Clara Ràfols

Solute–solvent and solvent–solvent interactions in binary solvent mixtures. Part 1. A comparison of several preferential solvation models for describing ET(30) polarity of bipolar hydrogen bond acceptor-cosolvent mixtures

The influence of solute–solvent and solvent–solvent interactions on the preferential solvation of solvatochromic indicators in binary solvent mixtures of Bipolar hydrogen bond acceptors has been studied. Several equations based on solvent exchange models that relate the transition energy of the Dimroth–Reichardt ET(30) indicator with the solvent composition are derived and compared. The models tested assume that the two solvents mixed interact to form a common structure with an ET(30) value not always intermediate between those of the solvents mixed. The solvatochromic indicator can be preferentially solvated by any of the solvents mixed or by the mixed solvent obtained. The parameters obtained explain the strong synergism observed for some of the mixtures with strong hydrogen bond donors (alcohols and chloroform).

Solute-solvent and solvent-solvent interactions in binary solvent mixtures. Part 7. Comparison of the enhancement of the water structure in alcohol-water mixtures measured by solvatochromic indicators

A preferential solvation model that takes into account the enhancement of the structure of water when small amounts of alcohol are added was applied to solvatochromic data for binary mixtures of water with 2-methylpropan-2-ol, propan-2-ol, ethanol and methanol. Application of the model allows the calculation of the effect of the enhancement of the water structure on solvatochromic solvent properties. It is demonstrated that the enhancement of water structure increases the solvent dipolarity/polarizability and hydrogen-bond donor acidity and decreases the solvent hydrogen-bond acceptor basicity. The effect decreases in the order 2-methylpropan-2-ol–water, propan-2-ol–water, ethanol–water and methanol–water.

Determination of mono- and disulphonated azo dyes by liquid chromatography–atmospheric pressure ionization mass spectrometry

Liquid chromatography–atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) with positive (PI) and negative (NI) modes of operation and liquid chromatography–high flow pneumatically assisted electrospray (ES) mass spectrometry with negative mode were used for the determination of the disulphonated azo dyes Acid Red 1, Mordant Red 9, Acid Red 13, Acid Red 14, Acid Red 73, Acid Yellow 23, Direct Yellow 28 and Acid Blue 113 and of the monosulphonated azo dyes Mordant Yellow 8, Mordant Black 11 and Mordant Black 17. High fragmentation was observed when using APCI with losses of one or two SO3Na groups, either attached to one ring or two different rings. Losses of Na and 2Na are common to all techniques used. High flow ES was the most sensitive technique for all the dyes studied, except for Mordant Red 9, with a linear range varying from 1–3 to 700–800 ng. APCI using negative ion mode was one order of magnitude less sensitive than ES, with a linear range varying from 50–70 to 2000–3000 ng, whereas positive ion mode APCI-MS showed the poorest linear range and sensitivity. The determination of Direct Yellow 28 and Acid Blue 113 in water samples was also reported by preconcentrating 500 ml of water with solid-phase disk extraction followed by LC–high flow pneumatically assisted ES-MS.

Linear free energy relationship analysis of microemulsion electrokinetic chromatographic determination of lipophilicity

Log κ′ values for microemulsion electrokinetic chromatography (MEEKC) with a microemulsion of 1.44% (w/w) SDS, 6.49% (w/w) butan-1-ol and 0.82% (w/w) heptane are well correlated through the linear free energy relationship equation, logk′ = −1.133 + 0.279 R2 −0.692πH2 −0.060∑αH2 − 2.805 ∑βO2 + 3.048Vx n = 53, ϱ = 0.9941, S.D. = 0.090, F = 791 The solute descriptors are R2, an excess molar refraction; π2H, the dipolarity/polarizability; Σα2H and Σβ2O, the overall or effective hydrogen-bond acidity and basicity and Vx, the McGowan characteristic volume. The coefficients in Eq. i are almost exactly the same as those in the correlation of water-octanol partition coefficients, as log Poct, confirming that the particular microemulsion system used is an exact model for water-octanol partition. The 53 solutes in Eq. i include phenols and heterocyclic bases (pyrimidines, pyrazines, pyrroles, indoles and furans) with no outliers at all, so that the microemulsion system can be used generally for the determination of solute lipophilicity.

Solute-solvent interactions in micellar electrokinetic chromatography. III. Characterization of the selectivity of micellar electrokinetic chromatography systems.

Several micellar electrokinetic chromatography (MEKC) systems (sodium dodecyl sulfate, lithium dodecyl sulfate, lithium perfluorooctanesulfonate, sodium cholate, sodium deoxycholate, tetradecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide) have been characterized by means of the solvation parameter model. It has been observed that the coefficients of the correlation equations depend strongly on the particular set of compounds analyzed. Principal component analysis has been used to characterize the 2975 compounds with available solute descriptors and to select an appropriate subset of compounds to be analyzed by MEKC. With this set of compounds, the MEKC systems have been characterized. Principal component analysis has also been used to show the similarities and differences between the properties of the surfactants characterized by MEKC.

A comparison between different approaches to estimate the aqueous pKa values of several non-steroidal anti-inflammatory drugs

Abstract Two different approaches, one involving the experimental pK determination in an organic or hydro-organic solvent and the other using the program SPARC, have been tested to estimate the aqueous pKa values of several α-arylpropionic acids with anti-inflammatory properties. In the first approach, isopropyl alcohol seems to be a very suitable solvent because of its good solvation properties and also because it avoids the preferential solvation shown by the binary solvents that affects each solute to a different degree. The results obtained using isopropyl alcohol agree very well with those obtained by means of the SPARC program, based on the analysis of the chemical structure.

Solute–solvent and solvent–solvent interactions in binary solvent mixtures. Part 3. The ET(30) polarity of binary mixtures of hydroxylic solvents

Solvent exchange models have been applied to the transition energy of the Dimroth–Reichardt ET(30) solvatochromic indicator in binary hydroxylic solvents at 25 °C. The models tested have been critically evaluated to show the simplifications and limitations inherent to each model. A general five-parameter model [two preferential solvation and three ET(30) parameters] has been selected as the most appropriate. This model fits well the 51 binary systems tested in this work and 19 different ones previously studied. It also allows a quantitative interpretation of the ET(30) behaviour of the binary systems in terms of solute–solvent and solvent–solvent interactions.

Effect of temperature on pH measurements and acid-base equilibria in methanol-water mixtures.

The knowledge of the acid-base equilibria in water-solvent mixtures of both common buffers and analytes is necessary in order to predict their retention as function of pH, solvent composition and temperature. This paper describes the effect of temperature on acid-base equilibria in methanol-water solvent mixtures commonly used as HPLC mobile phases. We measured the delta-correction parameter (delta = sw pH - ss pH = Ej - log sw(gamma)oh) between two pH scales: pH measured in the solvent concerned and referred to the same standard state, ss pH, and the pH measured in that solvent mixture but referred to water as standard state, sw pH, for several methanol compositions in the temperature range of 20-50 degrees C. These determinations suggest that the delta-term depends only on composition of the mixture and on temperature. In water-rich mixtures, for which methanol is below 40% (w/w), delta-term seems to be independent of temperature, within the experimental uncertainties, whereas for methanol content larger than 50% (w/w) the delta-correction decreases as temperature increases. We have attributed this decrease to a large increase in the medium effect when mixtures have more than 50% methanol. The pKa of five weak electrolytes of different chemical nature in 50% methanol-water at 20-50 degrees C are presented: the effect of temperature on pKa was large for amines, pyridine and phenol, but almost no dependence was found for benzoic acid. This indicates that buffers can play a critical role in affecting retention and selectivity in HPLC at temperatures far from 25 degrees C, particularlyfor co-eluted solutes.

Effect of temperature on the chromatographic retention of ionizable compounds: II. Acetonitrile–water mobile phases

The retentive behavior of weak acids and bases in reversed-phase liquid chromatography (RPLC) upon changes in column temperature has been theoretically and experimentally studied. The study focuses on examining the temperature dependence of the retention of various solutes at eluent pH close to their corresponding pKa values, and on the indirect role exerted by the buffer ionization equilibria on retention and selectivity. Retention factors of several ionizable compounds in a typical octadecylsilica column and using buffer solutions dissolved in 30% (v/v) acetonitrile as eluent at five temperatures in the range from 25 to 50 degrees C were carefully measured. Six buffer solutions were prepared from judiciously chosen conjugated pairs of different chemical nature. Their pKa values in this acetonitrile-water composition and within the range of 15-50 degrees C were determined potentiometrically. These compounds exhibit very different standard ionization enthalpies within this temperature range. Thus, whenever they are used to control mobile phase pH, the column temperature determines their final pH. Predictive equations of retention that take into account the temperature effect on both the transfer and the ionization processes are evaluated. This study demonstrates the significant role that the selected buffer would have on retention and selectivity in RPLC at temperatures higher than 25 degrees C, particularly for solutes that coelute.

Variation of acidity constants and pH values of some organic acids in water—2-propanol mixtures with solvent composition. Effect of preferential solvation

A general equation which relates the dissociation constant of an acid with the solvent composition in binary solvent mixtures is derived. The equation considers that the two solvents interact to form mixed structures. The electrolyte interacts with the mixed solvent formed and with the two pure solvents mixed. The degree of these interactions determines the preferential solvation of the electrolyte by any of the three solvents which can be present in the solvent mixture. Dissociation constants of 3-nitrophenol, 4-nitrophenol, acetic, benzoic, 3-bromobenzoic, 3,5-dinitrobenzoic and o-phthalic acid have been determined in several water—2-propanol mixtures and the proposed equation applies successfully to these acids. The parameters obtained allow an easy calculation of the dissociation constant of these acids at any solvent composition. The calculated constant can be directly used, without any experimental determination, wherever an accurate pK or pH value in water—2-propanol is needed, such as in some liquid chromatographie applications.