  We consider the surface pressure of a colloid-laden liquid interface. As
micron-sized particles of suitable wettability can be irreversibly bound to the
liquid interface on experimental timescales, we use the canonical ensemble to
derive an expression for the surface pressure of a colloid-laden interface. We
use this expression to show that adsorption of particles with only hard-core
interactions has a negligible effect on surface pressures from typical
Langmuir-trough measurements. Moreover, we show that Langmuir-trough
measurements cannot be used to extract typical interparticle potentials.
Finally, we argue that the dependence of measured surface pressure on surface
fraction can be explained by particle coordination number at low to
intermediate particle surface fractions. At high surface fractions, where the
particles are jammed and cannot easily rearrange, contact-line sliding and/or
deformations of the liquid interface at the length scale of the particles play
a pivotal role.
