Double layer polarization in “realistic” aqueous salt-free suspensions

Abstract

The analysis of the frequency dependent electric permittivity of colloidal suspensions as a function of the frequency of the externally applied field is a powerful tool, very sensitive to the particle–solution interface and can provide rich information on the dynamics of electrical double layers. Among the different physical situations that can manifest themselves in the permittivity spectrum, attention will be paid in this paper to the case of concentrated suspensions of spherical colloidal particles in “realistic” salt-free conditions. This means that the ionic species in the liquid phase will be: (1) the “released counterions” coming from the dissociation of surface groups of the particles, (2) the H + and OH − ions from water dissociation, and (3) the ions generated by the environmental CO 2 dissolved in the suspension if it is open to the atmosphere. Theoretical predictions based on the cell model indicate the presence of two relaxations in the permittivity spectrum when the particle charge is large enough. The lowest frequency peak (appearing at frequencies around 10 5 Hz) changes with the overall conductivity of the medium, while the faster relaxation (characteristic frequency in the range 10 7–10 8 Hz) only depends on the surface charge of the particle. This suggests that the first relaxation corresponds to the classical Maxwell–Wagner (MW) relaxation associated to interfacial polarization of the diffuse layer, whereas the second, higher-frequency one, relates to the polarization of a compact layer of counterions developed very close to the particle surface at high particle charges (ionic condensation effect). In order to really prove the physical origin of these dispersions we have compared the results with equivalent salt-added (or low salt) suspensions. These have been built using the same surface charge density values for the particles and an added salt concentration such that the DC conductivities of the salt-free systems are mimicked. Interestingly, it has been found for the first time that a clear alpha-process (observed at the kHz frequency region, and linked to the freezing of the concentration polarization of the double layer) is present together with the above mentioned two MW processes. The alpha relaxation is, however, absent in the case of the realistic salt-free suspensions. In these, the existence of links between the local concentrations of ions due to the weak character of the main electrolyte in solution, carbonic acid, apparently prevents or largely hinders the formation of concentration polarization clouds.