Performance of palmitoyl diglycol amide and its anionic and nonionic derivatives in reducing crude oil/water interfacial tension in absence of alkali

Palmitoyl diglycol amide (PDGA) and its anionic derivatives, palmitoyl diglycol amide sulfate (PDGAS), palmitoyl diglycol amide carbonate (PDGAC), and nonionic derivatives, palmitoyl monoethanol amide ethoxylates (PMEA-EOn, n = 2, 3, 4), were synthesized, characterized and incorporated into surfactant formulations for reducing Daqing crude oil/connate water IFT by mixing with a lipophilic surfactant, didodecylmethylcarboxyl betaine (diC12B), and a hydrophilic surfactant, cetyldimethylcarboxyl betaine (C16B), in absence of alkali. Suffering from solubility limit PDGA can be incorporated into the formulation at a molar fraction of 0.5 and ultra low IFT can be attained in a total surfactant concentration range, 0.01–0.3 wt.%. The nonionic derivatives, PMEA-EOn (n = 2, 3, 4), with improved solubility in water, can be incorporated into the formulation at a higher molar fraction, 0.6, and ultra low IFT can be attained in a wide total surfactant concentration range, 0.05–0.5 wt.%. The increase of EO number in PMEA-EOn can be compensated by increasing/decreasing the molar fraction of diC12B/C16B in the mixture. The anionic derivatives, PDGAS and PDGAC, however, can be incorporated into the formulations at a much lower molar fraction, 0.25 and 0.29, respectively. Although the increased hydrophilicity of the anionic derivatives can be compensated by increasing the molar fraction of diC12B in the mixtures, and ultra low IFT can be achieved in the same total surfactant concentration range, the overall IFT behavior is not so good as systems containing the nonionic derivatives. It seems that the IFT behavior of these systems correlates well with the cross section area, a∞, of PDGA and its derivatives, those with smaller a∞, give in general better IFT behavior, showing that the cross section area of a surfactant at interface is crucial for selecting surfactants for surfactant–polymer flooding free of alkali.