Interfacial polarization, computer

According to the Kirkwood theory of polar dielectrics, simple relations (23) between molecular dipole moment vectors and the mean-square total dipole moment of water clusters can be used to compute the static dielectric constant of water. As the normalized mean-square total dipole moment increases towards unity, theory predicts decreases in the static dielectric constant. Since MD results indicate that the mean-square total dipole moment of interfacial water is greater than that for bulk water (48), the static dielectric... 

Recently, detailed molecular pictures of the interfacial structure on the time and distance scales of the ion-crossing event, as well as of ion transfer dynamics, have been provided by Benjamin s molecular dynamics computer simulations [71, 75, 128, 136]. The system studied [71, 75, 136] included 343 water molecules and 108 1,2-dichloroethane molecules, which were separately equilibrated in two liquid slabs, and then brought into contact to form a box about 4 nm long and of cross-section 2.17 nmx2.17 nm. In a previous study [128], the dynamics of ion transfer were studied in a system including 256 polar and 256 nonpolar diatomic molecules. Solvent-solvent and ion-solvent interactions were described with standard potential functions, comprising coulombic and Lennard-Jones 6-12 pairwise potentials for electrostatic and nonbonded interactions, respectively. While in the first study [128] the intramolecular bond vibration of both polar and nonpolar solvent molecules was modeled as a harmonic oscillator, the next studies [71,75,136] used a more advanced model [137] for water and a four-atom model, with a united atom for each of two... 

Kakiuchi, T., M. Nakanishi, and M. Senda (1988). The electrocapillary curves of a phosphatidylcholine monolayer at a polarized oil-water interface. I. Measurement of interfacial tension using a computer-aided pendant-drop method. Bull. Chem. Soc. Jpn 61, 1845-1851. 

Luciani et al. (1998) critically examined the experimental methods used for the measurements of the interfacial coefficient in polymer blends as well as the theoretical models for its evaluation. A new working relation was derived that makes it possible to compute the interfacial tension from the chemical structure of two polymers. The calculations involve the determination of the dispersive, polar, and hydrogen-bonding parts of the solubility parameter from the tabulated group and bond contributions. The computed values for 46 blends were found to follow the experimental ones with a reasonable scatter of +/— 36 %. The authors mentioned also that since many experimental techniques have been developed for low-viscosity Newtonian fluids, most were irrelevant to industrial polymeric systems. For their studies, two were selected capillary breakup method and a newly developed method based on the retraction rate of deformed drop. 

It has been shown that for a correct description of the interfacial tensions of pure fluids a temperature dependent influence parameter must de used. This is especially required for more polar species like alcohols and water, while for non-polar species the temperature dependence of the influence parameter is small. In all cases the expression for the influence parameter has n optimized by fitting the computed interfacial tensions to the experimental values. It should be remarked that due to the fitting procedure the predictive value for pure species is lost. 

Abstract A new type of interfacial additive has been developed by the authors recently. The molecules of amphiphilic character are capable of forming chemical bonds at both the polar and the apolar sides. These reactive surfactants have been synthesised in a specially designed computer-controlled reactor system. The aim of the development was to combine the advantages of nonre-active surfactants and reactive coupling agents, making possible an... 

One consequence of interfacial segregation based on polarity is strongly nonuniform orientational distributions of amphiphiles. For example, the vector directed from the nonpolar to the polar end of the solute (e.g., from the methyl carbon to the hydroxyl oxygen atoms in alcohols) clearly points toward water. The distribution of this vector, calculated from computer simulations, peaks at the orientation perpendicular to the interface and rapidly decreases as the vector becomes parallel. The probability of finding the vector pointing towards the nonpolar phase is negligible. From equation (8) it follows that a considerable free energy is required to invert the orientation of an amphiphile at the interface. 

Modem computer simulation of aqueous interfaces date from only 1985. In this short period, they have yielded new insights into the unique properties of interfacial systems, which distinguish them from bulk phases. Perhaps the most important of these properties is the existence of very different environments, polar and nonpolar, in direct proximity. As a result, aqueous interfaces tend to concentrate and organize organic material. In particular, they provide ideal surroundings for amphiphilic molecules, which can simultaneously have their polar parts immersed in water and nonpolar parts immersed... 

Extensive theory and computer simulation work has been able to clarify the molecular mechanisms of solvation dynamics in bulk liquids over the past three decades.One of the most important conclusions from this body of work is that most of the contribution to polar solvation dynamics comes from the solute s first solvation shell. This conclusion and the earlier discussion about the prominent role the solute hydration shell plays in understanding vibrational and rotational dynamics at liquid interfaces suggest that surface effects on solvation dynamics will be muted as the solute s polarity is increased. An experimental validation of this are the similar solvation dynamics of C314 at the water liquid/vapor interface and in bulk water, mentioned above, where the highly polar excited state n = 12D) implicates an interfacial hydration structure similar to the bulk. 

Neumann s equation of state (Neumann et ai, 1974) assumes as a matter of principle that the surface tension, and the interfacial tension, must on no account be subdivided into an apolar and a polar component. The computer program pertaining to the equation of state, in addition, makes it impossible for AG to assume a positive value, making it in turn impossible for particle-particle interactions to be repulsive (i.e., for non-electrically charged particles to form stable suspensions) and for neutral polymers (e.g., PEO,... 

The higher amount of trapped water is in favor of model (b), where the particles are in closer contact with the interfacial layer. However, the NMR line of the adsorbed water could overlap that of the trapped water. In order to check this hypothesis, the number of water molecules per AOT was calculated. The spectra in Fig. 17 have been decomposed in two bands corresponding, respectively, to the bound water and to the trapped water. The difference in intensities of the two NMR lines corresponding to the trapped water in the spectra without and with AgBr particles gives the amount of water trapped or adsorbed on the particles. The number of AOT molecules per particle has been calculated using a spherical surface of 4.6 nm diameter and a surface area of 0.41 nm for the polar part of the AOT molecule [35]. It has been computed that if the whole line intensity corresponded to the trapped water there would be 2000 water molecules per AOT molecule. As the trapped water is considered to be in the form of a monomer or a dimer, this value is too high to correspond only to water molecules trapped in the interface. Hence, it has to be assumed that the additional water molecules... 

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