The interface of 16 nm colloidal gold nanoparticles with the aqueous solution in which they are suspended is investigated using second harmonic generation. The population of malachite green freely adsorbing to the gold nanoparticle interface as a function of concentration is obtained. The experimental results are in good agreement with the modified Langmuir model, which includes depletion of the adsorbate bulk concentration. The free energy of adsorption is determined to be - 15.4 +/- 0.4 kcal/mol with 1.13 +/- 0.04 x 10(3) sites per nanoparticle. These results provide the first direct measurement of the isotherm of molecules adsorbing to colloidal metallic nanoparticles. (C) 2011 Published by Elsevier B.V.