Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Figure 2: Schematic on the effects of two
different solvents on the surface adsorption
of hard-sphere ions. Our results indicate
that the driving force of ions to interfaces
is model dependent [2].
continuum theory to explain the surface tension of the
halide salts. Levin and co-workers [Physical Review
Letters 103, 257802 (2009)] have used a polarizable
ion dielectric continuum theory to also explain the
surface tension data of halide salts. Interestingly, the
single ion potential of mean force (PMF) for iodide
agrees quite well (regarding the depth of the shallow
local minimum at the interface) with these two very
different approaches. Our Recent DFT calculations
that intrinsically include polarization also show good
agreement with the aforementioned PMFs for iodide
[10]. The question regarding the role of polarization
and the connection of molecular simulation to
dielectric continuum theory still remains. To this end,
we constructed molecular simulations of charged hardspheres
(HS) in model solvents (SPC/E and
Stockmayer fluid) where the separation of cavitation
and dielectric response is clear (see Figure 2). We
proposed a dielectric theory that contains the free
energy of cavitation (proportional to volume of HS), and charging (proportional to square of the
charge) where the dielectric constants and volume scaling was determined by molecular
simulation [2]. Our results indicate that the underlying physics of anion adsorption is dependent
on the nature of the solvent (e.g. SPC/E or Stockmayer). The symmetric nature of the
Stockmayer fluid precludes the existence of a surface potential and it is only cavitation that
determines surface activity. SPC/E water has a surface potential due to the broken symmetry of
both the cavity and interface. It was shown in our study [2] and a recent independent study
[Arslagarin and Beck, Journal of Chemical Physics 136, 104503 (2012)] that this notional
“surface potential” provides an additional crucial driving force that allows point-charge models
in SPC/E to give rise to surface adsorption.
Future directions
The role of this notional “surface potential” is currently being explored in the context of DFT
and the use of inherently polarizable models for the solute and solvent (with G.K. Schenter).
Future research will be extended to elucidation of the solvation and surface properties of more
complex anions such as the oxyanions (with G.K. Schenter and J. Fulton) such as ClO3 - and
BrO3 - where both exhibit very interesting behavior in the context of the Hofmeister series. We
are currently investigating the role of hydrophobic solvation with John D. Weeks. We are using
multiple representations of the molecular interaction for water (e.g. TIP5P, SPC/E, flavors of
DFT) to understand the structural response to a model hydrophobe and the signatures of volume
to area scaling.
Acknowledgements. This work was performed with Yan Levin (Brazil) John D. Weeks (U.
Maryland), D.J. Tobias (UC-I), Ilja Siepmann (U. Minn.), and I.-F.W. Kuo (LLNL), Greg
Schenter (PNNL), Liem Dang (PNNL), Marcel D. Baer (PNNL), Abe Stern (UC-I), Joost
VandeVondele (U. Zurich), Greg Kimmel (PNNL), John Fulton (PNNL), Roger Rousseau
(PNNL) and Shawn M. Kathmann ( PNNL).
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