Monomer exchange and rotational isomerization of alkyl monoglycosides in water

Ultrasonic attenuation spectra between 100 kHz and 400 MHz are reported for solutions of hexyl-, heptyl, octyl-, and nonyl-beta-d-glucopyranoside in water. Results for a mixture of an octyl glucopyranoside with an octyl galactopyranoside are also presented. The analytical descriptions of the spectra in terms of relaxation spectral functions feature up to four relaxation regions. The high-frequency relaxation processes, with relaxation times at around 2 and 10 ns, respectively, correspond with molecular processes also existing in saccharide solutions. They are assigned to the exocyclic hydroxymethyl group isomerization as well as to the rotation around glycosidic bond angles. The low frequency terms, with relaxation times in the range from 30 ns to 3 mus, are characteristic of micelle solutions. They are discussed in terms of two modes of monomer exchange. One mode reflects the well-known exchange kinetics of systems with Gaussian size distribution of globular micelles. The other mode is considered to be due to the monomer exchange at micelle sites with closer packing of surfactants, as is characteristic of large micelles with nonglobular shape.