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

Abstract

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 of a foreign amphiphilic or lipophilic substance. The solubilization of lipophilic and amphiphilic substances as well as that of water can continue in the mesophases but the capacity of the various mesophase structures for incorporating these substances varies greatly. It is greatest in the lamellar mesophases, especially that with the neat phase structure. It is considerably smaller in the two-dimensional hexagonal mesophases; in the other mesophases with a particle structure the solubilization of organic substances and water or either of these substances is in most cases extremely limited—that is to say, the existence of the phases would seem to be associated with fairly definite molar or volume proportions between the water and amphiphile or between the organic components themselves. The effect of the solubilization of a foreign organic substance on a binary system of amphiphile and water is in some system particularly great. The regions of existence of the individual phases increase, their thermal stability changes, many of the properties of a phase may alter, and entirely new phases may form. Significant factors in this connection are whether or not the molecules of the solubilized compound contain a hydrophilic group, the nature of the polar groups of this component and the association colloid, and the kind of interaction between these groups. Particularly far-reaching is the effect exerted by the solubilization of liquid monohydric alcohols and fatty acids, two types of amphiphiles that, even in dilute aqueous solutions below the c.m.c., lead to the formation of soluble aggregates and mesophases. A critical examination of the experimental data on the phase equilibria and phase structures in binary and ternary systems of amphiphiles containing water shows that in binary systems of association colloids and water the presence of mesomorphous phases with a reversed particle structure has not been established with certainty, and that in binary systems of swelling amphiphiles and water, mesophases with normal particle structures appear not to have been found. On the other hand, there are both normal and reversed micellar and mesomorphous structures in a large number of ternary systems. It is primarily the above two types of amphiphile, namely liquid monohydric alcohols and fatty acids, whose solubilization in association colloid systems has proved to promote change from normal to reversed structures and vice versa. The lamellar neat-phase structure constitutes the natural connection between the normal and reversed mesomorphous and micellar structures. Moreover, in a number of systems a direct transformation between normal and reversed micellar structures occurs in the same homogeneous solution phase. The various stages of micellar association and mesomorphous aggregation succeed each other in a definite order, though some stages are often absent. In the binary systems of association colloid and water the reduction in the water content results in a rise in the association and aggregation. In the ternary systems the addition of the foreign amphiphile or lipophile substance increases the diversity of this series of aggregation stages, and in some systems leads to the transformation from the normal to the reversed structure. The stepwise change in the state of association and aggregation with concentration is one of the most characteristic features of the aqueous systems of amphiphilic substances. Among the factors that are responsible for the stepwise changes and that regulate the stability of the various stages, the part played by water, the importance of the molecular structure of the amphiphilic components, and the conditions in the layers of hydrophilic groups of the aggregations of the amphiphile molecules are discussed, together with the conclusions that may be drawn from the position and the extent of the regions of existence of the homogeneous phases, and from the composition of the latter.