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Model-Free Analysis of Critical Micellar Concentrations for Detecting Demixing in Surfactant Mixtures
journal contribution
posted on 2017-03-07, 00:00 authored by Erik Frotscher, Jonas Höring, Grégory Durand, Carolyn Vargas, Sandro KellerAqueous 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 combined 1H- and 19F-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.