Molecular modelling of the mechanism of action of organic clay-swelling inhibitors

It is well known that the sodium smectite class of clays swells macroscopic ally in contact with water, whereas under normal conditions the potassium f orm does not. In recent work using molecular simulation methods, we have pr ovided a quantitative explanation both for the swelling behaviour of sodium smectite clays and the lack of swelling of potassium smectites [1]. In the present paper, we apply similar modelling methods to study the mechanism o f inhibition of clay-swelling by a range of organic molecules. Experimentally, it is known that polyalkylene glycols (polyethers) of inter mediate to high relative molecular mass are effective inhibitors of smectit e clay swelling. We use a range of atomistic simulation techniques, includi ng Monte Carlo and molecular dynamics, to investigate the interactions betw een a selection of these compounds. water, and a model smectite clay minera l. These interactions occur by means of organised intercalation of water an d organic molecules within the galleries between individual clay layers. The atomic interaction potentials deployed in this work are not as highly o ptimised as those used in our clay-cation-water work [1]. Nevertheless, our simulations yield trends and results that are in qualitative and sometimes semi-quantitative agreement with experimental findings on similiar (but no t identical) systems. The internal energy of adsorption of simple polyether s per unit mass on the model clay is not significantly different from that For water adsorption; our Monte Carlo studies indicate that entropy is the driving force for the sorption of the simpler organic molecules inside the clay layers: a single long chain polyethylene glycol can displace a large n umber of water molecules, each of whose translational entropy is greatly en hanced when outside the clay. Hydrophobically modified polyalkylene glycols also enjoy significant van der Waals interactions within the layers which they form within the clay galleries. In conjunction with experimental studies, our work furnishes valuable insig hts into the relative effectiveness of the compounds considered and reveals the generic features that high performance flay-swelling inhibitors should possess. For optimal inhibitory activity, these compounds should be reason ably long chain linear organic molecules with localised hydrophobic and hyd rophilic regions along the chain. On intercalation of these molecules withi n the clay layers, the hydrophobic regions provide an effective seal agains t ingress of water, while the hydrophilic ones enhance the binding of the s odium cations to the clay surface; preventing their hydration and the ensui ng clay swelling.