Investigation of the Electrokinetic Properties of Paraffin Suspension. 1. In Inorganic Electrolyte Solutions Emil Chibowski,* Agnieszka Ewa Wiacek, Lucyna Holysz, and Konrad Terpilowski Department of Physical ChemistrysInterfacial Phenomena, Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland Received December 15, 2004. In Final Form: March 7, 2005 Although electrical properties of nonionogenic hydrophobic surface (solid or liquid) in water and/or electrolyte solutions have been studied for many decades, they are still not well recognized, especially as for the nature of the charge and potential origin. Similarly, water structure at such a surface is still extensively studied. One such system is paraffin wax/water (electrolyte). The zeta potentials and the particle diameters of this system were investigated in this paper. To obtain the suspension of paraffin in water or electrolyte solution (NaCl or LaCl3), the mixture was heated to ca. 70 °C and then stirred during cooling. For thus obtained suspensions, the zeta potential was determined as a function of time at 20 °C. Also the pH effect on the zeta potentials was investigated. The zeta potentials were calculated from Henry’s equation. The results obtained by us are in agreement with those obtained earlier by others. They confirm that although H + /OH - are not surface charge creating ions, OH - ions to some extent are zeta potential determining for the paraffin surface. By use of the potentials and diameters, the electric charge for a spherical particle in the shear plane was calculated. These values are small in the range of 10 -3 C/m 2 . On the basis of the findings of water structure near hydrophobic surface and the calculated charges, it is concluded that in fact the potential may be created by immobilized and oriented water dipoles. Introduction Electrokinetic phenomena at nonionogenic hydrophobic surface (solid or liquid) in water and/or electrolyte solutions have been studied for many decades. 1-12 However, they still are not well recognized, especially as for the nature of the potential origin and the shear plane location. The properties of such interfaces are very important for stability of dispersed systems (both suspensions and emulsions) as well as in many biochemical processes including living organisms. Frequently, it is very desirable to increase the zeta potential of suspension particles or emulsion droplets and thus the electrostatic repulsion to stabilize the system. Van Oss et al. 13-16 were among the first who offered a quantitative expression (“extended DLVO theory”) for the calculation of the force due to hydrogen bonding or, more generally, electron-donor/ electron-acceptor (Lewis acid-base) interactions between two similar or dissimilar surfaces interacting across a liquid phase. 17 Although there is no direct relationship between the zeta potential value and the acid-base interactions, according to van Oss et al. 13,18,19 changes in the zeta potential may correlate with changes of the acid-base interactions. According to them, hydrophilicity of a surface can be reduced by any means that reduces the zeta potential, because the surface charge is due to ionized polar groups present on the surface, which often interact also via hydrogen bonds, i.e., Lewis acid-base interaction, especially the electron-donor groups. 18,19 However, it seems this cannot be the case for nonionic hydrophobic surfaces at which several-dozen-milivolts zeta potentials are determined at very low ionic strengths. The latest data suggest that changes in water structure near hydrophobic surface and water dipoles orientation may play a signifi- cant role in the potential creation. 20-30 * To whom correspondence should be addressed. E-mail: emil@ hermes.umcs.lublin.pl. (1) Carruthers, C. J. Trans. Faraday Soc. 1938, 34, 300. (2) Dickinson, W. Trans. Faraday Soc. 1941, 37, 140. (3) Taylor, A. J.; Wood F. W. Trans. Faraday Soc. 1957, 53, 523. (4) Parreira, H. C.; Schulman, J. H. 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