Tracer Diffusion and Shear Viscosity for the System Isobutyric Acid–Water near the Critical Mixing Point

Tracer diffusion coefficients of ¹⁴C isobutyric acid in mixtures of isobutyric acid–water have been measured over the entire concentration range at ${\text{ε\hspace{0.17em}=\hspace{0.17em}3.9\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}$ above the critical point using the diaphragm cell technique. No anomaly was detected. The temperature dependence of the shear viscosity coefficient for mixtures of critical composition was determined over the temperature range ${\text{6.7\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−6}}{\text{\hspace{0.17em}<\hspace{0.17em}ε\hspace{0.17em}<\hspace{0.17em}2.8\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−2}}$ by a closed capillary viscometer. These data were carefully analyzed in terms of a general power law equation. This analysis suggests that the excess viscosity may not diverge as the critical point is approached from the one‐phase region. Severe rounding occurs for ${\text{ε\hspace{0.17em}<\hspace{0.17em}5\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−5}}$ .