Surface forces steric

Key words Polyelectrolyte-surfactant association - surface forces - steric forces - critical association concentration - adsorption... 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

Surface force profiles between these polyelectrolyte brush layers have consisted of a long-range electrostatic repulsion and a short-range steric repulsion, as described earlier. Short-range steric repulsion has been analyzed quantitatively to provide the compressibility modulus per unit area (T) of the poly electrolyte brushes as a function of chain density (F) (Fig. 12a). The modulus F decreases linearly with a decrease in the chain density F, and suddenly increases beyond the critical density. The maximum value lies at F = 0.13 chain/nm. When we have decreased the chain density further, the modulus again linearly decreased relative to the chain density, which is natural for chains in the same state. The linear dependence of Y on F in both the low- and the high-density regions indicates that the jump in the compressibility modulus should be correlated with a kind of transition between the two different states. 

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]. 

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. 

In practice the situation may be more complicated. The shear plane may actually lie about 20 nm further away from the surface than the Stern plane, closer to the Gouy plane [271]. Also, if particle surfaces are covered by long chain molecules (physically or chemically bonded to the surface) then steric repulsion between particles may be significant. This repulsion is due to an osmotic effect caused by the high concentration of chains that are forced to overlap when particles closely approach, and also due to the volume restriction, or entropy decrease, that occurs when the chains lose possible conformations due to overlapping. 

Micropore diffusion Diffusion within the small micropores of the adsorbent which are of a size comparable with the molecular diameter of the sorbate. Under these conditions the diffusing molecule never escapes from the force field of the solid surface and steric hindrance is important. For zeolites the terms micropore diffusion and intracrystalline diffusion are synonymous. Raffinate Product stream containing the less strongly adsorbed species. 

Surface forces in foam films from amphiphilic block copolymers 3.3.3.1. Transition from electrostatic to steric stabilisation in foam films from... 

On the contrary the plateau values for the two copolymers are very different. Since the higher copolymer gives thicker films a surface force component of steric origin may be evoked. However, the thickness hK is an effective parameter which is too crude. As a reasonable compromise between physical relevance and tractability, the three-layer model is adopted. Within the three-layer model the foam film is viewed as a symmetric sandwich structure [159] two adsorption layers symmetrically confine an aqueous core (Fig. 3.34). 

Microscopic foam films from amphiphilic ABA triblock copolymers have been used to assess steric interactions. Most of the work on copolymers [128,129] has been carried out with the Thin Liquid Film-Pressure Balance Technique (see Chapter 2, Section 2.1.8). Nevertheless, some intriguing results have been obtained with the dynamic method for surface force measurement [127]. 

At higher pressure some positive component outweighs the double layer repulsion. It might be speculated that this strong repulsion is due to steric interactions between the hydrophilic PEO brushes and (at least) three surface force components must be considered, i.e. n = n vw + n ei + rrst. 

For monodisperse or unimodal dispersion systems (emulsions or suspensions), some literature (28-30) indicates that the relative viscosity is independent of the particle size. These results are applicable as long as the hydrodynamic forces are dominant. In other words, forces due to the presence of an electrical double layer or a steric barrier (due to the adsorption of macromolecules onto the surface of the particles) are negligible. In general the hydrodynamic forces are dominant (hard-sphere interaction) when the solid particles are relatively large (diameter >10 (xm). For particles with diameters less than 1 (xm, the colloidal surface forces and Brownian motion can be dominant, and the viscosity of a unimodal dispersion is no longer a unique function of the solids volume fraction (30). 

In addition to the van der Waals and electrical forces, steric forces resulting from pro-tmding polysaccharide and protein molecules affect interactions specific interactions between charged groups on the cell surface and on the solid surface, hydrogen bonding or the formation of cellular bridges may all occur to complicate the picture. The ten possible forces of interaction between cells and surfaces have been listed as ... 

Consider a molecule diffusing in free space or a solute molecule diffusing in solution. Upon colliding with a surface, assume that the molecule is sufficiently entrained by surface forces that there results a reduction in dimensionality of its diffusion space from d = 3 to d — 2, and that in its subsequent motion the molecule is sterically constrained to follow the pathways defined by the lattice structure of the surface (or, perhaps, the boundary lines separating adjacent domains). If at some point in its trajectory the molecule becomes permanently immobilized, either because of physical binding at a site or because an irreversible reaction has occurred at that site, then, qualitatively, this sequence of events is descriptive of many diffusion-reaction processes in biology, chemistry and physics. 

Surface Forces. Surface forces can be measured with thin film balances where freely suspended horizontal films are submitted to controlled pressures. If the film drainage has been smooth enough, so that no early rupture has occurred, a regime where interactions between the two sides of the film become significant can be attained this is in a thickness region of about 100 nm and less. Typical interactions are van der Waals forces (attractive) and electrostatic forces (repulsive). Short range forces (steric, hydration) are also present. 

Surface Force Measurements. This technique enables the measurement of the force (10 mN accuracy) versus distance (0.1-0.2 nm accuracy) between two curved mica surfaces. The forces between two solid surfaces across an aqueous solution are highly sensitive to the structure of the solid/liquid interfaces. When such surfaces are covered with adsorbed protein layers, then, the analysis of the force/distance profiles may reveal the formation of protein bridges between the two surfaces, the occurrence of steric interactions, or any possible protein conformation change. 

The adsorbed surfactant molecules counteract flic drop coalescence in two ways (1,2). The presence of surfactant gives rise to repulsive surface forces (of either electrostatic, steric, or oscillatory structural origin) between the drops, thus providing a thermodynamic stabilization of the emulsion see also Refs 3 and 4. Moreover, the adsorbed surfae-... 

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