Molecular order and disorder of surfactants in clay nanocompositeswz

Nanocomposites of clay minerals and surfactants are of practical importance in many fields. Beside the use of these systems for pollution removal from waste water, they can also be used as catalysts, to stabilize clay formations while drilling for hydrocarbons, or for improving the strength of polymers. The nanocomposites are obtained through exchange of natural counter ions by ionic surfactants, such as alkylammonium. Despite the importance of such nanocomposites, their behaviour is not understood. It is commonly assumed that the distance between the clay platelets (basal spacing) increases stepwise or linearly with increasing alkylammonium chain length. The traditional explanation for this observation is that the surfactants form all-trans conformations between the clay platelets, either in layers or tilted in a liquid crystalline ordered phase. However, this explanation has recently been debated. X-Ray diffraction measurements show a complex intercalation behaviour. For example, when basal spacings are measured, corresponding to values in-between monolayer and bilayer structures, then these values are usually attributed to layer charge heterogeneity. In this case, the explanation is that different layer charges within a clay sample cause a varying surfactant loading in the interlayers between two clay platelets and therefore an ensemble of monoand bilayer structures, but this has never been directly confirmed experimentally. As X-ray diffraction peaks for such states overlap, the resulting peak is broad and difficult to interpret. Molecular simulations are ideally suited for obtaining more insight in the intercalation behaviour of surfactants in clay minerals. Several simulation studies have been carried out to compute the basal spacing for some clay minerals and some alkylammonium surfactants. These simulation studies show increasing disorder (phase transitions) of the surfactants in mica clay minerals with increasing temperature. Our simulations are complementary to the previous studies, in the sense that they cover an extensive range of surfactant chain lengths. Also, in contrast with mica, we consider clay minerals that are able of swelling. We find, for the first time, full quantitative agreement between the experimental and simulated swelling behaviour of alkylammonium in montmorillonite and vermiculite clay minerals. Based on our simulations, we give a new explanation for the gradual increase of the clay platelets distance. We show evidence that this is a true physical phenomenon, caused by disorder of the surfactant molecules. Out of several available molecular clay models, we choose a clay model that has proven to be successful in predicting the basal spacing of natural clay minerals with water, the swelling capabilities of clay minerals, and hysteresis in clay swelling. In our simulations, we use molecular dynamics to move the flexible alkylammonium molecules with a time step t of 1.0 fs at a constant temperature T of 298 K using the Nosé-Hoover thermostat. The SHAKE algorithm is included to constrain all bond lengths of the alkylammonium molecules. We describe the interactions between alkylammonium molecules and the clay mineral using the OPLS forcefield. For a correct sampling of the phase space of the long-chain alkylammonium molecules, we found it essential to use the configurational bias Monte Carlo scheme. Our simulations are performed in the isobaric–isothermal (NPzT) ensemble, with Pz=1.0 atm. The basal spacing changes are carried out with a Monte Carlo algorithm,