Bulk phase interfacial polarity

The electrical double layer (edl) at the oil-water interface is a heterogeneous interfacial region that separates two bulk phases of polarized media and maintains a spatial separation of charges. EDLs at such interfaces determine the kinetics of charge transfer across phase boundaries, stability and electrokinetic properties of lyophobic colloids, mechanisms of phase transfer or interfacial catalysis, charge separation in natural and artificial photosynthesis, and heterogeneous enzymatic catalysis [1-5]. 

By covalently attaching reactive groups to a polyelectrolyte main chain the uncertainty as to the location of the associated reactive groups can be eliminated. The location at which the reactive groups experience the macromolecular environment critically controls the reaction rate. If a reactive group is covalently bonded to a macromolecular surface, its reactivity would be markedly influenced by interfacial effects at the boundary between the polymer skeleton and the water phase. Those effects may vary with such factors as local electrostatic potential, local polarity, local hydrophobicity, and local viscosity. The values of these local parameters should be different from those in the bulk phase. 

Water-in-oil microemulsions (w/o-MEs), also known as reverse micelles, provide what appears to be a very unique and well-suited medium for solubilizing proteins, amino acids, and other biological molecules in a nonpolar medium. The medium consists of small aqueous-polar nanodroplets dispersed in an apolar bulk phase by surfactants (Fig. 1). Moreover, the droplet size is on the same order of magnitude as the encapsulated enzyme molecules. Typically, the medium is quite dynamic, with droplets spontaneously coalescing, exchanging materials, and reforming on the order of microseconds. Such small droplets yield a large amount of interfacial area. For many surfactants, the size of the dispersed aqueous nanodroplets is directly proportional to the water-surfactant mole ratio, also known as w. Several reviews have been written which provide more detailed discussion of the physical properties of microemulsions [1-3]. 

The physical depth of the microscopic interfacial region can be estimated to correspond to the distance over which interfacial molecular and ionic forces exert their influence. Although molecules or ions experience no net forces in the interior of the bulk phases, these forces become unbalanced as the ions or molecules move toward the interface. On the aqueous side of the interfaces, where monolayers of charged molecules or polar groups can be present, these forces can be felt over several nanometers. On the organic side of the interface, where van der Waals forces are mainly operative, the interfacial region generally extends for tens of nanometers. The van der Waals forces decrease with the seventh power of the intermolecular distances, so molecules experience essentially symmetric forces, once they are a few molecular diameters away from the interface. 

Fruitful interplay between experiment and theory has led to an increasingly detailed understanding of equilibrium and dynamic solvation properties in bulk solution. However, applying these ideas to solvent-solute and surface-solute interactions at interfaces is not straightforward due to the inherent anisotropic, short-range forces found in these environments. Our research will examine how different solvents and substrates conspire to alter solution-phase surface chemistry from the bulk solution limit. In particular, we intend to determine systematically and quantitatively the origins of interfacial polarity at solid-liquid interfaces as well as identify how surface-induced polar ordering... 

It is now time to reconsider the simple case of a two-phase system that contains two different types of molecules. If molecules of phase a are polar and molecules of phase [3 are nonpolar, the introduction of amphiphilic molecules that are capable of associating with either one of the two bulk phase molecules will result in an accumulation at the interface. Hence, these molecules will have a true excess concentration at the interface. Figure D3.5.4 illustrates that once surfactants adsorb at interfaces, the concentration within the interface may be larger than in any of the other phases. In order to predict the influence that these adsorbed surfactant molecules can have on the properties of the bulk system, interfacial chemists must be able to quantify the number of molecules that are adsorbed at the interface, that is, they must be able to measure the interfacial coverage. Unfortunately, it is extremely difficult, if not impossible, to directly measure the concentration of surface-active molecules adsorbed in a two-dimensional plane. This is where the thermodynamic concepts discussed earlier prove to be very useful, because a relationship between the interfacial coverage (G) and the interfacial tension (y) can be derived. 

In Fig. 8 the interfacial tension between styrene and aqueous acrylic acid solution as well as the stability of the concentrated emulsion is plotted against the concentration of acrylic acid in water. As the concentration of this polar monomer increases, the concentrated emulsion becomes more unstable, and finally the entire concentrated emulsion separates into bulk phases. 

SHG is a coherent process and in principle the experimental system needed to observe the response is very simple. The fundamental radiation from a laser source incident at an interface generates the harmonic beam via non-linear polarization of the medium. Typically, this beam is observed in reflection, but many studies have been undertaken in total internal reflection and transmission geometries. As the harmonic beam is well separated from the fundamental in frequency, it can be detected the difficulties arise due to the inherent inefficiency of the harmonic generation and the low intensities that need to be detected. The sensitivity and selectivity of SHG to the interfacial species in the presence of the same species in the bulk phase provides the driving force to overcome these experimental difficulties. 

Besides the ionic double layers that may be present at phase boundaries there Is also a second type of double layer, caused by polarization of the interfacial region, l.e. a double layer not attributable to free ions. An important contribution is the preferential orientation of solvent dipoles and multipoles close to the surface. These molecules may also have induced dipoles. In the surfaces of solids the centres of positive and negative charges are, as a rule, displaced as compared with the situation in the bulk. All these charge displacements together constitute the interfacial polarization. The associated potential difference across phase boundaries is called the interfactal potential (drop) or x-potential. 

The membrane in a broad sense is a thin layer that separates two distinctively different phases, i.e., gas/gas, gas/liquid, or liquid/liquid. No characteristic requirement, such as polymer, solid, etc., applies to the nature of materials that function as a membrane. A liquid or a dynamically formed interface could also function as a membrane. Although the selective transport through a membrane is an important feature of membranes, it is not necessarily included in the broad definition of the membrane. The overall transport characteristics of a membrane depends on both the transport characteristics of the bulk phase of membrane and the interfacial characteristics between the bulk phase and the contacting phase or phases, including the concentration polarization at the interface. The term membrane is preferentially used for high-throughput membranes, and membranes with very low throughput are often expressed by the term barrier. ... 

The effect of the curvation of the micelle on solubilization capacity has been pointed out by Mukerjee (1979, 1980). The convex surface produces a considerable Laplace pressure (equation 7.1) inside the micelle. This may explain the lower solubilizing power of aqueous micellar solutions of hydrocarbon-chain surfactants for hydrocarbons, compared to that of bulk phase hydrocarbons, and the decrease in solubilization capacity with increase in molar volume of the solubilizate. On the other hand, reduction of the tension or the curvature at the micellar-aqueous solution interface should increase solubilization capacity through reduction in Laplace pressure. This may in part account for the increased solubilization of hydrocarbons by aqueous solutions of ionic surfactants upon the addition of polar solubilizates or upon the addition of electrolyte. The increase in the solubilization of hydrocarbons with decrease in interfacial tension has been pointed out by Bourrel (1983). 

Microemulsions are not homogeneous at the molecular level in that they consist of microscopic domains of water and oil separated by a surfactant film. The reaction may occur in either of the two domains, as well as at the interface. However, if the solubility of the polar reactant in hydrocarbon and of the lipophilic component in water is negligible, the reaction can be assumed to be a purely interfacial reaction, i.e. no reaction occurs in the two bulk phases. 

Recent electrochemical studies of chemical reactions at the polarized liquid liquid interface have been briefly summarized. All chemical reactions seem to occur in either one of the two adjacent bulk phases before or after the interfacial charge transfer. No effect of the double layers at the interface on the chemical reactions has been confirmed. No evidence has been obtained for the specific role of adsorption in the interfacial chemical reactions. 

A compound is interfacially active when its concentration in the interfacial region exceeds those in the two adjacent bulk phases. In the limit of infinite dilution, interfacial activity can be identified by the presence of an interfacial minimum in the free energy of the solute, as a function of its position along the direction perpendicular to the interface. Traditionally, activity at the interface between water and a non-polar liquid has been associated with the concept of amphiphilicity. In an interfacial environment, the polar parts of amphiphilic solutes are... 

The interfacial potential difference can arise from a combination of charge separation across the interface, orientation of polar molecules on the solution side of the interface, and specific adsorption of ions. The thickness of the zones in which properties differ from those in the bulk phases is probably no greater than 10 m on the metal side and 10 m on the solution side. 

Extraneous molecules in solid phase polymer systems are not limited to plasticizer molecules or even exclusive to substances deliberately added. Impurities wdien present often affect the dielectric behaviour of pol mers and water in particular often has very significant effects on the dielectric spectrum. Poly(niethyl methacrylate) poly(oxymethylene) , and nylons to mention a few are influenced by moisture in this way. The influence of moisture on dielectric relaxation can be the result of interfacial polarization as well as dipolar mechanism. Further, this complication is not restricted to additives such as water but may occur whenever a combination of phase boundary and bulk or sur ce conductivity to or over the botmdaiy can take place. The proof that a relaxaticu is the result of interfacial polarization is not easy to establish, but there is evidence that mie of the relaxations in nylons and pol3 urethanes) are of this type. As expected, conductive fillers will introduce interfacial polarization and this effect has been well documented, especially in carbon filled rubbers . Indeed, as we shall disci later, electronic conductance when localized by interfacial boundaries does result in a form of interfacial polarization. Here, because of its large magnitude the phenomenon has been termed hyperelectronic polarization. 

Figure 8.16 Hypothetical structure of a molecular complexed interfacial film at a propellant water interface. From Sanders [90]. The oriented liquid crystal nature of molecular complexes with their attendant layers of oriented water molecules suggests that the interfacial region around an emulsified propellant droplet can be viewed as consisting of alternating shells of oriented water and molecular complex molecules. The propellant interface would consist of a monolayer of adsorbed molecular complex molecules with the polar heads oriented towards an adjacent hydration layer. The hydration layer of water molecules in turn would be surrounded with a bimolecular shell of complex molecules with the polar heads on one side of the shell oriented towards the inner hydration layer and the polar heads on the other side oriented towards an outer hydration shell. This configuration of alternating layers of oriented water and bimolecular complex molecules would extend into the bulk phase with diminishing orientation until it disappeared. ...

Both blocked anndno acids were found to be interfacially active. Their conformational preferences at the interface differ from those in the adjacent bulk phases and cannot be deduced from the stabilities of different conformers in these phases. This indicates that the interface exerts a unique effect on conformational equilibria in a single peptide unit. The orientations of the peptides are also affected. Nonpolar NANML is oriented such that its nonpolar side chain is buried in hexane, whereas the polar side chain of NANMQ is exposed to water. The free energies needed to rotate each peptide such that its side chain is immersed in the solvent of different polarity is substantial. 

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... 

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