View source for Electrostatics for Explorting the Nature of Water Adsorption on the Laponite Sheets' Surface

Entry by Haifei Zhang, AP 225, Fall 2009

== Soft matter keywords ==

Water Adsorption, Laponite

== Overview ==
In this paper, the authors studies the nature of the interaction of water with laponite surfaces using the topology of the electrostatic potential using density functional theory for periodic systems as well as an uncharged sheet model. The topological analysis predicts that for uncharged surfaces the adsorption mode is such that the water molecules are adsorbed almost parallel to the surface. For laponite surfaces, where there is a net charge, the adsorption mode involves electrostatic repulsion between the negative lone pairs on the water molecules and the ones on the surface oxygen atoms. As a consequence, the water molecules bind to the surface in a perpendicular and tilted approach, minimizing the repulsive interactions. The authors also discussed the advantage of using the topology of the electrostatic potential as an efficient method to describe the electrostatic interactions between adsorbates
and surfaces.

== Computational method ==
The electrostatic potential at <math>\bar r</math> generated by the total charge distribution, <math>\rho ^{tot}</math>, of a periodic system is given by 

<math>V(\bar r) = \sum\limits_n {\int {\rho ^{tot} (\bar r' - \bar R_n )} } |\bar r - \bar r'|^{ - 1} d\bar r'</math>

The summation extends to all direct lattice vectors, the prime on the integral sign indicating that an infinitesimal region about <math>\bar r = \bar r'</math> is excluded from the domain of integration to avoid divergent nuclear self-interaction terms that would otherwise arise in the electrostatic energy per cell. <math>\rho ^{tot}</math> may be decomposed into electronic and nuclear components.

In this paper, <math>V(r)</math> was calculated by means of the CRYSTAL98 program using the Khon-Sham Hamiltonian with the gradient-corrected Perdew-Becke-Ernzerhof (PBE) exchange potential. The topology of <math>V(r)</math> was analyzed using an algorithm developed in the authors' lab in the same way that those developed for the study of electronic density topology were used. The CPs were calculated using the Newton-Raphson (NR) technique.

== Modeling and results==
[[Image:Topic2fig1.jpg|300px|thumb|left|Structure of the unit cell of the idealized laponite platelet (a) Side view of a 1x1 cell. (b) Top view of a 3x3 cell. Red, yellow, green and white spheres denote O, Si, Mg, and H atoms, respectively. ]]

[[Image:Topic2fig2.jpg|300px|thumb|center|(a)Side view and (b) top view of the optimized unit cell of the laponite platelet. Red, yellow, green, white, pink, and dark-blue spheres denote O, Si, Mg, H, Li, and Na atoms, respectively.]]

[[Image:Topic2fig3.jpg|300px|thumb|left|Contour maps of the electrostatic potential (EP) for the H2O molecule. Blue and yellow zones denote the negative and positive values of the EP, respectively. (a) Side view of the negative-valued zone. (b) Top view of the positive-valued zone. Red spheres denote the EP minima critical points characterizing the oxygen lone pairs. Red and white cylinders denote O and H atoms, respectively. (c) Complete view of the H2O EP. Dotted contours denote the border of the region.]]

[[Image:Topic2fig8.jpg|300px|thumb|center|(a and b) Top view of the EP-predicted water adsorption modes on a ring of the laponite platelet surface. Blue and yellow zones denote negative and positive values of the EP, respectively. Red spheres denote EP minima critical points. Large red spheres denote lone-pair minima CPs of the water molecule. (c) Top view of a cylinder model showing the adsorption mode of a water (ball-and-stick models) monolayer on the laponite platelet surface. Yellow, red, green, violet, and white cylinders denote Si, O, Mg, Li, and H atoms, respectively.]]





== Summary ==
the nature of the interaction of water molecules
with the surface of the laponite sheets has been studied by
carrying out a systematic determination of the topology of V(r)
for the laponite platelet surface, water molecules, and an
uncharged sheet model using ab initio density functional theory
(DFT) methods for periodic systems. It is shown that the
computation of the topology of V(r) provides a reliable and
relatively inexpensive method (that does not require the fullgeometry
optimizations of the supermolecule) of studying the
nature of the interactions between adsorbates and extended
surfaces. To our knowledge, this novel methodology has not
been previously used to treat these systems.

== Soft matter details ==
The results obtained in this paper indicate that the analysis of the EP topology of the isolated interacting systems (water and surface) provides a reliable and efficient method to predict and understand the different adsorption modes of complicated systems involving molecules and surfaces dominated by electrostatic interactions. In water-clay systems such as the ones studied in this paper, the model provides a useful tool to understand the effect of charge in the system. As the results indicate, the presence of charge in the laponite surface drives the water adsorption toward perpendicular and tilted configurations, in contrast to the uncharged surface, where the only adsorption mode consist of water molecules lying parallel to the surface. The calculation can be extended to include polymer adsorption on hydrated clay surfaces, which is important in the design of shake gels.

== References ==
[1] Electrostatics for Explorting the Nature of Water Adsorption on the Laponite Sheets' Surface Yosslen Aray, Manuel Marquez, Jesus Rodriguez, Santiago Coll, Yamil Simon-Manso, Carlos Gonzalez and David A. Weitz, ''J. Phys. Chem.'' '''107''', 8946-8952 (2003).