Electromobility of Mica Particles Dispersed in Aqueous Solutions

-Mobility measurements were made for dry-ground mica particles dispersed in aqueous KC1, Cr(NO3)3, and cetyltrimethylammonium bromide (CTAB) solutions. In the latter solutions charge reversal occurred at about 2 x 10 -6 M and 3 x 10 -5 M, respectively. Negative zeta potentials in KC1 solutions calculated using the Smoluchowski equation are about one third of the corresponding values obtained from streaming potentials and force measurements using mica sheets. Good agreement, however, was obtained when positively charged groups created during grinding were neutralized, and the zeta potentials were corrected according to the procedure of O'Brien and White with an assumed average particle size of 0.25/~m. When the zeta potentials of positively charged particles were corrected in this way, agreement with values calculated from force measurements was also improved. Key Words--Electromobility, Force measurements, Mica, Smoluchowski equation, Streaming potential. I N T R O D U C T I O N The properties of clay-size muscovite have come under increased scrutiny in recent years because of the use of large (square centimeter size) cleaved sheets in the direct measurement of Derjaguin, Landau, Verwey, and Overbeek (DLVO) and "solvation forces" in aqueous solutions by Israelachvili and Adams (1978) and Pashley (198 la, 198 lb). The results of these force experiments show that the DLVO theory is generally accurate in dilute electrolyte solutions ( < 1 0 -2 M , depending on the electrolyte), but that at higher concentrations short-range (-<50 /~) non-DLVO forces arise. In such experiments the ion-exchange properties of the mica basal plane determined the nature of both the diffuse double-layer forces and the observed solvation forces (see Pashley, 198 l a). In dilute solutions where the forces can be accurately fitted by diffuse double-layer theory, an apparent surface potential can be calculated, because the Debye length of the solution is known. The variation of this potential with concentration and type of electrolyte gives a measure of ion adsorption at the mica surface and has been successfully described using a simple mass-action model by Pashley and Quirk (1984). This variation suggests that the mica basal plane is an almost ideal ion-exchange surface with only one type of surface site, a conclusion which was also reached by Goulding and Talibudeen (1980) from direct ionexchange measurements. Thus, the diffuse double-layer potential and charge observed on mica immersed in an aqueous electrolyte solution depend on the relative On leave from Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, ACT 260 l, Australia. Copyright 9 1985, The Clay Minerals Society concentrations and surface adsorption energies of hydronium ions and any other cations present in solution. These cations adsorb on the negative sites on the mica basal surface which are present at the high density of about 2.1 x 1014/cm2 (Gaines and Tabor, 1956). Hence, for complete neutralization a monovalent cation must adsorb at a coverage of one ion per 48 ,~2. Thus, even for quite a high surface potential (~ 100 mV) in dilute monovalent electrolyte solution (~ 10 -5 M), nearly all of these sites must be filled with adsorbed ions, and, in general, the diffuse layer charge is much less than the mica lattice charge (see Pashley, 198 la). In addition to the force method of obtaining surface potentials on mica, Lyons et al. (1981) used a radialflow streaming potential apparatus to obtain zeta potentials on large mica sheets immersed in KC1, HC1, and CaCl2 solutions. The potentials measured in KC1 and CaCI 2 solutions agree closely with those obtained from the force measurements, although in HC1 solution the force method gave significantly higher potentials (Pashley, 1981 b). The results from these quite different methods generally indicate good agreement and show that the basal plane is indeed capable of ion-exchange with a wide range of cations, as was also observed by Gaines 0957) and by Mokma et al. (1970) from ionexchange studies on mica powders. The ion-exchange properties of clays in general make them of some interest with respect to the effect of the diffuse double-layer potential on their water-swelling properties. Friend and Hunter (1970) measured electromobilities of Li+-vermiculite particles in an attempt to relate the observed zeta potential with the doublelayer swelling pressures observed by Norrish and Rausell-Colom (1963). The mobilities were found to vary in a complex manner with concentration and gave zeta potentials of much lower magnitude (~ 75 mV) than