Imidazolium-based ionic liquids (ILs) are environmentally friendly compounds and their gemini structure exhibit high surface/interface activity. In this contribution, a homolog series of gemini imidazolium ILs with different spacer lengths, [C4im-Cn-imC4]Br2, n = 2, 4 and 6, were synthesized and utilized for interfacial tension (IFT) reduction. The studied system was crude oil-water for which IFT reduction is crucial in the chemical enhanced oil recovery. The influence of temperature, within 298.2–328.2 K was considered. Results showed a drastic IFT reduction in the presence of ILs to the extent of 58.3, 72.3 and 63.9%, respectively, for n = 2, 4 and 6 at the corresponding CMCs, while temperature could still improve these effects. The observed stronger effect of the mid-length spacer IL (n = 4), at high concentrations, as well as higher CMC, can be attributed to adsorption tendency and compact arrangement at the interface. The Frumkin adsorption model was well fitted to the obtained data and the corresponding parameters were determined. Variation of the obtained parameters was presented at different temperatures and discussed consistently.
