Binary mixture (1:1) of Triton X100 and Propranolol hydrochloride in an aqueous solution of NaCl: Whether mixed micelles are formed, possible clarification in 1H DOSY NMR experiment

Abstract

Micellisation of the binary mixture (1:1) of non-ionic surfactant Triton X100 and surface-active drug Propranolol hydrochloride is examined in an aqueous solution of NaCl (0.3 molkg−1) in the temperature interval T = (273.15–313.15) K with steps of 5 K. Propranolol hydrochloride has two critical micellar concentrations (cmc) if the solution contains NaCl (0.3 molkg−1), while its binary mixture with Triton X100 (1:1) has one critical micellar concentration. The interaction coefficient (β) between structurally different micellar building units is β > 0, which means that there are antagonistic interactions in the binary mixed micelle, i.e., the molar excess Gibbs free energy is higher than zero, which probably originates from the increase in the degree of hydrophobic hydration when incorporating the naphthalene ring (part of Propranolol hydrochloride) into the Triton X100 micelle. The change of the molar Gibbs free energy of mixing is lower than zero. Thus the driving force for the formation of the binary mixed micelle originates from the entropy of mixing structurally different particles. The Diffusion-Ordered SpectroscopY (DOSY) experiment makes it possible to determine, based on the change (reduction) of the self-diffusion coefficient (D) of a surfactant concerning the D value in the monomeric state, whether the observed surfactant (whose concentration in the solution must be lower than its monocomponent cmc values) with another surfactant particle (from a binary mixture of surfactants) forms a binary mixed micelle. DOSY experiment was performed in an aqueous solution without NaCl. If the total concentration of the examined binary mixture is higher than its critical micellar concentration, it decreases the self-diffusion coefficient of both components, suggesting the formation of the binary mixed micelle. The self-diffusion coefficient of Triton X100 is significantly lower than that of propranolol hydrochloride, indicating that non-ionic surfactant dominates in the binary mixed micelle.