Quantifying the Critical Micelle Concentration of Nonionic and Ionic Surfactants by Self-Consistent Field Theory

Abstract

Quantifying the critical micelle concentration (CMC) and understanding its relationship with both the intrinsic molecular structures and environmental conditions are crucial for the rational design of surfactants. Here, we develop a self-consistent field theory which unifies the study of CMC, micellar structure and kinetic pathway of micellization in one framework. The long-range electrostatic interactions are accurately treated, which not only makes the theory applicable to both nonionic and ionic surfactants but also enables us to capture a variety of salt effects. The effectiveness and versatility of the theory is verified by applying it to three types of commonly used surfactants. For polyoxyethylene alkyl ethers (C$_m$E$_n$) surfactants, we predict a wide span of CMC from $10^{-6}$ to $10^{-2}$M as the composition parameters $m$ and $n$ are adjusted. For the ionic sodium dodecyl sulfate (SDS) surfactant, we show the decrease of CMC as salt concentration increases, and capture both the specific cation effect and the specific anion effect. Furthermore, for sodium lauryl ether sulfate (SLES) surfactants, we find a non-monotonic dependence of both the CMC and micelle size on the number of oxyethylene groups. Our theoretical predictions of CMC are in quantitative agreement with experimental data reported in literature for all the three types of surfactants.