Effect of Inorganic Salts on the Phase Behavior of an Aqueous Mixture

The effect of LiCl, NaCl, and CsCl on the phase behavior of an aqueous mixture of heptaethylene glycol dodecyl ether (C12E7) was investigated by means...
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Langmuir 2003, 19, 10487-10494

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Effect of Inorganic Salts on the Phase Behavior of an Aqueous Mixture of Heptaethylene Glycol Dodecyl Ether LiQiang Zheng,† Hiroyuki Minamikawa,‡ Kaori Harada,§ Tohru Inoue,*,§ and Galina G. Chernik| Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China, Nanoarchitectronics Research Center, AIST, Tsukuba Central-5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan, Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan, and Department of Chemistry, St. Petersburg State University, Universitetski pr. 2, Petrodvorets 198504, St. Petersburg, Russia Received April 25, 2003. In Final Form: September 2, 2003 The effect of LiCl, NaCl, and CsCl on the phase behavior of an aqueous mixture of heptaethylene glycol dodecyl ether (C12E7) was investigated by means of differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and Fourier transform infrared (FT-IR) spectroscopy. Phase diagrams for the mixtures containing the inorganic salts with a concentration of 1.0 M were constructed on the basis of the DSC experiments. The addition of the salts induced an expansion of the lamellar (LR) phase region toward higher temperatures and a shrinkage of the normal hexagonal (H1) and the bicontinuous cubic (V1) phase regions toward lower temperatures. The effectiveness of the salt species followed the sequence of CsCl < NaCl < LiCl for the expansion of the LR phase region and of LiCl < CsCl e NaCl for the shrinkage of the H1 and V1 phase regions. The influence of different salts on the phase behavior of the mixture is qualitatively interpreted in terms of the hydration of ions. The SAXS and FT-IR results revealed the salt effect on the molecular assemblies in the LR phase and H1 phase and on the conformational changes of the surfactant molecules in the two mesophases, respectively.

Introduction It is well-known that in aqueous mixtures of nonionic surfactants a variety of mesomorphic phases appear depending on the composition and temperature.1,2 The effect of inorganic salts on the phase behavior of nonionic surfactant-water mixtures is an interesting and important research subject in connection with the application of surfactant mesophases as a template to prepare mesoporous ceramic materials;3 these materials are usually produced by the reduction of metal salts dissolved in surfactant mesophases.4,5 Nevertheless, no systematic study has so far been reported for this problem except for a few cases.6,7 In general, the effect of inorganic salts on the solution properties of aqueous mixtures of nonionic surfactants is considered to be small. However, a large amount of inorganic salts would influence the solution properties through their effect on the properties of water, that is, the so-called salting-out effect. We have started to investigate * To whom correspondence should be addressed. Fax: +81-92865-6030. Phone: +81-92-871-6631. E-mail: [email protected]. † Shandong University. ‡ AIST. § Fukuoka University. | St. Petersburg State University. (1) Sjo¨blom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 369. (2) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994. (3) Leontidis, E. Curr. Opin. Colloid Interface Sci. 2002, 7, 81. (4) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (5) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R. Langmuir 1998, 14, 7340. (6) Iwanaga, T.; Suzuki, M.; Kunieda, H. Langmuir 1998, 14, 5775. (7) Alexeeva, M. V.; Churjusova, T. G.; Mokrushina, L. V.; Morachevsky, A. G.; Smirnova, N. A. Langmuir 1996, 12, 5263.

the salt effect for the phase behavior of aqueous mixtures of polyoxyethylene (POE)-type nonionic surfactants. In the present article, we report the results for the influence of a series of alkali chlorides, that is, LiCl, NaCl, and CsCl, on the aqueous phase behavior of heptaethylene glycol dodecyl ether. It is known that the viscosity B coefficient of an ion becomes a measure for the interaction of the ion with water.8 This coefficient represents the deviation of the viscosity of a salt solution from the predicted square-root law in the concentration dependence of the viscosity. Ions with a positive B coefficient act to increase the solution viscosity when dissolved in water, which in its turn suggests that these ions induce the formation of a kind of “network” structure in the solution; thus, they are called structure-making ions. Ions with a negative B coefficient exhibit the opposite action, and they are called structurebreaking ions. The viscosity B coefficients for Li+, Na+, and Cs+ are reported to be +0.150, +0.086, and -0.045, respectively.9 It was our interest in the present work to examine the relation between the effect of the salts on the properties of water and their effect on the aqueous phase behavior of a nonionic surfactant. Experimental Section Materials. Heptaethylene glycol dodecyl ether (C12E7) with a homogeneous chain length distribution was purchased from Nikko Chemicals (Tokyo, Japan) and used without further purification. Guaranteed grade LiCl‚H2O (99.9%), NaCl (>99.5%), and CsCl (99.9%) were obtained from Wako Pure Chemicals (Tokyo, Japan) and used as received. Water was purified by deionization followed by distillation twice, and that purified water was used to prepare the salt solutions. Salt solutions used for the (8) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, Revised Edition; Butterworth: London, 1965; p 304. (9) Kaminsky, M. Discuss. Faraday Soc. 1957, 24, 171.

10.1021/la030182l CCC: $25.00 © 2003 American Chemical Society Published on Web 11/13/2003

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Fourier transform infrared (FT-IR) experiments were prepared with heavy water (>99.75%) obtained from Wako Pure Chemicals (Tokyo, Japan). The salt concentrations were kept at 1.0 M unless otherwise stated. Samples of surfactant mixtures were prepared by mixing C12E7 and a salt solution to give desired compositions. Methods. Differential scanning calorimetry (DSC) was used to construct a T-X phase diagram of the aqueous C12E7 mixture in the presence of the salt species mentioned in the previous paragraph. The DSC measurements were carried out using a Seiko Denshi Model SSC5200 (Tokyo, Japan). DSC curves obtained by heating scans from approximately -30 to +80 °C at a rate of 1 °C/min were recorded for the mixtures of various compositions. The top temperatures of the endothermic peaks in the DSC curves were plotted as a function of the composition to give an estimation of the location of phase boundaries in the T-X phase diagrams of the mixtures. Structural aspects of the surfactant molecular assemblies in mesophases were examined by means of the small-angle X-ray scattering (SAXS) technique for mixtures with added LiCl and NaCl as well as for salt-free system. Samples for the SAXS experiments were prepared as follows. Mixtures of C12E7 and a water or salt solution were warmed to 60 °C and introduced into quartz capillaries (Glas, Berlin, 1.5-mm outer diameter, 1/100mm wall thickness). The capillaries were sealed by a flame. The samples in sealed capillaries were kept at room temperature for 1 day or 2 days before measurements. SAXS measurements were carried out with Cu KR radiation (wavelength λ ) 0.154 18 nm) generated by a Rigaku R-AXIS X-ray generator (Tokyo, Japan, 40 kV, 30 mA). The X-ray beam (0.5-mm diameter) was collimated with a side-by-side Kirkpatrick-Baez multilayer optics (Osmic, Inc.). The diffractograms were recorded with an imaging plate (Fuji Photo Films, HR-IIIN) in a flat camera (camera length of 145.5 mm). The recorded images were digitized on a Rigaku RINT2000 system. The specimen temperature was controlled with a Mettler FP82HT hot stage with an accuracy of (0.1 °C. Each sample was once cooled to -20 °C and then warmed in steps to the measurement temperatures at a rate of 1 °C/min. The sample was kept at each temperature for 5 min, and the diffraction signals were collected for 5 min. The d spacing was obtained from the position of the diffraction line on the axis representing the diffraction angle, 2θ. The errors in the d-spacing values come mostly from the errors in the reading of the diffraction lines because they tend to be somewhat broad in liquid-crystalline phases. The estimated errors in the d-spacing values were at most (3%. FT-IR spectroscopy was used to monitor the conformational change of the surfactant molecules associated with the temperature rise of the samples. A Bio-Rad model FTS165 (Cambridge, MA) was used for FT-IR measurements for the mixtures of C12E7 and salt solutions in D2O as a function of the temperature in the range from typically -20 to +40 °C. The samples were sandwiched between two silicone wafers of 3-cm diameter and were placed in a handmade cell holder, through which the water/ethylene glycol mixture of a constant temperature was circulated using a Neslab refrigerated circulation bath RTE-140 (Portsmouth, NH). The spectra were recorded with 50 scans at a 4-cm-1 resolution after the sample was kept at the desired temperature for more than 5 min. The formation of some mesophases in the present system was confirmed by polarized optical microscope (POM) observation. POM textures were obtained with an Olympus model BHSP POM (Tokyo, Japan) equipped with a Linkam Model THMS 600 temperature-controlling stage.

Results and Discussion DSC Results for the Mixture of C12E7 and NaCl Solutions. Figure 1 shows DSC curves obtained for the samples containing C12E7 and 1.0 M NaCl solutions of various compositions; the composition is expressed in terms of the weight percent of the 1.0 M NaCl solution. Many endothermic peaks in these DSC curves appear depending on the composition, which demonstrates that many phase transformations associated with the temperature rise take place in this system. The heat effect at a temperature of about -20 °C was not observed for the

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Figure 1. DSC curves obtained for the mixture of C12E7 and 1.0 M NaCl solutions. The composition expressed in terms of the weight percent of NaCl solution is indicated in the figure. The vertical scale was changed appropriately for each curve to compile many curves in the figure.

C12E7/H2O mixture10 and is seen for the NaCl solution without surfactant. Using the published phase diagram of the NaCl/H2O system,11 one can attribute the effect at -20 °C observed for the 1.0 M NaCl solution to the eutectic transition in which ice and crystal dihydrate form the NaCl solution upon heating (ice + NaCl‚2H2O(s) f solution). In addition to the distinct endothermic peaks shown in Figure 1, some small heat effects were detectable in the DSC curves in the high-temperature range drawn in a more expanded scale (refer to Figure 4b). The T-X phase diagram of this system was constructed by plotting the peak temperatures in the DSC curves as a function of the composition and is shown in Figure 2. In this figure, the DSC peak temperatures obtained for the mixture of C12E7 and water are also plotted for comparison,10 as well as those obtained for the mixtures of the C12E7/0.10 M NaCl solution and C12E7/0.50 M NaCl solution. The existence of the lamellar (LR) and the normal hexagonal (H1) phases was confirmed by the POM observation of the crossed flowerlike texture and fanlike texture, respectively. The temperatures at which these textures disappeared were in agreement with the phase boundaries determined from the DSC experiments. The mesophase existing between the LR and the H1 phases is the bicontinuous cubic (V1) phase;12 because of its optically isotropic nature, a dark field was observed by POM under the crossed Nicol condition. It should be noted here that the narrow two-phase regions between the two mesophases and between the mesophase and the liquid phase (usually (10) Inoue, T.; Matsuda, M.; Nibu, Y.; Misono, Y.; Suzuki, M. Langmuir 2001, 17, 1833. (11) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994; p 69. (12) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975.

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Figure 3. Transition enthalpy associated with the DSC peak at -20 °C versus the composition (in weight percent) of the NaCl solution in the mixture. The NaCl concentrations are 1.0 M (O) and 0.50 M (0).

Figure 2. Phase diagrams of aqueous C12E7 mixtures constructed by plotting the DSC peak temperature versus the composition of the mixtures. The meanings of symbols are the mixture with H2O (O and dotted line), 0.10 M NaCl solution (solid diamond), 0.50 M NaCl solution (crossed diamond), and 1.0 M NaCl solution (0 and solid line). For the two-phase regions between the mesophases and between the mesophase and the liquid phase, see the text.

found in aqueous surfactant systems) are simply expressed by single lines in Figure 2 and in other phase diagrams presented in the following to avoid the complexity caused by the crowdedness of the figure. Thus, one should consider that there exist narrow two-phase regions between the previously mentioned one-phase regions in these phase diagrams. Phase Diagram for the Mixture of the C12E7 and NaCl Solutions. It can be seen in Figure 2 that, concerning the upper phase boundaries of the mesophases, the addition of NaCl expands the LR phase region toward higher temperatures, while it shrinks the H1 and V1 phase regions toward lower temperatures, the extent of which increases with the increase in the NaCl concentration. On the other hand, the horizontal line corresponding to the eutectic phase transition (in which the H1 phase is formed from the solid phases) is shifted toward lower temperatures by the addition of NaCl in a concentrationdependent manner. This is attributed to the freezing point depression of the salt solution; the liquid water phase tends to appear at a lower temperature in the case of the salt/water mixture compared with pure water. It may be noteworthy to focus our attention on the horizontal phase boundary at about -20 °C. The present mixture is a three-component system, in which the composition of NaCl in water is kept at certain constant values. Thus, the phase diagram presented in Figure 2 for the mixture of the C12E7 and NaCl solutions should be regarded as a cross-sectional view of the trigonal column phase diagram for the ternary system of C12E7, NaCl, and water; the cross section differs depending on the NaCl concentration. According to the phase diagram of the aqueous NaCl mixture,11 the NaCl solution with the

concentration used in the present study splits upon freezing into two solid phases, that is, ice and crystal dihydrate, NaCl‚2H2O. In the present mixture, therefore, the three phases, that is, solid C12E7, ice, and NaCl‚2H2O, are coexisting as separate phases at the temperatures below -20.7 °C. The horizontal boundary at -20.7 °C is a line depicting the equilibrium of the solid surfactant, ice, NaCl‚2H2O, and the solution. The Gibbs phase rule predicts that one degree of freedom remains for a system of three components with four coexisting phases. Because this degree of freedom is used for pressure, no degree of freedom is available, and, thus, the temperature and the composition are fixed for this system. The fact that the temperature of this four-phase line is independent of the NaCl concentration (see Figure 2) is consistent with this interpretation. When the solid phases of the present mixture are heated, it is expected that a phase reaction, in which the NaCl solution is formed from ice and NaCl‚2H2O, takes place at the temperature corresponding to the four-phase line, and the reaction proceeds until NaCl‚2H2O disappears, keeping the temperature unaltered. If this is the case, the enthalpy change per unit weight of the mixture associated with this phase reaction should be proportional to the content (in weight percent) of the NaCl solution in the mixture. The values of ∆H expressed in Joules per gram of the mixture estimated from the area of the DSC peaks at -20 °C are plotted in Figure 3 as a function of the composition (in weight percent of NaCl solution). As expected, a proportional relationship is seen between ∆H and the mixture composition, although the data points are somewhat scattered. It should be noted that the cloud point curve must appear in the phase diagram presented in Figure 2; the cloud point for C12E7 is around 70 °C,13 and it is known that the cloud points for the POE-type nonionic surfactants are lowered by more than 15 °C in the presence of NaCl at 1.0 M.14-16 However, the heat effects associated with the clouding phenomenon, that is, the separation of the micellar solution into a dilute aqueous solution phase and a surfactant-rich solution phase, are rarely reported as (13) Inoue, T.; Ohmura, H.; Murata, D. J. Colloid Interface Sci. 2003, 258, 374. (14) Schott, H.; Royce, A. E.; Han, S. K. J. Colloid Interface Sci. 1984, 98, 196. (15) Weckstro¨m, K.; Zulauf, M. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2947. (16) Koshy, L.; Saiyad, A. H.; Rakshit, A. K. Colloid Polym. Sci. 1996, 274, 582.

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Figure 5. Phase diagrams of the mesophase region for the aqueous mixtures of C12E7 in the presence of and absence of the salts added. The salt concentration is 1.0 M. The salt species are none (O), LiCl (0 and dotted line), NaCl (2 and dashed line), and CsCl (]).

Figure 4. Effect of the salt species on the DSC curves for the mixtures of C12E7 and 1.0 M salt solutions. The salt species and composition (in wt % of the salt solutions) are indicated near each curve.

detected by the DSC technique. Usually, a phase transition in which a liquid splits into two liquid phases, such as the case of the clouding phenomenon, is calorimetrically invisible due to the fact that the phase separation is accompanied by small heat effects and, in some cases, takes place slowly. Thus, the possible two-phase region separated by the cloud point curve is not included in the phase diagram presented in Figure 2. Effect of the Salt Species on the Phase Behavior of the Aqueous C12E7 Mixture. Figure 4 shows the DSC curves obtained for C12E7 mixtures containing about 30 wt % 1.0 M solution of LiCl, NaCl, or CsCl. The heat effects observed at about -7 °C do not differ significantly for different salt species (Figure 4a). On the other hand, the heat effect observed in the high-temperature range depends on the salt species (Figure 4b); that is, the phase boundaries between the mesophases and between the mesophase and the liquid phase are affected by the salt species. The phase boundaries in the mesophase region, which were constructed by plotting the DSC peak temperature as a function of the composition, are compared in Figure 5 for the mixtures with and without added salts.

It can be seen in Figure 5 that the LR phase is expanded toward the higher temperature side by the presence of salts, and the effectiveness of the salt species for this expansion follows the sequence of none ≈ CsCl < NaCl < LiCl. On the other hand, the H1 phase and V1 phase shrink toward the lower temperature side in the presence of the salts, and the effectiveness of the salt species for this shrinkage obeys the sequence of none < LiCl < CsCl e NaCl. This salt effect on the temperature range of the mesophase stability may be interpreted qualitatively in terms of a change in the critical packing parameter of the surfactant molecule induced by the ions formed from the salts dissolved in aqueous media. The critical packing parameter, F, is defined by17

F ) v/(a0lc)

(1)

where v is the volume of the surfactant hydrocarbon chain, a0 is the optimum area occupied by the surfactant molecule at the interface between the hydrophobic and the hydrophilic regions of the surfactant molecular assemblies, and lc is the critical hydrocarbon chain length. The shape of the surfactant molecules is reflected by the F value.17 The value of F ) 1 corresponds to a cylindrical shape, and in this case the surfactant molecules are readily packed in a manner of planar bilayer arrangement; hence, the lamellar structure is preferable for the assemblies of this type of surfactant molecules. When F < 1/3, the surfactant molecules are cone-shaped, and they are packed with high curvature; thus, the spherical micelles are preferred by this type of surfactants. For 1/3 < F < 1, the shape of the surfactant molecules is a truncated cone, and the packing with low curvature is favorable; hence, the preferable molecular assemblies become cylindrical micelles or a (17) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1992; Chapter 17.

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Figure 6. X-ray diffraction spectra in the small-angle region obtained for the C12E7 samples. (a) The surfactant sample contains 20.5 wt % 1.0 M NaCl solution (top), 20.8 wt % 1.0 M LiCl solution (middle), and 20.6 wt % water (bottom). The temperature is 20.0 °C. (b) The surfactant sample contains 45.3 wt % 1.0 M NaCl solution (top), 45.2 wt % 1.0 M LiCl solution (middle), and 45.4 wt % water (bottom). The temperature is 10.0 °C.

flexible bilayer. In other words, this type of surfactant molecules tends to form the H1 phase or V1 phase. In the case of the POE-type nonionic surfactants, the F value is affected by the hydration of the POE chain because the bound water molecules contribute to the effective surface area, a0. When salts are added to the aqueous C12E7 mixture, the dehydration of the surfactant POE chain would be induced because the ions dissociated from the salts are strongly hydrated. This leads to a decrease in a0 and, hence, to an increase in the F value compared with that of the salt-free system. This change in the critical packing parameter brought about by the addition of salts is favorable for the surfactants to form the LR phase and unfavorable to form the H1 and V1 phases. Thus, the expansion of the LR phase and the shrinkage of the H1 and V1 phases induced by the addition of the salts may be explained, as a first approximation, in terms of the strong hydration of the ions, which causes the dehydration of the surfactant POE chain and the resultant increase in the critical packing parameter of the surfactant molecules. The viscosity B coefficient of the ions becomes a measure for the hydration ability of the ions. The B coefficient of the cations used in the present study increases in the sequence of Cs+ < Na+ < Li+, as mentioned in Introduction. This suggests that the amount of water molecules consumed for the hydration of the ions and, hence, the extent of the dehydration of the surfactant POE chain increase in the same order. Then, it is expected that the expansion effect for the LR region and the shrinkage effect for the H1 and V1 regions of these cations should obey the same sequence. Actually, the area of the LR region obtained in the presence of the salts increases in the sequence of CsCl < NaCl < LiCl, and this is in agreement with the previous prediction. However, the areas of the H1 and V1 regions decrease in the sequence of LiCl > CsCl g NaCl rather than CsCl > NaCl > LiCl which is expected from the previous consideration. Thus, it is suggested that some factor(s) other than the dehydration of the surfactant POE chain caused by the addition of the salts may participate

in establishing the stability regions of mesophases formed in the aqueous C12E7 mixture. It is of interest to compare the salt effect on the area of mesophase regions revealed by this work with that on the cloud point of aqueous nonionic surfactant solutions. The clouding phenomenon has been attributed to a more or less sudden dehydration of the POE chain occurring at the temperature called the cloud point, and the cloud point is depressed or elevated by the addition of inorganic salts depending on the salting-out or salting-in properties of the salts. Thus, the situations for the salt effect on the cloud point and that on the mesophase regions are analogous in a sense that the both effects are caused by the salt action to induce the dehydration of the surfactant POE chain. As for a series of alkali chlorides, it is known that the cloud point temperature is decreased by their addition in the sequence of LiCl > CsCl g KCl g NaCl.14-16 This sequence can be understood, except for LiCl, by considering the ability of cation species for their own hydration derived from their viscosity B coefficients. The weaker effect of LiCl than expected has been attributed to the complex formation between Li+ and the oxyethylene groups in the POE chain.14 The effect of alkali chlorides on the area of the H1 phase region is in accordance with the previous sequence regarding the salt effect on the cloud point. This might indicate that the complex formation of Li+ with the POE chain affects the region of the H1 phase. It seems that this complex formation does not exhibit such significant effect for determining the region of the LR phase. Effect of Salts on the Surfactant Molecular Assemblies in Mesophases Revealed by SAXS. Figure 6 shows the X-ray diffraction spectra in the small-angle region obtained for the mixtures of C12E7/H2O, C12E7/1.0 M LiCl solution, and C12E7/1.0 M NaCl solution. The spectra in Figure 6a were obtained for the mixtures with the composition of about 20% water or salt solution at 20 °C. The ratio of d-spacing values estimated from the peak positions (shown by arrows) is d1/d2 ≈ 1:1/2, which demonstrates that the surfactant molecular assembly has

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where vhc is the volume of hydrocarbon chain per one surfactant molecule. As for the H1 phase, under the assumption that this phase consists of infinitely long cylindrical micelles being packed in a hexagonal array, the following relation holds to estimate the radius of the hydrophobic part in the cylinder, rH, which corresponds to the hydrocarbon chain length in a micelle.

rH )

2 φ ) (x3π

1/2

hc

d

(5)

Then, as is given by

as ) 2vhc/rH

(6)

To estimate as values from previous relations, the volume fraction of the hydrophobic part, φhc, is needed. This can be calculated by

φhc )

Figure 7. Repeated spacing d as a function of the temperature obtained for the LR phase (a) and H1 phase (b). The salt species are: none (O), LiCl (0), and NaCl (4).

a lamellar structure under this condition, being consistent with the phase diagram. The X-ray diffraction patterns obtained for the mixtures containing about 45% water or salt solution at 10 °C are shown in Figure 6b. The ratio of d spacing is d1/d2/d3 ≈ 1:1/x3/1/2, which confirms the formation of a hexagonal phase under this condition as expected from the phase diagram. The d-spacing values determined from the peak position of the first-order diffraction were plotted in Figure 7 as a function of the temperature. The d spacing observed for the LR phase is somewhat increased by the addition of the salts (Figure 7a), whereas no systematic difference is seen for the H1 phase among the mixtures with and without added salts (Figure 7b), although the d spacing obtained for the system with added NaCl tends to increase at a high temperature. It has been reported elsewhere6 for the same system that the d spacing is increased by the addition of NaCl when observed at 25 °C. On the basis of these SAXS data, one can estimate the average area occupied by a surfactant molecule at the hydrophobic-hydrophilic interface, as, according to the standard procedure described by Luzzati et al.,18 as shown in the following. For LR phase, the thickness of the hydrophobic part in the lamellar phase, dhc, is estimated from the following relation with the knowledge of the volume fraction of the hydrophobic part of the surfactant molecules in the mixture, φhc.

dhc ) φhcd

(2)

The length of the surfactant hydrocarbon chain in the surfactant bilayer, lhc, would correspond to 1/2 of dhc, and, thus,

lhc ) 1/2φhcd

(3)

as ) vhc/lhc

(4)

Then, as is given by

(

)(

)

fw 1 - fw 1 - fw Vhc / VS + MS MS Fw

(7)

where MS, VS, and Vhc are the molecular weight, the surfactant molar volume, and the molar volume of its hydrocarbon chain, respectively, and fw and Fw are the weight fraction and the density of water or salt solution, respectively. The molar volumes of the POE-type nonionic surfactants have been frequently evaluated assuming that they are given as a sum of each group, that is,

VS ) Vhc + nVEO + VOH

(8)

where VEO and VOH are the molar volumes of the oxyethylene unit and the terminal hydroxyl group, respectively, and n is the number of the oxyethylene units. The validity of this assumption has been demonstrated elsewhere,19 and the numerical values of these molar volumes have been reported as Vhc ) 215 cm3 mol-1 for the dodecyl chain, VEO ) 38.8 cm3 mol-1, and VOH ) 8.8 cm3 mol-1.6 In addition, vhc is estimated to be 0.357 nm3 from the value of Vhc. The temperature dependence of the molar volumes in eq 8 has been discussed,20 according to which it is so small that the molar volumes can be regarded as constant in the present temperature range. Using these numerical values of the molar volumes and the density data of Fw ) 1.02 g cm-3 (1.0 M LiCl solution) and 1.04 g cm-3 (1.0 M NaCl solution), the as values were calculated from the previous equations, and they are plotted in Figure 8 as a function of the temperature. It can be seen that as in the LR phase is decreased by the addition of the salts (Figure 8a). This supports the previously mentioned interpretation regarding the origin of the salt-induced expansion of the LR region. That is, the dehydration of the surfactant POE chain caused by the salts would reduce the effective volume of the chain, and this results in the decrease in the cross-sectional area per one surfactant molecule at the hydrophobic-hydrophilic interface. Contrary to the case of the LR phase, no systematic trend is seen for the salt effect on as except for the mixture with NaCl in the high-temperature region (Figure 8b). It is likely that the cylindrical micelles have bigger clearance for the accommodation of surfactant (18) (a) Luzzati, V. In Biological Membranes; Chapman, D., Ed.; Academic Press: London, 1968; Vol. 1, p 71. (b) Luzzati, V.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta Crystallogr. 1960, 13, 660. (19) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (20) Minewaki, K.; Kato, T.; Yoshida, H.; Imai, M.; Ito, K. Langmuir 2001, 17, 1864.

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Figure 8. Average area occupied by a surfactant molecule at the hydrophobic-hydrophilic interface, as, as a function of the temperature obtained for the LR phase (a) and H1 phase (b). The salt species are: none (O), LiCl (0), and NaCl (4).

molecules compared with lamellar assemblies, because the surfactant molecules are packed with curvature. Then, the surfactant molecules would occupy a similar optimum area at the hydrophobic-hydrophilic interface regardless of the difference in the effective volume of the POE chain. Effect of Salts on the Conformational Change of the Surfactant Molecules Occurring in Mesophases. In previous works, the conformational changes of the surfactant molecules associated with the temperature rise of aqueous mixtures of the POE-type nonionic surfactants were studied, and it was suggested that the ordered conformational structure of surfactant molecules persists (in the temperature range of a few degrees) when solid phases transform to mesophases. According to the interpretation of FT-IR spectra, the order-disorder transfor-

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mation of both the hydrocarbon chain and the POE chain takes place rather gradually with the rise in temperature of the mesophases.10,21-23 The effect of added salts on this “chain-melting” process was examined by observing the IR spectral change as a function of the temperature. Figure 9 shows the IR spectral change with the temperature in three wavenumber regions obtained for the mixture of C12E7 and 1.0 M NaCl solutions in D2O, the composition of which is 20.9 wt % NaCl solution. The absorption bands around 2900 cm-1 (Figure 9a) are attributed to the methylene C-H stretching mode (νCH2), and can be used to monitor the conformational structure of the hydrocarbon chains of the surfactant molecules.24 The absorption bands around 1110 cm-1 (Figure 9b) and around 850 cm-1 (Figure 9c) are the coupled mode of the C-O stretching, C-C stretching, and methylene rocking (νCO + νCC + FCH2) and that of C-O stretching and methylene rocking (νCO + FCH2) of the POE chain, respectively, and these absorption bands reflect sensitively the conformational structure of the POE chain.25 Figure 9a demonstrates that the absorption peaks due to the C-H stretching vibration are rather sharp even after the completion of the transformation of the solid surfactant to the LR phase, which means that the conformational structure of the surfactant hydrocarbon chain is ordered to some extent even in the LR phase. The peak broadening and the short wave shift of the absorption band due to the C-H stretching mode complete at 18 °C, which suggests that the “melting” of the hydrocarbon chain is completed at this temperature for the LR phase of this mixture. A similar behavior of the conformational structure is also observed in the POE chain, as can be seen in Figure 9b,c, where the absorption peaks become less sharp at 18 °C and their sharpness disappears completely at 20 °C. It seems that the ordered structure of the POE chain is maintained up to a slightly higher temperature than that of the hydrocarbon chain. Figure 10 compares the IR spectra in the wavenumber region reflecting the conformational structure of the surfactant POE chain obtained for the LR phase in the absence of and presence of different salt species. In the absence of the added salt, the peak broadening (attributed to the order-disorder conformational change) is finished at 18 °C, while in the presence of NaCl and LiCl, the corresponding temperatures become 20 °C and higher than 20 °C, respectively. Namely, the salts that expand the LR

Figure 9. FT-IR spectra obtained for the mixture of C12E7 and 1.0 M NaCl solutions in D2O at various temperatures. The composition is 20.9 wt % NaCl solution. The temperature (°C) and the corresponding phase are indicated near each spectral line.

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Figure 10. FT-IR spectra obtained for the mixtures of C12E7 and D2O (a), 1.0 M NaCl solution in D2O (b), and 1.0 M LiCl in D2O (c) at various temperatures. The content of D2O or the salt solutions (in weight percent) is denoted in the figure. The temperature (°C) and the corresponding phase are indicated near each spectral line.

region elevate the temperatures of the conformational changes of the surfactant molecules observed in the IR spectra in the same order as the expansion effect. In terms of the phase behavior, the boundary of the transition from a biphasic state (solid + mesophase) to a mesophase slightly shifts to higher temperatures according to the sequence none < NaCl < LiCl. The substantial shift of the LR/liquid-phase boundary to the higher temperature side follows the same sequence. Conclusion In the present work, we investigated the effect of the inorganic salts LiCl, NaCl, and CsCl on the phase behavior of the aqueous C12E7 mixture. The addition of these salts expands the LR phase region in the phase diagram toward higher temperatures and shrinks the H1 and V1 phase regions toward lower temperatures. This behavior, induced by the addition of salts, is interpreted qualitatively as follows. The ions formed by the salt dissociation are strongly hydrated, which would cause the dehydration of the surfactant POE chain. This dehydration of the POE chain would result in an increase in the critical packing parameter of the surfactant molecule due to the reduction (21) Inoue, T.; Kawamura, H.; Matsuda, M.; Misono, Y.; Suzuki, M. Langmuir 2001, 17, 6915. (22) Zheng, L. Q.; Suzuki, M.; Inoue, T. Langmuir 2002, 18, 1991. (23) Zheng, L. Q.; Suzuki, M.; Inoue, T.; Lindman, B. Langmuir 2002, 18, 9204. (24) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (25) Matsuura, H.; Fukuhara, K. J. Polym. Sci., Part B 1986, 24, 1383.

of the effective surface area. The increase of the critical packing parameter is advantageous for the surfactant molecules to form the structure of the LR phase and disadvantageous to form the structures of the H1 and V1 phases, as compared with the salt-free situation. The effectiveness of the salt species to expand the LR phase region follows the sequence of CsCl < NaCl < LiCl. This trend is in accordance with the previous consideration, because the hydration of the cations is expected to become stronger in this order considering the viscosity B coefficient of these cations. According to the previous mechanism, the effectiveness of shrinking the H1 and V1 phases should obey the same order. However, actually, it follows the sequence of LiCl < CsCl e NaCl. This suggests that some other factor(s) must participate in the determination of the salt effect on the area of this mesophase region of the aqueous C12E7 mixture. The SAXS results support the previous interpretation, at least for the LR phase. The conformational changes of the surfactant molecules observed by FT-IR spectroscopy upon the addition of the salts are in agreement with the shifts of the phase boundaries detected by DSC. Acknowledgment. This work was supported in part by funds from the Central Research Institute of Fukuoka University (No. 015002), Natural Sciences Foundation of China (No. 20243005), and the Ministry of Science and Technology of China (No. G2000078104). The assistance of Ms. A. E. Yakuninskaya in the preparation of the manuscript is acknowledged. LA030182L