Non-specific Solute-solvent Interactions Research Articles

Spectrophotometric determination of preferential solvation and solvation shell composition of morin hydrate in some water-aliphatic alcohol mixed solvents

Absorption spectra of a flavonoid (morin hydrate) were studied in pure water, methanol, ethanol, 1-propanol, and 2-propanol and also in their aqueous binary mixtures at 25.0°C. Spectral changes were interpreted in terms of specific and non-specific solute-solvent interactions. The molar transition energy (ET) in the maximum absorption as a solvatochromic probe was measured in each binary mixture. The local mole fraction of each solvent composition calculated in the cybotactic region of the probe. The extent and importance of each solute-solvent interaction to ET was analyzed in the framework of the linear solvation energy relationships. Preferential solvation was detected as a non-ideal behavior of ET curve respective to the mole fraction of the alcohols in all the binary mixtures. The preference of morin hydrate to be solvated by one of the solvating species relative to the others was discussed in terms of solvent-solvent and solute-solvent interactions.

Enhanced antitumor effect of poly(L-glutamic acid)-rose bengal conjugate nanoparticle

The photophysical properties of two N-salicylidene p-X-anilines (XCN, COOH) were studied in a series of solvents with varying polarity and proticity by steady state absorption and emission spectroscopy. The two compounds are dually fluorescent in solution due to an excited state proton transfer. The solvatochromic shifts were used to connect the solvent polarity parameter to the dipole moment in excited state using several approaches. The multiparameter Kamlet–Taft and Catalán solvent scales with four parameters have been employed to analyze the specific and non-specific solvent–solute interactions. The absorption spectra are less sensitive to the substitution at the aniline ring and to the solvent properties than the fluorescence spectra. Especially the ratio between the normal and abnormal Stokes shifted fluorescence intensities is dependent on the polarity and hydrogen bonding abilities of the solvent. The increasing solvent viscosity led to a decrease in the ratio of the intensities of the two emissions.

Experimental and theoretical investigation of the molecular, electronic structure and solvatochromism of phenyl salicylate: External electric field effect on the electronic structure

The UV–vis absorption and steady state fluorescence spectra of phenyl salicylate (abbreviated as PS) have been recorded in a series of non-polar, polar protic and polar aprotic solvents at room temperature and the obtained spectral data are used to determine the solvatochromic behavior and the ground and excited state dipole moments. Basis set sensitive molecular structure along with X-ray crystal data are evaluated. The ground state and excited state dipole moments are determined by using Lippert-Mataga, Bakhshiev, Bilot-Kawski and Reichardt solvatochromic shift methods as a function of dielectric constant (ε) and refractive index (n) of the solvents. The larger excited state dipole moment value indicates the more polar PS in the excited state. The rate of μe/μg is found as 2.4239. Solvatochromic behavior of PS is enlightened by using Kamlet-Taft and Catalan models. Kamlet-Taft solvatochromic model indicates that non-specific solute solvent interactions are controlled by solvent dispersion-induction forces and specific interactions are directed by hydrogen-bond donor capacity of solvent. Catalan solvatochromic model designates that solute-solvent interactions are governed by solvent polarizability. Ground and excited state dipole moments are found theoretically by using DFT/B3LYP/6–311++G(d, p) and TDDFT/B3LYP/6–31++G(d, p) methods. External electric field effect on LUMO-HOMO band gap and dipole moment have been investigated by using B3LYP/6–311++G(d, p) method.

Solvent and substituent effect on intramolecular charge transfer in 5-arylidene-3-substituted-2,4-thiazolidinediones: Experimental and theoretical study

The substituent and solvent effect on intramolecular charge transfer (ICT) in 5-arylidene-3-methyl-2,4-thiazolidinediones (series 1) and 5-arylidene-3-phenyl-2,4-thiazolidinediones (series 2) was studied by using experimental and theoretical methodology. The effect of specific and non-specific solvent–solute interactions on the UV–vis absorption maxima shifts was evaluated by using the Kamlet-Taft and Catalán solvent parameter sets. Linear free energy relationships (LFERs) have been applied to the UV–vis and 13C NMR data by using SSP (single substituent parameter) and DSP (dual substituent parameters). Comparative LFER analysis of 10 styrenic series was performed in order to distinguish contribution of structural and electronic substituent effect on extent of π-polarization in a side chain (vinyl) group. Furthermore, the experimental findings were interpreted with the aid of ab initio MP2 and time-dependent density functional (TD-DFT) methods. TD-DFT calculations are performed to quantify the efficiency of intramolecular charge transfer (ICT) allowing us to define the charge-transfer distance (DCT), amount of transferred charge (QCT), and difference of dipole moments between the ground and excited states (μCT). It was found that both substituents and solvents influence electron density shift, i.e. extent of conjugation, and affect intramolecular charge transfer character in the course of excitation.

Solvatochromic properties of two related N-salicylidenanilines with dual fluorescence

The photophysical properties of two N-salicylidene p-X-anilines (XCN, COOH) were studied in a series of solvents with varying polarity and proticity by steady state absorption and emission spectroscopy. The two compounds are dually fluorescent in solution due to an excited state proton transfer. The solvatochromic shifts were used to connect the solvent polarity parameter to the dipole moment in excited state using several approaches. The multiparameter Kamlet–Taft and Catalán solvent scales with four parameters have been employed to analyze the specific and non-specific solvent–solute interactions. The absorption spectra are less sensitive to the substitution at the aniline ring and to the solvent properties than the fluorescence spectra. Especially the ratio between the normal and abnormal Stokes shifted fluorescence intensities is dependent on the polarity and hydrogen bonding abilities of the solvent. The increasing solvent viscosity led to a decrease in the ratio of the intensities of the two emissions.

Estimation of Ground-State and Singlet Excited-State Dipole Moments of Substituted Schiff Bases Containing Oxazolidin-2-one Moiety through Solvatochromic Methods

Absorption and fluorescence studies on novel Schiff bases (E)-4-(4-(4-nitro benzylideneamino)benzyl)oxazolidin-2-one (NBOA) and (E)-4-(4-(4-chlorobenzylidene amino)benzyl)oxazolidin-2-one (CBOA) were recorded in a series of twelve solvents upon increasing polarity at room temperature. Large Stokes shift indicates bathochromic fluorescence band for both the molecules. The photoluminescence properties of Schiff bases containing electron withdrawing and donating substituents were analyzed. Intramolecular charge transfer behavior can be studied based on the influence of different substituents in Schiff bases. Changes in position and intensity of absorption and fluorescence spectra are responsible for the stabilization of singlet excited-states of Schiff base molecules with different substituents, in polar solvents. This is attributed to the Intramolecular charge transfer (ICT) mechanism. In case of electron donating (-Cl) substituent, ICT contributes largely to positive solvatochromism when compared to electron withdrawing (-NO2) substituent. Ground-state and singlet excited-state dipole moments of NBOA and CBOA were calculated experimentally using solvent polarity function approaches given by Lippert-Mataga, Bakhshiev, Kawskii-Chamma-Viallet and Reichardt. Due to considerable π- electron density redistribution, singlet excited-state dipole moment was found to be greater than ground-state dipole moment. Ground-state dipole moment value which was determined by quantum chemical method was used to estimate excited-state dipole moment using solvatochromic correlations. Kamlet-Abboud-Taft and Catalan multiple linear regression approaches were used to study non-specific solute-solvent interaction and hydrogen bonding interactions in detail. Optimized geometry and HOMO-LUMO energies of NBOA and CBOA have been determined by DFT and TD-DFT/PCM (B3LYP/6-311G (d, p)). Mulliken charges and molecular electrostatic potential have also been evaluated from DFT calculations.

Preferential Solvation of a Highly Medium Responsive Pentacyanoferrate(II) Complex in Binary Solvent Mixtures: Understanding the Role of Dielectric Enrichment and the Specificity of Solute-Solvent Interactions.

In this work, the preferential solvation of an intensely solvatochromic ferrocyanide(II) dye involving a 4,4'-bipyridine-based ligand was examined in various binary solvent mixtures. Its solvatochromic behavior was rationalized in terms of specific and nonspecific solute-solvent interactions. An exceptional case of solvatochromic inversion was observed when going from alcohol/water to amide/water mixtures. These effects were quantified using Onsager's solvent polarity function. Furthermore, the sensitivity of the solvatochromism of the dye was determined using various solvatochromic parameters such as π* expressing the dipolarity/polarizability of solvents and α expressing the hydrogen-bond-donor acidity of solvents. This analysis was useful for the rationalization of the selective solvation phenomena occurring in the three types of alcohol/water and amide/water mixtures studied. Furthermore, two preferential solvation models were employed for the interpretation of the experimental spectral results in binary solvent mixtures, namely, the model of Suppan on dielectric enrichment [J. Chem. Soc. Faraday Trans. 1 1987, 83, 495-509] and the model of Bosch, Rosés, and co-workers [J. Chem. Soc., Perkin Trans. 2, 1995, 8, 1607-1615]. The first model successfully predicted the charge transfer energies of the dye in formamide/water and N-methylformamide/water mixtures, but in the case of MeOH/water mixtures, the prediction was less accurate because of the significant contribution of specific solute-solvent interactions in that case. The second model gave more insights for both specific solute-solvent as well as solvent-solvent interactions in the cybotactic region. The role of dielectric enrichment and specific interactions was discussed based on the findings.

Photophysical study of 6-amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile and estimation of ground-state and singlet excited-state dipole moments by solvatochromic approaches

An investigation of absorption and fluorescence behaviour of 6-amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (NDPPC) at room temperature was carried out using a series of twelve organic solvents with different polarities. Ground and excited state dipole moments were determined experimentally by using Lippert–Mataga polarity function, Bakhshiev solvent polarity parameter, Kawski-Chamma-Viallet solvent polarity parameter, Reichardt's microscopic solvent polarity parameter, Kamlet-Abboud-Taft multiple linear regression and Catalan dipolar polarizibility approach. Due to considerable π- electron density redistribution, the value of excited state dipole moment was found to be greater than that of the ground state. The ground state dipole moment value was determined by quantum chemical method which was used to estimate excited state dipole moment using solvatochromic correlations. Both non-specific solute-solvent interactions and hydrogen bonding interactions were investigated by using Kamlet-Abboud-Taft and Catalan method. TD-DFT (B3LYP/6-311G (d,p)) was used for the determination of HOMO-LUMO energies.

Synthesis, crystal structure and photophysical properties of (E)-4-(4-(2-hydroxybenzylideneamino)benzyl)oxazolidin-2-one

A new Schiff base, (4-(benzylideneamino)benzyl)oxazolidin-2-one has been synthesised from 4-(4-aminobenzyl)oxazolidin-2-one and salicylaldehyde by a simple condensation reaction. Single-crystal X-ray analysis of (E)-4-(4-(2-hydroxybenzylideneamino) benzyl)oxazolidin-2-one (HBOA) revealed that there is a 1-D, slipped, face-to-face motif with off-set, head-to-tail stacked columns. Detailed studies on photophysical properties of the synthesised compound in solutions indicate their potential applications in the field of organic light emitting devices and nonlinear optical materials. Absorption and fluorescence study of HBOA has been conducted in a series of solvents with increasing polarity at room temperature. Ground and excited state dipole moments have been determined experimentally by using Lippert–Mataga polarity function, Bakhshiev solvent polarity parameter, Kawskii–Chamma–Viallet solvent polarity parameter and Richardt׳s microscopic solvent polarity parameter. Due to the considerable π-electron density redistribution, the excited state dipole moment was found to be larger than that of the ground state. The ground state dipole moment value was determined by quantum chemical method which was used to estimate excited state dipole moment through solvatochromic correlations. Kamlet–Taft and Catalan methods were used to get the information of both non-specific solute–solvent interactions and hydrogen bonding interactions. TD-DFT (B3LYP/6-311G(d,p)) has been used for the determination of HOMO–LUMO energies. Mulliken charges and Molecular electrostatic potential were also evaluated from DFT calculations.

Solvatochromism and intramolecular hydrogen-bonding assisted dipole moment of phenyl 1-hydroxy-2-naphthoate in the ground and excited states

Absorption and steady state fluorescence spectra of phenyl 1-hydroxy-2-naphthoate (abbreviated as PHN) have been recorded in a series of non-polar, polar protic and polar aprotic solvents at room temperature. The pH effect on the excited state intramolecular proton transfer (ESIPT) has been discussed. FT-IR spectra in solid phase, CHCl3 and CDCl3 are analyzed. The ground state (μg) dipole moment is estimated by using Bilot-Kawski method and the excited state (μe) dipole moment is calculated by using Bilot-Kawski, Lippert-Mataga, Bakhshiev, Kawski-Chamma-Viallet and Reichardt solvatochromic shift methods as a function of dielectric constant (ε) and refractive index (n) of solvents. μe is observed as higher than μg for all of the used methods which indicates the more polar PHN in the excited state. The studied compound is observed to have intramolecular hydrogen bonding assisted dipole moments in the ground and excited state. The rate of μe/μg is found as 34.857. The ground and excited state dipole moment vectors are observed to make an angle with 3.842°. Solvatochromic behavior of PHN is investigated by means of linear solvation energy relationship (LSER) by using Kamlet-Taft and Catalan parameters. Kamlet-Taft solvatochromic model indicates that non-specific solute solvent interactions are controlled by solvent dispersion-induction forces and specific interactions are directed by hydrogen-bond acceptor capacity of solvent. Catalan solvatochromic model designates that solute-solvent interactions are governed by solvent dipolarity/polarizability and solvent acidity. Theoretical μg and μe have been determined by quantum chemical calculations using DFT-B3LYP/6-31G(d) and TDDFT-B3LYP/6-31G(d) level of theory, respectively. HOMO, LUMO, molecular electrostatic potential (MEP) of PHN are calculated by the same methods in the ground and excited states.

Specific and non-specific interaction effect on the solvatochromism of some symmetric (2-hydroxybenzilydeamino)phenoxy Schiff base derivatives

In this present work, solvatochromic behavior, specific and non-specific solute–solvent interactions and electronic structure of (2-hydroxybenzilydeamino)phenoxy Schiff base derivatives have been investigated by using electronic absorption spectra in solvent media with different polarities. Electronic absorption spectra are observed to exhibit three main electronic transition bands along with the fourth band which comes into existence from specific solute–solvent interactions. Solute–solvent interaction mechanism and solvatochromic behavior of the studied molecules have been analyzed and evaluated by means of four linear solvation energy relationship (LSER) methods such as Kamlet–Taft parameters, Catalan parameters, Marcus optical dielectric function and Dimroth–Reichardt ET solvent parameter. Achieved results indicate that the mechanisms which control the specific and non-specific solute–solvent interactions are considerably effected by the characteristics of substituents and molecule geometry. According to ET solvent parameter, electronic absorption spectra of (2-hydroxybenzilydeamino)phenoxy Schiff base derivatives exhibit positive solvatochromism. Tautomeric structure and excited state intramolecular proton transfer (ESIPT) have been elucidated by means of resonance structure. A 2D solute–solvent interaction model is investigated theoretically. 3D molecular structure, HOMO (the highest occupied molecular orbital), LUMO (the lowest unoccupied molecular orbital), molecular electrostatic potential (MEP) and solvent accessible surface (SAS) are calculated by using the DFT-B3LYP method.

N-(substituted phenyl)-2-chloroacetamides: LSER and LFER study

The UV absorption spectra of twelve N-(substituted phenyl)-2-chloroacetamides were recorded in eighteen solvents. The effect of specific and non-specific solvent–solute interactions on the absorption maxima shifts was evaluated by using the Kamlet–Taft solvent parameter set, i.e. applying linear solvation energy relationships (LSER) principles. Optimized geometries and experimental results were interpreted by using DFT (B3LYP/6-311+G(d,p) method) and time-dependent density functional (TD-DFT) method. Overall electron density in both ground and excited state was obtained by the use of Quantum Theory of Atoms in Molecules, i.e. Bader’s analysis. It was found that both solvent and substituents cause appropriate change of the extent of conjugation in the molecules that further affect their intra-molecular charge transfer (ICT) character. Linear free energy relationships (LFERs) were applied to the substituent-induced NMR chemical shifts (SCS) using single substituent parameter (SSP) and dual substituent parameter (DSP) model. Transmission mode of the electronic effects of substituent was discussed according to the results of theoretical calculations and results of LFER correlations. Comparative analysis of presented results with the ones published for structurally similar series of amide which contained cyano group, instead chlorine, provides additional information on the impact of present group to the properties of investigated compound.

Oxidative degradation of norfloxacin by a lipophilic oxidant, cetyltrimethylammonium permanganate in water–acetonitrile medium: A kinetic and mechanistic study

The present study reports the oxidative metabolism of an established antibacterial drug norfloxacin (NRF) by a lipid compatible lipophilic Mn(VII) oxidant, cetyltrimethylammonium permanganate (CTAP) in acetonitrile–water binary mixture in the presence of acetic acid. The metabolized products are identified as 7-amino-1-ethyl-6-fluoro-1,4-dihydro-4-oxoquinoline-3-carboxylic acid, formaldehyde, and ammonia. The kinetics of the reaction is studied in aqueous acetonitrile media in the presence of acetic acid by UV–vis spectroscopic method by monitoring the absorbance of Mn(VII) at 530nm under pseudo first-order condition. The reaction is found to be first-order with respect to CTAP and fractional order with respect to norfloxacin and acetic acid. Occurrence of Michaelis–Menten type kinetics with respect to norfloxacin confirmed the binding of oxidant and substrate to form a complex before the rate determining step. A suitable ionic mechanism is proposed based on the experimental findings. The proposed reaction mechanism is supported by the effect of solvent polarity and effect of temperature on the reaction rate. High negative entropy of activation (ΔS≠=−259 to −158JK−1mol−1) supported the existence of a forced, more ordered and extensively solvated transition state. Further, solvent polarity–reactivity relationship revealed (i) the presence of less polar transition state compared to the reactants, (ii) differential contribution from dipolar aprotic (acetonitrile) and polar protic (water) solvents toward the reaction process through specific and nonspecific solute–solvent interaction and (iii) presence of intramolecular H-bonding in oxidant–substrate complex in acetonitrile rich domain and intermolecular H-bonding between oxidant–substrate complex and water in water rich domain.

Ground and excited state hydrogen bonding effects of 6-aminocoumarin in water: An ab initio study

The hydrogen bonded complexes of 6-aminocoumarin (6AC) with several water molecules in their ground and lowest excited singlet electronic state have been investigated using the ab initio methods. It was shown that the complex containing five water molecules in the surrounding of the amino group of the 6AC molecule has the lowest stabilization energy in the ground electronic state. This complex can serve as a model of the first solvation shell of the 6AC molecule in bulk water. The energies of hydrogen bonds formed by the carbonyl group with up to two water molecules were also determined. The calculated change in the total hydrogen bond energy due to S0→S1 excitation is in good agreement with the experimental data [E. Krystkowiak, A. Maciejewski, Phys. Chem. Chem. Phys. 2011;13:11317–11324]. The maxima of absorption spectra of the hydrogen-bonded complexes, calculated taking into account nonspecific solute–solvent interactions, are in reasonable agreement with the experimental data.

Photo-physical behavior of some antitumor anthracycline in solvent media with different polarity

Absorption and emission spectra of three antitumour anthracyclines, with various substituent and stereoisomer groups, were studied in different solvents. The solute’s photo-physical behavior strongly depends on solvent–solute interactions and solvent’s nature. Solvatochromic method was used to investigate dipole moments of these materials in ground and excited states. Spectral variations were analyzed via means of linear solvation energy relationships concept, proposed by Kamlet and Taft. The results explain the nature of specific and non-specific solvent–solute interactions and functional groups’ reorientation of studied anthracyclines in different media.

Effect of hydroxyl group on the photophysical properties of benzo[a]xanthenes – Solvatochromic studies and estimation of dipole moment

Abstract Absorption and emission spectra of three substituted benzo[a]xanthenes were studied at 298 K in solvents of different polarity for the first time. The fluorescence of 2-hydroxy-9,9-dimethyl-12-(pyridin-2-yl)-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one was weak compared to 3-hydroxy-9,9-dimethyl-12-(pyridin-2-yl)-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one. It was observed that an increase in the solvent polarity led to an increase in the Stokes shift. The solvent effect on the spectral properties of benzo[a]xanthenes has been investigated by using the Lippert–Mataga and Reichardt–Dimroth methods. Bakhshiev's and Kawski–Chamma–Viallet's correlations were used to determine the excited state dipole moment, ground state dipole moment and their ratio. The nature and extent of solute–solvent interactions were described by multi-linear correlation using the three-parameter Kamlet–Taft and the new four-parameter Catalan polarity scales. Multiple regression analysis indicates that both non-specific solute–solvent interactions and specific solute–solvent interactions play an important role in the position of the Stokes shift. The polarizable continuum model using IEF-PCM was considered to calculate excitation energies in methanol. The HOMO and LUMO energies have been performed by TD-DFT (B3LYP/6-311G (d, p)) approach.

Solvatochromic and preferential solvation of fluorescein in some water-alcoholic mixed solvents

It is known that the hydrogen bonding has a significant effect on the spectral properties of fluorescein, however its relevance to solution behavior and preferential solvation of fluorescein is unclear in mixed solvents. In this work, absorption spectra of fluorescein dianion have been investigated in pure water, methanol, ethanol, 1-propanol, 2-propanol, dimethylsulfoxide, N,N-dimethylformamide, acetonitrile and in binary mixtures of water with the mentioned alcohols at 298.15K. Spectral changes were interpreted in terms of specific and nonspecific solute-solvent interactions. The energy of electronic transition in maximum absorption (ET) was calculated in each binary mixture. The extent and importance of each of solute–solvent interactions to ET were analyzed in the framework of the linear solvation energy relationships. Preferential solvation was detected as a nonideal behavior of ET curve respective to the analytical mole fraction of alcohols in all binary mixtures. The ET values were fitted to a solvent exchange model to calculate the preferential solvation parameters. The preference of fluorescein dianion to be solvated by one of the solvating species relative to others was discussed in terms of solvent–solvent and solute–solvent interactions.

Solution enthalpies of 1,4-dioxane: Study of solvent effects through quantitative structure–property relationships

Solution enthalpies of 1,4-dioxane have been obtained in 15 protic and aprotic solvents at 298.15K. Breaking the overall process through the use of Solomonov's methodology the cavity term was calculated and interaction enthalpies (ΔintH) were determined. Main factors involved in the interaction enthalpy have been identified and quantified using a QSPR approach based on the TAKA model equation. The relevant descriptors were found to be π* and β, which showed, respectively, exothermic and endothermic contributions.The magnitude of π*’ coefficient points toward non-specific solute–solvent interactions playing a major role in the solution process. The positive value of the β coefficient reflects the endothermic character of the solvents’ hydrogen bond acceptor (HBA) basicity contribution, indicating that solvent molecules engaged in hydrogen bonding preferentially interact with each other rather than with 1,4-dioxane.

Solvatochromic and quantum chemical investigations of newly synthesized succinimides: substituent effect on intramolecular charge transfer

Two series of 1-aryl-3-phenyl- and 1-aryl-3,3-diphenylpyrrolidine-2,5-diones were synthesized and their solvatochromic properties were studied in a set of 15 solvents of different polarity. The effect of specific and non-specific solvent–solute interactions on the position of their absorption bands was evaluated by using the solvent parameter sets of Kamlet and Taft. The interpretation of the effect of different substituent patterns on the solvatochromic properties of the investigated compounds was based on quantum chemical calculations performed by the density functional theory (DFT)/CAM-B3LYP method using the 6-311G(d,p) basis set. The theoretical absorption frequencies show very good agreement with the experimental values. The energy gaps between the HOMO and LUMO orbitals were also analyzed. It is demonstrated that different substituents change the conjugation effect and further determine the pathways of intramolecular charge transfer.

Deprotonation of para-sulphonatocalix[4]arene in water–methanol mixtures

The deprotonation of para-sulphonatocalix[4]arene was investigated in some aqueous methanol solutions by spectrophotometric method. Principal component analysis was performed on the spectral data matrices in order to determine the number of light absorbing species which are involved in the deprotonation process. The equilibrium constant of the deprotonation reaction together with the pure spectrum and concentration profile of each species were extracted by STAR programme utilising the multivariate curve fitting. The effect of specific and non-specific solute–solvent interactions on the pK a values was examined upon the basis of the linear solvation energy relationship concept. Thus, a multiple linear regression analysis was used to correlate the pK a variation with the macroscopic (dielectric constant) and microscopic Kamlet–Taft parameters (α, β and π*) of binary mixtures. Results indicate that the deprotonation of para-sulphonatocalix[4]arene in water–methanol mixtures is highly related with the non-specific long-range electrostatic solute–solvent interactions.