Micellization parameters of a novel cationic urethane gemini surfactant in the presence of insulin: Conductivity measurements

Gemini surfactants are newly developed amphiphilic molecules that have two hydrophilic head groups and two hydrophobic tails linked by a spacer [1]. Depending on the characteristics of the head moieties, gemini surfactants are classified in four groups: anionic, cationic, non-ionic and zwitterionic. Gemini surfactants feature interesting properties with respect to conventional surfactants, such as lower toxicity, higher degrees of biodegradability, optimal activity at extreme conditions of temperature, pH levels and salinity, etc. Therefore, they have attracted a great attention of the scientific and industrial communities [2].Studies of protein-surfactant interactions are significant in the fields of detergents, cosmetics, biosciences, foods and pharmaceutics [3]. Therefore, the aim of this work is to study the thermodynamic characteristics of cationic urethane gemini surfactant nano-micelles having the spacer length of 6 methylene units, in protein solution at neutral pH. The electrical conductivity method was used to determine the important system parameters, such as critical aggregation concentration (CAC), protein saturation point (PSP), the ionization degree of protein-surfactant complexes (α1) and ionization degree of free micelles (α2). The CAC is lower than the critical micelle concentration (CMC), which is determined in the absence of protein. The α1 is determined by the ratio of the slopes of the linear parts of the graph above and below the CAC, while the ratio of the slopes below the CAC and above the PSP is attributed to α2 [4]. The standard Gibbs energy for the surfactant-protein association process (ΔG0agg) as well as the standard Gibbs energy for the transfer of 1 mole of surfactant molecules from unperturbed micelles to protein-bound micelles (ΔG0ps) was also calculated.