Thermodynamic analysis of formation of crystal-like precious-opal structures
S.V. Vosel, D.V. Kalinin
Based on the theory of free volume, it is established that the Kirkwood-Alder transitions in concentrated model systems of noninteracting hard spheres are caused by external forces. Analysis shows that, when describing order-disorder transitions in real colloidal systems, the interaction of particles can be not taken into account if the potential energy of the particles in the forming dense phase (|uk|) is lower than 5–10 kT.
Two types of transition into dense, including regular, particle state are possible in real suspensions:
1) transitions (possible even in diluted suspensions) caused by strong interparticle attraction, with |uk|≥40 kT (coagulation), and
2) transitions caused by the action of external forces (Alder transitions) in concentrated suspensions of weakly interacting colloidal particles, |uk|<5010 kT (formation of regylar colloidal structures (RCS)).
Formation of dense monodisperse spherical silica particles (MSSP) from their fine suspensions belongs, probably, to the first-type transitions. Formation of RCS of noble opals from MSSP pertains to the second-type transitions. Formation of these RCS should pass through the stage of the suspension compaction. The conclusion is drawn that natural RCS should be a necessery stage in the formation of consolidated regular structures of noble opals.
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