Quantitative analysis using surfactant-particles interaction is the basis for many applications. In situ measurements of surfactant adsorption on nanoparticles are important to understanding adsorption kinetics. However, it is quite difficult to determine the individual numbers for each monomodal particles in a multimodal mixture system by current technologies. To cope with this problem, a new method, i.e. 2D differential centrifugal sedimentation (2D-DCS), has been developed and applied in situ to measure the number of CTAB molecules adsorbed on the surface of silica particles, assuming that the adlayer is composed of a compact CTAB monolayer. Results show that 2D-DCS can measure the adsorption amount for particles not only with single size distribution but also with multiple size distributions. The number of adsorbed CTAB per nm2 on silica particles determined by 2D-DCS are 1.4 and 3.9 for the monomodal particles of 210 and 1000 nm, respectively, which is similar to that measured by ζ-potential, DLS and UV-vis spectrometry, and 1.4, 2.3 and 2.5 for 210, 430 and 700 nm particles, respectively, for a trimodal particle system, where the size-dependent adsorption is difficult to be simultaneously measured by other technologies.