Surfaces, hydrophilic

We now return to the topic of rough surfaces and examine Wenzel s approach in a more critical light. We shall consider in succession the case of a hydrophilic surface (0 90°) and that of a hydrophobic surface (0 90°). All this is based on the model of Bico et al.  

A very rough hydrophilic surface is somewhat similar to a thin porous medium. We can imagine that some liquid escapes from the drop and penetrates into the nooks and crannies of the solid. The volume of film captured in the recesses is generally negligible, and in the end the drop finds itself essentially on a wet substrate viewed as a patchwork of solid and liquid. 

FIGURE 9.4. Hydrophilic porous surface. If the liquid spontaneously fills the grooves, a drop will find itself on a mixed solid/liquid surface. Equilibrium imposes a nearly vanishing Laplace pressure, implying a horizontal surface. 

We denote by and I the relative fractions of the solid and liquid phases underneath the drop. The Cassie-Baxter relation [equation (9.5)] can be applied to the mixed surface, with contact angles 6e and 0, respectively. We then deduce 9  

It becomes clear that, contrary to the predictions of Wenzel s law, we cannot induce total wetting 6 = 0) simply by means of a surface texture. Since we are in a partial wetting regime Oe 0), there will always remain islands that emerge above the absorbed film. 

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The transfer of Langmuir films from the air-liquid surface to a solid substrate has come to be known as Langmuir-Blodgett deposition, after its developers [130, 131]. The solid substrates are usually hydrophilic surfaces such as... 

The so-called self-assembly technique has its origin in 1946, when a paper was published by Bigelow et a] [116] and tluis is slightly younger tlian tlie LB teclmique. The autliors noted tliat a hydrophilic surface exposed to an amphiphilic compound dissolved in a non-polar solvent induces tlie amphiphilic material to fonn a monolayer on it. 

Increased water resistance Increased tensile strength Increased block resistance Increased solvent resistance Increased adhesion to hydrophilic surfaces... 

Poly(vinyl butyral), prepared by reacting poly(vinyl alcohol) with -butyraldehyde, finds wide appHcation as the interlayer in safety glass and as an adhesive for hydrophilic surfaces (161). Another example is the reaction of poly(vinyl alcohol) with formaldehyde to form poly(vinyl formal), used in the production of synthetic fibers and sponges (162). 

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. 

Figure 12.2 (a) Schematic drawing of membrane proteins in a typical membrane and their solubilization by detergents. The hydrophilic surfaces of the membrane proteins are indicated by red. (b) A membrane protein crystallized with detergents bound to its hydrophobic protein surface. The hydrophilic surfaces of the proteins and the symbols for detergents are as in (a). (Adapted from H. Michel, Trends Biochem. Sci. 8 56-59, 1983.)... 

The hydrophilic surface characteristics and the chemical nature of the polymer backbone in Toyopearl HW resins are the same as for packings in TSK-GEL PW HPLC columns. Consequently, Toyopearl HW packings are ideal scaleup resins for analytical separation methods developed with TSK-GEL HPLC columns. Eigure 4.44 shows a protein mixture first analyzed on TSK-GEL G3000 SWxl and TSK-GEL G3000 PWxl columns, then purified with the same mobile-phase conditions in a preparative Toyopearl HW-55 column. The elution profile and resolution remained similar from the analytical separation on the TSK-GEL G3000 PWxl column to the process-scale Toyopearl column. Scaleup from TSK-GEL PW columns can be direct and more predictable with Toyopearl HW resins. 

The neutral hydrophilic surface and the wide range of pore diameters available for SynChropak GPC allow many compounds from small peptides to nucleic acids and other polymers to be analyzed. Table 10.2 lists the approximate exclusion limits for both linear and globular solutes. Although this information... 

The basic requirements for an aqueous SEC column are (1) the beads must exhibit an extremely hydrophilic surface chemistry, (2) the beads should exhibit... 

The Jordi glucose-DVB column is a highly polar GPC column used for separating polar compounds. Modified glucose units are bonded to the DVB backbone to yield a hydrophilic surface (Fig. 13.16). 

PSt or PMMA, respectively, was coupled with polymethacrylate having a PEG side chain or methylammo-niumchloride side chain to prepare a block copolymer for giving a hydrophilic surface [55]. Also, PSt-b-PVP [36,37], PSt-b-(hydrophilic vinyl copolymer) [56], PSt-b-po y(sodium acrylate) (PNaA) [57], and PSt-b-PNaA-b-(polyperfluoroacrylate) (PFA) [58] were synthesized for the same application. 

The results of Popov etal.m are again in contrast with those above. In the case of Ag, dEff=0/9T is higher for (100) than for the (111). In this case, on the basis of Silva s interpretation, the more hydrophilic surface would be the (100), in contrast to the conclusion of the same authors based on the value of the capacitance. In the case of Cd, Popov et al.662 have found that dEa /bT is higher for the (0001) face than for the polycrystal-... 

Proteins derive their powerful and diverse capacity for molecular recognition and catalysis from their ability to fold into defined secondary and tertiary structures and display specific functional groups at precise locations in space. Functional protein domains are typically 50-200 residues in length and utilize a specific sequence of side chains to encode folded structures that have a compact hydrophobic core and a hydrophilic surface. Mimicry of protein structure and function by non-natural ohgomers such as peptoids wiU not only require the synthesis of >50mers with a variety of side chains, but wiU also require these non-natural sequences to adopt, in water, tertiary structures that are rich in secondary structure. 

The term "bioenertness" is a relative one since few if any synthetic polymers are totally biocompatible with living tissues. The terra is used here on the basis of preUminary in vitro and in vivo tests, together with chemical evaluations based on analogies with other well-tested systems. Two different types of polyphosphazenes are of interest as bioinert materials those with strongly hydrophobic surface characteristics and those with hydrophilic surfaces. These will be considered in turn. 

Intuitively it might be supposed that a poljrmer with a hydrophilic surface might be more biocompatible than one with a ("foreign ) hydrophobic surface. This simple interpretation is not always correct. Nevertheless, the study of hydrophilic polymers, especially those that promote endothelial tissue overgrowth or ingrowth, is an area of great interest. 

Water is a special liquid that forms unique bonds involving protons between the oxygen atoms of neighboring molecules, the so-called hydrogen bond. The solvation forces are then due not simply to molecular size effects, but also and most importantly to the directional nature of the bond. They can be attractive or hydrophobic (hydration forces between two hydrophobic surfaces) and repulsive or hydrophilic (between two hydrophilic surfaces). These forces arise from the disruption or modification of the hydrogen-bonding network of water by the surfaces. These forces are also found to decay exponentially with distance [6]. 

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