Molecular thermodynamic modeling of droplet-type microemulsions

Microemulsions are nanoheterogeneous, thermodynamically stable, spontaneously forming mixtures of oil and water by means of surfactants, with or without cosurfactants. The pledge to use small volumes of amphiphile molecules compared to large amounts of bulk phase modifiers in a variety of chemical and industrial processes, from enhanced oil recovery to biotechnology, fosters continuous investigation and an improved understanding of these systems. In this work, we develop a molecular thermodynamic theory for droplet-type microemulsions, both water-in-oil and oil-in-water, and provide the theoretical formulation for three-component microemulsions. Our thermodynamic model, which is based on a direct minimization of the Gibbs free energy of the total system, predicts the structural and compositional features of microemulsions. The predictions are compared with experimental data for droplet size in water-alkane-didodecyl dimethylammonium bromide systems.     

Interfacial composition, structural parameters and thermodynamic properties of water-in-oil microemulsions

The formation and structural characteristics of water-in-oil microemulsions comprising hexadecylpyridinium chloride (CPC), alkanols (C₄–C₆) and alkanes (C₅, C₈–C₁₀) have been investigated by the method of dilution. The compositions of the surfactant and the cosurfactant in the interfacial region (interphase) of the microemulsion droplets have been determined. The thermodynamics of transfer of the cosurfactants (alkanols) from the continuous oil (alkane) phase to the interface have been evaluated from dilution measurements at different temperatures. The structural parameters, radii of the droplet and the waterpool, aggregation numbers of CPC and the alkanols in the interphase of a droplet, and the nanoparticle density of solution have been estimated assuming monodispersity of the droplets. The thermodynamics and structural parameters have been examined in terms of the chain lengths of the alkanols and alkanes. Received: 12 September 2000 Accepted: 27 October 2000     

An investigation into the stability of microemulsions in electrophoresis

A typical oil‐in‐water microemulsion (ME) was applied for the capillary electrophoretic separation of nonsteroidal anti‐inflammatory drugs (NSAIDs). As the high‐pH ME was introduced into the capillary and a voltage was applied, we observed an unusual phenomenon during the preconditioning process: a sharp inflection point occurred at 56.2 min ( ± 4.8%) (N = 15). Both before (region A) and after (region B) that point, a steady state was observed. Highly reproducible results were obtained for this event. Two different absorbance patterns were observed for the hydrostatic injection of either freshly prepared ME or ME collected from the outlet of the capillary column during the steady state of region B. The latter had an inflection point at approximately 9 min, whereas the former had a constant absorbance over the entire range. To further characterize this property, regions A and B were used for the separation of NSAIDs. The results showed that both the detection limit and the reproducibility of the separation were superior in region B. For deep insight into the stability of the ME in an electric field, the effects of the oil, cosurfactant, pH, and voltage were systematically investigated in the neat ME. From our findings, it can be concluded that the inflection time might be the migration time of the ME in an electric field, and it might actually be the result of equilibration. Moreover, the existence of many unexpected phenomena seems to be the result of a change in the properties of ME droplet in an electric field.     

The effect of tricarbonylchromium on the thermodynamic stability of stable carbocations

The pK_R+'s for the ionization equilibria between a number of p,p′-substituted diphenylmethanols mono- and bis-complexed with the Cr(CO)₃ group and the corresponding carbocations in aqueous sulfuric acid were determined by spectrophotometric methods. The same pK_R+'s were measured for two ferrocenyltricarbonylchromiumarylmethanols in aqueous trifluoroacetic acid. The introduction of the Cr(CO)₃ group can make the complexed cation either more or less stable than the corresponding free ion, depending upon the structure. The effect of the complexation is explained in terms of electronic and conformational effects.     

The thermodynamic stability of clathrate hydrate. Encaging non-spherical propane molecules

The thermodynamic stability of a clathrate hydrate encaging non-spherical molecules has been investigated by examining the free energy of cage occupancy. In the present study, a generalized van der Waals and Platteeuw theory is extended to treat the rotational motion of guest molecules in clathrate hydrate cages. The vibrational free energy of both guest and host molecules is divided into harmonic and anharmonic contributions. The anharmonic free energy associated with the non-spherical nature of the guest molecules is evaluated as a perturbation from the spherical guest. Predicted thermodynamic properties are compared with measured values. It is shown that this anharmonic contribution is important in the free energy of the hindered rotation of the guests.     

On the thermodynamic stability of hydrogen clathrate hydrates

The cage occupancy of hydrogen clathrate hydrate has been examined by grand canonical Monte Carlo (GCMC) simulations for wide ranges of temperature and pressure. The simulations are carried out with a fixed number of water molecules and a fixed chemical potential of the guest species so that hydrogen molecules can be created or annihilated in the clathrate. Two types of the GCMC simulations are performed; in one the volume of the clathrate is fixed and in the other it is allowed to adjust itself under a preset pressure so as to take account of compression by a hydrostatic pressure and expansion due to multiple cage occupancy. It is found that the smaller cage in structure II is practically incapable of accommodating more than a single guest molecule even at pressures as high as 500 MPa, which agrees with the recent experimental investigations. The larger cage is found to encapsulate at most 4 hydrogen molecules, but its occupancy is dependent significantly on the pressure of hydrogen.     

The origin of virgin argan oil's high oxidative stability unraveled

To prepare either virgin edible or beauty argan oil, roasted or unroasted argan kernels are cold-pressed, respectively. Comparing the physicochemical parameters of edible and beauty argan oil immediately after preparation and after a two-year delay has led to the suggestion that phospholipids are a new and essential type of oil component participating in the excellent oxidative stability of edible argan oil, in addition to the already suggested Maillard-reaction products, phenols, and tocopherols.     

The number of Kekulé structures and the thermodynamic stability of conjugated systems

The dependence of Hückel π-electron energies, E_π, on the basic graph theoretical parameters N (the number of vertices), ν (the number of edges) and ASC (the algebraic structure count) is explored. The form with the ASC enters E_π is established and an equation for E_π is developed. It is shown how the early and apparent success of the (resonance) theory rested on the fortunate fact that all Kekulé structures for benzenoid hydrocarbons and acyclic polyenes have the same parity. The significance of ASC in determining chemical stability and reactivity is dicussed briefly.     

The thermodynamic stability of P8, A CBS‐Q study

The thermodynamic stabilities of P₂, P₄, and three P₈ cage structure were investigated through high‐precision CBS‐Q calculations. The CBS‐Q values for the bond energy of P₂ (ΔE_o: +115.7 kcal mol⁻¹) and the formation of P₄ from P₂ (Δ E_o:‐56.6 kcal mol⁻¹) were in excellent agreement with the experimental values (E_o: +117 and ‐56.4 kcal mol⁻¹ respectively). Among the P₈ cages, the cubane structure was the least stable (Δ E_o +37 kcal vs. 2×P₄). The most stable P₈ isomer adopts a cuneane structure resembling S₄N₄, and is more stable than white phosphorus at T = 0 K (Δ E_o −3.3 kcal mol⁻¹), but still unstable under standard conditions for entropic reasons (Δ G^o of +8.1 kcal mol⁻¹ vs. 2×P₄). The CBS‐Q energies represent significant revisions (6–20 kcal mol⁻¹) of previous computational predictions obtained by high‐level single method calculations. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:453–457, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20119     

Remarkable thermodynamic stability toward hydrolysis of tripodal titanium alkoxides

The monomeric titanium(IV) hydroxide complex, LTi(OH)(LH(3)= tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine), which is sterically inhibited from condensation to a mu-oxo dimer, cannot be prepared by hydrolysis of the alkoxide, with K(eq)= 0.012 for hydrolysis of the titanium methoxide in THF.     

On the thermodynamic and kinetic stability of nonequilibrium systems

A criterion of evolution of nonequilibrium systems in the region of stable states was formulated.     

Kinetic and thermodynamic stability of cluster compounds under heating

A new polymorphic form of Norfloxacin has been identified and fully characterized by a variety of methods including powder X-ray diffraction, vibrational spectroscopy (IR and Raman), thermal analysis (DSC and TG), SEM and solid-state NMR spectroscopy. The relationship between the new form C and the previously known forms A and B have been studied. Moreover, the crystal structure of the known form A has been solved by single-crystal methods.     

The thermodynamic properties and stability of tert-butyl peroxyesters of o-and m-carborane-1-carboxylic acids

The paper presents the results of experimental determination of the enthalpies of combustion and formation and the temperature dependence of the heat capacity of the o-and m-derivatives of tert-butyl peroxyesters of carborane-1-carboxylic acid. The entropies and Gibbs energies of formation of these compounds were calculated. The enthalpies, entropies, and Gibbs energies of the o-m transitions of the isomers in the condensed and gaseous states were determined.     

Light-sensitive microemulsions

A photodestructible surfactant, sodium 4-hexylphenylazosulfonate (C6-PAS), has been introduced to AOT-stabilized water-in-heptane microemulsions. Proton NMR spectra show that C6-PAS undergoes UV-induced decomposition, to yield a mixture of 4-hexylphenol and hexylbenzene. The photostationary state was determined by 1H NMR, indicating that nearly 90% of the initial photosurfactant had been destroyed, yielding non-surface-active hexylbenzene as the main product. This phototriggered breakdown gives rise to changes in adsorption and aggregation properties of C6-PAS, representing a novel route to induce microemulsion destabilization. When a series of microemulsions containing different amounts of C6-PAS were exposed to UV light, part of the dispersed water phase-separated. Small-angle neutron scattering (SANS) was used to follow the resulting UV-induced shrinkage of the water nanodroplets: a maximum volume decrease was found to be in the order of 60-70%. Kinetic SANS studies were also carried out in order to follow the changes in aggregation as a function of UV irradiation time. Multicontrast SANS experiments gave further insight; for example, it was demonstrated that the shell thickness remained constant. This study represents the first example of light-induced microemulsion destabilization.     

Thermal stability and related thermodynamic properties of N-ethylthiourea

The enthalpy and entropy of sublimation of N-ethylthiourea were obtained from the temperature dependence of its vapour pressure measured by both the torsion–effusion and the Knudsen effusion method in the temperature range 360–380 K. The compound undergoes no solid-to-solid phase transition or decomposition below 380 K. The pressure against reciprocal temperature resulted in lg(p, kPa) = (13.40 ± 0.27) − (6067 ± 102) /T(K). The molar sublimation enthalpy and entropy at the mid interval temperature were Δ_subH_m(370 K) = (116.1 ± 2.0) kJ mol⁻¹ and Δ_subS_m(370 K) = (218.0 ± 5.2) J mol⁻¹ K⁻¹, respectively. The same quantities derived at 298.15 K were (118.8 ± 2.1) kJ mol⁻¹ and (226.1 ± 5.5) J mol⁻¹ K⁻¹, respectively.     

Photo-stabilised microemulsions

Light-induced stabilisation of water-in-heptane microemulsions has been achieved with a UV-sensitive gemini photo-surfactant.     

Surfactant-free microemulsions

Surfactants (or amphiphiles) are generally believed to be a necessary component of microemulsions. However, research has found that in the absence of traditional surfactants, microemulsions can also form in a ternary system of two immiscible fluids and an “amphi-solvent”. Such microemulsions are called “surfactant-free microemulsions” (SFMEs). The so-called amphi-solvent is a solvent that is completely or at least partially miscible with each of the two immiscible fluids. The structures and properties of SFMEs are similar to those of traditional surfactant-based microemulsions (SBMEs) to some extent. This review focuses on recent advances in SFMEs, including their phase behavior, structures, properties and potential applications.     

Energetics and thermodynamic stability of the mixed valence ytterbium germanides

The results of an experimental study concerning the thermodynamic stability of the Yb germanides, described as intermediate valence compounds, complemented by a computational investigation for the Yb3Ge5 compound are reported. These compounds belong to the rare earth (RE) tetrelides (tetrel = Si, Ge, i.e., group 14 elements), a class of intermetallic materials showing unusual and promising physical properties (giant magnetocaloric effect, magnetostriction, and magnetoresistence). The high-temperature decomposition reactions of the Yb-Ge intermediate phases were studied experimentally by means of the KEMS (Knudsen effusion mass spectrometry) and KEWL (Knudsen effusion weight loss) techniques. From the reaction enthalpies derived by measuring the Yb(g) decomposition pressures as a function of temperature, the heats of formation of five out of six of the intermediate phases in the Yb-Ge system were calculated. From the computational side, the stability of the Yb3Ge5(s) compound has been investigated by DFT-LCAO-B3LYP (density functional theory-linear combination of atomic orbitals-hybrid b3lyp exchange-correlation functional) first principles calculations deriving its equilibrium geometry and the enthalpy of formation at 0 K in relation to the intermediate valence state of Yb in the lattice.     

On the thermodynamic stability and structural transition of clathrate hydrates

Gas mixtures of methane and ethane form structure II clathrate hydrates despite the fact that each of pure methane and pure ethane gases forms the structure I hydrate. Optimization of the interaction potential parameters for methane and ethane is attempted so as to reproduce the dissociation pressures of each simple hydrate containing either methane or ethane alone. An account for the structural transitions between type I and type II hydrates upon changing the mole fraction of the gas mixture is given on the basis of the van der Waals and Platteeuw theory with these optimized potentials. Cage occupancies of the two kinds of hydrates are also calculated as functions of the mole fraction at the dissociation pressure and at a fixed pressure well above the dissociation pressure.