Molecular Interactions + - Ionic interactions Dipole-dipole interactions Induced dipoles Hydrogen bond + - - + + - + - + - Intermolecular forces play very significant role in the solubility. There are Coulomb forces acting between charged molecules (ions). Intermolecular interactions between uncharged molecules includes dipole-dipole (Keesom forces), dipole - induced dipole (Debye interactions) and induced dipole - induced dipole interactions (London interactions). These interactions are collectively called Van-der-Waals forces. To account for the energy changes due to these interactions solution theories introduce various solubility parameters, which are determined in experiment. More information about solubility parameters is presented in “Physical Pharmacy” by A. Martin (Lea & Fibiger, Philadelphia, London, 1993, pp. 223-229).

Water Miscibility Solvents with relatively high polarity are miscible in water (acetic acid, ethanol, dioxane) Solvent with low polarity or non-polar are water-immiscible (hexane, petroleum, ether) General miscibility rule is that “similar likes similar”. Solvents with high polarity mix with water, while solvents low polarity or non-polar are water-immiscible. You can get better idea about water miscible and water immiscible solvents by studying the Solvent Miscibility Table.

Effects on Solubility Pressure: increases solubility of gases Temperature: solubility of most gases decreases with increasing T, solubility of liquids and solids may be affected both ways (usually increases with increasing T) Co-solvents: increase solubility by changing the solvent polarity. The solubility (miscibility) depend on the pressure, temperature and presence of third compounds such as co-solvents or salts.

Properties of Solutions Solution - homogeneous mixture Solute - the substance in the mixture present in lesser quantity Solvent - the substance present in the largest quantity Aqueous solution - solution where the solvent is water Solutions can be liquids as well as solids and gases

Examples of Solutions Air - oxygen and several trace gases are dissolved in the gaseous solvent, nitrogen Alloys - brass and other homogeneous metal mixtures in the solid state We will focus on liquid solutions as many important chemical reactions take place in liquid solutions

General Properties of Liquid Solutions Transparent, no visible particles May have color Electrolytes are formed from solutes that are soluble ionic compounds Nonelectrolytes do not dissociate Volumes of solute and solvent are not additive 1 L ethanol + 1 L water does not give 2 L of solution Why? the intermolecular forces of a mixture are different than the ones in the pure substances

Solutions and Colloids Colloidal suspension - contains solute particles which are not uniformly distributed Due to larger size of particles (1nm - 200 nm) Appears identical to solution from the naked eye

Colloidal suspension Solution Tyndall Effect the ability of a colloidal suspension to scatter light See a haze when shining light through the mixture Solutions: light passes right through without scattering Colloidal suspension Solution -light as haze, scatters light -no haze

Degree of Solubility Solubility - how much of a particular solute can dissolve in a certain solvent at a specified temperature Factors which affect solubility: Polarity of solute and solvent The more different they are, the lower the solubility Temperature Increase in temperature usually increases solubility Pressure Usually has no effect (solids and liquids) If solubility is of gas in liquid, directly proportional to applied pressure

Saturation Saturated solution - a solution that contains all the solute that can be dissolved at a particular temperature Supersaturated solution - contains more solute than can be dissolved at the current temperature How is this done? Heat solvent, saturate it with solute then cool slowly Sometimes the excess will precipitate out If it doesn’t precipitate, the solution will be supersaturated

Water as a Solvent Enthalpy of Solution. Water is often referred to as the “universal solvent” Excellent solvent for polar molecules Enthalpy of Solution. The enthalpy of solution of a substance is the energy change when one mole of a substance dissolves in water. endothermic (absorbs energy) AgNO3(s) + 22kJ/mol → Ag+(aq) + NO3-(aq) (NH4)NO3(s) → NH4+(aq) + NO3-(aq) + 2.42 kJ/mol Exothermic (releases energy)