Coexistence curves for the ternary system polystyrene (1)+polystyrene (2)+methylcyclohexane have been determined from measurements of refractive index. The polystyrene fraction of longer chain length was the same in all the measurements (M2=929000), but the shorter chain length fractions were varied (M1=13700, 23700, 29400, and 34000). The coexistence curves (in terms of total mass fraction of polymer) can be fitted to the same extended (classical) theory used in previous studies of quasi-binary hydrocarbon mixtures and can be extrapolated to a tricritical point at M2/M1=23 (M1=40000) and 36℃.

The turbidities of four bimodal mixtures mixtures of polystyrene in methylcyclohexane have been mea-sured in the three-phase region near the tricritical point, The results fromt eight different wave-lengths have been analyzed to obtain the correlation lengths. and the susceptibilities χ.χ and are strongly correlated. so it is their ratio x/that is determined most precisely. As the tricritical point is approached, alt three quantities diverge, as expected from the classical mean-field theory. Al-though the uncerainties are larger than wished for. it appears that the Griffiths first sums ger in-creasingly larger as the tricritical point is approached, and probably diverge in the same way as has been observed for other tricritical systems.

The coexistence curves of (NN dimethylaeet,tmide+hexane) have been determined within 10 K from the critical temperature by measurements of refractive index The crilical exponent βdeduced from (T, n), (T. x), and (T, φ) curves are consistent wlih theoretical predictions The results also have been analysed to obtain critical amplitudes and Wegner-eorreclion terms whenβ is fixed at the theoretical value and to compare the goodnesses of fit of the diameters of coexistence curves with τ1-2 andτ2β[τ=(T2-T)/T2]. It has been shown that the first Wegner-correction term is just significant and volume fraction possibly is a better order pararneter than is mole fraction.

A three-component microemulsion system can be regarded as a pseudobinary mixture at certain conditions. The coexistence curves of (T, n), (T,φ), and (T, ψ) (n and are refractive index and volume fraction, respectively; ψ is defined as ψ=φ/[φ+φc(1-φ)/(1-φc)]) for a ternary microemulsion (AOT-waterdecane) at constant pressure and a constant ratio of water to AOT have been determined within about 10 K from the critical temperature by measurements of refractive index. The critical exponent β was deduced precisely from (T, n), (T,φ), and (T, ψ) coexistence curves within 1.5 K above Tc. These values were 0.327, 0.324, and 0.325, respectively, and were consistent with the 3D Ising exponent. The volume fraction is a better choice of the concentration variable than ψ used for construction of the order parameter to fit the Ising behavior.