Langmuir 1992, 8, 2616-2622
2616
Phase Diagram and Phase Properties of the System
Water-Hexane-Aerosol OT
Camillo La Mesa,* Luigi Coppola, Giuseppe A. Ranieri, Mario Terenzi, and Giuseppe Chidichimo Dipartimento di Chimica, Universith della Calabria, 87030 Arcavacata di Rende, Cosenza, Italy Received February 14, 1992. In Final Form: June
1992
The phase diagram of the system water-hexane-Aerosol OT and information on its phase properties obtained by polarizing microscopy, turbidity, 23Na and quadrupole splitting, electrical conductance, and self-diffusion. The hydrocarbon has some influence on the extension and stability of the mesophases: it does not dissolve in the viscous isotropic phase, has a nonspecific influence on the extension of the lamellar one, and has a significant effect on extension and location of the reverse hexagonal phase. The reverse solution phase, extending in a wide part of the phase diagram, contains a molecular and a reverse micellar region; the dimensions of the aqueous droplets are 40 ± 15 A in diameter. The micellar region is of limited extension and able to dissolve small amounts of oil. Experimental evidence indicates that polyphasic lamellar dispersions and/or lamellar samples at high water content are extremely sensitive to shear stresses, transform into stiff lamellar-like structures, and retain such appearance for a long time. were
Langmuir 1992.8:2616-2622. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/23/19. For personal use only.
1,
12
345678H
Introduction
filtered, cooled, filtered again, and vacuum dried for 1 day at 70 °C; the whole procedure was repeated 2 times. The final product is an anhydrous white waxy paste, which has a melting point of 152 ± 2 °C, in fairly good agreement with previous studies.11 The appearance of dry AOT in polarizing microscopy is reported in Figure 1. This is one of the very few cases in which surfactant molecules show liquid crystalline organization without any solvent, at room temperature. Water was bidistilled, deionized, and degassed; its electrical conductance, k, is lower than 1 X 10-6 Q"1 cm-1, at 20 °C. Heavy water, 99.7% isotopic enrichment, and spectroscopic grade hexane, Merck, were used without further purification. Perdeuterated hexane was kindly supplied by Professor S. Romano (University of Messina). Chloroform and methanol, Carlo Erba, were of analytical purity. The samples were prepared by weighing the components in Pyrex glass tubes, which were frozen by gaseous nitrogen, sealed off, heated at 50 °C for 3 days and centrifuged back and fourth until macroscopic equilibrium was obtained. They were left at 20 ± 1 °C for 2 weeks, checked by inspection through crossed polaroids, and periodically controlled by quadrupole splitting or optical turbidity. The time required to reach equilibrium changes from phase to phase; we assume that the system is stable if 2H quadrupole splitting, or optical turbidity, is constant and reproducible after a week. Methods. Deuterium and sodium quadrupole splitting were measured by a Bruker WM 300, in FT mode. Typical experimental conditions are 50 pa acquisition time and 500 transients. Translational diffusion was measured by pulsed field gradient methods, PFG, on a Polaron spectrometer working at 15.0 MHz, on :H nuclei. The apparatus was connected to a Datalab transient recorder and to a Rockwell AIM 65 computer. The experimental accuracy on self-diffusion values is to within ±5%. Details on the apparatus setup and on measuring procedures are reported was
The self-organization of water and amphiphilic mole-
cules in polar and nonpolar domains and the consequent formation of lyotropic mesophases has been widely studied.1-5 Systematic investigation by NMR methods
allowed determination in detail of many dynamic properties of lyotropic and related solution phases.6-8 Most studies on these ternary systems deal with mixtures containing water, surfactant, and long chain alkanols;9’10 less information is available on hydrocarbon solubilization in the apolar region of lyotropic phases and on consequent modifications of their dynamic and thermodynamic properties. We report here an experimental investigation, relative to the system water-hexane-Aerosol OT. The surfactant has a hydrophilic lipophilic balance which allows the formation of either aqueous and nonaqueous solutions. The system was characterized by basic physicochemical methods and NMR; the experimental findings shed light on the molecular organization and dynamic properties of the different phases.
Experimental Section Materials. Aerosol OT, referred to as AOT, from Fluka, was purified by dissolution in hot methanol, filtered by warm fritted glass funnels, cooled,
product
filtered again, and vacuum dried. The in warm chloroform and the solution
was redissolved
(1) Luzzati, V.; Tardieu, A. Annu. Rev. Phys. Chem. 1974, 25, 79. (2) Ekwall, P. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1975; Vol. I, p 1. (3) Fontell, K. Prog. Chem. Fats Other Lipids 1979,16, 145. (4) Fontell, K. Mol. Cryst. Liq. Cryst. 1981, 63, 59. (5) Laughlin, R. G. In Surfactants; Tadros, Th., Ed.; Academic Press: New York, 1984; p 53. (6) Wennerstrdm, H.; Lindman, B.; Engstrom, S.; Sdderman, 0.; Lindblom, G.; Tiddy, G. J. T. In Magnetic Resonance in Colloid and Interface Science; Fraissard, J. P., Resing, H., Eds.; Reidel: Boston, MA, 1980; p 609. (7) Stilbs, P. Prog. NMR Spectrosc. 1986,19, 1. (8) Lindman, B.; Sdderman, 0.; Wennerstrdm, H. In Surfactant
elsewhere.12
Turbidity was measured by placing the solutions in optical grade cuvettes, located in a jacket thermostated to within ±0.1 °C. The light source was a Hg vapor lamp, Galileo. Small amounts of water, or hexane, could be added by a SOO-^L Hamilton syringe, located at the top of the cuvette; mixing was ensured by magnetic stirring.
Solutions. New Methods of Investigation; Zana, R., Ed.; M. Dekker; New York, 1986; Chapter VI, p 295. (9) (a) Mandell, L.; Ekwall, P. Acta Polytech. Scand., Chem. Incl. Metall. Ser. 1986,74,1. (b) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1969, 29, 639. (10) Khan, A.; Fontell, K.; Lindblom, G.; Lindman, B. J. Phys. Chem.
(11) Park, D.; Rogers, J.; Toft, R. W.; Winsor, P. A. J. Colloid Interface Sci. 1970, 32, 81. (12) Chidichimo, G.; De Fazio, D.; Ranieri, G. A.; Terenzi, M. Mol. Cryst. Liq. Cryst. 1985,135, 223.
1982, 86, 4266.
0743-7463/92/2408-2616$03.00/0
©
1992 American Chemical Society
Phase Properties of Water-Hexane-Aerosol OT
Langmuir, Vol. 8, No. 11, 1992 2617
Figure 2. Turbidity of the reverse solution phase versus weight
1. Appearance of dry AOT in polarizing microscopy, magnification 100. The sample was fused, slowly cooled from the melt (cooling rate 1 °C min '), allowed to stay at 20 ®C for 1 day and sealed between glass slides by an epoxy resin to avoid water uptake.
Figure
percent of added water, w, at 20 ®C. Data are taken relative to the solvent turbidity. The AOT content in the mother solution is 14.7, (•), 29.3, (O), and 46.2 wt % ( ), respectively. In the bottom is reported the composition dependence for mixtures containing 16.5 AOT wt % in equilibrium conditions (•) and under stirring at 300 rpm (O); the phase separation limit is indicated by an arrow.
Electrical conductance was measured by a Wayne-Kerr bridge, Model BM 905A, working in the range 4 X 102 to 1 X 104 Hz. The bridge was connected to a cell with platinum electrodes, thermostated to within ±0.01 °C by circulating water. No frequency dispersion was observed. Polarizing microscopy and conoscopic investigation were performed with a Leitz Laborlux microscope. In both cases, the samples were thermostated by a Linkam unit, Model 600A. The samples were uniformly pressed between previously cleaned microscope slides and covers, sealed by a heat-resistant epoxy resin, to avoid water and hexane evaporation, and allowed to stay at 20 °C for 20 min. In some cases the samples were sealed in glass capillaries having an inner diameter of 0.5 mm. The recognition of anisotropic textures was made according to Rosevear.13
Results
(A) Turbidity. Turbidity
composition plots, Figures 2 and 3, were used to study the solution phases, Li and L?, respectively. The onset of reverse micelles was inferred by first derivative plots of relative turbidity, d log T,/du>, versus weight percent of added water, w, Figure 3; the concentration above which oil-rich and water-rich phases coexist is the point at which the derivative is zero. Experiments made under stirring indicate that the two-phase limit is the point above which the turbidity steeply increases, Figure 2. This behavior is ascribed to the formation of a transient coarse emulsion. versus
(13) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 620.
Figure 3. Relative turbidity of the reverse solution
phase,
T„
in logarithmic scale, versus weight percent of added water, full line, and its first derivative plot, dotted line. The AOT content in the mother solution is 16.5 wt %. The arrow in the bottom indicates the concentration at which droplets do form.
(B) Electrical Conductance. It was used to determine micelle formation and counterion binding, to account for the formation of reverse micelles, and to define the related phase boundaries. Micelle formation and counterion binding were inferred from specific conductance versus C1^2 plots. The cmc is the intersection point of two conductance vs composition straight lines; the binding degree, 0, is the ratio of slopes
2618
La Mesa et al.
Langmuir, Vol. 8, No. 11,1992
__.——/—•
/
°
°
____Q
/
o
J
l
10
l1
•--S8-2-S-$-4
r
M-
1_
:-1-1-
20
r
Figure 4. Electrical conductivity of the reverse solution phase, in fi"1, as a function of w, at 20 °C. The mother solution is in Figure 3.
as
Figure 5. Water (O), hexane ( ) and AOT (A), self-diffusion, D, in 107 cm2 s'1, in the reverse solution phase as a function of added water, at 20 °C. The mother solution contains 29.3 AOT wt %.
above and below the cmc, respectively.14 The cmc is close to 7 X 10'3 mol kg-1 and the binding degree is 0.65 ± 0.07; the data accuracy does not allow the determination of whether 0 is composition dependent. A large conductance increase is observed when reverse micelles do form, Figure 4. The presence of some some water is required for partial ionization of AOT, the minimum value being about 7 to 8 water molecules per
surfactant. (C) Self-Diffusion. It was inferred by measuring the decay of spin-echo signal amplitude as a function of time between two gradient pulse sequences, A, according to15 In [A/Ag]
=
(7«G)2[A
-
6/3] DNMR
(1)
where Ag and A are the spin-echo signal amplitudes with and without applied gradient pulses, respectively, y is the gyro magnetic ratio of protons, 6 and G are the width and intensity of the field gradient pulse, respectively, A is the time between two gradient pulses, and Dnmb the measured self-diffusion value. Equation 1 is a simplified version obtained by the equation
Ag
=
exp(-(2t/T2i1)
-
(t6G)2(A
-
5/3)D,|
(2)
which relates the overall signal amplitude to the amount of nuclei in the ith site, P„ to their transverse relaxation time, Ty, and diffusivity, Z)„ respectively. Other symbols are as
above.
In wide-band conditions the major component diffusivity can be obtained with good accuracy if the minor component(s) contribution is close to the experimental accuracy. The above condition is fulfilled for water selfdiffusion in micellar and in lamellar phases. Conversely, the self-diffusion in reverse solution phase was inferred by comparison between two isomolar mixtures, in which a component had been replaced by its perdeuterated ho-
Figure 6. Sodium (O) and deuterium ( ) quadrupole splittings,
in kHz, as a function of weight percent of added hexane. The mother solution contains 42.4 AOT wt %.
component according to16
=
££>lSil
(3)
where (A) is the measured quantity, in kHz, £ the quadrupole coupling constant, Pi is the amount of nuclei in site i, and |S,j is the related bond order parameter. Experiments give |S,j values of D2O close to 0.01, irrespective of composition. Neither sodium and deuterium quadrupole splittings are sensitive to the amount of dissolved hexane, Figure 6, as if the interface region were completely unsensitive to the packing of hydrocarbon within the bilayer. A plot of 2H splittings versus surfactant mole fraction is linear from about 30 to 75 AOT wt %, Figure 7, and the number of bound water molecules can be evaluated by a two-site
approximation of eq
3 as
(D) Quadrupole Splittings. They were measured to define in detail the phase diagram and to determine the interaction modes of AOT with water, or counterion. The splitting amplitude is related to the amount of bound
(4) A)/|0.01£| = [X(AOT)/(l X(AOT))] where X(AOT) is the surfactant mole fraction and (n> the number of bound water molecules; these lie in the range 10-12. Use of eq 4 is possible in the case of hexane, when no modification of the interface occurs. Conversely, replacement of hexane with the homologous alkanol, or alkanoic acid, gives rise to significant modifications in < A) values, as indicated in the same figure. As to sodium quadrupole splittings, use of eq 3 gives binding constants in the range 0.8 ± 0.1, provided the
(14) Mukerjee, P.; Korematsu, K.; Okawauki, M.; Sugihara, G. J. Phys. Chem. 1985, 89, 5308. (15) Stejskal, G. O.; Tanner, A. J. Chem. Phys. 1965, 42, 288.
(16) (a) Wennerstrom, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974, 6,97. (b) Wennerstrom, H.; Persson, N. O.; Lindman, B. ACS Symp. Ser. 1976, No. 9, 253.
mologs. Proper data combination allows us to get water, hexane, and AOT self-diffusion values, respectively, as
indicated in Figure 5.
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