Micelle structures in aqueous solutions of glucose-based surfactants having an isoprenoid-type hydrophobic chain

Surfactant self-diffusion coefficients have been measured on a binary system of 1-O-β-3,7-dimethyloctyl-d-maltopyranoside (β-Mal2(Ger))/water and a mixed surfactant system of β-Mal2(Ger)/1-O-β-3,7-dimethyloctyl-d-glucopyranoside (β-Glc(Ger))/water at 25 °C. For comparison, measurements have also been made on 1-O-β-decyl-d-maltopyranoside (β-Mal2C10)/water and β-Mal2C10/1-O-β-decyl-d-glucopyranoside (β-GlcC10)/water. The hydrodynamic radius of β-Mal2(Ger) micelles obtained from the micellar diffusion coefficient is around 3 nm and nearly equal to that of β-GlcC10 micelles within experimental error. In the mixed surfactant systems, the hydrodynamic radii for both systems increase with increasing XGXG (the mole fraction of β-Glc(Ger) or β-GlcC10 in the total mixed solute) above XG≅0.4XG≅0.4 when the total surfactant concentration is kept constant at 2 wt%. The RHRH of β-Glc(Ger)/Mal2(Ger) micelles increases more rapidly than β-GlcC10/β-Mal2C10 micelles, and then phase separation occurs at XG≅0.65XG≅0.65. On the other hand, the RHRH of β-GlcC10/β-Mal2C10 micelles continues to increase until the phase separation occurs at XG≅0.92XG≅0.92. Measurements have also been performed as a function of total surfactant concentration at constant XGXG (=0.6). The CMC of the β-Glc(Ger)/Mal2(Ger) system is larger than that of the β-GlcC10/β-Mal2C10 system as expected from the results of the pure surfactant systems published previously. The RHRH increases with increasing total surfactant concentration for both systems. At higher concentrations, the RHRH of β-Glc(Ger)/Mal2(Ger) micelles increases more rapidly than β-GlcC10/β-Mal2C10 micelles. These results can be explained by the fact that the geranyl and decyl chains have the same volume but different chain lengths.

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