Solution pH and oligoamine molecular weight dependence of the transition from monolayer to multilayer adsorption at the air-water interface from sodium dodecyl sulfate/oligoamine mixtures.

Neutron reflectivity and surface tension have been used to investigate the solution pH and oligoamine molecular weight dependence of the adsorption of sodium dodecyl sulfate (SDS)/oligoamine mixtures at the air-water interface. For diethylenetriamine, triamine, or triethylenetetramine, tetramine mixed with SDS, there is monolayer adsorption at pH 7 and 10, and multilayer adsorption at pH 3. For the slightly higher molecular weight tetraethylenepentamine, pentamine, and pentaethylenehexamine, hexamine, the adsorption is in the form of a monolayer at pH 3 and multilayers at pH 7 and 10. Hence, there is a pH driven transition from monolayer to multilayer adsorption, which shifts from low pH to higher pH as the oligoamine molecular weight increases from tetramine to pentamine. This results from the relative balance between the electrostatic attraction between the SDS and amine nitrogen group which decreases as the charge density decreases with increasing pH, the ion-dipole interaction between the amine nitrogen and SDS sulfate group which is dominant at higher pH, and the hydrophobic interalkyl chain interaction between bound SDS molecules which changes with oligoamine molecular weight.

Adsorption of the linear poly(ethyleneimine) precursor poly(2-ethyl-2-oxazoline) and sodium dodecyl sulfate mixtures at the air-water interface: the impact of modification of the poly(ethyleneimine) functionality.

The adsorption of the polymer-surfactant mixture of poly(2-ethyl-2-oxazoline)-sodium dodecyl sulfate at the air-water interface has been studied by neutron reflectivity and surface tension. The observed patterns of adsorption more closely resemble those encountered in weakly interacting polymer-surfactant mixtures, rather than the pronounced enhancements in adsorption observed in strongly interacting polymer-surfactant mixtures, such as in the related poly(ethyleneimine)-sodium dodecyl sulfate mixtures. The adsorption was found to be strongly dependent on solution pH, polymer molecular weight, and polymer concentration. At the lower and higher molecular weights studied, there was little enhancement in the sodium dodecyl sulfate adsorption at low sodium dodecyl sulfate concentrations, whereas at the intermediate polymer molecular weights, some enhancement in the adsorption was observed. For the higher-molecular-weight polymers and at increasingly higher polymer concentrations, a significant reduction of the surfactant at the interface compared to pure sodium dodecyl sulfate occurred for sodium dodecyl sulfate concentrations between the critical aggregation concentration and the critical micellar concentration. The results illustrate the important role of modifying the functionality of poly(ethyleneimine) on surface adsorption.