Effect of Surface Tension of Organic Contaminant on the Movement of Water in Soil

Abstract

This study focuses on the Effect of Surface Tension of Butan-1-ol solutions on the movement of soil water using the drop number and capillary rise methods. In the drop number method, the number of drops of liquid falling from a small orifice is counted for a definite volume, keeping the drop rate constant, while in the capillary rise method, the liquid rises because of its surface tension in a capillary tube of small internal diameter immersed in it. The data by both methods were in excellent agreement. However, the drop number method provided more accurate results as opposed to the capillary rise method. The surface tensions for the capillary rise method were 7.16x10^-2, 6.70x10^-2, 6.10x10^-2, 5.60x10^-2, 4.40x10^-2, and 3.60x10^-2 N/m, while that of the drop number method gave 7.12x10^-2, 6.70x10^-2, 6.00x10^-2, 4.90x10^-2, 4.00x10^-2, and 2.90x10^-2 N/m, for butan-1-ol in soil water solutions of 0, 0.536, 0.105, 0.211, 0.433, and 0.854 M, respectively. It was generally observed that an increase in concentration of butan-1-ol in soil water solutions resulted in a decrease in surface tension. The surface excess concentration in each case was determined from the slope of surface tension graphs using the Gibbs adsorption isotherm; F = – 1/RT (dy/dlnC)_T   mol m^-2.  The surface excess concentration for the drop number method was found to be 5.4x10^-2 mol m^-2, while the capillary rise method was 4.8x10^-6 mol m^-2, respectively.  Surface tension is responsible for the shape of water drops and for holding the structures together, as plants may soak up the soil water. The study therefore, reaffirms the fact that substances which lower the surface tension of a solution are adsorbed at the surface and have greater effect on the surface properties hence surface excess of the solute is proportional to the concentration of solute multiplied by the rate of change of surface tension with respect to solute concentration.