Supersaturation

Supersaturation is a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapor of a compound that has a higher (partial) pressure than the vapor pressure of that compound. Supersaturation is a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapor of a compound that has a higher (partial) pressure than the vapor pressure of that compound. Special conditions need to be met in order to generate a supersaturated solution. One of the easiest ways to do this relies on the temperature dependence of solubility. As a general rule, the more heat is added to a system, the more soluble a substance becomes. (There are exceptions where the opposite is true). Therefore, at high temperatures, more solute can be dissolved than at lower temperatures. If this solution were to be suddenly cooled at a rate faster than the rate of precipitation, the solution will become supersaturated until the solute precipitates to the temperature-determined saturation point. The precipitation or crystallization of the solute takes longer than the actual cooling time because the molecules need to meet up and form the precipitate without being knocked apart by the solvent. Thus, the larger the molecule, the longer the solute will take to crystallize due to the principles of Brownian motion. The condition of supersaturation does not necessarily have to be reached through the manipulation of heat. The ideal gas law suggests that pressure and volume can also be changed to force a system into a supersaturated state. If the volume of solvent is decreased, the concentration of the solute can be above the saturation point and thus create a supersaturated solution. The decrease in volume is most commonly generated through evaporation. Similarly, an increase in pressure can drive a solution to a supersaturated state. All three of these mechanisms rely on the fact that the conditions of the solution can be changed quicker than the solute can precipitate or crystallize out. Supersaturated solutions will also undergo crystallization under specific conditions. In a normal solution, once the maximum amount of solute is dissolved, adding more solute would either cause the dissolved solute to precipitate out and/or for the solute to not dissolve at all. Similarly, there are cases wherein solubility of a saturated solution is decreased by manipulating temperature, pressure, or volume but a supersaturated state does not occur. In these cases, the solute will simply precipitate out. This is because a supersaturated solution is in a higher energy state than a saturated solution. A supersaturated solution of gases in a liquid may form bubbles if suitable nucleation sites exist. Supersaturation may be defined as a sum of all gas partial pressures in the liquid which exceeds the ambient pressure in the liquid. Crystallization will occur to allow the solution to reach a lower energy state.(Keep in mind that this process can be exothermic or endothermic). The activation energy comes in the form of a nuclei crystal being added to the liquid solution (or a condensation nuclei when the solution is gaseous). This nuclei can be either added from another source, which is known as seeding, or can spontaneously form within the solution due partly to ion and molecule interactions. This process is known as primary nucleation. It is necessary for the nuclei to be identical to the solute that is crystallizing. This will allow for the dissolved ions to build up on the nuclei and then each other in the process of crystal growth or secondary nucleation. There are a multitude of factors that will affect the rate and order of magnitude with which crystallization proceeds as well as the difference in formation of crystallites and single crystals. A crystallization phase diagram shows where undersaturation, saturation, and supersaturation occur at certain concentrations. Concentrations below the solubility curve result in an undersaturation solution. Saturation occurs when the concentrations are on the solubility curve. If the concentrations are above the solubility curve, the solution is considered supersaturated. There are three mechanisms with which supersaturation occurs: precipitation, nucleation, and metastable. In the precipitation zone, the molecules in a solution are in excess and will separate from the solution to form amorphous aggregates. The excess of molecules aggregate to form a crystalline structure when in the nucleation zone. In the metastable zone, the solution takes time to nucleate. In order to grow crystals while in the metastable zone, the conditions would require the formation of one nucleus while in the nucleation zone, just past the metastable region. The supersaturated solution can then return to the metastable region.