The properties of mixed surfactants of 1-tetradecyl-3-methylimidazolium bromide (C14mimBr) and the gemini surfactant N,N′-bis(dimethyldodecyl)-1,2-ethanediammonium dibromide (12-2-12) in water and in ethylammonium nitrate (EAN) were studied by surface tensiometry. The critical micelle concentration (cmc) values of the mixed surfactants were determined and found to be much larger in EAN than in water. The nonideal mixing behaviors were observed at the air/solvent interface and in the mixed micelle both in water and in EAN. The surface pressure at the cmc (πcmc), the maximum surface excess (Γmax), the minimum surface area per molecule (Amin), the standard Gibbs free energies of micellization (ΔGm0) and adsorption (ΔGads0), and the excess Gibbs free energy of mixed micelles (ΔGE) were determined. It was found that physicochemical properties of C14mimBr/12-2-12 mixed systems in water and in EAN were significantly different, which was discussed in terms of different properties of the solvents.

•Coexistence curve, heat capacity and turbidity measurements were performed.•RTIL solution showed solvophobic criticality.•Universal critical amplitude ratios were testified.•Asymmetric behavior of the diameter of coexistence curve was discussed.The liquid-liquid coexistence curve, heat capacity, and turbidity of binary solution {ethanol + butylpyridinium tetrafluoroborate]} have been precisely measured. The critical exponents and critical amplitudes corresponding to the heat capacity, width of coexistence curve, osmotic compressibility, and correlation length were obtained. The critical exponents and critical amplitude ratios showed well agreements with the theoretical values of the 3D-Ising universality class. The asymmetric behavior of the coexistence curve diameter was found to be well described by the complete scaling theory with the consideration of the heat capacity related term.

The liquid–liquid coexistence curve, heat capacity in the critical and noncritical regions, and the turbidity in critical one-phase and two-phase regions of the binary solution {iodobenzene + 1-decyl-3-methylimidazolium bis(trifluorosulfonyl)imide ([C10mim][NTf2])} have been precisely measured. From the data collected in the critical region, the critical exponents α, β, γ and ν, as well as the universal critical amplitude ratios RB and X were obtained and were shown to agree well with their theoretical values for the 3D-Ising universality class, which further confirmed the 3D-Ising criticality of ionic solutions even in a very low relative permittivity solvent. The coulombic character of the studied system was suggested by the small values of the RPM reduced critical temperature and density. Furthermore, it was found that both the asymmetric behaviors of the diameter of the coexistence curve and the osmotic compressibility could be well described using the complete scaling theory.

•Reactions of alkaline fadings of MG and CV were studied in different critical media.•Thermodynamic anomaly for reaction of MG in critical 2BE solution was observed.•No thermodynamic anomaly was detected for reaction of CV in critical microemulsion.•Critical anomalies of the reactions in different experimental designs were discussed.The reaction kinetics for alkaline fading of malachite green (MG) in the critical binary solution 2-butoxyethanol + water and of crystal violet (CV) in the critical microemulsion water/bis(2-ethylhexyl) sodium sulfosuccinate (AOT)/n-decane with the molar ratio of water to AOT being 30 have been studied by the three-wavelength UV-spectrophotometry at initial reaction stage and various temperatures. It was found that the reaction rates kobs for both above reactions were well described by the Arrhenius equation in the temperature ranges far away from the critical points. The critical slowing down effects were observed in the near-critical regions and the corresponding critical slowing down exponents were determined to be 0.146 ± 0.014 and 0.035 ± 0.006 for the two reactions, respectively. The former exhibited both the dynamic and thermodynamic critical slowing down effects; while the latter showed no thermodynamic singularity. It was attributed to the particular experimental design in this work that made the dimer/monomer droplet equilibrium in the critical microemulsion system no longer affect the reaction kinetics.

•Coexistence curve and heat capacities of {1-butanol + [C2mim][NTf2]} in the critical region were measured.•The values of the corresponding critical exponents were deduced to confirm the 3D-Ising criticality.•The asymmetry of the coexistence curve was discussed by the complete scaling theory.•The RPM reduced parameters were calculated to show the solvophobic criticality of the studied system.The liquid-liquid equilibrium data and the specific heat capacities for the binary room temperature ionic solution {1-butanol + 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide} have been reported. The corresponding critical exponents β and α were obtained and found to be well coincided with the 3D-Ising criticality. The critical complete scaling theory was applied to successfully describe the asymmetric behavior of the diameter of the coexistence curve with the consideration of the heat capacity related term. The large values of the reduced critical density, the reduced critical temperature and the critical amplitude related to the heat capacity were deduced, which indicated a solvophobic criticality of the studied system.

•The isobaric heat capacities and turbidities were measured.•Critical exponents and critical amplitudes were determined.•Universal critical amplitude ratios were tested.•Complete scaling theory was examined in significantly asymmetric systems.The isobaric heat capacities per unit volume in both one-phase and two-phase regions and the turbidities in the one-phase region for the significantly asymmetric binary mixtures of {water + 2,6-dimethylpyridine} and {water, or heavy water + 2,6-dimethylpiperidine} were carefully measured, from which the corresponding critical exponents were obtained and found to be consistent with the 3D-Ising values. The values of the system-dependent critical amplitudes for the heat capacity, the correlation length and the osmotic compressibility were deduced, which together with the values of the critical amplitudes related to the coexistence curves reported previously were used to test some universal critical amplitude ratios. Moreover, the importance of the heat capacity related term in the formalism of the complete scaling theory was confirmed. It was also found that the complete scaling formalism possibly becomes somewhat inapplicable for the significantly asymmetric systems.

•Coexistence curve, heat capacity and turbidity measurements were performed.•RTIL solution showed 3D-Ising criticality.•Universal critical amplitude ratios were testified.•Asymmetric behavior of the diameter of coexistence curve was discussed.The liquid-liquid coexistence curve for binary solution {bromobenzene + 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C6mim][NTf2])} has been measured in both critical and non-critical regions. The isobaric heat capacities per unit volume were measured in one-phase and two-phase regions. Moreover, the turbidity measurements were also performed in the near-critical one-phase region. From the liquid-liquid equilibrium, heat capacity and turbidity data collected in the critical region, the critical exponents α, β, γ and ν were obtained and shown to be consistent with those predicted for the 3D-Ising universality class. The universal critical amplitude ratios RB and X have been calculated and shown to reasonably coincide with the theoretical predicted values. The asymmetric behavior of the diameter of the coexistence curve was well described by the complete scaling theory, provided that the heat capacity related term was considered. Moreover, the coulombic criticality of the studied system was indicated by the small values of RPM reduced critical density and critical temperature.

The liquid–liquid phase equilibria of [water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-decane] with the molar ratio w0 of water to AOT being 37.9 and [water/AOT/ethoxylated-2,5,8,11-tetramethyl-6-dodecyne-5,8-diol(Dynol-604)/n-decane] with w0 = 37.9 and the mole fraction α of Dynol-604 in the total surfactants being 0.158 were measured in this study. From the data collected in the critical region, the critical exponent β corresponding to the width of the coexistence curve was determined, which showed good agreement with the 3D-Ising value. A thermodynamic approach based on the Carnahan–Starling–van der Waals type equation was proposed to describe the coexistence curves and to deduce the interaction properties between droplets in the microemulsions. The interaction enthalpies were found to be positive for the studied systems, which evidenced that the entropy effect dominated the phase separations as the temperature increased. The addition of Dynol-604 into the (water/AOT/n-decane) microemulsion resulted in the decrease in the critical temperature and the interaction enthalpy. Combining the liquid–liquid equilibrium data for (water/AOT/n-decane) microemulsions with various w0 values determined previously, it was shown that the interaction enthalpy decreased with w0 and tended to change its sign at low w0, which coincided with the results from the isothermal titration calorimetry investigation. All of these behaviors were interpreted by the effects of entropy and enthalpy and their competition, which resulted from the release of solvent molecules entrapped in the interface of microemulsion droplets and were dependent on the rigidity of the surfactant layers and the size of the droplet.

The volumetric properties of the mixed surfactants bis(2-ethylhexyl) sodium sulfosuccinate and ethoxylated-2,5,8,11-tetramethyl-6-dodecyne-5,8-diol (Dynol-604) at various compositions in isooctane have been studied by density measurements at 303.15 K. The apparent specific volumes and the critical reverse micelle concentrations of the binary mixed surfactants were determined. The results were used to calculate the compositions, excess properties of mixing, activity coefficients and the interaction parameter between the two surfactants in the mixed micelles, based on Rubingh’s formalism.

The aggregation behaviors of surface active ionic liquid 1-butyl-3-methylimidazolium bis(2-ethylhexyl) sulfosuccinate in aqueous solutions were investigated by conductometry, densimetry, fluorimetry, 1H nuclear magnetic resonance (1H NMR) spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM), which confirmed two distinguished critical concentrations. The first critical concentration was believed to be the critical aggregation concentration (CAC), where two different types of aggregates were formed, namely, micelles with the size of 10 nm and vesicles with the size of 100 nm. The second critical concentration at two-fold CAC was suggested to be resulting from the insertion of the imidazolium cations into aggregates.

The interactions between sodium polyacrylate (PANa) and mixed surfactants of polyoxyethylene tert-octyl phenyl ether (TX100) and dodecyltrimethylammonium bromide (DTAB) in 40 mM NaBr aqueous solutions were studied using isothermal titration calorimetry (ITC). It was found that whether DTAB was titrated into PANa/TX100 or TX100/DTAB was titrated into PANa, three endothermic peaks were detected; representing three processes: (1) binding of DTAB monomers to PANa chains through electrostatic interactions, (2) polymer-induced micellization, and (3) cross-linking of polymer chains. The interaction mechanism was interpreted using a thermodynamic model, and it was found that when the molar ratio of bound DTAB to the carboxylate group of the polymer (CpolyDTAB/Cpoly) reached about 0.5, the polymer-induced micellization occurred; while when CpolyDTAB/Cpoly reached about 1, indicating complete neutralization of the electrostatic charges on the polymer chains, the cross-linking of the polymer chains started and precipitation was observed; finally, as CpolyDTAB/Cpoly reached 1.3, the precipitate was redissolved slowly due to reversion of the charge ratio of the polymer chains.

•Liquid–liquid coexistence curves and heat capacities of the binary solution {nitromethane + 3-pentanol} were measured.•Asymmetric criticality of coexistence curves were analyzed by the complete scaling theory.•The importance of the heat capacity related term in description of the diameter of the coexistence curve was confirmed.•An unusually large asymmetric coefficient a3 related to the compressibility was observed.The liquid–liquid coexistence curves and the isobaric heat capacities per unit volume of the binary solution {nitromethane + 3-pentanol} have been measured in a wide temperature range. The values of the critical exponents were derived and found to be in good agreements with the theoretical ones of the 3D-Ising universality. It was found that the asymmetric behavior of the diameter of the coexistence curve was well described by the complete scaling theory with the consideration of the heat capacity related term. Moreover, analysis of the asymmetric behavior of the diameters of coexistence curves showed an unusual large asymmetric coefficient indicating the compressibility of this solution was not negligible, which was significantly different from that of {polar + nonpolar} binary solutions we investigated previously.

A new surface active ionic liquid 4-butylazobenzene-4′-hexyloxy-trimethyl-ammoniumtrifluoro-acetate (BHATfO) was synthesized to achieve light-responsive and reversible micelle–vesicle transformation, which was verified by means of UV-spectroscopy, surface tension, conductivity, dynamic light scattering and transmission electron microscopy.

•The volumetric properties of surfactants in monomeric and micellar states were measured.•CMCs of mixed surfactant systems of C12mimBr/12-2-12 were determined.•The properties of mixed micelles were analyzed by Margules equation.•The volumetric and calorimetric methods for studying mixed micelles were compared.The micellization of the binary mixed surfactants comprising of the Gemini surfactant N,N′-bis(dimethyldodecyl)-1,2-ethanediammonium dibromide and 1-dodecyl-3-methylimidazolium bromide has been studied by measurements of density. The apparent molar volumes were calculated for various surfactant concentrations and used to determine the critical micelle concentrations of the mixed surfactants at various compositions. An attractive effect was suggested by negative deviations of the experimental CMC values from the ideal ones. The Margules equation was applied to evaluate the micelle compositions, the activity coefficients of both components, and the excess molar Gibbs free energies of the mixed micelles. The stability of mixed micelles was shown to be enhanced as compared to those formed by single surfactants from the negative values of the excess Gibbs free energy. The comparison of the results obtained from the volumetric and ITC measurements indicated a reasonable good accordance with each other and confirmed the reliability of both methods for investigation on the properties of the mixed micelles.

•Coexistence curves, heat capacities and turbidities were measured.•Deuterium effect on coexistence curves was discussed.•Universal critical amplitude ratios were tested.•Asymmetry of coexistence curves was analyzed by the complete scaling theory.The (liquid + liquid) coexistence curves, the isobaric heat capacities per unit volume and the turbidities for the binary solution of {heavy water + 2,6-dimethylpyridine} have been precisely measured. The values of the critical exponents were obtained, which confirmed the 3D-Ising universality. It was found that the critical temperature dropped by 5.9 K and the critical amplitude of the coexistence curve significantly increased as compared to the binary solution of {water + 2,6-dimethylpyridine}. The complete scaling theory was applied to well describe the asymmetric behavior of the diameter of the coexistence curve as the heat capacity contribution was considered. Moreover, the values of the critical amplitudes of the correlation length and the osmotic compressibility were deduced, which together with the critical amplitudes of the coexistence curve and the heat capacity to test universal amplitude ratios.

The liquid–liquid coexistence curves for binary solutions {1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) + 1-propanol} and {1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) + 2-propanol} have been precisely measured. The values of the critical exponents β obtained from the liquid–liquid equilibrium data in the critical region confirmed the 3D-Ising universality. The isobaric heat capacities per unit volume were measured for {[C4mim][BF4] + 1-propanol (or 2-propanol, 1,3-propanediol, 1,4-butanediol)} in both critical and non-critical regions. The experimental results indicate a major solvophobic contribution to the criticality for the studied ionic solutions. The complete scaling theory was applied to well represent the asymmetric behavior of the diameter of the coexistence curves with the consideration of the heat capacity contribution. It was found that the contribution of the heat capacity related term in the ionic solution decreased with the increase of the permittivity of alcohol and was more important in the description of the asymmetry of the coexistence curve of the ionic solutions than that of the molecular solutions.

The interactions between the anionic polymer ι-carrageenan (IC) and the cationic surfactants 1-dodecyl-3-methylimidazolium bromide (C12mimBr), dodecyltrimethylammonium bromide (DTAB) and ethyl-α,ω-bis(dodecyldimethylammonium)dibromide (12-2-12) have been studied by fluorimetry and isothermal titration calorimetry. Our experimental results showed that at a low surfactant concentration, the monomers adsorbed on the IC chains through the electrostatic attraction, followed by the formation of induced micelles on the IC chains through the hydrophobic interaction until the IC chains are saturated by surfactant molecules; after that the added surfactant formed free micelles in the solution. A pseudo-phase-equilibrium thermodynamic model was proposed to explain the experimental results and to understand the mechanisms of the interactions in these three systems. Moreover, the salt effect on the interactions was investigated and found that it changed the critical concentrations but not the interaction mechanism.

The measurements of the liquid–liquid coexistence curve and the heat capacity for binary mixture {1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) + 1-propanol} have been precisely performed. The values of the critical exponents α and β in the critical region were obtained and coincided well with the 3D-Ising ones. The complete scaling theory was applied to well represent the asymmetric behavior of the diameter of the coexistence curve, indicating an important role of the heat capacity related term in the complete scaling formulizm. A comparison of the reduced critical parameters to those predicted by the restricted primitive model clearly showed the solvophobic criticality of the studied system.

•Interactions of surfactants with BSA were investigated by various techniques.•Property of the spacer in Gemini surfactant played an important role.•Gemini surfactants can interact much stronger than single-chain surfactants.The interactions of Gemini surfactants 1-dodecyl-N,N′-[(1,4-dioxo-1,4-butanediyl)bis(oxy-2,1-ethanediyl)]bis[N,N′-dimethylammonium]dibromide (12-su-12) and 1-dodecyl-N,N′-[((2E)-1,4-dioxo-2-butene-1,4-diyl)bis(oxy-2,1-ethanediyl)]bis[N,N′-dimethylammonium]dibromide (12-fo-12) and the corresponding single-chain surfactant 1-dodecyl-N-[2-(acetyloxy)ethyl]-N,N-dimethyl ammonium bromide (DTAAB) with bovine serum albumin (BSA) were investigated by conductometry, fluorimetry, UV–vis and CD spectrometry, and dynamic light scattering. It was found that the bindings of all three surfactants to BSA were cooperatively driven by electrostatic and hydrophobic interactions. The Gemini surfactants, carrying more charges and hydrophobic tails, showed stronger interactions with BSA than the single-chain surfactant. The presence of the double bond in the spacer of 12-fo-12 enhanced the interaction between the surfactant and BSA and also significantly varied the binding sites as compared to 12-su-12 and DTAAB, which indicated an important role of the property of the spacer in the interaction between the surfactant and BSA.

The liquid–liquid coexistence curve for nitrobenzene + n-nonane has been measured in a wide temperature range. The values of the critical exponent β obtained from the liquid–liquid equilibrium data in the critical region were found to be in good agreement with that of the 3D Ising universality. The monotonous crossover behavior from Ising criticality to mean-field one was observed. The isobaric heat capacities per unit volume were measured, with which the asymmetric behavior of the diameter of the coexistence curve was well-described by the complete scaling theory. Moreover, the turbidity data near the critical point were obtained to deduce the critical amplitudes of the correlation length and the osmotic compressibility, which together with the amplitudes related to the coexistence curve and the heat capacity were used to calculate the universal scaling ratios and to confirm their theoretical predictions.

•Coexistence curves of dimethyl carbonate + n-undecane (or + n-tridecane) were measured.•Isobaric heat capacity per unit volume of critical binary solutions dimethyl carbonate + n-undecane (or + n-tridecane) were determined.•The critical exponent β are consistent with the 3D-Ising value.•The asymmetry of the coexistence curves were discussed by the complete scaling theory.The (liquid + liquid) coexistences and the critical behavior of isobaric heat capacity per unit volume for critical binary solutions {dimethyl carbonate + n-undecane, or n-tridecane} have been studied. The critical exponents β and α were deduced and found to be consistent with the 3D-Ising values. The critical amplitudes were determined and used to test the asymmetric criticality of coexistence curves. It was found that the heat capacity does play an important role in describing the asymmetric criticality of the coexistence curves.Graphical abstract

The aggregation behaviors of two cationic surfactants, namely, N-dodecyl-N-2-hydroxyethyl-N,N-dimethyl ammonium bromide (C12HDAB) and N-dodecyl-N,N-2-dihydroxyethyl-N-methyl ammonium bromide (C12DHAB), were studied by measurements of density, conductivity, and steady-state fluorescence quenching. The aggregation number, the volumetric properties, and the standard Gibbs energy (ΔGmic0) of micellization were obtained, and the surface area (Amic) of the surfactant headgroups and the radius (r) of the micelles were estimated. It was found that substituting the methyl group with the hydroxyethyl group in the head of the surfactant led to the increase of the volume change of micellization and the decreases of the critical micelle concentration, association degree of the counterion, and the value of ΔGmic0, while the aggregation number of those surfactants in aqueous solutions was little affected by introducing the hydroxyethyl group. It was also found that the values of Amic and r increase with the hydroxyethyl group.

Highlights•Densities of mixed micelle system of AOT/Dynol-604 were measured.•Experimental values of CMC and volumetric properties were determined.•Excess thermodynamic properties of the surfactant mixtures were reported.Volumetric properties of mixed surfactants of Bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and Ethoxylated-2,5,8,11-tetramethyl-6-dodecyne-5,8-diol (Dynol-604) in aqueous solutions with various compositions have been studied at 298.15 K by density measurements. The apparent molar volumes, the partial molar volumes, the critical micelle concentrations, and the apparent molar volumes in the monomeric and micellar states of the binary mixed surfactants with various compositions were determined. Moreover, the results were used to calculate the micellar compositions, the excess free energies of mixing, the activity coefficients and the interaction parameter between two surfactants for the mixed micelles based on the Rubingh’s formulism.

Densities of binary mixtures {x dimethyl adipate + (1 − x) n-heptane} and {x dimethyl adipate + (1 − x) n-octane} were measured at various temperatures. The excess molar volumes were calculated from the density data and fitted with the Redlich–Kister equation. The critical anomaly of density was detected for {x dimethyl adipate + (1 − x) n-octane} binary solutions with the critical compositions in a narrow temperature range of the near-critical one-phase region. However, the density anomaly and the excess properties were taken into account in a test of the complete scaling theory and found to be negligible. The isobaric heat capacities per unit volume of the {x dimethyl adipate + (1 − x) n-heptane} and {x dimethyl adipate + (1 − x) n-octane} binary solutions at the critical compositions were also determined over the temperature range both near and far away from the critical points. It was found that the heat capacity plays an important role in describing the asymmetry of the coexistence curves.

Two Gemini surfactants with very similar structure but different spacer rigidity, namely 1-dodecanaminium,N,N′-[[(2E)-1,4-dioxo-2-butene-1,4-diyl]bis(oxy-2,1-ethanediyl)]bis[N,N-dimethyl-,bromide] (12-fo-12) and 1-dodecanaminium, N,N′-[(1,4-dioxo-1,4-butanediyl)bis(oxy-2,1-ethanediyl)] bis[N,N-dimethyl-, bromide] (12-su-12), and their monomeric counterpart 1-dodecanaminium, N-[2-(acetyloxy)ethyl]-N,N-dimethyl-, bromide (DTAAB) were synthesized and their aggregation behavior in aqueous solutions was studied by measurements of surface tension, conductivity, isothermal titration calorimetry, dynamic light scattering, and transmission electron microscopy. It was found that the Krafft point of 12-fo-12 was 18.6 °C, significantly higher than that of 12-su-12 (7.6 °C) and DTAAB (<0 °C). The minimum surface areas per surfactant Amin at the water–air interface of DTAAB, 12-su-12, and 12-fo-12 were determined. It was found that the value of Amin of DTAAB was larger than half that of 12-su-12 but smaller than half that of 12-fo-12. The values of the degree of association β of the three surfactants were found to be in a sequence of DTAAB > 12-su-12 > 12-fo-12, which was in accord with the sequence of the entropy of micellization. The enthalpies of micellization of the two Gemini surfactants were found to be more negative than double that of DTAAB, and 12-fo-12 had the most negative standard enthalpy of micellization. It was also found that 12-su-12 and DTAAB formed micelles in aqueous solutions, while 12-fo-12 could form micelles and vesicles dependent on the concentration.Graphical abstractHighlights► Two Gemini surfactants and their monomeric surfactant were synthesized. ► The rigidity was isolated from the other properties of the spacer. ► The surfactant with rigid spacer showed a higher surface activity. ► The dependence of the thermodynamics of aggregation on the rigidity was discussed. ► Different aggregation morphologies were found for different spacer rigidities.

Turbidities and isobaric heat capacities per unit volume, in a wide temperature range, have been measured for critical binary solutions of {nitrobenzene + n-undecane} and {nitrobenzene + n-dodecane}. The critical exponents and the system-dependent critical amplitudes were deduced. The non-critical and the critical-fluctuation induced contributions to the background heat capacities were determined. We also obtained the coupling constant \( \bar{u} \) by analyzing the coexistence curve data with crossover theory. These parameters were used to test some universal amplitude ratios and together with the coexistence-curve data to test the complete scaling theory. It is shown that the contribution from heat capacity plays an important role in describing the asymmetric criticality of the coexistence curve.

Liquid–liquid coexistence curves and turbidity measurements for the binary fluid mixture of {benzonitrile + tridecane} were conducted, from which the corresponding critical exponents and critical amplitudes were obtained. It was found that the critical exponents were in accordance with the theoretic predictions and the critical amplitudes supported the well-known two-scale-factor universality. Moreover, the coexistence curve plotted with temperature against mole fraction was found to be well-represented by the formulism proposed by Gutkowski et al. in the frame of the crossover theory, and the asymmetry of its diameter confirmed the complete scaling theory.

(Liquid + liquid) coexistence curve, turbidity, and isobaric heat capacity per unit volume for the critical solution of {benzonitrile + n-undecane} have been measured. The critical exponents β, ν, γ, and α have been deduced, which were found to be consistent with the theoretic predictions. Meanwhile, the experimental data have also been analyzed to obtain the system-dependent critical amplitudes B, ξ0, χ0, A±, and D corresponding to the difference of the general density variable of two coexisting phases Δρ, the correlation length ξ, the osmotic compressibility χ, the isobaric heat capacity per unit volume CpV−1, and the first term of correction-to-scaling for the isobaric heat capacity per unit volume, which were used to test some universal ratios. It was found that the coexistence curve may be well described by the crossover model proposed by Gutkowski et al. The critical-fluctuation induced contribution to the background heat capacity Bcr was obtained and used to analyze the asymmetric behavior of the diameter of the coexistence curve. The result indicated that the asymmetry of the coexistence curve can be well described by the complete scaling theory proposed by Anisimov et al., and the heat capacity does make a significant contribution to this asymmetric behavior.Highlights► Coexistence curve, isobaric heat capacity and turbidity measurements have been reported. ► Asymmetry of the coexistence curves has been analyzed by the complete scaling theory. ► Heat capacity has been shown to be important in describing the asymmetric criticality. ► Universal amplitude ratios have been tested.

The interactions of curcumin with cationic micelles of gemini surfactant trimethylene-1,3-bis-(dodecyldimethyl ammonium bromide) (C12C3C12Br2) and single-chain surfactant dodecyltrimethylammonium bromide (DTAB) were investigated. At pH 13, the trianionic curcumin (Cur3−) had rapid degradation to about 21.5% with an observed rate constant (kobs) of 0.0402 h−1 in water, whereas DTAB micelle and C12C3C12Br2 micelle can suppress Cur3− degradation to 9.6% with kobs = 0.0174 h−1 and to 5.0% with kobs = 0.00782 h−1, respectively. Cur3− also showed higher intensities of characteristic peaks with the position changes in absorption and fluorescence spectra as well as increased anisotropy (r) in two surfactant micelles than in water. These results were ascribed to the solubilization of Cur3− in the palisade layer of DTAB and C12C3C12Br2 micelles through the attractive electrostatic interactions between negatively charged groups of Cur3− and quaternary ammonium headgroups of surfactants, which was also supported by the values of lowest pKa (pKa1) and corresponding thermodynamic parameters of curcumin in water, DTAB and C12C3C12Br2 micelles. The stronger electrostatic attractions between Cur3− and the micelle of C12C3C12Br2 brought about more increased stability of Cur3− and more remarkable changes in absorption and fluorescence spectra of Cur3− in C12C3C12Br2 micelle than in DTAB micelle.Graphical abstractHighlights► Stability of trianionic curcumin can be improved by cationic surfactant micelles. ► Trianionic curcumin is located in micelle palisade layer by electrostatic attraction. ► Gemini surfactant micelle leads to more remarkable changes in curcumin's spectra. ► Structure and aggregation of surfactant molecules have an important role.

A new experimental design for the measurement of the real heat of dilution of the microemulsion droplets by isothermal titration microcalorimetry (ITC) has been reported and used to study the interaction enthalpies of the droplets for the system of water/sodium bis(2-ethylhexyl)-sulfosuccinate (AOT)/toluene. The results are in good agreement with those determined from light-scattering experiments.

The critical behavior of isobaric heat capacities per unit volume for a series of critical binary solutions {benzonitrile + octane, or dodecane, or hexadecane} and {dimethyl carbonate + nonane, or decane, or dodecane} were studied. The corresponding exponent was obtained to be in consistent with the 3D-Ising value. The amplitudes in one-phase and two-phase regions were deduced, which were used to test some critical amplitude ratios. Analysis of the dependence of the effective critical exponent of the heat capacity on the temperature indicated a critical crossover from the 3D-Ising to the mean-field for all the studied systems. It was found that the heat capacity does play an important role for describing the asymmetric criticality of coexistence curves by the complete scaling theory.

The liquid–liquid coexistence for dimethyl carbonate + nonane and dimethyl carbonate + dodecane have been measured. The critical amplitudes and the critical exponents are obtained from the measured experimental data. The critical exponents corresponding to the coexistence curve β are consistent with the 3D-Ising values. The liquid–liquid coexistence results have also been analyzed to determine Wegner-correction terms when β and Δ are fixed at their theoretical values and to examine the diameters and the asymmetry of the coexistence curves.

The liquid−liquid coexistences of temperature vs refractive index (T−n), temperature vs mole fraction (T−x), and temperature vs volume fraction (T−ϕ) for dimethyl carbonate + hexadecane and dimethyl carbonate + octadecane have been determined within 10 K from the critical temperatures. The critical amplitudes B and the critical exponents β have been deduced from the coexistence curves. The results have also been analyzed to determine Wegner correction terms and to examine the asymmetric behavior of the diameters of the coexistence curves. Experimental values of β are consistent with the 3D Ising value in the whole temperature range investigated for (T−n) and (T−ϕ), while the tendency of crossover to the mean field value was detected for (T−x). The data of coexistence curves (T−x) were fitted with the crossover equation proposed by Anisimov and Sengers to obtain the crossover parameters. It was found that the contribution of the regular term was significant as the temperature was far away from the critical point and the value of effective critical exponent increased with the distance from the critical point, exhibiting the monotonic crossover from the Ising to the mean-field critical behaviors.

The liquid−liquid coexistence curves of the binary solutions of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) + 1,3-propanediol and [C4mim][BF4] + 1,4-butanediol were determined by measuring the temperature dependence of the refractive index in the two phases of the samples with critical compositions. The critical amplitude B and the critical exponent β have been deduced from the coexistence curves. The experimental values of β are consistent with theoretical prediction. The results have also been analyzed to determine the Wegner-correction terms and to examine the asymmetric behavior of the diameters of the coexistence curves.

The liquid−liquid coexistence of dimethyl carbonate + n-decane and dimethyl carbonate + n-tetradecane has been determined within 10 K from the critical temperature by measurements of refractive index n. The critical amplitude B and the critical exponent β have been deduced from the coexistence curves. Experimental values of β are consistent with theoretical prediction. The results have also been analyzed to determine Wegner-correction terms when β and Δ are fixed at the theoretical values and to examine the asymmetric behavior of the diameters of the coexistence curves.

The liquid–liquid coexistence curves for binary solutions {1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) + 1-propanol} and {1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) + 2-propanol} have been precisely measured. The values of the critical exponents β obtained from the liquid–liquid equilibrium data in the critical region confirmed the 3D-Ising universality. The isobaric heat capacities per unit volume were measured for {[C4mim][BF4] + 1-propanol (or 2-propanol, 1,3-propanediol, 1,4-butanediol)} in both critical and non-critical regions. The experimental results indicate a major solvophobic contribution to the criticality for the studied ionic solutions. The complete scaling theory was applied to well represent the asymmetric behavior of the diameter of the coexistence curves with the consideration of the heat capacity contribution. It was found that the contribution of the heat capacity related term in the ionic solution decreased with the increase of the permittivity of alcohol and was more important in the description of the asymmetry of the coexistence curve of the ionic solutions than that of the molecular solutions.