  We show that the interaction potential between sterically stabilized, nearly
hard-sphere [poly(methylmethacrylate)-poly(lauryl methacrylate) (PMMA-PLMA)]
colloids at a water-oil interface has a negligible unscreened-dipole
contribution, suggesting that models previously developed for charged particles
at liquid interfaces are not necessarily applicable to sterically stabilized
particles. Interparticle potentials, $U(r)$, are extracted from radial
distribution functions [$g(r)$, measured by fluorescence microscopy] via
Ornstein-Zernike inversion and via a reverse Monte Carlo scheme. The results
are then validated by particle tracking in a blinking optical trap. Using a
Bayesian model comparison, we find that our PMMA-PLMA data is better described
by a screened monopole only rather than a functional form having a screened
monopole plus an unscreened dipole term. We postulate that the long range
repulsion we observe arises mainly through interactions between neutral holes
on a charged interface, i.e., the charge of the liquid interface cannot, in
general, be ignored. In agreement with this interpretation, we find that the
interaction can be tuned by varying salt concentration in the aqueous phase.
Inspired by recent theoretical work on point charges at dielectric interfaces,
which we explain is relevant here, we show that a screened $\frac{1}{r^2}$ term
can also be used to fit our data. Finally, we present measurements for
poly(methyl methacrylate)-poly(12-hydroxystearic acid) (PMMA-PHSA) particles at
a water-oil interface. These suggest that, for PMMA-PHSA particles, there is an
additional contribution to the interaction potential. This is in line with our
optical-tweezer measurements for PMMA-PHSA colloids in bulk oil, which indicate
that they are slightly charged.
