Thus small changes in energy can lead to large changes in the position of the equilibrium. T is the temperature in Kelvin, and D S is the entropy. Conversely, repulsive interactions will disfavor binding. Entropy is a thermodynamic measure of the degree of disorder of a system. Since a decrease in the number of particles corresponds to less disorder, complexation that decreases the number of particles in the system is thermodynamically disfavored.
Molecular recognition relies on how well two molecules fit together.
If the guest is too large to fit into the cavity of the host, complexation cannot occur. If the guest is too small, it may not form strong noncovalent attractions to the host because the distance between the atoms of the host and guest may be too far for strong attraction. This is a measure of the strength of the noncovalent boning between the host and guest. The type of bonding is the same in each case, namely ion - dipole. These features influence the observed binding selectivity.