The influence of flexible segments on liquid crystalline properties in terms of solubility parameters. An attempt at quantitative interpretation

The phase behavior of some rodlike block molecules has been reviewed with reference to the polarity of constituent segments. It was found that the ability of the mesophase formation is connected with differences in polar character between the flexible chains and rigid cores. Thus the polar poly(oxyethylene) group connected with the polar rigid core reduces mesophase stability but is advantageous when put together with some apolar building blocks. An attempt at quantitative estimation of the incompatibilities of different parts of molecules by means of Hansen solubility parameters delta and Flory interaction parameters chi has also been made. On the basis of chi parameters the Gibbs free energies of mixing of these segments were calculated. The changes of Gibbs free energy reflecting the compatibility of segments and their tendency to the phase separation and the volume fraction of mesogenic rigid core reflecting their ability to arrangement in one direction appear to be crucial in terms of type of the mesophase formation.     

Predicted electronic and thermodynamic properties of a newly discovered Zn8Sb7 phase

A new binary compound, Zn(8)Sb(7), has recently been prepared in nanoparticulate form via solution synthesis. No such phase is known in the bulk phase diagram; instead, one would expect phase separation to the good thermoelectric semiconductors ZnSb and Zn(4)Sb(3). Here, density functional calculations are employed to determine the free energies of formation, including effects from vibrations and configurational disorder, of the relevant phases, yielding insight into the phase stability of Zn(8)Sb(7). Band structure calculations predict Zn(8)Sb(7), much like ZnSb and Zn(4)Sb(3), to be an intermetallic semiconductor with similar thermoelectric properties. If sufficient entropy or surface energy exists to stabilize the bulk material, it would be stable in a limited temperature window at high temperature.     

A quasichemical approach for protein-cluster free energies in dilute solution

Reversible formation of protein oligomers or small clusters is a key step in processes such as protein polymerization, fibril formation, and protein phase separation from dilute solution. A straightforward, statistical mechanical approach to accurately calculate cluster free energies in solution is presented using a cell-based, quasichemical (QC) approximation for the partition function of proteins in an implicit solvent. The inputs to the model are the protein potential of mean force (PMF) and the corresponding subcell degeneracies up to relatively low particle densities. The approach is tested using simple two and three dimensional lattice models in which proteins interact with either isotropic or anisotropic nearest-neighbor attractions. Comparison with direct Monte Carlo simulation shows that cluster probabilities and free energies of oligomer formation (DeltaG(i) (0)) are quantitatively predicted by the QC approach for protein volume fractions approximately 10(-2) (weight/volume concentration approximately 10 g l(-1)) and below. For small clusters, DeltaG(i) (0) depends weakly on the strength of short-ranged attractive interactions for most experimentally relevant values of the normalized osmotic second virial coefficient (b(2) (*)). For larger clusters (i"2), there is a small but non-negligible b(2) (*) dependence. The results suggest that nonspecific, hydrophobic attractions may not significantly stabilize prenuclei in processes such as non-native aggregation. Biased Monte Carlo methods are shown to accurately provide subcell degeneracies that are intractable to obtain analytically or by direct enumeration, and so offer a means to generalize the approach to mixtures and proteins with more complex PMFs.     

Single-ion solvation free energies and the normal hydrogen electrode potential in methanol, acetonitrile, and dimethyl sulfoxide

The division of thermodynamic solvation free energies of electrolytes into contributions from individual ionic constituents is conventionally accomplished by using the single-ion solvation free energy of one reference ion, conventionally the proton, to set the single-ion scales. Thus, the determination of the free energy of solvation of the proton in various solvents is a fundamental issue of central importance in solution chemistry. In the present article, relative solvation free energies of ions and ion-solvent clusters in methanol, acetonitrile, and dimethyl sulfoxide (DMSO) have been determined using a combination of experimental and theoretical gas-phase free energies of formation, solution-phase reduction potentials and acid dissociation constants, and gas-phase clustering free energies. Applying the cluster pair approximation to differences between these relative solvation free energies leads to values of -263.5, -260.2, and -273.3 kcal/mol for the absolute solvation free energy of the proton in methanol, acetonitrile, and DMSO, respectively. The final absolute proton solvation free energies are used to assign absolute values for the normal hydrogen electrode potential and the solvation free energies of other single ions in the solvents mentioned above.     

Liquid-liquid phase separation by nucleation and growth in solutions of poly(2,6 dimethyl-1,4 phenylene oxide) in toluene

In solutions of poly(2,6 dimethyl-1,4 phenylene oxide) in toluene, the nucleation of the newly formed phase during liquid-liquid phase separation takes place after induction periods which vary between several minutes (at temperatures close to the spinodal) and several hours (at temperatures close to the cloudpoint). The growth of the nuclei in the initial stages is diffusion controlled. The diffusion coefficients and the activation energy of diffusion were calculated. From these values, together with the calculated volume free energies and the experimental induction times, an estimate could be made of the size of a critical nucleus and of the surface free energy of the nuclei.     

Mechanism of amorphisation in Cu–Ru,a binary alloy with a positive heat of mixing

Cu–Ru has a positive heat of formation and does not form equilibrium alloys. Nevertheless, amorphous alloys have been obtained by He (Phys. Rev. B 75, 045431 (2007)) by ion mixing of multilayers. Analysis of the free energies of the competing phases (the glass and the crystalline solid solutions based on Cu and Ru) leads us to propose that formation of glasses occurs as a result of kinetic frustration between the hcp and fcc solid solutions. These two have lower free energies than the glass, but those free energies are very similar, so a strong driving force for the formation of a particular crystalline phase does not exist. In addition, formation and growth of hcp and the fcc phases appears equally difficult from a kinetic point of view. Very small embryos can form but their growth will be frustrated by the presence of embryos of the other phase.     

Computational methods in organic thermochemistry. 1. Hydrocarbon enthalpies and free energies of formation

Standard state enthalpies and free energies of formation can be computed with reasonable accuracy (usually within 4 and often 2 kJ/mol) using high level model chemistries. A comparison set of nearly 300 organic compounds ranging from 1 to 10 carbon atoms having a variety of functional groups for which enthalpy and free energy literature values are available has been examined using G2, G2MP2, G3, G3MP2, G3B3, G3MP2B3, CBS-QB3, and density functional (B3LYP/6-311+G(3df,2p)) model chemistries. G3 gives an average mean absolute deviation of 3.0 and 13.4 kJ/mol for the enthalpies and free energies, respectively, using the atomization method and 3.1 and 3.7 kJ/mol when bond separation reactions are employed. G3 and G3B3 are the most accurate overall; the related G3MP2 and G3MP2B3 are nearly as accurate and can compute larger molecules. CBS-QB3 was also found to be accurate but is more limited in the size of molecules that can be computed. The density functional energies were found to have large deviations from the literature values using either the atomization or the bond separation method. Regardless of the model employed, the free energies are increasingly underestimated by computation as the size of the molecule increases. A series of corrections applied to the aliphatic hydrocarbons is presented, which usually reduces the deviations to less than 4 kJ/mol regardless of the size of the molecule.     

Phase equilibria and phase separation dynamics in a polymer composite containing a main-chain liquid crystalline polymer

Various topological phase diagrams of blends of main-chain liquid crystalline polymer (MCLCP) and flexible polymer have been established theoretically in the framework of Matsuyama–Kato theory by combining Flory–Huggins (FH) free energy for isotropic mixing, Maier–Saupe (MS) free energy for nematic ordering in the constituent MCLCP, and free energy pertaining to polymer chain-rigidity. As a scouting study, various phase diagrams of binary flexible polymer blends have been solved self-consistently that reveal a combined lower critical solution temperature (LCST) and upper critical solution temperature (UCST), including an hourglass phase diagram. The calculated phase diagrams exhibit liquidus and solidus lines along with a nematic–isotropic (NI) transition of the constituent MCLCP. Depending on the strengths of the FH interaction parameters and the anisotropic (nematic–nematic) interaction parameters, the self-consistent solution reveals an hourglass type phase diagram overlapping with the NI transition of the constituent MCLCP. Subsequently, thermodynamic parameters estimated from the phase diagrams hitherto established have been employed in the numerical computation to elucidate phase separation dynamics and morphology evolution accompanying thermal-quench induced phase separation of the MCLCP/polymer mixture. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3621-3630, 2006     

Equation-of-motion coupled-cluster study on exciton states of polyethylene with periodic boundary condition

Equation-of-motion coupled cluster with singles and doubles (EOM-CCSD) method has been applied to exciton states of polyethylene using ab initio crystal Hartree-Fock method with one-dimensional periodic boundary condition. Full transformation of two-electron integrals from atomic-orbital basis to crystal-orbital basis has been performed for EOM-CCSD calculations. In order to make transformed integrals to have correct properties of translational symmetry, a lattice summation scheme has been proposed. The EOM-CCSD excitation energies have been obtained for the lowest singlet and triplet exciton states of polyethylene. The excitation energies converge with system size much faster than oligomer calculations using n-alkanes. Quasiparticle energy-level calculations by second-order many-body perturbation theory and by solving the inverse Dyson equation have also been performed to obtain exciton binding energies. Basis set dependencies on excitation energy, quasiparticle band gap, and exciton binding energy have been investigated. At the 6-31+G level, the excitation energy of the lowest singlet-exciton state and its binding energy are calculated to be 8.1 and 3.2 eV, respectively. The calculated excitation energy is well comparable with the corresponding experimental value, 7.6 eV.     

Study of charge transfer complexes of menadione (vitamin K3) with a series of anilines

Menadione (vitamin K(3)) has been shown to form charge transfer complexes with N,N-dimethyl aniline, N,N-dimethyl p-toluidine and N,N-dimethyl m-toluidine in CCl(4) medium. The CT transition energies are well correlated with the ionisation potentials of the anilines. The formation constants of the complexes have been determined at a number of temperatures from which the enthalpies and entropies of formation have been obtained. The formation constants exhibit a very good linear free energy relationship (Hammett) at all the temperatures studied.     

Phase change of Lennard-Jones nano clusters containing non magic numbers

Phase changes of Lennard-Jones clusters containing 4N ³ (N= 1?20) identical atoms in terms of solid and liquid phase-like forms have been studied by performing molecular dynamics (MD) simulation at sharply-bounded range of temperatures between freezing temperature (T _f) and melting temperature (T _m) and at constant pressure. The small differences between the free energies of clusters in different phase-like forms and also the non-rigidity of the cluster (0 ≤ γ ≤ 1) as an order-parameter, which characterizes the phase transition, have been calculated. Plots of the free energy of phase change versus the non-rigidity indicate that the free energy is a continuous function of the non-rigidity and also different crystalline-like cores with different free energies correspond to the same non-rigidity factor at any given temperature.     

Mixing Properties of Two-Dimensional Lattice Solutions of Amphiphiles

Lattice Monte Carlo simulations of two-dimensional amphiphile solutions are used to examine the accuracy of the mixing properties predicted by lattice theories such as the Flory-Huggins theory, random-solution approximation, and quasichemical approximation. The internal energy, Helmholtz free energy, and entropy of mixing have been calculated from the configurational energy data obtained from the simulations, and the effect of nonrandom mixing on these properties has been determined. The quasichemical approximation predicts the entropy and Helmholtz free energy of mixing accurately for the amphiphile solution, but fails to predict the energy of mixing, due to the presence of microphase (self-aggregation) separation, which is beyond the reach of the quasichemical approximation, a mean-field theory. Helmholtz free energy of mixing is predicted accurately, and the shielding of the solvophobic segments in the microphase leads to small energies of mixing compared to the entropy of mixing. Copyright 2000 Academic Press.     

A new class of pressure-sensitive adhesives based on interpolymer and polymer-oligomer complexes

On the basis of previous concepts concerning the molecular nature of pressure-sensitive adhesion, a simple method of preparing new adhesives with the desired mechanical and adhesive behavior and water-absorbability via mixing of nonadhesive polymers has been developed. Pressure-sensitive adhesion is related to the combination of a high energy of cohesion and a large free volume, which leads to a high molecular mobility. This method is based on the formation of interpolymer or polymer-oligomer complexes during mixing of macromolecules capable of hydrogen, electrostatic, or ionic bonding. In interpolymer complexes, a high cohesion results from the formation of bonds between macromolecules carrying complementary groups in main chains, whereas free volume is related to defectiveness of the resulting network and formation of loops. In complexes formed by a high-molecular-mass polymer and an oligomer carrying complementary reactive groups at ends of short chains, a high energy of cohesion is related to their interaction with mainchain functional groups of the polymer, whereas a relatively large free volume is associated with the length and flexibility of intermacromolecular crosslinks via oligomer chains. The adhesive and viscoelastic properties of adhesives and their water absorbability are regulated by changes in the composition of mixtures of a film-forming polymer with a polymer or oligomer crosslinker and a plasticizer. In this case, an increase in cohesive strength is achieved owing to an increase in the crosslinker concentration, while the enhancement of free volume is ensured by the increasing plasticizer content in the blend. Adhesive materials capable of adherence to wet substrates, hydroactivated adhesives, and adhesion moisture sorbents have been prepared for the first time.     

Ion-specific weak adsorption of salts and water/octanol transfer free energy of a model amphiphilic hexapeptide

An amphiphilic hexapeptide has been used as a model to quantify how specific ion effects induced by addition of four salts tune the hydrophilic/hydrophobic balance and induce temperature-dependant coacervate formation from aqueous solution. The hexapeptide chosen is present as a dimer with low transfer energy from water to octanol. Taking sodium chloride as the reference state in the Hofmeister scale, we identify water activity effects and therefore measure the free energy of transfer from water to octanol and separately the free energy associated to the adsorption of chaotropic ions or the desorption of kosmotropic ions for the same amphiphilic peptide. These effects have the same order of magnitude: therefore, both energies of solvation as well as transfer into octanol strongly depend on the nature of the electrolytes used to formulate any buffer. Model peptides could be used on separation processes based on criteria linked to "Hofmeister" but different from volume and valency.     

Revisiting the Frenkel-Ladd method to compute the free energy of solids: the Einstein molecule approach

In this paper a new method to evaluate the free energy of solids is proposed. The method can be regarded as a variant of the method proposed by Frenkel and Ladd [J. Chem. Phys. 81, 3188 (1984)]. The main equations of the method can be derived in a simple way. The method can be easily implemented within a Monte Carlo program. We have applied the method to determine the free energy of hard spheres in the solid phase for several system sizes. The obtained free energies agree within the numerical uncertainty with those obtained by Polson et al. [J. Chem. Phys. 112, 5339 (2000)]. The fluid-solid equilibria has been determined for several system sizes and compared to the values published previously by Wilding and Bruce [Phys. Rev. Lett. 85, 5138 (2000)] using the phase switch methodology. It is shown that both the free energies and the coexistence pressures present a strong size dependence and that the results obtained from free energy calculations agree with those obtained using the phase switch method, which constitutes a cross-check of both methodologies. From the results of this work we estimate the coexistence pressure of the fluid-solid transition of hard spheres in the thermodynamic limit to be p*=11.54(4), which is slightly lower than the classical value of Hoover and Ree (p*=11.70) [J. Chem. Phys. 49, 3609 (1968)]. Taking into account the strong size dependence of the free energy of the solid phase, we propose to introduce finite size corrections, which allow us to estimate approximately the free energy of the solid phase in the thermodynamic limit from the known value of the free energy of the solid phase with N molecules. We have also determined the free energy of a Lennard-Jones solid by using both the methodology of this work and the finite size correction. It is shown how a relatively good estimate of the free energy of the system in the thermodynamic limit is obtained even from the free energy of a relatively small system.     

Energetics of formation of TiGa3As4 and TiGa3P4 intermediate band materials

Using density functional theory quantum methods, total energy values and vibrational properties have been computed, and thermodynamic properties evaluated, for Ti-substituted GaAs and GaP, proposed as candidates for intermediate band photovoltaic cells. The calculations predict that the formation of these materials from the binary compounds implies an increase in total energy (that is ascribed largely to the change in coordination undergone by Ti, from six-fold to four-fold), and thus phase separation rather than mixed compound formation would be favored. However, the mentioned increase is not larger (for the arsenide case it is actually smaller) than that predicted for Mn-substituted GaAs, a material which has been experimentally made, and therefore the obtention of these Ti-substituted materials is expected to be feasible as well. Vibrational and disorder entropy contributions to the formation free energy of the ternary compounds have been also computed; they compensate partially for the total energy increase, and indicate that the thermodynamic feasibility of the materials synthesis improves for low Ti concentrations and high temperature conditions.     

AM1 and PM3 Study of Tautomerism of Hypoxanthine in the Gas and Aqueous Phases

Heats of formation, entropies, Gibbs free energies, relative tautomerisation energies, tautomeric equilibrium constants, dipole moments, and ionization potentials for the eight possible tautomers of hypoxanthine have been studied using semiempirical AM1 and PM3 quantum-chemical calculations at the SCF level in the gas and aqueous phases, with full geometry optimization. The COSMO solvation model was employed for aqueous solution calculations. The calculations show that the two keto tautomers H17 and H19 are the predominant species at room temperature in the gas and aqueous phase. However, the tautomer H17 is the more dominant species in gas phase, while the H19 tautomer is the more dominant species in the aqueous phase. Comparison with available experimental data provides support for the results derived from theoretical computations. The entropy effect on the Gibbs free energy of hypoxanthine is very small and there is little significance for the tautomeric equilibria of the base. The enthalpic term is dominant in the determination of the equilibrium constant.     

Photodissociation dynamics of the HCNN radical

The photodissociation dynamics of the diazomethyl (HCNN) radical have been studied using fast radical beam photofragment translational spectroscopy. A photofragment yield spectrum was obtained for the range of 25,510-40,820 cm(-1), and photodissociation was shown to occur for energies above 25,600 cm(-1). The only product channel observed was the formation of CH and N2. Fragment translational energy and angular distributions were obtained at several energies in the range covered by the photofragment yield spectrum. The fragment translational energy distributions showed at least two distinct features at energies up to 4.59 eV, and were not well fit by phase space theory at any of the excitation energies studied. A revised C-N bond dissociation energy and heat of formation for HCNN, D0(HC-NN)=1.139+/-0.019 eV and DeltafH0(HCNN)=5.010+/-0.023 eV, were determined.     

Comparison of yttrium,lanthanides and actinides in respect to unit cell volumes of isostructural compounds and thermodynamic functions of complex formation

The investigations carried out till now and presented in this paper show that apart from the well known itinerant properties of yttrium in respect to free energy of complex formation, also actinides(III) change their position in the lanthanide series in respect to G. It has also been shown that yttrium and actinides exhibit itinerant behaviour in respect to unit cell volumes. Evidence has been presented that delocalization of 4f and 5f orbitals is the reason for the two types of migratory properties. Since the itinerant behaviour of yttrium and actinides(III) in respect to stability constants (free energies of complex formation) is the basis for yttrium-lanthanides and lanthanides-actinides group separations, a better qualitative understanding of the mechanism involved may contribute to the development of more efficient separation procedures.