Adsorption of Dodecyl Sulfate Surfactants with Monovalent Metal Counterions at the Air-Water Interface Studied by Neutron Reflection and Surface Tension

Abstract

Neutron reflection and surface tension have been used to study the adsorption at the air-water interface of the dodecyl sulfates of the alkali metals (MDS). The critical micelle concentration (cmc) at 33°C was found to vary from 8.5 × 10-3M for LiDS to 5.9 × 10-3M for CsDS, in agreement with the values in the literature. Neutron reflection was used to measure the adsorption of LiDS, NaDS, and CsDS at their cmc's at a temperature of 25°C for LiDS and NaDS, and 33°C for CsDS. The area per molecule(Acmc) was found to be 50, 45, and 38 ± Å2 respectively. Surface tension measurements were made for LiDS, NaDS, KDS, RbDS, and CsDS at varying temperatures and Acmc has also been calculated using the Gibbs equation. The values corresponding to the neutron reflection measurements are 51, 48, and 39 ± 3 Å2 when the counterions are Li+, Na+, and Cs+, respectively. The effect of temperature on Acmc was found to be smaller than the change of counterion. The variation of the surface tension with temperature was used to determine the thermodynamic parameters ΔGads, ΔHads, and ΔSads and ΔGmic, ΔHmic, and ΔSmic for adsorption and micellization respectively. The most noteworthy feature is that an increase in temperature decreases the surface tension of LiDS but increases it for all the other species. The structure of the adsorbed layer was determined by neutron reflection. Two methods of analysis of the data were used. One is the optical matrix method which fits a single structural model to the reflectivity profiles from a set of isotopic species at a given concentration, and the other by a more direct approach based on the kinematic theory. Both methods give identical structures for all three systems. The thickness of the layers at their corresponding cmc's are 18 Å for LiDS, 18 Å for NaDS, and 19 ± 1 Å for CsDS and the number of water molecules associated with each surfactant head group region is respectively eight, seven, and four. The distance from the center of the alkyl chain distribution to that of the solvent is found to be 6.5 Å for LiDS, 7.5 Å for NaDS, and 8.0 ± 1 Å for CsDS. The extent to which the alkyl chain is immersed in water decreases with the area per molecule and is about 35% for LiDS and below 30% for CsDS at their cmc's.