SURFACE VISCOELASTIC RELAXATION ON AQUEOUS-SOLUTIONS OF TETRAHYDROFURAN

Surface light scattering was used to study thermally excited capillary waves on aqueous solutions of tetrahydrofuran. The data indicate the presence of various surface viscoelastic relaxation processes in this system. In particular, the surface dilatational elastic modulus shows clear evidence of a relaxation with time constant similar to 5 mu s, w hich is here associated with reorientation of the adsorbed tetrahydrof uran molecules. The time constant of this process increases significan tly for surface films which are older than 5 h; this change is associa ted with other changes in the surface viscoelasticity, including the r ate of increase of the surface pressure. The dilatational surface visc osity deduced from the surface light scattering data is negative, as h as been found in other recent studies of soluble surfactants. These ne gative surface viscosities are interpreted as effective properties onl y, reflecting a reduction in the dilatational wave damping due to some process or processes not included in the theory used in the data anal ysis. The transverse shear surface viscoelastic modulus exhibits a com plex frequency dependence, more than one relaxation process of differe nt time constant being involved. Possible molecular mechanisms are dis cussed. (C) 1997 American Institute of Physics.