Calorimetric study of a closed reentrant nematic region

Binary mixtures of 4-n-undecyloxy-(4'-isothiocyanatophenyl)-benzoate (11.O.NCS) and 4-n-decyloxyphenyl-4'-(4''-cyanobenzyloxy)-benzoate (10OPCBOB) exhibit a closed reentrant nematic (Nᵣ) region---a ``nematic lake''---between X=51 and 79, where X is the mole percent of 10OPCBOB. A high-resolution ac calorimetric study has been carried out on six mixtures with this composition range to characterize the smectic-Ad (Sm-Ad)--Nᵣ and Nᵣ₋₋smectic-A₁ (Sm-A₁) transitions plus one mixture (X=80.7) that undergoes a direct Sm-Ad--Sm-A₁ transition. Of special interest is the Nᵣ₋₋Sm-A₁ transition, which exhibits three-dimensional XY critical heat-capacity behavior at X=53.8 and 64.5, shows crossover behavior with nonuniversal effective exponents {α}{}0.26 for X=74.5 and {α}{}0.31 for X=76.4, and seems to approach tricritical behavior near the entry to the lake. The presence of such a tricritical point is consistent with theoretical predictions of the multicritical topology near a Sm-Ad--{N}ᵣ₋₋$Sm-${A}₁$ point.