Energy-related engineering applications involving shales pose ever-increasing challenges for the constitutive modelling of these geomaterials. The present work proposes a new definition of effective stress developed by using a thermodynamic approach associated with the concepts of continuum mechanics. The developed expression makes explicit the dependence of the effective stress on the chemical composition of the pore water. The paper shows that the proposed expression is able to improve the interpretation of unsaturated shear strength envelopes that would be achieved by using other formulations.