The negative adsorption of anions by clay suspensions

Abstract-Simple equations are presented which allow double-layer potentials of clays to be derived from co-ion exclusion measurements in monovalent and divalent electrolyte solutions. These equations have been used to re-interpret earlier results for illite and montmorillonite. The potentials derived follow the lyotropic series for the various homoionic modification of each clay. We have demonstrated that the Schofield equation, which assumes high double-layer potentials, cannot be applied to co-ion exclusion in clay systems. Re-analysis of earlier measurements has shown that for a given homoionic clay the potentials are almost independent of concentration over the range 0.3 to 0.003 molar. Thus, clay surfaces appear to behave more like constant-potential than constant-charge surfaces.

Section: Discussionmentioning

confidence: 83%

Surface Potentials Derived from Co-Ion Exclusion Measurements on Homoionic Montmorillonite and Illite

Chan

1984

“…Also, coion exclusion measurements on montmorillonite and illite clays for a wide range of cations indicate quite constant surface potentials over a concentration range of 0.3 to 0.003 M (see Chan et al, 1984). Callaghan and Ottewill (1974) obtained constant zeta potentials ( -4 7 ___ 3 mV) from mobility measurements on Na +-montmorillonite panicles in NaCI solutions from 10-to 10 -4 M. If only cations adsorb at the basal plane surface, as shown by Bolt and Wakentin (1958), then in order for the potential to remain constant these adsorbed ions must de-sorb as their concentration in bulk solution increases, in contradiction with the laws of mass action.…”

Section: Pashleymentioning

confidence: 89%

Electromobility of Mica Particles Dispersed in Aqueous Solutions

Pashley

1985

Abstract--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.

“…Thus, if 1 g of powderwithasurfaceareaof8 • 105 m2/kg (approximately that of a smectite) were added to 1 ml of a potassium and magnesium nitrate solution, the Mg concentration would increase by approximately 12% relative to that of the K. Under more typical experimental conditions, e.g., a solid to liquid ratio (w/v) of 0.04, the increase is only 0.2%. Concern for the 'geometric' effect might be the unexpressed reason why in experiments upon negative adsorption some workers (e.g., Bolt and Warkentin, 1958;Edwards and Quirk, 1962;De Haan and Bolt, 1963) preferred to start with the sample of clay already in contact with electrolyte, whereas others used dry clays (e.g., Bower and Goertzen, 1955;Posner and Quirk, 1964;Schofield and Talibuddin, 1948). There appears to be no significant difference in the magnitude of the results from either approach.…”

Section: Acw(_) _ Ac-(w~ or Acwmentioning

confidence: 99%

“…Since the bulk solution mu st be electrically neutral, this identity is true when AC+(w), the total increase in cation concentration, is substituted for AC_(w). The link between NIC and negative adsorption can be exploited to explain the relationship between work performed under the heading of negative adsorption of anions (Bolt and Warkentin, 1958;Wiklander, 1964) and that under the heading of water adsorption or water uptake (Dufey and Laudelout, 1976;Edwards and Quirk, 1962;Norrish, 1954). This link has not yet been explained adequately and can give an impression that two distinctly different phenomena exist: water adsorption and anion repulsion.…”

mentioning

confidence: 99%

Thermodynamic Characterization of Clay/Electrolyte Systems

Neal

Thomas

Truesdale

1982

Abstract--Clay/electrolyte systems are best characterized using a new variable, notional interfacial content (NIC). The NIC approach emphasizes that any division of the total amount of a species present in a clay/ electrolyte system into that part which 'belongs' to the clay and that part which 'belongs' to solution is necessarily arbitrary. It carries out this division by choosing one of the species as a reference species and defining it as being completely in a notional bulk solution which had the same composition, but not extent, as the real bulk solution. The NIC of each of the other species present, that is that part of their total amount not in the notional bulk solution, therefore represents the amounts of those species 'belonging' to the clay.The NIC concept is universal and hence encompasses several other, older terms. For example, by choosing different reference species, the variables hitherto called 'water adsorption' and 'negative adsorption' (which have been used to describe the same phenomenon) may be obtained. Similarly, certain earlier definitions of exchangeable and adsorbed cations (some being pragmatic are not included) as well as that of ion surface excesses may be accounted for. The NIC approach thus provides a rationalization of several earlier terms which are, in fact, plagued by a multiplicity of definition which makes their use very complicated.