We determined the effect of selected counterions (Cl-, Br-, and HSO4 -) on the sorption of the cationic surfactant hexadecyltrimethylammonuium (HDTMA) on clinoptilolite zeolite and on the subsequent sorption of chromate by HDTMA−zeolite. The HDTMA sorption on the zeolite, as characterized by the Langmuir sorption maximum, followed the trend HDTMA-Br > HDTMA-Cl > HDTMA-HSO4 (208, 151, and 132 mmol/kg, respectively). The same counterion trend was observed for HDTMA sorption on KGA-1 kaolinite. Measurement of counterion sorption indicated that HDTMA-Br and HDTMA-Cl formed complete bilayers on the zeolite, whereas HDTMA-HSO4 showed less than full bilayer formation. Competitive sorption between HDTMA-Br and HDTMA-Cl on the zeolite also showed a preference for the Br- counterion. The counterion stabilization of HDTMA admicelles on the zeolite surface follows the same trends as the counterion stabilization of micelles in solution. Chromate sorption was also strongly influenced by the HDTMA−zeolite counterion, with chromate sorption maxima decreasing in the order HDTMA-HSO4 > HDTMA-Cl > HDTMA-Br (28, 16, and 11 mmol/kg, respectively). The sorption of chromate and other divalent anions on HDTMA−zeolite results from a combination of entropic, Coulombic, and hydrophobic effects, all of which are functions of the initial HDTMA counterion. In the design of surfactant-modified clays and zeolites for environmental applications, the strong influence of the surfactant counterion must be considered.

No abstract

The sorption of nonpolar hydrophobic organic compounds by soil organic matter has long been attributed to a partitioning mechanism, with the sorption coefficient proportional to the fractional organic carbon content of the soil. However, deviations from this linear proportionality have been observed and reported in the literature by many authors. In our study a natural zeolite was modified with a cationic surfactant to achieve different fractional organic carbon contents and different surfactant molecule configurations on the surface. The sorption of perchloroethylene (PCE) by the surfactant-modified zeolite (SMZ) was found to be dependent on the bound surfactant molecule configuration as well as on the fractional organic carbon content. Below monolayer coverage by the surfactant, the PCE sorption coefficient on SMZ was proportional to the fractional organic carbon content. Above monolayer coverage, increasing fractional organic carbon content resulted in minimal further increase in the PCE sorption coefficient. The change in PCE sorption behavior was attributed to the structural differences between sorbed surfactant monolayers and bilayers. The surfactant surface configuration has a significant impact on the effective volume and density of the bound organic phase that is responsible for partitioning nonpolar organic compounds. The ratio of the organic carbon-based distribution coefficient (K oc) for the monolayer versus that for the bilayer systems was 1.7, similar to the estimated bilayer to monolayer hydrocarbon density of 1.3. Our results reinforce the notion that the structure of natural organic matter as well as its quantity controls the sorption of nonpolar organics to soils and sediments.

A quantum critical point is a point in a system's phase diagram at which an order is completely suppressed at absolute zero temperature (T). The presence of a quantum critical point manifests itself in the finite-T physical properties, and often gives rise to new states of matter. Superconductivity in the cuprates and in heavy fermion materials is believed by many to be mediated by fluctuations associated with a quantum critical point. In the recently discovered iron-pnictide superconductors, we report transport and NMR measurements on BaFe 2 À x Ni x As 2 (0rxr0.17). We find two critical points at x c1 ¼ 0.10 and x c2 ¼ 0.14. The electrical resistivity follows r ¼ r 0 þ AT n , with n ¼ 1 around x c1 and another minimal n ¼ 1.1 at x c2 . By NMR measurements, we identity x c1 to be a magnetic quantum critical point and suggest that x c2 is a new type of quantum critical point associated with a nematic structural phase transition. Our results suggest that the superconductivity in carrier-doped pnictides is closely linked to the quantum criticality.