Hydrophobic-hydrophilic boundaries

The balance between the interfacial energy of the hydrophobic/hydrophilic boundary and the entropic loss when the polymeric chains form the more ordered strucmres determines the vesicle membrane thickness, d. Helfand and Wasserman obtained a power law for the membrane thickness as follows ... 

The sidewalls and bottom surfaces of the apertures were chemically treated to be hydrophilic. The top surfaces of the apertures were naturally hydrophobic based on material surface properties, thereby pinning the liquid-air interfaces along the hydrophobic-hydrophilic boundaries at the top edges of the apertures. 

Formation of microlens at interface between oil and aqueous solution. Interface is pinned stably at a hydrophobic-hydrophilic boundary along circular aperture. Volumetric changes of hydrogel microposts cause flexible aperture slip to bend in the z direction. The pinned water-oil interface is pressed downward or upward, thus tuning the focal length. (Source Dong, L. and H. Jiang. 2006. Applied Physics Letters, 89(21), 211120. With permission.)... 

Methyl ester ethoxylates are similar in structure to conventional alchol ethoxylates, but the structural differences that do exist have an important impact on their performance. As shown in Fig. 1, methyl ester ethoxylates contain an ester linkage at the hydrophobe-hydrophile boundary of the molecule in place of the ether linkage in alcohol ethoxylates. This ester linkage sterically constrains the molecule, which reduces the tendency of the surfactant to micellize and leads to a higher critical micelle coneentration... 

Complementing the equilibrium measurements will be a series of time resolved studies. Dynamics experiments will measure solvent relaxation rates around chromophores adsorbed to different solid-liquid interfaces. Interfacial solvation dynamics will be compared to their bulk solution limits, and efforts to correlate the polar order found at liquid surfaces with interfacial mobility will be made. Experiments will test existing theories about surface solvation at hydrophobic and hydrophilic boundaries as well as recent models of dielectric friction at interfaces. Of particular interest is whether or not strong dipole-dipole forces at surfaces induce solid-like structure in an adjacent solvent. If so, then these interactions will have profound effects on interpretations of interfacial surface chemistry and relaxation. 

Figure 8.6 (a) Segmented flow of oil and water flow of oil and water in a channel with in a channel with a medium hydrophobic hydrophilic properties-the interface boundary... 

When liquid-liquid, hydrophobic-hydrophilic interfaces exist, the poly(ethylene oxide) molecule can have different conformations in the different phases. In the case of solubility in both phases, the helical or more ordered conformation exists in the water-phase boundary and a more random conformation exists in the organic-phase boundary. 

An ABA (hydrophihc-hydrophobic-hydrophilic) triblock copolymer has been studied extensively because of its ability to form a hydrogel. As an example, aqueous solutions of commercially available PEO-PPO-PEO polymers (Plurorucs (BASF) or Poloxamers (ICI)) demonstrate phase transitions from sol to gel (low temperature sol-gel boundary) and gel to sol (high temperature gel-sol boundary) as monotonically increasing temperature when the polymer concentration is above a critical value [4,10. Continuous heating the polymer solutions in a temperature range above the high temperature boundary makes the... 

This result makes it clear that particle stress is strongly dependent on the interaction between the particles and the interface, so that electrostatic and also hydrophobic and hydrophilic interactions with the phase boundary are particularly important. This means that the stress caused by gas sparging and also by boundary-layer flows, as opposed to reactors with free turbulent flow (reactors with impellers and baffles), may depend on the particle system and therefore applicability to other material systems is limited. 

Figure 16.9b. Since Fc- and Fc -covered surfaces are known to be hydrophobic and hydrophilic, respectively [42, 43], the wetting gradient is expected to be formed around P. Therefore, we call P the wetting boundary. The position of the wetting boundary and the magnitude of the wetting gradient can be reversibly controlled as functions of FofFset and respectively, where I is the length of the substrate in the...

In emulsion polymerization, a solution of monomer in one solvent forms droplets, suspended in a second, immiscible solvent. We often employ surfactants to stabilize the droplets through the formation of micelles containing pure monomer or a monomer in solution. Micelles assemble when amphiphilic surfactant molecules (containing both a hydrophobic and hydrophilic end) organize at a phase boundary so that their hydrophilic portion interacts with the hydrophilic component of the emulsion, while their hydrophobic part interacts with the hydrophobic portion of the emulsion. Figure 2.14 illustrates a micellized emulsion structure. To start the polymerization reaction, a phase-specific initiator or catalyst diffuses into the core of the droplets, starting the polymerization. 

HydrophiIic versus hydrophobic coadsorption. The contrast between the hydrophilic and hydrophobic coadsorption seen on Rh(111) and Pt(lll), if confirmed under normal electrochemical conditions, might be of electrocatalytic importance. On Rh(lll), where net attractive CO-HgO interactions produce a mixed phase in which CO is displaced to a three-fold binding site which is not occupied in the absence of water, CO and water appear to occupy adjacent binding sites. Such thorough mixing of the oxygen source (water) and the intermediate [or poison] (CO) should improve electrooxidation rates for C 0 H fuels (11). On Pt(lll), where net repulsions cause condensation of CO and water into separate patches, reaction between the adsorbed species could occur only at the boundaries between patches, and one would expect slower kinetics. 

Only a handful of models treat the diffusion media as at least partially hydrophobic - - - the others treat the medium as entirely hydrophilic. Although in principle it does not change the approach, the physical picture and boundary conditions must change. An entirely hydrophilic medium means that the liquid pressure must always be below the gas pres-... 

The physical sense of the distribution potential can be demonstrated on the example of the distribution equilibrium of the salt of a hydrophilic cation and a hydrophobic anion between water (wt) and an organic solvent that is immiscible with water (org). After attaining distribution equilibrium the concentrations of the anion and the cation in each of the two phases are the same because of the electroneutraUty condition. However, at the phase boundary an electrical double layer is formed as a result of the greater tendency of the anions to pass from the aqueous phase into the organic phase, and of the cations to move in the opposite direction. This can be characterized quantita tively by quantities—and — AGJJ. ", for which... 

At the phase boundary between water and an organic solvent immiscible with water, an electric double layer may also be formed if one of the solutions in contact contains an amphiphilic ion, i.e. an ion that contains both hydrophilic (usually electrically charged) and hydrophobic (nonpolar) groups (for example the dodecyltrimethylammonium ion). Ions with these properties are simply adsorbed on the phase boundary with the nonpolar group oriented into the organic solvent and the ionized group in the aqueous phase. 

It follows from study of the kinetics of transfer of ions across the phase boundary between two immiscible electrolyte solutions (see chapter 9) that ion-exchanger ions, where the ion is as nearly as possible symmetrically surrounded by hydrophobic groups on all sides, are especially suitable. Amphiphilic (amphipathic) substances, in whose molecules the hydrophobic part is separated from the hydrophilic part, are less suitable because they have a tendency to become adsorbed on the membrane/water phase boundary, thus retarding ion transfer across this boundary. 

It is postulated that the main thermodynamic driving force for particle adsorption at the liquid-liquid interface is the osmotic repulsion between the colloidal particles and hydrophilic starch polymer molecules. This leads to an effective depletion flocculation of particles at the boundaries of the starch-rich regions. At the same time, the gelatin has a strong tendency to adsorb at the hydrophobic surface of the polystyrene particles, thereby conferring upon them some degree of thermodynamic... 

Porous Membrane DS Devices. The applicability of a simple tubular DS based on a porous hydrophobic PTFE membrane tube was demonstrated for the collection of S02 (dilute H202 was used as the scrubber liquid, and conductometric detection was used) (46). The parameters of available tubular membranes that are important in determining the overall behavior of such a device include the following First, the fractional surface porosity, which is typically between 0.4 and 0.7 and represents the probability of an analyte gas molecule entering a pore in the event of a collision with the wall. Second, wall thickness, which is typically between 25 and 1000 xm and determines, together with the pore tortuosity (a measure of how convoluted the path is from one side of the membrane to the other), the overall diffusion distance from one side of the wall to the other. If uptake probability at the air-liquid interface in the pore is not the controlling factor, then items 1 and 2 together determine the collection efficiency. The transport of the analyte gas molecule takes place within the pores, in the gas phase. This process is far faster than the situation with a hydrophilic membrane the relaxation time is well below 100 ms, and the overall response time may in fact be determined by liquid-phase diffusion in the boundary layer within the lumen of the membrane tube, by liquid-phase dispersion within the... 

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