Hydrophilic Chain Length for Octylphenol Polyoxyethylene\nEther Adsorption at the <i>n</i>‑Hexadecane–Water\nInterface: Theoretical and Experimental Study

With\nthe advancement of technologies for developing tight and shale\nreservoirs, nonionic surfactants have garnered significant attention\ndue to their remarkable properties. The structure of these surfactants\nplays a crucial role in determining the characteristics of the oil–water\ninterface, particularly influencing emulsification behavior and crude\noil recovery. This study investigates the effect of varying the number\nof hydrophilic polar groups (<i>n</i> = 10, 20, 30, 50)\nin octylphenol polyoxyethylene ether (OP-<i>n</i>) on its\nadsorption behavior at the <i>n</i>-hexadecane–water\ninterface using molecular dynamics simulation. The impact of these\nvariations on interfacial properties was further analyzed through\nmeasurements of interfacial tension and observations of emulsion droplet\nmorphology. The study results indicate that variations in the number\nof hydrophilic polar groups significantly affect interfacial properties.\nIncreasing the number of hydrophilic polar groups led to a notable\nincrease in the thickness of the <i>n</i>-hexadecane or\nwater phase, as well as the thickness of the water or oil layer and\nthe surfactant layer. Moreover, when the number of hydrophilic polar\ngroups reached 20, the OP-<i>n</i> molecules exhibited a\nmore curled conformation at the interface, enhancing their ability\nto encapsulate water and resulting in a decrease in the diffusion\ncoefficient of the molecules in each phase. Additionally, interfacial\ntension was found to be positively correlated with the number of hydrophilic\npolar groups and remained unchanged beyond a certain emulsion diameter.\nThis study provides a theoretical basis and reference data for optimizing\nsurfactant structures to improve crude oil recovery.