Water and Solubility PDF

Water and Solubility Prof. Dr. Özlem Dalmızrak Department of Medical Biochemistry Faculty of Medicine Near East University  WATER Structure and Properties Water constitutes 45%-75% of total human body weight. It is distributed in intr...

Water and Solubility Prof. Dr. Özlem Dalmızrak Department of Medical Biochemistry Faculty of Medicine Near East University  WATER Structure and Properties Water constitutes 45%-75% of total human body weight. It is distributed in intracellular (55%) and extracellular (45%) components. As the biological solvent water plays a major role in all aspects of metabolism: absorption, transport, digestion, and excretion of organic and inorganic substances. Hydrogen bonds between water molecules provide the cohesive forces that make water a liquid at room temperature.  Polar molecules dissolve readily in water because they can replace water-water interactions with more energetically favourable water-solute interactions. In contrast, nonpolar biomolecules interfere with water-water interactions but are unable to form water-solute interactions. As a result nonpolar molecules are poorly soluble in water. In aqueous solutions, nonpolar molecules tend to cluster together. Tetrahedral  Hidrogen Bond Formation Donor Acceptor Hydrogen (H) bonds are relatively weak. Bond dissociation energies of; Hydrogen bonds: 23 kJ/mol Covalent O-H bond: 470 kJ/mol Covalent C-C bond: 348 kJ/mol Lifetime of each H bond is 1-20 picoseconds (1 ps= 10-12 s) Almost tetrahedral arrangement of orbitals about the oxygen atom allows each water molecule to form hydrogen bonds with as many as four neighbouring water molecules (The fact is 3.4 molecules).  Hydrogen Bonding in Water................................................ Hydrogen Bonding in Ice  Water forms hydrogen bonds with polar solutes Hydrogen bonds are not unique to water. They form between an electronegative atom (the hydrogen acceptor, usually oxygen or nitrogen) and hydrogen atom bound to another electronegative atom (the hydrogen donor) in the same or another molecule. Alcohols, aldehydes, ketones and compounds containing N-H bonds all form hydrogen bonds with water molecules and tend to be soluble in water. Direction of the hydrogen bond  Water as a Solvent The processes of life require solubilization of large numbers of ions, large and small molecules. Since water is a dipolar molecule and has a high tendency to form hydrogen bonds, it is an excellent solvent for intracellular and extracellular medium. Any molecules that carry groups capable of forming hydrogen bonds can be dissolved in water. Therefore water dissolves hydroxylic compounds, amines, sulfhydryl compounds, esters and ketones. Aliphatic and aromatic hydrocarbons and many of their derivatives, which cannot form hydrogen bonds, are not water soluble.  Not only hydrogen bond acceptors or donors that dissolve well in water. In contrast to most organic liquids, water is an excellent solvent for ionic compounds. Substances like NaCl are very stable. They readily dissolve in water. This is caused by dipolar nature of water. Dipoles interact with ions, so cations and anions in aqueous solution are hydrated. They are surrounded by shells of water molecules, called hydration shells. High solubility of NaCl is caused by two factors: First, the formation of hydration shells is energetically and thermodynamically favorable. Secondly, high dielectric constant of water decreases the charge-charge interaction between Na and Cl ions. Substances that are readily dissolved in water are “hydrophilic” or “water-loving” substances. Water dissolves salts such as NaCl by hydrating and stabilizing the Na+ and Cl- ions, weakining the electrostatic interactions between them.  Nonpolar and nonionic compound does not dissolve in water. Substances like aliphatic and aromatic hydrocarbons, therefore, are called “hydrophobic” or “water-fearing” compounds. A most interesting and important class of molecules are the ones that exhibit both hydrophilic and hydrophobic properties simultaneously. Such amphipathic compounds have a hydrophilic or polar head group and a hydrophobic tail – usually a hydrocarbon. Hydrophylic WATER groups Fatty acid Detergent Phospholipid  WATER Micelles liposome Solutes of all kinds alter certain physical properties of the solvent (Vapor pressure, boiling point, melting point and osmotic pressure). Osmosis: Water movement across semi-permeable membrane driven by differences in osmotic pressure