Chapter 5
Solvatochromism and Solvation Dynamics in CO -Expanded Liquids Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: January 6, 2009 | doi: 10.1021/bk-2009-1006.ch005
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Chet Swalina , Sergei Arzhantzev , Hongping L i , and Mark Maroncelli * 1,
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Department of Chemistry, The Pennsylvania State University, University Park, PA 16803 Department of Chemistry, Zhengzhou University, No. 100 Science Road, Zhengzhou, Henan 450001, China
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Molecular dynamics (MD) simulations and electronic spectroscopy are used to investigate solvatochromism and solvation dynamics in CH CN+CO mixtures. Experimental results for the probe solute trans-4-(dimethylamino)-4'cyanostilbene (DCS) reveal a nonlinear dependence of the solvatochromic shifts upon composition, suggesting substantial preferential solvation of DCS by CH CN. MD simulations show that analyzing these shifts using the common assumption of spectral additivity leads to grossly exaggerated estimates of the extent of this preference. Solvation dynamics has been measured using Kerr-gated emission spectroscopy and calculated using MD simulations. Simulation-experiment comparisons offer an interpretation of the observed dynamics in terms of nonspecific preferential solvation. 3
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© 2009 American Chemical Society
In Gas-Expanded Liquids and Near-Critical Media; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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96 Introduction Gas-expanded liquids (GXLs) are mixtures of a liquid organic solvent and a near-critical gaseous component such as C 0 (1-4). A surprising attribute of GXLs, shown in the top panel of Figure 1, is that gas expansion leads to an increase in mass density compared to the original liquid (5). But, as shown in the bottom panel, simulations reveal that this increased density is accompanied by a decrease in the packing fraction, the fraction of the total volume occupied by the van der Waals volumes of the constituent molecules. At high x , the packingfractionsdecrease to values -60% of those original liquids. Thus, there is substantially more "free-volume" in the C0 -expanded liquids than in the original liquids. The unique capabilities of GXLs such as enhanced solubilities of gases and increased fluidity arise from this extra space. Combined with the ability to readily pressure-tune the amount of expanding gas in the liquid, these features make GXLs potentially useful green solvent alternatives (1,2,4). The focus of this paper is on exploring molecular aspects of solvation in these rarified solvents. One objective is to describe preferential solvation in GXLs, the enrichment of one solvent component in the cybotactic region relative to the bulk. The extent of preferential solvation in mixtures can be determined from knowledge of how gas-to-solution spectral shifts depend on local compositions. Typically, spectral shifts in homogeneous mixtures are assumed to be linearly related to the shifts observed in the pure component solvents, weighted by their molefractionsin the mixture,
Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: January 6, 2009 | doi: 10.1021/bk-2009-1006.ch005
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V = JC(1M1) + JC(2)I/(2)
(1)
Inversion of this equation provides an approximate expression for the local mole fraction of a particular solvent component (6), x:(\) =
[v-v(2)]/[v(\)-v(2)]
(2)
Throughout this paper, the subscript u denotes the cybotactic region of the solute and (1) and (2) refer to the liquid and gaseous components of a GXL mixture, respectively. The linear interpolation in eq 1 implicitly assumes that contributions to the spectral shift from each component are additive with respect to the numbers of solvent molecules in the cybotactic region, v = N (1)£(1) + N (2)5(2) u
u
(3)
Here, 8(i) is the per-molecule or per-atom contribution to the spectral shift and N (i) is the number of molecules or atoms in the first solvation shell associated u
In Gas-Expanded Liquids and Near-Critical Media; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
97 0.9 \- M a s s Density s o
Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: January 6, 2009 | doi: 10.1021/bk-2009-1006.ch005
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