Liquid-vapor equilibrium and surface tension of nonconformal molecular fluids

Molecular dynamics simulations at constant temperature have been performed on the liquid-vapor interface for fluids characterized by a recently introduced three-parameter potential. This potential is a modification of the well-known spherical Kihara interaction and is termed approximate nonconformal (ANC). It has been used successfully to describe many real molecules in the gaseous phase. Besides the usual molecular energy and size, the ANC potential introduces a third parameter s, called softness, to measure the form of the potential profile. Study of these systems shows that their critical and interfacial properties follow very closely those of four selected substances: argon, methane, propane, and hexane. Deviations of the properties predicted from the experimental values are analyzed and their probable causes are determined. The critical properties of ANC fluids and their dependence on s are also obtained via first-order perturbation theory in the form of an augmented van der Waals model. Analysis of the results shows that ANC potential functions can be used as reliable effective interactions for real dense fluids.     

Non-monotonic behaviour with concentration of the surface tension of certain binary liquid alloys

The surface tension σ(c) of most liquid binary alloys usually varies with concentration c in a monotonic way between the values σ₁ and σ₂ of the two pure metals, and this behaviour is well explained by current models. Some alloys show deviations from this ideal behaviour. One of those is Fe–B. The surface tension of this liquid alloy shows a minimum at 17 atomic % B, which corresponds well with the composition of the eutectic point in the phase diagram, followed by a maximum at a concentration of 24 atomic % B or higher. The usual models for the surface tension of liquid binary alloys do not explain those exceptional features, and we propose that a model involving the concentration fluctuations in the liquid alloy has the proper ingredients to account for the features in Fe–B and similar alloys.     

Survismeter for simultaneous viscosity and surface tension study for molecular interactions

Survismeter simultaneously measures viscosities and surface tensions of several standard solvents (AR, methanol, ethanol, glycerol, ethyl acetate, n‐hexane, diethyl ether, chloroform, benzene, CCl₄ and formic acid) and freshly prepared solutions of urea (U), 1‐methylurea (MU), 1,3‐dimethylurea (DMU) at several temperatures. Analysis for accuracies and hydrophobic interactions were made with data of solvents and solutions respectively. It replaces the use of viscometers and stalagmometer for viscosity and surface tensions individually. A decrease with one ? CH₃ of MU and an increase with two ? CH₃ of DMU in viscosities for 288.15–298.15 K with reverse trend for 303.15–308.15 K are noticed. Surface tension decreases from U to MU and increases with DMU at a slightly higher rate, but decreases with temperature. The ? CH₃ is noticed to weaken hydrophilic interactions and strengthening hydrophobic interactions with stronger structure effecting changes in MU and DMU. Copyright © 2008 John Wiley & Sons, Ltd.     

A simple method for calculation of the surface tension of molecular inorganic liquids

A method was proposed for calculation of the temperature dependence of the surface tension using a single experimental characteristic of a substance, its boiling point.     

Universal scaling features of spectroscopic constants for diatomic systems

Based on a new criterion that was proposed to search for the universality of spectroscopic constants for bound ground-state diatomics [R. H. Xie and P. S. Hsu, Phys. Rev. Lett. 96, 243201 (2006)], we have found universal scaling relations between spectroscopic constants of diatomic systems with s-, p-, and d-type valence-shell constituents. Our study suggests a useful empirical approach for the prediction of molecular spectroscopic constants.     

The correlations of the enthalpy of vaporization and the surface tension of molecular liquids

Correlation relations based on Stefan's rule, which defined dependence between the enthalpy of vaporization, the surface tension, the molar volume and the molar mass of a substance, were obtained. For development of the correlation equations two computational procedures were used: a method of the least squares and a method of artificial neural networks. The method of artificial neural networks was shown to give somewhat better results than the linear least-squares procedure. The average deviation of the calculated values from the experimental ones did not exceed 6% for training set of substances and 10% for control set (the method of the least squares). For the method of artificial neural networks it is 3% and 8%, respectively.     

Simultaneous formation behaviour of surface structures and molecular alignment by patterned photopolymerisation

ABSTRACT

Highly functional soft materials with fine control of molecular alignment are of great interest for the applications in various fields. However, the current method of molecular alignment still has some challenges. Recently, we have developed a new alignment process based on a concept of scanning wave photopolymerisation (SWaP). When a sample is irradiated with spatially selected light, a polymerisation occurs only in an irradiated region, leading to a molecular motion between the irradiated and unirradiated regions. Such molecular motion generates the alignment of liquid crystal molecules. Moreover, a surface relief structure is formed in the polymer film by the molecular motion. In this study, we simultaneously formed the surface structure and molecular alignment by the patterned photopolymerisation. We compared the degree of molecular alignment with the shape of the created surface structure, and revealed that the degree of molecular alignment was maximized at the boundary of irradiated and unirradiated regions. This method enables one to form both molecular alignment patterning and surface structuring in a single step.     

Universal scaling laws of diffusion: application to liquid metals

This work focuses on the universal scaling laws, which relate scaled diffusivity to excess entropy in fluids and their mixtures. The derivation of the new scaling law for diffusivity proposed recently [A. Samanta, Sk. M. Ali, and S. K. Ghosh, Phys. Rev. Lett. 92, 145901 (2004)] is discussed in details highlighting the nature of approximations involved. Also the applicability of the scaling law is extended to a new class of liquids, viz., liquid metals. The results calculated based on the scaling laws are shown to be in very good agreement with the simulation results for liquid Rb and Cs metals along the liquid-vapor coexistence curve corresponding to a wide variation of temperature and density. The new universal scaling law discussed here is superior to the earlier empirically proposed scaling laws and provides a very simple route to calculate a dynamical quantity such as diffusivity from an equilibrium property such as the radial distribution function.     

On relativistic surface tension

On the basis of special relativity and classical thermodynamics, the surface tension was shown to be a relativistic invariant.     

Surface tension of polymers with long unbranched side chains

The surface tension of polymers having long unbranched side chains was measured by a contact-angle technique. It ranges between 20–29 dynes/cm and is almost entirely due to dispersion forces. At the lower end of this range the side chains are attached to every other carbon atom of the main chain and the structure is characterized by extended alkyl side chains at right angles to the main chain with methyl groups forming the interface. As the spacing frequency of the side chains increases and the main chain becomes the site of other bulky groups, this structure becomes less ordered and the surface tension increases to the high twenties. The same effect, to a lesser extent, is observed when the length of the side chains is gradually diminished to 12 carbon atoms from 21. A sudden increase of the polar force contribution of the surface tension at 10–12 carbon atoms indicates collapse of the ordered side-chain structure.     

Comparative study of the effect of tail corrections on surface tension determined by molecular simulation

We report results from a comparative study of the influence of tail corrections on the surface tension of the Lennard-Jones fluid. We find that cutoff-independent surface tensions can be obtained by applying a set of tail corrections recently introduced by Janecek at each step of an interfacial Monte Carlo (MC) or molecular dynamics (MD) simulation. The effect of tail corrections on an alternative methodology for calculating surface tension, the combination of finite-size scaling and grand-canonical transition-matrix Monte Carlo (FSS/GC-TMMC), was also investigated. Using this indirect method, surface tensions were calculated with standard (bulk-fluid) tail corrections and lattice sums, the latter usually considered more accurate but computationally more intensive than the former. With standard tail corrections, we find that the surface tension decreases with increasing cutoff distance, reaching a limiting value corresponding to the maximum cutoff possible, namely half the simulation box length. In contrast, surface tension values obtained with the lattice summation were cutoff-independent. More importantly, these values were equivalent to those surface tension values obtained using standard tail corrections and a cutoff distance of half the box length. We also find that the surface tension values obtained here are in agreement with those found in the literature. Last, we find that surface tension values obtained by MD and FSS/GC-TMMC are in decent agreement so long as the appropriate tail correction schemes are used, and that the relative uncertainties in the surface tensions calculated by MD are generally an order of magnitude greater than those calculated by FSS/GC-TMMC. However, the time required by MD on a single central processing unit is less than that required by FSS/GC-TMMC.     

Relationship between surface tension and surface coverage

Surfactant action is caused in part by a dramatic reduction in surface tension. Using surface excess measurements from a radioactive surfactant, it was possible to show that (a) the surface tension declines only slightly when the occupancy of the air/water interface increases from 0 to 60% of the maximum and (b) the steep drop in surface tension in region B (Figure 1 ), frequently observed to be linear, begins at about 80% occupancy. Surfactant continues to enter the interface cooperatively up to and past the critical micelle concentration. Linearity in region B is not indicative of surface saturation despite a seemingly constant surface excess throughout the region. The disparity between interfacial areas determined by surface tension and by other methods is discussed in terms of these results.     

Impact of molecular surface characteristics on the reversed-phase retention behaviour of synthetic dyes

The lipophilicity and specific hydrophobic surface area of 42 synthetic dyes were determined on reversed-phase alumina layers using water-methanol mixtures as eluents, and their relationship with the molecular surface characteristics was elucidated by principal component analysis followed by a two-dimensional nonlinear map and cluster analysis. Four dyes remained on the origin in each eluent system. Except for two dyes, the majority showed regular retention behaviour with their retention decreasing steadily with increasing concentration of methanol in the mobile phase. Calculations indicated that both hydrophobicity parameters decrease with increasing polar surface area of the molecules.     

Rubbing-induced molecular reorientation on an alignment surface of an aromatic polyimide containing cyanobiphenyl side chains

Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical second harmonic generation (SHG) experiments have been utilized to study the molecular orientation and conformation changes at a rubbed polyimide alignment-layer surface. This aromatic polyimide containing pendent cyanobiphenyl mesogens was synthesized via a polycondensation of 2,2'-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) with bis[omega-[4-(4-cyanophenyl)phenoxy]hexyl] 4,4'-diamino-2,2'-biphenyldicarboxylate (nCBBP, n = 6), abbreviated as 6FDA--6CBBP. Uniform alignment layers, possessing high pretilt angles ranging from 39 degrees to 43 degrees, have been achieved after mechanical rubbing of the polyimide thin film surface at room temperature and subsequent annealing. This is the first time that high pretilt angles have been detected to possess a negative angle (-theta(c)) with respect to the rubbing direction (i.e., opposite to the rubbing direction), considerably different from the conventional pretilt angle (theta(c)) observed along the rubbing direction. This observation is confirmed using magnetic null and SHG methods. Combined polyethylene (PE) SLD and atomic force microscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-on PE lamellar crystals is oriented normal to the rubbing direction, indicating that the PE chains are aligned parallel to the rubbing direction. This SLD technique probes the anisotropic surface orientation of the outermost molecules of the rubbed polyimide layer. The SERS results show that prior to rubbing the surface, both the pendent cyanobiphenyls in the side chains and backbones possess nearly planar chain conformations at the polyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylene structure, but also significant conformational rearrangements of the pendent side chains at the surfaces. The molecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms a uniformly tilted conformation on the rubbed surface, and the polar cyano groups point down toward the layer surface deduced from SHG phase measurements. This conformational rearrangement of the side chains results in the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenyls having the -theta(c) pretilt angle with respect to the rubbing direction.     

Dynamical scaling of single chains on adsorbing substrates: diffusion processes

We study the dynamics of tethered chains of length N on adsorbing surfaces, considering the dilute case; for this we use the bond fluctuation model and scaling concepts. In particular, we focus on the mean-square displacement of single monomers and of the center of mass of the chains. The characteristic time tau of the fluctuations of a free chain in a good solvent grows as tau approximately N(a), where the coefficient a obeys a=2nu+1. We show that the same coefficient also holds at the critical point of adsorption. At intermediate time scales single monomers show subdiffusive behavior; this concurs with the behavior calculated from scaling arguments based on the dynamical exponent a. In the adsorbed state tau(perpendicular), the time scale for the relaxation in the direction perpendicular to the surface, becomes independent of N; tau(perpendicular) is then the relaxation time of an adsorption blob. In the direction parallel to the surface the motion is similar to that of a two-dimensional chain and is controlled by a time scale given by tau(parallel) approximately N(2nu(2)+1)L(-2Delta(nu/nu)), where nu(2) is the Flory exponent in two dimensions, nu is the Flory exponent in three dimensions, and Deltanu=nu(2)-nu. For the motion parallel to the surface we find dynamical scaling over a range of about four decades in time.