Molecular recognition probes of solvation thermodynamics in solvent mixtures

High-throughput UV-Vis experiments using four molecular recognition-based probes, made by the combination of two hydrogen bond acceptors, tri-n-butylphosphine oxide and N,N'-bis(2-ethylhexyl)acetamide, and two hydrogen bond donors, 4-phenylazophenol and 4-nitrophenol, were performed. The association constants for the 1 : 1 H-bond interaction involved in each probe system were measured in mixtures of a polar and non-polar solvent, di-n-hexyl ether and n-octane, respectively. Similar behaviour was observed for all four systems. When the concentration of the polar solvent was low, the association constant was identical to that observed in pure n-octane. However, once the concentration of the polar solvent exceeded a threshold, the association constant decreased linearly with the concentration of di-n-hexyl ether. Selective solvation in mixtures can be understood based on the competition between the multiple competing equilibria in the system. In this case, solvation thermodynamics are dominated by competition of the ether for solvation of H-bond donors. For the more polar solute, 4-nitrophenol, the selective solvation starts at lower concentrations of the polar solvent compared with the less polar solute, 4-phenylazophenol. Thus the speciation and hence the properties of systems containing multiple solutes and multiple solvents can be estimated from the H-bond properties and the concentrations of the individual functional groups.     

Molecular energy parameters by solution thermodynamics

The relationships between statistical thermodynamics and equilibrium constants, either cumulative, stepwise or specific site constants are investigated. In order to show the link between equilibrium constants and statistical thermodynamic microscopic properties, a distinction has been introduced between non-reacting and reacting systems. The non-reacting systems are those for which continuous statistical distributions of enthalpies can be assumed. The distribution function can be obtained as an integral of the interparticle potential extended to the whole ensemble. Molecular partition functions ζ_A, are used to describe the properties of the ensembles. The reacting ensemble is represented by distinct distributions of enthalpies, each distribution being grouped around a mean value. Each level is representative of one species. Around each mean level the distribution is continuous as in non-reacting ensembles. The reacting ensemble of particles is described by a grand canonical molar partition function Z_M=(1+kγ(i)[A])^t where k is the specific site constant, γ(i) is the cooperativity function, [A] is the concentration of free ligand, and the power t indicates the maximum number of i sites in one class. The specific site constant k is proportional to the affinity of binding and is related to the depth of the minimum of the potential function. The factor γ_i is the cooperativity factor given by the value of the cooperativity function γ(i) at the ith level and indicates how the depth of the potential function is affected by previous binding of a ligand. The values of the stability constant and cooperativity factor can be optimized by a computer program. The derivatives of the partition function Z_M with respect to ln[A] correspond to the formation function 〈n〉 (first derivative) and to the buffer capacity B_C (second derivative). The derivatives of the partition function Z_M with respect to temperature are reaction enthalpy ΔH (first derivative with −(1/T)) and apparent heat capacity ΔC_p,app (second derivative with ln T). The denaturation heat obtained by integration of ΔC_p,app dT for many proteins explains why the denaturation enthalpy depends linearly upon T.     

Molecular and thermal diffusion coefficients of alkane-alkane and alkane-aromatic binary mixtures: effect of shape and size of molecules

New molecular and thermal diffusion coefficients of binary mixtures of normal decane-normal alkanes and methylnaphthalene-normal alkanes are measured at atmospheric pressure and T = 25 degrees C. The normal alkanes used in this work include nC5-nC20. Thermal diffusion coefficients were measured in a thermogravitational column. Molecular diffusion coefficients were measured using an open-ended capillary tube technique. Results show a significant effect of molecular shape and size on thermal and molecular diffusion coefficients. Molecular diffusion coefficients show a monotonic behavior in both aromatic-normal alkane and normal decane-normal alkane mixtures. Thermal diffusion coefficients reveal a nonmonotonic trend with molecular size in the normal decane-normal alkane mixtures. This is the first report of the nonmonotonic behavior in the literature. The data presented in this paper provide an accurate self-molecular diffusion coefficient for nC10 from binary data.     

Vibrational energy storage in high pressure mixtures of diatomic molecules

CO/N₂, CO/Ar/O₂, and CO/N₂/O₂ gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T_V=1900–2300 K for N₂, T_V=2600–3800 K for CO, and T_V=2200–2800 K for O₂. The translational–rotational temperature of the gases does not exceed T=700 K. Line-of-sight averaged CO vibrational level populations up to v=40 are inferred from infrared emission spectra. Vibrational level populations of CO (v=0–8), N₂ (v=0–4), and O₂ (v=0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement.     

Molecular thermodynamics on the basis of an equation of state for argon. II. Binary mixtures of weakly nonspherical molecules

Using a molecular perturbation theory based on an equation of state for pure argon, excess properties and vapor-liquid equilibria are predicted for various binary mixtures composed of weakly nonspherical molecules. The results are rather satisfactory and generally much better than obtained using typical empirical methods. It is further demonstrated that a binary parameter in the dispersion energy results in only modest improvement     

Statistical thermodynamics of mixtures of polar liquids in the model of hindered rotation of molecules

The model of hindered rotation of molecules was used to calculate the internal energy of mixtures of dipolar hard spheres. A comparison of the analytic equations obtained with the data of Monte Carlo simulations and hypernetted chain theory calculations showed the importance of various correlation effects caused by electrostatic and steric forces.     

Molecular interaction in binary surfactant mixtures containing alkyl polyglycoside

Surface tensions were measured for several binary mixtures of a multidegree polymerized alkyl polyglycoside, C12G1.46' with different types of surfactants in 0.1 M NaCl at 25 degrees C. Based on regular solution theory, using a dimensional crystal model and a phase separation model, the molecule exchange energy in mixed monolayer formation (epsilon) and mixed micellization (epsilon(m)) were determined. Surfactants used in the mixtures with C12G1.46 in this study are C12E3S (trioxyethylenated dodecyl sulfonate), C12TAC (dodecyl trimethylammonium chloride), BE-6 (hexaoxyethylenated trisiloxane surfactant), and TMN-6 (hexaoxyethylenated-2,6,8-trimethylnonanol). The mixtures show exchange energy in mixed monolayer formation (epsilon) and mixed micellization (epsilon(m)) ranging from -660 to -1410 J/mol, indicating a decrease in surface energy upon mixing. The decreases in surface energy are in the order C12G1.46/C12E3S > C12G1.46/C12TAC, C12G1.46/C12TAC > C12G2/C12TAC and C12G1.46/BE-6 > C12G1.46/TMN-6. The ability of the mixed monolayer formation relative to the mixed micelle formation of the same binary mixture, measured by the (epsilon-epsilon(m)) values, is in the order C12G1.46/BE-6 > C12G1.46/TMN-6 > C12G1.46/C12E3S-->0 > C12G1.46/C12TAC.     

Temperature and composition dependence of viscosity I. Propylene carbonate-dimethoxyethane mixtures and thermodynamics of fluid flow

Concentration and temperature dependence of viscosity of propylene carbonate (PC)/dimethoxyethane (DME) mixtures were studied at temperatures between –45 and 25°C. Among various semiempirical (x)-functions available for data analysis, only the McAllister equation fits the data precisely. Thermodynamic quantities obtained from such an analysis results in an interpretation of the structure of this binary system consistent with results from other methods.     

Molecular size and shape properties of diamondoid molecules occurring in crude oil

The molecular size and shape for 27 diamondoids molecules (adamantanes and alkyladamantanes, diamantine and alkyldiamantanes) were calculated by computational quantum mechanical modeling method of dispersion correction B3LYP-D3/6-311+G**, in which the family of adamantanes presents molecular sizes in terms of width of 6.8–7.9 Å, and for the family of diamantanes 6.8–7.4 Å, in terms of length the size for adamantanes are 7.6–9.5 Å and for diamantanes 9.3–10.0 Å and in terms of height are rounded from 7.4 to 9.6 Å for adamantanes and 7.4 to 8.5 Å for diamantanes. This size depends on the alkyl substitution either in CH₂ or CH bridgehead positions. A measure of spherical shape deviation in terms of ovality, (O = 1 for sphere shape) was calculated, in which for adamantane, methyladamantanes, dimethyladamantanes, and multi-substituted adamantanes, is 1.17, 1.21, 1.24, and 1.26–1.31, respectively and ovality value for diamantane is 1.20 and 1.22–1.27 for methyl substituted diamantanes. Ovality (shape) and molecular size differences between adamantane, methyladamantanes, dimethyladamantanes, multi-substituted adamantanes, and the corresponding diamantanes allow suggesting a dynamic model for separation from the linear alkanes in a mixture into a slit pore shape typical for microporous carbons.     

Structure and the thermodynamics of non-electrolyte mixtures

A review is given of thermodynamic effects arising from the following types of order in liquid components or their mixtures: quadrupolar order in benzene as shown by the benzene-cyclohexane system, orientational order in long-chain normal alkanes revealed by their mixtures with globular molecules, non-randomness in mixtures approaching phase separation, alcohol multimers in inert solvents, water-structuring around hydrophobic solutes, and micelles.Presented at the Patterson Symposium, 76^th CSC Congress, Sherbrooke, Quebec, May 30–June 3, 1993     

Molecular simulation of the high-pressure phase equilibria of binary atomic fluid mixtures using the exponential-6 intermolecular potential

Molecular simulation results using the exponential-6 intermolecular potential are reported for the phase behaviour of the atomic binary mixtures of neon+xenon, helium+neon, helium+argon and helium+xenon. These binary mixtures exhibit both vapour–liquid and liquid–liquid phase equilibria up to very high pressures. Comparison with experiment indicates good overall agreement. The results indicate that the exponential-6 intermolecular potential is a useful generic potential for molecular simulation.     

The generalized potential energy function for diatomic molecules

A new generalized potential energy function is suggested for diatomic molecules. The Dunham, Simons—Parr—Finlan, Thakkar and Ogilvie potentials are shown to be particular cases of the generalized potential energy function. It is also shown that the function suggested may reproduce the path of the potential curve with sufficient accuracy even for the cases of small expansion length.     

Local structure and thermodynamics of a core-softened potential fluid: theory and simulation.

Phase behavior and structural properties of homogeneous and inhomogeneous core-softened (CS) fluid consisting of particles interacting via the potential, which combines the hard-core repulsion and double attractive well interaction, are investigated. The vapour-liquid coexistence curves and critical points for various interaction ranges of the potential are determined by discrete molecular dynamics simulations to provide guidance for the choice of the bulk density and potential parameters for the study of homogeneous and inhomogeneous structures. Spatial correlations in the homogeneous CS system are studied by the Ornstein-Zernike integral equation in combination with the modified hypernetted chain (MHNC) approximation. The local structure of CS fluid subjected to diverse external fields maintaining the equilibrium with the bulk CS fluid are studied on the basis of a recently proposed third order+second order perturbation density functional approximation (DFA). The accuracy of DFA predictions is tested against the results of a grand canonical ensemble Monte Carlo simulation. Reasonable agreement between the results of both methods proves that the DFA theory applied in this work is a convenient theoretical tool for the investigation of the CS fluid, which is practically applicable for modeling numerous real systems.     

Performance of multiplicity-based energy correctors for molecules containing second-row elements

We introduce a posteriori multiplicity-based corrections to ab initio energies in order to reproduce experimental atomization energies. This simple approach, as compared to the alternative ones to improve density functionals and standard correlated methods, requires less computational resources than higher levels of theory. We extend our approach to include molecules containing second-row elements. Molecules are taken from the Gaussian sets for which experimental values are known with errors of less than 1 kcal/mol. We postulate that inexpensive multiplicity-based corrections can account for effects that are not accounted because of the low level of theory of the method or because of the small basis used for the calculations.     

Non-ideality in Born-free energy of solvation in alcohol-water and dimethylsulfoxide-acetonitrile mixtures: Solvent size ratio and ion size dependence

Recent extension of mean spherical approximation (MSA) for electrolyte solution has been employed to investigate the non-ideality in Born-free energy of solvation of a rigid, mono-positive ion in binary dipolar mixtures of associating (ethanol-water) and non-associating (dimethylsulfoxide-acetonitrile) solvents. In addition to the dipole moments, the solvent size ratio and ion size have been treated in a consistent manner in this extended MSA theory for the first time. The solvent-solvent size ratio is found to play an important role in determining the non-ideality in these binary mixtures. Smaller ions such as Li⁺ and Na⁺ show stronger non-ideality in such mixtures compared to bigger ions (for example, Cs⁺ and Bu₄N⁺). The partial solvent polarization densities around smaller ions in tertiary butanol (TBA)-water mixture is found to be very different from that in other alcohol-water mixtures as well as to that for larger ions in aqueous solutions of TBA. Non-ideality is weaker in mixtures consisting of solvent species possessing nearly equal diameters and dipole moments and is reflected in the mole fraction dependent partial solvent polarization densities.     

Theory of the thermodynamic properties of binary mixtures of organic molecules with size mismatch

The Flory expression for the Gibbs free energy of mixing of a binary mixture is improved by introducing a hard-sphere form for the entropy of mixing. The resulting expression is used to describe the characteristic features of organic mixtures of globular molecules with size mismatch. In particular, we show that the above model, with an interchange energy depending on temperature, accounts for the thermodynamic properties and concentration fluctuations of a number of octamethylcyclotetrasiloxane-based mixtures.     

Linker molecules in surfactant mixtures

Linker molecules are amphiphiles that segregate near the microemulsion membrane either near the surfactant tail (lipophilic linkers) or the surfactant head group (hydrophilic linkers). The idea of the lipophilic linkers was introduced a decade ago as a way to increase the surfactant–oil interaction and the oil solubilization capacity. Long chain (>9 tail carbons) alcohols were first used as lipophilic linkers. Later it was found that the solubilization enhancement plateaus (saturates) above a certain lipophilic linker concentration. Hydrophilic linkers have been recently introduced as a way to compensate for the saturation effect observed for lipophilic linkers. Hydrophilic linkers are surfactant-like molecules with 6–9 tail carbons that coadsorb with the surfactant at the oil/water interface, thereby increasing the surfactant–water interaction, but have a poor interaction with the oil phase due to their short tail. A special synergism emerges when combining hydrophilic and lipophilic linkers, which further increases the solubilization enhancement over lipophilic linkers alone. We will discuss the profound impact of linker molecules on interfacial properties such as characteristic length, interfacial rigidity and dynamics (coalescence, solubilization and relaxation experiments) of the interface. We also demonstrate how these properties affect the performance of cleaning formulations designed around linker molecules. We describe linker-based formulations for a wide range of oils, including highly hydrophobic oils (e.g. hexadecane) that have proven very hard to clean. We also report on the use of ‘extended’ surfactants as an alternative to self-assembled linker systems.     

On the thermodynamics of alcohol solutions. Phase equilibria of binary and ternary mixtures containing any number of alcohols

Nagata, I., 1985. On the thermodynamics of alcohol solutions. Phase equilibria of binary and ternary mixtures containing any number of alcohols. Fluid Phase Equilibria, 19: 153–174.Binary vapor—liquid and liquid—liquid equilibrium data for alcohol solutions includin one or two alcohols are correlated with the UNIQUAC associated solution theory (Nagata and Kawamura). The theory uses pure liquid association constants determined by the method of Brandani and a single value of the enthalpy of the hydrogen bond equal to ?23.2 kJ mol ^?1 for pure alcohols. For alcohol-active nonassociating component mixtures and alcohol—alcohol mixtures the theory involves additional solvation constants. The theory is extended to contain ternary mixtures with any number of alcohols. Ternary predictions of vapor—liquid and liquid—liquid equilibria are performed using only binary parameters. Good agreement is obtained between calculated and experimental results for many representative mixtures.