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Dehydration of water from surface Mg²⁺ is most likely the rate-limiting step in the dolomite growth at low temperature. Here, we investigate the role of polysaccharide in this step using classical molecular dynamics (MD) calculations. Free energy (potential of mean force, PMF) calculations have been performed for water molecules leaving the first two hydration layers above the dolomite (104) surface under the following three conditions: without catalyst (Fig. 1), with monosaccharide (mannose) and with oligosaccharide (three units of mannose) (Fig. 2). MD simulations reveal that there is no obvious effect of monosaccharide in lowering the dehydration barrier for surface Mg²⁺. However, we found that there are metastable configurations of oligosaccharide, which can decrease the dehydration barrier of surface Mg²⁺ by about 0.7-1.1 kcal/mol. In these configurations, the molecule lies relatively flat on the surface and forms a bridge shape. The hydrophobic space near the surface created by the non-polar –CH groups of the oligosaccharide in the bridge conformation is the reason for the observed reduction of dehydration barrier.

Our simulations have focused on the dehydration of surface Mg and we have shown that the polysaccharide is able to decrease the dehydration barrier for the surface Mg by its hydrophobic –CH groups, which is a possible reason why polysaccharide is important in the dolomite growth at low temperature. Nevertheless, the pathway for carbonate ions to supplant the surface water molecules in the presence of polysaccharide is complex and needs to be further investigated.