  We analyze the possibility of evaluation of the thickness and refractive
index of the stagnant layer in concentrated suspensions of nanoparticles
through the transport characteristics of scattered light photons. The analysis
is based on a physically-transparent generalization of the concept of the
single scattering intensity off systems in which the number of particles within
regions with linear sizes of order of the wavelength in the medium greatly
exceeds unity. This generalization is carried out within the notion of compact
groups of particles, makes it possible to go beyond the traditional Born
approximation, and take into account many-particle effects contributed from
those ranges of integration variables in the terms of the iteration series for
the scattered field where the internal propagators have delta-function-type
behavior. The evaluation of the photon transport characteristics becomes
possible without a detailed modeling of many-particle scattering and
correlation processes in the system. The photon mean-free-path length, $l$, is
investigated as a function of the stagnant refractive index and that of the
layer thickness to show a noticeable effect of both parameters on it. As the
layer refractive index is increased at a fixed layer thickness, $l$ decreases
because the suspension optical density increases. As a function of the layer
thickness, $l$ reveals different types of behavior, depending on the relation
between refractive indices of the stagnant layer and base liquid. If the former
is smaller than the latter, this behavior is increasing; in the opposite case,
it is decreasing. An experimentally observed increase of $l$ with the particle
concentration is explained as a manifestation of higher correlation effects.
Our theory reveals that the stagnant layer make the situation more complicated,
for both factors may either enhance or diminish each other.
