Basic Research Needs for Geosciences - Energetics Meetings and ...

CROSSCUTTING ISSUE:THERMODYNAMICS OF THE SOLUTE-TO-SOLID CONTINUUMCourtesy of Alexandra Navrotsky, UC DavisFigure 47: Experimentally measured enthalpies of formation for a variety of iron oxide minerals relative to bulkhematite plus liquid water as a function of surface area. The crossovers in energy for both anhydrous and hydrousconditions show that different phases become stable at the nanoscale. This reflects competition among the closelybalanced energetics of polymorphism, surface energy, and hydration and indicates a complex energy landscape ofaccessible states (more discussion is found in Majzlan et al. 2003 and Mazeina et al. 2006).defined, though again different from those of the bulk phase. Thus, for example, in the ironoxides, which are common in the geological environment, maghemite rather than hematite, andferrihydrite rather than goethite, may dominate the chemistry and transport of minor constituents,such as actinides, which may themselves also form colloidal phases.Colloid chemistry was developed by applying concepts borrowed from continuum mechanics tothe nanoscale: electrical double layer, point of zero charge, Gibbs-Thomson effect, etc. Theseconcepts work amazingly well considering that they relate the notion of uniform distributions offorce and charge on spherical surfaces to particles which are inherently atomistic, faceted, andheterogeneous. If one distinguishes nanoscience from colloid science, it is that the formerfocuses more on molecular scale interactions and structural heterogeneity through its use ofmodern spectroscopic and computational techniques to build an atomistic description. But thechallenge remains to link this microscopic structural description to thermodynamics anddynamics.A specific example of a quasi-thermodynamic description of chemical reactions at the solidsolutioninterface is given by so-called “surface complexation models.” These models quantifyreactions between solutes and surface species with free energy changes that include, in additionto an intrinsic chemical term, a coulombic term calculated on the basis of electrical double layertheory. In effect the coulombic term is an activity coefficient for the surface species that isneither unity (as that of a pure solid phase) nor that of a solute surrounded by water molecules.This approach also has worked remarkably well in that the average electrical potential calculatedfor a continuous smooth surface is applied to individual molecules. But these models are limitedin their applications since they do not explain the “chemical reality” of many surfacephenomena. For example, the adsorption of solutes on surfaces can result in the formation ofBasic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 167