ENTROPICALLY DRIVEN MICROPHASE TRANSITIONS IN MIXTURES OF COLLOIDAL RODS AND SPHERES

Although the idea that entropy alone is sufficient to produce an order ed state is an old one in colloid science(1), the notion remains count er-intuitive and it is (often assumed that attractive interactions are necessary to generate phases with long-range order, The phase behavio ur for both rods and spheres has been studied experimentally(1-7), the oretically(8,9) and by computer simulations(10). Here we describe the phase behaviour of mixtures of colloidal rodlike and sphere-like parti cles (respectively viruses and polystyrene latex or polyethylene oxide polymer) under conditions in which they act like hard' particles(2,3) . We find a wealth of behaviour: bulk demixing into rod-rich and rod-p oor phases and microphase separation into a variety of morphologies. O ne microphase consists of layers of rods alternating with layers of sp heres(11); in another microphase of unanticipated complexity, the sphe res reversibly assemble into columns, which in turn pack into a crysta lline array. Our experiments, and previous theory and computer simulat ions(11), suggest that this phase behaviour is entropically driven by steric repulsion between particles. The phenomena are likely to be qui te general, applying also for example to low-molecular-mass liquid cry stals(12). This kind of microphase separation might also be relevant t o systems of amphiphiles(13) and block copolymers(14), to bioseparatio n methods and DNA partitioning in prokaryotes(15), and to protein crys tallization(16,17) and the manufacture of composite materials.