Some properties of water - PDF Free Download

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

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 2

Water is asymmetric and polar Almost all electron density is shifted from hydrogen towards oxygen: average electron density around O in a water molecule about 10 times that around H. 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. 4

Water is asymmetric and polar Almost all electron density is shifted from hydrogen towards oxygen: average electron density around O in a water molecule about 10 times that around H. 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. 5

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 6

Water is asymmetric and polar Water is highly polar High net dipole moment gives water highly 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). 7

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. 8

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 locked in a perfect crystal featuring void spaces. Reason for anomalous decrease of density when water cooled below 4 C. 9

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! 10

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. 11

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. 12

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! 13

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. 14

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. 15

Water H-bond network Liquid water compromise between H-bond optimization and entropy 16

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. 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: - 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. 18

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 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. 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 Energetic cost not significant. Entropic cost important! 1. Waters lining cage cannot point any of four H-bonding site towards solute and still remain fully H-bonded. 2. Outside nonpolar surface water H-bonds are constrained to lie parallel to surface of solute. 23

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 Energetic cost not significant. Entropic cost important! 1. Waters lining cage cannot point any of four H-bonding site towards solute and still remain fully H-bonded. 2. Outside nonpolar surface water H-bonds are constrained to lie parallel to surface of solute. Loss of orientational freedom by water. 24

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. Propane C 3 H 8 dissolved in water: ΔH=-3.2 k B T -TΔS=9.6 k B T ΔF=6.4 k B T 26

Solvation of nonpolar molecules Large non polar objects (C 60 ) are also embedded by clathrate cages 28

Solvation of nonpolar molecules Clathrates have different but well-defined structures 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. 31

Solvation of nonpolar molecules Type I Unit Cell 33

Solvation of nonpolar molecules Type II Unit Cell 34

Solvation of nonpolar molecules Type H Unit Cell 35

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. 36

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) The translational and rotational entropies of every molecule of solute increase upon solvation. Heptanol (C 7 H 15 OH) 37

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) The translational and rotational entropies of every molecule of solute increase upon solvation. However, for any solute molecule many water molecules decrease their orientational entropies. 38

Solvation of nonpolar molecules Entropy key to hydrophobic solvation 6 different ways for any H 2 O to H-bond with two nearest neighbors in liquid water. 40

Solvation of nonpolar molecules Entropy key to hydrophobic solvation 6 different ways for any H 2 O to H-bond with two nearest neighbors in liquid water. 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 42

Solvation of nonpolar molecules Entropy key to hydrophobic solvation Hydrophobic molecules stick together to minimize water-exposed area. 43

Solvation of nonpolar molecules Entropy key to hydrophobic solvation Hydrophobic molecules stick together to minimize water-exposed area. Less surface area exposed: - fewer water molecules loose entropy - lower energetic penalty 44

Solvation of nonpolar molecules Solvation free energy 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. Free energy cost of creating interface (and cavity) roughly proportional to surface area. 47

Large nonpolar molecules Clathrate structures can only form around sufficiently small solutes. When objects too large waters cannot maintain tetrahedral arrangement of H-bonds. 49

Large nonpolar molecules Clathrate structures can only form around sufficiently small solutes. When objects too large waters cannot maintain tetrahedral arrangement of H-bonds. Being less constrained, entropic cost is not as high as for small nonpolar solvents. Enthalpic component of solvation becomes key to solvation (smaller value of average number of H-bonds). 51

Hydrophobicity General definition based on thermodynamic preference for polar vs. non-polar solvents: Hydrophobic substance characterized by positive Gibbs energy of transfer from a non-polar to a polar solvent (depends on solvents!). Quantify hydrophobicity of a small molecular group R by its hydrophobicity constant π = log Soct/wat S 0 oct/wat S ratio of molar solubility of compound R-A in octanol (non-polar) to that in water S 0 ratio of molar solubility of compound H-A in octanol to that in water 54

Hydrophobicity π = log Soct/wat S 0 oct/wat Positive values of π indicate hydrophobicity, negative values indicate hydrophilicity (thermodynamic preference for water as a solvent). 55

Hydrophobicity π = log Soct/wat S 0 oct/wat Positive values of π indicate hydrophobicity, negative values indicate hydrophilicity (thermodynamic preference for water as a solvent). Experiments show that π of most groups do not depend on nature of A. Additive properties of groups: R CH 3 CH 3 CH 2 CH 3 (CH 2 ) 2 CH 3 (CH 2 ) 3 CH 3 (CH 2 ) 4 π 0.5 1.0 1.5 2.0 2.5 57

Solvation of polar molecules Small molecules such as sugars are soluble at room T Some H-bonds are formed between water and solute molecules. Compensate for entropic penalty due to presence of sites functioning only as acceptors or donors on surface of molecule. 59

Solvation of polar molecules Small polar molecules interact electrostatically with water, even if no H-bonds are formed If no H-bonds are formed, entropic penalty must be paid as for nonpolar objects. However, electrostatic interactions compensate for the loss of orientational entropy by waters. 61

Solvation of ions Strong electrostatic (ion-dipole) interactions stabilize the structures formed by water around ions. 62

Solvation of ions Strong electrostatic (ion-dipole) interactions stabilize the structures formed by water around ions. Water molecules around small cations highly polarized Strengthening in their donor H-bonding 63

Solvation of ions 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 2 bond. 64

Solvation of ions Dominant forces on ions (and polar molecules) in aqueous solution Short range chemical interactions involving effective partial charge transfer from the ion or charged atom to water Namely - Spare outer electrons on water interact with cations/positively charged atoms. - H-bonds donated from water interact with anions/negatively charged atoms. 66

Solvation of ions Resulting interactions with water rather 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. 67

Solvation of ions 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 difference from bulk water. 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 - 7-9 shells [~(H 2 0) 400 ] for strongly hydrated cations (but good H-bonding capabilities retained). 70

Solvation of ions (H 2 0) 20 water clathrates surround monovalent cations. 71

Solvation of ions (H 2 0) 20 water clathrates surround monovalent cations. Also in this case, several defined structures possible: 72

Solvation of ions (H 2 0) 20 water clathrates surround monovalent cations. Also in this case, several defined structures possible: - Tetrahedral cavity (c) in puckered water dodecahedra 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. 74

References Books and other sources Atkins, Physical Chemistry 9 th ed., chap. 17 http://www1.lsbu.ac.uk/water/water_structure_science.html 76