Non-associated Liquids Research Articles

Anisotropic inertial term of the solvation energy in binary solutions

An expression based on the Frohlich theorem is given for the anisotropic inertial solvation potential of solutions. The principle of the additivity of the anisotropic inertial solvation potentials of solution components is put forward and substantiated. A model thermodynamic function of the anisotropic inertial solvation potential of a binary solution is suggested. The effect of formation of 1:1 complexes and bimolecular associates on the anisotropic inertial solvation potential of a binary solution is analyzed. The composition dependences of the anisotropic inertial solvation potentials of binary solutions of nitrobenzene, acetonitrile, nitromethane, and tetrachloromethane in associated and nonassociated polar and nonpolar solvents and in water are determined. The dependences obtained are compared to the corresponding model functions. Changes in the contribution of specific intermolecular interactions to the anisotropic inertial term of the Helmholtz energy of solvation of binary solutions are revealed by this method. Previously unknown anisotropic inertial solvation potentials are obtained for associated and polar nonassociated liquids in relation to their content in hexane. Conclusions on the magnitude and character of changes in the microstructure of solutions are made. The transformation of the anisotropic inertial to isotropic noninertial term of the Helmholtz energy of solvation is noted by the example of a solution with the ethanol volume fraction in hexane of 0.13.

Viscoelastic studies of some phenols from dielectric measurements

The variation of dielectric relaxation time with the viscosity of the medium is being exploited in drawing certain quantitative conclusions regarding molecular motion and intermolecular forces in liquids, liquid mixtures, dilute solutions and multi-component polar solutes in dilute solution. With no perfect empirical or theoretical equation in place for the variation of dielectric relaxation time with viscosity, the experimental investigations on different systems only can give an insight. In the present study the results of dielectric measurements carried out on pure samples of o-ethyl phenol, 2-n-butylphenol, 4-n-butylphenol, and 2,6-butylphenol in dilute solutions in different mixed solvents (benzene + paraffin) and on binary mixtures (1 : 1) of p-2-n-butylphenol + 4-n-butylphenol and p-bromonitrobenzene + 2,6-butylphenol, are reported. For comparison, the results on p-bromonitrobenzene + m-bromonitrobenzene as an example of mixture of non-associative liquids was also carried out and the results are presented. Different parameters determined using these dielectric measurements are also presented using different models and these studies indicate that the dielectric behavior at microwave frequencies favor the concept of dynamic viscosity and a single viscoelastic relaxation time for the system under study.

On the effect of temperature on the dielectric relaxation time of some phenols and certain of their binary mixtures

Dielectric relaxation behavior of polar molecules in a non-polar solvent, or mixtures of these substances at different microwave frequencies and over a range of temperatures and concentrations gives an idea about inter-, and intra-molecular forces. Also such studies enable one to calculate thermodynamic parameters such as change of activation energy of dipole orientation (Δ G*), enthalpy (Δ H*) and entropy (Δ S*). Such studies in the case of binary-, ternary-, etc. mixtures of polar molecules in pure liquid phase or in dilute solution phase in a non-polar solvent help in drawing certain quantitative conclusions regarding their relaxation behavior as to whether a single component is responsible for observed microwave absorption or a cooperative phenomenon (average) by all the dipoles of the mixture contributes to it. An experimental investigation is necessary to draw quantitative conclusions regarding the system of the molecules which are not studied so as to examine if the results obtained are in favor or against to the general conclusions already arrived at in other systems. With this in view, systematic dielectric measurements in a range of temperatures are carried out at a single microwave frequency on a single weight fraction in benzene of the four substituted phenols, namely, o-ethyl phenol, 2- n-butyl phenol, 4- n-butyl phenol and 2,6-dimetoxy phenol and on binary (1 : 1) mixtures of [ p-bromonitrobenzene + m-bromonitrobenzene; both non-associated liquids], [ p-bromonitrobenzene + 2,6-dimetoxy phenol; one non-associated and other associated liquid] and [4- n-butyl phenol + 2,6-dimetoxy phenol; both associated liquids] in benzene as solvent at different temperatures. The results are presented and discussed.

Dielectric studies on binary polar mixtures of propanoic acid with esters

It is interesting to see the nature of intermolecular interactions between associative and non-associative polar liquids. Binary polar–polar liquid mixtures of ethyl acetate, ethyl benzoate, ethyl acrylate, ethyl butyrate, n-butyl acetate and benzyl benzoate, each with propanoic acid were subjected to dielectric studies at temperatures 25 °C, 35 °C and 45 °C. Static permittivity ( ε o) and dielectric constant at high frequency ( ε ∞) were found through dielectric measurements for different concentrations of each system. The Bruggeman dielectric factor, Kirkwood correlation factor and the excess permittivity were determined. Deviations from the linearity of various models suggest molecular association through hydrogen bonding between the polar–polar constituents of the mixtures. The formation of cyclic and linear α-multimers in the above systems were identified. The results and their temperature dependence were interpreted accordingly.

Relative Permittivity of Polar Liquids. Comparison of Theory, Experiment, and Simulation

A molecular-based second-order perturbation theory is applied to calculate the relative permittivity of polar liquids. Our basic model is the dipolar hard sphere fluid. The main purpose of this work is to propose various approaches to take into account the molecular polarizability. In the continuum approach, we apply the Kirkwood-Fröhlich equation and use the high-frequency relative permittivity. The Kirkwood g-factor representing molecular correlations is calculated by a perturbation theory. In the molecular approach, the molecular polarizability is built into the model on the molecular level (the polarizable dipolar hard sphere fluid). To calculate the relative permittivity of this system, an equation obtained from a renormalization procedure is used. In both approaches, we apply a series expansion for the relative permittivity and show that these series expansions give results in better agreement with simulation data than the original equations. After testing our theoretical equations against our own Monte Carlo simulation results, we compare the results obtained from our theoretical equations and simulations to experimental data for amines, ethers, and halogenated, sulfur, and hydroxy compounds. We propose a procedure to calculate potential parameters (hard sphere diameter, reduced polarizability, and reduced dipole moment) from experimental data such as the permanent dipole moment, refractive index, density, and temperature. We show that for compounds of low relative permittivity the polarizable dipolar hard sphere (PDHS) model and the continuum approach give reasonable results. For nonassociative liquids of higher relative permittivity, the PDHS model overestimates experimental data due to unsatisfactory representation of the shape of the molecules. In the case of associative liquids, the PDHS model works well, and in some cases it underestimates the experimental values due to the unsatisfactory treatment of electrostatic interactions.

Fluidity of water and of hydrated ions confined between solid surfaces to molecularly thinfilms

In contrast to non-associating liquids such as oils or organic solvents, whose viscositydiverges when they are confined by solid surfaces to films thinner than about ten moleculardiameters, recent studies reveal that salt-free water remains fluid, with a viscosity close toits bulk value, even when confined to films down to only one or two monolayers thick.For the case of high concentration aqueous salt solutions compressed down tosubnanometre films between confining planar surfaces, the hydration sheathsabout the ions (trapped between the oppositely charged surfaces) also remainextremely fluid: this behaviour is attributed to the tenacity of water molecules in thehydration layers together with their rapid relaxation/exchange time. Relatedexperiments on highly compressed, polyelectrolyte brushes in aqueous media reveal aremarkable lubricity which is in large measure attributed to similar hydrationlayers about the charged segments: this water of hydration strongly resists beingsqueezed out, but at the same time it may rapidly exchange with adjacent watermolecules, thereby remaining quite fluid and acting as a molecular lubricant.

Influence of excited molecules on the process of Frictional flow of Nonassociated liquids

It is shown that the viscosity of various nonassociated liquids can be described adequately by taking into account both the influence of the rate of the straight-line formation of holes’ volume caused by irregular jumpings of excited molecules and the increase in the number of bimolecular collisions which is connected with intramolecular conformational transitions in the liquid phase. The proposed equations allow the calculation of this transport property at different temperatures.

Fluidity of Water Confined Down to Subnanometer Films

A surface force balance with extremely high sensitivity and resolution for measuring shear forces across thin films has been used to investigate directly the dynamic properties of salt-free water (so-called conductivity water) in a gap between two atomically smooth solid surfaces. Our results reveal that no shear stress can be sustained by water (within our resolution and shear rates) down to films of thickness D = D0 = 0.0 +/- 0.3 nm. At short range (D < 3.5 +/- 1 nm), an attractive van der Waals (vdW) force between the surfaces causes a jump into a flat adhesive contact at D0, at which the surfaces rigidly couple. Analysis of the jump behavior reveals that the viscosity of water remains within a factor of 3 or so of its bulk value down to D0. This contrasts sharply with the case of confined nonassociating liquids, whose effective viscosity increases by many orders of magnitude at film thicknesses lower than about five to eight monolayers. We attribute this to the fundamentally different mechanisms of solidification of organic liquids and of water. In the former case, the density increase induced in the films by the confinement promotes solidification, while, in the case of water, such densification (due to vdW attraction between the liquid molecules and the confining walls), in agreement with bulk behavior, suppresses the tendency of the water to solidify.

Volume and temperature as control parameters for the dielectric α relaxation of polymers and molecular glass formers

Methods of analysing the dielectric α relaxation and equation-of-state data for supercooled glass-forming materials, to determine the relative contribution of volume and temperature, are illustrated using measurements on poly[(o-cresyl glycidyl ether)–co-formaldehyde] (PCGE). The ratio E V /E P = 0.66 of apparent activation energies and the ratio |α τ |/α P = 1.7, of the thermal expansivities, evaluated at τ = 1 s and P = 0.1 MPa respectively, reveal that for PCGE the volume and temperature both exert a substantial influence on the relaxation times. In this regard, PCGE is similar to other polymers and non-associated molecular liquids for which such analyses have been carried out. This means that the underlying bases of both free-volume and thermal activation models are not correct. The exceptional behaviour is that of hydrogen-bonded liquids, such as polyalcohols, for which competing effects minimize the sensitivity to pressure, whereby temperature becomes the dominant control variable.

Specific and unspecific interactions in polar fluids in view of wideband dielectric far-infrared spectra

A simple molecular analytical theory of dielectric relaxation in strongly polar fluids is considered in terms of a semi-phenomenological approach. Theoretical spectra ε( ν), α( ν) of complex permittivity and absorption coefficient are fully determined by a form of intermolecular potential well, in which a dipole reorients. In a recent publication by V.I. Gaiduk, O.F. Nielsen, and T.S. Perova [J. Molliq 95 (1002) 1–25] the wideband spectra of liquid H 2O and D 2O were described in terms of a composite model comprising the rectangular and the cosine squared potential wells. Much better results are achieved in this work, where the rectangular well is replaced by a well with a rounded bottom termed the hat-curved well. The spectrum of the auto-correlation function (ACF) is calculated for such a potential. The proposed theory of a composite model, comprising hat-curved and parabolic wells, is applied for liquid water. This model is capable for describing the Debye relaxation region, the second relaxation region in the submillimeter wavelength range, and the far infra-red (FIR) ε( ν), α( ν) spectra, where an intense librational band and an additional weak band are placed, respectively, near 700 cm −1 and 200 cm −1. The latter band reflects the features of so-called specific (viz. directly related to H-bonds) interactions and the former band reflects the features of unspecific interactions. The physical mechanisms connected with these types of interactions are discussed in terms of two relevant types of water structure (types of molecular rotation). The proposed theory is also applied to a non-associated liquid in terms of one hat-curved potential well.

Response to “Comment on ‘An interpretation of the low-frequency spectrum of liquid water’ ” [J. Chem. Phys. 118, 452 (2003)

We remark the fact that even if the frequency peak around 60 cm−1 in the low-frequency spectrum of liquid water at room temperature can have a transversal component, it is a general feature of all sorts of liquids at ambient conditions. Conversely, the frequency around 170 cm−1 is only present in the spectra of associated liquids at ambient conditions, since although this mode is seen in the spectra of simple nonassociated liquids at supercooled states, it vanishes as temperature rises.

New universal relationships for surface tension and vaporization enthalpy of non-associated liquids

The methodology of thermodynamic similarity has been used to study the relation between the surface tension of a liquid and the vaporization enthalpy calculated per unit volume of the liquid phase. Simple reliable relationships that do not contain substance dependent parameters have been found. A new universal one-parameter relationship for the temperature dependence of the vaporization enthalpy has been suggested. The results of calculations with the use of these relationships are compared with experimental data.

Viscosity of Near-Boiling NonAssociated Liquids: Dependence on Surface Tension, Molecular Mass and IntraMolecular Conformational Transitions

The equation proposed for near-boiling nonassociated liquids describes a new functional dependence of their viscosity on such physico-chemical characteristics as surface tension and molecular mass. It is shown that the viscosity of liquids with conformationally flexible molecules can be calculated adequately by taking into account the influence of intramolecular conformational transitions on the number of bimolecular collisions in the liquid phase.

Dynamics of water, hydrated-ions and charged polymers in highly-confined films, and their role in friction modification

ABSTRACTRecent studies have revealed that, in contrast to non-associating liquids such as oils or organic solvents, salt-free water retains a viscosity close to its bulk value even when confined to films thinner than some 3 nm, indeed down to only one or two monolayers thick [1,2]. For the case of high concentration aqueous salt solution compressed down to subnanometer films between charged surfaces, the trapped hydrated ions serve to act as molecular ball-bearings, sustaining a large load while remaining very fluid under shear [3]. This behaviour is attributed to the tenacity of the hydration sheaths together with their rapid relaxation time. Finally, a very recent study [4] has shown that when charged polymer brushes in aqueous media are compressed and slid past each other, they provide a lubrication that is considerably superior to that afforded by neutral brushes: This is attributed on the one hand to the resistance to mutual interpenetration of the chains due to entropic barriers in the good-solvent conditions, and, on the other hand, to the hydration-sheaths on the charged polymer segments which can act – as noted above – as molecular ball-bearings.

New considerations of the Barclay–Butler rule and the behavior of water dissolved in organic solvents

The Barclay–Butler (B-B) rule, which states that a linear relationship exists between the standard ΔHvap and ΔSvap for simple, non-associated liquids and their solutions, has been used to distinguish associated (‘abnormal’) liquids from simple (‘normal’) liquids. The exact character of the B-B plots depends on the standard states chosen for the liquid/solution and vapor. We examine the effects of using number density for both vapor and liquid states for pure liquids, non-aqueous solutions, aqueous solutions and solutions in which water is the solute. The utility of B-B plots to detect solute-induced order is strengthened, and we also find remarkable changes in the modified B-B relationship: (1) the points for small, H-bonded liquids, including water, are pulled below the general B-B line; (2) many solutions containing small, simple solutes have negative entropies of vaporization; and (3) solutions of water in several organic solvents, relevant to studies of proteins and micelles, appear ‘abnormal’.

Complexation Thermodynamics of Cyclodextrins in the Framework of a Molecular Size-Based Model for Nonassociative Organic Liquids That Includes a Modified Hydration-Shell Hydrogen-Bond Model for Water

The complexation thermodynamics of a large number of guest molecules with natural α-, β-, and γ-cyclodextrins (CD) can be well-described within the framework of a recently introduced, unified, molecular size-based model for nonassociative liquids that also includes a modified hydration-shell hydrogen-bond model for water (Buchwald, P.; Bodor, N. J. Am. Chem. Soc. 2000, 122, 10671). With increasing guest size, 1:1 complex stability, as measured by ln K or ΔG°, tends to increase linearly up to a size limit characteristic for each CD. For α- and β-CD, the corresponding slopes and intercepts are in excellent agreement with those predicted by the model. For larger guest structures, values level off and are scattered around an average value depending on shape, goodness of fit, and possibly lipophilicity and some specific effects (e.g., such as those caused by presence of phenol functionality). It is an important achievement in the description of interactions in liquids that the very same interaction constants d...

Theoretical Insights into the Formation, Structure, and Energetics of Some Cyclodextrin Complexes

To investigate the physicochemical aspects relevant for the formation of various cyclodextrin inclusion complexes and to search for corresponding general structure–complex-stability relationships, stability data of 1 : 1 complexes for 179, 310, and 51 guest molecules with unsubstituted α-, β-, and γ-cyclodextrin were collected. Statistical analysis using structure-based parameters such as molecular size, hydrophobicity, rotatable bonds, electronic properties, and the presence or absence of more than 150 various functional or structural moieties were performed. The complexation thermodynamics could be well described within the framework of our recently introduced molecular size-based model for nonassociative liquids. With increasing guest size, 1 : 1 complex stability, as measured by ln K or Δ G0, increases linearly up to a size limit characteristic for each CD, and the corresponding slopes and intercepts are in agreement with those predicted by the model. For larger structures, values level off and are scattered around an average value depending on shape, goodness of fit, and possibly lipophilicity and some specific effects (e.g. such as those caused by presence of phenol functionality). The complexation between β-cyclodextrin and certain large steroidal guest molecules, especially a brain-targeted estradiol chemical delivery systems (E2-CDS) that is under clinical development, was investigated in details based on fully relaxed semiempirical AM1 quantum chemical calculations. A deformation index (DI) of the CD ring computed using these fully optimized host-guest geometries could be used to characterize the conformational change of the guest.

Conformational and dielectric analysis of hydrogen bonded polar binary mixtures of methyl benzoate with N-methylaniline

The dielectric properties of the hydrogen bonded binary system of non-associated polar liquid methyl benzoate (ester) and associated polar liquid N-methylaniline (secondary amine) were studied on both pure and equimolar binary mixtures of methyl benzoate and N-methylaniline in dilute solutions of benzene. AM 1 semi-empirical calculations were performed on various conformers of the hydrogen bonded complex. The minimum enery conformer was selected and self-consistent reaction field calculations were performed on the resultant geometry to include solvent effects. The experimental value of dipole moment is compared with the calculated value for the conformers.

A dynamic light scattering study of the hypersonic relaxation in liquid toluene

Brillouin spectra of liquid toluene have been measured between 288 and 358 K using a dynamic light scattering technique. The polarized spectra have been modeled according to Mountain’s theory. The most relevant quantity of the theory, namely the relaxation time, has been used to check the validity of several molecular models proposed to interpret vibrational–translational energy exchanges observed in nonassociated liquids.

Fluidity of water confined to subnanometre films.

The fluidity of water in confined geometries is relevant to processes ranging from tribology to protein folding, and its molecular mobility in pores and slits has been extensively studied using a variety of approaches. Studies in which liquid flow is measured directly suggest that the viscosity of aqueous electrolytes confined to films of thickness greater than about 2-3 nm remains close to that in the bulk; this behaviour is similar to that of non-associative organic liquids confined to films thicker than about 7-8 molecular layers. Here we observe that the effective viscosity of water remains within a factor of three of its bulk value, even when it is confined to films in the thickness range 3.5 +/- 1 to 0.0 +/- 0.4 nm. This contrasts markedly with the behaviour of organic solvents, whose viscosity diverges when confined to films thinner than about 5-8 molecular layers. We attribute this to the fundamentally different mechanisms of solidification in the two cases. For non-associative liquids, confinement promotes solidification by suppressing translational freedom of the molecules; however, in the case of water, confinement seems primarily to suppress the formation of the highly directional hydrogen-bonded networks associated with freezing.