Thermodynamics of equilibrium adsorption and surface tension of single and binary ionic surfactant systems
Abstract
Understanding of the equilibrium adsorption and tension behavior of single ionic surfactants and of binary ionic surfactant mixtures is important for many applications in foaming, detergency, coating flows, and lung surfactant formulations. Certain lung surfactant components, such as DPPG and sodium palmitate, are ionic, as are certain potential replacement lung surfactants, such as sodium myristate. The equilibrium adsorption and tension behavior of two typical ionic surfactants, sodium dodecylsulfate and sodium dodecylsulfonate, has been investigated both theoretically and experimentally. The single component models are fitted to surface tension data and predict adsorbed surface densities and surface potentials. These predictions compare favorably to experimentally determined surface densities obtained from ellipsometry and infrared reflection absorption spectroscopy, as well as to the experimentally determined voltage drop across the air/solution interface. The extension of the single component equilibrium tension models to binary ionic surfactant mixtures was performed via the general, thermodynamically consistent framework of the ideal and nonideal adsorbed solution theories. The Nonideal Adsorbed Solution (NAS) model of Siddiqui and Franses [1996], valid for binary mixtures of nonionic surfactants at the air/water interface, was extended to include binary mixtures of ionic surfactants (iNAS). For simplicity, only symmetric ionic surfactants of the same ionic character (charge) were formally considered, although the model can be further extended to any valence type. This new ionic adsorbed solution model is valid below the mixed cmc and also above the mixed cmc when combined to a micellization model. The surface tensions, surface coverages, mixed monolayer compositions, and surface potentials are calculated with the iNAS model, using only parameters for the pure components as model inputs. Counterion binding to monolayers or micelles is an important component of the model. The iNAS model is tested with tension data for an equimolar mixture of sodium dodecylsulfate and sodium dodecylsulfonate. The predictions of the model are further evaluated with independent data on the densities and compositions of the adsorbed monolayers for equimolar mixtures.
Degree
Ph.D.
Advisors
Franses, Purdue University.
Subject Area
Chemical engineering
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