Interfacial behaviour of surfactants and enzymes: studies at model surfaces

This thesis presents results from experimental studies of the interfacial behaviour of surfactants and enzymes at model surfaces. The main technique used throughout the work was ellipsometry.

Studies of surfactant adsorption in static and dynamic wetting situations were performed to investigate adsorption behaviour close to the three-phase contact line (tpl) during dynamic wetting. It was demonstrated for a hydrophobic substrate and a non-ionic surfactant that the dynamic wetting behaviour is strongly affected by surfactant transport over the advancing tpl. This transport route appears to determine the adsorption at the interfaces joining in the tpl and thus also the dynamic wetting tension. It was concluded that the adsorption rate-determining step was the diffusion of surfactant from the bulk solution to the liquid/vapour interface. In contrast to the hydrophobic substrate surfactant carry-over across the tpl to hydrophilic silica substrates was much less efficient. Apparently the reassembly into the surface bilayer/micellar structures which form at the hydrophilic surface is a slower process than simply transferring surfactants from one monolayer at the liquid-vapour interface to another at the hydrophobic solid-liquid interface. Binary surfactant systems comprising cationic and non-ionic surfactants were also studied at hydrophobic solid surfaces in static and dynamic wetting situations. Adsorption isotherms indicated that below the cmc the surface layer consisted almost exclusively of the low solubility non-ionic surfactant. At higher surfactant concentrations the cationic surfactant mixes into the layer. Furthermore, the equilibrium adsorption behaviour was mirrored in the kinetics of desorption. The measured desorption curves indicated a transitional compositional changes and back-diffusion of the non-ionic surfactant due to changes in the concentration gradients of the two components within the diffusion zone. Adsorption and wetting force measurements, measured during substrate immersion were found to correlate to the equilibrium adsorption behaviour.

In a comparison of surfactant adsorption measurements performed by quartz crystal microbalance (QCM) and ellipsometry, the question of consistency and complementary of the two techniques were addressed. It was shown that the frequency shift obtained from the QCM experiments results in an overestimation of the adsorbed mass, due to the presence of water coupled to or trapped within the adsorbed layer. This effect was shown to be larger for a hydrophilic surface than for a hydrophobic substrate consistent with less dense surface layers and/or surface bound water.

In order to facilitate studies of the interfacial behaviour of cellulose active enzymes, different methods for producing model surfaces was assessed and elaborated. The substrates produced by our modified spin-coating procedure fulfilled the demands of being chemically pure, stable in aqueous solutions, smooth and reflecting thereby allowing studies of surfactant and polymer adsorption as well as enzymatic degradation. Investigation of a commercial cellulase mixture at the cellulose model surface, showed that subsequent to an initial adsorption phase the cellulose film began to degrade with time in a concentration dependent manner. In a study where the structure of the enzyme was varied the carbohydrate-binding module (CBM) of the enzyme was shown to play a major role for the binding at the cellulose interface, as well as for the subsequent degradation process. Removing the CBM resulted in a lower adsorption of the cellulase and a slower rate of degradation. Removal of the CBM also resulted in elimination of the pH dependent degradation observed for the native enzyme.