Distance Dependence of Polymeric Steric Interactions

Steric interactions between emulsion droplets are conveniently divided into three regimes, according to the separation of the surface of the bare droplets (h) relative to the thickness of the polymer layers (8):

FIGURE 3.16 Interdroplet pair potential due to steric polymeric interactions. At intermediate separations, the steric polymeric interaction can be either attractive or repulsive depending on the quality of the solvent because of the mixing contribution, but at short separations it is strongly repulsive because of the elastic contribution.

FIGURE 3.16 Interdroplet pair potential due to steric polymeric interactions. At intermediate separations, the steric polymeric interaction can be either attractive or repulsive depending on the quality of the solvent because of the mixing contribution, but at short separations it is strongly repulsive because of the elastic contribution.

1. Zero interaction regime (h > 28). At sufficiently large droplet separations, the polymer layers do not overlap with each other and the steric interaction between the droplets is zero.

2. Interpenetration regime (8 < h < 28). When the droplets are sufficiently close together for the polymer layer on one droplet to interpenetrate that on another droplet, without significantly compressing it, the major contribution to the steric interaction is the mixing contribution (wmix), which may be either positive or negative depending on the quality of the solvent

3. Interpenetration and compression regime (h < 8). When the droplets get so close together that the polymer layers start to compress each other, the overall steric interaction is a combination of elastic and mixing contributions, although the strongly repulsive elastic component usually dominates, and so the overall interaction is repulsive.

It should be stressed that the length of the interpenetration and elastic regions actually depends on the precise nature of the polymer molecules present at the interface (Claesson et al. 1995). Flexible biopolymer molecules will have relatively large interpenetration regions, whereas compact globular proteins will have relatively small ones. Consequently, the choice of 8 as the distance where the elastic contribution first contributes to the interaction is arbitrary, and different values will be more appropriate for some systems. As mentioned earlier, the only way these values can accurately be established for a particular system is by measuring the force between two polymer-coated surfaces as they are brought closer together (Israelachvili 1992, Claesson et al. 1995, 1996).

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