A thermodynamic theory of association to a molecule immobilized near a surface has been developed. Exact equations for the binding enthalpy, entropy and equilibrium reaction constant for an immobilized complex are derived. Using Linear Poisson-Boltzmann theory of the electric double-layer interaction between an ion-penetrable sphere and a hard plate allows a closed form evaluation. We briefly discuss application of the theory to a DNA chip at high (1 M NaCl) and low (0.01 M NaCl) ionic strength for dielectric and metallic substrates. Predicted strong electrostatic effects suggest the feasibility of electronic control of DNA hybridization and design of chips avoiding the DNA folding problem.