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• Study investigates the molecular level interaction of crude oil (heavy and light) with different surfactants. • The key findings demonstrate the effect of crude oil components, surfactant type, salinity and pH on oil solubilization. • Crude oil composition influences the micellar solubilization for designing injection fluids for effective EOR. Surfactant assisted enhanced oil recovery holds significant potential for the future. The phase behavior study along with core flooding experiments are the general practice in the petroleum industry for proper selection of customized surfactant solutions for enhanced oil recovery (EOR). However, the detailed molecular-level mechanisms of oil solubilization within the micellar microenvironment of the formed microemulsion is still not well understood for EOR application with different oil composition, or for developing algorithms for appropriate selection of surfactant for optimum recovery from crude oil reservoirs. Therefore, an attempt has been made in this work to understand the interaction mechanisms of crude oil solubilization in surfactant solution under different physicochemical conditions. This study characterizes two crude oil samples (heavy oil − HO and light oil − LO) and reports their interaction behavior with surfactant solutions (commercial surfactant-SA and non-ionic surfactants-SN) using UV–visible and fluorescence spectroscopic techniques under saline and pH condition. The results demonstrate that the commercial surfactant (SA) entraps polycyclic aromatic hydrocarbons (PAH) with comparatively smaller fused aromatic ring (FAR) into the surfactant micelle, whereas, the non-ionic surfactant (SN) incorporates medium and large size FAR from both heavy (HO) and light crude oil (LO). The addition of salt (sodium chloride) to the non-ionic surfactant shifts the peak of the spectra towards a higher wavelength demonstrating the entrapment of large size FAR. Further, the addition of alkali (NaOH) in the saline-surfactant solution cause change in pH of the solution where at higher pH relatively smaller size PAH are solubilized in the micellar microenvironment. The present study demonstrated that oil solubilization changes with change in the physicochemical condition prevailing in the reservoir condition and has deep-rooted implications for designing surfactant solutions for EOR from crude oil reservoirs. [ABSTRACT FROM AUTHOR]