Effects of Salts on the Micellization of a Short-Tailed Nonionic Ethoxylated Surfactant: An Intradiffusion Study |
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Authors: | Riccardo Imperatore Giuseppe Vitiello Donato Ciccarelli Gerardino D’Errico |
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Institution: | 1. Department of Chemical Sciences, University of Naples “Federico II”, Complesso di Monte S. Angelo, Via Cinthia, 80126, Naples, Italy 2. CSGI - Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase, University of Florence, Via della Lastruccia 3, 50019, Florence, Italy
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Abstract: | In commercial formulations, surfactants are often co-formulated with inorganic electrolytes, which are included as inexpensive thickeners. Salts affect the surfactant’s aggregative and functional behavior. However, while the electrolyte effect on the self-aggregation of ionic surfactants can be rationalized in terms of electrostatic interactions, in the case of nonionic surfactants the molecular determinants are still unclear. In this work, we investigate the effects of alkali and alkaline–earth metal chlorides on the micellization of the nonionic surfactant hexyl penta(oxyethylene) ether, C6E5, in aqueous solution. To this aim, the C6E5 intradiffusion (also named self-diffusion) coefficient in aqueous mixtures of various alkali and alkaline–earth metal chlorides was measured by pulsed gradient spin-echo NMR. The results show that all the considered electrolytes cause a decrease of the surfactant critical micellar concentration, cmc, while the micellar size is almost unaffected. The experimental evidence can be interpreted in terms of de-hydration of the apolar alkyl tails with a minor contribution arising from the dehydration of the poly(ethylene oxide) headgroups. The order of effectiveness of the different cations follows the Hofmeister series, some aspects of which are briefly discussed. |
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