Experimental Determination of a Two-Dimensional Liquid-Vapor Critical-Point Exponent

The liquid-vapor coexistence boundary of submonolayer C${\mathrm{H}}_4$ adsorbed on graphite was mapped out in an extensive ac heat-capacity study. The order-parameter exponent $\beta$ that describes the shape of the liquid-vapor coexistence boundary was found to be 0.127 ± 0.020. This is in good agreement with $\frac{1}{8}$, the exact value calculated for a two-dimensional Ising-model system.