Controlling self-assembly of colloidal particles to create ordered structures over large areas has attracted considerable attention due to the low cost and wide variety of applications. Latex particles can form well ordered 2D crystalline arrays at a solid-liquid interface over areas of about 20cm2. Quartz crystal microbalance data suggests that the particles are separated from a silica interface by a layer of water. In this study, the separation of colloidal structures from the interface will be probed and modified by changing the concentration of the salt in the dispersion. GiSANS measurement will be used to investigate the near surface structures of the particles, in particular exploiting high resolution of the incident wave vector near the critical momentum transfer for total reflection. This permits probing different depths and hence the particle separation from the interface.