SURFACE VISCOELASTICITY OF HYDROXYPROPYLCELLULOSE AND HYDROXYETHYLCELLULOSE MONOLAYERS AT THE AIR-WATER-INTERFACE

The surface viscoelastic properties of hydroxypropylcellulose (HPC) and hydroxyethylcellulose (HEC), spread and adsorbed as monolayers at the air/water interface, have been determined using the electrocapillary wave diffraction (ECWD) technique over the wave frequency range 0.1-4.0 kHz. Evidence is presented to show that monolayers formed by adsorption from solution produce surface elasticities and surface viscosities that are essentially identical to those values obtained by spread monolayers at the same surface pressure. In all studies the more hydrophobic HPC exhibited greater surface elasticity and surface viscosity than HEC. Distinct changes in these parameters occur as the surface concentration exceeds values at which extensive looping of monomer segments into the underlying aqueous phase appears to take place. This suggests that the concentration and state of these monomer segments in the aqueous phase play the dominant role in the surface viscoelasticity of adsorbed monolayers which exist at these higher surface concentrations. Such conclusions are supported by the significant changes in viscoelasticity for HPC monolayers when the temperature is raised from 25 to 45-degrees-C, conditions that cause a phase separation of HPC in solution. On the other hand, HEC, which does not exhibit a lower critical solution temperature in this temperature range, does not exhibit such changes in monolayer properties.