Self-assembly, surface activity and structure of n-octyl-β-D-thioglucopyranoside in ethylene glycol-water mixtures

Abstract

The effect of the addition of ethylene glycol (EG) on the interfacial adsorption and micellar properties of the alkylglucoside surfactant n-octyl-β-D-thioglucopyranoside (OTG) has been investigated. Critical micelle concentrations (cmc) upon EG addition were obtained by both surface tension measurements and the pyrene 1:3 ratio method. A systematic increase in the cmc induced by the presence of the co-solvent was observed. This behavior was attributed to a reduction in the cohesive energy of the mixed solvent with respect to pure water, which favors an increase in the solubility of the surfactant with EG content. Static light scattering measurements revealed a decrease in the mean aggregation number of the OTG micelles with EG addition. Moreover, dynamic light scattering data showed that the effect of the surfactant concentration on micellar size is also controlled by the content of the co-solvent in the system. Finally, the effect of EG addition on the microstructure of OTG micelles was investigated using the hydrophobic probe Coumarin 153 (C153). Time-resolved fluorescence anisotropy decay curves of the probe solubilized in micelles were analyzed using the two-step model. The results indicate a slight reduction of the average reorientation time of the probe molecule with increasing EG in the mixed solvent system, thereby suggesting a lesser compactness induced by the presence of the co-solvent.

Figure 1

Molecular structure of n-octyl-β-d-thioglucopyranoside (OTG).

Figure 2

(a) Surface tension isotherms of OTG and (b) plots of the pyrene 1:3 ratio index as a function of surfactant concentration in several solvent systems whit different EG contents at 25 °C. The adsorption isotherm in 40% EG and the plot of the pyrene 1:3 ratio index in pure water were omitted for the sake of clarity.

Figure 3

Effect of EG addition on the relative cmc values of OTG and TX-100. (cmc)₀ is the cmc in pure water. Data plotted are the cmc values obtained by the pyrene 1:3 ratio method.

Figure 4

Plots of the Gibbs energy of micellization ΔG_mic⁰, as a function of the solvophobic parameter, S_p, and of the Gordon parameter, G, in various EG-water solvent mixtures.

Figure 5

Apparent hydrodynamic radius of micelles, R_H, as a function of the micellar concentration in different water-EG solvent mixtures. The solid lines are the best linear fit to the experimental data.

Figure 6

Debye plots for OTG micellar solutions in different water-EG solvent mixtures. The solid lines are the best linear fit to the experimental data according to Equation (14) (see Experimental section).

Figure 7

Steady-state emission spectra of C153 in OTG micellar solutions (30 mM) at different water-EG solvent mixtures (λ_exc = 405 nm) and 25 °C. The spectrum in water is plotted but matched by that in 20% EG. The spectra in 10% and 30% EG are omitted for the sake of clarity.

Figure 8

Fluorescence decay of C153 in OTG micellar solutions (30 mM) in pure water and in a solvent system with 40% EG at 25 °C. The solid lines through the data points are the best fit to a single-exponential function. The corresponding weighted residuals are also shown. IRF is the instrumental response function.

Figure 9

Fluorescence anisotropy decays of C153 in OTG micellar solutions (30 mM) in pure water and in a solvent system with 30% EG at 25 °C. The solid lines through the data points are the best fit to a biexponential function. The corresponding weighted residuals are also shown.