Cosurfactant‐Mediated Interfacial Flexibility: A Key to Controlling the Morphology of Anisotropic Nanostructures in Cetyltrimethylammonium Bromide Reverse Micelles

Reverse micelles (RMs) have emerged as versatile soft templates for engineering anisotropic nanostructures. However, insights into how cosurfactant-mediated interfacial flexibility governs morphology remain limited, even though this parameter is critical for the rational design of anisotropic nanomaterials. In this study, we examine how cosurfactant-mediated interfacial dynamics in cetyltrimethylammonium bromide (CTAB)-based RMs direct the growth of copper oxalate (CuOx) nanostructures. By employing two distinct cosurfactants-1-butanol and 1-octanol-we demonstrate a morphology switch from nanorods to nanocubes, respectively. Using the solvatochromic probe Coumarin 343 (C343), we probe solvation dynamics to assess interfacial environments of RMs. The observed biexponential solvation behavior indicates heterogeneous water populations and interfacial perturbations during nanostructure formation. RMs containing 1-octanol exhibit significantly retarded water reorientation, indicating a more rigid interface compared to those with 1-butanol, likely due to stronger interactions between the longer alkyl chain of octanol and the CTAB tails. These interfacial differences correlate with the formation of cube-shaped and rod-shaped nanostructures, respectively. Complementary in situ NMR spectroscopy reveals distinct water microenvironments in butanol- and octanol-based RM systems, aligning with the morphological anisotropy observed. Small-angle neutron scattering (SANS) further confirms cosurfactant-induced changes in the micellar size and packing. Collectively, these findings highlight the pivotal role of cosurfactant-mediated interfacial flexibility in governing the formation of anisotropic nanostructures and provide comprehensive insights for controlling morphology in soft template systems.