Solutions of alkali soaps and water in fatty acids VIII. Correlations of X-ray data with other experimental observations

Abstract

In the system sodium octanoate-octanoic acid-water an obvious parallel exists between the low-angle X-ray observations in the isotropic liquidL 2-phase and its composition, properties and structure. This parallel does not exist only with respect to the total content of acid sodium octanoates (with stoichiometric compositions 1NaC8∶2HC8∶x H2O, and 1NaC8∶3 HC8∶x H2O, where NaC8=sodium octanoate and HC8=octanoic acid) but also with respect to their amphiphilic composition and their water content (the value of x). In the region of 55–70% water, where the structure of the phase closely resembles, in many respects, that of the adjacent liquid crystalline lamellar D-phase, the dependence of of the quasi-Bragg values on concentration parallels that of the Bragg value in the mesophase, in spite of the disorder of aggregates in the isotropic phase. Furthermore, the numerical values of the quasi-Bragg spacings at given concentrations of the acid sodium octanoates are almost of the same sizes as those of the real Bragg spacings in the D-phase. In this region, the quasi-Bragg values give information of the same type about structure as do the real Bragg values in the D-phase; that is, information about the interplanar distances of adjacent double layers of acid soap, separated by intermediate layers of water. At low water contents (up to about 40%) where a direct comparison with a liquid crystalline phase is not possible, the quasi-Bragg values obviously increase in parallel with the molecular volume of the hydrated acid sodium actanoates and thus reflect one of the structural dimensions of these molecules. If one also considers those changes inside the molecules that, according to other observations, are caused by the incorporation of water, the mentioned parallel becomes still more obvious and it may be primarily the length of the hydrated acid soap molecules that is reflected in the quasi-Bragg values. At water contents above 40%, where increased amounts of “unbound„, bulk-water begin to appear in the phase, this parallel ceases. Above 55% water, where the phase obtains the character of an aqueous solution of hydrated acid sodium octanoates, the quasi-Bragg values are then shown to reflect the thickness of the aggregates of hydrated acid sodium octanoates (1 NaC8∶2 HC8∶x H2O and 1NaC8∶3 HC8∶x H2O) together with their intercalated layers of bulk-water. Based upon the comparison of X-ray phenomena with previously obtained experimental evidence, it is possible to draw some conclusions about the structure of theL 2-phase. The most important is that the elongation of hydrated acid sodium actanoate molecules, caused by the incorporation of water up to about 40%, is strongly supported.