26194
J. Phys. Chem. B 2006, 110, 26194-26203
Re-entrant Nematic Behavior in the 7OCB+9OCB Mixtures: Evidence for Multiple Nematic-Smectic Tricritical Points Paul Cusmin,† Josep Salud,† David O. Lo´ pez,*,† Maria R. de la Fuente,‡ Sergio Diez,† Miguel A. Pe´ rez-Jubindo,‡ Marı´a Barrio,† and Josep Ll.Tamarit† Laboratori de Caracteritzacio´ de Materials (LCM), Departament de Fı´sica i Enginyeria Nuclear, E.T.S.E.I.B. UniVersitat Polite` cnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain, and Departamento de Fı´sica Aplicada II, Facultad de Ciencias, UniVersidad del Paı´s Vasco, Apartado 644, E-48080 Bilbao, Spain ReceiVed: July 5, 2006; In Final Form: October 4, 2006
Themetastablephasediagramofthetwo-componentsystemheptyloxycyanobiphenyl(7OCB)+nonyloxycyanobiphenyl (9OCB) was determined by means of modulated differential scanning calorimetry (MDSC) and optical microscopy measurements. It was experimentally established that the 7OCB+9OCB two-component system exhibits a monotropic re-entrant nematic behavior. A complete quantitative thermodynamic analysis, through Oonk’s equal G analysis, was performed, allowing for the calculation of the monotropic re-entrant behavior and the prediction of two tricritical points, one of them experimentally accessible for the SmAd-to-N transition and the other non-experimentally accessible for the RN-to-SmAd transition. From specific-heat measurements, latent heats were obtained for those mixtures displaying a first-order SmAd-to-N transition. Additionally, for some mixtures, the specific-heat critical exponents (R), through the second-order SmAd-to-N transition, were obtained. Both batches of data enable us to access to the experimental tricritical temperature for the SmAdto-N transition.
1. Introduction There exist certain kinds of liquid crystal materials that display some common features. One such feature is that the constituent molecules have strongly polar end groups and a smectic A phase in which the layer spacing d varies, l
