Hydration water structure, hydration forces, and mechanical properties of polysaccharide films

We have used ellipsometry at controlled relative humidity (RH) to measure the equilibrium swelling of ultrathin films of polysaccharides, including native and modified phytoglycogen nanoparticles as well as dextran and hyaluronic acid. At high humidities (RH > 70%), the RH-driven swelling of hydrophilic polymers can be described by exponentially decaying interstitial hydration forces. We have used two complementary approaches to analyse this high RH swelling regime to measure the length scale λ0 that characterizes exponentially decaying hydration forces and to quantify the associated bulk modulus Kp of the films. We further probe this swelling regime using attenuated total reflection infrared (ATR-IR) spectroscopy to investigate the hydrogen bond structure of the hydration water in the polysaccharide films. By combining ellipsometry and ATR-IR spectroscopy we find that the structuring of the hydration water hydrogen bond network in this regime correlates with both the hydration force decay length and film mechanical stiffness. These measurements provide insight into the relationship between the hydration water structure, hydration forces, and mechanical properties of polysaccharides, and suggest that a more ordered water hydrogen bond network leads to a smaller λ0 and larger Kp.