Cationic Micelles Modulated in the Presence of α,ω‐Alkanediols: A SANS, NMR and Conductometric Study

Abstract

AbstractThe solution behavior of a typical cationic surfactant, tetradecyltrimethylammonium bromide, in mixed solvent systems composed of water and varying concentrations of α,ω‐alkanediols; 1,2‐ethanediol (ED), 1,4‐butanediol (BD), 1,6‐hexanediol (HD) and 1,8‐octanediol (OD) was examined via electrical conductance measurements, 13C‐NMR spectroscopy and small angle neutron scattering (SANS) measurements. The critical micelle concentration (CMC) values and degree of counterion dissociation (α) indicate that both ED and BD oppose micellization, whereas HD and OD enhance micelle formation. Changes in the 13C‐NMR chemical shifts (∆δ values) reveal that the short chain diols reside almost exclusively in the bulk phase and hence, affect the formation of micelles by altering the solvent properties in the bulk of the solution, whereas HD and OD partition between the pseudomicellar phase and the bulk phase. SANS studies indicated that both the micellar size and aggregation number (Nagg) decrease in the presence of all diols. ED and BD behave like cosolvents and increase the α and CMC values and decrease Nagg. We note that the effect of HD and OD on the properties of the micelles is concentration dependent; at low concentrations, these diols interact with the micelles and behave as cosurfactants (as evidenced by the trends in the micellar properties), while at higher concentrations, they enhance the surfactant solubility and behave as a cosolvent.