CRITICAL EXPONENTS FOR THERMAL-EXPANSION AND ISOTHERMAL COMPRESSIBILITY NEAR THE NEMATIC TO SMECTIC-A PHASE-TRANSITION

High-resolution measurements of the molar volume, V, measured as a fun ction of temperature at two different pressures allowed the calculatio n of the isobaric thermal expansion coefficient, alpha(p)(T), and isot hermal compressibility coefficient, beta(T)(T). Critical exponents for observed singularities in both thermodynamic quantities were found. I t was confirmed that the same critical exponent a, usually assigned to heat capacity, describes singularities of the thermal expansion and t he isothermal compressibility coefficients. Two compounds from the two different homologous series, nCB and (m) over bar S5, were studied. V (T) and beta(T)(T) variations in 8CB showed a continuous phase transit ion with the effective critical exponent a = alpha' = 0.32 +/- 0.02. F or 10S5 a weakly first-order transition with a volume jump Delta V = 0 .127 +/- 0.001 cm(3)/mol (Delta V/V = 2.6 x 10(-4))was observed and an exponent a = alpha' = 0.50 +/- 0.02 was found.