TIRM is an optical technique invented by Prieve and Walz for measuring interaction (free) energies between a single colloidal sphere and a flat substrate. Briefly, in TIRM a diffusing colloidal sphere is exposed to an evanescent wave originating at the substrate-solvent interface. The scattered intensity from the sphere varies exponentially with its height above the substrate and the sphere samples different heights in the 'potential well' created by gravity and electrostatic repulsion from the bottom substrate. It follows that the interaction energy can be evaluated as a function of separation by recording the intensity over a period of time and by measuring the intensity for the same particle attached to the substrate. In contrast to most other force measurement techniques, TIRM gives access to weak forces and large distances - the range that is most relevant for colloidal interactions in equilibrium. In this project we are exploring surfactant-mediated effects on these interactions.