Understanding the influence of electrostatic interactions on observed pKa shifts in surfactant aggregates using classical simulations

Abstract

Surfactants exhibit complex phase behaviour, forming aggregates when in solution of varying sizes and shapes. This can lead to a dramatic increase in the $p{K}_a$ of those surfactants with weak acid head groups, a straightforward example of which is fatty acids. It has long been known that fatty acids have $p{K}_a$s far above that of a typical carboxylic acid, despite having very similar chemical structures. Here, we study the effect of aggregation on the $p{K}_a$ of oleic acid, a C-18 fatty acid using constant pH molecular dynamics simulations. Using a single, simple reference state, we are able to accurately predict the increase in $p{K}_a$ from a single surfactant to an aggregate, agreeing with previous work, and thereby showing the transferability of such a method. We then study the effect of adding salt (KCl) on the solution and find that this leads to a reduction in this $p{K}_a$ upshift, in agreement qualitatively, though not quantitatively, with Debye-Hückel theory. We attribute the former to a rise in electrostatic interactions between the head groups of the oleic acid in larger aggregates, which are screened when KCl is added to the solutions due to clustering of K⁺ ions at the interface, which leads to a reduction in the overall electrostatic potential. Our results highlight the non-ideality of the surfactant solutions and will help in predicting the impact that the solution's ionic strength has on the phase diagram of ionic surfactants.