PHASE-BEHAVIOR OF A SYMMETRICAL BINARY MIXTURE OF HARD-RODS

The phase behaviour of long hard rods is independent of their length t o breadth ratio in the limit that this ratio is very large. We form a binary mixture of rods with different length to breadth ratios but the same second virial coefficient. As the second virial coefficient is t he same for both components, their phase behaviour in the pure state i s identical. However, the difference in their shapes-one is longer and thinner than the other-results in an increased interaction between a pair of rods of different components. As the difference in shape of th e two components is increased, first isotropic-isotropic coexistence i s observed (with a critical point), then in addition nematic-nematic c oexistence. At first there is a nematic-nematic critical point but thi s point reaches the isotropic-nematic transition, creating a four phas e region. Gibbs' phase rule, as usually stated, permits a maximum of t hree phases to coexist simultaneously in a binary athermal mixture. He re, the symmetry between the two components allows four to coexist. (C ) 1996 American Institute of Physics.