Solvatochromic indicator

It should be born in mind, however, that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different signs from those of the decrystalUzation proper. Specifically, the contribution to the activation parameters of the interactions that occur in the solvent system should be taken into account. Consider the energetics of association of the solvated ions with the AGU. We may employ the extra-thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols [99]. Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in Eq. 7 is endothermic ... 

As well as density the solvent strength and selectivity of the fluid will affect both the solubilizing power of the fluid for specific solutes and retention in SFC. Solvatochromic indicators have been used to rank fluids commonly used in SFC in terms of... 

Polymers can be characterized via the Kamlet-Taft approach which describes the ability of a species to act as a hydrogen bond acid (ai), the ability to act as a hydrogen bond base (Pj), the dipolarity/polarizability (7ti ), and the size of a species. These parameters are obtained by dissolving solvatochromic indicator dyes in the respective polymer and by measuring the shift of their absorbance maxima18. 

A similar strategy has been adopted for multivariate detection of vapors with a set of metalloporhyrins with different central metal atom (Rakow and Suslick, 2000). They act as solvatochromic indicators that is, their spectrum changes with the vapor molecules solvating the indicator. 

The primary standard betaine dye (44) is only sparingly soluble in water and less polar solvents it is insoluble in nonpolar solvents such as aliphatic hydrocarbons. In order to overcome the solubility problems in nonpolar solvents, the more lipophilic penta-t-butyl-substituted betaine dye (45) has additionally been used as a secondary reference probe [174]. The excellent Hnear correlation between the Ej values of the two dyes allows the calculation of t(30) values for solvents in which the solvatochromic indicator dye (44) is not soluble. Introduction of electron-withdrawing substituents e.g. Cl [323], F, CF3, C6F13 [324]) in the betaine molecule reduces the basicity of its phenolate moiety, which allows the direct determination of x(30) values for somewhat more acidic solvents. Moreover, the Hpophilic and fluorophilic penta(trifluoromethyl)-substituted betaine dye (46) is more soluble in nonpolar solvents e.g. hexafluoro-benzene) than the standard dye (44) [324]. Conversely, the solubility in aqueous media can be improved through replacement of some of the peripheral hydrophobic phenyl groups in (44) by more hydrophilic pyridyl groups, to yield the more water-soluble betaine dye (47) [325]. The Ej values of these new secondary standard betaine dyes correlate linearly with the x(30) values of (44), which allows the calculation of x(30) values for solvents in which only betaine dyes (45)-(47) are sufficiently stable and soluble for the UV/Vis spectroscopic measurements [324, 325]. 

The same authors also introduced a r scale of solvent dipolarity/polarizability [84a]. This n scale is so named because it is derived from solvent effects on the n n electronic transitions of a selection of seven positively solvatochromic nitroaromatics of the type D-C6H4-A, where D and A stand for electron-donor e.g. NMe2) and electron-acceptor e.g. NO2) groups, respectively 4-nitroanisole, A,A-diethyl-3-nitroaniline, 4-methoxy-/ -nitrostyrene, 1-ethyl-4-nitrobenzene, A-methyl-2-nitro-p-toluidine, N,N-diethyl-4-nitroaniline, and 4-(dimethylamino)benzophenone. Given a solvatochromic indicator compound, the n value for a solvent S was defined according to Eq. (7-32) ... 

Interestingly, a statistical principal component analysis of the solvatochromic shift data sets previously used by Kamlet and Taft in defining the n scale has shown that, rather than one [n ), two solvent parameters (0ik and 02k) are necessary to describe the solvent-induced band shifts of the studied solvatochromic indicators [236]. This is not unexpected since the n parameters are assumed to consist of a blend of dipolarity and polarizability contributions to the solute/solvent interactions. 

Laurence et al. [237, 238] have shown that some of the absorption bands of the solvatochromic indicator dyes previously used by Kamlet and Taft to fix the reference lines exhibit a solvent- and temperature-dependent vibrational fine structure, which makes the determination of the band maxima, v ax, difficult. Furthermore, the indicator dyes used are thermo-solvatochromic, i.e. v ax is temperature-dependent. No allowance for this temperature-dependence of the a, / , and n values has been made previously. 

Both Kamlet, Abboud, and Taft et al. s [224, 226] and Swain et aVs [265] multiparameter solvent effect treatments have an inherent weakness in so far as the solvent parameters a, and n as well as and are averaged and statistically optimized parameters the former are derived from various types of solvatochromic indicator dyes. 

All three methods give similar values of interfacial potentials typical results for some of micelles and vesicles are listed in Table 3. Also listed are estimates of interfacial dielectric constants (e), determined by comparing the position of absorption bands of solvatochromic indicators in the surfactant assemblies with that of reference 1,4-dioxane water mixtures with known e values. More generally, luminescence probe analysis [49], thermal leasing [50] and absorption spectroscopy [47, 51] are techniques that have all been utilized to measure local polarities in micelles and vesicles. It is important to note that these methods presume knowledge of the loca-... 

Mixed solvents The addition of a small amount of a co-solvent to a supercritical fluid can increase solubilities of certain substances from several percent to several orders of magnitude (32.33.34.35). Spectroscopic data, which have been obtained recently, indicate that preferential solvation by a co-solvent contributes to the large increases. The co-solvents acetone, methanol, ethanol, and n-octane, were investigated by Kim and Johnston(36) using the solute phenol blue as a solvatochromic indicator. In Figure 7, it is apparent that the red shift (solvent strength) exceeds the value which is obtained from linear behavior, i.e. the concavity is positive. This means that the local concentration of co-solvent near the solute... 

Before using a solvatochromic probe in the SCF state, it is important to perform calibrations in well-defined one-phase liquid systems. A variety of solvatochromic indicators have been used previously to probe AOT reverse micelles at atmospheric pressure. For example El Seoud et al. (2 investigated malachite green and thymol blue, which are hydrophobic. They are insoluble in heptane despite their hydrophobicity. Accordingly it was found that these probes are absorbed at the surfactant/oil interface, but do not partition into the water pool. The Xmax and UV absorbance for thymol blue change significantly for Wo below 6, but become constant for Wq from 8.3 to 22.2. UV-VIS and positron annihilation studies showed that even less hydrophobic probes such as nitrophenols are also solubilizied in the surfactant/oil interface. 

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)... 

In order to take into acconnt this ionic liqnid effect on the solvatochromic property (T) of the mixtures, the wave numbers of maximum absorptions of the solvatochromic indicators (A,iV-diethyl-4-nitroaniline, 4-nitroaniline and Reichardt s betaine dye) at different compositions of SI (alcohol) and S2 (IL) were fitted to the equations. 

Buhvestov U, Rived F, Rafols C, Bosch E, Roses M (1998) Solute-solvent and solvent-solvent interactions in binary solvent mixtures. Part 7. Comparison of the enhancement of water structure in alcohol-water mixtures measurement by solvatochromic indicators. J Phys Org Chem 11 185-192... 

Roses M, Buhvestov U, Rafols C, RivedF, Bosch E (1997) Solute-solvent rmd solvent-solvent interactions in binary solvent mixtures. Part 6. A quantitative measurement of the enhrmce-ment of the water structure in 2-methylpropan-2-ol-water and pioprm-2-ol-water mixtures by solvatochromic indicators. J Chem Soc Perkin Trans 2 1341-1348... 

Migron, Y. Marcus, Y. Two reintroduced solvatochromic indicators for hydrogen-bond-donation and acceptance. J. Phys. Org. Chem. 1991. 4. 310-314. 

Based on structure (A) the compound may be considered as another bridged sesquifulvalene or as a bridged [14]annulene. It is a deep blue-violet compound, which is solvatochromic, indicating a strongly polar ground state [277]. Its n.m.r. spectra may also indicate dipolarity in the molecule, which would be accentuated by the presence of the cyano groups attached to the five-membered ring. 

Lagalante et al. proposed the use of 4-nitropyridine N-oxide as a suitable solvatochromic indicator of solvent acidity. The hypsochromic shifts determined by these authors for 43 of the solvents in Table 10.3.1 are due largely to the acidity of solvent and, to a lesser extent, also to its basicity based on the following equation, however, solvent polarity induces a bathochromic shift in the band ... 

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