Orientational self-assembly of nanoparticles in nematic droplets.

We demonstrate experimentally that the anchoring of a nematic liquid crystal on a solid substrate together with the anchoring of the liquid crystal on a nanoparticle surface induces orientational self-assembly of anisometric nanoparticles in liquid crystal droplets. The observed phenomenon opens a novel route for fabrication of thin colloidal films with tailored properties.

The viscous consequence of different trends in clustering of 1,2-diol and 1,n-diol molecules.

This paper presents the molecular basis for the quite different behavior of the viscosity of 1,2- and 1,n-diols in dependence of the length of the alkyl part of the molecules of these compounds. The experimental data on the dipolar orientational effects revealed a decidedly different role of that part of the molecules in creating a microstructure of both the hydrogen-bonded liquids. In the case of 1,n-diols, an increase in the alkyl radical length, i.e. an increasing of the distance between the OH groups within the molecule, highly stimulates molecular self-assembly in form of gradually longer and wider ribbon-like clusters. This effect yields a quite important increase in the viscosity of 1,n-diols as n increases. In the case of 1,2-diols, due to gradual separation of the hydrophilic and hydrophobic parts of the molecules, the situation is quite different. Two OH groups situated on one of the ends of the hydrocarbon radical form the clusters of a micelle-like shape, however, the dipole moment is not compensated. Along with an increase in the hydrocarbon part in 1,2-diol molecules, one only observes an increase in the intermolecular consolidation within the micelle-like entities. This manifests as a gradual decrease in the polarity of these clusters. So, actually, there are no relevant reasons for essential differences of viscosities in the series of 1,2-diols.

From supramolecular to conventional polymers: polyethylene glycol.

Increasing the polymerization degree of polyethylene glycol (PEG), HO-(CH2CH2O-)nH, entails lowering the number of hydroxyl groups per unit volume; therefore, supramolecular polymers are gradually replaced by longer and longer conventional polymers. This paper concerns an estimation of the polymerization degree (n) where PEG changes its nature from a supramolecular polymer to a conventional polymer. It was found that that virtual transformation takes place when n reaches the value of about 9. The conclusion follows from the different thermal behaviors of the dipolar orientational effects in liquid PEG detected for n below and above 9. The result reflects a diametrically different impact of the temperature on the linkages between the monomers in supramolecular and conventional polymers.

Alternating current magnetic susceptibility of a ferronematic.

We report on experimental studies focusing on the dynamic ac magnetic susceptibility of a ferronematic. It has been shown recently, that in the isotropic phase of a ferronematic, a weak dc bias magnetic field of a few oersteds increases the ac magnetic susceptibility. This increment vanishes irreversibly if the substance is cooled down to the nematic phase, but can be reinduced by applying the dc bias field again in the isotropic phase [Tomasovicová, N. et al. Soft Matter2016, 12, 5780-5786]. The effect has no analogue in the neat host liquid crystal. Here, we demonstrate that by doubling the concentration of the magnetic nanoparticles, the range of the dc bias magnetic field to which the ferronematic is sensitive without saturation can be increased by about two orders of magnitude. This finding paves a way to application possibilities, such as low magnetic field sensors, or basic logical elements for information storage.

Does water belong to the homologous series of hydroxyl compounds H(CH2)nOH?

The main objective of this paper is to find a source of anomalously high value of the equilibrium permittivity of water. The source is identified to be the unusually high deformation polarizability. The conclusion follows from the analysis of the behavior of the orientational entropy increment induced by an external electric field applied to the liquids belonging to the homologous series of hydroxyl compounds H(CH2)nOH at the end of which water is located. The finding reflects the "indecision" of water about its dielectric relationship with the alcohol family: the value of the permittivity of water absolutely does not fit into alcohols (is too high), while the dipolar orientation effects (which normally determine the permittivity level) fit into alcohols quite well. It results from the presented experimental data that among all the diversity of intermolecular hydrogen-bonded structures existing in liquid water, predominant are the polar entities, i.e. the structures which more or less resemble the chains. Otherwise, the dipolar orientational effects would behave in a quite different way than what is observed in the experiment. The result is convergent with the conclusion of Wernet et al., based on the high-performance X-ray studies of water (Science, 2004).

Dipolar Self-Assembling in Mixtures of Propylene Carbonate and Dimethyl Sulfoxide as Revealed by the Orientational Entropy.

This article presents the results of static dielectric studies performed on mixtures of two strongly polar liquids important from a technological point of view: propylene carbonate (PC) and dimethyl sulfoxide (DMSO). The dielectric data were analyzed in terms of the molar orientational entropy increment induced by the probing electric field. It was found that the two polar liquids in the neat state reveal quite different molecular organization in terms of dipole-dipole self-assembling: PC exhibits a dipolar coupling of the head-to-tail type, whereas in DMSO one observes extreme restriction of dipolar association in any form. In PC + DMSO mixtures, the disintegration of the dipolar ensembles of PC molecules takes place and the progress of that process is strictly proportional to the concentration of DMSO. The static permittivity of mixtures of such differently self-organized liquids exhibits a positive deviation from the additive rule and the deviation develops symmetrically within the concentration scale.

Biasing a ferronematic - a new way to detect weak magnetic field.

The magnetic properties of a ferronematic, i.e., a nematic liquid crystal doped with magnetic nanoparticles in low volume concentration are studied, with the focus on the ac magnetic susceptibility. A weak dc bias magnetic field (a few Oe) applied to the ferronematic in its isotropic phase increases the ac magnetic susceptibility considerably. Passage of the isotropic-to-nematic phase transition resets this enhancement irreversibly (unless the dc bias field is applied again in the isotropic phase).

Hierarchical Structure of Supramolecular Polymers Formed by N,N'-Di(2-ethylhexyl)urea in Solutions.

Supramolecular chain polymers formed by N,N'-di(2-ethylhexyl)urea (EHU) dissolved at low concentrations (up to 0.1 mole fraction) in heptane were investigated with the use of the dielectric spectroscopy. The experimental data show an exceptional ability of the chains for the antiparallel self-aggregation due to dipole-dipole interactions, leading to an anomalous dependence of the static permittivity of EHU + heptane solutions on temperature and concentration of the urea. The primary molecular assembly into polymeric chains is therefore followed by a secondary bundling of the chains which facilitates a longitudinal translation of the chains. That peculiarity and an asymmetry of the alkyl substituent in the EHU molecule making the system a mixture of diastereoisomers of unfavorable packing of the side group, are the most probable molecular mechanisms which prevent the crystallization of EHU-the only known liquid urea derivative.

Compliance of the Stokes-Einstein model and breakdown of the Stokes-Einstein-Debye model for a urea-based supramolecular polymer of high viscosity.

Impedance spectroscopy was used for the study of the static and dynamic behavior of the electrical conductivity of a hydrogen-bonded supramolecular polymer of high viscosity. The experimental data are discussed in the frame of the Stokes-Einstein and Stokes-Einstein-Debye models. It was found that the translational movement of the ions is due to normal Brownian diffusion, which was revealed by a fulfillment of Ohm's law by the electric current and a strictly exponential decay of the current after removing the electric stimulus. The dependence of the dc conductivity on the viscosity of the medium fulfills the Stokes-Einstein model quite well. An extension of the model, by including in it the conductivity relaxation time, is proposed in this paper. A breakdown of the Stokes-Einstein-Debye model is revealed by the relations of the dipolar relaxation time to the viscosity and to the dc ionic conductivity. The importance of the C=O···H-N hydrogen bonds in that breakdown is discussed.

Capacitance changes in ferronematic liquid crystals induced by low magnetic fields.

The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNT-COOH), and SWCNT functionalized with Fe(3)O(4) nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was φ(1)=10(-4) and φ(2)=10(-3). Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Fréederiksz transition threshold) has been observed, which is not present in the undoped nematic.

Phase transition from liquid to disordered solid phase of cyclooctanone studied with dielectric methods.

The dielectric studies performed for cyclooctanone in its static dielectric regime have shown that at the phase transition from liquid to disordered (plastic) solid phase of the compound, the following singular phenomena occur: (a) the static permittivity (ε(s)) exhibits a small increase at the transition, instead of that usually observed for polar liquids, a strong decrease of the permittivity at the transition to the crystalline solid phase; (b) temperature dependence of the permittivity, ε(s)(T), is practically the same in both phases, reflecting the same dipolar orientational entropy increment induced by the probing electric field; (c) a distinct jump occurs in the slope of the frequency dependence (in log-log scale) of the dielectric losses due to an ionic current, from the "ohmic" value -1 in the liquid phase to about -0.9 just after the transition to the plastic phase; (d) a similar jump is observed in the shape of the electric modulus spectra of cyclooctanone. The results clearly show the liquid-like freedom in the molecular dynamics in the plastic phase as well as similar intermolecular interactions in both phases of cyclooctanone. The differences in the dielectric losses frequency behavior may reflect the change in the ions dynamics: from the normal translational Brownian diffusion in the liquid phase to the subdiffusional dynamics in the plastic phase.

On similarity of hydrogen-bonded networks in liquid formamide and water as revealed in the static dielectric studies.

The paper presents the experimental verification of the result obtained with the molecular dynamics simulation which revealed the differences in the topology of the hydrogen-bonded networks in liquid formamide and water, namely, the differences in their intermolecular cyclization process (I. Bakó, et al. J. Chem. Phys. 2010, 132, 014506). It is shown in our paper that the difference in the (simulated) size distribution of the hydrogen-bonded molecular rings in water (a relatively sharp maximum at about 6 molecules) and formamide (a broad maximum at about 11 molecules) strongly manifests itself in the experimental values of the Kirkwood correlation factor of the compounds. A much larger number of molecules included in the cyclic species (of more or less compensated dipole moment) leads to significant decrease of the Kirkwood correlation factor of formamide in comparison to that of water. Besides, as a consequence of an enhancement in formation of the cyclic multimers of formamide, one observes an essential reduction of the orientational entropy increment of that liquid, in comparison to the entropy effect related to liquid amides where the chain multimers are formed.

On intermolecular dipolar coupling in two strongly polar liquids: dimethyl sulfoxide and acetonitrile.

The paper presents the results of studies of the electric and dielectric properties of dimethyl sulfoxide (DMSO) and acetonitrile (ACN), two strongly polar liquids composed of the molecules of the same dipole moment value (µ ≈ 4 D) but of a quite different static dielectric permittivity (ε(S)(DMSO) >> ε(S)(ACN)). It was shown that the activation energies for both the dc ionic conductivity (σ(DC)) and the viscosity (η) are two times higher for DMSO than for ACN; however, for both of the liquids, the temperature dependence of the product σ(DC)η is quite close to the prediction of the Stokes-Einstein relation. The dielectric results are interpreted in terms of the intermolecular dipole-dipole coupling. An exceptional behavior of DMSO most certainly results from its "monomolecularity", i.e., from the lack of the dipolar coupling in that strongly polar liquid. The effect is a consequence of a very specific structure of the DMSO molecule where its rotational dynamics makes the intermolecular dipole-dipole coupling very unfavorable, in contrast to the ACN molecules.

Fractional Stokes-Einstein-Debye relation and orientational entropy effects in strongly hydrogen-bonded liquid amides.

The impedance spectroscopy studies performed for two strongly hydrogen-bonded liquid amides: N-methylpropionamide (NMP, CH(3)·NH·CO·C(2)H(5)) and N-ethylacetamide (NEA, C(2)H(5)·NH·CO·CH(3)) have shown that the two centers of the peptide linkage, -NH·CO-, active in the C=OH-N hydrogen bonds formation, exhibit quite different sensibilities to the steric screening effects. In contrast to the oxygen atom, a relatively small change (CH(3)- to C(2)H(5)-) in the screening of the hydrogen atom leads to an essential decrease of the degree of the amide self-association. As a consequence, both the static dielectric permittivity and the orientational entropy increment of NEA are essentially lower than those of NMP. However, it was found that the dynamic processes studied are only weakly influenced (in the case of dc conductivity, σ(DC)) or totally not influenced (the dielectric relaxation time, τ(D)) by the different degrees of NMP and NEA self-association. The experiment shows that for both the amides, the logσ(DC) vs. logτ(D) dependence is nonlinear and can be described with the fractional Stokes-Einstein-Debye relation, σ(DC)τ ≅ const, with the exponent s varying from about -0.8 to about -0.6 in the temperature range from 5 °C to 110 °C.

Molecular dynamics and entropy effects in hydrogen-bonded supramolecular polymer N,N'-di(2-methyl-2-pentylheptyl)urea dissolved in nonpolar medium.

On the basis of the static dielectric permittivity temperature behavior of the supramolecular polymer formed by N,N'-di(2-methyl-2-pentylheptyl)urea (MPHU) in carbon tetrachloride diluted solution (10% in mole fraction) and that of neat acetonitrile, it was found that two liquids of the same macroscopic polarity (expressed by the same value of the dielectric permittivity approximately 35) exhibit an essential difference in the field-induced orientational entropy change. A much higher entropy effect is observed for the liquid composed of not too numerous but highly polar molecular entities (MPHU + CCl(4)) than for the liquid composed of numerous but less polar entities (neat acetonitrile). The analysis of the dielectric relaxation spectra of MPHU + CCl(4) solutions was performed with the Davidson-Cole (DC) model. It was found that the DC exponent beta changes its value in a quite important range of 0.4-0.8, depending on MPHU concentration and temperature. This reflects an important deviation of the relaxation mechanism occurring in the supramolecular system investigated from that corresponding to the normal Brownian rotational diffusion (beta = 1).

Conductivity dynamics and static dielectric permittivity of highly conducting molecular liquids studied with impedance spectroscopy. Formamides.

Impedance spectroscopy was used for studies of the frequency dependence of the electric conductivity of two highly conducting liquid amides: N-methylformamide (NMF), strongly self-associated via the hydrogen bonds C=O...N-H, and N,N-dimethylformamide (DMF), which is not able to self-organize by hydrogen bonding. The studies were performed in the frequency range f = 50 kHz-100 MHz and in the temperature range T = 278.15-363.15 K. It was found that, at a given temperature, NMF exhibits static conductivity (sigma(0)) of 2 orders of magnitude higher, and conductivity relaxation time (tau(sigma)) of 2 orders of magnitude shorter, than those of DMF. On the basis on sigma(0) and tau(sigma) values, the static dielectric permittivity (epsilon) was calculated for both liquids studied and the values obtained perfectly agree with those measured with the classical dielectric methods. An empirical equation for epsilon(T) dependence for NMF and DMF is proposed.

Nonlinear harmonic components in dielectric relaxation of polyelectrolytes.

The problem of the nonlinear ac dielectric response in polyelectrolytes is carried out by considering these electrically charged systems acted on by a strong dc bias electric field superimposed on a weak ac field. An infinite set of coupled (translation-rotation) kinetic equations is derived, and by using a perturbation procedure, we establish a new expression for the complex dielectric increment of an assembly of nonpolar rodlike macroions surrounded by small counterions. By considering the stationary regime, we show that the electric polarization of the nonpolar macroion is given by calculating the expectation value of P(1)(cos theta)H(1)(x), where P(1) and H(1) stand for the first Legendre and Hermite polynomials, respectively. The results so obtained provide new relaxation features, which are presented in the form of analytic expressions and illustrated by Cole-Cole-like diagrams and three-dimensional dispersion and absorption plots in order to show the influence of the coupling parameter a measuring the importance of the rotational motion over the translational one.

Structural changes in the 6CHBT liquid crystal doped with spherical, rodlike, and chainlike magnetic particles.

In this work the 4-(trans- 4'-n -hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) liquid crystal was doped with differently shaped magnetite nanoparticles. The structural changes were observed by capacitance measurements and showed significant influence of the shape and size of the magnetic particles on the magnetic Fréedericksz transition. For the volume concentration phi= 2 x 10(-4) of the magnetic particles, the critical magnetic field was established for the pure liquid crystal, and for liquid crystals doped with spherical, chainlike, and rodlike magnetic particles. The influence of the magnetic field depends on the type of anchoring, which is characterized by the density of anchoring energy and by the initial orientation between the liquid crystal molecules and the magnetic moment of the magnetic particles. The experimental results indicated soft anchoring in the case of spherical magnetic particles and rigid anchoring in the case of rodlike and chainlike magnetic particles, with parallel initial orientation between the magnetic moments of the magnetic particles and director.

Temperature behavior of the electric field-induced entropy increment within a homologous series of nematogenic compounds.

Based on the temperature behavior of the entropy increment induced by the probing electric field in the isotropic phase of mesogenic compounds belonging to the homologous series C(n)H(2n+1)PhCOOPhCN ( n = 4-10), it is found that an ability to the dipolar aggregation of the molecules depends on the alkyl tail length, and, in particular, the ability is strongly reinforced when the number n changes from 7 to 8. The role of the molecular structure in the self-assembling process is discussed.

The entropy effects in binary mixtures of polar mesogenic solvent/nonpolar solute.

The paper concerns two aspects of the entropy in mesogenic systems: (i) the entropy jump (Delta S (0) NI) at the phase transition from the isotropic liquid (I) to the nematic liquid crystalline state (N), and (ii) the entropy increment (Delta S) caused by the ordering action of the probing electric field applied to the dipolar system. The system studied are the mixtures of strongly polar mesogenic solvent n-hexylcyanobiphenyl (C 6H 13PhPhCN, 6CB) and the nonpolar nonmesogenic admixture 4-ethylcyclohexyl-4'- n-nonylphenyl (C 2H 5CyHxPhC 9H 19, 2CyPh9). The entropy jump at the I-N phase transition in pure 6CB [Delta S (0) NI= 1.52 J/(mol K)] was evaluated from the analysis of the phase diagram of the mixture 6CB + 2CyPh9 with use of the Landau-Lifshitz theory; the resulting value of the transition enthalpy (Delta H (0) NI = T NIDelta S (0) NI = 0.50 kJ/mol) agrees well to that obtained with the calorimetric methods. The field-induced entropy increment (Delta S) was calculated, at the given temperature, from the static dielectric permittivity derivative value (depsilon s/d T), with use of the Fröhlich theory. The singularities in dependence of the entropy increment on the temperature and on the mixtures composition are discussed in terms of the prenematic molecular self-organization extent in mesogenic liquids of different density of dipoles.