mixed solvent at constant temperature. The addition of salt also causes the same structural transition of the microemulsions18 if the salt has the salting-out effect. However it is still unclear that the effect of added alcohol on the surfactant hydrophilicity is the same as that of inorganic salts. In this context, we investigated how water-soluble alcohols affect the surfactant aggregation and the struc(10) Blokhus, A. M.; Høiland, H.; Gjerde, M. I.; Backlund, S.; Ruths, M. Douhe´ret, G. Prog. Colloid Polym. Sci. 1990, 82, 243. (11) Attwood, D.; Florence, A. T. Surfactant Systems; Chapman and Hall: London, 1983; p 94. (12) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. J. Phys. Chem. 1995, 99, 6220. (13) Sagitani, M.; Ikeda, Y.; Ogo, Y. Yukagaku 1984, 33, 30. (14) Iwanaga, T.; Suzuki, M.; Kunieda, H. Langmuir 1998, 14, 5775. (15) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (16) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (17) Martino, A.; Kaler, E. W. Langmuir 1995, 11, 779. (18) Shinoda, K.; Kunieda, H. J. Colloid Interface Sci. 1987, 118, 586.
10.1021/la9900573 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999
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ture of the aggregates by phase study, cmc measurement, small-angle X-ray scattering (SAXS), and pulsed field gradient NMR. As the water-soluble alcohols, we chose a series of alcohols having a three-carbon backbone but with different number of hydroxide units, namely, glycerol, propylene glycol, 1-propanol. Experimental Section Materials. Octaethylene glycol dodecyl ether (C12EO8) was purchased from Nikko Chemicals Co. For making phase diagrams, cmc measurements, and SAXS measurements, guaranteed grades of glycerol (Wako Pure Chemicals, Tokyo, 99.0%), propylene glycol (1,2-propanediol, Wako Pure Chemicals, Tokyo, 99.0%), 1-propanol (Tokyo Kasei Kogyo, Tokyo, 99.5%) were used. For self-diffusion measurement, glycerol (Prolab, France, 99.5%), 1,2-propanediol (SIGMA, 99.8%), and 1-propanol (Fisons, England 99.93%) were used. Hexamethyldisilane (MERCK, 95%) was also used for a self-diffusion study. All reagents are used without further purification. Samples were prepared with distilled water for making phase diagrams, cmc measurements, and SAXS measurements and with Millipore-filtered water for self-diffusion measurement. Procedures. Phase Diagram. All chemicals were weighed and sealed in a test tube. These samples were mixed by heating and shaking for micellar solution or by repeating centrifugation for the samples containing liquid crystals to obtain a homogeneity. They were kept in a water bath controlled at constant temperature, 25 ( 0.05 °C, for several hours to several days, which depends on phase separations. A hexagonal liquid crystal was distinguished by the observation of optical birefringence under crossed polarizers and the relative peak positions in a SAXS spectrum. A cubic phase was distinguished by its very high viscosity and optical isotropy. cmc Measurement. Several surface tension values (γ) at 25 °C, measured by a Wilhelmy-type surface balance (Kyowa Interface Science, CBVP-Z), were plotted on a log C vs γ diagram, where C is the molarity of surfactant. A cmc value was determined as the C value at a break of a γ curve. SAXS Measurement. SAXS measurements were performed on a slit-collimation small-angle X-ray camera with 15 kW Cu KR radiation (Rigaku, RINT 2500). Samples were sandwiched between two layers of polyethylenetelephthalate film (Mylar seal method). Self-Diffusion Measurement. Self-diffusion coefficients were measured by the pulsed field gradient NMR method (PFG-NMR method)19 on a Bruker DMX-200 spectrometer operating at proton resonance frequency of 200 MHz. A standard 90°-180° pulse sequence and stimulated echo pulse sequence were used for the measurement in the n-propanol system and in the other systems, respectively. Viscosity Measurement. The relative viscosity of an aqueous propylene glycol solution to pure water at 25 °C was measured using a Schott-Gera¨te capillary viscometer. The viscosities of water-glycerol and water-propanol mixtures were taken from the literature.20
Results and Discussion Phase Behavior at Low Surfactant Volume Fraction. Phase diagrams in water/alcohol/C12EO8 systems at 25 °C are shown in Figure 1a-c. Alcohols used are glycerol (Gly), propylene glycol (PG), and 1-propanol (C3OH). The volume fraction of C12EO8 in the system, φs, is plotted horizontally and, the volume fraction of alcohol in water + alcohol, φ, is plotted vertically. In the Gly system, an aqueous micellar solution phase (Wm) is split into two isotropic phases at a high Gly content. This suggests that Gly is a less good solvent than water for the ethylene oxide chains at 25 °C. The phase diagram resembles the well-known φs-temperature diagram of the binary water/ (19) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1. (20) The Chemical Society of Japan Kagaku Binran; Maruzen: Tokyo, 1993; Vol. II, pp 47-48.
Figure 1. Phase diagrams in water/alcohol/C12EO8 systems at 25 °C as a function of the volume fraction of surfactant in the system, φs, and of the volume fraction of alcohol in the water-alcohol mixture, φ. Alcohols used are (a) glycerol, (b) propylene glycol and (c) 1-propanol. Phase notations, Wm, H1, V1, II, and Wm + H1 indicate a micellar phase, a hexagonal liquid crystalline phase, a bicontinuous cubic phase, two-phase equilibrium of solvent-rich solution and surfactant-rich solution, and two phase equilibrium of Wm and H1, respectively.
C12EO8 system.21 On the other hand, such a phase separation is not observed in the PG and C3OH systems as is shown in parts b and c of Figure 1. A water/CmEOn surfactant system has a lower critical solution temperature (LCST) or clouding temperature due
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to the attractive force between polyoxyethylene chains.22 The volume fraction of surfactant at LCST, i.e., the critical volume fraction, φc, reports on the degree of the micellar growth. It was theoretically shown that φc should be 0.1213 for a suspension of colloidal hard spheres having shortrange attractive interactions (adhesive hard sphere model, AHS model).23 Thus, we expect φc ) 0.1213 if the micelles remain spherical near the critical point. On the other hand, the Flory-Huggins theory predicts that φc in a polymer solution is equal to 1/(1 + n1/2), where n is the number of monomers in a polymer chain.24 It means that a longerchain-polymer system shows smaller φc. Therefore if micelles grow with approaching the clouding curve, φc should be less than 0.1213. For instance, φc is about 0.0125 in the water/C12EO5 system in which an extensive micellar growth was observed26 and 0.032 in the water/C12EO8 system.27 The location of the critical volume fraction of surfactant in the present system is estimated to 0.06 ( 0.01 since, on the vicinity of the clouding curve in the two-phase region, a small amount of surfactant-rich phase separates at φs ) 0.05 and a small portion of the aqueous phase separates at φs ) 0.07. This is lower than the prediction of the AHS model, indicating nonspherical micelles are formed in the Wm phase near the clouding curve. Phase Transition from Hexagonal Liquid Crystal Phase to Isotropic Phase. A hexagonal liquid crystal phase (H1) turns into an isotropic phase (Wm) with increasing φ. When temperature goes up, a liquid crystal generally turns into an isotropic solution.28 This “melt” is induced by the increase in the thermal motion of surfactant, which decreases the molecular order of surfactant in the liquid crystal.29 However this explanation is not applied to the present system since the temperature is constant. To understand the “melting” mechanism at constant temperature, we measured an interlayer spacing, d, of the H1 phase at φs ) 0.52 by SAXS, and the results are shown in Figure 2. When it is assumed that infinitely long cylindrical micelles are packed in a hexagonal array and all of surfactant molecules are in the aggregates, the following equations are introduced.30
( )
d ) rL
x3π 2φL
1/2
or d )
( )
x2vL x3π as φL
1/2
(1)
where rL is the radius of the lipophilic part of a cylinder, as is the effective cross sectional area per amphiphilic molecule at the polar-apolar interface, vL is the volume of the lipophilic part of an amphiphilic molecule, φL is the volume fraction of the lipophilic part of amphiphilic molecules in the system. Note that when more than two (21) Mitchel, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T. A.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (22) Sjo¨blom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactants Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; Chapter 7. (23) Barboy, B. J. Chem. Phys. 1974, 61, 3194. (24) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactant and Polymers in Aqueous Solution; Wiley: Chichester, 1998; Chapter 9. (25) Strey, R.; Schoma¨cker R.; Roux, D.; Nallet F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (26) Kato, T.; Anzai, S.; Takano, S.; Seimiya, T. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2499. (27) Corti, M.; Minero, C.; Degiorgio, V. J. Phys. Chem. 1984, 88, 309. (28) Laughlin, R. G. Aqeous Phase Behavior of Surfactant; Academic Press: London, 1994; p 121. (29) McMillan, W. L. In Liquid Crystals and Ordered Fluids; Johnson, J. F., Porter, R. S., Eds.; Plenum Press: New York, 1974; pp 141-146. (30) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952.
Figure 2. Interlayer spacing, d, of H1 phase measured by SAXS plotted as a function of φ. φs for all samples is kept at 0.52. Filled circles, open circles, and triangles indicate systems with Gly, PG, and C3OH, respectively.
kinds of molecules compose cylinders, the values of rL, as, and vL are average values among the molecules. Three possibilities for the change in d upon addition of alcohol should be considered. If alcohol molecules penetrate into the palisade layer of the cylinders with increasing φ, φL increases and rL decreases. Therefore d decreases. The number of hydrated water molecules to ethylene oxide chains decreases monotonically with decreasing water content in a water/C12EO8 system.31,32 Hence, the dehydration could happen in the present systems with increasing φ. Kabalnov et al.12 suggested that the salt or polyol,which is depleted from a surfactant layer, dehydrates the layer by an osmotic effect. Such a dehydration can explain the decrease in as33 and corresponding increase in d. An increase in d can also arise from an increased monomer solubility in the mixed solvent, by decreasing the effective φL. Alcohols used in the present study have lower γ/V1/3 values (15.3 for Gly, 8.7 for PG, and 5.6 for C3OH) than those obtained in water. When these alcohols are mixed with water, the mixed solvent has a lower γ/V1/3 value than that of pure water, resulting in an increase of the cmc and monomer solubility in the mixed solvent. As is shown in Figure 2, with increasing Gly content, d increases gradually. This means that the dehydration and/or the increase in cmc happen with increasing φ. The cmc increases with the Gly content as described later. However the increase is too small compared to the surfactant concentration for the measurement, φs ) 0.52, and has little effect on the d value. Therefore it can be considered that the increase of d is mainly due to the dehydration. As described before, the dehydration causes the shrinkage of as followed by the induction of less curved surface of the aggregate, which is suggested by the surfactant parameter, vL/asl, where vL and l are the volume and the length of the lipophilic part of a surfactant molecule, respectively. To attain the less curved aggregates, cylinders in the H1 phase connect with each other. As a result, the H1 phase loses the periodicity and then turns into the Wm phase. On the other hand, d decreases monotonically in the PG and C3OH systems. This means the penetration of (31) Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1983, 87, 4756. (32) Jonstro¨mer, M.; Jo¨nsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293. (33) Kunieda, H.; Umizu, G.; Yamaguchi, Y.; Suzuki, M. Yukagaku 1998, 47, 879.
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Figure 3. Self-diffusion constant of surfactant (Ds, open circle), water (Dw, filled circle), glycerol (DGly, triangle), propylene glycol (DPG, triangle), and 1-propanol (DC3OH, triangle) at 25 °C for the system with glycerol at φs ) 0.05 and 0.3 ((a) and (b), respectively), with propylene glycol at φs ) 0.05 and 0.3 ((c) and (d), respectively), with 1-propanol at φs ) 0.05 and 0.3 ((e) and (f), respectively). Self-diffusion constant of HMDS (DHMDS, cross) is also plotted in Figure 3b. Lower and higher dotted lines are the loci of Dmic and Dsmono, respectively (see text).
alcohol molecules happens with increasing φ. It is known that an elasticity of surfactant film decreases with mixing shorter-length molecules.34 Hence the phase transition from H1 phase to Wm phase is mainly induced by the penetration of PG and C3OH molecules. PFG-NMR Self-Diffusion Measurement. We performed a PFG-NMR self-diffusion measurement in the
Wm region at constant surfactant concentrations, φS ) 0.05 and 0.3, at 25 °C with changing the water/alcohol ratio, and the results are shown in Figure 3a-f. The self-diffusion constant of colloidal particles in a solvent is given by the following Stokes-Einstein relation35
(34) Szleifer, I.; Kramer, D.; Ben-Shaul, A.; Roux, D.; Gelbart, W. M. Phys. Rev. Lett. 1988, 60, 1966.
(35) Lindman, B.; Olsson, U. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 344.
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D)
kT (1 - ksφparticle + ...) 6πηRH
Aramaki et al.
(2)
where k is Boltzmann constant, T is the absolute temperature, η is the solvent viscosity, RH is the hydrodynamic radius of a particle, ks is a dimensionless constant which depends on the particle geometry and interactions, and φparticle is the particle volume fraction in the system. Assuming that any structural change of micelles does not happen with increasing φ at constant surfactant volume fraction and temperature, RH, ks, and φparticle are unchanged and the self-diffusion constant of the micelle, Dmic, depends only on the viscosity of solvent. We obtain the theoretical value of Dmic as the following equation.
Dmic )
η0 D 0 η mic
(3)
where η0 and η are the viscosities of solvent at φ ) 0 and at a certain φ, respectively. Dmic0 is the self-diffusion constant of the micelle at φ ) 0, which is equal to the surfactant self-diffusion constant, Ds, at φ ) 0 within a reasonable uncertainty because the cmc of C12EO8 in water is considerably low and we can neglect contributions from surfactant monomers. The locus of Dmic is shown in Figure 3. A self-diffusion constant of C12EO8 monomer in pure water at 25 °C,36 3.5 × 10-10 m2‚s-1, is also applied to eq 3, and the locus is shown as Dsmono in Figure 3. Structural Change of Micelles in the Glycerol System. In the Gly system, Ds values at φs ) 0.05 and 0.3 are almost the same as the theoretical value, Dmic, up to φ ) 0.4 and 0.3, respectively, and then are deviated downward in the former system and upward in the latter system, as is shown in parts a and b of Figure 3, respectively. The deviation at φS ) 0.05 is easily understood by the effect of micellar growth. Kato et al.26 reported that the micellar growth in a dilute aqueous CmEOn surfactant solution starts at a specific temperature, TC - 30 °C, where TC is the lower critical solution temperature. Cloud temperature curves at various φ in the Gly system are shown in Figure 4. Figure 4 shows that TC - 30 °C is almost equal to 25 °C at around φ ) 0.5 and the deviation of Ds from Dmic is started at a little bit lower φ. It is considered that the micellar growth would start at lower φ when the surfactant concentration is high because of the lower influence of translational entropy. This is inferred by a fluorescence quenching study37 which shows that C12EO8 micelles in a 37 wt % aqueous surfactant solution start to grow at around 30 °C, which is much lower than Tc - 30 °C. For φs ) 0.3, Ds first decreases, goes through a minimum, and then increases with increasing φ. The reason for the increase in Ds at higher φ is probably the formation of a multiconnected (bicontinuous) structure which is known to occur in many systems including aqueous solutions of C12EO538 and C16EO7.39 Gly dehydrates surfactant and this could make as smaller, accompanied by a less curved layer which allows the above structure. We also measured the diffusion coefficient of hexamethyldisilane (HMDS) which is added to the system by a trace amount. The result is also plotted in Figure 3b. HMDS molecules diffuse much faster than surfactant (36) Nilsson, P.-G.; Wennerstro¨m, H.; Lindman, B. J. Phys. Chem. 1983, 87, 1377. (37) Medhage, B.; Almgren, M.; Alsins, J. J. Phys. Chem. 1993, 97, 7753. (38) Brown, W.; Pu, Z.; Rymde´n, R. J. Phys. Chem. 1988, 92, 6086. (39) Kato, T.; Terao, T.; Tsukada, M.; Seimiya, T. J. Phys. Chem. 1993, 97, 3910.
Figure 4. A series of clouding curves at various φ in the Gly system. Data for pure water is taken from ref 21. Table 1. cmc of C12EO8 in Several Water-Alcohol Mixturesa alcohol used glycerol propylene glycol 1-propanol
φ
cmc (mol/L)
0 0.4 0.6 0.4 0.6 0.8 0.2
1.09 × 10-4 1.46 × 10-4 2.10 × 10-4 5.05 × 10-4 1.45 × 10-2 1.12 × 10-1 5.11 × 10-4
Note that the cmc data for φ ) 0 are taken from the book of ref 5 and the surface tension did not change with the surfactant concentration enough to detect the cmc in the 1-propanol system above φ ) 0.4. a
molecules. In the case of O/W microemulsion (oil/water weight ratio is 1/9) with C12EO5,40 when the temperature closes to the transition temperature to lamellar phase, the structure of the aggregates becomes bicontinuous and the diffusion coefficient of solubilizates (cycrohexane and hexadecane) is much larger than that of the surfactant. This also supports the above interpretation in the present Gly system. Other possible explanations for the increase of Ds at higher φ, like a decrease in micellar size and an increase of the surfactant monomer concentration can be excluded. The diffusion coefficient of surfactant and HMDS should be the same extent if only the micellar size decreases under the condition that HMDS is not soluble in a water-glycerol mixture even at high Gly content, which is verified by the NMR spectrum that we could not detect any signal of HMDS for the water-glycerol mixture (φ ) 0.7) which is equilibrated with HMDS. We also measured the cmc value at various φ, and the results are shown in Table 1. Although the cmc at φ ) 0.6 in the Gly system increases by a factor of 2 from the value at φ ) 0, it is not enough for the increase in surfactant diffusion observed with considering φs ) 0.3. An increase in Ds due to an increase in the rate of molecular exchange upon collisions can also be excluded. The effect of the molecular exchange between two micelles on the diffusion constant of the molecules, which are solubilized in the micelles, was analyzed by Clarkson et al.,41 and the following relation was obtained.42 (40) Olsson, U.; Nagai, K.; Wennerstro¨m, H. J. Phys. Chem. 1988, 92, 2, 6675. (41) Clarkson, M. T.; Beaglehole, D.; Callaghan, P. T. Phys. Rev. Lett. 1985, 54, 1722.
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Figure 5. Ds/Dmic (open circle) and DHMDS/Dmic (cross) ratios are plotted against φ in the Gly system (φS ) 0.3). The dotted line represents the upper limit, 2.2, in the case of the molecular exchange between micelles (see text).
D/Dm ) 1 + P 4φm
(4)
Here D and Dm are the observed self-diffusion constant of an exchangeable species and the self-diffusion constant of a micelle. P (0 e P e 1) is the probability factor of the exchange. φm is the volume fraction of micelles. φs can be considered to be equal to φm since the cmc is extremely low even at high glycerol content. Therefore, in the case of φs ) 0.3, the second term on the right-hand side of the eq 4 becomes 1.2P, then the possible range of the D/Dm ratio is led as 1 e D/Dm e 2.2 under the condition 0 e P e 1. In the present study, we can substitute Ds and DHMDS for D and Dmic for Dm and the Ds/Dmic and DHMDS/Dmic ratios are plotted against φ in Figure 5. The dotted line is the upper limit, 2.2, in the case of the exchange. Both Ds/Dmic and DHMDS/Dmic ratios become much higher than the upper limit at high φ. Therefore the exchange of HMDS and surfactant between micelles are not the case for the increase of the diffusion coefficients. As is shown in parts a and b of Figure 3, Ds in the Gly system at φs ) 0.05 is higher than that at φs ) 0.3 below φ ) 0.55, whereas the former value becomes small above φ ) 0.55. The same tendency was observed in the binary water/C12EO5 system with increasing temperature,38 which is explained by increasing the contribution of lateral diffusion of surfactant molecules on the surface of multiconnected micelles at higher surfactant concentration. This similarity of the φs dependence of surfactant diffusion between our present system and the water/C12EO5 system also supports the above conclusion. However such a multiconnected structure is not formed in a binary water/C12EO8 system with increasing temperature in spite of the fact that both the increase in temperature and the addition of glycerol in water dehydrate surfactant and the phase behavior of the present glycerol system are similar to that of the binary water/ C12EO8 system with temperature. The reason of the difference of the micellar growth may be the larger degree of dehydration with φ than with temperature. In fact, when 10% NaCl aqueous solution is employed as solvent, the phase behavior of the NaCl solution/C12EO6 system as a function of temperature becomes similar to the water/ C12EO5 system,43 suggesting that the surfactant behaves as a shorter ethylene oxide one when the water content (42) Jonstro¨mer, M.; Olsson, U.; Prker, W. O., Jr. Langmuir 1995, 11, 61. (43) Kahlweit, M.; Strey, R.; Haase, D. J. Phys. Chem. 1985, 89, 163.
Figure 6. Small-angle X-ray scattering spectra from the Wm phase at φs ) 0.52 in the (a) Gly, (b) PG, and (c) C3OH system. q is the scattering vector.
in solvent decreases. In the present system, the Ds increase starts above φ ) 0.3, thus the surfactant C12EO8 behaves as the surfactant having much shorter ethylene oxide at high φ. Normally, one observes a micellar growth and a decrease of the surfactant self-diffusion coefficient prior to the connected structure.38,40 For the φs ) 0.3 sample, this does not seem to be the case. The reason for this is at present unclear. Micellar Breakdown in the Propylene Glycol and 1-Propanol Systems. In the PG and C3OH systems, roughly speaking, Ds is deviated upward from Dmic, then reaches values similar to Dsmono at high φ as is shown in Figure 3c-f. It is shown in Table 1 that the cmc increases
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rapidly with increasing the PG and C3OH content. Furthermore the SAXS spectra of the isotropic solution above the H1 region (Figure 6) shows that the scattering intensity of the peak decreases in the PG and C3OH systems whereas the intensity increases in the Gly system. These results suggest that the fraction of monomeric surfactant increases and, finally, micelles cannot form any more in alcohol-rich solvent in the PG and C3OH system. In the C3OH system, Ds increases and the scattering intensity decreases more rapidly than in the PG system, suggesting that the C3OH interrupts micellar formation more, which coincides with the fact that C3OH has less cohesive energy density than PG. Thus, the effect of adding PG and C3OH is to decrease the amphiphilicity of the surfactant, similar to decreasing the lipophilic chain length; i.e., cmc increases extensively with decreasing the carbon number of CmEOn surfactant and finally the ability to form micelles is lost. At φ ) 0.45, the peak intensity of the Gly system is larger than that of the PG system. It is known that the scattering intensity is proportional to the number of micelles, the form factor, and the structure factor.44 Considering the sharpness of the peaks and the surfactant concentration for the measurement, the structure factor is almost the same in these systems. Although the form factor has some effect on the intensity, it is speculated that the number of micelles in the Gly system is larger than that in the PG system even just after the melt of the H1 phase. In the present systems, the diffusion coefficient of water, Dw, and of Gly, DGly, depend only on the variation of the viscosity of solvent, namely, vary parallel to Dmic or Dsmono. However the diffusion constants of PG, DPG, and of C3OH, DC3OH, show different trend. They do not decrease as much as the viscosity variation predicts, which is particularly shown at φs ) 0.3. It might be because Gly molecules are only in bulk aqueous phase whereas some of PG and C3(44) Magid, L. J. In Nonionic Surfactant Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 688.
Aramaki et al.
OH molecules are trapped in micelles at low φ, then, with increasing φ, trapped alcohol molecules are released by the micellar breakdown. Mechanism of “Salting Out” and “Salting In” by Alcohols. We mentioned in the previous section that Gly shows the “salting-out” effect on the clouding phenomenon whereas PG and C3OH show the “salting-in” effect. The SAXS and PFG-NMR results revealed that the addition of Gly induces the dehydration from surfactant. This is consistent with the Kabalnov proposal,12 i.e., the saltingout is induced by the depletion of the added species from a surfactant film. On the other hand, with the addition of PG and C3OH, the size of micelles decreases and the monomeric solubility of surfactant in solvent increases rapidly. This interrupts micellar growth and could increase the cloud temperature. Conclusion In the Gly system, the surfactant remains strongly amphiphilic but becomes increasingly hydrophobic upon replacing water with Gly due to the dehydration from the polyoxyethylene chain (Gly is a less good solvent than water for the ethylene oxide chains than water at room temperature.) In particular, it is suggested that multiconnected micelles are formed at high Gly and surfactant concentration, which is very different from the micellar behavior with increasing temperature in the binary water/ C12EO8 system although the increase in temperature also induces the dehydration. On the other hand, when replacing water with PG or C3OH, the amphiphilicity of the surfactant strongly decreases and eventually surfactant does not have an ability to form micelles in the alcoholdominant solvent. On the basis of our discussion, the mechanism of the “salting-out” effect by glycerol is almost similar to that by inorganic salts; however, the “saltingin” effect is essentially different from that by salt. LA9900573
