We study the splitting of single-particle energies between spin-orbit partners (Is splitting) for the isotopic chains of some even-even closed shell nuclei in the Hartree-Fock-Bogolyubov framework. Z = 8, 20, 28 and 50 magic proton nuclei of stability valley and their even-even neighbours are selected and a limited systematic investigation is performed for the isotopic chains of C, O, Si, Ca, Ni, Sr, Sn, Te, and Ce isotopes. They are compared with existing data. The modification of proton spin-orbit partners energy splitting of those isotopes is investigated with valence neutron numbers extending to the neutron drip-line. Our calculation shows that the proton Is splitting in chosen nuclei becomes smaller with the increasing neutron number. This reduction is accounted for two different effects, namely, the effect of neutron diffuseness and tensor interaction. The neutron diffuseness values are also calculated for the nuclei under investigation. The deduced ls splitting reduction is compared for the nuclei with the same isospin number. The larger reduction is attributed to the tensor interaction assuming that the Is reduction due to the neutron surface thickness will be the same for the nuclei having the same neutron diffuseness parameter. The contribution from tensor interaction is mainly accounted for the interaction between the spin-orbit partner proton orbital and the neutron orbital having different angular momentum. Briefly, the effect of increased valence neutron number on proton effective single particle levels is discussed.