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A molecular parameter to scale the Gibbs free energies of adsorption and micellization for nonionic surfactants
Journal article   Open access   Peer reviewed

A molecular parameter to scale the Gibbs free energies of adsorption and micellization for nonionic surfactants

Zachary R. Hinton and Nicolas J. Alvarez
Colloids and surfaces. A, Physicochemical and engineering aspects, v 609, p125622
20 Jan 2021
DOI: https://doi.org/10.1016/j.colsurfa.2020.125622
url
https://doi.org/10.1016/j.msea.2015.11.070View
Accepted (AM)Open Access (Publisher-Specific),  Open

Abstract

Chemistry Chemistry, Physical Physical Sciences Science & Technology
A long standing goal in the interfacial thermodynamics community is free energies of adsorption and micellization in terms of molecular parameters. Historically, correlations for CiEj'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 (Gamma(infinity)). However, no such correlations to date are capable of capturing a large dataset of CiEj 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, CiEj surfactant chemistry. An extensive review of the literature is performed to generate interfacial thermodynamic parameters for a wide-range of CiEj 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, N-C, with the theorized dependence on hydrophilic-lipophilic balance (HLB), represented as y(phob), 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 CiEj surfactants. Furthermore, the proposed models for the free energies of micellization and adsorption are capable of predicting the maximum surface concentration of CiEj 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 CiEj's and the resulting theory quantitatively explains many hypotheses about their interfacial thermodynamics.

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