In the framework of the Ginzburg-Landau (GL) theory in the limit of a GL parameter much larger than unity, we study theoretically the critical behavior of a mesoscopic superconducting ring with negligible width in the presence of a magnetic field applied perpendicularly to the ring plane. It is assumed that the inner ring edge is in contact with a material whose properties are accounted in the de Gennes boundary condition with a parameter b (de Gennes extrapolation length), while the outer edge is in contact with vacuum. Special attention is devoted to the influence of the different materials contacting the inner ring edge of the superconductor on the features of the phase diagram, as well as the effect of the confinement on such thermodynamic property.