(484b) Aqueous Carbon Dioxide Environments Under Silica Nano Confinement

         The
CO₂ geological capture and sequestration have been considered potential
approaches toward the mitigation of the greenhouse effects of CO₂ release
into the atmosphere, a process that relies on the low hydraulic permeability of
caprocks resulting from fluid-substrate interfacial
and confinement behavior. Fluids at interfaces and under confinement exhibit
microstructural, dynamical, and thermophysical
behavior quite different from their bulk counterparts, a feature that also highlights
the inherent inability of current modeling approaches to describe properly the
fluid-caprock interfacial mechanisms underlying the
geological CO₂ sequestration.

         Experimental
and modeling studies most often focus on the interactions of either water-rich
CO₂ or pure CO₂ environments with representative caprock substrates, and the ensuing modeling is based on
that assumption, i.e., the impact of the CO₂-contaminants are
rarely addressed, despite the fact that CO₂ always carries small
quantities of 'contaminants' including SO₂, H₂S, NO_x whose impact in terms of rock interactions
cannot be ignored.

         The
overarching goal of this investigation is to address key fundamental issues
regarding the behavior at aqueous-CO₂ solutions at the fluid-silica
interface, including (a) how the degree of surface polarity and/or surface
mismatch affect/s the interfacial structure, (b) how the overlapping of
interfacial structures affects the confined fluid composition, and (c) how
contaminants might affect the preferential adsorption and composition of the
interfacial layers.  For that
purpose we carry out a molecular-based study of the microstructural and
dynamical behavior of CO₂-aqueous solutions at silica surfaces under
extreme confinement at conditions relevant to geologic capture and sequestration of carbon
dioxide, involving
either water-rich CO₂ or CO₂-rich phases.  This effort comprises (globally)
isobaric-isothermal and (locally) grand-canonical molecular dynamics
simulations of aqueous-CO₂ systems whose unlike-pair interactions
are described by a recently optimized force-field parameterization that provides
an accurate and simultaneous representation of the water-rich CO₂ and
CO₂-rich phases at LLE conditions [1].  Moreover, we employ three types of slit-pore configurations to
represent two extreme cases of surface polarity, and a mismatched pair of
plates, to interrogate the fluid behavior at and confined between heterogeneous
surfaces.  Based on this study we illustrate
how the interplay between these types of fluid-surface interactions and extreme
fluid confinement, i.e., strong
overlapping of interfacial structures, can induce a drying out of the pore
environment whose immediate consequence is a significant enhancement of the pore
CO₂
concentration relative to that of the corresponding bulk environment [2].  This behavior highlights some often overlooked implications including (a)
the pore fluid environments cannot be represented by that of a bulk counterpart
at the prevailing pore state conditions when either interpreting or modeling
the process at a macroscopic level, (b) the chemical processes occurring in nano-pore aqueous environments strongly depend on the
nature of the confining mineral surfaces, (c) the fluid environment in contact
with the mineral surface behaves considerably different from either that a few
molecular diameters away from the mineral surface or that representing the corresponding
bulk, and (d) the competitive co-adsorption of CO₂ contaminants at
the mineral surfaces can radically modify the interfacial equilibrium and
dynamical behavior, and consequently, the mineral surface chemical
stability.

            [1]       Vlcek, L.;
Chialvo, A. A.; Cole, D. R. Journal of
Physical Chemistry B 2011, 115, 8775.

            [2]       Chialvo, A.
A.; Vlcek, L.; Cole, D. R. Journal of
Physical Chemistry C 2012, Submitted.