High precision density studies near the smectic A–nematic tricritical point

Abstract

We have carried out very accurate density measurements (with a precision of ±5 × 10⁻⁵g cm⁻³) near the smectic A-nematic transition in binary mixtures of 4-n-nonyl-4′-cyanobiphenyl (9CB) and 4-n-decyl-4′-cyanobiphenyl (10CB). The transition crosses over from second to first order as the temperature range of the nematic phase decreases. For mixtures with the shortest nematic range the data deviate noticeably from a single power law behaviour. Such a deviation is an indication of the first order nature of the transition. Very good fits to a single power law have been obtained for pure 9CB and the x = 0·04 mixture where x is the mole fraction of 10CB in 9CB. The critical exponent obtained from the power law fitting has enabled us to locate the tricritical point to be very close to x=0·04, which is in agreement with the results obtained previously by high resolution calorimetric [1] and X-ray scattering studies [2].