MEASUREMENTS OF THE ELECTRIC CHARGE AND SURFACE-POTENTIAL ON SMALL AQUEOUS DROPS IN THE AIR BY APPLYING THE MILLIKAN METHOD

The electric charge and the surface potential on small liquid drops fo rmed from pure waters and various aqueous solutions in the air were me asured by applying the well-known Millikan oil drop method. The measur ed values were then discussed in relation to the pH values, the valenc es and concentrations of different electrolytes, and ionic types of su rfactants in the aqueous solutions. Pure water drops were all negative ly charged in the air and have the strongest specific electric charge and surface potential. Their negative charge is caused by the specific adsorption of OH- anions onto the water-air interface. The electric c harge on the deionized water (DIW) drops in the air was found to decre ase once its pH value deviates from the equilibrium pH of the pure DIW . The aqueous drops were negatively charged if they were formed from t he DIW solutions with an anionic surfactant, sodium dodecyl sulphate ( SDS), and an electrolyte of NaCl or CaCl2, respectively. However, the electric charge on the aqueous drops became positive if the electrolyt e AI(2)(SO4), was used instead. The positive charge may be due to the selective adsorption of trivalent cations, Al3+, onto the liquid-air i nterfaces. As expected, the aqueous drops were positively charged when they were formed from the solutions containing a cationic surfactant, cetyltrimethylammonium bromide (CTAB), and sodium sulphate (Na2SO4). The absolute value of the electric charge on all the aqueous drops dec reased with increasing the electrolyte concentrations in the aqueous s olutions. All these measured results are in excellent accordance with the zeta potential of the air bubbles generated in the same aqueous so lutions. (C) 1998 Elsevier Science B.V.