Dynamic properties of monomolecular films

Abstract

Two different types of cationic polysoaps together with their monomers were obtained. These species consist of divalent quaternary ammonium repeat units carrying a hydrocarbon or a fluorocarbon side chain, respectively. Dynamic surface tensions and surface dilatational rheology were measured by means of drop shape analysis of a sessile bubble, using specially developed hardware and software. Surface dilatational rheology was measured by the oscillating bubble method. Both the monomers and polymers show strong surface activity. The hydrocarbon monomer shows a maximum surface pressure of 44 mN/m and the fluorinated monomer of 55 mN/m. The polymers show lower surface pressures, 28 and 47 mN/m for the hydrocarbon and fluorocarbon polymers, respectively. The fluorinated polymer has peculiar adsorption characteristics, especially at low concentrations. All substances have very high surface dilatational elasticity, up to 180 mN/m for the fluorinated polymer, whilst the hydrocarbon substances give approximately half of this value. The specific surface areas are relatively high, and adsorption of all these substances is considerably slower than for ordinary surfactants, probably due to their particularly bulky head-group and charged nature. The monomers form micelles with CMCs of 0.3 and of 3.0 gL-1 for the fluorinated and hydrocarbon monomer, respectively. With increasing bulk concentration, a maximum in surface elasticity is observed for the monomers, but this is decreasing to almost zero at high concentrations. Both this decrease and most of the frequency dependence of the modules are believed to be caused by molecular transport between the surface and the bulk. It is more pronounced for the monomers than for the polymers. Deviations from the transport theory are observed, and are believed to be caused by in-surface relaxation processes.