Computational prediction of the critical micelle concentration (CMC) of surfactants using the non-Bornian solvation model.

The non-Bornian solvation model was used to predict the Gibbs energy change for the adsorption-desorption processes of ionic (14 anionic and 9 cationic) surfactants and 19 non-ionic surfactants at the interface between oil (O) (=nitrobenzene) and water (W). Except for 10 non-ionic surfactants (polyoxyethylenes) having semi-hydrophobic -OC2H4- groups, both ionic and non-ionic surfactants showed a clear energy minimum in their adsorption-desorption processes, providing reliable values of Gibbs energies, and , for the two adsorption processes from their respective bulk phases to the interface (I). It was then found that the critical micelle concentration (CMC) for surfactants (especially for the ionic ones) is linearly related to the two independent variables, i.e., and .

Redox reactions between ABTS•+ and dihydroxybenzenes as studied by cyclic voltammetry.

Redox reactions between several types of polyphenols and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS•+) used in a trolox equivalent antioxidant capacity (TEAC) assay were monitored by continuous cyclic voltammetry. The TEAC assay is one of the effective methods for clarifying the radical scavenging reaction mechanism of antioxidants. We obtained information on whether the reaction was a simple electron transfer, an electron transfer involving a subsequent chemical reaction of the antioxidant itself, or an electron transfer involving a coupling reaction between ABTS•+ and the antioxidant.

DFT Study of α-Keggin-type Iso-polyoxotungstate Anions [HnW12O40](8-n)- (n =1-4): Can [H4W12O40]4- Exist?

The Keggin-type iso-polyoxotungstate (iso-POT) anions [HnW12O40](8-n)- 's, in which their central vacancies are occupied by protons, are attractive materials. It is of importance to reveal if the vacancies can be fully occupied by four protons. For further understanding the speciation of these iso-POT anions, relative stabilities and proton transfer reactions between H1[Hn-1W12O40](8-n)- and [HnW12O40](8-n)- were examined in detail by using the first-principles calculations (the nudged elastic band method, the synchronous transit-guided quasi-Newton method, the intrinsic reaction coordinate method, and frequency analysis calculations). Thermochemistry analysis of the proton transfer was also performed. [HnW12O40](8-n)- was energetically more stable than H1[Hn-1W12O40](8-n)-. This held for n = 4. According to the results of thermochemistry analysis, the rate constant and the Wigner correction were respectively 3.1 × 101 s-1 and 2.2 at T = 298.15 K for the proton transfer from H1[H3W12O40]4- to [H4W12O40]4-, indicating that [H4W12O40]4- can exist when H1[H3W12O40]4- is formed by protonating [H3W12O40]5-.

A Theoretical Approach to the Fluorophilicity of Ions via the Gibbs Energy of Ion Transfer at the Fluorous Solvent/Water Interface.

The non-Bornian solvation model has been applied to a theoretical consideration of the Gibbs free energy for the transfer of fluorinated anions, non-fluorinated cations, and non-fluorinated anions at the 2H,3H-decafluoropentane (DFP)/water (W) and 1,2-dichloroethane (DCE)/W interfaces. According to our previous experimental results, the fluorinated anions are more stable in DFP than DCE, while the non-fluorinated cations and anions are less stable in DFP. To understand this characteristic feature of DFP, energy decomposition analyses have been performed for the hypothetical transfer of ions at the DFP/DCE interface. In conclusion, the characteristics of DFP as a fluorous solvent should be explained in terms of the higher repulsive interaction of the solvent molecule with ions, particularly with non-fluorinated ions.

Fluorination Effect on the Gibbs Transfer Energy for Methylene Group from 1,2-Dichloroethane or 1,1,1,2,3,4,4,5,5,5-​Decafluoropentane to Water.

The ion-transfer reactions of alkyl and perfluoroalkyl carboxylate ions (CH3(CH2)n-2COO- and CF3(CF2)n-2COO-) at 1,2-dichloroethane (DCE) | water (W) and 1,1,1,2,3,4,4,5,5,5-decafluoropentane (DFP) | W interfaces were investigated. These ions gave reversible or quasi-reversible voltammetric waves due to their ion transfer across the interfaces, and the formal potentials and the formal Gibbs transfer energies from a non-aqueous solvent to water, ΔG°'tr,α→W (α = DCE and DFP), were determined. The ΔG°'tr,α→W for CH3(CH2)n-2COO- and CF3(CF2)n-2COO- linearly increased with n, allowing for estimating ΔG°'tr,α→W for methylene groups. The estimated value of ΔG°'tr,DCE→W for the -CH2- group was higher than that of ΔG°'tr,DCE→W for the -CH2- group, whereas the ΔG°'tr,DCE→W for the -CF2- group was lower than that of ΔG°'tr,DCE→W for the -CF2- group, indicating that the -CH2- (or -CF2-) group is more favorably (or unfavorably) solvated in the DCE phase compared to the DFP phase. From the estimated values, the fluorination effect of alkyl chains on partitioning the alkyl group between the biphase media has also been discussed.

Ion-Transfer Voltammetry at Fluorous Ether | Water Interfaces.

This paper describes that fluorous ethers, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, can be used for a non-aqueous medium in electrochemistry at a liquid | liquid interface. These solvents dissolved a high concentration of tetraalkylammonium salts with highly-fluorinated anions, such as bis(nonafluorobutanesulfonyl)imide and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ions, and the solution had a high conductivity. The fluorous ether | water interfaces exhibited a substantial polarizable potential window, and various ions, including tetraphenylarsonium and tetraphenylborate ions, gave a reversible voltammetric wave due to their ion transfer across the interfaces. Using the tetraphenylarsonium-tetraphenylborate assumption, the formal potentials for the ion transfer and the formal Gibbs energies of ion transfer from the ethers to water were estimated. The Gibbs energies were close to those from a previously reported fluorous solvent, 1,1,1,2,3,4,4,5,5,5-decafluoropentane; the fluorous ethers also exhibited a higher affinity for fluorinated ions. Because of a lower volatility, the fluorous ethers would be more advantageously used in two-phase electrochemistry, particularly concerning analytical purposes.

Computational Prediction of Adsorption Equilibrium for Nonionic Surfactants at the Oil/Water Interface.

The non-Bornian solvation model has been applied for predicting the adsorption equilibrium for nonionic surfactants at the oil (O)/water (W) interface. In the non-Bornian model, the small contribution from the long-range electrostatic interaction is ignored, and the solvation or resolvation energy is formulated based on the short-range solute molecule (or ion)-solvent interactions-cavity formation, Coulomb, polarization, charge transfer, etc. These interaction energies are given by zero, first, and second-order functions of the local electric field (Ei) on the molecular surface, which can be estimated by density functional theory calculation. In the present study, we considered an adsorption process as "partial" transfer of a molecule across the O/W interface. Using a non-Bornian, semi-empirical equation for the Gibbs energy of transfer of nonionic molecules, the adsorption states of alkyl alcohols (1-dodecanol, 1-octanol, and 1-hexanol) at the 1,2-dichloroethane/W interface were successfully predicted. The orientation angle (θ), the rotation angle (ω), and the penetration depth into the O phase (d) of the alcohols in the adsorption state could be estimated. Furthermore, the energies for the adsorption from O and W (ΔGad°,O→I and ΔGad°,W→I) could be estimated theoretically. The values of ΔGad°,O→I for the alcohols studied were in good agreement with those determined experimentally by the drop-weight method.

Gibbs Transfer Energies of Ions from a Mixed Solvent of 2H,3H-Decafluoropentane and 1,2-Dichloroethane to Water.

The transfer of hydrophilic, lipophilic, and fluorophilic ions at the interface between water (W) and a mixed solvent (MIX) of 2H,3H-decafluoropentane (DFP) and 1,2-dichloroethane (DCE) was studied voltammetrically and potentiometrically, and the formal Gibbs transfer energies of the ions from MIX to W, ΔG0'tr,MIX→W, were determined. The ΔG0'tr,MIX→W values of all the ions tested were higher than those from DFP to W. Namely, the ions would exist more stably in MIX than DFP, even for fluorophilic ions. This is due to the addition of DCE, which has a higher dielectric constant. A comparison of ΔG0'tr,MIX→W with that from DCE to W showed a superior affinity of fluorophilic ions to the fluorous solvent in spite of equivolume addition of DCE. Therefore, the mixed solvent would be a practically superior extraction medium for fluorophilic ions. In practice, the MIX | methylene blue+ (W) system showed higher extractability of a fluorophilic ion C8F17SO3- than the DFP | W and DCE | W systems.

Is the Oil | Water Interface the Simplest and Best Suited Model for Understanding Biomembranes?

Many studies have been conducted by using the oil (O) | water (W) interface as a simple model for understanding ion transfer (IT) or electron transfer (ET) across biomembranes. In this review, we revisit the usability of the O | W interface as a biomembrane model. For understanding biomembrane IT, the O | W interface is the simplest and best suited model. For example, the standard Gibbs transfer energy of drug ions at the O | W interface is a useful measure for evaluating their membrane permeability in a conventional in vitro assay, called PAMPA. However, the O | W interface is not necessarily a good model for understanding biomembrane ET. This is because no net current can be observed with the O | W interface, owing to the ET-coupled proton transfer. In such a case, the self-assembled monolayer (SAM) formed on a metal electrode serves as a better model for understanding biomembrane ET.

Prediction of the Standard Gibbs Energy of Ion Transfer across the 1,2-Dichloroethane/Water Interface.

The standard Gibbs energy of ion transfer at the 1,2-dichloroethane/water interface (ΔGtr°,W→O) was determined for 26 organic cations and 24 anions by means of ion-transfer voltammetry with a micro oil/water interface. Based on the data sets, a theoretical analysis was performed with the non-Bornian solvation model, in which the solvation energy of an organic ion is evaluated from local electric fields on the surface of the ion. The semi-empirical equations thus obtained are available for relatively accurate prediction of ΔGtr°,W→O for organic ions. The mean absolute error was 1.9 or 3.1 kJ mol-1 for cations or anions, respectively, corresponding to the error of ∼20 or ∼30 mV in the standard ion-transfer potential. In this paper, energy decomposition has been performed to discuss different contributions to ΔGtr°,W→O from the "hydrated" (strongly charged) and positively and negatively charged "non-hydrated" (moderately charged) surfaces as well as from the hydrophobic interaction (cavity formation energy).

Solvate and protic ionic liquids from aza-crown ethers: synthesis, thermal properties, and LCST behavior.

In recent years, solvate and protic ionic liquids (ILs) have attracted much attention. We synthesized both types of ILs from alkyl aza-crown ethers (L = N-propyl-1-aza-15-crown-5 (L1) and N-C6F13C2H4-1-aza-15-crown-5 (L2)). The solvate ILs [ML][Tf2N] (M = Na+, K+) were solids (Tm = 58-68 °C), whereas the solvate ILs [ML][Tf2N] (M = Li+, Ag+) and protic ILs [HL][Tf2N] were liquids with low glass transition temperatures. The ILs containing Na ions were more crystalline and exhibited higher melting points than the other ILs. The decomposition temperatures of the protic ILs were higher than those of the solvate ILs. A protic IL with a paramagnetic anion, [HL1][FeCl4] (Tm = 70.5 °C), was also synthesized and its crystal structure was determined. The solvate ILs [NaL2][X] (X = Cl-, CF3CO2-, TsO-, PhSO3-) exhibited a lower critical solution temperature (LCST)-type behavior in water. The effects of salt addition on the LCST of L2 were also investigated. The LCST of these ILs generally increased with increasing hydrophilicity or basicity of the counter anion. This tendency, which is nearly opposite to that of ILs with quaternary onium cations, is ascribed to the amphiphilic nature of the cation. The corresponding protic ILs did not exhibit LCST behavior.

Can Electron-Rich Oxygen (O2-) Withdraw Electrons from Metal Centers? A DFT Study on Oxoanion-Caged Polyoxometalates.

The answer to the question "Can electron-rich oxygen (O2-) withdraw electrons from metal centers?" is seemingly simple, but how the electron population on the M atom behaves when the O-M distance changes is a matter of controversy. A case study has been conducted for Keggin-type polyoxometalate (POM) complexes, and the first-principles electronic structure calculations were carried out not only for real POM species but also for "hypothetical" ones whose heteroatom was replaced with a point charge. From the results of natural population analysis, it was proven that even an electron-rich O2-, owing to its larger electronegativity as a neutral atom, withdraws electrons when electron redistribution occurs by the change of the bond length. In the case where O2- coexists with a cation having a large positive charge (e.g., P5+(O2-)4 = [PO4]3-), the gross electron population (GEP) on the M atom seemingly increases as the O atom comes closer, but this increment in GEP is not due to the role of the O atom but due to a Coulombic effect of the positive charge located on the cation. Furthermore, it was suggested that not GEP but net electron population (NEP) should be responsible for the redox properties.

Determination of the Electrostatic Potential of Oil-in-Water Emulsion Droplets by Combined Use of Two Membrane Potential-Sensitive Dyes.

The fluorescence behaviors of potential-sensitive dyes including anionic DiBAC4(3) (denoted by dye A), DiSBAC2(3) (dye B), and zwitterionic di-4-ANEPPS (dye C) were studied in oil-in-water (O/W) emulsions. In this study, the equilibrium Galvani potential difference (ΔOWφeq) of the O/W-emulsion droplets was controlled by changing the ratio of the concentrations of electrolytes added to the O (=1,2-dichloroethane) and W phases. When using an adequate combination of the dyes, i.e., B and C, we could observe that the ratio of their fluorescence peak intensities was changed from 1.08 to 1.38, depending on the change of (ΔOWφeq from 26 to 73 mV. It is desirable to apply this method to study the potential-dependent ion or electron-transfer reactions occurring at vesicles or liposomes, and also to biomembranes.

Photoinduced Charge-Transfer State of 4-Carbazolyl-3-(trifluoromethyl)benzoic Acid: Photophysical Property and Application to Reduction of Carbon-Halogen Bonds as a Sensitizer.

The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluoromethyl)benzoic acid was confirmed. It showed a higher catalytic activity in terms of yield and selectivity in the photochemical reduction of alkyl halides compared to the parent carbazole. Even unactivated primary alkyl bromides could be reduced by this photocatalyst. The high catalytic activity is rationalized by considering the slower backward single-electron transfer owing to the spatial separation of the donor and acceptor subunits.

Evaluation of the artificial membrane permeability of drugs by digital simulation.

A digital simulation method has been developed for evaluating the membrane permeability of drugs in the parallel artificial membrane permeation assay (PAMPA). The simulation results have shown that the permeability coefficient (log Ppampa) of drugs is linearly increased with increasing their distribution coefficient (log KD,M) to the lipid membrane, i.e., the hydrophobicity of the drug molecules. However, log Ppampa shows signs of leveling off for highly hydrophobic drugs. Such a dependence of log Ppampa is in harmony with the reported experimental data, and has been well explained in terms of the change in the rate-determining step from the diffusion in the membrane to that in the unstirred water layer (UWL) on both sides of the membrane. Additionally, the effects of several factors, including lag time, diffusion coefficient, pH, and pKa, on the permeability coefficient have been well simulated. It has thus been suggested that the proposed method should be promising for in silico evaluation of the membrane permeability of drugs.

Prediction of the Standard Gibbs Energy of Transfer of Organic Ions Across the Interface between Two Immiscible Liquids.

The non-Bornian solvation model was applied for evaluation of the standard Gibbs energy (ΔGtr°,W→O) of transfer of organic ions from water (W) to organic solvent (O = nitrobenzene). The solvation energy of an ion in either W or O is basically formulated as the energy required for the formation of a nanosized ion­solvent interface around the ion; however, many organic ions with strongly charged groups (e.g., -SO3-, -CO2-, -NH3+) are preferentially hydrated in O. Here we divided the surface of an ion into "hydrated" and "non-hydrated" surfaces and then carried out regression analyses with experimental values of ΔGtr°,W→O. In the analyses, the local electric field on the surface of an organic ion was evaluated through density functional theory calculation. Good regression results were then obtained with the mean absolute error of 1.9 and 2.4 kJ mol-1 for 34 anions and 63 cations, respectively. These errors correspond to the error of ∼20 mV in the standard ion-transfer potential (ΔOWϕ°), being only two times larger than the typical experimental error (∼10 mV) in the voltammetric measurement. This non-Bornian model is promising for theoretical prediction of ΔGtr°,W→O (or ΔOWϕ°) for organic ions and possibly of the biomembrane permeability for ionic drugs.

Coextraction of water into nitrobenzene with organic ions.

Various organic anions (sulfonates (RSO3(-)), carboxylates (RCO2(-)), and phenolates (RO(-))) and ammonium cations (RNH3(+), R2NH2(+), and R3NH(+)) were distributed in the nitrobenzene (NB)-water system by using Crystal Violet and dipicrylaminate, respectively. The number of water molecules (n) being coextracted into NB with an ion was then determined by the Karl Fischer method. The n values determined and those reported previously showed the variation from 0.51 to 3.4, depending on not only the charged groups but also the noncharged R-groups. In this study, we focused our attention to the strong electric field on the charged group and its facilitation effect for binding water molecules in NB. The local electric field (Ei) on the surface of an organic ion was evaluated by using Gaussian09 program with a subprogram developed in our recent study. It was found that the n values showed a clear dependence on the average value of Ei on oxygen or hydrogen atoms, respectively, of an anionic or cationic group.

How can multielectron transfer be realized? A case study with keggin-type polyoxometalates in acetonitrile.

Theoretical consideration and computational simulation have been performed on the voltammetric properties of Keggin polyoxometalates (POMs), and the conversion from successive one-electron transfer in unacidified media to four-electron transfer (through two-electron transfer) in acidified media has been discussed. Perfect simulation of the cyclic voltammograms of POMs could be achieved using the standard formal potentials and the protonation constants, systematically evaluated by the equations, in which "simple (intrinsic)" and "synergistic (extrinsic)" electron-withdrawing effects of the µ4-oxygen were taken into consideration. In the proposed model, the formal potential of the one-electron redox waves for the ith reduction step is presented by Ei°(z0, s) = Ei** + 0.51(z0 ­ i + 1) + 1.067s (i = 1, 2, 3, 4; E1** = E2** = 0.577 V; E3** = E4** = 0.377 V), where z0 is the initial ionic charge of a Keggin POM and s is the mean bond valence of the µ4-O­W bonds in the POM. The values of Ei**s are related to the energy levels of the two lowest unoccupied molecular orbitals (LUMOs) of a hypothetical Keggin POM with null charge and null bond valence. Then it was revealed that the LUMOs have small on-site repulsion, which may be an important factor that makes multielectron transfer feasible. These findings would give a big clue in developing novel redox materials exhibiting multielectron transfers.

Combined use of two membrane-potential-sensitive dyes for determination of the Galvani potential difference across a biomimetic oil/water interface.

The fluorescence behavior of anionic membrane-potential-sensitive dyes, bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)) and bis-(1,3-diethylthiobarbituric acid)trimethine oxonol (DiSBAC2(3)), at a biomimetic 1,2-dichloroethane (DCE)/water (W) interface was studied by the mean of potential-modulated fluorescence (PMF) spectroscopy. The respective dyes gave a well-defined PMF signal due to the adsorption/desorption at the DCE/W interface. It was also found that the potentials where the two dyes gave the PMF signals were different by about 100 mV. We then attempted a combined use of the two dyes for determination of the Galvani potential difference across the DCE/W interface. When 40 µM DiBAC4(3) and 15 µM DiSBAC2(3) were initially added to the W phase, distinctly different spectra were obtained for different interfacial potentials. The ratio of the PMF signal intensities at 530 and 575 nm (the fluorescence maximum wavelengths for the respective dyes) showed a clear dependence on the interfacial potential. These results suggested the potential utility of the combined use of two dyes for the determination of membrane potentials in vivo.

Sophisticated design of PVC membrane ion-selective electrodes based on the mixed potential theory.

The mixed potential (MP) theory was successfully utilized to design an ionophore-based polyvinyl chloride (PVC) membrane K(+) ion-selective electrode (ISE). Prior to the application of the MP theory, the transfer of K(+) and interfering ions (Na(+), Li(+), and H(+)) facilitated by bis(benzo-15-crown-5) (BB15C5) or dibenzo-18-crown-6 (DB18C6) at a micro PVC membrane/water interface was studied by ion-transfer voltammetry (ITV). The reversible half-wave potentials were then obtained for the facilitated transfer of the ions. Using such voltammetric data and the literature data about diffusion coefficients of ions, we could well-predict the potential responses of the BB15C5- or DB18C6-based K(+) ISE, as the function of the concentrations of primary and interfering ions, and also of the counterion for K(+) [e.g., tetrakis(4-chlorophenyl)borate] added to the membrane. Thus, the MP theory has been proven to be useful to optimize the membrane composition for a higher ion selectivity and a lower detection limit. It has also been found that the leaching of ions from an inner solution is too small to affect the detection limit, at least for the designed PVC membrane ISE.