Preferential Solvation In Binary Mixtures Research Articles

Synergism in the solvation of solvatochromic probes in binary mixtures with ionic liquids

The preferential solvation of two perichromic dyes, 4-amino-N-methylphthalimide (4-AMP) and Nile Red (NR) in binary mixtures of an ionic liquid ([bmim][X], where X = BF4, PF6, Tf2N and DCA) with a protic or non-protic solvent, was investigated, in search of examples of a synergetic or “hyperpolar” behavior. Synergism was generally observed in those mixtures where the two pure components had similar polarity values, according to the employed probe. A parallel was drawn between the observed synergism in these IL mixtures and the phenomenon of hydrotropism, also observed with organic solutes in aqueous IL mixtures. More generally, in binary mixtures with organic solvents, ionic liquids can act as solvatotropes, promoting the dye solvation by formation of solute-specific aggregates with all species present in the mixture. By stabilizing the dipolar excited state of 4-AMP or NR through hydrogen-bonds and electrostatic interactions in these aggregates, a “hyperpolar” milieu is generated, which surpasses the polarities of the two pure components. Solvatotropism seems to offer a more realistic picture than the solvent-exchange model, originally developed for simpler systems, to the phenomenon of preferential solvation in binary mixtures with ionic liquids.

Study of molecular interactions and preferential solvation in binary mixtures of cyclohexane and (C5–C10) 1-alkanol by Kirkwood-Buff integrals

Mixtures formed by cyclohexane + 1-alkanol have been investigated in the framework of the Kirkwood–Buff theory of solution. The alcohols considered are 1-pentanol up to 1-decanol. Data for the Kirkwood–Buff integrals, Gij excess or deficit number of molecules around the central ones, Δnij and coefficients of linear solvation, δij are reported. For solutions with a given cyclohexane, the increase in the CH2 groups of alcohols chain causes a decrease in G12 by the following sequences: 1-decanol > 1-nonanol > 1-octanol > 1-heptanol > 1-hexanol > 1-pentanol. The comparison of G12 and other calculated parameters for cyclohexane + 1-alkanol mixtures reveals that the interactions between similar molecules are more outstanding and the decrease in the alkyl chain of alcohols leads to the weakening of interactions related to the self–association of 1-alkanol, differences in the free volumes, hydrophobic interactions, and structural changes. These trends are consistent with experimental data on the excess molar volumes.

Unusual Reverse Photochromic Behavior of Indolinobenzospiropyran 6-Carboxylates in Aqueous Binary Solvents

Thermoand photochromic indolinobenzospiropyran dyes recently have become important in connection with the rapid development of information recording systems, such as high-density optical data storage, optical switching, displays and non-linear optics. These molecules are known to be in equilibrium with their metastable colored forms. The absorption of ultraviolet light shifts the equilibrium towards the merocyanine colored form (MC), while the reverse process shifting them toward the colorless form (SP) is induced by visible light, heat, or can occur spontaneously (normal photochromism). The color of the MC form as well as the rate of rearrangement back to the SP form are both dependent on the polarity of the solvent. The stabilization of the MC form in polar solvents leads to it having a larger energy of activation and a slower MC↔SP transition as compared to that in non-polar solvents. Occasionally, the merocyanine form is further stabilized by factors such as hydrogen bonding, combination with a crown ether or cyclodextrin and complexation with a metal. Thus, the initially colorless solutions of spiropyrans become deeply colored upon the spontaneous formation of the merocyanine (reverse photochromism). Continuing our previous works on the solvatokinetic and solvatochromic behaviors of spiropyran derivatives in various neat organic solvents, we examined the photochromic behaviors of spiropyran derivatives in binary aqueous solvent media. Since merocyanine dyes in solution usually exist in dipolar ionic forms, their preferential solvation in binary mixtures was expected to affect their photophysical behavior. Although many studies have been conducted on the photochromic behaviors of spiropyran derivatives in various neat organic solvents, to the best of our knowledge, no papers have previously been published on the photochromic behaviors of spiropyrans in aqueous binary solvent systems. Herein, we describe the unusual photochromic behaviors of indolinobenzospiropyran 6-carboxylates 1-2 in aqueous binary solvent media, as depicted in Scheme 1. The electronic absorption spectra of the spiropyran carboxylates 1-2 showed no absorbance in the visible region on irradiation in neat solvents, whereas, they unexpectedly showed chromotropism in aqueous binary solvent mixtures such as water/methanol, water/DMSO, water/acetonitrile, etc. The UV-Vis. spectral changes for the SP → MC conversion of SP-1 in water-methanol solvent mixtures (50/50 v/v %) are shown in Figure 1, as a representative example. Initially, the water-methanol solution of SP-1 has an absorption band only in the 300 nm region. However, the colorless solution becomes spontaneously deeply colored upon the ring-opening reaction of SP-1 to MC-1. MC-1 has a strong absorption at 522 nm in 50/50% aqueous methanol. Reproducible results for the ring-opening reaction of spiropyran to merocyanine were obtained through the development of a protocol that maximized the formation of the colored form, as indicated by the solid line in Figure 1. The colored merocyanine solutions revert to the colorless spiropyran state upon their exposure to visible light of the appropriate wavelength. In addition to its major absorption at 500-550 nm, SP-2 has an extra absorption band in the 400-450 nm region in aqueous binary solvent mixtures. This extra absorption band is probably due to the proton transferred MC form (MCHCOO−). The proton might migrate from the 6-substituent COOH to the oxygen atom of the phenoxide anion in aqueous solvents, since the COOH group is more acidic than the OH group of the phenyl ring. This was confirmed by the UV-Vis spectral behavior of spiropyran both in HCl/aqueous

Study of Preferential Solvation in Binary Mixtures by Means of Frequency-Domain Fluorescence Spectroscopy

The preferential solvation of 8-N,N-(dimethylamino)-11H-indeno[2,1-a]pyrene, Py(S)DMA, in its transient charge transfer (CT) state in binary solvents such as toluene/DMSO liquid mixtures was studied by means of frequency-domain fluorometry. The data obtained were considered within the following kinetic scheme: the preferential solvation was described by the system of consecutive reversible reactions of which each step is associated with the absorption of one DMSO molecule in the first solvation shell of the fluoresent Py(S)DMA dipolar CT molecule. The rate constants of the first two reversible elementary processes (i.e., the decay of solvation complexes of Py(S)DMA with one and two polar molecules, k21 5 1.1 10 9 s 21 and k22 5 1.4 10 9 s 21 ) were determined.

A simple kinetic model of preferential solvation in binary mixtures

Based on experimental data a kinetic model is presented that adequately describes the main features of preferential solvation of a charge-transfer dipole in binary mixtures. 8-( N, N-dimethylamino)-11H-indeno[2,1-a]pyrene is used as fluorescent probe in toluene/DMSO mixtures. The observed saturation behavior can be understood in terms of microclusters of polar molecules surrounding the solute dipolar species. In such an approach, the size of the microclusters is mainly determined by interfacial surface tension.

Characterization of solvent mixtures. Part 8 — preferential solvation of chemical probes in binary solvent systems of a polar aprotic hydrogen-bond acceptor solvent with acetonitrile or nitromethane. Solvent effects on aromatic nucleophilic substitution reactions

The use of chemical probes for the characterization of chemical properties was explored for completely non-aqueous aprotic binary solvent mixtures. The Dimroth–Reichardt ET(30) betaine dye, 4-nitrophenol, 4-nitroanisole, 4-nitroaniline and N,N-diethyl-4-nitroaniline were used to study preferential solvation in binary mixtures of a polar aprotic hydrogen bond-acceptor solvent (DMSO, DMF, acetone, butanone and ethyl acetate) with acetonitrile or nitromethane at 25 °C over the whole range of solvent composition. The indicators were employed to determine ET(30), π*, α and β solvatochromic parameters of the mixtures. Each solvent system was analysed according to its deviation from ideal behaviour due to the preferential solvation of the indicators and also from the complicated intermolecular interactions of the mixed solvent. The validity of the concept of an intrinsic, absolute property of a solvent mixture and whether such a property can be defined by means of chemical probes is discussed. These results were related to the solvent effects on some aromatic nucleophilic substitution reactions. The kinetics of the reactions between 1-fluoro-2,4-dinitrobenzene and primary or secondary amines were studied in ethyl acetate– acetonitrile and N,N-dimethylformamide–acetonitrile mixtures, respectively, taken as representatives of two clearly different solvation models. Copyright © 1999 John Wiley & Sons, Ltd.

Preferential solvation in binary mixtures. Comparison between the quasi-lattice quasi-chemical model and the stepwise solvent exchange model

The quasi-lattice quasi-chemical (QLQC) model and the stepwise solvent exchange (SSE) model are both used in solvation chemistry to describe preferential solvation of solutes in binary mixtures. A critical assessment of the SSE model reveals a basic flaw. The usual assumption in the SSE model, that the activity coefficients of the solvent molecules in the solvation shell equal that of the bulk liquid, can be regarded as being a correction for this flaw. Comparison of the QLQC model with the SSE model explains the cause and meaning of the adjustable, extra parameters, which have been added to the SSE model in order to account for experimental data.

Preferential solvation in mixed solvents. Part 6.—Binary mixtures containing methanol, ethanol, acetone or triethylamine and another organic solvent

The preferential solvation in binary mixtures containing methanol, ethanol, acetone or triethylamine and another organic solvent has been examined by means of the quasi-lattice quasi-chemical (QLQC) and inverse Kirkwood–Buff integrals (IKBI) methods. The results are expressed in terms of the composition of the environment close to a given molecule in the mixtures, and are rationalized in terms of the interactions that take place in them.