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 ζ of local concentration fluctuations in seven 2‐butoxyethanol (C 4 E 1 )/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≤( y‐y c )≤0.075; ( y c = 0.2947; y , mass fraction of C 4 E 1 ; 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 ζ and the quantity χ* T which is proportional to the generalized susceptibility χ T can be described by power laws of the form ζ = ζ' 0 l ps −v' (ζ' 0 , amplitude; v', exponent) and χ amplitude; χ, exponent) with l ps = ( T pc ‐ T P / T ps ); ( T P , temperature of phase separation). T ps is obtained as one of the parameters of a three parameter fit (ζ' 0 , v', T ps ; χ *' T, 0 , γ' T ps ) of the corresponding power laws to experimental ζ−[ T‐T P (vis)] and χ* 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 ζ' 0 v′, and y ' are independent of concentration (ζ 0 = (0.46±0.04) nm; v′ = (0.61±0.01); y ' = (1.20±0.09)). They are only slightly different from the corresponding values of the 2‐C 4 E 1 /H 2 O mixture of critical composition (ζ 0 = (0.44±0.04) nm, v = (0.63±0.03); y = (1.22±0.02); universal value for binary liquid mixtures, Landau‐Ginsburg‐Wilson model, v = 0.630; y = 1.241). (b) On the basis of a scaled equation of state (linear model). This model describes qualitatively the temperature dependence of ζ of the mixtures of non‐critical composition.