Adsorption energetics of potassium sulfate dye inclusion crystals
Introduction
During the past several years we have studied the process of dyeing crystals [1], especially simple ionic salts that have adsorbed and oriented sulfonated dyes and luminophores during growth from solution. Mixed crystals of this sort, so called dye inclusion crystals, have been investigated in the context of studies on the nature of pleochroism, mechanisms of crystal growth, silver halide photosensitization, ceramics crystallization, colloid stabilization, habit modification, epitaxy, explosives preparation and kidney stone inhibition [1]. Despite the great scope of dye inclusion crystal research, our understanding of dyed crystals has been limited to the inferences that we have drawn from single crystal optical and magnetic resonance spectroscopies of as-grown crystals [2]. An alternative approach, adopted by Mauri and Moret [3], [4], is the analysis of surface chemistry of growing crystals in real time using scanned probe microscopies. Yet another strategy, explored herein, is the computational evaluation of the adsorption process using algorithms that emphasize the electrostatic interactions between adsorbate and host surfaces. In this work, we have calculated the energetics of the replacement of surface ions on various K2SO4 facets by 2-aminobenzenesulfonate (1). The results of the computations are compared with observations of the guest luminescence associated with particular growth sectors.
Adsorbate 1 was chosen as the focus of this study because it has been studied extensively as an additive in growing K2SO4 crystals [2] and because it is comparatively small with few geometric degrees of freedom. When adsorbed through different K2SO4 crystallographic facets (Fig. 1), 1 displayed distinct photophysical properties (Fig. 2) suggesting that the chromophores were in different environments, and conformations that were dependent on the absorption mechanisms characteristic of particular facets.
Section snippets
Computational methodology
The computer code marvin [5] was used to study the interactions of 1 with β-K2SO4 (orthorhombic, Pmcn, with a=5.572 Å, b=10.072 Å, c=7.483 Å) [6]. Crystals grow from aqueous solution with {010}, {110}, {021}, {001}, {011}, and {111} forms. The basic component of marvin is the simulation cell, which has planar 2D periodic boundary conditions parallel to the surface. The simulation cell consists of a number of blocks, split into two regions (1 and 2) that contain structural units consisting of ions
Results
The habit of K2SO4 is highly variable, depending upon temperature, rate of evaporation or cooling of saturated solutions, and additive concentration. Gurney ascertained, on the basis of the brightness of growth sectors in crystals grown in many ways in the presence of 1, that the decreasing tendency of the faces in the [001] zone to recognize 1 followed the sequence (001)>(021)∼(011)>(010) [16]. These observations were semi-quantitative—the quantum efficiencies of 1 are growth-sector dependent,
Discussion
Luminescence intensities indicate that the trend for incorporation of 1 in K2SO4, from most favorable to least favorable growth sector, is (001)>(011)∼(021)>(010) [16]. Our modeling predicts a similar trend; (010) is always the worst (least exothermic) substrate for replacement while (001) is the best or second best. In fact, on relaxed surfaces (010) is the only face for which the computed replacement energy is endothermic (although it must be remembered that the solvation energies of the
Acknowledgements
BK thanks the US National Science Foundation and the Petroleum Research Fund of the American Chemical Society. JDG and ALR are indebted to the Australian Research Council for an International Researcher Exchange Scheme grant. Finally JDG is grateful to the Royal Society for a University Research Fellowship.
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