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Figure 7.7 Enthalpic stabilisation: representation of en-thalpic stabilisation of particles with adsorbed hydrophilic chains. The hydrated chains of the polyoxyethylene molecules —(OCH2CH2)n OH protrude into the aqueous dispersing medium. On close approach of the particles to within 2d (twice the length of the stabilising chains), hydrating water is released, resulting in a positive enthalpy change which is energetically unfavourable.

Figure 7.6 The influence of the surface potential (^>0) on the total potential energy of interaction of two spherical particles.

Figure 7.7 Enthalpic stabilisation: representation of en-thalpic stabilisation of particles with adsorbed hydrophilic chains. The hydrated chains of the polyoxyethylene molecules —(OCH2CH2)n OH protrude into the aqueous dispersing medium. On close approach of the particles to within 2d (twice the length of the stabilising chains), hydrating water is released, resulting in a positive enthalpy change which is energetically unfavourable.

to a steric interaction when the chains interact. The repulsive forces may not always be enthalpic in origin. Loss of conformational freedom leads to a negative entropy change (-AS). Each chain loses some of its conforma-tional freedom and its contribution to the free energy of the system is increased, leading to repulsion. This volume restriction is compounded by an 'osmotic effect' which arises as the macromolecular chains on neighbouring particles crowd into each other's space, increasing the concentration of chains in the overlap region. The repulsion which arises is due to the osmotic pressure of the solvent attempting to dilute out the concentrated region: this can only be achieved by the particles moving apart.

Quantitative assessments of the steric effect depends on three parameters:

• The hydrophilic polymer chain length, d

• The interaction of the solvent with the chains

• The number of chains per unit area of interacting surface

The steric effect does not come into play until H = 2d, so the interaction increases suddenly with decreasing distance. There are many problems in applying such equations in practice, the main ones being the lack of an

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