Thermodynamic Study on Micelle formation of N-Nonyl-β-D-Thiomaltoside in Water and Water/alcohol Mixtures and on Adsorption at Air/water Interface : a Study Performed by Comparing with MEGA-10 and Alkyl Glucosides

By means of surface tension measurements (drop volume method), micelle formation and adsorption at air/water interface were investigated as a function of temperature of aqueous solution of the nonionic surfactant, n-nonyl-β-D-thiomaltoside (NTM), used as a membrane-protein solubilizer. For comparison with NTM, such surface tension measurements were performed even for aqueous solutions of decanoyl-N-methylglucamide (MEGA-10). Examination was also made of the effect of adding alcohols (methanol, ethanol and 1-butanol) on critical micellization concentration (CMC) and on surface activity. As for the behaviour of surface activity as well as CMC, NTM was found more similar to ionic surfactants than typical nonionic surfactants such as polyoxyethylene alkyl ethers (POE's); NTM exhibited no cloud point at least up to 100°C, but showed a minimum CMC at 28.3°C in the curve of temperature vs CMC. Thermodynamic pararneters upon micellization such as the standard Gibbs energy, enthalpy and entropy changes (ΔG⁰_m, ΔH⁰_m, and ΔS⁰_m, respectively) were estimated, and a strictly linear compensation relation between ΔH⁰_m, and ΔS⁰_m, was found to hold for both the surfactants as ΔS⁰_m = (1/300)ΔH⁰_m + (entropy change at the temperature giving ΔH⁰_m = 0). In the light scattering measurement, the Debye plot resulted in a comparatively small aggregation number of 16.4 at 25° C, indicating that the present thermodynamic analysis based on the phase separation model (PSM) may involve a rough approximation. However, a thermodynamical feature of NTM may be understood from PSM and facilitates comparison with MEGA-10 and alkyl glucosides. In addition, the hydrophilic-lipophilic balance (HLB) value which had not been indicated by Davies was determined for sulfur in amphiphilic organic compounds (-S-) as 1.5.