THERMODYNAMICS AND PHASE BEHAVIOR OF SINGLE AND BINARY SURFACTANT SYSTEMS (MICELLE, LIQUID CRYSTAL)
Abstract
The solution and phase behavior single and binary surfactants with water is investigated theoretically and experimentally. A hierarchy of thermodynamic models of micellization of binary surfactants is developed. The mass-action model with one finite micelle size and the pseudo-phase separation model are analyzed. General conditions for micelle demixing, demicellization, and azeotrope micellization are developed. The mixed critical micellization concentration(s), the monomer, micelle, and counterion inventories, and the micelle compositions are calculated as a function of total surfactant concentration and total composition. Sample calculations are presented for examining the effects of micelle size and variable degree of counterion binding. The mass-action model should be used for surfactant systems with micelle aggregation numbers of 10 or less and possibly up to 50. A variable degree of counterion binding must be used for mixed nonionic/ionic surfactant systems. The models are used to determine surfactant mixing parameters for experimental data and then mixed micelle and mixed liquid crystal compositions and micellar pseudo-phase diagrams for aqueous binary surfactant systems. The solution and phase behavior of the single surfactants sodium di-2-ethylhexylsulfosuccinate (AOT), sodium dodecylsulfate (SDS), and sodium 4-(1'-heptylnonyl)benzenesulfonate (SHBS) with water and the binary surfactant systems AOT/SDS and SHBS/SDS with water are investigated at 25(DEGREES)C. Conductimetry, surface tensiometry, sodium-activity measurements, isopiestic vapor pressure measurements, polarizing microscopy, and X-ray diffraction are mainly used. Dried SDS can have at least three different polymorphic structures which depend on the solvent and thermal history. Hydrated SDS remains crystalline and absorbs at most 6 wt% water. The SDS micelles can solubilize AOT in mixed micelles in weight ratios up to 1:1. AOT lamellar liquid crystals can solubilize SDS in mixed liquid crystals in ratios up to 1:2. The ternary AOT/SDS/water phase diagram originally delineated by Hart (1982) is examined further, with the three-phase region more firmly established. Demicellization is observed experimentally for the SHBS/SDS/water system. Experimental and calculated pseudo-phase diagrams are compared.
Degree
Ph.D.
Subject Area
Chemical engineering
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