The adsorption of zinc dialkyldithiophosphates on partially stabilized zirconia from hydrocarbon solution

Kinetics, isotherms and enthalpy of adsorption of zinc di-n-alkyl(di-n-alkylphenyl, di-n-dialkylphenyl) dithiophosphates (ZDTPs) lubricating oil additives on zirconia partially stabilized by yttria (PSZ) powder from hydrocarbon solution at temperatures 25 degrees C and 35 degrees C were done. The adsorption enthalpy has been determined using a Montcal calorimeter. ZDTP adsorption isotherms are Langmuir-like. The adsorption quantities and differential molar enthalpies (Delta H-d(a)) of ZDTPs adsorption on PSZ are low for n-alkyl derivatives, the adsorption decreases with the increasing length of the hydrocarbon chain of the additive ZDTP, the unexpectedly for di-n-propyl ZDTP the differential molar enthalpies of adsorption at 25 degrees C are very low, near zero and for di-n-dialkylphenyl ZDTP only at lower coverage ratios are endothermic, at higher surface coverage for this and remaining additives are exothermic. At 35 degrees C all n-alkyl ZDTPs adsorb on PSZ endothermically endothermic, while both aryl ZDTPs exhibit exothermic effects. All ZDTPs are reversibly physisorbed at 25 degrees C and at 35 degrees C except for both mono- and dialkylphenyl ZDTPs, which are weakly chemisorbed.