Ammonium-functionalised monodisperse silica spheres have been prepared and spread together with sodium dodecylsulfate at the air-water surface. The resulting particulate films have been deposited on various substrates. The Langmuir isotherm has been used to estimate the density and refractive index of the particles. The films exhibit vivid optical diffraction due to a high degree of ordering of the particles. The lattice constant of this two-dimensional opal structure has been determined by atomic force microscopy at 536 nm. Optical diffraction of the films has been established and explained in terms of a Bragg-like diffraction. The lattice constant, as determined by optical diffraction, is found to be in good agreement with the AFM measurement, within experimental error. We consider this Langmuir-Blodgett deposition of 2D opal as a promising route for the preparation of active layers in photonic band gap devices and materials.