Bernard Brun

Some aspects on the hydration of surfactant micelles

A review of the hydration properties of surfactant micelles is presented. Water self-diffusion studies are foundto give global hydration numbers corresponding approximately to the hydration of the counterions and the polar head-groups. The hydration numbers are inconsistent with water penetration into micelles and other arguments against such an effect are discussed as well. Furthermore, the hydration of counterions and the effect of solubilization on hydration are discussed. Sodium cholate differs considerably in its self—association behaviour from micelle-forming amphiphiles and this is found to apply also for the hydration characteristics.

Translational motion and association in aqueous sodium dodecyl sulphate solutions

SummaryThe translational motion of surfactant anions and water molecules has been studied at 25 °C in aqueous solutions of sodium dodecyl sulphate. Self-diffusion coefficients were determined with the open-ended capillary tube method employing radioactive labelling. Anion diffusion coefficients follow a simple model for micelle formation except that a marked decrease in translational mobility is observed well below the critical micelle concentration thus giving evidence for pre-micellar aggregation. From the anion self-diffusion coefficients micellar diffusion coefficients were calculated. These latter decreased with increasing soap concentration in a way theoretically predicted for intermicellar interactions. Addition of sodium dodecyl sulphate retards water translational motion most effectively at submicellar concentrations. A change in micellar hydration is indicated at about 70 · 10−3 mol/kg. The critical micelle concentration is found both from anion and water diffusion to be 8.5. 10−3 mol/kg. In acetate buffer the critical micelle concentration is considerably lower.ZusammenfassungDie Translationsbewegung von oberflächenaktiven Anionen und Wassermolekülen wurde bei 25 °C in wäßrigen Lösungen von Natriumdodecylsulfat untersucht. Die Koeffizienten der Selbstdiffusion wurden nach der „Offenen-Kapillar”-Methode mit radioaktiver Markierung bestimmt. Die Koeffizienten der Aniondiffusion gehorchen einem einfachen Modell der Mizellenbildung mit der Ausnahme, daß eine deutliche Abnahme der Translationsbeweglichkeit schon unterhalb der kritischen Mizellkonzentration beobachtet wurde. Dies läßt auf eine prämizellare Aggregation schließen. Aus den Koeffizienten der Selbstdiffusion der Anionen wurden die Diffusionskoeffizienten der Mizellen berechnet; letztere nahmen mit steigender Seifenkonzentration in der Weise ab, wie theoretisch für die intermizellaren Wechselwirkungen erwartet wurde. Der Zusatz von Natriumdodecylsulfat hemmt die Translationsbewegung des Wassers, besonders stark bei submizellaren Konzentrationen. Eine Änderung des Hydratationszustandes der Mizellen deutet sich bei 70 · 10−3 mol/kg an. Als kritische Mizellkonzentration wurde sowohl aus den Messungen der Anionenwie der Wasserdiffusion 8,5 · 10−3 mol/kg gefunden. Im Acetatpuffer ist die kritische Mizellkonzentration beträchtlich niedriger.

Translational motion and association in aqueous sodium cholate solutions

Abstract Self-association in aqueous sodium cholate solutions was studied at 25°C by determining the translational self-diffusion coefficients of water molecules, cholate ions, and sodium ions. Association was found to be much more gradual than for simple surfactants. Thus association starts at a concentration of about 0.014 mole/kg and progresses continuously up to the highest concentrations studied (0.5 mole/kg). The counterion association degree, which was derived from the sodium ion diffusion coefficient, is much lower than for typical micelle-forming substances. From the concentration dependence of the water self-diffusion coefficient the micellar hydration number was estimated. Hydration was found to be much more extensive than for simple surfactant micelles.

Micellar Structure in Intestinal Bulk — Relations with Mucosal Uptake

From measurements of translational self-diffusion coefficients and from solubility characteristics of bile salt mixed micelles, we have investigated some molecular associations between the major lipids of the intestinal aqueous content in an attempt to correlate the cholesterol flux into the intestinal mucosal cells with the structure of micellar solutions.