STATIC LIGHT-SCATTERING EXPERIMENTS WITH 2-BUTOXYETHANOL WATER MIXTURES OF NEAR-CRITICAL COMPOSITION IN THE VICINITY OF THE LOWER CRITICAL-POINT

Measurements of the concentration and temperature dependence of the correlation length xi of local concentration fluctuations in seven 2-butoxyethanol (C4E1)/water mixtures of different non-critical but near-critical compositions are carried out in the vicinity of the lower critical point of the system in the composition range -0.016 less-than-or-equal-to (y-y(c)) less-than-or-equal-to 0.075; (y(c) = 0.2947; y, mass fraction of C4E1; y(c), critical composition). The data are analysed in two ways: (a) On the basis of the concept of the pseudo-spinodal temperature T(ps). The temperature dependence of xi and the quantity chi(T)* which is proportional to the generalized susceptibility chi(T) can be described by power laws of the form xi = xi0'l(ps)-nu'BAR (xi0', amplitude; nu', exponent) and chi(T)* = chi(T,0)*'l(ps)-gamma'BAR (chi(T,0)*', amplitude; gamma', exponent) with l(ps)BAR = (T(ps)-T(P))/T(ps); (T(P), temperature of phase separation). T(ps) is obtained as one of the parameters of a three parameter fit (xi0', nu', T(ps); chi(T,0)*', gamma', T(ps)) of the corresponding power laws to experimental xi-[T-T(P)(vis)] and chi(T)*-[T-T(P)(vis)] data sets. T(ps) is larger than the visually determined temperature of phase separation T(P)(vis). As expected, the temperature difference (T(ps) - T(P)(vis)) > 0 has its largest value for the mixture with the largest value \y-y(c)\ and its smallest value for the mixture with the smallest value \y-y(c)\. The values of xi0', nu', and gamma' are independent of concentration (xi0' = (0.46 +/- 0.04) nm; nu' = (0.61 +/- 0.01); gamma' = (1.20 +/- 0.09)). They are only slightly different from the corresponding values of the 2-C4E1/H2O mixture of critical composition (xi0 = (0.44 +/- 0.04) nm, nu = (0.63 +/- 0.03); gamma = (1.22 +/- 0.02); universal value for binary liquid mixtures, Landau-Ginsburg-Wilson model, nu = 0.630; gamma = 1.241). (b) On the basis of a scaled equation of state (linear model). This model describes qualitatively the temperature dependence of xi of the mixtures of non-critical composition.