The influence of the nanocurvature on the surface interactions and molecular dynamics of model liquid confined in cylindrical pores

Abstract

Currently, a great effort is put on the understanding of the effect of the nanometrical constraint on the dynamics of the soft materials. Recent studies by Alexandris et al. [Macromolecules,2016, 49, 7400–7414] and some of us [J. Phys. Chem. C, 2019,123, 5549–5556] revealed that the enhancement of the molecular mobility and depression in the glass transition of various low and high molecular weight glass formers infiltrated into Anodic Aluminum Oxide (AAO) templates strongly correlates with the strength of the interfacial interactions between the materials and the porous medium. However, in those investigations, one very important and fundamental aspect related to the variation in the wettability, the surface tension as well as the interfacial energy due to surface curvature has been completely overlooked. Herein, we have performed systematic and unique Atomic Force Microscopy (AFM) measurements on the model glass-forming liquid, glycerol (GLY), incorporated into porous AAO membranes of varying pore diameter. It was found that with increasing degree of confinement, the adhesion force between GLY and AFM tip decreases significantly. It seems that as indicated by the Tolman relation, the interfacial tension of the confined GLY drops leading to a better wetting of polyalcohol within the smallest pores. Interestingly, for this particular system (confined within templates of d = 10 nm), a complementary Differential Scanning Calorimetry (DSC) investigations revealed the presence of the two glass transition temperatures upon heating runs. This indicated that the enhancement of the interactions between alumina and alcohol leads to the formation of the interfacial layer that vitrifies at higher temperature (Tg,interfacial) than the bulk material. Interestingly, the observed double Tgs appeared only on heating. Moreover, different thermal protocols revealed the variation of the heat capacity jump corresponding to the glass transition temperature of the interfacial layer upon the annealing experiments, indicating the ongoing desorption process. In addition, it was also found that the structural dynamics of GLY incorporated within d = 10 nm starts to deviate below Tg,interfacial from the behavior of the non-confined sample. Reported data are the first experimental evidence on the correlation between variation of the surface interactions with the substrate's curvature and enhancement of the dynamics of the confined liquids. Thus, a better understanding of the dynamics of confined soft matter, especially the relation between the finite size and surface interactions can be gained.