ELECTROPHORETIC MOBILITY MEASUREMENTS IN LOW CONDUCTIVITY MEDIA

Determining the sign and magnitude of the electric charge of colloidal particles suspended in low conductivity liquids (picosiemens/meter range) by electrophoresis continues to be difficult and controversial. We have measured the electrophoretic mobility of suspensions of carbon black in dodecane under various conditions using two slightly different techniques. The first technique (Coulter DELSA 440) uses a cell consisting of a narrow rectangular flow channel placed between two electrodes and a dual laser beam with heterodyne optics to measure particle velocity. The second technique uses parallel plate electrodes immersed in the colloid system with a commercially available laser Doppler velocimeter from TSI with homodyne optics. We find that the uniform electric field, easily produced in the parallel plate cell, is satisfactorily reproduced in the rectangular cell as long as the cell is electrically isolated, i.e., not in contact with a constant temperature water bath. Furthermore we find that a common technique of measuring electrophoretic mobilities a long time after the electric field has been applied leads to errors caused by flocculation and/or sedimentation of the suspended particles. We have measured the effect of Chevron's OLOA 1200 on the charging of carefully cleaned carbon black. OLOA 1200 causes carbon black to become negatively charged; the amount of charge increases with OLOA 1200 concentration, but the magnitude of charging is less than that previously reported.