The solvent molecules in the immediate vicinity of a solute particle often have a much different ordering than the rest of the solvent, and this area of differently ordered solvent molecules is called the cybotactic region. This stabilizes the system and creates a solvation shell (or hydration shell in the case of water) around each particle of solute. Polar solvent molecules can solvate polar solutes and ions because they can orient the appropriate partially charged portion of the molecule towards the solute through electrostatic attraction. The part with more electron density will experience a partial negative charge while the part with less electron density will experience a partial positive charge. Polar solvents have molecular dipoles, meaning that part of the solvent molecule has more electron density than another part of the molecule. Solvent polarity is the most important factor in determining how well it solvates a particular solute. Photographer: Armin Kübelbeck, CC-BY-SA, Wikimedia Commons Nile red at daylight (top row) and UV-light (second row) in different solvents. The similarity or complementary character of these properties between solvent and solute determines how well a solute can be solvated by a particular solvent. Which of these forces are at play depends on the molecular structure and properties of the solvent and solute. Solvation involves different types of intermolecular interactions: hydrogen bonding, ion–dipole interactions, and van der Waals forces (which consist of dipole–dipole, dipole–induced dipole, and induced dipole–induced dipole interactions). Solvents and intermolecular interactions The units for solubility express a concentration: mass per volume (mg/mL), molarity (mol/L), etc. The typical unit for dissolution rate is mol/s. The consideration of the units makes the distinction clearer. Solubility quantifies the dynamic equilibrium state achieved when the rate of dissolution equals the rate of precipitation. Solvation or dissolution is a kinetic process and is quantified by its rate. Solvation is, in concept, distinct from solubility. The concept of the solvation interaction can also be applied to an insoluble material, for example, solvation of functional groups on a surface of ion-exchange resin. Solvated species can often be described by coordination number, and the complex stability constants. In the solvated state, an ion or molecule in a solution is surrounded or complexed by solvent molecules. Distinction from solubility īy a IUPAC definition, solvation is an interaction of a solute with the solvent, which leads to stabilization of the solute species in the solution. Solubility of solid compounds depends on a competition between lattice energy and solvation, including entropy effects related to changes in the solvent structure. Solvation of a solute by water is called hydration. Solvation is the process of reorganizing solvent and solute molecules into solvation complexes and involves bond formation, hydrogen bonding, and van der Waals forces. Ions are surrounded by a concentric shell of solvent. The surrounded solute particles then move away from the solid solute and out into the solution. If the attractive forces between the solvent and solute particles are greater than the attractive forces holding the solute particles together, the solvent particles pull the solute particles apart and surround them. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the solute, including solubility, reactivity, and color, as well as influencing the properties of the solvent such as its viscosity and density. Solvation describes the interaction of a solvent with dissolved molecules.
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