Dielectric Properties of Montmorillonites Saturated by Bivalent Cations

R. Calvet
1975 Clays and clay minerals  
This study deals with the analysis of dielectric measurements made on montmorillonites saturated by bivalent cations. These measurements are performed between -150~ and +30~ at frequencies ranging from 300 to 10,000 Hz. Their interpretation is based on a numerical analysis allowing determination of the dielectric losses due to free charge carriers conductivity phenomena and losses due to relaxation phenomena. The free charge carriers conductivity is due to the movement of protons. It depends
more » ... tons. It depends very much on the nature of compensating cations and on the water content and seems to be closely related to the characteristics of the swelling. Two types of relaxation phenomenon are described: a Debye relaxation due to electric dipole rotations and a Maxwell-Wagner relaxation due to heterogeneity effects. The analysis of the first phenomenon leads to the examination of the values of the relaxation time. It appears that the rotations of water molecules are difficult with bivalent cations. This essentially is shown by the high activation energy of the phenomenon. The discussion of these parameters shows that the state of adsorbed water molecules are certainly different as compared to the state of water molecules in ice or in liquid water. The characteristics of the second relaxation phenomenon are closely dependent on the free carriers charge conductivity. Among the physico-chemical methods used in clay mineral studies, dielectric measurements offer many possibilities for obtaining information about the surface properties of layer-silicates. Dielectric measurements have been especially useful in the analysis of some characteristics of adsorbed water on montmorillonite (Fripiat et al., 1965; Weiler and Chaussidon, 1968; Mamy, 1968) . In an alternating electric field, a clay sample is characterized by a complex dielectric constant: E* = E'-je", where e' is the real dielectric constant of the material and E" is the loss factor. This last term is proportional to the energy dissipated in the dielectric medium (Bottcher, 1952) , and is made up of several components due to conductivity and relaxation phenomena. As a consequence, the results of the measurements cannot be directly interpreted, and the experimental curves log (C)= f (reciprocal temperature) have to be numerically decomposed into different parts corresponding to the conductivity and relaxation phenomena. EXPERIMENTAL RESULTS The clay used is the montmorillonite of Camp Berteau, the formula of which is (Si4) [(All.46 ,3 + Feo.15 )3 + Mgo.33 ]2 + O10 (OH) 2 M+39 ' The measured exchange capacity is 116 m-equiv,/ 100 g of clay dried at 1000~ Dielectric properties were determined on clay samples made of pressed powder (30 kg/cm2). These samc.c.~l. 23 4 pies were equilibrated over sulfuric acid solutions in order to fix their water content. The measurements were made between -150~ and +30~ for the frequencies: 300, 500, 1000, 2000, 5000 and 10,000 Hz. The technological details of the cell used have been published by Mamy (1968). The apparatus consists of a generator Wayne-Kerr S 121, an electrical bridge Wayne-Kerr B 221 and a wave analyser Wayne-Kerr A 321. The experimental results are represented by two sets of curves: log C =f(1/T) and tO6 = e"/e'= f'(T). Such curves are obtained for each cation (Mg 2 § Ca 2+, Sr 2 § Ba 2 § at one frequency, for a given water content (it will be always expressed in g water/100 g of clay dried at 250~ The variation of log E" are given in Fig. 1 . In previous work (Calvet, 1972 a) we have shown that it was possible to describe two conductivity phenomena for montmorillonites saturated by monovalent cations (Mamy, 1968) . One of these conductivities is dominant at low temperatures (< -100~ and is designated by al, the other is dominant at high temperatures and is designated by a2. The corresponding dielectric losses are e~ and e~. We have also described two absorption phenomena. The first, is weak and can be attributed to a Debye relaxation (e~ = dielectric loss), the second, much stronger is only visible for water contents greater than 9"5~o and is due to a Maxwell-Wagner relaxation (dielectric loss = E~). Thus, the total dielectric loss e" is: 257 (1)
doi:10.1346/ccmn.1975.0230401 fatcat:k2wg2r6wofccfn5ffajrmot5qu