Dispersion of interaction

Multiple scattering is a considerable problem in extracting structural information from the dynamic light-scattering of dispersions of interacting parti-cles. " The effect of particle interactions on lg (K, t) has been studied by several authors, most notably Pusey and Ackerson. Experimentally, g K, t) is found no longer to be of a simple exponential form. The difficulty theoretically is to separate out the effects of particle interactions, polydispersity, and multiple scattering. 

Eq. IV-9 would use the surface tensions that liquids A and B would have if their inter-molecular potentials contained only the same kinds of interactions as those involved between A and B (see Refs. 20, 22-24). For the hydrocarbon-water system, Fowkes [20] assumed that Uh arose solely from dispersion interactions leaving... 

Two nucleation processes important to many people (including some surface scientists ) occur in the formation of gallstones in human bile and kidney stones in urine. Cholesterol crystallization in bile causes the formation of gallstones. Cryotransmission microscopy (Chapter VIII) studies of human bile reveal vesicles, micelles, and potential early crystallites indicating that the cholesterol crystallization in bile is not cooperative and the true nucleation time may be much shorter than that found by standard clinical analysis by light microscopy [75]. Kidney stones often form from crystals of calcium oxalates in urine. Inhibitors can prevent nucleation and influence the solid phase and intercrystallite interactions [76, 77]. Citrate, for example, is an important physiological inhibitor to the formation of calcium renal stones. Electrokinetic studies (see Section V-6) have shown the effect of various inhibitors on the surface potential and colloidal stability of micrometer-sized dispersions of calcium oxalate crystals formed in synthetic urine [78, 79]. 

Between any two atoms or molecules, van der Waals (or dispersion) forces act because of interactions between the fluctuating electromagnetic fields resulting from their polarizabilities (see section Al. 5, and, for instance. 

Vincent B, Edwards J, Emmett S and Greet R 1988 Phase separation in dispersions of weakly-interacting particles in solutions of non-adsorbing polymers Colloid Surf. 31 267-98... 

The effect of polarity in enhancing the energy of interaction has been discussed by Kiselev and his associates who distinguish between non-specific adsorption, where only dispersion and repulsive forces are involved 4>d and and specific adsorption, where coulombic contributions (some or all of (p, [Pg.11]

The enhancement of interaction energy in micropores was discussed in some detail in Chapter 4. It was emphasized that the critical pore width d at which the enhancement first appears increases with increasing diameter a of the adsorbate molecule, since the relevant parameter is the ratio d/a rather than d itself. The quantity a is involved because the magnitude of the dispersion interaction increases as the polarizability, and therefore the size, of the molecule increases (cf. p. 5). 

When two or more molecular species involved in a separation are both adsorbed, selectivity effects become important because of interaction between the 2eobte and the adsorbate molecule. These interaction energies include dispersion and short-range repulsion energies (( ) and ( )j ), polarization energy (( )p), and components attributed to electrostatic interactions. 

An important property of a pigment is its ability to maintain its color when exposed to light, weather, heat, and chemicals. This property is seldom measured for pigments alone. Rather it is determined for the dispersion of a pigment in a desired medium, eg, paints or plastics, and in many cases it is compared to the performance of a standard pigment. The observed changes are the result of complex pigment and media reactions and their possible interactions. In aH evaluations, time of exposure plays a role. 

Dispersive Interactions. For pairs of nonpolar polymers, the intermolecular forces are primarily of the dispersive type, and in such cases the energy of interaction between unlike segments is expected to be closely approximated by the geometric mean of the energies of interaction between the two like pairs (98). In this case, the Flory-Huggins interaction energy between this polymer pair can be expressed in terms of the solubiUty parameters 5 of the pure components. 

Dispersion of a soHd or Hquid in a Hquid affects the viscosity. In many cases Newtonian flow behavior is transformed into non-Newtonian flow behavior. Shear thinning results from the abiHty of the soHd particles or Hquid droplets to come together to form network stmctures when at rest or under low shear. With increasing shear the interlinked stmcture gradually breaks down, and the resistance to flow decreases. The viscosity of a dispersed system depends on hydrodynamic interactions between particles or droplets and the Hquid, particle—particle interactions (bumping), and interparticle attractions that promote the formation of aggregates, floes, and networks. 

Polymer/Polymer Complexes. PVP complexes with other polymers capable of interacting by hydrogen-bonding, ion-dipole, or dispersion forces. For example mixing of PVP with poly(acryHc acid) (PAA) in aqueous solution results in immediate precipitation of an insoluble complex (113). Addition of base results in dismption of hydrogen bonding and dissolution (114—116). Complexes with a variety of poly-acids (117) and polyphenols (118) have been reported. The interest in compatibiHty on a molecular level, an interesting phenomenon rarely found to exist between dissimilar polymers, is favored by the abiHty of PVP to form polymer/polymer complexes. 

An estimate of the enthalpy change which conesponds to the activation energy of the collision theory analysis of 167kJmoP may be made by assuming that the formation of tire dimer from two molecules of the monomer is energetically equivalent to tire dipole-dipole and dispersion interactions of two HI molecules. These exothermic sources of interaction are counterbalanced... 

This dispersion interaction must be added to the dipole-dipole interactions between molecules, such as HCl, NH3 and H2O which have a permanent dipole, fi. The magnitude of die dipole moment depends on tire differences in electronegativity of the atoms in the molecule. Here again, the energy of interaction varies as (orientation effect). 

Whilst the aliphatic nylons are generally classified as being impact resistant, they are affected by stress concentrators like sharp comers which may lead to brittle failures. Incorporation of mbbers which are not soluble in the nylons and hence form dispersions of rubber droplets in the polyamide matrix but which nevertheless can have some interaction between mbber and polyamide can be most effective. Materials described in the literature include the ethylene-propylene rubbers, ionomers (q.v.), polyurethanes, acrylates and methacrylates, ABS polymers and polyamides from dimer acid. 

Dispersion forces are ubiquitous and are present in all molecular interactions. They can occur in isolation, but are always present even when other types of interaction dominate. Typically, the interactions between hydrocarbons are exclusively dispersive and, because of them, hexane, at S.T.P., is a liquid boiling at 68.7°C and is not a gas. Dispersive interactions are sometimes referred to as hydrophobic or lyophobic particularly in the fields of biotechnology and biochemistry. These terms appear to have arisen because dispersive substances, e.g., the aliphatic hydrocarbons, do not dissolve readily in water. Biochemical terms for molecular interactions in relation to the physical chemical terms will be discussed later. 

The theory of molecular interactions can become extremely involved and the mathematical manipulations very unwieldy. To facilitate the discussion, certain simplifying assumptions will be made. These assumptions will be inexact and the expressions given for both dispersive and polar forces will not be precise. However, they will be reasonably accurate and sufficiently so, to reveal those variables that control the different types of interaction. At a first approximation, the interaction energy, (Ud), involved with dispersive forces has been calculated to be... 

The interactive character of a molecule can be very complex and a molecule can have many interactive sites. These sites will comprise the three basic types of interaction, i.e., dispersive, polar and ionic. Some molecules (for example, large molecules such as biopolymers) can have many different interactive sites dispersed throughout the entire molecule. The interactive character of the molecule as a whole will be... 

The dispersion of a solute band in a packed column was originally treated comprehensively by Van Deemter et al. [4] who postulated that there were four first-order effect, spreading processes that were responsible for peak dispersion. These the authors designated as multi-path dispersion, longitudinal diffusion, resistance to mass transfer in the mobile phase and resistance to mass transfer in the stationary phase. Van Deemter derived an expression for the variance contribution of each dispersion process to the overall variance per unit length of the column. Consequently, as the individual dispersion processes can be assumed to be random and non-interacting, the total variance per unit length of the column was obtained from a sum of the individual variance contributions. 

The technique of IGC may be employed to obtain acid-base information, as suggested by Schultz and Lavielle [99], by using acid and base probe gases on a solid for which the alkane line has already been obtained. If acid-base interaction is involved in the adsorption, the retention volume should be greater than that corresponding to the dispersion force interaction alone, which should be the same as that of the equivalent alkane , i.e. the hypothetical alkane for which the value... 

There are three types of interactions that contribute to van der Waals forces. These are interactions between freely rotating permanent dipoles (Keesom interactions), dipole-induced dipole interaction (Debye interactions), and instantaneous dip le-induced dipole (London dispersion interactions), with the total van der Waals force arising from the sum. The total van der Waals interaction between materials arise from the sum of all three of these contributions. 

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