Detection of Two Glass Transitions on Triton X-100 under Confinement

Abstract

Because of the strong tendency of Triton X-100 to crystallize, inclusion in a SBA-15 matrix of 5.7 nm in pore diameter was used as a means to suppress crystallization. Clear evidence that Triton X-100 exists under confinement in the amorphous and supercooled state is given by X-ray diffraction supported by differential scanning calorimetry and dielectric relaxation spectroscopy. From the thermogravimetric analysis, a loading degree of 50% (wt) was estimated; the decomposition of confined Triton X-100 follows a two-step profile, indicating that molecules are partitioned between bulklike and adsorbed, the latter fraction being in a higher proportion. This allowed the unequivocal detection by DSC of two well-resolved glass transitions (∼20 K apart), which is a remarkable result obtained by conventional calorimetric analysis in confined systems. The two molecular populations have different mobilities giving rise to two different dielectric relaxation processes: an αTr/SBA bulklike process associated with molecules located more in the center of the pores, slightly slowed down relative to bulk α-relaxation, and an S-process due to molecules absorbed at the walls with significantly hindered mobility. Moreover, an MWS process is detected due to interfacial polarization that builds up between the adsorbed Triton X-100 molecules and SBA pore walls whose temperature dependence seems to be correlated with the one of the S-process. The inclusion in SBA-15 revealed to be a good strategy to simultaneously avoid crystallization and achieve a long-term stabilization of the disordered form.