TRANSITIONAL BEHAVIOR IN ADSORBED LAYERS OF N-DECANOIC ACID AT THE AIR WATER INTERFACE/

The surface viscoelasticity of aqueous solutions of surface chemically pure n-decanoic acid has been studied by light scattering from therma lly excited capillary waves of frequency between 5 x 10(4) and 9 x 10( 5) s(-1). The surface dilatational elastic modulus measured by light s cattering, corresponding to such frequencies, agrees with the equilibr ium value derived from the pi-ln(c) variation for concentrations below 8 x 10(-6) M and above 6 x 10(-5) M, but systematically exceeds the e quilibrium variation between these concentrations. This intermediate c oncentration range has been described as one of transition between two different states of adsorption (Lunkenheimer and Hirte, J. Phys. Chem . 1992, 96, 8683). The behavior of the elastic modulus can be explaine d by relaxation involving molecular reorientation within the adsorbed layer, with a relaxation time >> 4 mu s. Such reorientation would be e ntirely consistent with a change of adsorption state, such as has been suggested for this system. Other aspects of the surface viscoelastici ty are briefly discussed.