  We examine the organization and dynamics of binary colloidal monolayers
composed of micron-scale silica particles interspersed with smaller-diameter
silica particles that serve as minority component impurities. These binary
monolayers are prepared at the surface of ionic liquid droplets over a range of
size ratios ($\sigma=0.16-0.66$) and are studied with low-dose minimally
perturbative scanning electron microscopy (SEM). The high resolution of SEM
imaging provides direct tracking of all particle coordinates over time,
enabling a complete description of the microscopic state. In these bidisperse
size mixtures, particle interactions are non-additive because interfacial
pinning to the droplet surface causes the equators of differently sized
particles to lie in separate planes. By varying the size ratio we control the
extent of non-additivity in order to achieve phase behavior inaccessible to
additive 2D systems. Across the range of size ratios we tune the system from a
mobile small-particle phase ($\sigma<0.24$), to an interstitial solid
($0.24<\sigma<0.33$), to a disordered glass ($\sigma>0.33$). These distinct
phase regimes are classified through measurements of hexagonal ordering of the
large-particle host lattice and the lattice's capacity for small-particle
transport. Altogether, we explain these structural and dynamic trends by
considering the combined influence of interparticle interactions and the
colloidal packing geometry. Our measurements are reproduced in molecular
dynamics simulations of 2D non-additive disks, suggesting an efficient method
for describing confined systems with reduced dimensionality representations.
