Dissolution State of Nonionic Surfactants

The most important properties of a surfactant are its solubility and its interfacial activity. When a nonionic surfactant is added to a two-phase liquid-liquid system, it preferentially adsorbs at the interface forming an adsorbed layer. At low surfactant concentrations an equilibrium exists between surfactant monomers dissolved in the oil phase, surfactant monomers dissolved in the water phase and the interfacial surfactant. In the case of a separated system (constant interfacial area), as the concentration of surfactant in the system increases, the amount of surfactant at the interface reaches a maximum possible concentration and, on further increase in surfactant concentration, excess surfactant will now form micelles in either the oil or aqueous phase, or form a surfactant phase depending on its affinity for the oil and water phases. The break point is termed the critical micelle concentration (CMC). 

A surfactant micelle is generally pictured as a sphere of surfactant molecules with a liquid phase core, e.g. an aqueous micellar solution has continuous structure containing micelles with an oil phase core. This phase is thermodynamically stable and the oil within the micelle is termed solubilised. For conditions below the CMC, the effect of the adsorbed interfacial surfactant on the nSOW system s interfacial tension is governed by the Gibbs adsorption equation  

To stabilise an emulsion, the surfactant must be present at a concentration above the CMC hence, we shall be mainly concerned with systems of this sort. The phase in which the surfactant forms micelles is dependent on the surfactant s affinity for oil and water. The surfactant s affinity is controlled by a number of factors. Winsof first addressed the problem of describing surfactant affinity. He introduced the concept of interaction energies between surfactant molecules adsorbed at the interface and the oil and water phases. Salager identified different types of interactions. The ratio of the total interaction energies (per unit area of interface) of the surfactant for the oil and water phases is known as Winsor r (symbols are defined below)  

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K. Shinoda The Correlation between the Dissolution State of Nonionic Surfactant and the Type of Dispersion Stabilized with the Surfactant. J. Colloid Interface Sci. 24, 4 (1967). 

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