Water interfacial region

In emulsion polymerization the compartmentalization of reaction loci and the location of monomer in polymer particles favor the growth and slow down termination events. The contribution of solution polymerization in the continuous phase is strongly restricted due to the location of monomer in the monomer droplets and/or polymer particles. This gives rise to greatly different characteristics of polymer formation in latex particles from those in bulk or solution polymerization. In emulsion polymerization, where polymer and monomer are mutually soluble, the polymerization locus is the whole particle. If the monomer and polymer are partly mutually soluble, the particle/water interfacial region is the polymerization locus. 

Flavor Partitioning The perception of a flavor depends on the precise location of the flavor molecules within an emulsion. The aroma is determined by the presence of volatile molecules in the vapor phase above an emulsion (122, 126). Most flavors are perceived more intensely when they are present in the aqueous phase, rather than in the oil phase (127, 128). Certain flavor molecules may associate with the interfacial region, which alters their concentration in the vapor and aqueous phases (129). It is therefore important to establish the factors that determine the partitioning of flavor molecules within an emulsion. An emulsion system can be conveniently divided into four phases between which the flavor molecules distribute themselves the interior of the droplets, the continuous phase, the oil-water interfacial region, and the vapor phase above the emulsion. The relative concentration of the flavor molecules in each of these regions depends on their molecular structure and the properties of each of the phases (130, 131). 

Interfacial Viscosity, The foregoing discussion of rheology has dealt with the bulk viscosity properties. A closely related and very important property is the interfacial viscosity, which can be thought of as the two-dimensional equivalent of bulk viscosity, operative in the oil-water interfacial region. As droplets in an emulsion approach each other, the thinning of... 

Detailed studies of the ice-water interfacial region, in the presence and absence of the mixture, are presently under way. Finally, any consideration of the functional models for antifreeze glycoprotein, which now includes potentiation, also must be tested on AFP. Potentiating substances in the sera of species with AFP probably would be very different from those in the species with AFGP. 

Metal oxide semiconductor electrodes also differ from bare metal electrodes with respect to interactions with water. Interfacial region in which water properties differ significantly from those found in the bulk phase is generally more extensive than for metal electrodes. Significant interfacial water structure can extend to several molecular layers from oxide surfaces. Also, the inner monolayer of water can be rotationally immobile due to hydrogen bonding, a feature that is absent at pure metal surfaces. 

In addition to bulk viscosity properties, a closely related and very important property is the interfacial viscosity, which can be thought of as the two-dimensional equivalent of bulk viscosity, operative in the oil-water interfacial region. As droplets in an emulsion approach each other the thinning of the films between the drops, and their resistance to rupture, are thought to be of great importance to the ultimate stability of the emulsion. Thus, a high interfacial viscosity can promote emulsion stability by retarding the rate of droplet coalescence, as discussed in later sections. Further details on the principles, measurement, and applications to emulsion stability of interfacial viscosity are reviewed by Malhotra and Wasan [32]. 

It is known that the position of the maximum fluorescence of ketocyanine dyes in a solution is highly solvent sensitive and the energy of maximum fluorescence, E(F), is linearly dependent on the solvatochromic parameters, a and %. The dyes are solnbilized in the micelle-water interface [95], It is believed that the polarity characteristics of the micelle-water interfacial region resemble those of aqneons alkanols [49], The values of E(F) of ketocyanine dyes have been determined in varions pnre n-alkanols and mixed binary aqneons alkanol systems for which the values of solvatochromic parameters (a and % ) are known [96], An MLRA of E(F) with the... 

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

The unexpected preference for the interfacial region at lower concentrations of benzene has prompted speculation. It has been demonstrated that aromatic compounds are capable of forming weak hydrogen bonds with water. This ability favours uptake in the aqueous interface over solubilisation in the interior. Alternatively, some authors have attributed the binding behaviour of benzene to its... 

If equation 7 holds, then the soHd is exclusively in the aqueous phase equation 8 defines the condition at which the soHd resides in the oil phase whereas if equation 9 is satisfied then the soHd collects at the water—oil interfacial region. Figure 16 is the flow sheet of a bench-scale study that demonstrates the concept of two-Hquid flotation (40). 

Because the core of an aqueous micelle is extremely hydrophobic, it has the abiHty to solubiHze oil within it, as weU as to stabilize a dispersion. These solubilization and suspension properties of surfactants are the basis for the cleansing abiHty of soaps and other surfactants. Furthermore, the abiHty of surfactants to stabilize interfacial regions, particularly the air—water interface, is the basis for lathering, foaming, and sudsing. 

Figure 5 Electron density distributions along the bilayer normal from an MD simulation of a fully hydrated liquid crystalline phase DPPC bilayer. (a) Total, lipid, and water contributions (b) contributions of lipid components in the interfacial region.

By small-angle neutron scattering experiments on water/AOT/hydrocarbon microemulsions containing various additives, the change of the radius of the miceUar core with the addition of small quantities of additives has been investigated. The results are consistent with a model in which amphiphilic molecules such as benzyl alcohol and octanol are preferentially adsorbed into the water/surfactant interfacial region, decreasing the micellar radius, whereas toluene remains predominantly in the bulk hydrocarbon phase. The effect of n-alcohols on the stability of microemulsions has also been reported [119],... 

Girault and Schiffrin [4] proposed an alternative model, which questioned the concept of the ion-free inner layer at the ITIES. They suggested that the interfacial region is not molecularly sharp, but consist of a mixed solvent region with a continuous change in the solvent properties [Fig. 1(b)]. Interfacial solvent mixing should lead to the mixed solvation of ions at the ITIES, which influences the surface excess of water [4]. Existence of the mixed solvent layer has been supported by theoretical calculations for the lattice-gas model of the liquid-liquid interface [23], which suggest that the thickness of this layer depends on the miscibility of the two solvents [23]. However, for solvents of experimental interest, the interfacial thickness approaches the sum of solvent radii, which is comparable with the inner-layer thickness in the MVN model. 

---