Direct measurement of the spreading pressure of organic liquids on water

Abstract

A method is presented to measure the pressure corresponding to the spreading coefficient which quantifies the interfacial tension forces that develop when an organic liquid is deposited on a water surface. The method uses a hydrophobic porous probe to introduce a spreading (positive spreading coefficient) organic liquid, at pressures less than atmospheric pressure, to a water surface. The porous probe is filled with the organic liquid and connected to a pressure transducer, which is used to measure the negative pressure required to prevent the organic liquid from spreading on the water surface; that is, the null pressure. The method is demonstrated with two hydrocarbons, Soltrol-220 and diesel fuel. A theory that predicts the thickness of an organic liquid lens for negative spreading coefficients and one-dimensional spreading rates for positive spreading coefficients was used to obtain an estimate of the spreading pressure (force per unit area). With positive spreading coefficients, the predicted values are between 0.0 and 0.2 cm H 2O (200 dyn/cm 2). The porous probe measured values of near 45 cm H 2O for Soltrol-220 and 340 cm H 2O for diesel fuel. The measurements indicate a large discrepancy between the current theoretical understanding and measured values.