9130
J. Phys. Chem. 1995, 99, 9130-9135
Solubilization of Polymer in the Lyotropic Lamellar Phase: The AOT/PEO/Water System Kewei Zhang* and Per Linse Physical Chemistry I , Chemical Center, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received: September 12, 1994; In Final Form: January 27, 1995@
The solubilization of a nonionic polymer, poly(ethy1ene oxide) (PEO), in a lamellar phase formed by sodium and 23Na-NMR, small-angle X-ray bis(2-ethylhexyl) sulfosuccinate (AOT) in water has been studied by 2Hscattering (SAXS), and polarizing microscopy. It has been found that (i) a substantial amount of the PEO polymers can dissolve in the lamellar phase, (ii) the solubilization of the polymers in the lamellar phase diminishes with increasing molecular weight of the polymer, (iii) it is evident that the polymers are entrapped in the aqueous domains of the lamellar phase, and (iv) SAXS measurements indicate that the solubilization of the PEO polymers in the lamellar phase exerts no apparent influence on the width of the aqueous domain of the lamellar phase. In an attempt to further understand the experimental results, a simple model of the stability of the lamellar phase based on a mean-field lattice theory and the Poisson-Boltzmann equation was used. The qualitative results of the model were in line with the experimental data. Moreover, it appeared that the electrostatic interaction is significant, and its coupling with nonelectrostatic contributions is important for the description of the solubilization of the PEO polymers in the lamellar phase formed by AOT in water.
Introduction Surfactant-polymer interactions in aqueous systems are being extensively investigated in recent years due to their importance in industrial applications, such as paints, foodstuffs, enhanced oil recovery, pharmaceuticals, and detergents, as well as for a deeper understanding of biological systems.'-6 Most of these investigations are focused on dilute systems, in particular, on how physicochemical properties of a polymer solution are varied as a function of the concentration of added surfactant. At high surfactant concentrations, the phase behavior of binary surfactant-water systems in general becomes quite intricate with liquid crystalline phases appearing. Up to now, very few studies concerning interactions between polymers and liquid crystalline phases are reported. It is well-known that small molecules can be dissolved in, and some of them even stabilize, the liquid crystalline For polymers, it is expected that the entropy cost due to constraints on the chain conformations would reduce the dissolution of polymers in liquid crystalline phases. Interestingly, however, it has been reported that both lamellar and hexagonal phases can incorporate a considerable amount of p ~ l y m e r . ~ . ~ . ' ~ Kekicheff et al. studied the solubility of poly(ethy1ene oxide) (PEO) of different molecular weight in the lamellar phase formed by sodium dodecyl sulfate (SDS) in water? They found that as much as 7 wt % PEO could be dissolved into this lamellar phase, and moreover the solubility is independent of the molecular weight of the PEO polymer. They suggested that in the lamellar phase PEO polymers deform the lamellar bilayers and further penetrate through the bilayers, rather than being confined in the aqueous domains of the lamellar phase. There exist several reasons for further investigations of this phenomenon: (i) The dissolution of polymers in the liquid crystalline phase provides an attractive method to confine polymers in a two-dimensional space. A good understanding of restricted polymer diffusion is of significance in fundamental research as well as in various applications."-12 (ii) It may serve as a valuable guide to a deeper understanding of the interactions
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, May 1, 1995.
398
358 318 2H20
50
AOT
WtOm
Figure 1. Binary phase diagram of AOTPH2O with concentration and temperature as variables: D, lamellar phase region; F, reverse-hexagonal phase region; 12, isotropic cubic phase region: LI and L2, isotropic solution phase regions (adapted from ref 17 with permission).
between synthetic neutral polymers with lipid vesicles, which is of great interest in cell-cell recognition, aggregation (or agglutination), and f ~ s i o n . ' ~ -(iii) ' ~ The mechanical properties of liquid crystals may be modified significantly by altering the amount and/or type of added polymers. In this report, we present experimental results of the solubilization of PEO in the lamellar phase formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in water (*H20). AOT is an amphiphilic lipid possessing branched alkyl chains with a low solubility in water. It forms a lamellar liquid crystalline phase covering a wide concentration and temperature regime as shown in Figure l.17 The width of the aqueous domain in the lamellar phase decreases monotonically from about 180 8, at 13 wt % to about 29 8, at 69 wt % AOT.I8 PEO is soluble in water in all proportions at room temperature and is frequently used in solubilization of biological membra ne^.'^,*^ We have also examined the phase equilibrium between the lamellar phase and an isotropic and homogeneous aqueous solution of PEO by employing a simple model. By minimizing the free energy
0022-3654/95/2099-9130$09.00/0 0 1995 American Chemical Society
The AOTPEONater System
J. Phys. Chem., Vol. 99, No. 22, 1995 9131
with respect to the distribution of PEO and water between the two phases, the equilibrium between the phases was modeled.
a
b
Experimental Section Materials. Sodium bis(Zethylhexy1) sulfosuccinate (AOT) was obtained from Fluka (Germany) and purified by recrystallizing in a methanovwater solution for three times and then dried at 60 "C for 3 days. The final product is an anhydrous white waxy paste. Two PEO polymers with molecular weights of 6000 (PEO6) and 20000 (PE020) were used. They were obtained from Fluka and used without further purification. Electrical conductivity measurements performed for dilute PEO solutions indicated that there were no electrolyte impurities. 2H20 was purchased from Norsk Hydro, Norway, and its isotropic enrichment was at least 99.8%. Sample Preparation. The samples were prepared by mixing appropriate amounts of AOT with PEO bulk solutions into glass tubes which were immediately flame-sealed. The samples were thoroughly mixed by repeated centrifugation for several days and were subsequently kept at 25 "C for 3 months before the experiments were performed. After several months some samples were rechecked again by *H-NMR measurements. It is worth noting that the samples in the dilute lamellar phase region (below ca. 25 wt % AOT) had to be treated very carefully. They required the longest time without perturbation in order to give quadrupolar splittings. Only broad-band pseudoisotropic spectral shapes were obtained when the samples were slightly perturbed. Methods. The phase diagrams were determined by ocular inspection, polarizing microscopy,21 and analysis of 2H and 23Na-NMR s p e ~ t r a . ~ * % * ~ 2H and 23Nahave nuclear spin quantum numbers I 1 1 and thus possess electric quadrupole moments. For powder samples, the 2H quadrupole splitting, A, is given by24,25A = ICpiY&il, where pi is the fraction of nuclei in site i with the effective quadrupole coupling constant YQ. The order parameter Si is
AOT: 15.0 % PEO6: 17.0 %
Figure 2. Representative water 2H-NMR spectra for samples in the lamellar phase (a) AOT/PEO$H20 = 15/17/68 and (b) AOTPEOd 'HzO = 19.8/6.1/74.1.
obtained by measuring the peak-to-peak distance in the corresponding NMR spectra. Figure 2 shows representative water 2H-NMR spectra obtained for samples kept at 25 "C without disturbing for about 8 months. The texture of the lamellar phases was examined under the polarizing microscope (Axioplan Universal of Zeiss) equipped with a differential interference contrast (DIS) unit, a camera (MC 100) for direct imaging, and a video system with image analysis facilities for automatic documentation and registration of the results. SAXS measurements were performed at 25 & 1 "C with a camera with pinhole collimation after Kiessig. The X-ray radiation used was nickel-filtered copper K a (A = 1.542 A). The sample-to-film distance was 0.208 m. We have also tried to establish the location of tie lines by preparing samples in the region 15-30% AOT and 20-30% PEO. The samples were allowed to stay at 25 "C for about 6 weeks before the examinations. At AOT concentration above ca. 25%, we did not obtain a visible phase separation, whereas given by Si = 1/2(3 cos2 8 - l), where 8 is the angle between below 25% a visible phase separation occurred. The upper the symmetry axis of the mesophase and the electric field phase was AOT-rich and displayed birefringence, whereas the gradient and the bar denotes an ensemble average. For *H, V Q lower phase was a clear isotropic solution. The PEO contents has a fixed known value, while it is not straightforward to in the two phases were determined by 'H-NMR spectroscopy. estimate its value for the counterions. As a first approximation, The areas of the proton signals specific for PEO and AOT were one can use the conventional two-site model to distinguish measured on a Nicolet XL-300 spectrometer operating at 25 between free (subscript f ) and bound (subscript b) molecules "C. The contents of PEO and AOT were determined from the or ions so that A = lPbYQ,Sb P ~ Y Q , , ~Usually ". the second calibration curves prepared from measurements on the correterm can be neglected since Sf is close to zero. For *H sponding solutions of known concentrations. The relative error quadrupole splittings, one obtains A = nv~S(1 - X D ~ O ) / X D ~ O , of this determination was estimated to be less than 5%. where Y Q has a value of ca. 220 kHz. n is the average hydration number of amphiphilic components, and XD,O is the total mole Theoretical Model fraction of water. Water and counterion quadrupole splittings thus give direct In an attempt to capture some essential features deduced from insight into hydration and ionic interactions, but the main the experiments, a simple model of the stability of the lamellar significance for the present work is that they provide direct phase of the AOTPEO/water system was constructed. In insight into the phases present in a sample. Since the splitting particular, the exclusion of PEO and water from the aqueous is a very sensitive measure of phase anisotropy, it is straightregion of the lamellar liquid crystal into an isotropic and forward to distinguish between lamellar, hexagonal, and isohomogeneous aqueous solution of PEO is examined. The central quantity is the free energy difference between (i) the tropic phases and to detect phase 2H-NMR has, therefore, become a standard technique for phase diagram one-phase lamellar system and (ii) a two-phase system consisting determination. of a lamellar phase and the isotropic and homogeneous aqueous solution of PEO, as shown in Figure 3. This free energy 2H- and 23Na-NMRspectra were recorded for a large number difference is monitored as a function of the relative sizes of the of samples over the composition range investigated. The 2Htwo phases, keeping the distribution of PEO and water in and 23Na-NMRmeasurements were performed at a resonance equilibrium between the phases. frequency of 15.351 and 26.49 MHz, respectively, with a Bruker MSL- 100 pulsed spectrometerworking in the Fourier-transform The model system consists of AOT, PEO, and water mode. The NMR results were c o n f i i e d by polarizing micros(including the counterions), and the sum of their volumes is equal to the total volume of the system, Le., V = VAOT+ VPEO copy. The magnitude of the quadrupolar splitting, A, was
+
Zhang and Linse
9132 J. Phys. Chem., Vol. 99, No. 22, 1995
+t
l ~+~ ; . .
Figure 3. Schematic illustration of a one-phase lamellar liquid crystal formed by AOT. water. and counterions with dissolved PEO in the aqueous domain (top) in equilibrium with a two-phase system consisting of a lamellar liquid crystalline phase and an isotropic phase of PEO and water (bottom).
+
VW. Moreover, the total volume of the system can also be POM, divided between two phases according to V = where PAM denotes the volume of the lamellar phase and PoM that of the isotropic phase. Starting from the case of only the lamellar phase present (VAM = V, and hence P O M = 0), we successively transfer the volume 6 V from the lamellar phase to the homogeneous one. After the nth step, the volume AV = n(dV) has been transferred, of which the volume AV!;, is PEO and AV($ is water. At each step the sum of the free energy of the lamellar and the isotropic phases given by
+
2H2O
10
20
30
40
AOT
%AOT+ Figure 4. Partial ternary phase diagram of the AOT/PEO/'H?O system at 25 "C: D. lamellar phase region; LI, isotropic solution phase region; LI + D. two-phase region consisting of a lamellar phase and an isotropic solution;
