Surface force hydrophobic interaction

In filtration, the particle-collector interaction is taken as the sum of the London-van der Waals and double layer interactions, i.e. the Deijagin-Landau-Verwey-Overbeek (DLVO) theory. In most cases, the London-van der Waals force is attractive. The double layer interaction, on the other hand, may be repulsive or attractive depending on whether the surface of the particle and the collector bear like or opposite charges. The range and distance dependence is also different. The DLVO theory was later extended with contributions from the Born repulsion, hydration (structural) forces, hydrophobic interactions and steric hindrance originating from adsorbed macromolecules or polymers. Because no analytical solutions exist for the full convective diffusion equation, a number of approximations were devised (e.g., Smoluchowski-Levich approximation, and the surface force boundary layer approximation) to solve the equations in an approximate way, using analytical methods. 

Experimental studies of the thermodynamic, spectroscopic and transport properties of mineral/water interfaces have been extensive, albeit conflicting at times (4-10). Ambiguous terms such as "hydration forces", "hydrophobic interactions", and "structured water" have arisen to describe interfacial properties which have been difficult to quantify and explain. A detailed statistical-mechanical description of the forces, energies and properties of water at mineral surfaces is clearly desirable. 

Figure 25 Processes occurring in the deposition of nanoparticles in flow conditions as a function of the range of interaction of forces (imi) and adhesion times. At the start, mass transport to the surface occurs, initial adhesion following through electrostatic attraction and van der Waals forces. Hydrophobic interactions can play their part as well as specific receptor-ligand interactions, which are short-range interactions. Source. From Ref. 116.

As for van der Waals forces, hydrophobic interactions are individually weak (0.1 to 0.2 kJ moF for every square angstrom of solvent-accessible hydrocarbon surface ), but the total contribution of hydrophobic bonds to drug-receptor interactions is substantial. Similarly, the overall strength of the hydrophobic interaction between two molecules is very dependent on the quality of the steric match between the two molecules. If this is not sufficiently close to squeeze all of the solvent from the interface, a substantial entropy penalty must be paid for each of the trapped water molecules. 

In addition to electrostatic forces, hydrophobic interactions are also implied in the interaction of proteins with soil constituents and this results in an interplay between different driving forces in adsorption. For example, hydrophobic interactions with clays can result from an electrostatic exchange of the hydrophilic counter-ions on the clay surface, leaving a hydrophobic siloxane surface (Staunton and Quiquam-poix, 1994). The rearrangement of the enzyme structure on the surface can be facilitated when hydrophobic amino acids come into contact with the clay hydrophobic siloxane layer and remain shielded from the water molecules of the solution. If this structural modification is accompanied by a decrease in ordered secondary structures, it will result in an additional increase in conformational entropy. This will lower the Gibbs energy of the system. The combination of all these different sub-processes gives rise to an irreversibility of the modification of conformation of enzymes on clay surfaces. 

Electrostatic interactions appear when the adsorptive is an electrolyte that is dissociated or protonated in aqueous solution under the experimental conditions used. These interactions, either attractive or repulsive, are strongly dependent on the charge densities for both the carbon surface and the adsorptive molecule and on the ionic strength of the solution. The non-electrostatic interactions are always attractive, and include van der Waals forces, hydrophobic interactions and hydrogen bonding. 

Unlike starch, where destructuring leads to an amorphous fluid, denaturation of proteins exposes core structural groups which can be more hydrophobic than surface groups (depending on the environment). In a polar environment the structure forms with polar residues in the surface and hydrophobie ones inside, and vice versa. Furthermore, with an increase in temperature, ehains become more mobile but their movement is restricted because of newly formed intermolecular forces. Hydrophobic interaction intensifies due to temperature and denaturation which is followed by coagulation. ... 

Protein adsorption has been studied with a variety of techniques such as ellipsome-try [107,108], ESCA [109], surface forces measurements [102], total internal reflection fluorescence (TIRE) [103,110], electron microscopy [111], and electrokinetic measurement of latex particles [112,113] and capillaries [114], The TIRE technique has recently been adapted to observe surface diffusion [106] and orientation [IIS] in adsorbed layers. These experiments point toward the significant influence of the protein-surface interaction on the adsorption characteristics [105,108,110]. A very important interaction is due to the hydrophobic interaction between parts of the protein and polymeric surfaces [18], although often electrostatic interactions are also influential [ 116]. Protein desorption can be affected by altering the pH [117] or by the introduction of a complexing agent [118]. 

The (I)-(III)-samples sorption ability investigation for cationic dyes microamounts has shown that for DG the maximum rate of extraction is within 70-90 % at pH 3. The isotherm of S-type proves the physical character of solution process and a seeming ionic exchange. Maximal rate of F extraction for all samples was 40-60 % at pH 8 due to electrostatic forces. The anionic dyes have more significant affinity to surface researching Al Oj-samples comparatively with cationic. The forms of obtained soi ption isotherms atpH have mixed character of H,F-type chemosorption mechanism of fonuation of a primary monolayer with the further bilayers formation due to H-bonds and hydrophobic interactions. The different values of pH p for sorbents and dyes confirm their multifunctional character and distinctions in the acid-base properties of adsoi ption centers. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

The main contributions to AadsG for a globular protein are from electrostatic, dispersion, and hydrophobic forces and from changes in the structure of the protein molecule. Although in this section these contributions are discussed individually, strict separation of the influence of these forces on the overall adsorption process of a protein is not possible. For instance, adsorption-induced alteration of the protein structure affects the electrostatic and hydrophobic interaction between the protein and the surface. When the sorbent surface is not smooth but is covered with (polymeric)... 

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