Enthalpy of Demixing of a Binary Liquid Mixture of Critical Composition: Study of the System Iso‐Butoxyethanol/Water

Abstract A fast pressure jump technique is used to measure calorimetrically the temperature dependence of the enthalpy of demixing of an iso‐butoxyethanol(i‐C 4 E 1 )/water mixture of critical composition (x c (i‐C 4 E 1 ) = 0.0664; T c = 26.46 °C) in the vicinity of its lower critical point. It is found that the enthalpy of demixing has a positive sign as expected thermodynamically for a lower critical point. A linear relation is observed between the molar enthalpy of demixing Δ demix H m (X c ) and the temperature difference ( T – T c ) ( T c , critical temperature; T , temperature; ( T > T c ) with a slope of (∂ Δ demix H m (X c )/∂ T ) p = (5.5 ± 0.3) J mol −1 K −1 . The enthalpy of demixing Δ demix H m (X c ) becomes zero at the critical temperature T c . The value of (∂ Δ demix H m (X c )/∂ T ) p is of the same order of magnitude as the absolute value of (∂ Δ demix H m (X c )/∂ T ) p of the system isobutyric acid/water (upper critical point; (∂ Δ demix H m (X c )/∂ T ) p = −(5.2 ± 0.4) J mol −1 K −1 ) using the same technique.

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