Interfacial properties of some carbohydrate-based surfactants and polymers: Experiments and Theory

The aim of this thesis is primarily to investigate the adsorption of surfactants, in particular to the airwater interface. The effects of the head-group and the hydrocarbon chain are of special interest. Using thermodynamical models we were able to predict that the transition from the Henry range to the liquid-expanded phase is controlled by the formation of small clusters of surfactants oriented in the plane of the surface. Hence, the transition from the Henry range can not be of first-order, and additionally the phase formed after the onset of the micelle formation does have a liquid-like character. We refer to this phase as the dilute surface micellar range. Interactions between the micelles in the dilute surface micellar range can be accounted for by excluded area interactions using the harddisc model. The stability of the micelles can be attributed to the line tension acting on the micelle edge, which stops unlimited growth of the surface micelles. With increasing density of surface micelles a new phase is formed, referred to as the granular phase. The granular phase exhibits very low compressibilty and an almost constant area/molecule in an extended surfactant bulk concentration interval. For three sugar-based surfactants carrying decyl hydrocarbon tails at room temperature the defining area/molecule is in the close vicinity of 79 A. In the granular phase the lines of the surface micelles are intact, but distorted probably into a more hexagonal shape to cover the surface. For a sugar-based surfactant carrying dodecyl hydrocarbon tail the defining area/molecule in the granular range was found to be close to 70 A. For one of the decyl surfactants 70 A was also observed, indicating the existence of a few seemingly discrete adsorption modes for surfactants in the granular range. The transition from the granular range to the true liquid-expanded phase, defined by a complete coverage of the entire surface of fluid hydrocarbon tails, is seemingly a first-order transition. This is indicated in the surface tension isotherm by a “knee” with an abrupt change from a constant surface density (granular phase) to a lower molcular area and an increasing adsorption with increasing surfactant chemical potential (true liquid-expanded phase). In the liquid-expanded phase the role of the hydrocarbon chain restriction in the surface was investigated by applying the hard-disc model on mixtures of sugar-based decyl surfactants to account for the head-group surface fractions obtained by applying Gibbs surface tension equation to mixtures of sugar-based surfactants. The behaviour of the head-groups at the interface was accurately described by a hard-disc model, and hence this resulted in a prediction of the decyl hydrocarbon chain configurational pressure as a function of molecular area. Applying the deduced configurational pressure on pure surfactants yielded accurate models on the surface pressure and excess free energy contribution from the head-groups. Futhermore, the limits of the prediction capability of the model separation of head-group and hydrocarbon tail phase was tested by means of systems exhibiting partial interpenetration of the head-group into the hydrocarbon tail phase. For ethylene-oxide-based surfactants a weak interpenetration effect was observed. However, for the thiomaltoside head-group a large effect due to interpenetration was encountered. This was deemed to be related to the reorientation of the head-group due to the preferential mixing of the sulphur with the hydrocarbon tail phase. From measurements of surface tension after the cmc the average micelle size of n-dodecyl-β-D-maltopyranoside was determined to be 76 monomers. Surface forces measurements using crystalline hydrophobic surfaces in sugar-based surfactant solution proved that surfactants desorb at low applied loads. Surface forces measurements on adsorbed layers of cationic polymer containing β-cyclodextrin (β−CD-EPN) adsorbed onto mica in solutions of admantanegrafted polyethylene-oxide polymers (Ad-PEO) were performed to investigate the interaction between β-cyclodextrin and admantane. In the absence of Ad-PEO very low adhesion was observed between the β−CD-EPN coated mica surfaces, on addition of Ad-PEO a significant increase was detected in both layer thickness and adhesion. This was attributed to the interaction between admantane and βcyclodextrin.