  Single-particle tracking and optical tweezers are powerful techniques for
studying diverse processes at the microscopic scale. The stochastic behavior of
a microscopically observable particle contains information about its
interaction with surrounding molecules, and an optical tweezer can further
facilitate this observation with its ability to constrain the particle to an
area of interest. Although these techniques found their initial applications in
biology, they can also shed new light on colloid and interface phenomena by
unveiling nanoscale morphologies and molecular-level interactions in real time,
which have been obscured in traditional ensemble analysis. Here we demonstrate
the application of single-particle tracking and optical tweezers for studying
molecular interactions at solid-liquid interfaces. Specifically, we investigate
the behavior of surfactants at the water-glass interface by tracing their
interactions with gold nanoparticles that are optically trapped on these
molecules. We discover the underlying mechanisms governing the particle motion,
which can be explained by hydrophobic interactions, disruptions, and
rearrangements among surfactant monomers at the interfaces. Such
interpretations are further supported by statistical analysis of an individual
trajectory and comparison to theoretical predictions. Our findings provide new
insights into the surfactant dynamics in this specific system but also
illustrate the promise of single-particle tracking and optical manipulation in
studying nanoscale physics and chemistry of surfaces and interfaces.
