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Amenta V.,University of Sheffield | Cook J.L.,University of Sheffield | Hunter C.A.,University of Sheffield | Low C.M.R.,Imperial College London | And 2 more authors.
Journal of the American Chemical Society | Year: 2013

The association constants for formation a 1:1 complex between 4-phenyl azophenol and tri-n-butylphosphine oxide were measured in mixtures of n-octane and n-decanol, n-octane and n-hexanoic acid, and n-octane and 2-ethylhexyl acetamide. The experiments provide insight into the competition between solvent self-association and solvent-solute interactions in these systems. The solvation properties of the three polar solvents are quite different from one another and from polar solvents that do not self-associate. Carboxylic acids form dimers in concentrated solution (>1 mM in alkanes). Carboxylic acid dimers have exposed H-bond acceptor sites that solvate H-bond donor solutes with a similar binding affinity to carboxylic acid monomers. The carboxylic acid H-bond donor site is inaccessible in the dimer and is not available to solvate H-bond acceptor solutes. The result is that solvation of H-bond acceptor solutes is in competition with solvent dimerization, whereas solvation of H-bond donor solutes is not. Secondary amides form linear polymers in concentrated solution (>10 mM in alkanes). The solvation properties of the secondary amide aggregates are similar to those of carboxylic acid dimers. Solvation of H-bond acceptor solutes must compete with solvent self-association, because the amide H-bond donor site is not accessible in the middle of a polymeric aggregate. However, the amide aggregates have exposed H-bond acceptor sites, which solvate H-bond donor solutes with similar binding affinity to amide monomers. Alcohols form cyclic tetramers at concentrations of 100 mM in alkanes, and these cyclic aggregates are in equilibrium with linear polymeric aggregates at concentrations above 1 M. The alcohol aggregates have exposed H-bond acceptor sites that solvate H-bond donor solutes with similar binding affinity to alcohol monomers. Although the alcohol H-bond donor sites are involved in H-bond interactions with other alcohols in the aggregates, these sites are sufficiently exposed to form a second bifurcated H-bond with H-bond acceptor solutes, and these interactions have a similar binding affinity to alcohol monomers. The result is that self-association of alcohols does not compete with solvation of solutes, and alcohols are significantly more polar solvents than expected based on the properties of alcohol monomers. © 2013 American Chemical Society. Source


Buurma N.J.,University of Sheffield | Cook J.L.,University of Sheffield | Hunter C.A.,University of Sheffield | Low C.M.R.,Imperial College London | Vinter J.G.,Cresset Biomolecular Discovery
Chemical Science | Year: 2010

High throughput experiments using a molecular recognition probe reveal a simple relationship between solvent functional group concentration and selective solvation. The 1:1 association constant for the H-bonding interaction between tri-n-butylphosphine oxide and 4-phenylazophenol was measured in 1088 different alkane-ether mixtures using a UV-Vis plate reader. Although the stability of the complex decreased with increasing concentration of the more polar ether cosolvent as expected, the results show that it is the functional group composition rather than the constitution of the solvent molecules or the properties of the bulk liquid that determines the solvation thermodynamics. Thus the solvent properties of a simple ether can be reproduced by an appropriate mixture of a polyether and an alkane that has the same net concentration of ether oxygen functional groups. The results suggest that solvation may be understood at the molecular level simply by considering the polarities and the concentrations of the functional groups present in the solvent, because these are the parameters that affect local solvation interactions with the solutes. © The Royal Society of Chemistry 2011. Source


Amenta V.,University of Sheffield | Cook J.L.,University of Sheffield | Hunter C.A.,University of Sheffield | Low C.M.R.,Imperial College London | Vinter J.G.,Cresset Biomolecular Discovery
Organic and Biomolecular Chemistry | Year: 2011

High-throughput UV-Vis experiments using four molecular recognition-based probes, made by the combination of two hydrogen bond acceptors, tri-n-butylphosphine oxide and N,N′-bis(2-ethylhexyl)acetamide, and two hydrogen bond donors, 4-phenylazophenol and 4-nitrophenol, were performed. The association constants for the 1:1 H-bond interaction involved in each probe system were measured in mixtures of a polar and non-polar solvent, di-n-hexyl ether and n-octane, respectively. Similar behaviour was observed for all four systems. When the concentration of the polar solvent was low, the association constant was identical to that observed in pure n-octane. However, once the concentration of the polar solvent exceeded a threshold, the association constant decreased linearly with the concentration of di-n-hexyl ether. Selective solvation in mixtures can be understood based on the competition between the multiple competing equilibria in the system. In this case, solvation thermodynamics are dominated by competition of the ether for solvation of H-bond donors. For the more polar solute, 4-nitrophenol, the selective solvation starts at lower concentrations of the polar solvent compared with the less polar solute, 4-phenylazophenol. Thus the speciation and hence the properties of systems containing multiple solutes and multiple solvents can be estimated from the H-bond properties and the concentrations of the individual functional groups. © 2011 The Royal Society of Chemistry. Source


Amenta V.,University of Sheffield | Cook J.L.,University of Sheffield | Hunter C.A.,University of Sheffield | Low C.M.R.,Imperial College London | Vinter J.G.,Cresset Biomolecular Discovery
Journal of Physical Chemistry B | Year: 2012

The association constants for formation of 1:1 complexes between a H-bond acceptor, tri-n-butylphosphine oxide, and a H-bond donor, 4-phenylazophenol, have been measured in a range of different solvent mixtures. Binary mixtures of n-octane and a more polar solvent (ether, ester, ketone, nitrile, sulfoxide, tertiary amide, and halogenated and aromatic solvents) have been investigated. Similar behavior was observed in all cases. When the concentration of the more polar solvent is low, the association constant is identical to that observed in pure n-octane. Once a threshold concentration of the more polar solvent in reached, the logarithm of the association constant decreases in direct proportion to the logarithm of the concentration of the more polar solvent. This indicates that one of the two solutes is preferentially solvated by the more polar solvent, and it is competition with this solvation equilibrium that determines the observed association constant. The concentration of the more polar solvent at which the onset of preferential solvation takes place depends on solvent polarity: 700 mM for toluene, 60 mM for 1,1,2,2-tetrachloroethane, 20 mM for the ether, ester, ketone, and nitrile, 0.2 mM for the tertiary amide, and 0.1 mM for the sulfoxide solvents. The results can be explained by a simple model that considers only pairwise interactions between specific sites on the surfaces of the solutes and solvents, which implies that the bulk properties of the solvent have little impact on solvation thermodynamics. © 2012 American Chemical Society. Source

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