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Abstract
The critical micelle concentration of sodium oleate-hexadecyltrimethylammonium bromide mixtures was determined as a function of the mixture composition and analyzed with Rubingh's theory, showing a strong nonideality. The hydrophile-lipophile balance (HLB) of the mixtures was also measured and the validity of the additivity rule for the HLB of surfactants mixtures was tested. Results were compared with the aggregation behavior of that mixture. Both the HLB and the critical micelle concentration dependencies on the mixture composition exhibit a negative deviation when compared with ideal behavior, which is explained on the basis of the interactions between components and surfactant partitioning between the bulk and the interface.
Keywords Hydrophile-lipophile balance * Critical micelle concentration * Cationic-anionic mixed surfactants * Sodium oleate * Hexadecyltrimethylammonium bromide
Introduction
In technological applications, surfactants mixtures are commonly used. Thus, a better knowledge of the factors affecting the properties of these mixtures is of paramount importance. In most applications the use of anionic-cationic surfactant mixtures may be of interest, but in many situations the use of this kind of systems is limited by die risk of precipitation or instability at or near the 1:1 proportion [I]. As a consequence, the conditions to obtain an anionic-cationic mixture which does not precipitate in all the composition range may be interesting for the design of systems suitable for many applications, because their combination could exhibit some synergy [I]. One method of obtaining information about the interactions in surfactant mixtures is the study of the dependence of the critical micelle concentration (CMC) on mixture composition. We have selected this method to study the particular system sodium oleate (NaOL)/hexadecyltrimeÜiylammonium bromide (CTAB). Since one of the applications in which surfactant mixture is recommended is emulsification, we also studied the influence of ttie composition on the hydrophilic-lipophilic balance (HLB).
Emulsions formed by two immiscible liquids usually require the presence of an emulsifier, if stability has to be attained. Hundreds of surface-active agents are available and may potentially aid emulsion formation and stability. Although this variety contributes to flexibility, certain principles of selection are needed if the task of choosing an emulsifier is to be rendered manageable.
The selection of the best surfactant for die stabilization of a particular emulsion is largely a trial and error process aided by classificatory...