  We present a model for quasielastic neutron scattering (QENS) by an aqueous
solution of compact and inflexible molecules. This model accounts for
time-dependent spatial pair correlations between the atoms of the same as well
as of distinct molecules and includes all coherent and incoherent neutron
scattering contributions. The extension of the static theory of the excluded
volume effect [A. K. Soper, J. Phys.:Condens. Matter 9, 2399 (1997)] to the
time-dependent (dynamic) case allows us to obtain simplified model expressions
for QENS spectra in the low Q region in the uniform fluid approximation. The
resulting expressions describe the quasielastic small-angle neutron scattering
(QESANS) spectra of D2O solutions of native and methylated cyclodextrins well,
yielding in particular translational and rotational diffusion coefficients of
these compounds in aqueous solution. Finally, we discuss the full potential of
the QESANS analysis (that is, beyond the uniform fluid approximation), in
particular, the information on solute-solvent interactions (e.g., hydration
shell properties) that such an analysis can provide, in principle.
