Hydration of sugar based surfactants under osmotic stress: A SAXS study

The effect of adding salts to micelles made of a sugar based surfactant, n-octyl-beta-glucoside, has been investigated using small angle X-ray scattering (SAXS) and dynamic light scattering (DLS) techniques. It was shown that adding salts leads to change the outer shell hydration of the micelles i.e. where the glucose moieties are situated. No ion adsorption at the micelle surface was detected. Three regimes have been determined as a function of the ionic strength (IS): (i) at low IS (<1 M), no significant effect on the hydration of the surfactant sugar moieties was detected while at (ii) intermediate IS (1–1.8 M) a salting-in effect was observed, i.e. a strengthening of the hydration of the glucose polar heads at the micelle surface, and at (ii) higher IS a strong salting-out effect is detectable i.e. strong dehydration of the polar head inducing an increase in the micelle size. It is noteworthy to remark that such a transition from a salting-in to a salting-out effect is observed with classical polyethoxylated surfactants (CiEj) when measuring the cloud point as a function of a salting-in electrolyte concentration i.e. the cloud point shows an increase and then a decrease by addition of the electrolyte.