Microwave enhancement of evaporation of a polar liquid. II

The evaporation of a polar liquid at low pressure upon exposure to microwave radiation has been investigated. The conventional theories previously developed to describe the non-equilibrium conditions of the evaporating interface are found to be in poor agreement with the experimental results. It is possible to take into account the influence of microwave fields on the evaporating surface by modifying the expression for the entropy production. Another approach, based on the concept that microwaves would increase the pressure, has also been found to be satisfactory.     

Lattice model of a polar liquid

The molecular model of a polar liquid based on the lattice gas theory was used to study the liquid—gas phase equilibrium. In this theory, the key unknown variables are the probabilities of various mutual pair positions of the molecules. The molecules are considered as non-polarizable point dipoles. The intermolecular interaction potential includes a dipole—dipole component depending on the molecular orientation and the Lennard-Jones contribution stabilizing the system with dipole interactions. The theory provides qualitative agreement with experimental data on the water—vapor equilibrium.     

Considering induced dipoles in a discrete model of a polar liquid

The lattice gas model is generalized to describe the equilibrium distributions of polar solution components with allowance for Lennard-Jones and dipole-dipole potential interactions with constant and induced moments. It is shown that including induced dipoles potential results in an effective many-particle interaction potential, depending on the spatial distribution of solution components. The distributions of all solution components are calculated in a quasi-chemical approximation allowing for the spatial correlation of interacting particles. A procedure for reducing the dimensionality of a set of algebraic equations is considered, and expressions for vapor-liquid equilibrium isotherms are obtained. Expressions for the rates of elementary mono- and bimolecular chemical reactions are derived using the transition state theory in systems with induced dipoles for rapidly overcoming the activation barrier in the permanent state of solvent molecules’ atomic subsystems. Ways of considering the internal motions (vibrations, rotation, and displacements) of molecules in a polar liquid are discussed.     

The absorption coefficient of a polar liquid with solvated electrons

An equation for the absorption coefficient of a polar liquid with excess (solvated) electrons is derived. It is taken into account that (1) each excess electron can for a certain time reside on only one liquid molecule (during this time, the molecule is in the anion-resonance state), and (2) a polar liquid is electrostatically nonuniform because it has different local potentials, which can be calculated for each molecule. The probabilities of quantum movements of excess electrons in a liquid from one molecule to another caused by the absorption of photons are considered.     

Ion-pair evaporation from ionic liquid clusters

A differential mobility analyzer (DMA) is used in atmospheric pressure N₂ to select a narrow range of electrical mobilities from a complex mix of cluster ions of composition (CA)_n(C⁺)_z. The clusters are introduced into the N₂ gas by electrospraying concentrated (～20 mM) acetonitrile solutions of ionic liquids (molten salts) of composition CA (C⁺ = cation, A^? = anion). Mass analysis of these mobility-selected ions reveals the occurrence of individual neutral ion-pair evaporation events from the smallest singly charged clusters: (CA)_nC⁺→(CA)_{n? 1}C⁺+CA. Although bulk ionic liquids are effectively involatile at room temperature, up to six sequential evaporation events are observed. Because this requires far more internal energy than available in the original clusters, substantial heating (～10 eV) must take place in the ion guides leading to the mass analyzer. The observed increase in IL evaporation rate with decreasing size is drastic, in qualitative agreement with the exponential vapor pressure dependence predicted by Kelvin’s formula. A single evaporation event is barely detectable at n = 13, while two or more are prominent for n ≤ 9. Magic number clusters (CA)₄C⁺ with singularly low volatilities are found in three of the four ionic liquids studied. Like their recently reported liquid phase prenucleation cluster analogs, these magic number clusters could play a key role as gas-phase nucleation seeds. All the singularly involatile clusters seen are cations, which may help understand commonly observed sign effects in ion-induced nucleation. No other charge-sign asymmetry is seen on cluster evaporation.     

Substrate evaporation induced neck-shape evolution of a liquid/solid interface

The wetting behavior and interface interaction in the CaF₂/Me and NaCl/Me systems were studied using the sessile drop method. It was observed that liquid Bi, In, Sn and Ga do not wet the CaF₂ and NaCl substrates at 1000 K and liquid Cu does not wet the CaF₂ substrate in the 1423–1573 K temperature range. Nevertheless, different spreading behavior was observed during experiments. For the CaF₂/Me systems, at 1000 K, the contact angle remains constant with time, while for the NaCl/Me systems, a non-monotonic spreading behavior was detected. For these systems the contact angle increases initially and then rapidly decreases. For the CaF₂/Cu system at 1423 K the contact angle remains constant, while it increases with time at 1573 K. It was established that the wetting behavior is attributed to the evaporation of the solid substrate, which leads to the formation of a neck-shape interface. The experimental results were well accounted for by a model, which considered the geometrical characteristic of the metal/ceramic interface and thermo-physical properties of the metals and the substrates.     

Reversed-phase high-performance liquid chromatography of polar lipids. II. Procedure for a phospholipid derivative of methotrexate

Phospholipid derivatives of methotrexate (MTX) having enhanced membrane penetration (DP-71 being the most important) are potential prodrugs for treatment of autoimmune and inflammatory diseases as well as diseases involving abnormal cell proliferation. The previously published reversed-phase HPLC methods for similar compounds, phospholipid derivatives of valproic acid and non-steroidal anti-inflammatory drugs (NSAIDs), could not be used for MTX derivatives due to highly basic character of the MTX core molecule. The new HPLC procedure using gradient elution was developed as a compromise between the pharmacopoeial method for MTX and the previous "generic" procedure for phospholipid derivatives of NSAIDs. The newly developed method is sensitive, selective, reproducible, and stability indicating. Identification of major related compounds was carried out. The bioanalytical applications of this method, as well as of the derived isocratic procedure, are discussed and illustrated by examples of pharmacokinetic studies.     

The spontaneous polarization of a polar liquid close to a hydrophobic surface

The phenomenological vector model is used to discuss parallel orientation and spontaneous polarization of molecular dipole moments of a polar substance close to a hydrophobic surface.     

Complexes of acid amides with polar aprotic solvents. I

Complexes of bis(N-phenyl)-pyromellitic acid diamide with dimethylformamide were synthetized and studied by thermal mass-spectrometry. It was found that, depending on the precipitating agent applied, the composition of the complexes corresponded to 1 acid amide: 2 dimethylformamide or to 1 acid amide: 1 dimethylformamide. The effects of complex formation on the thermal cyclodehydration of bis(N-phenyl)-pyromellitic acid diamide and on the accompanying degradation processes are discussed.     

The evaporation/condensation transition of liquid droplets

The condensation of a supersaturated vapor enclosed in a finite system is considered. A phenomenological analysis reveals that the vapor is found to be stable at densities well above coexistence. The system size at which the supersaturated vapor condenses into a droplet is found to be governed by a typical length scale which depends on the coexistence densities, temperature and surface tension. When fluctuations are neglected, the chemical potential is seen to show a discontinuity at an effective spinodal point, where the inhomogeneous state becomes more stable than the homogeneous state. If fluctuations are taken into account, the transition is rounded, but the slope of the chemical potential versus density isotherm develops a discontinuity in the thermodynamic limit. In order to test the theoretical predictions, we perform a simulation study of droplet condensation for a Lennard-Jones fluid and obtain loops in the chemical potential versus density and pressure. By computing probability distributions for the cluster size, chemical potential, and internal energy, we confirm that the effective spinodal point may be identified with the occurrence of a first order phase transition, resulting in the condensation of a droplet. An accurate equation of state is employed in order to estimate the droplet size and the coexisting vapor density and good quantitative agreement with the simulation data is obtained. The results highlight the need of an accurate equation of state data for the Laplace equation to have predictive power.     

Diagrammatic kinetic theory for a lattice model of a liquid. I. Theory

We present a diagrammatic formalism for the time correlation functions of density fluctuations for an excluded volume lattice gas on a simple d-dimensional hypercubic lattice. We consider a multicomponent system in which particles of different species can have different transition rates. Our theoretical approach uses a Hilbert space formalism for the time dependent dynamical variables of a stochastic process that satisfies the detailed balance condition. We construct a Liouville matrix consistent with the dynamics of the model to calculate both the equation of motion for multipoint densities in configuration space and the interactions in the diagrammatic theory. A Boley basis of fluctuation vectors for the Hilbert space is used to develop two formally exact diagrammatic series for the time correlation functions. These theoretical techniques are generalizations of methods previously used for spin systems and atomic liquids, and they are generalizable to more complex lattice models of liquids such as a lattice gas with attractive interactions or polymer models. We use our formalism to construct approximate kinetic theories for the van Hove correlation and self-correlation function. The most simple approximation is the mean field approximation, which is exact for the van Hove correlation function of a one component system but an approximation for the self-correlation function. We use our first diagrammatic series to derive a two site multiple scattering approximation that gives a simple analytic expression for the spatial Fourier transform of the self-correlation function. We employ our second diagrammatic series to derive a simple mode coupling type approximation that provides a system of equations that can be solved for the self-correlation function.     

On the evaporation and motion of a volatile droplet near a volatile liquid surface

A problem concerning the free evaporation or condensation growth of a droplet near an infinite planar surface of the same liquid is solved. The behavior of the droplet is considered at vapor temperature and concentration gradients preset at an infinite distance from it. The boundary conditions take into account effects that are linear with respect to the Knudsen number. Equations are derived for the rate of variations in the radius of the droplet and the velocity of its steady motion induced by nonuniform temperature and concentration of the vapor. Dependences of the rate of variations in the radius and the velocity of the steady motion of the droplet on the distance from the planar surface are presented for a droplet 1 ??m in radius suspended in air.     

Models for polyesterification kinetics. I. Esterfication kinetics in polar medium

The kinetics of the reaction of 1-octadecanol with octadecanoic acid and w-carboxy polyxyethylene with w-hydroxy polyoxyethylene in the absence of a cataylst were studied as models of polyesterification. Both rreactions are third order. Order two in acid and one in alcohol have been experimentally established by studies of nonstoichiometric systems. These results are not in agreement with recent work related to polyesterification kinetics. The mechanism involves autocatalysis by the acid and the presence of nondissociated ion pairs. Activation enthalpies and entropies are given.     

Non-symmetric ether-linked liquid crystalline dimers with a highly polar end group

A new class of non-symmetric dimeric compounds derived from 4-cyano-4′-hydroxybiphenyl in which two rigid parts are connected via flexible spacers have been designed and synthesised. These materials possess trialkoxy chains attached at one end of the molecule, while the other end consists of a biphenyl moiety terminated with the highly polar cyano group. The molecular structures of these dimers have been confirmed by elemental analysis and spectroscopic data and their phase behaviour has been characterised by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Almost all of the synthesised materials exhibit liquid crystalline properties depending on the number of carbon atoms in the terminal chains, where all short chains derivatives form nematic phases and depending on the length of the internal spacer long terminal chains homologues display crystalline or unidentified smectic phase.     

Direct measurement of the force between solid surfaces in a polar liquid

The force between mica sheets in a polar liquid (propylene carbonate) with various electrolyte concentrations is found to be the sum of an electrostatic double-layer force, accurately described by Gouy—Chapman theory, and a short-range oscillatory salvation force, qualitatively similar to that found in non-polar liquids.     

Armstrong effect as a tool for structural elucidation of liquid polar dielectrics

The effect that appears when high-voltage electric field is applied to liquid polar dielectrics was described. The effect was suggested to be called the Armstrong effect after the name of the scientist who discovered it. Experimental data on the Armstrong effect in pure water were considered. The possible composition of megaclusters of liquid water was evaluated. The potential applications of the effect were suggested.     

Conformational properties of polar polymers. I. Poly(vinyl chloride)

A recently developed method for including polar bonds in conformational energy calculations is applied to poly(vinyl chloride). Inductive effects on dipole moments and the effects of intervening atoms on electrostatic interaction energies are represented by polarizability centers in conjunction with bond centered dipoles. Solvation energies are estimated by means of a continuum dipole–quadrupole electrostatic model. Calculated energies of a number of conformations of meso and racemic 2,4-dichloropentane and the iso, syndio, and hetero forms of 2,4,6-trichloroheptane give satisfactory representations of isomer and conformer populations. Electrostatic effects are found to be quite important. However they appear to be effectively of sufficiently short range that the calculated conformer energies are found to be fit well by a linear combination of interaction parameters (consisting of gauche, skew chlorine, four-bond CH₂…CH₂, CH₂…Cl, and Cl…Cl interactions) conventional to vinyl polymers and a special four-bond interaction that arises when the bond sequence Cl? CH? CH²? CH? Cl is (nearly) coplanar. These interaction parameters when assembled into statistical weight matrices lead to calculated values of both the characteristic ratio and the dipole moment ratio in satisfactory agreement with experiment. Least energy paths for transitions between the most stable conformations are also calculated.     

A molecular beam study of the evaporation of water from a liquid jet

A method to maintain a clean surface of a liquid in a high vacuum is described. Using a very thin and fast liquid jet it is not only possible to prevent freezing of the liquid but also to reduce the number of collisions between evaporating molecules to negligibly small values. Thus many of the standard, vacuum dependent, particle probing techniques for solid surfaces can be used for studies of rapidly vaporizing, high vapor pressure liquids. In a first molecular beam investigation we have used time-of-flight analysis to measure the velocity distribution of H₂O molecules vaporizing from thin jets of pure liquid water. The experiments were carried out for liquid jet diameters between 50 and 5 µm. In this range the expanding vapor is observed to undergo the transition to the collision-free molecular flow regime. From the measured velocity distributions the local surface temperature is determined to be less than 210 K. This appears to be the lowest temperature ever reported for supercooled liquid water.     

Heat and mass transfer with liquid evaporation into a turbulent air stream

Several recent experiments demonstrated the enhancement of turbulent heat transfer with evaporation as compared with that of a dry body. The present study was an attempt to obtain more accurate data on the heat and mass transfer coefficients with water evaporation. In addition, a method of error evaluation was developed and applied to correct the experimental errors in the recently reported results. Most of the corrected values and the present experimental data revealed that the heat transfer coefficient with evaporation agreed with that of the dry body without evaporation within experimental error.