A protocol for cluster size distribution analysis was developed in order to parametrize local two-dimensional (2D) order in a quantitative manner, using mean cluster sizes and fractional hcp cluster formation (f(hcp)). Cluster size analysis was performed on 2D arrays of An nanoparticles encapsulated in resorcinarene tetrathiol, which were organized into close-packed films at aqueous interfaces. The degree of monolayer formation and 2D order within the self-assembled nanoparticle arrays was observed to be strongly dependent on the amount and type of electrolyte (chloride and/or citrate) adsorbed on the nanoparticle surface, prior to encapsulation and extraction to the solvent interface. Increasing the concentration of adsorbed electrolyte could promote monoparticulate film formation but had a variable effect on local 2D order.