Hydration repulsion is the universal force that acts between well-solvated surfaces in water and balances other surface attractions such as the van der Waals forces in the nanometer range which ultimately prevents the aggregation and precipitation of suspensions. Although this phenomenon is observed through a multitude of experimental methods in various systems, an atomistic-level understanding of the mechanism remains elusive. Furthermore, it is well known that temperature and specific ions can drastically change the hydration repulsion. Using atomistic molecular dynamics simulations to calculate accurate pressures and thermodynamic energetics for approaching carbohydrate surfaces in DI water or in salt solutions at different temperatures, we show here that hydration repulsion decreases at high temperatures due to lowering of the dehydration energy for the first hydration layer. In salt solutions, the carbohydrate-complexing Ca2+ ions increase hydration repulsion due to extended water structures. Experimental studies investigating the colloidal stability and swelling behavior of polysaccharide-coated nanoparticles confirm the optimal hydration repulsion for the polysaccharide coatings in CaCl2 solution; when subjected to high temperatures in DI water the polysaccharide coating de-swells, and at the same elevated temperatures in MgCl2 solution the nanoparticles lose their colloidal stability and aggregate. Both instances of de-swelling and aggregation indicate less hydration repulsion. Our results provide mechanistic evidence relating temperature and specific ion effects on the hydration repulsion, which has great implications for regulating colloidal stability of suspensions in all aqueous systems.

Hydration repulsion between carbohydrate surfaces mediated by temperature and specific ions

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.