nonionized

Nonionized, also called unionized, describes a molecule that carries no net electrical charge under the conditions considered. It usually refers to the neutral form of a compound that can exist in both charged (ionized) and uncharged states through protonation or deprotonation.

In aqueous solutions, many weak acids and bases exist in an equilibrium between nonionized and ionized forms. The ratio is influenced by pH and the compound’s acid dissociation constant (pKa) for acids or the base dissociation constant (pKa) for bases. For an acid HA, lowering pH favors the nonionized HA, while raising pH favors the ionized A−. For a base B, lowering pH favors the ionized BH+, while raising pH favors the nonionized B form.

Nonionized molecules typically have greater lipophilicity and can cross lipid membranes more readily than their ionized counterparts, which are usually more water-soluble and less permeable to membranes. This distinction is important in pharmacokinetics, as absorption, distribution, and excretion depend on the balance of ionized and nonionized forms at physiological pH. Environmental chemistry also considers ionization for mobility and bioavailability in water bodies.

Examples include weak acids such as acetic acid, where the nonionized form predominates at pH below the pKa, and the ionized acetate form dominates at higher pH. For weak bases such as amines, the nonionized form prevails at higher pH, while the ionized ammonium form dominates in acidic conditions. Some amino acids can be zwitterionic at physiological pH, carrying both positive and negative charges, and are not typically described as nonionized.