Surface tension of a binary liquid mixture in the vicinity of its critical point

Results of measurement of the temperature and composition dependence of the surface tension sigma(alpha,beta gamma) of 2,6-dimethylpyridine (abbreviated 2.6-dp)/water mixtures are reported. The index alpha refers to the vapor phase and the beta gamma to the homogeneous 2,6-dp/water mixture. The experiments are carried out at constant temperatures T below and at the critical temperature T-c (= 33.87 degrees C) in the entire composition range (0 less than or equal to x less than or equal to 1; x, mole fraction of 2.6-dp). For mixtures with compositions in the range -0.04<(x-x(c))<0.10(x(c) = 0.640) the experimental sigma(alpha,beta gamma)(x,T-c) data can be represented by the theoretically established equations sigma(alpha,beta gamma) = sigma(c) + C(x(c)-x)(mu*/beta*) for x<x(c) and sigma(alpha,beta gamma) = sigma(c) + D(x-x(c))(mu*/beta*) for x>x(c) (Nagarajan, Webb, Widom, J. Chem Phys. 77, 5771 (1982)). The constants C and D have different values of opposite signs. The value of the exponent (mu*/beta*) obtained from experimental data with compositions x<x(c) is consistent with the theoretically predicted value (mu*/beta*) = 3.88.