Traditional and novel methods for estimating the layer charge of smectites

Several methods for characterizing layer charge of smectites were compared. Ten samples of various Na+- and Li+-saturated smectites (four montmorillonites, one nontronite, two saponites and three hectorites) were used. The smectites included minerals isolated from bentonites, as well as some synthetic materials. Cation exchange capacities (CECs) were determined as a reference parameter. These data express the total charge of cations that balance the negative charge sites of smectites. Four CEC methods were used, based on ion exchange reactions with solutions of barium chloride, ammonium acetate, methylene blue and a chelate complex of Cu(II). The layer charge distribution in the octahedral and tetrahedral sheets was estimated from structural formulae that were calculated from chemical analyses. Charge calculation from a structural formula is sensitive to mineral impurities and depends on how the formula is calculated. Moreover, only trace deviations from an ideal structure, such as an inhomogeneous octahedral sheet may significantly affect charge values calculated from structural formulae. Layer charge distribution and average charge densities were determined using the alkylammonium method. After intercalation of n-alkylammonium ions, the interlayer distance depends on the length of alkyl chains and the layer charge of the mineral. The charge distributions of all smectite samples were heterogeneous. The layer charge determined by the alkylammonium method reflects the permanent charge. The estimated CECs calculated from alkylammonium layer charge values were systematically lower than measured CEC values, which include contributions from both the permanent and variable charge sites and some factors related to the alkylammonium method itself. The layer charge distribution was qualitatively estimated using the visible spectra of methylene blue/smectite suspensions. Trends in the spectra indicate that dye spectral properties that reflect molecular aggregation can potentially be used to qualitatively estimate or compare the layer charges of smectites. All the methods include some advantages and disadvantages and differ in their requirements for sample type, purity and composition, time and expenses needed for the analysis. In general, method selection depends on the specific application and whether the need is to characterize layer charge, charge density, charge distribution, or for comparing the charge of similar samples or samples after a chemical treatment or modification.