A model for charging of insulative materials has been developed which quantitatively links the surface chemistry of materials to their work functions, and to the triboelectric charge exchange charge in two-component developers. The proposed model fits within the standard high-density of states model for charging, but proposes bidirectional electron transfer from basic sites on one contacting material to acidic sites on the second contacting material, and equally, from the basic sites on the second material to acidic sites on the first. The result is an "effective" work function for the material surface, the average of the work functions of the surface acidic and basis sites. This model predicts that both surface work functions and negative toner charge increase linearly with the logarithm of the ratio of the toner acid/base constants, K_a/K_b, which can be experimentally measured by inverse gas chromatography. Charge exchange is zero when the toner and carrier acid-to-base constants are identical. The model implies that an insulator's surface Lewis base and acid functional groups are responsible for triboelectric charge exchange. The model can also be applied to insulator to metal charge exchange.