Surfaces, hydrophobic

Besides this drag reduction potential in pressure-driven flows, it has recently been predicted that hydrophobic slippage could be best exploited in surface-driven transport. 

This expression can be generalized to other interfacial transport phenomena, such as diffusio-osmosis and thermo-osmosis, as it has been well discussed in. These correspond to the induction of a flow by the gradient of a solute concentration for the former and by a 

the surface is mixed and we can resort to the Cassie-Baxter relation [equation (9.5)] to determine the angle B. The two phases involved (solid and air) are characterized by their respective contact angles Be and tt. 

FIGURE 9.5. Hydrophobic porous surface. The liquid does not necessarily fill the pores, and a drop rests on a composite of solid and air. 

We have described two possible mechanisms amplifying the hydrophobic properties of a solid by means of a surface texture. Surface roughness can readily make a solid more hydrophobic (Wenzel s relation, equation (9.3)] it can also promote the formation of air pockets under a drop, which further reinforces the hydrophobic nature of the surface [equation (9.9)]. It is important to know in practice which of the two mechanisms applies. 

If z acos kx) describes the profile of the solid surface, we can work out a one-to-one correspondence between the value of the roughness and the threshold for air pockets formation. The maximum slope of the profile is ak (in absolute value), and the condition for air trapping (that is to say, the establishment of a horizontal liquid/vapor line of contact) for a given Oe reads 

FIGURE 9.6. Ideal solid surface with a sinusoidal profile. For low amplitudes, the liquid is able to follow the waviness. Beyond a certain amplitude threshold, air pockets can form underneath the liquid. 

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Proteins often have the same high-affinity isotherms as do synthetic polymers and are also slow to equilibrate, due to many contacts with the surface. Proteins, however, have the additional complication that they can partially or completely unfold at the solid-liquid interface to expose their hydrophobic core units to a hydrophobic surface... 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

Fig. 7.23 In simulations of stearic add on a hydrophobic surface hydrogen bonding between the head groups is important in controlling the orientation of the molecules [Kim et al, 1994b],...

Hydrophilic and Hydrophobic Surfaces. Water is a small, highly polar molecular and it is therefore strongly adsorbed on a polar surface as a result of the large contribution from the electrostatic forces. Polar adsorbents such as most zeoHtes, siUca gel, or activated alumina therefore adsorb water more strongly than they adsorb organic species, and, as a result, such adsorbents are commonly called hydrophilic. In contrast, on a nonpolar surface where there is no electrostatic interaction water is held only very weakly and is easily displaced by organics. Such adsorbents, which are the only practical choice for adsorption of organics from aqueous solutions, are termed hydrophobic. 

Increased flexibility Increased dispersing power Increased water sensitivity Increased adhesion to hydrophobic surfaces... 

Synthetic polymeric adsorbents have a high porosity, large surface area, and an inert hydrophobic surface. These resins can be regenerated ... 

Most molded plastics have a very smooth, hydrophobic surface that must be modified. Chemical etchants are used to oxidize and roughen the surface. The resultant hydrophilic surface promotes good metal-to-plastic adhesion. The etchant is usually a solution of chromic acid and sulfuric acid pure chromic acid can also be used. 

The thioredoxin domain (see Figure 2.7) has a central (3 sheet surrounded by a helices. The active part of the molecule is a Pa(3 unit comprising p strands 2 and 3 joined by a helix 2. The redox-active disulfide bridge is at the amino end of this a helix and is formed by a Cys-X-X-Cys motif where X is any residue in DsbA, in thioredoxin, and in other members of this family of redox-active proteins. The a-helical domain of DsbA is positioned so that this disulfide bridge is at the center of a relatively extensive hydrophobic protein surface. Since disulfide bonds in proteins are usually buried in a hydrophobic environment, this hydrophobic surface in DsbA could provide an interaction area for exposed hydrophobic patches on partially folded protein substrates. 

The lipophilicity (7 m value) and specific hydrophobic surface area of 1 ]/f-pyrido[2,]-fi]quinazolin-] 1-one and its isomeric 6//-pyrido[l, 2-u]qui-nazolin-6-one were determined by reversed-phase thin-layer chromatography (98MI4). 

K. Benedek, S. Dong and B. E. Kaiger, Kinetics of unfolding of proteins on hydrophobic surfaces in reversed-phase liquid chiomatography , /. Chromatogr. 317 227-243 (1984). 

When the polymer was prepared by the suspension polymerization technique, the product was crosslinked beads of unusually uniform size (see Fig. 16 for SEM picture of the beads) with hydrophobic surface characteristics. This shows that cardanyl acrylate/methacry-late can be used as comonomers-cum-cross-linking agents in vinyl polymerizations. This further gives rise to more opportunities to prepare polymer supports for synthesis particularly for experiments in solid-state peptide synthesis. Polymer supports based on activated acrylates have recently been reported to be useful in supported organic reactions, metal ion separation, etc. [198,199]. Copolymers are expected to give better performance and, hence, coplymers of CA and CM A with methyl methacrylate (MMA), styrene (St), and acrylonitrile (AN) were prepared and characterized [196,197]. 

With few exceptions it is more efficient and economical to use at least two different pretreatments, i.e. degreasing processes which deal with a) and b), and pickling processes which remove oxide and corrosion films. Degreasing comes first, as pickling processes fail on hydrophobic surfaces. 

With increasing alcohol concentration non-bulk electrostatic contributions become relevant. Because these non-bulk electrostatic contributions depend on the concentration of the cosolvent as well on the size of the alkyl-group, one can conclude that there is a relation to the smaller free energy necessary for exposing hydrophobic surfaces to the medium. It yields ... 

Chaperones bind to exposed hydrophobic surfaces of polypeptide substrates, and through either ATP-dependent or ATP-independent mechanisms facilitate the folding/assembly, intracellular transport, degradation, and activity of polypeptides. 

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