title: Model-Free Analysis of Critical Micellar Concentrations for Detecting Demixing in Surfactant Mixtures.
abstract: Aqueous mixtures of two or more surfactants are often employed for research or industrial purposes because such mixtures offer advantages over single-surfactant systems. This is particularly true for mixtures of fluorocarbon (FC) and hydrocarbon (HC) surfactants, which display a broad range of mutual miscibilities in mixed micelles. Unfortunately, the prediction and even the experimental elucidation of the micellar mixing behavior of surfactant mixtures remain challenging, as evidenced by conflicting results and conclusions derived from diverse, and often complex, mixing models. One of the most intriguing questions is whether certain combinations of FC and HC surfactants form only one type of mixed micelle or rather demix into two micelle populations, namely, FC-rich and HC-rich ones. Here, we demonstrate a novel approach to the model-free analysis of critical micellar concentrations (CMCs) of surfactant mixtures that is based on a fit of the experimental data with cubic splines using a stringent thermodynamic criterion for mixing. As a proof of principle, we analyze CMC values determined by isothermal titration calorimetry and confirm the conclusions with the aid of combined1H- and19F-NMR spectroscopy. Specifically, we show that aqueous mixtures of an FC maltoside and an HC maltoside conform with the assumption of only one type of micelle regardless of the mixing ratio, whereas combining the same FC surfactant with an HC surfactant carrying a zwitterionic phosphocholine headgroup gives rise to two coexisting micelle populations at high mole fractions of the FC maltoside.
authors.affiliation: Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany.
authors.forename: Erik
authors.initials: E
authors.lastname: Frotscher
authors.affiliation: Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany.
authors.forename: Jonas
authors.initials: J
authors.lastname: Höring
authors.affiliation: Equipe Chimie Bioorganique et Systèmes Amphiphiles, Université d'Avignon et des Pays de Vaucluse , 33 Rue Louis Pasteur, 84000 Avignon, France.|Institut des Biomolécules Max Mousseron , UMR 5247 CNRS-UM-ENSCM, 15 Avenue Charles Flahault, 34093 Montpellier Cedex 05, France.
authors.forename: Grégory
authors.identifier: 0000-0001-6680-2821
authors.initials: G
authors.lastname: Durand
authors.affiliation: Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany.
authors.forename: Carolyn
authors.initials: C
authors.lastname: Vargas
authors.affiliation: Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany.
authors.forename: Sandro
authors.identifier: 0000-0001-5469-8772
authors.initials: S
authors.lastname: Keller