Zeta Potential 

You said similar size nano particles of three different metals (Ag, Cu, Al) disperse in polar solvent (methanol). First of all, you should make sure what are you synthesizing means is it metal oxide nanoparticles, metal dioxide nanoparticle or unioxidized metal core. for example Cu nanoparticles may b CuO, Cu2O, Cu2O3 or Cu nanoparticles. Based on the oxidation state of your nanoparticles you can further go for EDL study. In case of unoxidized metal core nanoparticles, there is no effect of EDL. Also dispersing your nanoparticles in polar solvent will only be feasible if your nanoparticles are hydrophillic. If you synthesize metal nanoparticles using solvothermal method using oleic acid, oleylamine or benzyl ether then your nanoparticles wont get dispersed in polar solvent without suitable surface modification.

As far as DLVO theory is concerned, metal nanoparticles will follow DLVO theory cause theory is more concerned with formation of EDL on surface. If you want to study this phenomenon, concentration on QCM analysis of metal nanoparticles along with XPS analysis. QCM will help you study surface interaction phenomenon based on charge on nanoparticles. Zeta potential wont be so useful as it will give you sum total of charge on nanoparticles which wont be suitable for analyzing surface interaction of charged nanoparticles.

The EDL theory is devoted to a huge number of works. If simply the EDL consists of an adsorption and a diffuse layer. The behavior of ions near the surface is determined by three factors. 1. Intermolecular ( specific) forces.  2..Electrostatic mutual interaction of ions with a charged surface and with each other. 3. Thermal motion of  ions.

The theory of DLVO is designed to assess the stability of disperse systems with an electrical stability factor. It will calculate the energy between two similarly charged surfaces with diffuse ion layers. The repulsion of diffuse ions and their molecular attraction are compared.

The stability of disperse systems is determined by other factors of stability except for the electrical factor..

EDL and DLVO are complementary.

Thank you Yuri Mirgorod sir.

I have three system. 

1) Ag nanoparticles disperse in methanol

2) Al NPs disperse in methanol

3) Cu NPs disperse in methanol

Pure methanol is a polar solvent and do not have ions.

I have measured Zeta potentials and found for silver negative value and for other is positive.

How should I calculate resultant barrier at 7 pH of methanol dispersed metal NPs, using DLVO theory. while it require ions concentration and  oxidation state of ions of in the solvent. My system have just polar solvent not electrolyte.

which factor will involve to differentiate different metals system.