Reciprocal relation of surface tension and dissolved organic matter originating from top soil layer leachates

The surface-active characteristics of naturally occurring dissolved organic matter (DOM) was studied in DOM solutions leached out from four OM-containing topsoils. A reciprocal relationship was obtained between the leaching dynamics of the dissolved organic carbon (DOC) and the surface tension (γL) of these solutions: when the DOC decreased with cumulative leachate pore volumes, the γL increased and approached γL of the displacing solution (i.e., water). For all soil solutions, the corresponding γL versus log(DOC) exhibited a linear relationship, from which the surface excess (Γ) and the average surface area (A, square nanometer) occupied by the surface-active DOM molecules were calculated based on the Gibbs equation. The calculated A values were of the same order of magnitude as those reported for some humic acids, but smaller. The nature of the displaced DOM solutions was further examined in terms of the effect of pH on the γL, which exhibited a sigmoidal reduction with decreasing pH. From the E2/E3 ratio, it was deduced that with decreasing pH, the molecular size of the DOM molecules decreases, and consequently, so does A. Accordingly, the reduction in γL with increasing pH was explained by the protonation of acid functional groups, which imparts a more hydrophobic nature to the DOM molecules, but also by the reduction in molecular size, and consequently, in A.