Microscopic solvent properties

Baker, S.N., Baker, G.A., Bright, F.V., Temperature-dependent microscopic solvent properties of dry and wet l-butyl-3-methylimidazolium hexafluo-rophosphate Correlation with ET(30) and Kamlet-Taft polarity scales. Green Chem., 4,165-169, 2002. 

Lu, J., Liotta, G.L., Eckert, G.A., Spectroscopically probing microscopic solvent properties of room-temperature ionic liquids with the addition of carbon dioxide, /. Phys. Chem. A, 107, 3995-4000, 2003. 

Over the last few years, the development of solvents of desired properties with a particular use in mind has been challenging. To evaluate the behaviour of a liquid as solvent, it is necessary to understand the solvation interactions at molecular level. In this vein, it is of interest to quantify its most relevant molecular-microscopic solvent properties, which determine how it will interact with potential solutes. An appropriate method to study solute-solvent interactions is the use of solvatochromic indicators that reflect the specific and non-specific solute-solvent interactions on the UV-Vis spectral band shifts. In this sense, a number of empirical solvatochromic parameters have been proposed to quantify molecular-microscopic solvent properties. In most cases, only one indicator is used to build the respective scale. Among these, the E (30) parameter proposed by Dimroth and Reichardt [23] to measure solvent dipolarity/polarisability which is also sensitive to the solvent s hydrogen-bond donor capability. On the other hand, the n, a and P (Kamlet, Abboud and Taft)... 

Using ethylammonium nitrate (EAN) as PIL ( . , =0.95, ji = 1.12, a=1.10, P = 0.46), following types of binary mixture models were selected for the analysis and quantification of the microscopic solvent properties (a) [molecular aprotic solvent with HBA ability + PIL cosolvent], (b) [molecular aprotic solvent with both HBD and HBA ability + PIL cosolvent] and (c) [molecular protic solvent + PIL cosolvent] [31]. The molecular solvents included in this analysis were dimethylsul-phoxide (DMSO) ( ., =0.44, 7r = 1.00, a=0.02 and p=0.76) as a polar aprotic HBA solvent, acetonitrile (AN) ( .,. =0.46, 7t =0.75, a=0.19, p=0.40) as polar aprotic HBA/HBD solvent and methanol ( .,. =0.76, 7t =0.60, a=0.98, p=0.66) as a protic solvent. EAN is a N-H-bond donor. In all cases, the pure component part of the mixtures was capable of forming associated species through hydrogen-bonding interactions. For the explored solvent mixtures, empirical parameters . n, a and P were calculated from the wave numbers of the absorbance maxima of the corresponding chemical probes at 25°C. 

For the reaction performed in DMF + [lmim][PFg] nuxtures, the k response patterns reveals a continuous decrease with the increase in the concentration of IL. Moreover, it was of interest to relate the kinetic data to the molecular-microscopic solvent properties in order to aid in the interpretation of the solvent effects in the reaction under study. In order to quantitatively interpret the influence of the solvent effects on the reaction, the reported kinetic data and the quantified molecular-microscopic solvent properties were correlated by means of the multiparametric approach developed by Kamlet-Abboud-Taft [26]. 

The multiparametric correlation treatment of the kinetic data with the molecular-microscopic solvent properties allowed determining the incidence of each type of solvent property on the kinetics of reaction. For polar aprotic molecular solvent + [bmim][BF ] or [bmim][PF ], the solvation effects are dominated by both the dipo-larity/dipolarisabihty and the basicity of the media. Such solvent properties contribute to accelerate the chemical process. 

With a given set of potential functions we can evaluate various average properties of the solvent. In particular, we would like to simulate experimentally observed macroscopic properties using microscopic solvent models. To do this we have to exploit the theory of statistical mechanics... 

Among many molecular theories so far proposed to calculate Cr, a theory by Gierer and Wirtz is well-known in relation to the concept of "microscopic viscosity." In their theory, the solvent flow is divided into successive layers each with the thickness equal to the diameter of the solvent molecule. Such an artificial treatment is not satisfactory in both molecular and continuum senses and we ought to improve the model. Before turning to molecular theories, however, it is worthwhile to investigate Cr by the extended continuum model since, in solution chemistry, the concept of "microscopic viscosity" is an intuitive extension of the primitive continuum model based on the uniform solvent properties. 

In Figure 2 (a), we plot Cj against for the two functions of the "microscopic viscosity " The correlation curves for Eqs. (6) and (7) are roughly identical, the correlation between Cj and Cr is not so sensitive to the functional form From these correlation curves, we can estimate the significance of the effect of non-uniform solvent properties on Cr-Such effects are usually concealed by large effects of the solute shape and the surface boundary condition which is often treated as an adjustable parameter. 

Bulk solvent properties such as dielectric constant (e) may sometimes correlate with solvolysis rate constants, but microscopic dielectric constants in the vicinity of ions can be very different from bulk values. For water within 1.5 A of an ion, e has been estimated to fall to 5, rising to —80 for a water 4 A from the ion (252). One notable effect of the lowering of the dielectric constant of a solvent, however, is to encourage ion-pair formation (222, 285). Correlations that employ a... 

For the association, it is essential that A and R are in close proximity, i.e., their distance must be shorter than a critical radius. If this condition is satisfied, there is a certain probability that within a imit time interval A and R will form a complex. For a set of given environmental conditions (temperature, pressure, solvent properties) this probabihty is the same for all neighboring pairs of A and R, provided we do not consider microscopic conditions such as their mutual orientation or their instantaneous speeds of translation and rotation. For a given receptor the probabihty that any molecule of analyte appears within the critical distance is proportional to the concentration of A. The total number of associations per time interval in a particular region is proportional... 

Supercritical fluids in the highly compressible regime are of particular interest, because it is in this regime that one can easily access the intermediate solvent densities, and thus the associated intermediate solvent properties, which are obscured in subcritical fluids by the liquid-vapor coexistence curve. However, a large macroscopic compressibility arises from a balance of energetic and entropic forces which, concomitantly, give rise to interesting microscopic behaviors. These microscopic consequences must be accounted for if one is to accurately predict reaction rates in compressible SCFs. 

Interpretation of this pressure dependence as an effective volume change from the reactants to the barrier top is generally unenlightening, however, as the microscopic origin of this behavior is more clearly understood in terms of solvation free energies and the dependence of these quantities on the solvent properties at different pressures. 

A more productive approach is mesoscopic, which is what we used so far. Molecules can be viewed as collections of state and transition dipole moments which are coupled to the environment, which itself is described by a Hmited number of fields and parameters. This point of view has several advantages. It is used in many experimental studies of tautomerism. It allows us to find trends and make predictions of behavior in other environments, and to take a design stance in which we can build molecular systems for sensing and reporting. The molecular parameters can be measured with some accuracy, and the environment modeled in a variety of ways. The solvent is described by a Hmited number of fields density, velocity, polarization and a limited number of parameters viscosity, dielectric constant, and proton donating/accepting abiHty. These parameters themselves can, in principle, be derived from microscopic molecular properties, but it is easier to view them as measurable quantities, independent of the particular reaction studied. 

Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

Since the interface behaves like a capacitor, Helmholtz described it as two rigid charged planes of opposite sign [2]. For a more quantitative description Gouy and Chapman introduced a model for the electrolyte at a microscopic level [2]. In the Gouy-Chapman approach the interfacial properties are related to ionic distributions at the interface, the solvent is a dielectric medium of dielectric constant e filling the solution half-space up to the perfect charged plane—the wall. The ionic solution is considered as formed... 

Most modern discussions of solvent effects rely on the concept of solvent polarity. Qualitative ideas of polarity are based on observations such as like dissolves like and are well accepted. However, quantification of polarity has proven to be extraordinarily difficult. Since the macroscopic property polarity arises from a myriad of possible microscopic interactions, this is perhaps unsurprising. Hence, it is important that care is taken when measuring the polarity of any liquid to ensure that it is clearly understood what is actually being measured. 

The relative importance of the hafide anion - HO - Cell interactions can be inferred from application of the Taft-Kamlet-Abboud equation to the UV-Vis absorbance data of solvatochromic probes, dissolved in cellulose solutions in different solvent systems, including LiCl/DMAc and LiCl/N-methyl-2-pyrrolidinone [96]. According to this equation, the microscopic polarity measured by the indicator, Ej (indicator), in kcalmol is correlated with the properties of the solvents by Eq. 1 ... 

Epoxidized oils were also used to modify PLA Ali et ah (2009) reported that its use as a plasticizer to improve flexibility. Thermal and scanning electron microscope analysis revealed that epoxidized soybean oil is partially miscible with PLA. Rheological and mechanical properties of PLA/epoxidized soybean oil blends were studied by Xu and Qu (2009) Epoxidized soybean oil exhibited a positive effect on both the elongation at break and melt rheology. Al-Mulla et al. (2010b) also reported that plasticization of PLA (epoxidized palm oil) was carried out via solution casting process using chloroform as a solvent. The results indicated that improved flexibility could be achieved by incorporation of epoxidized palm oil. 

Metals are insoluble in common liquid solvents but can dissolve in each other (like dissolves like). A mixture of substances with metallic properties is called an alloy. Some alloys are true solutions, but microscopic views show that others are heterogeneous mixtures. Brass, for instance, is a homogeneous solution of copper (20 to 97%) and zinc (80 to 3%), but common plumber s solder is a heterogeneous alloy of lead (67%) and tin (33%). When solder is examined under a microscope, separate regions of solid lead and solid tin can be seen. When brass is examined, no such regions can be detected. 

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