When the addition of the differential amount of component $$i$$ $$\mathrm{d}n_{i}^{\sigma }$$ or $$\mathrm{d}n_{i}^{\text{s}}$$ is effected at constant pressure $$p$$, the differential molar @E02141@ of adsorption, $$\Delta _{a}H_{i}^{\sigma}$$ or $$\Delta _{a}H_{i}^{\text{s}}$$ also called the isosteric @E02141@ of adsorption ($$q^{\text{st}}$$) is defined as: $\Delta _{a}H_{i}^{\sigma} = -q^{\text{st},\sigma} = U_{i}^{\sigma} - H_{i}^{\text{g}}$ $\Delta _{a}H_{i}^{\text{s}} = -q^{\text{st},\sigma} = H_{i}^{\sigma} - H_{i}^{\text{g}}$ where $$H_{i}^{\text{s}}=(\frac{\partial H^{\text{s}}}{\partial n_{i}^{\text{s}}})_{T,p,m,n_{j}^{\text{s}}}$$ and $$H_{i}^{\text{g}}$$ is the partial molar @E02141@ of component $$i$$ in the gas phase, i.e. $$(\frac{\partial H^{\text{g}}}{\partial n_{i}^{\text{g}}})_{T,p,n_{i}^{\text{g}}}$$