A quantification of immersion of the adsorbed ionic surfactants at liquid|fluid interfaces

The electrical-double-layer (EDL)-based adsorption models for ionic surfactants propose all surfactant heads ideally aligned in the Stern layer, creating a single surface potential which controls the distribution of counterions within the diffuse layer. Despite their successful application to many surfactant systems, these models exhibit a noticeable shortcoming when it comes to explaining the larger surface excess of ionic surfactants at air vertical bar water interface than oil vertical bar water interface. Experiments and computation simulations have already shown that some surfactant head groups tend to immerse into the deeper layers of interfacial water at high surface coverage. Such an immersion alters the surface potential distribution and characteristics of EDL. However, a theoretical study of this phenomenon is not available yet. This paper presents a useful modeling approach to quantification of the surfactant immersion. We combined the ionic surfactant adsorption models with the theory of equilibrium adsorption constant. We modified several non-ionic surfactant adsorption models for describing the ionic surfactant adsorption by accounting for their immersion. The adsorption parameters were determined by fitting to experimental adsorption data and were then used along with the theory of equilibrium adsorption constants to successfully explain the reported difference in the surface excess of sodium dodecyl sulphate (SDS) at air vertical bar water and oil vertical bar water interfaces. The surfactant immersion directly affected the intermolecular interaction parameter which was closely related with the adsorption energy of surfactants at the interfaces. The collaborative effect of these two parameters was also found to be responsible for the immersion of surfactants and shaping their different concentration profiles across various liquid vertical bar fluid interfaces. The immersion of surfactants was found to enhance their surface adsorption effectively. Also, our model successfully elucidated why SDS adsorption at the oil vertical bar water interface unexpectedly decreases as the hydrophobicity of the oil phase increases. (C) 2016 Elsevier B.V. All rights reserved.