Molecular Dynamics Simulation Of Interlayer Structure Of Vermiculite

In nature,the negative layer charges of vermiculite are usually balanced by interlayer metal cations.They can be replaced by organic cations through ion exchange,resulting the organo-vermiculites.The intercalation of organic cations changes vermiculite from hydrophilic to organophilic,thus improve the uptake capacity for organic matter.However,the microscopic properties of vermiculites and organo-vermiculites are still unknown.In order to get a well understanding of the adsorption mechanism of vermiculite to metal cations and organic cations,molecular dynamics simulations were performed to study the swelling behavior,interlayer structures and dynamics of interlayer species.As for Na-vermiculite,the results indicate that water molecules can form monolayer and bilayer hydration state,and Na+ ions form inner-sphere and outer-sphere complexes accordingly.The self-diffusion coefficient of Na+ ions and water molecules increase as the water content increasing,but they are both smaller than the bulk values,indicating that the mobility of interlayer species is restricted by vermiculite to some degree.At the same hydration state,the mobility of interlayer species of vermiculite is slightly lower than that of smectite,the different amount of layer charge and different layer-charge distribution between vermiculte and smectite are responsible for this difference.As for alkylammonium-intercalated vermiculite,the layering behavior of alkylammonium is uncovered.The alkyl chains form monolayers,bilayers and pseudo-trimolecular layers as the carbon-chain length increasing.The layering behaviors of alkyl chains are similar in alkylammonium-intercalated vermiculites and smectites,and they are controlled by both the amount of layer charge and the carbon-chain length.The ammonium groups were fixed firmly over two locations:over the six-member rings(?90%)and over the substitution sites(?10%).In smectites,all ammonium groups were located above the six-member rings.This difference is caused by different layer-charge distributions between vermiculites and smectites.The ammonium groups have low mobility and the alkyl groups are slightly more mobile.The research of the adsorption of alkylammonium-intercalated vermiculite for benzene indicates that the appearance of benzene in the interlayer space has no influence on the layering behavior of alkyl chains and the distribution of ammonium groups.Similarly to the situation of alkylammonium-intercalate smectite,the sorption mechanisms of benzene in the interlayer of alkylammonium-intercalated vermiculite can be divided into surface adsorption and partitioning,and they are determined by the layer charge and the carbon-chain length.The two mechanisms generally work together to influence the adsorption of the organoclay for benzene.The motions of interlayer species are restricted,so they have very low mobility.The order of mobility of interlaye species is:benzene>alkyl groups>ammonium groups.Vermiculite limits the motion of organic cations as well as metal cations,which provides theoretical basis for using vermiculite to absorb harmful metal cations and organic cations.The microscopic structural information will be useful for the further research of the adsorption of vermiculite for other kinds of metal cations and positive charged organic molecules.