Some properties of water

Some properties of water Hydrogen bond network Solvation under the microscope 1

NB Queste diapositive sono state preparate per il corso di Biofisica tenuto dal Dr. Attilio V. Vargiu presso il Dipartimento di Fisica nell A.A. 2014/2015 Non sostituiscono il materiale didattico consigliato a piè del programma. 2

References Books and other sources Biological Physics (updated 1 st ed.), Philip Nelson, Chap. http://www1.lsbu.ac.uk/water/water_structure_science.html Movies Exercise 3

Water solutions Oil and water does not mix at equilibrium essentially due to entropy Substances that does not mix with water hydrophobic Lipids. Substances that does mix with water hydrophilic Hydrogen peroxide Carbohydrates Salt 4

Water is asymmetric and polar Almost all electron density is shifted from hydrogens towards oxygen: average electron density around O in a water molecule about 10 times that around Hs. The latter are essentially naked protons. Water molecule is highly polar Water molecule (slightly) is bent The angle between O and the two Hs is almost equal to the value of 109.5 found in perfect octahedron. 5

Water is asymmetric and polar Almost all electron density is shifted from hydrogens towards the oxygen: average electron density around O in a water molecule about 10 times that around Hs. The latter are essentially naked protons. Water molecule is highly polar Water molecule (slightly) is bent Deviation due to steric hindrance of O lone pairs which repel bond between this atom and H. 6

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 a net dipole moment of ~ 1.8 D In addition to dipole, water has higher non zero spherical multipole moments: - linear quadrupole - square quadrupole - linear octupole - cubic octupole 7

Water is asymmetric and polar Water is highly polarizable High net dipole moment gives water highly polarizability. 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). 8

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 arise which creates open structure and 3D bonding network. Solid water (ice) features indeed tetrahedral arrangement of waters, with two Hs at ~0.1 nm and two others at ~ 0.18 nm from a central oxygen. 9

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, that becomes like naked proton. Tetrahedral arrangement of water arise when T lowers and thermal disorder becomes less dominant water molecules get locket in a perfect crystal featuring void spaces. Reason for anomalous decrease of density when water cooled below 4 C. 10

H-bond forming liquids Boiling points of elements of Groups 5, 6, 7 bound to H atoms Boiling point of compounds containing first element in each group abnormally high (particularly true for water). Electronegativity decreases monotonically from top to bottom in periodic table. Some additional intermolecular forces of attraction must be significant in addition to induced dipole (van der Waals) forces H-bonds due to permanent dipoles! 11

H-bond forming liquids Boiling points of elements of Groups 5, 6, 7 bound to H atoms Why HF boils at higher T than NH 3? F significantly more electronegative than N. H-F bond more polar than N-H. NH 3 has dipole moment ~1.4 D, HF has ~1.9D. Very strong H-bonding occurs between HF molecules leading to higher boiling point. 12

H-bond forming liquids Boiling points of elements of Groups 5, 6, 7 bound to H atoms Why H 2 O boils at higher T than HF? Despite F is more electronegative than O, H-bond interactions in HF are so strong that HF exists as H-bonded dimers (and even larger clusters) in vapour, whereas H 2 O is only H-bonded when liquid or ice. Not all H-bonds need to broke to boil HF lower boiling point than H 2 O. 13

H-bond forming liquids H 2 O*: N mol 2N mol H-bonds NH 3 : N mol N mol H-bonds HF: N mol N mol H-bonds Why H 2 O boils at higher T than HF and NH 3? Each H 2 O molecule has two H atoms and the O atom has two lone pairs each water molecule can make two H-bonds. HF has only 1 H and NH 3 has only one lone pair on N both can only make only one H bond. * NB: These pictures are only meant to show the H-bond possibilities of each molecule! 14

Water H-bond network H-bond in water stronger than in other similar H-bonding liquids H-bonds in water stronger than in other H 2 X solvents: e.g. hydrogen sulfide (H 2 S) has 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 H 2 S molecules interact via vdw forces. H 2 S 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 in bulky residues. None show anomalous behavior of thermodynamic, kinetic or structural properties like those observed in water. 15

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. 16

Water H-bond network Liquid water compromise between H-bond optimization and entropy 17

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 the 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 shown by modeling. 18

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 up of four-h-bonded tetrahedrally coordinated clusters. - There is 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. 19

Solvation of nonpolar molecules When solvated, small nonpolar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped inside "cages" of H-bonded, frozen water molecules, called clathrates Water molecules reorganize around the solute forming a clathrate cage (a chemical substance consisting of a lattice trapping or containing molecules) Allows maintaining H-bonds with each other with nearly preferred tetrahedral orientation: average number of H-bonds not drop very much when small nonpolar object introduced. 20

Solvation of nonpolar molecules When solvated, small nonpolar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped inside "cages" of H-bonded, frozen water molecules, called clathrates Despite energetic cost not significant, entropic one must be considered, as water loose some orientational freedom. Waters lining cage cannot point any of 4 H-bonding site towards solute and still remain fully H-bonded. Indeed outside nonpolar surface water H- bonds are constrained to lie parallel to surface of solute. 21

Solvation of nonpolar molecules When solvated, small nonpolar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped inside "cages" of H-bonded, frozen water molecules, called clathrates Balance between optimizing electrostatic interactions and reduce loss of entropy. At room T entropic term dominates ΔF of solvation for small nonpolar objects (e.g. when propane C 3 H 8 dissolves we have ΔF=6.4 k B T, ΔH=-3.2 k B T, -TΔS=9.6 k B T. TΔS source of hydrophobic effect: poor water solubility of nonpolar molecules at room T. 22

Solvation of nonpolar molecules Large non polar objects (C 60 ) are also embedded by clathrate cages 23

Solvation of nonpolar molecules Different types of clathrates found Gas hydrates usually form two crystallographic cubic structures (types I and II) and seldom, a third hexagonal structure (type H): Unit cell of type I made of 46 water molecules, forming two types of cages (small and large). Typical guests forming type I are CO 2 and CH 4. Unit cell of type II made of 136 waters, forming also two types of cages (small and large). Formed by gases like O 2 and N 2. Unit cell of type H made of 34 waters, forming three types of cages two small of different type and one huge. Requires cooperation of two guest gases (large and small) to be stable. 24

Solvation of nonpolar molecules Type I 25

Solvation of nonpolar molecules Type II 26

Solvation of nonpolar molecules Type H 27

Solvation of nonpolar molecules Entropy key to hydrophobic solvation Butanol (C 4 H 9 OH) Pentanol (C 5 H 11 OH) Hexanol (C 6 H 13 OH) Heptanol (C 7 H 15 OH) Hydrophobic solvation very complex phenomenon, but something can be understood from experiments, showing a decrease of solubility as T increases. 28

Solvation of nonpolar molecules Entropy key to hydrophobic solvation Butanol (C 4 H 9 OH) Pentanol (C 5 H 11 OH) Hexanol (C 6 H 13 OH) Heptanol (C 7 H 15 OH) Every solute molecule increases its translational and rotational entropies when solvated. However, for any solute molecule many water molecules decrease their orientational entropies. Dissolving more solute particles lead to a larger net decrease of entropy, and thus increasing T makes this cost higher, decreasing solubility 29

Solvation of nonpolar molecules Entropy key to hydrophobic solvation In liquid water 6 different ways for H 2 O to H-bond with two nearest neighbors. When nonpolar molecule inserted in water, number of possible H-bonding configurations reduces to 3, because waters constrained on surface. 30

Solvation of nonpolar molecules Entropy key to hydrophobic solvation In liquid water 6 different ways for H 2 O to H-bond with two nearest neighbors. When nonpolar molecule inserted in water, number of possible H-bonding configurations reduces to 3, because waters constrained on surface. ΔS = S only water S water+hydrophobic ( ) ΔS = N A k B lnw only water k B lnw water+hydrophobic ( ) ΔS = N A k B ln6 k B ln3 ΔS = N A k B ln2 TΔS 0.42 kcal / mol 31

Solvation of nonpolar molecules Entropy key to hydrophobic solvation Hydrophobic molecules stick together to minimize area exposed to water. Less surface area exposed: - fewer water molecules suffer loss of entropy - the lower the energetic penalty Leads to poor solubility and eventually total phase separation. 32

Solvation of nonpolar molecules Solvation roughly proportional to solute surface area Due to short range of H-bond interaction, H-bond network disrupted only in first layer surrounding nonpolar object. Thus, free energy cost of creating interface (and cavity) roughly proportional to surface area. Verified experimentally as solubility of hydrocarbon chains decreases with increasing chain length. 33

Large nonpolar molecules Clathrate structures can only form around sufficiently small solutes. When objects are too large, this strategy does not work, and waters cannot maintain tetrahedral arrangement of H-bonds. Enthalpic component of solvation becomes significant. However, being less constrained, entropic cost is not as high as for small nonpolar solvents. Magnitude of hydrophobic effect for large solutes similar to that for small molecules 34

Solvation of polar molecules Small polar molecules interact electrostatically with water, although do not form H-bonds Since no H-bonds are formed, entropic penalty must be paid as for nonpolar objects. However, electrostatic interactions are formed, which compensate for the loss of orientational entropy by waters Small polar molecules such as sugars are soluble at room T 35

Solvation of ions Strong electrostatic (ion-dipole) interactions stabilize the structures formed by water around ions. Water molecules around small cations are highly polarized, and this polarization causes strengthening in their donor H-bonding: - e.g. H-bond energies of Zn 2+ (H 2 0) 5 HO-H OH 2, ZnCl + (H 2 0) 4 HO-H OH 2, ZnCl 2 (H 2 0) 3 HO-H OH 2 are 426%, 277% and 23% stronger than HO-H OH H-bond. 36

Solvation of ions Dominant forces on ions (and polar molecules) in aqueous solution are short range chemical interactions involving involve effective partial charge transfer from the ion, or charged atom, to water. Namely: - Spare outer electrons on water interact with cations and positively charged atoms. - H-bonds donated from water interact with anions and negatively charged atoms. Resulting interactions with water quite different: - Anion/H-bond interactions enthalpically much greater than cation-lone pair electron interactions for the same size ions due to closer approach of atoms in first case. Solvation energy of monovalent cations and anions well described by continuum model that includes electrostatic, dispersion, and cavity contributions showing that the water molecules outside these influences has little net effect difference from bulk water. 37

Solvation of ions Presence of ions stabilize water clusters over their state in bulk, as they reduce H-bonding exchanges and proton mobility of affected water molecules. Effect on clustering extends out to 3-4 hydration shells (~(H 2 0) 130 ) for weakly hydrated ions and 7-9 shells (~(H 2 0) 400 ) for strongly hydrated cations (but retaining good H-bonding). (H 2 0) 20 water clathrates surround monovalent cations: - Tetrahedral cavity (c) in puckered water dodecahedra may be used by H 3 O + and NH 4 + - Octahedral cavity (d) could be occupied by many monatomic cations and anions having six waters in their (inner) hydration shell (Na +, K +, Cs +, Ca 2+, Cl -, Br - ), whilst allowing a fully hydrogen-bonded second shell - Cubic cavity (b) may be occupied by 3+ charged lanthanoid or actinoid ions with coordination number of eight. 38