A Molecular Parameter to Scale the Gibbs Free Energies of Adsorption and Micellization for Nonionic Surfactants

A long standing goal in the interfacial thermodynamics community is free energies of adsorption and micellization in terms of molecular parameters. Historically, correlations for CiE j ‘s, a common class of nonionic surfactants, have been proposed between the number of ethylene glycol groups and/or the number of carbons and thermodynamic parameters, such as the critical micelle concentration (CMC), the equilibrium constant (K), and the maximum surface concentration ( ). However, no such correlations to date are capable of capturing a large dataset of CiE j thermodynamic parameters. In this paper we propose the functional form of the critical micelle concentration and adsorption isotherm parameters in terms of molecular parameters for nonionic, CiE j surfactant chemistry. An extensive review of the literature is performed to generate interfacial thermodynamic parameters for a wide-range of CiE j chemistries. The data are used to parameterize and validate the molecular models. More specifically, we combine the well known linear dependence of CMC and K on the number of carbons, NC, with the theorized dependence on hydrophilic-lipophilic balance (HLB), represented as yphob, which is capable of collapsing all experimental data onto mastercurves. The resulting free energy models represent the first successful phenomenological theory which is able to collapse a broad range of experimental data for CiE j surfactants. Furthermore, the proposed models for the free energies of micellization and adsorption are capable of predicting the maximum surface concentration of CiE j surfactants at the air–water interface, thus providing a link between the physics of adsorption and self-assembly. This theory is a step towards engineering interfacial thermodynamics by chemical design which enables prediction of interfacial properties of novel surfactants without extrapolation. In addition, this paper summarizes the many years of experimental and theoretical understanding of surfactant structure property relationships for CiE j’s and the resulting theory quantitatively explains many hypotheses about their interfacial thermodynamics.