Phase Transitions of Hard Spheres Plus Colloids

Phase Transitions of Hard Spheres Plus Colloids 

In the previous chapter we considered the elfect of added non-adsorbing polymers on the phase behaviour of suspensions of spherical colloids. The depletion effect is also operational in mixtures of large and small (hard) spheres, see Fig. 5.1 where two big spheres in a sea of small spheres are brought together. As the big spheres get close, the smaller spheres can no longer enter the gap between the big ones. The outside small particles then push the big spheres together. 

The added small colloids may be of a similar colloid shape (i.e., spheres) or a different shape such as rod-like colloids. In Sect. 2.4 we found that rod-Uke colloids give rise to a strong depletion interaction and in this chapter we will see that rod-Uke colloids influence the phase behaviour of a colloidal suspension significantly at very low concentrations. 

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In this chapter we discuss the basics of the phase behaviour of hard spheres plus depletants. Phase transitions are the result of physical properties of a collection of particles depending on many-body interactions. In Chap. 2 we focused on two-body interactions. As we shall see, depletion elfects are commonly not pair-wise additive. Therefore, the prediction of phase transitions of particles with depletion interaction is not straightforward. As a starting point a description is required for the thermodynamic properties of the pure colloidal dispersion. Here the colloid-atom analogy, recognized by Einstein and exploited by Perrin in his classical experiments, is very useful. Subsequently, we explain the basics of the free volume theory for the phase behaviour of colloids -I- depletants. In this chapter we treat only simplest type of depletant, the penetrable hard sphere. 

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