Mixed interfacial layers

In addition to these experimental methods, there is also a role for computer simulation and theoretical modelling in providing understanding of structural and mechanical properties of mixed interfacial layers. The techniques of Brownian dynamics simulation and self-consistent-field calculations have, for example, been used to some advantage in this field (Wijmans and Dickinson, 1999 Pugnaloni et al., 2003a,b, 2004, 2005 Parkinson et al., 2005 Ettelaie et al., 2008). 

At the point of maximum synergism or maximum antagonism in surface or interfacial tension reduction effectiveness, the composition of the mixed interfacial layer equals the composition of the mixed micelle, i.e., X E = the mole... 

Mixed interfacial layers can be formed in various ways, as shown schematically in Fig. 4.4. 

Fig. 4.4 Formation of mixed interfacial layers a) from a mixed bulk solution, b) as penetration of one component into an insoluble layer, c) from two bulk solutions at the joint interface...

Another possible beneficial effect of interfacial complex formation, in addition to the improved surface energetic just mentioned, is that such structures may possess a greater mechanical strength than will a simple mixed interfacial layer. Closer molecular packing densities and a greater extent of lateral interaction between hydrophobic chains may result in significant decreases in the mobility of molecules... 

The non-steady-state optical analysis introduced by Ding et al. also featured deviations from the Butler-Volmer behavior under identical conditions [43]. In this case, the large potential range accessible with these techniques allows measurements of the rate constant in the vicinity of the potential of zero charge (k j). The potential dependence of the ET rate constant normalized by as obtained from the optical analysis of the TCNQ reduction by ferrocyanide is displayed in Fig. 10(a) [43]. This dependence was analyzed in terms of the preencounter equilibrium model associated with a mixed-solvent layer type of interfacial structure [see Eqs. (14) and (16)]. The experimental results were compared to the theoretical curve obtained from Eq. (14) assuming that the potential drop between the reaction planes (A 0) is zero. The potential drop in the aqueous side was estimated by the Gouy-Chapman model. The theoretical curve underestimates the experimental trend, and the difference can be associated with the third term in Eq. (14). 

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. 

A simple test to estimate the interfacial layer thickness is to measure the thickness of the bottom layer before and after spinning, exposure, and development of the top layer. The difference is taken to be the thickness of the interfacial layer for comparison purposes. In reality, the mixing is continuous and the development of the top layer stops inside the interfacial layer instead of at its edges precisely. Furthermore, the test in Reference 26 relies on the IBM Film Thickness Analyzer to measure the resist thickness for convenience. Since this tool operates on the principle of spectral reflectivity changes caused by film thickness changes, a uniform refractive index is important. When some part of the interfacial layer still remains, the measurement can be erroneous in principle. 

Ettelaie, R., Akinshina, A., Dickinson, E. (2008). Mixed protein-polysaccharide interfacial layers a self consistent field calculation study. Faraday Discussions, 139, 161-178. 

Kotsmar, Cs., Pradines, V., Alahverdjieva, V.S., Aksenenko, E.V., Fainerman, V.B., Kovalchuk, V.E, Kragel, J., Leser, M.E., Noskov, B.A., Miller, R. (2009). Thermodynamics, adsorption kinetics and rheology of mixed protein-surfactant interfacial layers. Ach cmces in Colloid and Interface Science, 150, 41-54. 

During ageing of the mix, interfacial milk protein hydration also increases simultaneously with protein desorption from the fat globules. The water content of the isolated cream layers after centrifugation of ice cream mix can be analyzed by Karl Fischer titration. From such analyses, interfacial protein hydration can be calculated (Figure 13). 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

Monte Carlo and molecular dynamics calculations of the density profile of model system of benzene-water [70], 1,2-dichloroethane-water [71], and decane-water [72] interfaces show that the thickness of the transition region at the interface is molecu-larly sharp, typically within 0.5 nm, rather than diffuse (Fig. 4). A similar sharp density profile has been reported also at several liquid-vapor interfaces [73, 74]. The sharpness of interfaces thus seems to be a general characteristic of the boundary between two stable phases and it is likely that the presence of supporting electrolytes would not significantly alter the thickness of the transition region at an ITIES. The interfacial mixed solvent layer [54, 55], if any, would probably have a thickness comparable with this thin inner layer. 

One problem associated with the PCM scheme is that during application of a photoresist such as AZI350J onto a PMMA film, a thin layer of PMMA is redissolved and mixed with the photoresist so that a thin interfacial layer is formed that remains after development of the photoresist layer and inhibits proper exposure and development of the PMMA layer. Because the PMMA developer, such as chlorobenzene or toluene, used in the capped process is chosen to be a nonsolvent for the photoresist, such a solvent cannot remove the interfacial layer. Therefore, some process, like plasma treatment, is required to remove the interfacial layer prior to the blanket exposure of the bottom PMMA layer (83, 85). 

Wijdenes and Geomini (170) examined the effects of the phenolic resin composition, its molecular weight distribution, solvent composition, and prebake temperature on the interfacial layer formation. They found that combined use of poly(p-vinylphenol) (structure 3.10) as matrix resin and cyclohexanone as the casting solvent in the diazoquinone resist formulation minimizes mixing of the two layers and yields a capped PCM structure without any plasma treatment. 

The interfacial characteristics between an oil drop and aqueous mixed emulsifier solutions were studied with a spinning drop interfacial tensiometer. An interfacial layer was observed at the oil/aqueous phase interface, as evidenced by the formation of "tails" on the rotating drop. The length of these "tails" increased with spinning time and rotation speed. The interfacial tensions between styrene and aqueous mixed emulsifier solutions were unexpectedly high, 5 to 13 dynes/cm, whereas tensions in the range of 10 2 dynes/cm were measured between the "tails" and the aqueous solution. 

Static, equilibration studies also indicated that a molecular association forms at the oil/water interface in the presence of mixed emulsifiers. Spinning drop experiments with pre-equilibrated oil and aqueous phases suggested that the presence of oil in association with the mixed emulsifier molecules in the aqueous phase affects the formation of an interfacial layer. 

The objective of this research program was to investigate the characteristics of the interfacial films observed in our miniemulsion systems. This study of oil/aqueous mixed emulsifier solution interfacial properties included the effects of mixed emulsifier molar ratio and concentration, fatty alcohol initial location and chain length, and oil phase water solubility. The effect of equilibration on the formation of interfacial layers was also studied. 

Interfacial Layer Visualization. One of the key results of extensive spinning drop experiments between aqueous mixed emulsifier solutions and styrene was visual evidence for the formation of mixed emulsifier interfacial films. This interfacial layer is depicted in Figure 1 by the formation of "tails" as a function of time on the rotating styrene drop in an aqueous solution of 1 1 SLS/CA, based on lOmM SLS. 

The chemical structure of the oil phase itself has a pronounced effect on the formation of an interfacial layer. Spinning drop experiments with oil phases of different chemical structure and the same water solubilities illustrated this effect. Comparison of Figures 1 and 3 shows that interfacial layers having significantly different characteristics form between the same 1 1 SLS/CA solution and different oil phases (styrene and EHA, respectively) with approximately the same water solubilities. This may be the result of different types of specific interactions between the various components of the mixed emulsifier system and either one of the two types of oils. NMR studies will be conducted in order to investigate this point. 

The relatively large interfacial tension values given in Table I and depicted in Figure 4 may actually be an indication of the interfacial tension between the oil droplet and the mixed emulsifier interfacial layer. This hypothesis is supported by the low interfacial tensions measured for "tails" which have detached themselves from the oil drop. The interfacial tension between these detached, free-... 

Equilibration Studies, Because the formation of an interfacial layer is a dynamic phenomenon, experiments were conducted to study the effect of oil-water phase equilibria on interfacial properties. Two experiments were carried out where aqueous solutions of 1 1 SLS/LA were equilibrated with both styrene and toluene in sealed containers, without agitation for five weeks. Several important observations were made (a) Despite the presence of mixed emulsifiers,... 

Similar experiments with a fresh styrene drop, which had not been pre-equilibrated, in the same pre-equilibrated, 1 1 SLS/LA mixed emulsifier solution, did not yield any visible interfacial layer. Therefore, the diffusion of the mixed emulsifiers into the oil phase evidently does not effect the formation of an interfacial layer. Therefore, the controlling factor appears to be the diffusion of the mixed emulsifiers in association with oil molecules into the aqueous phase. 

Interfacial tension values between styrene and several mixed emulsifier solutions were relatively high, 5-13 dynes/cm, while the apparent interfacial tensions between the aqueous phase and the resulting interfacial layer were substantially less than 1 dyne/cm. 

The theoretical treatment presented (Eqs 4.1-4.5) is applicable also for direct wet electrochemistry on Pt cathode in aprotic electrolyte solution [12,13] (Table 4.1) and for some other chemical reductants, Rj, viz. benzoin dianion [14] and sodium dihydronaphthylide [15] (Table 4.1). Apparently, the decision between chemical and electrochemical carbonization may not be straightforward. The latter scenario requires a compact solid electrolyte with mixed electron/ion conductivity to be present at the interface. This occurs almost ideally in the reactions of solid fluoropolymers with diluted alkali metal amalgams [3]. If the interfacial layer is mechanically cracked, both electrochemical and chemical carbonization may take place, and the actual kinetics deviates from that predicted by Eq. 4.4 [10]. There is, however, another mechanism, leading to the perturbations of the Jansta and Dousek s electrochemical model (Eq. 4.4). This situation typically occurs if gaseous perfluorinated precursors react with Li-amalgam [4,5], and it will be theoretically treated in the next section. 

FIG. 3 Different models of interfacial ET. (A) Aqueous and organic redox species are separated by the sharp interfacial boundary. (B) Interfacial potential drop across a thin ion-free layer between redox reactants. (C) ET reaction occurs within a nm-thick mixed solvent layer. No potential drops between reactant molecules. 

---