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Pronounced Microheterogeneity in a Sorbitol–Water Mixture Observed through Variable Temperature Neutron Scattering
In this study, the structure of concentrated d-sorbitol–water
mixtures is studied by wide- and small-angle neutron scattering (WANS
and SANS) as a function of temperature. The mixtures are prepared
using both deuterated and regular sorbitol and water at a molar fraction
of sorbitol of 0.19 (equivalent to 70% by weight of regular sorbitol
in water). Retention of an amorphous structure (i.e., absence of crystallinity)
is confirmed for this system over the entire temperature range, 100–298
K. The glass transition temperature, Tg, is found from differential
scanning calorimetry to be approximately 200 K. WANS data are analyzed
using empirical potential structure refinement, to obtain the site–site
radial distribution functions (RDFs) and coordination numbers. This
analysis reveals the presence of nanoscaled water clusters surrounded
by (and interacting with) sorbitol molecules. The water clusters appear
more structured compared to bulk water and, especially at the lowest
temperatures, resemble the structure of low-density amorphous ice
(LDA). Upon cooling to 100 K the peaks in the water RDFs become markedly
sharper, with increased coordination number, indicating enhanced local
(nanometer-scale) ordering, with changes taking place both above and
well below the Tg. On the mesoscopic (submicrometer) scale, although
there are no changes between 298 and 213 K, cooling the sample to
100 K results in a significant increase in the SANS signal, which
is indicative of pronounced inhomogeneities. This increase in the
scattering is partly reversed during heating, although some hysteresis
is observed. Furthermore, a power law analysis of the SANS data indicates
the existence of domains with well-defined interfaces on the submicrometer
length scale, probably as a result of the appearance and growth of
microscopic voids in the glassy matrix. Because of the unusual combination
of small and wide scattering data used here, the present results provide
new physical insight into the structure of aqueous glasses over a
broad temperature and length scale, leading to an improved understanding
of the mechanisms of temperature- and water-induced (de)Âstabilization
of various systems, including proteins, pharmaceuticals, and biological
objects