Leo Mandell

Some observations on binary and ternary aerosol OT systems

Abstract Phase equilibria diagrams at 20°C are presented for the systems AOT- p -xylene-water, AOT-caprylic acid-water, and AOT- n -decanol-water. In all these systems occur 2 solution phases, L 1 and L 2 , and 3 mesophases, F, I 2 , and D. The mesophase F, which includes pure AOT, has a reversed two-dimensional hexagonal structure with long mutually parallel AOT rods where the sodium sulfonate groups are oriented toward the center. Water is incorporated in the polar cores, whereby the rods swell. Hydrocarbon is located around the rods and between their outer paraffin chains. The isotropic mesophase I 2 has a body-centered cubic structure; also in this phase the AOT molecules are oriented with the polar groups inwards. The mesophase D has a lamellar structure of the neat-soap type. In the AOT- p -xylene-water system the water-free solution in region L 2 is micellar and contains cylindrical, nearly spherical, and largely unsolvatized micelles of the reversed type. The aggregation number is 23 and the diameter about 22–23A. The arrangement of the AOT molecules in them resembles that in the aggregates of the water-free mesophase of region F. When water is solubilized it is taken up in the polar core and the micelles will swell; their nearly spherical shape is retained. Only when the volume fraction of micellar substance exceeds about 0.74 will the micelles deform and grow out to cylindrical aggregates of the same type as in the water-rich region of phase F. In the L 2 solutions of the AOT-caprylic acid-water and AOT-decanol-water systems the interaction between the hydrophilic groups of the added organic substances and the polar groups of the AOT will split up the original AOT aggregates and result in small composite entities. In the presence of water their aggregation will increase. The “microemulsions” of Schulman are in many cases identical in principle with the micellar solutions of reversed type that occur in the L 2 solutions of many ternary amphiphile systems. The stabilizing amphiphilic layer consists in these “microemulsions” of a combination of association colloid and alcohol whereas this layer in the system of AOT-hydrocarbon-water consists only of one component. In the case of thermodynamically stable micellar solutions the term “micro-emulsions” is a misnomer and should not be used in this connection.

The aqueous cetyl trimethylammonium bromide solutions

Abstract The dependence on the concentration of the density, viscosity, refractive index, and light scattering has been studied in aqueous solutions of cetyltrimethylammonium bromide between 0.036% (c.m.c.) and 26%. The results are discussed in relation to results of other studies. Above the c.m.c. there are more or less spherical, strongly hydrated micelles composed of about 95 CTAB molecules. From 9% to 11% upwards the micelles are deformed to rod-shaped micelles owing to the increasing crowding in the solution. At 26% the isotropic micellar solution is transformed via a narrow two-phase region to mesomorphous middle soap with rod-shaped hydrated CTAB aggregates in hexagonal array.

Solubilization in Micelles and Mesophases and the Transition from Normal to Reversed Structures

Abstract The relationship between various kinds of solubilization and the formation of mesophases is elucidated. In aqueous solutions of association colloids there are three processes by which mesomorphous phases form: (i) the separation of the micellar substance in mesomorphous form on reduction of the water content, (ii) the separation of the micellar substance at lower concentrations, from the c.m.c. upwards, as a result of the changes that the micelles undergo on solubilization of foreign amphiphilic and lipophilic substances, and (iii) below the c.m.c. through the formation of aggregates between the solubilizate and the molecules or ions of the association colloids. The last of these processes is typical of the solubilization of fatty acids and liquid monohydric alcohols. In systems of sparingly soluble amphiphilea that do not give micellar aqueous solutions but take up water with swelling, mesophases often form through this “solubilization” of water, a process that is often promoted by the presence ...

Effect of micellar shape and solubilization on counter-ion binding studied by 81Br NMR

Abstract The nuclear quadrupole relaxation of 81Br in aqueous cetyltrimethylammonium bromide solutions has been studied as a function of surfactant concentration and temperature. The experimental data can be correlated with the counter-ion binding, and indirectly with the charge density of the micellar surface, and are used to extract information concerning micellar shape and location of solubilized molecules. The change in shape from small globular to long cylindrical micelles is shown to lead to an enforced counter-ion binding. Variable temperature studies show that the shape transition concentration is increased with increasing temperature. Benzene, N,N-dimethylaniline and hexanol appear to promote this shape transition, cyclohexane does not. The relaxation data suggest that benzene, N,N-dimethylaniline and hexanol are located near the micellar surface; cyclohexane, on the other hand, appears to be incorporated in the interior of the micelles. These conclusions are compared with results from other studies. The motion responsible for nuclear quadrupole relaxation is relatively rapid and measurements with D2O as solvent show that the motion of the bromide ions is correlated with the motion of the water molecules. The energies of activation obtained for the relaxation process show that the relaxation mechanism is even independent of drastic changes in phase structure.

The cetyltrimethylammonium bromide-hexanol-water system

An investigation has been carried out of the phase equilibria in the three-component system cetyltrimethylammonium bromide-hexanol-water at 25°C. Separation of the phases was effected by centrifugation and the composition of them was determined by analysis and their internal structure by X-ray diffraction measurements. Their consistency and gross appearance were examined and their texture was observed under the polarizing microscope. The investigation disclosed besides crystalline cetyltrimethylammonium bromide two homogeneous solution phases, the aqueous solution, L1, and the hexanolic solution, L2, and two homogeneous mesomorphous phases with different internal structure. One of the mesophases displayed a lamellar structure and the other a two-dimensional hexagonal structure. The vertices of the three-phase triangles and the direction of the tie-lines in the two-phase zones between the regions with homogeneous phases have been determined.

Ternary systems of potassium soap, alcohol, and water: I. Phase equilibria and phase structures

Abstract Phase equilibria have been studied at 20°C in the systems potassium caprylate-decanol-water, potassium caprate-octanol-water, and potassium olcate-decanol-water. The boundaries of the regions of the different phases were as a rule determined by centrifuging specimens from the two- and three-phase zones and subsequent analysis of the separated layers. Besides solid crystalline soap, all three systems contain two solution phases and a number of (4–6) mesomorphous phases. X-ray diffraction studies of the structure of the mesophases disclosed two lamellar mesophases and several mesophases with particle structure; of the latter three with Bragg spacing ratios of 1: 1 2 : 1 3 two with a two-dimensional hexagonal structure, and two that are optically isotropic and have a cubic structure. The homogeneous mesophases differ distinctly in appearance under the polarizing microscope.

The solution phase with reversed micelles in the cetyl trimethylammonium bromide-hexanol-water system

Abstract The homogeneous isotropic hexanol-rich solutions of region L2 in the system consisting of cetyl trimethylammonium bromide (CTAB), hexanol, and water have been examined at 25°C by viscosity and density measurements and X-ray diffraction in the low-angle region. The results obtained were compared with those of simultaneous nuclear magnetic resonance measurements in the same solution region. CTAB dissolves to the extent of up to 7% in water-free hexanol with the formation of unhydrated ions firmly bound together. In the presence of 4–5 moles of water per mole of CTAB the solubility is increased about sevenfold and micelles are formed containing hexanol as well as CTAB and water. Most of the hexanol is located outside the micelles as intermicellar liquid. The micelles are of the reversed oleophilic type, with a core of hydrated hydrophilic groups respectively water, and their size increases with the water content. At low water contents the bromide ions in this core are bound to hydrated ion pairs, whereas at high water contents they are located in the water cores but are nonetheless fairly firmly bound.

A study of counter-ion binding to reversed micelles by nuclear magnetic quadrupole relaxation of 81Br.

Abstract Measurements of the nuclear magnetic relaxation (nmr) of 81Br in the isotropic solution phase extending from the hexanol corner in the cetyltrimethylammonium bromide/n-hexanol/water system are presented. The effect on the 81Br nuclear magnetic resonance line width of varying the resonance frequency, the temperature, and the concentrations of the components in the solution has been investigated. It is found that except for high hexanol contents the bromide ions are located in the water cores of reversed micelles. At the highest hexanol concentrations the relaxation rate is very fast. This is found to be due to ion pair formation. At high water concentrations the bromide ion binding is not affected by changing the solution composition. Lowering the water concentration in the part of the solution phase rich in cetyltrimethylammonium bromide leads to a strong cation-anion interaction. A comparison with viscosity and X-ray data demonstrates that the enforced cation-anion coupling is due to a firm binding of the anions to the micellar surface. A simple model in which the bromide ions are rapidly exchanging between two sites, provides a description of the observations. The extensive association in these solutions also is reflected in the observed apparent energies of activation of the relaxation process. These are very high compared to those observed in aqueous solutions of alkali bromides.

Phase equilibria in the ternary system sodium caprylate-1, 8-octane diol-water

Abstract The phase equilibria and phase structures in the system sodium caprylate-1,8-octane diol water at 20°C have been investigated. Besides the solid crystalline sodium caprylate and octane diol phases there occur a solution phase with water as solvent and two mesophases. The mesophases display a two-dimensional hexagonal and a lamellar structure, respectively. The last mentioned seems to be composed of coherent single molecular amphiphile layers alternating with water layers.

Ternary systems of potassium soap, alcohol and water : II. structure and composition of the mesophases

Abstract In all the ternary systems examined, potassium caprylate-decanol-water, potassium caprate-octanol-water, and potassium oleate-decanol-water, lamellar mesophases with the neat-soap structure occur and appear under extremely varying concentration conditions. Their properties, especially the ability to incorporate water, change greatly with the molar ratio of alcohol to soap. Hexagonal mesophases of the midle-soap type in equilibrium with concentrated micellar aqueous solutions also occur in all three systems. The same is the case for the the type B lamellar mesophases, which are in equilibrium with dilute aqueous solutions below the c.m.c. The regions of existence of these two types of mesophase are, however, more restricted. The other mesophases, all of which have particle structures, occur less commonly. They are as a rule restricted to certain ratios between two or three of the components of the systems.