Dipole moments of symmetrical molecular systems

It is shown that quantization of nuclear motion causes the intrinsic dipole moment of a molecular system to depart from the classical representation, e.g., it is different from zero for symmetrical molecules. A formula is derived for the mean dipole moment ¯p as a function of temperature with allowance for the internal motion of the nuclei, which is functionally related to the dipole moment. Calculations are performed for ammonia with allowance for the inversion splitting, which is due to tunneling between two equivalent equilibrium configurations having their dipole moments in opposite directions. The temperature coefficient of ¯p may be positive or negative, in accordance with the relation between the tunneling frequency and the temperature; the formula usually employed is valid only in the limiting case of low frequencies and high temperatures. A deduction is given for the criterion for instability of the maximally symmetrical configuration with respect to odd nuclear displacements (dipole distortions); this is based on a simple model system having an inversion center, a totally symmetric ground state, and a triply degenerate odd excited state of the T_1u type. The experimental consequences of the results are discussed, as well as the concept of symmetry for a molecular system in which the maximally symmetrical configuration is unstable.     

Precision Dilatometry of Microemulsions with Anionic Surfactants

Density ρ, isothermal compressibility β_Т, and thermal expansion coefficient α are measured with high accuracy at 15 and 25°C for water-n-octane-sodium dodecyl sulfate-n-pentanol microemulsions with a water/oil weight ratio varying from 0.05 to 4. The internal pressure of the microemulsions is shown to be calculable from the results of dilatometric measurements. The phenomenon of temperature inversion is revealed for the temperature coefficient of internal pressure ΔP _i/ΔT of the microemulsions. The analysis of this coefficient makes it possible to distinguish between three regions of the structural state in the microemulsions and a continuous structural transition in the associated microemulsions.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 479–484.Original Russian Text Copyright © 2005 by Kartsev, Shtykov, Shtykova.     

Carbon isotopic composition effects on interatomic interactions and the properties of diamond

The interatomic interaction potential parameters were determined for ¹²C and ¹³C in diamond. The results were used to obtain the isotopic dependences of such diamond properties as the Debye temperature, molar heat capacity, thermal expansion coefficient, energies of vacancy formation and self-diffusion, surface energy, and longitudinal velocity of sound. The isotopic dependence of isochoric heat capacity disappeared as the temperature increased. Sign inversion was observed for the isotopic dependence of the thermal expansion coefficient at a certain temperature: its growth changed into a drop. This approach was also used to estimate changes in the interatomic interaction potential and crystal bulk compression modulus of lithium in going from ⁷Li to ⁶Li. The isotopic dependences of phase transition parameters and the whole p-T phase diagram of a simple substance were predicted.     

Computation of some thermodynamics, transport, structural properties, and new equation of state for fluid neon using a new intermolecular potential from molecular dynamics simulation

A new pair potential energy function of neon has been determined via the inversion of reduced viscosity collision integrals at zero pressure and fitted to obtain an analytical potential form. The pair potential reproduces the second virial coefficient, viscosity, thermal conductivity, and self-diffusion coefficient of neon in a good accordance with experimental data over wide ranges of temperature and density. We have also performed molecular dynamics simulation to obtain some thermodynamics, transport, and structural properties of fluid neon at different temperatures and densities using our calculated pair potential supplemented by quantum corrections following the Feynman–Hibbs approach. The significance of this work is that the three-body expression of Wang and Sadus (J Chem Phys 125:144509–1, 2006) can be used to improve the prediction of the pressures of neon without requiring an expensive three-body calculation. The molecular dynamics simulation of neon has been also used to determine a new equation of state for neon. Our results are in a good agreement with experiment and literature values.     

Determination of the Phase Inversion Temperature of Orange Oil/Water Emulsions by Rheology and Microcalorimetry

Abstract

In low-energy emulsification processes, phase inversion occurs when the phases of a dispersion exchange, because of changes in the medium's properties. This paper reports experiments to determine the phase inversion temperature (PIT) of orange oil/water emulsions stabilized by nonionic surfactants. Two techniques were employed: rheology, which is already commonly used to obtain the PIT, and microcalorimetry, which has been proposed as a new technique. Continuous monitoring of the emulsions' viscosity permitted identifying different phenomena that occur while the temperature varies. For all the dispersions prepared, the rheological curves obtained showed two peaks, one attributed to the phase separation process and the other to the phase inversion phenomenon. The microcalorimetry technique showed two endothermic transitions as the dispersion's temperature increased. The initial temperatures were comparable to those obtained by rheology. The influence of the surfactant concentration and the hydrophilic-lipophilic balance (HLB) of the mixture of surfactants and the reduction in volume of the phases at the phase inversion temperature were also evaluated. In general, both methods used to evaluate the phase inversion of the orange oil/water systems (rheology and microcalorimetry) presented concordant results, both for the phase separation process and the phase inversion temperature.     

Salting-out effect induced by temperature cycling on a water/nonionic surfactant/oil system

This paper presents original effects induced by temperature cycling on the transitional phase inversion of emulsions, stabilized by a nonionic polyethoxylated C18E6 surfactant model. The phase inversion follow-up is performed by electrical conductivity measurements, which involves focusing the study on the shape and location of the emulsion inversion region. In that way, new observations are brought out as a gradual evolution of the emulsion inversion along the cycling process. Two alternative approaches are considered for tackling these results: (i) first, a molecular approach regarding the particular organization and rearrangement of water clusters surrounding the surfactant polymer polar head, and (ii) second, a thermodynamic approach only considering the whole Gibbs free energy of the system. The volumic approaches are transposed, here, to the water/oil interface, and disclose that the phase inversion zone is included in a metastable region, able to stabilize for a given temperature, either metastable O/W emulsions or stable W/O ones. In that way, this study proposes novel and complementary insights into the phenomena governing the emulsion phase inversion.     

无皂相反转乳化法制备高分子水基分散体系

高分子树脂水基分散体系是指高分子树脂以微粒形式分散于水中 ,包括高分子乳液、高分子悬浮液等体系 .由于高分子树脂水基分散体系具有低成本 ,高性能 ,无污染等优点 ,一直受到普遍关注 .高分子树脂乳液一般通过乳液聚合制备 .该方法只适用于由含双键的单体制备的加聚产物 .而在制备缩聚产物的乳液时 ,乳液聚合方法局限性很大 ,而最近发展起来的相反转乳化技术 ,适用于制备包括加聚产物和缩聚产物在内的大多数高分子水基分散体系 ,拓宽了高聚物水基分散体系的范围 [1] .杨振忠等 [2～ 5] 利用相反转乳化技术制备了未固化和可固化的环氧树脂…     

Temperature dependence of the second-order susceptibility in calamitic ferroelectric liquid crystals

The dij coefficients of the second-order susceptibility tensor for second harmonic generation were detemined for a calamitic ferroelectric liquid crystal (FLC) specifically designed for applications in non-linear optics. The measurements were performed at different temperatures in the SmC* phase. In accordance with the design methodology of these materials, the coefficient along the polar axis d22 is the greatest, and depicts an expected behaviour in the whole range of the ferroelectric phase. On the other hand, the temperature dependence of d21, d23, and d25, is anomalous to some extent, including a sign inversion of d23 at a certain temperature. These results, which contrast with those reported recently for non-calamitic FLCs, were qualitatively interpreted in the light of different conformations which, by virtue of plausible distortions of the molecular core, could coexist in the sample.     

About temperature dependency of surface tensions in mixtures of lecithin in 3-hydroxypropionitrile

The surface tension temperature dependency of lecithin in 3-hydroxypropionitrile (3-HPN) was studied by means of the BP-2 bubble pressure tensiometer developed by KRUESS. The dynamic surface tension curves of dilute lecithin solutions show a surprising temperature dependency within the range of 5–40 °C at 5 °C intervals and an "inversion point" was noticed. At higher concentrations of lecithin the observed inversion point disappears. The surface tensions recorded for long time periods are in agreement with selected data determined by means of the Wilhelmy plate method. The dynamic surface tensions at 50 s were fitted by the general exponential function y=A exp (?x/b)+c with x as the concentration. The obtained parameter b hereby equals the concentration intersection point of the asymptotes ( x→∞ and x→0) and yields the value of the critical aggregation concentration (cac). The strong linear temperature dependency of the determined cac values within the range of 0.22 mmol/kg (5 °C) to 0.716 mmol/kg (40 °C) is remarkable.Diffusion coefficients of the dilute solutions were calculated by the Ward and Tordai equation assuming a diffusion controlled adsorption mechanism. Similar to the determined surface tensions at 50 s, the diffusion coefficients of the dilute solutions show itself an unusual temperature dependency. This may support the assumption of a change in orientation of the lecithin lipid at the surface. Above the cac the temperature dependency of the diffusion coefficient follows the general Arrhenius law. The calculated molar fractions and absolute amounts of lecithin on the surface show an expected temperature dependency [1] in distinction of the surface excess amounts.     

影响环氧树脂E-44相反转乳化的因素

研究了乳化剂的HLB值、乳化剂浓度、乳化温度、搅拌强度对环氧树脂E-44相反转乳化中临界含水量Rf值与乳状液稳定性的影响,探讨了环氧树脂E-44相反转机理。实验结果表明环氧树脂E-44从W/O向O/W相反转的发生与乳化剂的HLB值与浓度、乳化温度、搅拌强度等因素都有关,其Rf值随着乳化条件的不同是动态变化的。     

Joule-Thomson inversion curve prediction by using equation of state

In this paper five equations of state are tested for checking their ability to predict the Joule-Thomson inversion curve.These five equations of state are:Mohsennia-Modarres-Mansoori(MMM),Ji-Lemp(JL),modified Soave-Redlich-Kwang(SRK)equation of state by Graboski(MSRK1),modified SRK equation of state by Peneloux and Rauzy(MSRK2),and modified Peng-Robinson (PR)equation of state by Rauzy(PRmr).The investigated equations of state give good prediction of the low-temperature branch of the inversion curve,except for MMM equation of state.The high-temperature branch and the peak of the inversion curve have been observed,in general,to be sensitive to the applied equation of state.The values of the maximum inversion temperature and maximum inversion pressure are calculated for each component used in this work.     

The conformational analysis of tetrahydro-1,4,2-dioxazines

A conformational study of some tetrahydro-1,4,2-dioxazines by use of ¹H and ¹³C NMR spectroscopy is reported. The conformational characteristics of this ring are compared to those of the related systems, tetrahydro-1,2- and 1,3-oxazine. A study of model compounds allows the assignment of ring and nitrogen inversion processes in the variable temperature NMR spectra. Ring inversion is found to be a lower energy process than nitrogen inversion. The barriers to these processes are measured in several derivatives and the implications of the results for studies of nitrogen inversion in other 6-membered rings are pointed out. The conformational free energy differences of N-Me and N-Et groups are measured and discussed. It is somewhat easier to put an N-Et group axial than N-Me (ca.0.25 kcal mole^?1). It also appears than an Et group at C-6 goes axial more readily than does a Me.     

Konformative Beweglichkeit Flexibler Ringsysteme–XI Untersuchungen mit Hilfe der Protonenresonanzspektroskopie Ringinversion bei Methyl- und Alkoxylcyclohexanen

The ring inversion of cyclohexane derivatives with one, two and three pairs of geminal methyl and alkoxyl groups has been studied by investigations of the temperature dependence of the NMR signals. The rate of the inversion depends on the number and the relative positions of the ligand pairs as well as on their size. A single pair of geminal ligands has only a very slight effect on the inversion rate. Two ligand pairs hinder the inversion when they are in the 1 and 4 positions relative to one another, but facilitate it when they are in the 1 and 3 positions. In both cases the effects are smaller with methoxyl groups than with methyl groups. Three ligand pairs significantly increase the inversion rate when they are in the 1, 3 and 5 positions, but reduce it when they are in the 1, 2 and 4 positions. The effect of 1,3 substituents is attributed to an increase in the energy of the ground state of the molecules whereas the effect of 1,4 arranged substituents is attributed to an increase in the energy of the transition state of the chair inversion.     

Design of a new rotary molecular machine based on nitrogen inversion: a DFT investigation

Ab initio calculations are employed to investigate nitrogen inversion as a configuration change that can supply an extremely useful switchable control mechanism for some complex systems. In this paper, the design of a new artificial rotary molecular machine based on nitrogen inversion is discussed. The introduced design of a molecular rotator is based on the reciprocating motion of a substituent due to the inversion phenomenon, leading to the rotary motion in the molecule. Since simple secondary amines easily face the inversion process at room temperature, aziridine is selected as the initial driver for the molecular motion. The most obvious finding from this study is that, following the displacement of the substituent attached to the aziridine nitrogen atom, two rotary motions occurr in the molecule, one clockwise and another counterclockwise with a 39.52° to 150.09° angle domain.     

Molecular dynamics simulation of the thermodynamic properties of mercury at pressures below 2.5 GPa and temperatures below 10000 K

Models of mercury were constructed by molecular dynamics using the interparticle potential of the embedded atom model (EAM) at temperatures below 10 000 K and pressures below 2.5 GPa. The thermodynamic properties of the models were presented on the isobars of 0.5, 1.0, 1.5, 2.0, and 2.5 GPa. The compressibility factors Z = pV/(RT) were calculated; the coordinates of the inversion points of the Joule–Thomson coefficient below 5600 K were found from the positions of minima on the Z(p, T) isobars. At densities above 8–9 g/cm³, the results of simulation agreed well with experiment; at lower densities there were discrepancies associated with a loss of metal properties by real mercury. The behavior of the models was analyzed in the region of the van der Waals loop. The calculated critical temperature of mercury was found to be significantly overestimated relative to the experiment. Modeling the “meta-mercury” with the EAM potential with excluded embedded potential contribution gave better agreement with the equation of state of mercury at lower densities. The states with Z = 1 can be observed below 1.0 GPa. The calculated temperature of the inversion of the Joule–Thomson coefficient increased monotonically to 5600 K as the pressure increased to 2.5 GPa.     

Layer inversion in organic heterostructures

Thermally activated layer inversion of ultrathin pentacene/para-sexiphenyl organic heterostructures is observed using a combination of reflectance difference spectroscopy and scanning tunneling microscopy. The heterostructures are formed by deposition of sub-monolayer pentacene (PEN) on top of well ordered para-sexiphenyl (p-6P) layers on Cu(110) at 15 K. When the sample temperature is raised, these heterostructures invert, with pentacene molecules diffusing through the para-sexiphenyl buffer layer and getting in direct contact with the substrate. The observed irreversible inversion demonstrates that the p-6P/PEN/Cu(110) is energetically preferred over PEN/p-6P/Cu(110). Furthermore, the onset temperature of the inversion increases with the layer thickness of para-sexiphenyl indicating a corresponding increase of the kinetic barrier for the inversion. Our results demonstrate the strong influence of the configuration of organic heterostructures on their thermal stability, especially for the very thin layers.     

The far-infrared vapour phase spectra of aniline-ND2 and aniline-NHD

Room temperature vapour phase infrared spectra of aniline-ND₂, aniline-NHD, and aniline-NH₂ from 12 cm^?1 to 650 cm^?1 are presented. Transitions in the inversion vibration are assigned. Barriers to inversion are calculated and found to be slightly lower for aniline-NH₂.     

Transferability of coarse-grained force fields: the polymer case

A key question for all coarse-graining methodologies is the degree of transferability of the resulting force field between various systems and thermodynamic conditions. Here we present a detailed study of the transferability over different thermodynamic states of a coarse-grained (CG) force field developed using the iterative Boltzmann inversion method. The force field is optimized against distribution functions obtained from atomistic simulations. We analyze the polymer case by investigating the bulk of polystyrene and polyamide-6,6 whose coarse-grained models differ in the chain length and in the number of atoms lumped in one bead. The effect of temperature and pressure on static, dynamic, and thermodynamic properties is tested by comparing systematically the coarse-grain results with the atomistic ones. We find that the CG model describing the polystyrene is transferable only in a narrow range of temperature and it fails in describing the change of the bulk density when temperature is 80 K lower than the optimization one. Moreover the calculation of the self-diffusion coefficient shows that the CG model is characterized by a faster dynamics than the atomistic one and that it overestimates the isothermal compressibility. On the contrary, the polyamide-6,6 CG model turns out to be fully transferable between different thermodynamic conditions. The transferability is checked by changing either the temperature or the pressure of the simulation. We find that, in this case, the CG model is able to follow all the intra- and interstructural rearrangements caused by the temperature changes. In addition, while at low temperature the difference between the CG and atomistic dynamics is remarkable due to the presence of hydrogen bonds in the atomistic systems, for high temperatures, the speedup of the CG dynamics is strongly reduced, leading to a CG diffusion coefficient only six times bigger than the atomistic one. Moreover, the isothermal compressibility calculated at different temperatures agrees very well with the experimental one. We find that the polymer chain length does not affect the transferability of the force field and we attribute such transferability mainly to the finer model used in describing the polyamide-6,6 than the polystyrene.     

相反转乳化技术制备环氧树脂交联多孔微球

多孔微球在分离吸附、催化、涂料印刷等多方面具有潜在应用价值 .最近研究表明 ,当微孔孔径在 2 0 0～ 80 0ｎｍ范围时 ,由于强烈的光散射作用 ,多孔微球可以作为遮光剂使用[1] .Ｏｋｕｂｏ等用酸碱逐步处理法制备了一些共聚物的亚微米级多孔球[2～ 4] .Ｏｋｕｂｏ又用动态溶胀种子聚合法制备了微米级的聚合物微球 ,用类似的方法得到了多孔结构[5,6] .但这种方法涉及了许多烦琐过程并且产生了很多副产物需要处理 ,使得其费时费力而且成本相当高 .Ｓｃｈｌａｒｂ等发展了一种新的制备方法 ,在乳液聚合过程中引入有机溶剂 ,在实验后期除掉有机…     

New diastereomeric compound with cholesteric twist inversion

The synthesis and characterization of a new diastereomeric compound are reported; this exhibits a temperature induced inversion of the cholesteric twist. Measurements of the cholesteric pitch and handedness are presented and qualitatively discussed in terms of the nature of the cholesteric twist inversion in pure compounds. Preliminary measurements of properties of the ferroelectric S*c phase of this compound are also reported.