solvation layer

Meaning: A solvation layer (solvation shell; a hydration shell when the solvent is water) is the group of solvent molecules packed around a dissolved particle and oriented by attraction to it. In water, oxygen (δ−) faces cations and hydrogens (δ+) face anions. The attractions are ion–dipole for ions and hydrogen bonding or dipole–dipole for polar solutes. The energy released offsets the energy needed to separate solute particles and solvent molecules, so favorable solvation promotes dissolution. Higher charge density (larger charge, smaller radius) gives stronger solvation.

On the MCAT: Passages ask why NaCl dissolves in water but not in hexane, which ion is more strongly hydrated (Li⁺ over Cs⁺, Mg²⁺ over Na⁺), or how hydrated size affects mobility. Apply "like dissolves like": polar solvents stabilize ions and polar solutes, while nonpolar solvents cannot form ion–dipole attractions.

Common confusion: The shell is not covalently bonded to the ion. These are intermolecular attractions, and solvent molecules in the shell exchange with the bulk solvent. Also, hydrated size typically runs opposite to bare ionic size: small Li⁺ attracts more water, so hydrated Li⁺ is larger than hydrated Cs⁺.