Molecular order and disorder of surfactants in clay nanocomposites

Abstract

Disorder of intercalated surfactant molecules in clay minerals causes gradual swelling, rather than commonly assumed swelling in discrete steps.Nanocomposites of clay minerals and surfactants are of practical importance in many fields.1 Beside the use of these systems for pollution removal from waste water, 2 they can also be used as catalysts, 3 to stabilize clay formations while drilling for hydrocarbons, 4 or for improving the strength of polymers. 5,6The nanocomposites are obtained through exchange of natural counter ions by ionic surfactants, such as alkylammonium. 7Despite 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. 8 The traditional explanation for this observation is that the surfactants form all-trans conformations between the clay platelets, 8 either in layers or tilted in a liquid crystalline ordered phase. However, this explanation has recently been debated. 9 X-Ray diffraction measurements show a complex intercalation behaviour.7 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.10 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. 6,[11][12][13] These simulation studies show increasing disorder (phase transitions) of the surfactants in mica clay minerals with increasing temperature.6,12 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, 6,14,15 we choose a clay model 15 that has proven to be successful in predicting the basal spacing of natural clay minerals with water, 15,16 the swelling capabilities of clay minerals, 17,18 and hysteresis in clay swelling. 17,19 In our simulations, we ...

Structure and dynamic properties of water saturated CTMA-montmorillonite: molecular dynamics simulations

“…2a and 6a). The alkyl chains have a smaller percentage of gauche conformations with layered configuration due to confinement, which is consistent with previous simulation of dry OMt (Tambach et al, 2006). This also indicates that a small quantity of water scarcely affects the alkyl chain configuration at very low loading levels.…”

“…Layered arrangements of alkylammonium chains in the interlayer space of montmorillonite (Mt) have been discovered in several studies (Heinz et al, 2003(Heinz et al, , 2005(Heinz et al, , 2007Zeng and Yu, 2008;Zeng et al, 2004). Furthermore, the conformation of alkyl chains has been studied in detail and different distribution of gauche-trans conformations with respect to basal spacing has been revealed (Heinz et al, 2007;Tambach et al, 2006). These simulations reproduced XRD measurements and revealed the layering configuration of alkyl chains.…”

“…The average fraction of gauche conformations at normal temperature is found to be between the value in a hydrocarbon crystal (about 5% gauche conformation) and those in liquid alkanes (about 25% gauche conformation) (Vaia et al, 1994). Compared to previous simulations of dried OMt (15-25% gauche conformations of CTMA + at 1.00 CEC) (Heinz et al, 2007;Tambach et al, 2006), our results show a more ordered conformation, which are consistent with the experimental conclusions (Kung and Hayes, 1993). It can be explained that water molecules enlarge the interlayer space and make alkyl chains easier to stretch to all-trans conformation.…”

“…Therefore, in the water saturated systems, the arrangement of surfactant is different from the traditional layered configurations as shown in dry systems. The alkyl chains form an inclined paraffin-type arrangement with average tilt angle of 35°with the surface (Liu et al, 2007;Tambach et al, 2006;Zeng et al, 2003Zeng et al, , 2004. This arrangement turns the interlayer space environment into a more hydrophobic medium which contributes to the adsorption of organic contaminants, corresponding to experimental studies (Zhu et al, 2008a).…”

“…Recently, molecular simulations were introduced into the study of the structure of dried OC and proved to be powerful methods in complementing experiments, especially in probing the details about molecular arrangements and conformations at the atomic level (Greenwell et al, 2005;Hackett et al, 1998;He et al, 2005;Heinz et al, 2003Heinz et al, , 2005Heinz et al, , 2007Liu et al, 2007;Tambach et al, 2006;Zeng and Yu, 2008;Zeng et al, 2003Zeng et al, , 2004. Layered arrangements of alkylammonium chains in the interlayer space of montmorillonite (Mt) have been discovered in several studies (Heinz et al, 2003(Heinz et al, , 2005(Heinz et al, , 2007Zeng and Yu, 2008;Zeng et al, 2004).…”