Dynamic contact angles in mercury/carbon tetrachloride/solution systems. III. The mechanism and kinetics of spreading

The surface areas (A) per molecule of several typical anionic and cationic surface-active agents at carbon tetrachloride/water, and mercury/water interfaces have been determined by the application of Gibb's adsorption equation. Whereas a limiting value for A at the carbon tetrachloride/water interface was of the order 54 A2, values at the mercury/water interface were three to five times higher. This was probably due to the incomplete orientation of the molecules at the mercury/water interface. Spreading in the mercury/carbon tetrachloride/solution system was controlled by the magnitudes of the interfacial forces. Because of the rapid rate of establishment of equilibrium excesses at the interfaces, compared with the rate of interfacial expansion (or contraction) these forces were effectively constant. The spreading processes were exponential and followed first-order kinetics of the form: dθ dt = K(θ eq − θ t ) , where θeq is the value of θ at t∞, θt the value at time t, and K a constant only for a particular concentration of surface-active agent; it is proportional to the rate of the spreading process. The energy of activation of the spreading process was found to increase with increase in concentration of each surface-active agent. At a unique and reproducible “critical” concentration of each of the ionic surface-active agents, the mechanism of the spreading process changed abruptly, and the magnitude of the energy of activation then effectively remained constant, i.e., independent of concentration. At low surface concentrations of the ionic surface-active agents (apparently chemisorbed on the mercury surface) a mechanism is proposed in which these molecules are compressed ahead of the spreading drop. As the surface concentration increased, however, a situation was reached where this compression mechanism was no longer possible, due to the dense packing of surface-active molecules. After this stage, the drop proceeded to spread over the adsorbed layer of surface-active molecules. It has been shown that spreading coefficients can not only predict the position of the equilibrium stage but in some cases the speed of a spreading process as well. Better agreement may have been obtained if the difficulties involved in a change in mechanism of the spreading process had not occurred.