At adsorption  change  enthalpy and entropy of surface. The sum of these values is the change in Gibbs free energy of the surface. Experimentally easier to determine the enthalpy change.

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In order for adsorption to occur, the Gibbs free energy change must be negative if the process occurs at constant pressure, or the Helmholtz free energy change must be negative if it occurs at constant volume. But, in either case, equivalently and more simply, the change in total entropy must be positive. But adsorption concentrates the substance being adsorbed, and hence is always accompanied by a decrease in configurational entropy. Thus there must always be an overcompensating increase in thermal entropy: adsorption must always be exothermic.