Dielectric relaxation of some rigid polar molecules in a polystyrene matrix

Dielectric absorption studies have been carried out on solutions of twelve polar solutes dispersed in an atactic polystyrene matrix over a range of temperatures in the frequency region 50 to 10⁵Hz and in one case also in the range of 10⁴ to 10⁷Hz. Rigid molecules of varying size and molecular dipole moments halobenzenes, p-halotoluenes, p-halobiphenyls, p-nitrobiphenyl and p-bromoethylbenzene - have been examined. All these solute molecules have in common the fact that the molecular dipole moment lies along the main principal axis. The relaxation times and energy barrier parameters for molecular dipole relaxations are determined. The Eyring enthalpy of activation, ΔH_E, is found to range from 9 for fluorobenzene to 102 kJ mol⁻¹ for p-iodobiphenyl. Relaxation data of these molecules indicate a linear correlation between enthalpy and entropy of activation, and the former is found to depend upon the volume needed for the reorientation of molecules. Present results for halobenzenes and p-halotoluenes reveal a linear dependence of log (relaxation time) on the mean moment of inertia. The enthalpy data for these rigid molecules are useful in the study of flexible molecules when a decision has to be made as to whether the absorption is to be ascribed to molecular or intramolecular or overlap of the two relaxation processes.     

Dielectric behavior of some aromatic polyimide films

Aromatic polyimides were prepared by polycondensation reaction of two aromatic diamines, such as 4,4′-diaminodiphenylmethane (DDM) and 3,3′-dimethyl-4,4′-diaminodiphenylmethane (MDDM), with aromatic dianhydrides, such as 4,4′-isopropylidene-diphenoxy-bis(phthalic anhydride) (6HDA), benzophenonetetracarboxylic dianhydride (BTDA) and hexafluoroisopropylidene-bis (phthalic anhydride) (6FDA). These polymers are soluble in polar aprotic solvents and can be cast into thin films from such solutions. The polyimides show high thermal stability, with decomposition temperature being above 430 °C in air, and high glass transition temperature being in the range of 200–287 °C. The free standing films, having the thickness of tens of micrometers, exhibited good mechanical and electrical insulating properties. The dielectric constant, molecular mobility and AC conductivity of thin films prepared from these polymers were investigated in detailed. The study of their dielectric behavior evidenced low dielectric constant values, in the range of 2.88–3.48 at 1 Hz at room temperature, and three relaxation processes (γ,β₁ and β₂) were observed at sub-glass temperatures for polyimides based on 6HDA and 6FDA and only two (γ and β) relaxations were detected for polyimides based on BTDA. The cooperativity of the molecular motions associated with the relaxation processes was discussed.     

Interaction of cis-diastereomers of dicyclohexano-18-crown-6 with maleic anhydride and some “small” polar molecules

The cis-anti-cis diastereomer of dicyclohexano-18-crown-6 with maleic anhydride or formamide gives a crystalline complex of ratio 12. The three-dimensional structure of the former was found from x-ray studies. The formation of complexes was used for the separation of the individual cis-syn-cis and cis-anti-cis diastereomers from a mixture. Crystallization of a cis-anti-cis diastereomer from DMFA, DMSO, and N-methylpyrrolidine gave a high melting form.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1329–1332, October, 1989.     

Study of localizability for some small molecules

Localized charge densities of some small molecules at their experimental and calculated equilibrium geometries have been studied using the 6-31G and 6-31G/d basis sets. Introducing polarization functions it has been found that the bond-pair charge distributions are “more”, while the lone-pair- ones are “less” localized at their theoretically determined total-energy minima than at the corresponding experimental geometries.     

Dielectric behavior of some ferrofluids in low-frequency fields

The dielectric behavior of a ferrofluid with magnetite particles dispersed in kerosene was analyzed taking into account the Schwarz model, concerning the low-frequency dielectric behavior in systems consisting of colloidal particles suspended in electrolytes. For this reason, the complex dielectric permittivity and dielectric loss factor, in the frequency range of 10 Hz-500 kHz, at different temperatures between 20 degrees C and 100 degrees C were measured. Based on these experimental results, the experimental dependencies on both temperature of the relaxation time and activation energy of the relaxation process were analyzed. The obtained results show that the Schwarz model can be applied, in order to explain the low-frequency dielectric behavior of a ferrofluid with magnetite particles in kerosene, if the change of counterion concentration at the surface of colloidal particles is taken into account. Consequently, it is shown that the dielectric spectroscopy can be used in order to analyze the presence of particle agglomerations within ferrofluids.     

Dielectric studies on some mesogenic,non-mesogenic and organic molecules

Two important molecular parameters, namely, the dielectric relaxation time τ and electric dipole moment are highly useful in having insight into the molecular structure, size, shape, apart from the inter- and intra-molecular forces, etc. With these in view dielectric measurements in benzene at room temperature on the pure samples of o-ethyl phenol, 2-n-butylphenol, 4-n-butylphenol, 2,6-dimethoxyphenol and 3,4-difluorophenol were carried out at a frequency of 9.98 GHz by employing concentration variation method. Similar measurements, on a single weight fraction of each of them at 9.98 GHz and also at 8.74 GHz are carried out. Measurements on a single weight fraction in benzene of each of the liquid crystal samples, namely, EPCP.car (Butyl-p-(p-ethoxy phenoxy corbonyl)-phenyl carbonate), PPPB (p-pentylphenyl-p-propyl benzonate), EPAP.Hp (p-(p-ethoxyphenyl azo) phenyl heptenate) and EPAP. Und (p-(p-ethoxyphenyl azo) phenyl undecylenate were also carried out at the said two frequencies. Using the obtained values of ε′, ε′′ relaxation time and dipole moment were determined using different methods. The obtained results are compared with earlier results wherever available.     

Entanglement of polar symmetric top molecules as candidate qubits

Proposals for quantum computing using rotational states of polar molecules as qubits have previously considered only diatomic molecules. For these the Stark effect is second-order, so a sizable external electric field is required to produce the requisite dipole moments in the laboratory frame. Here we consider use of polar symmetric top molecules. These offer advantages resulting from a first-order Stark effect, which renders the effective dipole moments nearly independent of the field strength. That permits use of much lower external field strengths for addressing sites. Moreover, for a particular choice of qubits, the electric dipole interactions become isomorphous with NMR systems for which many techniques enhancing logic gate operations have been developed. Also inviting is the wider chemical scope, since many symmetric top organic molecules provide options for auxiliary storage qubits in spin and hyperfine structure or in internal rotation states.     

Generalized phase behavior of small molecules and nanoparticles

Prediction and understanding of the thermodynamic properties and kinetics of phase transitions in molecular systems depends on tuning intermolecular interactions such that the desired structures are assembled. These interactions can depend on the solvent temperature and composition and are difficult to determine in an a priori manner. This is especially true for large and complex molecules and nanoparticles with functionalized surfaces. Here, we demonstrate the use of the pair contribution of the long-time self-diffusivity determined by pulsed-field gradient spin-echo nuclear magnetic resonance as a probe of these interactions. Materials with high solubilities have scaled long-time self-diffusivity, D2, values that are close to hard sphere values and decrease as the solubility decreases. We find a remarkable correlation between solubility and D2 for a wide range of hydrogen-bonding solutes that crystallize upon quenching solutions from high temperature. This generalized phase behavior can be understood in terms of the solutes' interacting with attractive forces that have an extent that is only a small fraction of their diameters.     

Study of the retention behavior of small polar molecules on different types of stationary phases used in hydrophilic interaction liquid chromatography

The retention behavior of a large group of analytes (35) with varied properties (pK_a and logP) was studied on eight hydrophilic interaction LC columns with different surfaces, stationary phase chemistries, and types of particles. The acetonitrile content (5–95%), buffer concentration (0.5–200 mM), and pH of the mobile phase (3.8 and 6.8) were evaluated for their effects on the retention behavior. The type of stationary phase had a significant impact on the selectivity and retention time of the tested analytes. Completely different selectivity was observed on the aminopropyl stationary phase. In this study, the influence of the buffer concentration was similar for all tested columns, except for the aminopropyl stationary phase. Increasing the buffer concentration led to decreased retention times for the basic compounds and increased retention times for the acidic compounds, while the inverse behavior was observed on the aminopropyl stationary phase. The selectivity of the individual stationary phases was evaluated at pH 3.8 and 6.8. Much lower selectivity differences between the stationary phases were observed at pH 6.8 than pH 3.8. Bare silica stationary phases were used in the comparison of the particles (fused‐core and fully porous particles of 3 and 1.7 μm) and the columns provided by different manufacturers.     

A REMPI study of solvation of small Hg n clusters by polar molecules

Free Hg _n (DME) _m clusters (where DME=dimethyl-ether,n=1, 2, 3,m=1÷5) formed in a supersonic expansion were studied by the REMPI (Resonance-Enhanced Multi-Photon Ionization) technique. A large decrease of ionization energies due to solvation of Hg _n clusters is observed. Preliminary results are discussed in terms of different equilibrium configurations of the electronic ground, excited and ionic states of clusters.     

On the angular shape of van der Waals dimers of small polar molecules

Ab initio calculations on twenty van der Waals dimers of small polar molecules at the experimentally observed intermolecular separation R, using Hartree-Fock molecular moments, show that the minima in the electrostatic interaction expanded up to R⁻⁶ converges to angular structures which are close to those observed experimentally for such complexes.     

Open-tubular capillary electrochromatography of small polar molecules using etched, chemically modified capillaries

The migration characteristics of small polar molecules are evaluated on etched, chemically modified capillaries with four different moieties (C5, C18, diol and cholesterol) bonded onto a silica hydride surface. The effects of pH on migration are used to determine the possible contributions of eletrophoretic mobility, electroosmotic flow (EOF) and analyte/bonded phase interactions. The EOF on etched capillaries is more complicated than on ordinary fused capillaries because it changes from anodic to cathodic as the pH is raised. A mixture of neurotransmitters and related compounds is used to further evaluate the effects of the bonded moiety on the separation properties of this particular electrophoretic format.     

Dielectric response of one-dimensional polar chains

We propose a theory for the dielectric constant of materials made of parallel infinite one-dimensional chains of dipoles. Each dipole is allowed to rotate in three dimensions. Monte Carlo simulations show that the Kirkwood factor of the chain grows with increasing dipole moment much faster than in the case of three-dimensional polar fluids. With increasing dipole moment or cooling the one-dimensional chain undergoes a continuous order-disorder transition to the ferroelectric phase, in which the dielectric constant is limited by the size of ferroelectric domains along the chain.     

Calculation of the fourth-rank molecular hypermagnetizability of some small molecules

A computational scheme has been developed within the framework of Rayleigh-Schr?dinger perturbation theory to evaluate nonlinear interaction energy contributions for a molecule in the presence of an external spatially uniform, time-independent magnetic field. Terms connected with the fourth power of the perturbing field, representing the fourth-rank hypermagnetizabilities of five small molecules, have been evaluated at the coupled Hartree-Fock level of accuracy within the conventional common-origin approach. Gaugeless basis sets of increasing size and flexibility have been employed in a numerical test, adopting two different coordinate systems to estimate the degree of convergence of theoretical tensor components.     

Polarizabilities of interacting polar molecules

Collision-induced polarizability in the electro-optical Kerr effect is shown to be the dominant contributor to the second Kerr virial coefficient B_K of dipolar gases. It gives a positive B_K and may be orders of magnitude larger than the contribution due to the intrinsic anisotropy in the polarizability of the free molecules, thus resolving a long-standing discrepancy between experiment and theory. The collision-induced contribution provides a reasonable fit to the observed B_K for the fluoromethanes if a simple Stockmayer-type potential is used.     

Rotational relaxation of polar molecules

The method of obtaining rotational energy relaxation times from experimental thermal conductivities using the Wang Chang-Uhlenbeck theory of transport coefficient for polyatomic gases is considered. For polar gases the method turns out to be useful only if serious calculations of the inelastic collisions involved are performed. Using a semiclassical, partly statistical collision treatment the results for the rotational energy relaxation numbers for halogen hydrides taking dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions into account are presented. It is characteristic for the results that hardly any temperature dependence or isotope effects are observed, a behaviour different from earlier investigations.     

Dendritic polyamine architectures with lipophilic shells as nanocompartments for polar guest molecules: A comparative study of their transport behavior

Dendritic core-shell architectures were synthesized by simple melt reactions of polyethylenimine (PEI) with different fatty acids. These systems were investigated towards their ability to encapsulate various guest molecules. Parameters, such as the length of the attached alkyl chain, size of the polymer core, concentration of the core-shell architecture in solution, pH, and nature of the guest molecule were investigated and compared. Guest molecules that bear anionic groups, such as carboxylate, phosphate, sulfonate, or acidic OH groups, as present in phenol units, are readily encapsulated and transferred from the aqueous phase to the organic phase because of multiple anionic–cationic interactions. Hyperbranched polymer architectures exhibit enhanced encapsulation properties when compared with their linear counterparts. In case of the hyperbranched system PEI₂₅C16 amide higher transport capacities were observed at lower concentrations of the polymer, for example, 26 guest molecules at 10⁻⁵ mol L⁻¹ versus 143 at 10⁻⁶ mol L⁻¹, suggesting formation of larger aggregates. The aggregation behavior of these polar nanocompartments were investigated at different concentrations by AFM showing particle aggregates of typically 250 nm at a concentration of 10⁻⁵ M. The individual particle sizes in these aggregates are similar to the particle size at 10⁻⁶ M concentration, typically 4.0–5.5 nm (AFM height). This indicates that aggregate formation takes place at concentrations higher than 10⁻⁸ M and transport might be mediated by small aggregates rather than unimolecular particles. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2287–2303, 2007     

Dielectric permittivity profiles of confined polar fluids

The dielectric response of a simple model of a polar fluid near neutral interfaces is examined by a combination of linear response theory and extensive molecular dynamics simulations. Fluctuation expressions for a local permittivity tensor epsilon(r) are derived for planar and spherical geometries, based on the assumption of a purely local relationship between polarization and electric field. While the longitudinal component of epsilon exhibits strong oscillations on the molecular scale near interfaces, the transverse component becomes ill defined and unphysical, indicating nonlocality in the dielectric response. Both components go over to the correct bulk permittivity beyond a few molecular diameters. Upon approaching interfaces from the bulk, the permittivity tends to increase, rather than decrease as commonly assumed, and this behavior is confirmed for a simple model of water near a hydrophobic surface. An unexpected finding of the present analysis is the formation of "electrostatic double layers" signaled by a dramatic overscreening of an externally applied field inside the polar fluid close to an interface. The local electric field is of opposite sign to the external field and of significantly larger amplitude within the first layer of polar molecules.     

How small polar molecules protect membrane systems against osmotic stress: the urea-water-phospholipid system

We investigate how a small polar molecule, urea, can act to protect a phospholipid bilayer system against osmotic stress. Osmotic stress can be caused by a dry environment, by freezing, or by exposure to aqueous systems with high osmotic pressure due to solutes like in saline water. A large number of organisms regularly experience osmotic stress, and it is a common response to produce small polar molecules intracellularly. We have selected a ternary system of urea-water-dimyristoyl phosphatidylcholine (DMPC) as a model to investigate the molecular mechanism behind this protective effect, in this case, of urea, and we put special emphasis on the applications of urea in skin care products. Using differential scanning calorimetry, X-ray diffraction, and sorption microbalance measurements, we studied the phase behavior of lipid systems exposed to an excess of solvent of varying compositions, as well as lipid systems exposed to water at reduced relative humidities. From this, we have arrived at a rather detailed thermodynamic characterization. The basic findings are as follows: (i) In excess solvent, the thermally induced lipid phase transitions are only marginally dependent on the urea content, with the exception being that the P(beta) phase is not observed in the presence of urea. (ii) For lipid systems with limited access to solvent, the phase behavior is basically determined by the amount (volume) of solvent irrespective of the urea content. (iii) The presence of urea has the effect of retaining the liquid crystalline phase at relative humidities down to 64% (at 27 degrees C), whereas, in the absence of urea, the transition to the gel phase occurs already at a relative humidity of 94%. This demonstrates the protective effect of urea against osmotic stress. (iv) In skin care products, urea is referred to as a moisturizer, which we find slightly misleading as it replaces the water while keeping the physical properties unaltered. (v) In other systems, urea is known to weaken the hydrophobic interactions, while for the lipid system we find few signs of this loosening of the strong segregation into polar and apolar regions on addition of urea.