The molecular structure of water controls its thermodynamic properties. Owing to the molecular shape and dipole moment, water readily forms an H-bonding network, where one molecule can associate with up to four other water molecules. Due to these dipole-dipole interactions, thermodynamic properties of water at temperatures near the freezing point deviate from normal liquid behavior tends. These properties include the density, isothermal compressibility, and heat capacity. Similar to temperature effects, water confined in meso- and nano-pores (&lt; 50 nm) experience unique behavior. Studies have shown when water resides in nano-pores the density, surface tension, and dielectric constant decrease relative to bulk water. Further, vibrational spectroscopy, by probing O-H vibrational modes, demonstrates H-bonding disturbances occurring when water resides in nano-scale pores.<br/>In this study, we explore the relationship between pore size and the impact of nano-scale confinement on water properties. Experimentally we utilized mesoporous silicas with a narrow pore size distribution ranging from 8 to 2 nm. These include SBA-15 series (&lt; 150 mm particles, pores; 8 nm, 4 nm), and <i>ms</i>-silica series (3 mm spherical particles, pores; 4 nm, 2 nm). We measured nitrogen and water adsorption on these silicas, using Brunauer-Emmet-Teller (BET) to quantify surface area and the density of water in the pores. Thermogravimetric analysis and direct scanning calorimetry were performed to assess water desorption and de-hydroxylation as a function of pore size. Lastly, Raman and attenuated total reflection Fourier transform infrared (ATR-FTIR) vibrational spectroscopies were used to probe the H-bonding network of water residing in nano-pores. In sum our results indicate that thermodynamic and physical properties of water, including freezing temperature and desorption of water are pore-size dependent, and when water is confined in nano-scale pores, systematic and predicable deviations occur, likely caused by disruptions in the H-bonding network.

Properties of nano-scale confined water: How confinement disrupts hydrogen bonding networks

256th National Meeting (2018)

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