Dynamic surface tension of aqueous surfactant solutions: 3. Some effects of molecular structure and environment

Abstract

The effect of surfactant molecular structure and environment on the parameters, γ m, t∗, and n in the equation, γ t = γ m + ( γ 0 − γ m /[1 + ( t t∗ ) n], describing the surface tension, γ t at time t, of an aqueous solution at constant surfactant concentration, C, is explored. From the equation, t∗ is the time at which the rate of change of γ t with log t reaches a maximum, equal to 0.576 ν( γ 0 − γ m), where γ 0 and γ m are the surface tension of the solvent and the solution at mesoequilibrium, respectively. A variety of anionic surfactants in swamping amounts of electrolyte and nonionic surfactants were studied. At a given value of C, t∗ decreases with a decrease in the equilibrium surface excess concentration, Γ eq, of the surfactant; at a given Δ eq/ C value, it generally increases with an increase in the hydrophobic character of the surfactant. The value of n also increases with an increase in the hydrophobic character of the surfactant. The value of γ m is affected by the same factors that determine the value of γ eq, the equilibrium surface tension. The maximum rate of surface tension reduction with log time, which depends upon the values of n and γ m, consequently increases with an increase in the hydrophobic character of the surfactant. For low γ t values, γ m must be low and t∗ less than t.