H20

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Flow Birefringence in the L2Phase of the Aerosol-OT/Isooctane/H20 System A. C. Hall, E. Tekle, and Z. A. Schelly* Center for Colloidal and Interfacial Dynamics, Department of Chemistry, Uniuersity of Texas a t Arlington, Arlington, Texas 76019-0065 Receiued January 13,1989 Using apparatus well suited to rapid examination of samples, we have investigated flow birefringence in the I.1 phase of some systems of Aerosol-OT, isooctane or pxylene, and water. When the phese boundary is approached from the side, the presence of flow birefringence appears to correlate with the development of liquid-crystalline phases (LC) beyond the phase boundary and its absence with the formation of two isotropic fluid phases (L,, I.1). The intensity of flow birefringence in the I.1 phase increases with proximity to the phase boundary.

Introduction Aerosol-OT (AOT, sodium bis(Eethylhexy1) sulfosuccinate) is a well-balanced surfactant whose phase behavior in ternary systems with water and various organic solvents has been elucidated in more or less detail by a number of authors.14 Although at room temperature AOT has only low solubility in water, solutions of AOT in nonpolar solvents are capable of solubilizing large amounts of water by incorporating it within inverse micelles. Eventually, however, with further addition of water to solutions containing fixed amounts of AOT, a limiting concentration is reached, beyond which no more water can be accommodated by micelles. In this case, a second phase, either homogeneous liquid, L,, or liquid crystal, LC (generally lamellar), appears, in equilibrium with L2. The location of the corresponding boundary in the ternary phase diagram depends on the relative concentrations of water and nonpolar solvent and on temperatwe. Although energetic and structural factors governing surfactant aggregation have been analyzed: the stability, structure, and mechanisms of formation of various aggregate species have not been fully established.

Experimental Section Materialn and Apparatus. The AOT used in cur expvimenta wan obtained from Fluka (purum, >98%). It WBS further purified according to methods previously described! The hydrocarbon solvents were of Fluka HPLC grade. Water was double deionized were a 250-W and distilled from quartz apparatus. Light soinmdeseent lamp or a l&mW cw HeNe laser from Melles Grint Both Polaroid and Glan polarizers were used. Photographs were macrolens. taken with a Nikon FM m e n equipped with a 55" using Kodak T-Max P3200 high-speed film. Preliminary kinetic observations were made by using a Dumnut KM2433 photo. multiplier whose signal was recorded on a Nicolet 204 digital oscillwope. Temperature was controlled to fO.l "C by a Haake FK2 thermostat bath. Procedure. Solutions to be examined for flow birefringence were made by adding water to stock solutions of AOT in iaooctane. The solutions were placed in 1-cm square W spectrophotometer (1) E e W , P.; Mandsl. L.; FonteU. K.J. Colloid Interface Sei. 1970, 33.115. (2) Kunidn. H.; Shin&. K.J. Colloid Interface si. 1979, 70,577. (3) Tamamvnhi,9.;WataMbe. N. Colloid F'olym. Sci. 198O,WS, 174. (4) "ux. D.;@ U q , M: Ph~sicsof Amphiphiks: Mkellea. Veaielea and Microemulstom; DiGiorgto. V.. Cortl. M., Eds.; North-Holland: Amsterdam. 1985, p 842. (5) Israelachvili, 3. N.; Mitchell. D.3.; Ninham. B. W. J . Chem. Soe.. Faroday Tram. 1976. 72, 1525. (6) Ueda. M.;Schally. 2. A. h n g m i r 1988,4,653.

Figure 1. View of the needle immersed in the sample solution (analyzer removed). cuvettes, which were brought to mnstant temperature in a brass cell holder. Fluid fromthe cuvette was withdrawn vertically into a 5-mL hypodermic syringe (ala0 thermostated). taking care to exclude air by maintaining the liquid level in the cuvette above the level of the needle tip. Subsequently, the fluid was injected back into the cuvette by rapidly depressing the syringe plunger. If fluid leaving the needle in a stream perpendicular to a linearly polarized, collimated light beam is flow birefringent, it becomes visible between crossed polarizen as a luminous jet. With a 20-gauge needle, mean fluid velocities of 5 m s-' are easily attainable, corresponding to radial flow gradients of at least 1 X 104 s-l.

Discussion Figure 1 shows the cuvette and a 20-gauge needle in place. When flow birefringence occurs, the form of the luminous jet depends on the orientation of the plane of

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Letters

1261 Longmuir, Vol. 5, No.5, 1989

Figure 3. oSeiUoseopetrace of the intensity of flow birefringence as a function of time. Increasing light intensity is toward the bottom. The arrow indicates the instant when flow stops. Full scales: ordinate, 800 mV; abscissa, 200 ms. AOT

Figure 4. Partial phaw diagram of the AOT/isooetane/HsO system at 25 OC. We investigated the L,phase below the dashed line mmponding to constant AOT/isooCtan e = 34% w/w. The shaded area indicates where flow birefringence has so far been observed. direction of flow. In Figure 2b, the incident beam is p larized a t 45O to the flow. Both illustrate the case of a homogeneous 0.15 M AOT in isooCtane w/o microemulsion with R = [H,O]/[AOT] = 60,at 12 'C. Thii is 3 0.1 "C below the temperature a t which phase separation occurs. The time dependence of the flow birefringence is illustrated in the oscilloscope trace shown in Figure 3. In the partial phase diagram of Figure 4, the shaded area indicates the domain where flow birefringence was observed a t 25 'C. The method outlined above has the advantage of simplicity and thus can be used to qualitatively examine large numbers of samples in short order. Its main disadvantage is that the complex and ill-defined flow pattern does not lend itself to quantitative analysis of the kind possible with Couette or even tube flow. Moreover, due to the limited path length a c r w the jet, weak birefringence, such as that typical of pure solvents. produces no visible effect. Nevertheless, it has been possible to observe that flow birefringence occurs well within the phase boundary of the regime in AOT/isooctane/H,O systems. As composition within L2 becomes progressively water-rich; Le., as one approaches the phase boundary along a line of constant AOT/isooctane ratio, flow birefringence grows stronger until, at the phase boundary, liquid crystal appears and with it. of course, permanent birefringence. In contrast,

*

Figure 2. Luminous jet observed between crossed polarizers. Plane of polarization of the incident beam: (a) parallel or perpendicular, (b) 45*, to the direction of downward flow. polarization of the incident beam, and with white light there are also chromatic effecta. Both phenomena will be analyzed in a forthcoming communication. Figure 2a shows typical flow birefringence observed with the incident beam polarized either parallel or perpendicular to the

Langmuir 1989, 5 , 1265-1267

in an analogous domain of the phase diagram (AOT 18.5-41.57’0, p-xylene 74.7-37.2%, H 2 0 6.8-21.3% w/w) we have so far been unable to detect flow birefringence in the L2 phase of the AOT/p-xylene/H,O system. For this case, however, addition of water leads not to mesomorphous forms but to two-phase systems of L2 in equilibrium with dilute aqueous L1 solution.’ Application of more sensitive techniques should show whether this contrast in behavior reflects a qualitative difference in the nature of the two systems or merely a quantitative one. As has been observed previously2 in the AOT/isooctane/H20 system, the area in the phase diagram occupied by the LC phase increases with temperature a t the expense of the L2 phase. This is consistent with the increasing solubility of AOT in water with temperature as well as with the tendency of two-tailed surfactants, which have similar affinities for water and hydrocarbon to form lamellar phases. For a given L2 composition, we observe that flow birefringence increases strongly with temperature until, as with the addition of water, permanent birefrin-

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gence supervenes. Clearly, the observed flow birefringence may be due either to the creation of anisotropic structures by the flow field or to hydrodynamic orientation of liquid crystalline precursors already present. We may mention, in this connection, that light-scattering results suggest the presence of anisotropic precursors of liquid crystal in aqueous solutions of nonionic ~ u r f a c t a n t s . ~

Acknowledgment. This material is based in part upon work supported by the Texas Advanced Research Program under Grant No. 1766. Additional support by the National Science Foundation (Grant No. CHE-8706345) and the R. A. Welch Foundation is gratefully acknowledged. We thank Dr. R. E. Crick for taking the photographs and D. Miller for technical assistance. Registry No. AOT, 577-11-7; isooctane, 540-84-1. (7) Richtering, W. H.; Burchard, W.; Jahns,E.; Finbelman, H. J.Phys. Chem. 1988,92,6032.

Historical Notes C t S C Folklore. 2. Einstein’s Last Contribution to Surface Chemistry

friend of Einstein, was a practicing physician and the son of a pharmacist.

Karol J. Mysels

The Article

Chemistry Department, University of California, San Diego, La Jolla, California 92093-031 7 Received April 12, 1989

Einsteins contributions to what we now call colloid chemistry are well-known and appreciated. They deal with Brownian motion, diffusion, sedimentation, and light scattering, Le., bulk properties. Milton Kerker has recently reviewed them.’ Less known are his papers dealing with surface chemistry despite the fact that they include his first publicationS2 This is understandable, as the influence of these papers on todays interests is certainly small. Perhaps least generally known (cited about once per decade recently), but admired in a small circle of membranologists, is his 1923 paper with M i i h ~ a m .‘This ~ paper has many remarkable features, not the least of which is its title “The Experimental Determination of the Pore Size of Filters”, which hardly fits the popular picture of Einstein the theorist. It appeared in the German Medical Weekly, hardly a normal forum for a leading physicist. Furthermore, it is not the product of youthful exuberance but was written when Einstein was in his forties after he had published the general theory of relativity and had returned from his triumphal trip through the United States. The probable explanation of these abnormalities lies in the fact that the coauthor, Dr. Hans Muhsam, a personal (1)Kerker, M. Langmuir 1985,1,531; J. Colloid Interface Sci. 1989, 129, 291.

(2) Einstein, A. Ann. Phys. 1901,4, 513. (3)Einstein, A.;Miihsam, H. Deutsche Medizinische Wochenschrift 1923,49,1012.

The authors point out that the then current determination of pore size of membrane filters by permeability to water and of solid filters by retention of colloids of approximately known size can give only very rough results. A &sureand easy” method would be one based on capillarity. If a filter is first saturated with a liquid, the minimum pressure of air required to press the liquid out is 2u/r-, where r- is the radius of the smallest constriction of a pore, and this is precisely the dimension determining the smallest particle retained by this pore. u is, of course, the surface tension, but, as was often done in their days, the authors call it the capillary constant, which is now generally reserved for ( 2 ~ / p g ) O . ~ . For a filter composed of many pores of different sizes, the first passage of air corresponds to the widest of the smallest constrictions (the filter size). They point out that the significance of this filter size remains unaffected by the fact that the pores form a network. The authors then report that they used an apparatus outlined in Figure 1 and observed the appearance of bubbles of air which crossed the wall of the filter. This happened at a pressure of about 1 atm. This pressure would now be called the “bubble point”. Ether was used as the liquid because of its low surface tension, and separate experiments showed that it wetted completely the filter. Neglecting any deviations from circularity in the cross section, the experiment gave a filter size of 0.7 pm. A footnote points out that if this pore diameter is estimated hydrodynamically, based on Poisseuilles law and the experimentally determined porosity and viscous resistance of the filter, assuming pores of uniform diameter, a roughly 10-fold higher value is obtained. This is not surprising,

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