Hydrophobic and Hydrophilic Surfaces

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. 

The crystal structure of talc, illustrated in Figure 4, consists of repeating layers of a sandwich of bmcite [1317-43-7], Mg(OH)2, between sheets of silica [7631-86-9], SiOj. The layers of silica are not strongly bonded to each other (except for van der Waals forces) and thus it is easy to fracture talc along this surface, which corresponds to delamination. This surface is covalent and hydrophobic. If talc is fractured across the bmcite layer, the surfaces generated are ionic and hydrophilic in nature. Thus talc has a natural balance of hydrophilic and hydrophobic surfaces, giving it surfactant properties and consequendy the name soapstone which is used in many parts of the wodd. 

Adsorption of the enzymes subtilisin BPN and lysozyme onto model hydrophilic and hydrophobic surfaces was examined using adsorption isotherm experiments, infrared reflection-absorption spectroscopy (IRRAS), and attenuated total reflectance (ATR) infrared (IR) spectroscopy. For both lysozyme and BPN, most of the enzyme adsorbed onto the model surface within ten seconds. Nearly an order-of-magnitude more BPN adsorbed on the hydrophobic Ge surface than the hydrophilic one, while lysozyme adsorbed somewhat more strongly to the hydrophilic Ge surface. No changes in secondary structure were noted for either enzyme. The appearance of carboxylate bands in some of the adsorbed BPN spectra suggests hydrolysis of amide bonds has occurred. 

Figure 5.11 Illustrations of possible water structure at hydrophilic and hydrophobic surfaces. Bulk water is shown by pentagonal and partial pentagonal circuits, which indicate structural entities being in equilibrium with monomeric water represented by arrows. Dipole-dipole interaction at a hydrophilic surface causes ordering of water molecules, leading to a notable disordered zone. Water molecules at a hydrophobic surface have extensive clathrate-like structure with a minimal disordered zone. From Nguyen and Schulze [53]. Copyright 2004, Dekker.

The nature of the adsorption of surfactants at both the hydrophilic and hydrophobic solid surfaces has been subject to extensive studies, and a number of excellent recent reviews exist [27-29]. The structure of the adsorbed layer, at both the hydrophilic and hydrophobic surfaces, has been the subject of much conjecture. From the form of the adsorption isotherm at the hydrophilic surface, the cooperative nature of the adsorption was established, and the evolution of the structure with concentration was inferred [30] (see Fig 3). 

In contrast to the periphery of the cyclodextrins, the internal cavities, with diameters of 5 to 8 A (Ihble 18.1), have hydrophobic character because they are lined by the methylene C-H groups and by the ether-like 0(4) and 0(5) oxygen atoms. The distribution of hydrophilic and hydrophobic surfaces, together with the annular shapes of the cyclodextrins, gives rise to the microheterogeneous environment [555] which is the reason for some of their most interesting properties (Box 18.1). 

Ideally, models of vicinal water should eventually "explain all established experimental facts. There is a long way to go However, some general observations have been made. One is that, against a variety of hydrophobic phases (silver iodide, mercury, air) water molecules appear to be oriented with the negative ends of the molecules pointing outward (sec. 3.9). In other words, the polarization of water adjacent to silver iodide and mercury is similar to the spontaneous polarization of water surfaces. The implication is that near such surfaces water-water interactions play at least an important role as water-surface interactions. Another observation, relevant for the interpretation of electroklnetic phenomena, is that tangentially immobile surface layers do occur near both hydrophilic and hydrophobic surfaces. 

The shape of the meniscus of water at the plate may either be depressed away or advanced toward the dry surface. In general, a hydrophobic surface would result in a depressed meniscus (0D,a,i > 90 degrees, cos 0D,a,i < 0) whereas a hydrophilic surface would result in an advanced meniscus (0D,a,i < 90 degrees, cos 0D,a,i > 0)- The meniscus can be clearly seen visually for extremely hydrophilic and hydrophobic surfaces. Surfaces with moderate hydrophilicity/hydrophobicity exhibit flat meniscus, which seems to be perpendicular to the surface (0D,a,i = 90 degrees, cos 0D,a,i =0). 

The affinity to hydrophilic and hydrophobic surfaces is particularly useful in the hydrophilization of a wide range of substances, ranging from hydrophobic tablet cores - to permit sugar or film-coating, to medical plastics. 

Figure 1.27 Scheme showing the use of charged head groups to switch between hydrophilic and hydrophobic surfaces by changing the potential applied to the surface. (From [144] J. 

ADSORPTION OF ANTIFREEZE PROTEIN AND A COMMERCIAL LOW DOSAGE HYDRATE INHIBITIOR ON HYDROPHILIC AND HYDROPHOBIC SURFACES... 

Various surfaces occur during the production, purification, and preparation of proteins as well as in the formulation process, for example, exposure to glass surfaces in vials and air-liquid interfaces in the formulation. AU these t5rpes of surfaces have various adsorption properties. A major distinction is made between hydrophilic and hydrophobic surfaces as well as air-liquid, liquid-liquid, and solid-liquid surfaces. Naturally, the type of interface also influences the observed structural changes (56,60,61). 

Fig. 13 Stress-slip velocity variations for a microgel paste sheared along hydrophilic closed circles) and hydrophobic surfaces open circles). The dashed arrow denotes the yield stress, (7y = 34 Pa. The solid arrows denote the stress below which the paste adheres to the surface as = 0.7 Pa and 5.8 Pa for the hydrophilic and hydrophobic surface, respectively. The solid lines represent the quadratic (Vs and linear (Vs a) fits to the data for the hydrophobic and hydrophilic surfaces respectively...

Jass, J., Tjarnhage, T., and Puu, G. 2000. From liposomes to supported, planar bilayer structures on hydrophilic and hydrophobic surfaces An atomic force microscopy study, Biophys J 79,3153-3163. 

Figure 5.11 Illustrations of possibie water zones near hydrophilic and hydrophobic surfaces. Nguyen and Schulze [11] have suggested that dipole-dipole interaction at a hydrophilic surface causes ordering of water...

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