Interfaces between disparate phases of matter offer large electrostatic field and density gradients changing the local free energy surface and therefore form a challenging set of problems in current chemical physics. Experiments on DNA microarrays have revealed substantial differences in hybridization thermodynamics between DNA free in solution and surface tethered DNA. We have developed a mean field model of the Coulomb effects in 2-D DNA arrays to understand the binding isotherms and thermal denaturation of the double helix. We find that the electrostatic repulsion of the assayed nucleic acid from the array of DNA probes dominates the binding thermodynamics and thus causes a Coulomb induced blockage of the hybridization. The results explain the effects observed in DNA microarrays: dramatic decrease of the hybridization efficiency and the thermal denaturation curve broadening as the probe surface density grows. We demonstrate application of the theory for evaluation and optimization of the sensitivity, specificity and the dynamic range of DNA array devices.