Surfactants at non-polar surfaces

Abstract: The aim of this thesis work was to investigate theadsorption of surfactants to different nonpolar interfaces.Particularly, the effects of the polar group and the nature ofthe hydrophobic interface were elucidated. The interfacialbehavior of the liquid-vapor interface was investigated bymeans of surface tension measurements. Here the effect of thepolar group and the hydrocarbon chain length was investigatedin a systematic manner. It was found that the shorter of thetwo chains examined, decyl, generated a larger surface pressurecontribution than the longer, dodecyl. Furthermore, the sugarbased surfactants behaved differently as compared to theethylene oxide based ones. The former could be modelled byassuming a hard disc behavior of the head group while thelatter displayed polymeric behavior. The influence of saltconcentration on the surface tension behavior of an ionicsurfactant, sodium dodecyl sulphate, was investigated. Theresult could be rationalized by employing the Gouy- Chapmanmodel to the polar region. Furthermore, mixtures of two sugarbased surfactants were investigated by surface tensionmeasurements and the adsorbed amount of the two components atthe interface atdifferent concentrations and fractions in thebulk were obtained by applying the Gibbs surface tensionequation. It was found that the molecule with the smaller headgroup adsorbed preferentially, and more so as the totalsurfactant concentration was increased. These findings could beexplained by considering the interactions generated by thedifferent head groups. The adsorption of sugar surfactants toan isolated hydrophobic surface was studied by means of wettingmeasurements and the behavior was similar to that at theliquid-vapor interface. Wetting isotherms were measured on twodifferent hydrophobic surfaces where the covalently attachedhydrophobic layers were in a crystalline and fluid state,respectively. The wetting results revealed that the sugarsurfactants anchored in the fluid hydrophobic layer. This had asignificant influence on the force profile. For example, at thecrystalline surface the surfactant monolayers were easilyremoved as the surface came into contact at relatively lowapplied loads. This was not the case when the hydrophobic layerwas in a fluid state. Here a significant fraction of thesurfactants remained between the surfaces. Disjoining pressureisotherms were measured using a sugar based surfactant thatwere thoroughly purified and compared to the as receivedsample. Even the purified sample showed a double-layer forcealthough lower as compared to the as received, one. Asignificant difference in foam stability was also observed.