Steric force, polymer-covered surfaces

The third contribution to the free energy comes from the steric repulsion in the oil layers between the hydrocarbon tails of the surfactant adsorbed on the interface. Extending the simple formula suggested by de Gennes18 for the steric force between long-chain polymer-covered surfaces, to relatively short chains, one can write... 

Repulsive steric forces are encountered when the outer segments of two polymer-covered surfaces begin to overlap. These interactions usually lead to a repulsive force due to the unfavorable reduction in entropy associated... 

Protrusion Model for the Hydration Force. Recently Is-raelachvili and Wennerstrom (IW) proposed that the origin of the hydration force is due to the head-group protrusion of the phospholipid molecules into the solvent region (27), and, therefore, the force is more akin to a steric force acting between polymer-covered surfaces. Because such an explanation of the nature of the hydration force does not involve electrostatic concepts, we will not present the IW theory here. A detailed description of a protrusion model is available in a recent review by Israelachvili and Wennerstrom (28), and a critique of IW theory of protrusions can be found in Parsegian and Rand (29). 

In effect, we have mainly covered attractive interactions introduced when adding polymer to a system. This is because our interest here was mainly focused on retention aids which function by enhancing the attractive interactions in the papermaking system. Polymeric systems tuned to increase the repulsive force between surfaces by way of steric interactions (caused by chain overlap and loss of conformational entropy on compression), electrosteric interactions (between charged polymer brushes), and/or electrostatic repulsions (due to charge over-compensation), are equally important in practical applications. However, repulsive polymeric forces should not by any means be viewed as purely... 

The onset of the steric exclusion force depends on the means of attaching the polymer chains to the substrates. For physically adsorbed polymer chains covering bodh surfaces, the steric exclusion force becomes detectable around 6Rg, where Rg is the unperturbed radius of gyration of the random polymer coil in solution (12), For terminally attached polymer chains, the repulsion commences around 12Rg (14), These values are approximate and depend on a number of factors including solvent quality, temperature, surface concentration and type of polymer chains attached to the surfaces. 

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