Study of Non‐Equilibrium States of a Homogeneous 2‐Butoxyethanol/Water Mixture of Critical Composition in the Vicinity of Its Lower Critical Point Using Fast Pressure Jumps

Abstract

AbstractThe pressure dependence of the critical temperature is used to bring a homogeneous 2‐butoxyethanol/ water mixture of critical composition (xc = 0.0598; mole fraction of 2‐butoxyethanol; critical temperature Tc (P = 1 bar) = 49.39°C) from a state away from its critical temperature to a state close to the critical temperature. This is accomplished by fast pressure jumps (expansions) with an amplitude in the range of −50 bar < ΔP < −5 bar (ΔP = (Pf‐Pi); Pf, hydrostatic pressure in the final state (Pf = 1 bar); Pi, hydrostatic pressure in the initial state) and a characteristic time constant of τp ≃ 0.3 ms. The coefficient characterizing the pressure dependence of the critical temperature has a value of (dTc/dP)c.l. = 43 mK bar−1. The delayed increase of the intensity of scattered light (“critical opalescence”) is measured as function of time at different scattering angles (10° ≤ θ ≤ 60°) and different temperature differences (Tc ‐ Tf) (Tc. critical temperature; Tf, final temperature after the pressure jump). The fast pressure jump leads to a small decrease of the temperature of the sample ((∂T/∂P)s = 6 mK bar−1). The measured relaxation curves are found to be single exponentials for times short compared with the relaxation time τT of the heat conduction process bringing the system back to its initial temperature Tf (τT is in the order of seconds). – For small values of the scaling variable (qζf) < 0.7 (q, absolute value of scattering wave vector; q = (4μñ/λ0) sin (0/2); 0, scattering angle; ñ, index of refraction; λ0, wavelength of incident light in vacuum; ζ, correlation length of local concentration fluctuations) the structure factor relaxes as expected on the basis of linear response (τhyd/τexp = 1; τhyd, hydrodynamic relaxation time calculated from independent dynamic and static light scattering experiments; τexp, experimental relaxation time). At larger values of (qζf) ((qζf) > 0.7) the structure factor relaxes more slowly than theoretically expected (i.e. τhyd/τexp < 1). This behaviour is similar to that of the systems 3‐methyl‐pentane/nitroethane and 2, 6‐dimethylpyridine/water.