Joachim Venzmer

Certain trisiloxane surfactants have the remarkable property of being able to superspread: small volumes of the surfactant solution rapidly wet large areas of hydrophobic surfaces. The molecular properties of the surfactants that govern this technologically relevant process are still under debate. To gain a deeper understanding, the surfactant behavior during the spreading process needs to be studied at molecular length scales. Here, we present neutron reflectivity analyses of two trisiloxane surfactants of similar chemical structure, of which only one exhibits superspreading properties. We present an approach to determining the composition of the adsorbed surfactant layer in spread surfactant films at the solid-liquid interface, accounting for contributions from attenuated back-reflections of the neutron beam in films with thicknesses in the range of several tens to hundreds of micrometers. Differences between superspreading and non-superspreading surfactants with regard to their volume fraction profiles at the solid/liquid interface obtained in the self-consistent analysis of the reflectivity curves are in agreement with a simple explanation of the difference in spreading behavior based on thermodynamics.

Certain trisiloxane surfactants have the remarkable property of being able to superspread: Small volumes of water rapidly wet large areas of hydrophobic surfaces. The molecular properties of the surfactants which govern this technologically relevant effect are still under debate. To gain a deeper understanding, the surfactant behaviour during the spreading process needs to be studied at molecular length scales. Here, we present neutron reflectivity analyses of two trisiloxane surfactants of similar chemical structure, of which only one exhibits superspreading properties. We present an approach to determining the composition of the adsorbed surfactant layer in spread surfactant films at the solid-liquid interface, accounting for contributions from attenuated back-reflections of the neutron beam in films with thicknesses in the range of several tens to hundreds of micrometers. Differences between superspreading and non-superspreading surfactants with regard to their volume fraction profiles at the solid/liquid interface obtained in the self-consistent analysis of the reflectivity curves are in agreement with a simple explanation of the difference in spreading behaviour based on thermodynamics.

The microstructure of the aqueous solutions of purified acidic Sophorolipid (SL) has previously been studied using highly sophisticated methods such as SANS and Cryo-TEM. We were interested in whether (a) the main findings also apply to commercially available SL (which is a mixture of acidic and lactonic SL) and (b) more readily available methods such as DLS can be used to gain insight into the molecular aggregation of SL. Our work was motivated by the increasing interest in biosurfactants for applications in personal and household care. Moreover, the origin behind the more or less lack of rheological response to changes in pH is of practical relevance, as it is somewhat unusual for a carboxylate-group containing surfactant. By using DLS microrheology, we could elucidate the aggregation structure and dynamics of the surfactant on a microscopic scale. Surprisingly, the different degrees of protonation only impacted the microscopic properties such as exchange kinetics and the plateau values of the storage moduli.

Vesicles have recently found widespread use in applications such as conditioning of textiles, paper and hair, as well as transdermal drug delivery. The mode of treatment in several such cases involves the application of droplets of aqueous dispersions of vesicles onto dry porous substrates like paper and textiles. One of the factors which affects the performance of such treatments is the rate at which the droplets spread and imbibe on the porous substrate. Depending upon the specific purpose of the treatment either a fast or slow droplet spreading kinetics could be desired. Therefore, it is important to have a good understanding of the droplet spreading process and the factors which influence it. In this work, an experimental investigation of the simultaneous spreading and imbibition of vesicle dispersion droplets on cellulose filter papers is carried out. Two different types of vesicles which are composed of similar lipid molecules but exhibit contrasting lipid bilayer phase behavior are used. Two different grades of filter papers with comparable porosities but different thicknesses are used as porous substrate. It is found that the droplet spreading behavior is of the “complete wetting” type on the thicker porous substrate, whereas it is of the “partial wetting” type on the thinner substrate. Furthermore, it is observed that the spreading of droplets containing vesicles with liquid-crystalline phase bilayers occurs faster than that of vesicles with solid-gel phase bilayers. The secondary radial penetration which commences after the initial droplet spreading is complete is also investigated and discussed.