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OCR for page 3358
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3358-3364, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Surface geochemistry of the clay minerals
GARRISON SPOSITO*T, NEAL T. SKIPPERT, REBECCA SUMMONS, SUNG HO PARK*, ALAN K. SOPER§,
AND JEFFERY A. GREATHOUSE,
*Earth Sciences Division, Mail Stop 90/1116, Ernest Orlando Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720; "Department
of Physics and Astronomy, University College, Gower Street, London WC1E 6BT, United Kingdom; QISIS Facility, Rutherford Appleton Laboratory, Chilton,
Didcot, Oxfordshire OX11 OQX, United Kingdom; and Department of Chemistry, University of the Incarnate Word, 4301 Broadway, San Antonio, TX 78209
ABSTRACT Clay minerals are layer type aluminosilicates
that figure in terrestrial biogeochemical cycles, in the buff-
ering capacity of the oceans, and in the containment of toxic
waste materials. They are also used as lubricants in petroleum
extraction and as industrial catalysts for the synthesis of many
organic compounds. These applications derive fundamentally
from the colloidal size and permanent structural charge of
clay mineral particles, which endow them with significant
surface reactivity. Unraveling the surface geochemistry of
hydrated clay minerals is an abiding, if difficult, topic in earth
sciences research. Recent experimental and computational
studies that take advantage of new methodologies and basic
insights derived from the study of concentrated ionic solutions
have begun to clarify the structure of electrical double layers
formed on hydrated clay mineral surfaces, particularly those
in the interlayer region of swelling 2:1 layer type clay minerals.
One emerging trend is that the coordination of interlayer
cations with water molecules and clay mineral surface oxygens
is governed largely by cation size and charge, similarly to a
concentrated ionic solution, but the location of structural
charge within a clay layer and the existence of hydrophobic
patches on its surface provide important modulations. The
larger the interlayer cation, the greater the influence of clay
mineral structure and hydrophobicity on the configurations of
adsorbed water molecules. This picture extends readily to
hydrophobic molecules adsorbed within an interlayer region,
with important implications for clay-hydrocarbon interac-
tions and the design of catalysts for organic synthesis.
The clay minerals are layer-type aluminosilicates, ubiquitous
on our planet in geologic deposits, terrestrial weathering
environments, and marine sediments (1, 2~. Their name de-
rives from the micrometer-sized particles into which they
crystallize. This small particle size, in turn, endows these
minerals with an important surface reactivity that plays a
major role in the terrestrial biogeochemical cycling of metals,
in the chemical homeostasis of the oceans, and in a broad
variety of managed processes, including oil and gas production,
industrial catalysis, pharmaceutical delivery, and radioactive
waste disposal. Metal nutrients such as K+ or Ca2+ are
retained in temperate-zone soils on negatively charged clay
mineral surfaces but eventually can be released for consump-
tion in the biosphere or for buffering these soils against excess
acidity brought in by applied fertilizers or contaminated
rainwater (34. Clay minerals precipitated from seawater in
nearshore depositional environments can similarly influence
the geochemical cycles of metal cations such as K+ (4), as well
as the oceanic buffering of atmospheric CO2 on a global scale
(5~. In engineered settings, clay mineral swelling promoted by
Na+ adsorption plays a significant role in petroleum extraction
PNAS is available online at www.puas.org.
(6) and in the construction of environmental liners (74. Major
impact on industrial organic synthesis comes in the many
designed catalysts developed from clay minerals with adsorbed
polymeric cations (8~.
Certain clay minerals are isostructural with mica (1, 2) but
are not as well crystallized because of random isomorphic
cation substitutions in their structure (9, 10~. These cation
substitutions lead to a negative net surface charge that induces
an electrical double layer on clay mineral surfaces when they
are exposed to aqueous electrolyte solutions (i.e., to natural
waters). Water molecules can be intercalated between clay
layers to create an interlayer ionic solution that causes swelling
phenomena related to electrical dale lever nronertie.~ (?
11, 12~.
-I -- r- -r ~a- ~ -A
The structure of the double layer that forms in swelling clay
mineral interlayers has been the object of much geochemical
research (12), but only in recent years, aided especially by
insights gained from studies of concentrated aqueous ionic
solutions (13), has the powerful tandem mix of spectroscopy
and molecular modeling been able to clarify matters. Two
particularly effective innovations have been isotopic-
difference neutron diffraction and Monte Carlo computer
simulation (13~. The present article is a brief account of our
own recent efforts in applying these two innovations to the
hydrated clay minerals, which are of widespread importance in
terrestrial surface geochemistry. The emphasis here is on a
molecular picture of clay hydrate interlayer structure, partic-
ularly the issues of how similar this structure is to that in
concentrated ionic solutions and how different it is from the
tetrahedral hydrogen-bonded network characteristic of liquid
water in bulk. This kind of fundamental understanding is
essential to improved modeling of global elemental cycles and
better design of engineered clay materials (5-8~.
Crystal Structures of Clay Minerals. Clay minerals are
stacked, polymeric sandwiches of tetrahedral and octahedral
sheet structures (3, 9-12~. They are classified first into "layer
types," differentiated by the number of tetrahedral and octa-
hedral sheets that have combined, and then into "groups,"
differentiated by the kinds of isomorphic cation substitution
that have occurred (10~. Layer types are sketched in Fig. lA,
and the groups are identified in Table 1. The 1:1 layer type
consists of one tetrahedral sheet fused to an octahedral sheet.
It is represented in Table 1 by the kaolinite group, whose
generic chemical formula is
ESinIAl4-nI lAln] + n2-4Fe(III)n3o~otoH)~ nH2O
where cations enclosed in square brackets are located in the
tetrahedral sheet, and the stoichiometric coefficients (ni, i =
Abbreviation: MC, Monte Carlo.
lTo whom reprint requests should be addressed at: Hilgard Hall
#3110, University of California, Berkeley, CA 94720-3110. e-mail:
gsposito@nature.berkeley.edu.
3358
OCR for page 3359
Colloquium Paper: Sposito et al.
A
AX b
~)~ o2
~Si4+
1:1 LAYER
(9~
R
FIG. 1. (A) Crystal structure of 1:1 and 2:1 layer type clay minerals,
where X (shaded circles) is usually OH and M can be Al, Mg, Fe, etc.
(B) Siloxane cavity in the basal plane of a tetrahedral sheet.
1, 2, 3), which may be fractions, are constrained by charge
neutrality and mass balance conditions given in Table 1. The
octahedral sheet has two-thirds cation-site occupancy (dioc-
tahedral sheet; full occupancy gives a trioctahedral sheet), and
the structural hydration coefficient n = 0, except for lo-A
halloysite (1 A = 0.1 nary), for which n = 4. Normally, there is
no significant isomorphic substitution for Si in the tetrahedral
sheet (nl = 4) or for Al in the octahedral sheet (n2 = 4), and,
therefore, no significant negative structural charge occurs in
this dioctahedral clay mineral (9, 10~.
The 2:1 layer type has two tetrahedral sheets fused to an
octahedral sheet. Three clay mineral groups having this struc-
ture are illite, vermiculite, and smectite (9, 10~. Their generic
chemical formula is
Cxtsin~Alg-n~]Alnl +n2-~3Fe(III)n3Fe(II)n4MgnsMn6o2o~oH)4
Proc. Natl. Acad. Sci. USA 96 (1999J 3359
where charge and mass balance constraints again appear in
Table 1. The symbol Cx represents x moles of monovalent
cation charge that balances negative structural charge created
by isomorphic replacement of tetrahedral Si by Al; dioctahe-
dral Al by Mg, Fe(II); or trioctahedral Mg by the unspecified
structural cation M (for example, Li in the trioctahedral
mineral hectorite). The layer charge x is thus the number of
moles of excess electron charge per chemical formula unit that
is produced by isomorphic cation substitutions (Table 1~. It is
balanced by cations adsorbed on or near the basal plane of a
tetrahedral sheet.
The three 2:1 groups are differentiated in two principal
ways. As indicated in Table 1, the layer charge generally
decreases in the order illite ~ vermiculite > smectite. The
vermiculite group is further distinguished from the smectite
group by a much greater degree of isomorphic substitution in
the tetrahedral sheet (9, 10~. Among dioctahedral smectites,
those for which substitution of Al for Si in the tetrahedral sheet
exceeds that of Fe(II) or Mg for Al in the octahedral sheet are
called beidellite, and those for which the reverse is true are
called montmorillonite (11~. The emphasis in the present
article will be on vermiculite and smectite clay minerals
because of their importance in terrestrial weathering processes
and designed industrial applications (5-8~.
Siloxane Surface Reactivity. The plane of oxygen ions
bounding each side of a 2:1 clay mineral layer (i.e., the basal
plane of a tetrahedral sheet) is called a siloxane surface (12~.
A reactive site associated with this surface is the hexagonal
cavity formed by the bases of six corner-sharing Si tetrahedra
(Fig. 1B). It has a diameter of ~0.26 nm and is bordered by six
oxygen ions, with a hydroxyl group rooted at the bottom in the
octahedral sheet. In dioctahedral clay minerals, this hydroxyl
group points toward the empty metal site in the octahedral
sheet whereas, in trioctahedral clay minerals, it points perpen-
dicularly to the siloxane surface (9~.
The reactivity of the siloxane surface depends on the nature
of the local charge distribution in the clay layer (12~. In the
absence of nearby isomorphic cation substitutions that create
negative charge, a siloxane surface functions only as a mild
charge donor. If isomorphic substitution of Al by Fe(II) or Mg
occurs in the octahedral sheet, the resulting excess negative
charge makes it possible for the surface to form reasonably
strong adsorption complexes with cations and water molecules.
If isomorphic substitution of Si by Al occurs in the tetrahedral
sheet, with the excess negative charge thereby localized much
nearer to the periphery of the siloxane surface, much stronger
adsorption complexes with cations and stronger hydrogen
bonds to vicinal water molecules become possible. Charged
sites also exist on the edges of clay mineral crystallites (12~.
Their role in clay mineral surface geochemistry ranges from
critical to subordinate as the layer type shifts from 1:1 to 2:1
(9, 11, 12~.
Cation adsorption complexes can be classified as either
inner-sphere or outer-sphere (3~. An inner-sphere surface
complex has no water molecule interposed between the sur-
face functional group and the small cation or molecule it binds
whereas an outer-sphere surface complex has at least one such
interposed water molecule. Outer-sphere surface complexes
thus comprise solvated adsorbed cations. Surface complexes
involving metal cations are illustrated in Fig. 2 for a 2:1 layer
type clay mineral.
Ions bound in surface complexes are distinguished from
those adsorbed in the diffuse portion of the electrical double
layer (also illustrated in Fig. 2) because the former species
remain immobilized on a siloxane surface over molecular time
scales that are long when compared wish, for example, the 4-10
ps required for a single diffusive step by a solvated ion in
aqueous solution (13~. The well known outer-sphere surface
complex formed by bivalent metal cations (for example, Ca2+)
in the interlayer region of montmorillonite (compare the left
OCR for page 3360
3360 Colloquium Paper: Sposito et al.
Table 1. Some clay mineral groups
Proc. Natl. Acad. Sci. USA 96 (1999)
Group Layer type Layer charge, x Structural charge balance* Structural mass balance*
Kaolinite 1:1 <0.01 x = 4 - nil i6=~ ni = 8!
Illite 2:1 1.4-2.0 x = 8 - nl + n4 + n5 + n6! >6=~ ni = 12t
Vermiculite 2:1 1.2-1.8 x = 8 - nl ~ n4 + nS + n6§ i6=~ ni = 12!
Smectite 2:1 0.5-1.2 x = 8 - nl + n4 + nS + n61 26_~ ni = 121
l
*Based on an O~o(OH)s (1:1) or 020~0H)4 (2:1) unit cell formula.
iSee text for definitions of ni (i = 1, . . ., 6~.
tThe formulas given are dioctahedral clay minerals with bivalent M in the octahedral sheet. For trioctahedral smectite and
vermiculite with monovalent M in the octahedral sheet, x = 2~8 - nl) - n2 - n3 + n6 and >6=~ ni = 14.
side of Fig. 2) is immobile on the molecular time scale of ~100
ps probed by electron spin resonance spectroscopy and by
quasielastic neutron scattering (14, 15~. These three types of
clay mineral surface species inner sphere complex, outer-
sphere complex, and diffuse-layer" represent different modes
of adsorption of aqueous cations that contribute to the for-
mation of an electrical double layer on charged siloxane
surfaces (3~.
Interlayer Surface Structure by Neutron Diffraction. The
surface species "cartooned" in Fig. 2 emerged conceptually
from the results of in situ spectroscopic studies (14, 15~. The
consensus of these studies is that hydrated smectite and
vermiculite interlayers are in many ways similar in structure to
a concentrated ionic solution (14~. Aqueous solution molec-
ular structure, in turn, has been explored especially well over
the same time-period by isotopic-difference neutron diffrac-
tion (13, 16~. In this methodology, isotopic substitution of one
or more diffracting atoms is performed (for example, substi-
tution of hydrogen by deuterium in interlayer water) to create
differences in coherent neutron scattering cross section that
facilitate locating the atom accurately in relation to its dif
CATION SURFACE
SPECIES ON 2:1
LAYER TYPE CLAY
MINERALS AND
THEIR RESIDENCE
TIMES BY SPECTRAL
METHODOLOGIES
~X'
Interlayer Complex
o
Cl)
o
c~
cn
WATER
MOLECULE (L)
7/~/~///~/~
TIME SCALE IN
SECONDS {log)
X-RAY,W
IR SPECTRA
NEUTRON
SCATTERING
fracting neighbors (13, 16~. Isotopic-difference neutron dif-
fraction methods are reviewed concisely by Skipper et al. (17)
for hydrated 2:1 clay minerals. Neutron diffraction studies of
the interlayers in these systems have already provided valuable
insight as to molecular structure in the electrical double layer
formed on siloxane surfaces (17-19~.
Fig. 3 is a visualization of the arrangement of water mole-
cules around Ca2+ adsorbed in the interlayer region of the
two-layer hydrate of trioctahedral vermiculite, based on the
results of H/D isotopic-difference neutron diffraction exper-
iments (18~. Because the water protons could be identified
separately from those in the clay mineral structure and were
distinguishable from other interlayer atoms, it was possible to
show rather clearly that the Ca2+ are octahedrally coordinated
to their nearest-neighbor water molecules, as also occurs in
concentrated, as opposed to dilute, aqueous solutions of CaCl2
(13, 184. Moreover, four of the solvating water molecules form
hydrogen bonds with the siloxane surfaces, as expected for clay
minerals with isomorphic substitutions only in the tetrahedral
sheets (12~. Because the two-layer hydrate of vermiculite has
about eight water molecules per Ca2+ in the interlayer region
ps ns ~s ms s
~ I 1 1 1 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
15 - 13 -I 1 -9 -7 -5 -3 -I O
ESR SPECTRA
N.IR ~PECTRA
_
RESIDENCE
L O15` ISC
OSC
tg DIFFUSE ION(DI)
INNER- SPHERE
COMPLEX ( ISC)
Mineral ~---~-
Externol Basal Plane Complexes
OUTER- SPHERE COMPLEXES (OSC]
T"Stern
Layer
FIG. 2. Cartoon of the three types of small cation adsorption by a 2:1 layer type clay mineral. The "Stern Layer" comprises only surface
complexes, which can form in the interlayer region (left) as well as on single siloxane surfaces (right). Characteristic residence time scales of the
three adsorbed species are compared at upper right to the time scales of in situ spectroscopic methods used to detect them.
OCR for page 3361
Colloquium Paper: Sposito et al.
_~
~9_:
I' -A
|,
-
FIG. 3. Visualization of Ca2+ (large black sphere) in an octahedral
salvation complex (water molecules, with smaller black spheres as O
and red spheres as H) in the interlayer region of Ca-vermiculite.
Portions of the opposing clay mineral layers are shown, with structural
protons also indicated in red. Compare to the left side of the cartoon
in Fig. 2.
(18), two water molecules per Ca2+ must be nonsolvating.
Skipper et al. (18) were able to find these water molecules
tucked into the cavities of the siloxane surface. Other neutron
diffraction studies of trioctahedral vermiculite hydrates have
also reported this phenomenon (17-19~.
These structural inferences are in accord with and improve
on the results of earlier neutron-scattering, electron spin
resonance "facilitated by Cu(II) doping], and x-ray diffraction
studies of the two-layer hydrate of Ca-vermiculite (20-22), the
last of which places Ca2+ precisely between a triad of surface
O on one siloxane surface and a hexagonal cavity in the other.
Neutron-scattering and electron spin resonance studies also
confirm the immobilization of the interlayer Ca(H2O)62+
salvation complex on a 100-ps timescale, which is an order of
magnitude longer than a diffusive time step for Ca2+ in
aqueous solution (13~.
Interlayer Surface Structure by Computer Simulation.
Monte Carlo (MC) computer simulations are well known as
essential components of research on aqueous ionic solutions
(134. The underlying paradigm in these simulations is to
construct intermolecular potential functions that represent
parametrically all of the known interactions in a system then
devise a strategy for sampling the phase space of the inter-
acting system to compute its chemical properties. In a typical
MC simulation, the configuration space of the system is
sampled randomly under the guidance of an algorithm based
in equilibrium statistical mechanics (23~. Convergence of the
Proc. Natl. Acad. Sci. USA 96 (1999J 3361
simulation is monitored by examining the stability of calcu-
lated system properties (for example, the layer spacing in the
case of hydrated clay minerals) as sampling proceeds.
The synergistic relationship between experiment and mo-
lecular modeling indigenous to the study of aqueous ionic
solutions has not been possible in the study of hydrated clay
minerals until very recently, directly after the appearance of
fourth-generation supercomputers and the emergence of con-
venient parametric models for water-smectite and cation-
smectite potential functions based on quantum mechanical
insight (24-26~. These developments encouraged the under-
taking of systematic simulation studies of low-charge smectite
hydrates, particularly the ubiquitous Wyoming montmorillo-
nites, which have both octahedral and tetrahedral charge sites
(6, 27-34~. Prototypical Wyoming montmorillonite corre-
sponds to At = 7.75, n2 = 3.75, n3 = n4 = 0, nS = 0.5, and n6 =
0 in the chemical formula given above for 2:1 layer type clay
minerals, with x = 0.75 as the layer charge. The results of these
simulations have generated a number of fundamental ques-
tions that require additional molecular-scale experimentation,
thus sustaining the theory-experiment dialogue that defines
fundamental geochemical research. The computer simulations
summarized in the present article were performed by using the
code MONTE (35), developed by N. T. Skipper and K. Refson,
with phase-space sampling strategies as described by Skipper
et al. (27, 28) and Chang et al. (25, 26~. Figs. 4 and 5 are
"snapshots" of equilibrium interlayer configurations based on
MC simulations (34) of the two-layer hydrates of Na- and
K-Wyoming montmorillonite. The species shown in Fig. 4 is
Na+ bound in an outer-sphere surface complex to an octahe-
dral charge site of the clay mineral. This visualization, like Fig.
3, confirms the spectroscopy-inspired cartoon in Fig. 2. Fig. 4
includes only water molecules in a salvation shell confined to
within 3.2 A of the central Na+, the individual Na-H2O
separations varying from 2.2 to 2.5 id. These vicinal water
molecules form a distorted octahedron, in agreement with
Na-H2O distances and coordination numbers determined for
concentrated NaCl solutions with the same H2O/Na ratio ~ 10
(13). Fig. 5 illustrates K+ bound in an inner-sphere surface
complex to an octahedral charge site on Wyoming montmo-
rillonite. Portions of both siloxane surfaces are shown, and, in
this case, water molecules both within and outside the primary
salvation shell of K+ (K-O separations varying from 2.8 to 3.7
\) are depicted. The coordination number of K+ with O is
eight, but two are contributed by oxygen ions in the siloxane
surface. Thus, K+ is in distorted cubic coordination with its
neighboring O. consistent with geometric concepts based on
the K+/O radius ratio (3~.
Strongly solvating monovalent cations like Na+ or Li+ have
a tendency to form only inner-sphere surface complexes with
tetrahedral charge sites and only outer-sphere surface com-
plexes with octahedral charge sites on smectites (28, 33~. This
trend can be related to partial desolvation facilitated by the
smaller distance of closest approach between an interlayer
cation and a charge site that exists for a tetrahedral as opposed
to an octahedral site, which necessarily lies deeper in the clay
layer. For large cations like K+, however, the desolvation
process is always facile because of a weak interaction with
water molecules (13, 31), and inner-sphere surface complex-
ation is not so dependent on the location of the charge site in
the clay layer.
Fig. 6 is a snapshot, based on MC simulation (33), of the
interlayer configuration in Li-hectorite, a trioctahedral smec-
tite having only octahedral charge sites. The view in the figure
is along the clay layer c axis, with only one of the two opposing
siloxane surfaces shown but with all cations and water mole-
cules in a MC simulation cell depicted. This stable hectorite
hydrate, which has been investigated extensively by a variety of
spectroscopic techniques, has an average of three water mol-
ecules per Li+, which corresponds to a very concentrated ionic
OCR for page 3362
3362 Colloquium Paper: Sposito et al.
Proc. Natl. Acad. Sci. USA 96 (1999J
_
<_ f:
{~':
/` ~
FIG. 4. Visualization of Na+ bound in an outer-sphere surface complex in the interlayer region of Wyoming montmorillonite, based on MC
simulation. A portion of the siloxane surface structure also is shown.
_~_
~-~ ~ ~
.,ic-5~
~'~
FIG. 5. Visualization of K+ bound in an inner-sphere surface complex in the interlayer region of Wyoming montmorillonite, based on MC
simulation. Green lines extend from K+ (black sphere) to nearest-neighbor O in the surface complex. Dashed lines indicate hydrogen bonds between
water molecules. Portions of the opposing two siloxane surfaces also are shown, with the beige sphere at the bottom of the figure (center) indicating
a site of All+ substitution for Si4+.
OCR for page 3363
Colloquium Paper: Sposito et al.
FIG. 6. Visualization of the interlayer configuration in Li(H2O)3-
hectorite, based on MC simulation (33~. The Li+ are bound in
outer-sphere surface complexes with two water molecules. Other
water molecules are keyed into the siloxane surface cavities.
solution (~18.5 molal). Fig. 6 conforms to the trend expected
for small interlayer cations, in that only outer-sphere surface
fN
)
-
~ ~'_-
:~~N
~ _ ~t ~ \
.
~. ~
K~ ('of ~
Proc. Natl. Acad. Sci. USA 96 (1999) 3363
complexes have formed, comprising just two solvating water
molecules, in agreement with the average hydration number of
2.3 found in isotopic-difference neutron diffraction studies of
very concentrated LiCl solutions (16~. The nonsolvating water
molecules have keyed themselves into the hexagonal cavities of
the siloxane surface (one such molecule is at the center of the
simulation cell), reminiscent of the situation in trioctahedral
vermiculite (17-19~.
Much experimental and theoretical information points to an
inherent hydrophobicity of the siloxane surface tsee, for
example, the summary by Jaynes and Boyd (36~] were it not for
the presence of layer charge. Pyrophyllite, the uncharged
analog of montmorillonite, and talc, the uncharged analog of
vermiculite, both have hydrophobic siloxane surfaces (36, 374.
Studies of the effect of layer charge on the adsorption of both
water and hydrocarbon molecules by smectites (36, 38) indeed
show that surface hydrophobicity increases as the layer charge
decreases. Recent molecular dynamics simulations of ion
salvation and mobility in aqueous solution (39, 40) suggest that
large cations like K+ tend to interact with water molecules not
only through their positive charge but also through solvent
cage formation, which is just what hydrocarbon molecules do
(41-434. This hydrophobic tendency may be the basis for K+
associating directly with clay mineral O (Fig. 5) instead of
forming a well organized salvation complex near octahedral
charge sites, as does Na+ in Fig. 4. Fig. 7 exposes the inherent
hydrophobicity of the siloxane surface even more directly
through a MC snapshot of a methane molecule adsorbed in the
interlayer region of the three-layer hydrate of Na-montmoril
~/ In
1~
_- ~
hi/
~ _/
C_
_-E3
\~
fit
.1
. A~
~ _
_~~\
t:
M. .
A_
FIG. 7. Visualization of a methane molecule adsorbed in the interlayer region of the three-layer hydrate of Na-montmorillonite, based on MC
simulation. The typical 20-fold coordination between CH4 and O occurs, but with nearly half of the O being in the siloxane surface.
OCR for page 3364
3364 Colloquium Paper: Sposito et al.
lonite (H2O/CH4 = 23~. The well known 20-fold coordination
cage induced by CH4 in bulk water (41, 42) was reproduced
successfully by the CH4-O potential function used in the MC 10.
simulation (43~. In the interlayer of a Na-montmorillonite
hydrate, however, methane coordinates to eight clay mineral O
and approximately a dozen water molecule O to form this cage
in a highly distorted coordination structure. This kind of
hydrophobic association, which may be favored for CH4 over
a purely solvent-based arrangement of neighboring O atoms,
could play an important role in the chemical evolution of
organic molecules as mediated by clay minerals (44~.
The multifaceted nature of interactions within clay mineral
interlayers leads necessarily to complexity in the structure of
adsorbed water. This complexity is well illustrated by a con-
sideration of water molecule orientations in the two-layer
hydrate of Na-montmorillonite as revealed by MC simulation
(29~. Sodium-water molecule interactions in this system pro-
duce a local coordination structure like that in concentrated
aqueous solutions of NaCl (13, 29), but Na+ interactions with
tetrahedral charge sites are still strong enough to allow inner-
sphere surface complex formation with oxygen ions in the
siloxane surface. The configuration of water molecules differs
between inner-sphere and outer-sphere surface complexes.
When these two species are forced to cohabit within the
constrained spatial domain that exists in an interlayer region,
disorder in the water network is likely, with distorted H-bonds
and an array of water dipole orientations taking on almost
every possible direction (29~. This disorder is enhanced by an
evident attraction between water molecules and the cavities in
the siloxane surface, which gives rise to nonsolvating water
molecules keyed into these holes irrespective of the type of
adsorbed cation (6, 18, 19, 29-31, 33~. The characteristics of
adsorbed water on 2:1 clay minerals also reveal the competi-
tion between interlayer cations and clay mineral structure for
intercalated water molecules, as well as that between hydro-
philic and hydrophobic interactions. This competition pro-
duces a complex electrical double layer structure whose origins
and behavior are beginning to be understood at a fundamental
level as the basis for progress toward improved design in
applications (7, 8) that provide palpable benefits for human-
k~nd.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
Without the unfailing support of Dr. John Maccini (U.S. National
Science Foundation) and Dr. Sally Benson (Lawrence Berkeley Na-
tional Laboratory), the simulation research described herein would not
have been possible. The authors thank the National Energy Research
Scientific Computing Center for allocations of time on its Cray
supercomputers and Angela Zabel for excellent preparation of the
typescript. The research reported in this paper was supported in part
by National Science Foundation Grant EAR-9505629 and in part by
the Director, Office of Energy Research, Office of Basic Energy
Sciences, Geosciences Division of the U.S. Department of Energy
under Contract DE-AC03-76SF00098.
1.
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Representative terms from entire chapter:
clay minerals
