Order-Parameter Exponentβ1of a Binary Liquid Mixture at a Boundary

We have measured the transition from partial to complete wetting of Pyrex capillaries in a critical mixture of 2,6-lutidine and water with different amounts of dissolved potassium chloride. With increasing concentration of KCl the difference between the critical and wetting temperatures decreases. At a concentration of 7.3 × ${10}^{-2\mathrm{}}$ mol/l KCl we observe only partial wetting and can thus determine the difference between the critical exponent of the order parameter, ${\beta }_1$, at the (Pyrex) boundary, and $\beta$, in the bulk, from the capillary rise close to $T_c$. The result ${\beta }_1=0.83\mathrm{}\pm 0.05$ is in good agreement with theoretical predictions.