An Equation Involving Cohesive Force for the Surface Pressure of Long Chain Ampholytes at Oil/Water Interface and Its Verification

An equation for surface pressure, involving cohesive force, at oil/water interface has been derived for ampholytes and weak electrolytes. The equation has been tested with the experimental results obtained by Payens on monolayers of stearylphosphonic acid spread on oil/water interface and has been found to be fairly satisfactory. Assuming stcarylphosphonic acid to be an ampholyte its isoelectric point and the dissociation constant ka have been evaluated from the data on surface pressure and area obtained by using different ionic strengths at pH 5·8. The pKa and the pH corresponding to the isoelectric point have been found to be 4·66 and 3·73 respectively