Recently, the structure of a binary liquid of tert-butanol/toluene was explored in a confined situation and compared with the bulk liquid [1]. Using contrast matching from neutron scattering, it has been shown that a mixture of toluene (TOL) and tert-butanol (TBA) with a TBA/TOL volume fraction composition of 56/44% confined in cylindrical silica (MCM-41) of radius of 24Å can separate [1], whereas this mixture is fully homogeneous in the bulk phase. Using a core-shell (CS) model, we have also depicted a possible full nanosegregation such that TBA is located close to the silica surface, whereas TOL is found at the center of the pore. This peculiar structure is ascribed to the HBs between the TBA molecules and the silanol (SiOH), where the interfacial density is approximately 1.5 and 3.5 OH/nm2[2,3]. Although the combination of neutron diffraction experiments and contrast matching have highlighted the presence of heterogeneity, the local organization and microscopic processes controlling this peculiar CS structure remain to be clarified.
Therefore, using advanced molecular dynamics (MD) and Monte Carlo simulations, we captured the CS structure and clarified the microscopic driving force that governs it. MD simulations of pure TBA, pure TOL and TBA/TOL mixtures were thus conducted in confined and bulk phases.For consistency with the experiments, the confined medium corresponds to a hydrophilic cylindrical silica nanopore with a radius of 24Å.
We show that the nano-confinement induces strong heterogeneity, causing the absence of the extinction of Bragg's peak from neutron diffraction measurements. Ultimately, we highlight the underlying mechanism of formation of nanophases.
[1] A. R. A. Hamid, R. Mhnna, R. Lefort, A. Ghoufi, C. Alba-Simionesco, B. Frick, andD. Morineau, J. Phys. Chem. C 120, 9245 (2016).
[2] A. Ghoufi, I. Hureau, D. Morineau, R. Renou, and A. Szymczyk, J. Phys. Chem. C 117, 15203 (2013).
[3] A. Ghoufi, D. Morineau, R. Lefort, and P. Malfreyt, J. Chem. Theory Comput. 6, 3212 (2010).
Advances in molecular simulation , Interfacial and confined phenomena
