Where does affinity collide?

In chemistry, affinity is the tendency of substances to react or combine: the electron affinity of an atom is the energy released when it gains an electron, and in thermodynamics De Donder's affinity is a state function that measures how far a reaction is from equilibrium and which way it will run. Hotop and Lineberger's 1985 Binding energies in atomic negative ions: II compiled measured electron affinities across the elements, one concrete form of the reactive tendency. Kondepudi and Prigogine's Modern thermodynamics sets out the affinity of a reaction as the negative change in Gibbs energy per unit of advancement. The referent is a physical driving force toward combination, measured in energy units, not the closeness of a molecular bond already formed, a person's liking, or a mined co-purchase.[1][2]

Unit

A reaction's driving force toward combination, in energy per mole

Recovery means

A reaction proceeds until its affinity reaches zero at equilibrium, when the forward and reverse tendencies balance

Measured by

Electron affinities from photodetachment and spectroscopy; reaction affinity from the change in Gibbs energy, or from how far a system sits from equilibrium

In immunology and biochemistry, affinity is the strength of a single binding interaction between two molecules, such as one antibody arm and its antigen or a ligand and its receptor, and it is summarised by the equilibrium dissociation constant Kd, where a smaller Kd means tighter binding. This is distinct from avidity, the combined strength of many such bonds acting together, so a low-affinity antibody with several arms can still bind a target firmly through high avidity. Rudnick and Adams's 2009 Affinity and avidity in antibody-based tumor targeting separates the two and shows how each shapes where a targeting molecule goes and how long it stays. The referent is the tightness of one molecular interaction at equilibrium, not a bulk reactive tendency, a social liking, or a data pattern.[3]

Unit

The tightness of one binding interaction, as an equilibrium constant Kd

Recovery means

A bound pair dissociates at a rate set by the off-rate, and Kd is the ratio of off-rate to on-rate at equilibrium

Measured by

Estimated from binding kinetics by surface plasmon resonance, equilibrium dialysis, or titration, and reported as Kd

In marketing and sociology, affinity is a natural liking or felt connection: affinity marketing builds on a person's existing attachment to a group, cause, or brand, as when a card or product is offered through an alumni body or charity that its members already identify with. Macchiette and Roy's 1992 Affinity marketing: What is it and how does it work? describes how a marketer partners with an affinity group so that loyalty to the group transfers to the offer. The idea is close to homophily, the tendency of similar people to associate, and to brand affinity, a consumer's felt closeness to a brand. The referent is a psychological and social liking between people, groups, and brands, not an energy of reaction, a molecular binding constant, or an algorithm's rule.[4]

Unit

A person's felt liking or closeness to a group, cause, or brand

Recovery means

Not applicable

Measured by

Gauged by self-reported identification and preference surveys, and by behaviour such as membership, repeat purchase, and response to affinity offers