Phase Diagrams with Closed Loop Smectic — Nematic Phase Boundaries

A T, x diagram with a closed loop smectic‐nematic phase boundary is observed in mixtures of 4‐n‐heptyloxy‐4′‐cyanobiphenyl and a mixture of 4‐cyanobenzylidene‐4′‐n‐octyloxyaniline and 4‐n‐pentylphenyl 4‐n‐octyloxybenzoate. Application of pressure reduces the smectic phase area and, at a critical pressure of 145 bar, the area is reduced to a point (double critical point) in the T, x diagram. Thermodynamics leads to an appropriate description of the phase boundary.