Molecular dynamics simulation of short hydrophobic chain extended surfactants: Effects of PO and EO groups

Short hydrophobic chain extended surfactants ( s -CES) exhibit excellent interfacial properties and good solubility, and have great potential for development and application in enhanced oil recovery . In this work, we designed three kinds of ethylhexyl polyoxypropylene polyoxyethylene ether carboxylates (C 8 P 15 E m C, m=15/10/5), and analyzed the effects of partial substitution of the hydrophobic chain with oxypropylene (PO) groups by molecular dynamics (MD) simulations and interfacial tension (IFT) experiments. The simulation calculated the stretch degree, spindle tilt angle, and rising height of PO chain, as well as the overall stretch degree of oxyethylene (EO) chain. The results revealed that the PO chain had a coiled spiral configuration with two PO units rotated by 90° or eight PO units in a helical loop. In systems with shorter EO chains (m=10/5), the orientation of PO units near the oil phase was more parallel to the interface. However, the EO chain extended obliquely to the aqueous phase and became flatter as it went deeper into the water. Concurrently, the middle EO units tended to curl when the EO chain was longer (m=15/10). The IFT experiments found that the extended surfactants with an optimal PO chain showed good interface performance when PO units were 15. The antagonism of hydrophilic-lipophilic balance and size effect made the s -CES require a 10 EO chain to improve the interfacial activity, which had the greatest effect on IFT reduction. This study has important guiding significance for the molecular design of s -CES and their application in enhanced oil recovery. The long PO chains of three short hydrophobic chain extended surfactants have a helical structure with the eight PO units coiled in a loop. The EO chains are inclined towards the aqueous phase, and the middle part of longer EO chains is curled. C 8 P 15 E 10 C with the best spatial matching of hydrophilic and hydrophobic parts is the most effective in IFT reduction.