The zeta potential of extended dielectrics and conductors in terms of streaming potential and streaming current measurements

The electrical characterization of surfaces in terms of the zeta potential (zeta), i.e., the electric potential contributing to the interaction potential energy, is of major importance in a wide variety of industrial, environmental and biomedical applications in which the integration of any material with the surrounding media is initially mediated by the physico-chemical properties of its outer surface layer. Among the different existing electrokinetic techniques for obtaining zeta, streaming potential (V-str) and streaming current (I-str) are important when dealing with flat-extended samples. Mostly dielectric materials have been subjected to this type of analysis and only a few papers can be found in the literature regarding the electrokinetic characterization of conducting materials. Nevertheless, a standardized procedure is typically followed to calculate zeta from the measured data and, importantly, it is shown in this paper that such a procedure leads to incorrect zeta potential values when conductors are investigated. In any case, assessment of a reliable numerical value of zeta requires careful consideration of the origin of the input data and the characteristics of the experimental setup. In particular, it is shown that the cell resistance (R) typically obtained through a.c. signals (R-a.c.), and needed for the calculations of zeta, always underestimates the zeta potential values obtained from streaming potential measurements. The consideration of R-EK, derived from the V-str/I-str ratio, leads to reliable values of zeta when dielectrics are investigated. For metals, the contribution of conductivity of the sample to the cell resistance provokes an underestimation of R-EK, which leads to unrealistic values of zeta. For the electrical characterization of conducting samples I-str measurements constitute a better choice. In general, the findings gathered in this manuscript establish a measurement protocol for obtaining reliable zeta potentials of dielectrics and conductors based on the intrinsic electrokinetic behavior of both types of samples.