There is good correlation of contact angle measurements and contact angles calculated from surfactant adsorption density data for an electrically neutral surface, as reported in a previous paper for the system hematite-aqueous solution-ketone, with surfactant hexadecyl sulfonic acid. The same method is not sufficient when the hematite surface is electrically charged. Data was collected to develop the appropriate form of an electrostatic term for the analysis. Acid-base titration was used to evaluate surface electrical properties versus pH for the hematite used in the study. Surfactant adsorption isotherms were measured at pH of 4.5, 5.5, 6, and 7 to use in developing an equation for effect of surface potential on contact angle. After adding a term for the contribution of the electric field, the contact angles calculated from adsorption data follow the measured contact angles well. (C) 2011 Elsevier Inc. All rights reserved.