Effects of Salts on the Micellization of a Short-Tailed Nonionic Ethoxylated Surfactant: An Intradiffusion Study Riccardo Imperatore • Giuseppe Vitiello • Donato Ciccarelli • Gerardino D’Errico Received: 19 November 2013 / Accepted: 10 January 2014 / Published online: 25 January 2014 Ó Springer Science+Business Media New York 2014 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, C 6 E 5 , in aqueous solution. To this aim, the C 6 E 5 intradiffusion (also named self-diffusion) coefficient in aqueous mix- tures 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(eth- ylene oxide) headgroups. The order of effectiveness of the different cations follows the Hofmeister series, some aspects of which are briefly discussed. Keywords Nonionic surfactants  Micelles  Electrolytes  Hofmeister series  Intradiffusion  Self-diffusion 1 Introduction In the last decades, the application of nonionic surfactants has steadily increased in many fields, including cosmetics, pharmaceuticals and food preparation. Specifically, R. Imperatore  G. Vitiello  D. Ciccarelli  G. D’Errico (&) Department of Chemical Sciences, University of Naples ‘‘Federico II’’, Complesso di Monte S. Angelo, Via Cinthia, 80126 Naples, Italy e-mail: gerardino.derrico@unina.it R. Imperatore  G. Vitiello  D. Ciccarelli  G. D’Errico CSGI - Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase, University of Florence, Via della Lastruccia 3, 50019 Florence, Italy 123 J Solution Chem (2014) 43:227–239 DOI 10.1007/s10953-014-0133-z