Influences of Dilute Organic Adsorbates on the Hydration of Low-Surface-Area Silicates

Competitive adsorption of dilute quantities of certain organic molecules and water at silicate surfaces strongly influence the rates of silicate dissolution, hydration, and crystallization. Here, we determine the molecular-level structures, compositions, and site-specific interactions of adsorbed organic molecules at low absolute bulk concentrations on heterogeneous silicate particle surfaces at early stages of hydration. Specifically, dilute quantities (similar to 0.1% by weight of solids) of the disaccharide sucrose or industrially important phosphonic acid species slow dramatically the hydration of low-surface-area (similar to 1 m(2)/g) silicate particles. Here, the physicochemically distinct adsorption interactions of these organic species are established by using dynamic nuclear polarization (DNP) surface-enhanced solid-state NMR techniques. These measurements provide significantly improved signal sensitivity for near-surface species that is crucial for the detection and analysis of dilute adsorbed organic molecules and silicate species on low-surface-area particles, which until now have been infeasible to characterize. DNP-enhanced 2D Si-29{H-1}, C-13{H-1}, and P-31{H-1} heteronuclear correlation and 1D Si-29{C-13} rotational-echo double-resonance NMR measurements establish hydrogen-bond-mediated adsorption of sucrose at distinct nonhydrated and hydrated silicate surface sites and electrostatic interactions with surface Ca2+ cations. By comparison, phosphonic acid molecules are found to adsorb electrostatically at or near cationic calcium surface sites to form Ca(2+)phosphonate complexes. Although dilute quantities of both types of organic molecules effectively inhibit hydration, they do so by adsorbing in distinct ways that depend on their specific architectures and physicochemical interactions. The results demonstrate the feasibility of using DNP-enhanced NMR techniques to measure and assess dilute adsorbed molecules and their molecular interactions on low-surface-area materials, notably for compositions that are industrially relevant.