Many physical chemistry lab courses include an experiment in which students measure surface tension as a function of surfactant concentration. In the traditional experiment, the data are fit to the Gibbs isotherm to determine the molar area for the surfactant, and the critical micelle concentration is used to calculate the Gibbs energy of micelle formation. In the experiment described here, students fit their data to the Szyszkowski equation, allowing them to also calculate the Gibbs energy of surface adsorption. In addition, we use this experiment to introduce nonlinear custom curve fitting. (Contains 3 figures and 1 note.)