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Some Properties of Water

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Some Properties of Water Some properties of water Hydrogen bond network Solvation under the microscope Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 1 Water solutions Oil and water does not mix at equilibrium essentially due to entropy Substances that does not mix with water Substances that does mix with water hydrophobic hydrophilic – Lipids. – Hydrogen peroxide – Carbohydrates – Salts Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 2 Water is asymmetric and polar Water molecule is highly polar • Almost all electron density shifted from hydrogen towards oxygen. • Average electron density around O in a water molecule about 10 times that around H. • Latter are essentially naked protons. Water molecule (slightly) bent Angle between O and two Hs almost equal to value of 109.5° found in perfect octahedron. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 3 Water is asymmetric and polar Water molecule is highly polar • Almost all electron density shifted from hydrogen towards oxygen. • Average electron density around O in a water molecule about 10 times that around H. • Latter are essentially naked protons. Water molecule (slightly) bent Deviation due to steric hindrance between O lone pairs, which repel bond between this atom and H. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 4 Water is asymmetric and polar Water molecule has net permanent dipole moment Two highly polarized OH bonds forming an angle other than π add up to create 1.8 D net dipole moment In addition to dipole, water has higher non-zero spherical multipole moments: - linear quadrupole - square quadrupole - linear octupole - cubic octupole Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 5 Water is asymmetric and polar Water is highly polar High net dipole moment gives water high polarization. Water molecules align themselves: • With respect to one another and with respect to ions. • In external electric field (partly counterbalances thermal agitation). Without electric field With external field • Microwave oven use this principle to induce high frequency oscillations of water molecules (eventually contained in food). Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 6 Water H-bond network A water molecule can form up to four H-bonds • Each water can accept two Hs via O lone pairs, and donate two Hs. • Since lone pairs and OH bonds sit on almost perpendicular planes, local tetrahedral order arises, which creates open structure and 3D bonding network. • Solid water (ice) features indeed tetrahedral arrangement of waters, with two Hs at ~0.1 (arms) nm and two at ~ 0.18 nm (legs) from central oxygen. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 7 Water H-bond network • Typical distance of H-bond in water (0.18 nm between H and O) longer than covalent bond and shorter than sum of H and O atomic radii (0.26 nm). • Due to very small size of H in water, which behaves almost like naked proton. • Tetrahedral arrangement of water arises when T lowers and thermal disorder becomes less dominant water molecules get locked in a perfect crystal featuring void spaces. • Reason for anomalous decrease of density when water cooled below 4 °C. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 8 H-bond forming liquids Boiling points of elements of Groups 15, 16, 17 bound to H atoms Boiling point of compounds containing first element in each group abnormally high (particularly true for water). Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 9 Water H-bond network H-bond in water stronger than in similar H-bonding liquids • H-bonds in water stronger than in other H2X solvents: e.g. hydrogen sulfide (H2S) much weaker H-bond capability than water because of lower electronegativity of S (2.6) compared to O (3.5). • Despite higher bending angle of two Hs around S (92° vs. 104.5°), weaker dipole (~1D) than water. Essentially H2S molecules interact via vdW forces. • H2S gas at room temperature even though it has twice the molecular mass of water. Other H-bonding liquids cannot form so many bonds • Hydrogen fluoride, ammonia, methanol cannot form four hydrogen bonds, either due to an inability to donate/accept hydrogens or due to steric effects. • None shows anomalous behavior of thermodynamic, kinetic or structural properties like those observed in water. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 10 Water H-bond network Liquid water compromise between H-bond optimization and entropy • When T > 273 K water melts because thermal energy partly disrupts ordered arrangement of tetrahedrons into hexagonal array. • Liquid water as collection of many small ice-like fragments. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 11 Water H-bond network Liquid water compromise between H-bond optimization and entropy Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 12 Water H-bond network Partially ordered state maintained in liquid phase. • Each water molecule surrounded on average by ~4 nearest neighbors. • Each water molecule forms on average ~2.5 out of 4 possible H-bonds at any time: - Arrangement may consist of one pair of more tetrahedrally arranged strong H-bonds (one donor D and one acceptor A) with remaining hydrogen bond pair being either ~6 kJ/mol weaker, less tetrahedrally arranged, or bifurcated. - Division of water into higher (4-linked) and lower (2-linked) H-bond coordinated water at any time seen in computer simulations of water dynamics at room temperature. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 13 Water H-bond network Partially ordered state maintained in liquid phase. • Each water molecule surrounded on average by ~4 nearest neighbors. • Each water molecule forms on average ~2.5 out of 4 possible H-bonds at any time: - At room T, X-ray spectroscopy shown that 80% of molecules in liquid water have one (cooperatively strengthened) strong H-bonded OH group and one non-, or only weakly, bonded OH group at any instant (sub-fs averaged). - Remaining 20% made of four-H-bonded tetrahedrally coordinated clusters. - Much debate as to whether such structuring represents the more time-averaged structure... even if instantaneous H-bonded arrangement is tetrahedral, distortions to electron density distribution may cause H-bonds to have different strengths. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 14 Solvation of nonpolar molecules When solvated, small nonpolar molecules (typically gases) or polar molecules with large hydrophobic moieties get trapped into "cages" of H-bonded, frozen water molecules, called clathrates • Water molecules reorganize around solute forming a clathrate cage (chemical substance consisting of lattice trapping or containing molecules). • Allows atoms to maintain H-bonds with each other in nearly preferred tetrahedral orientation: average number of H-bonds not drop very much when small nonpolar object introduced. Biophysics Course held at Physics Department, University of Cagliari, Italy. Academic Year: 2017/2018. Dr. Attilio Vittorio Vargiu PLEASE NOTE! This material is meant just as a guide, it does not substitute the books suggested for the Course. 15 Solvation of nonpolar molecules When solvated, small nonpolar molecules (typically gases) or polar molecules with large hydrophobic moieties get trapped into "cages" of H-bonded, frozen water molecules, called clathrates • Energetic cost not significant. • Entropic cost important! 1. Waters lining cage cannot point any of four H-bonding sites towards solute and still remain fully H-bonded.
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