On the Decoupling of Relaxation Modes in a Molecular Liquid Caused by Isothermal Introduction of 2 nm Structural Inhomogeneities

Abstract

To support a new interpretation of the origin of the dynamic heterogeneity observed pervasively in fragile liquids as they approach their glass transition temperatures T g , we demonstrate that the introduction of ~2 nm structural inhomogeneities into a homogeneous glassformer leads to a decoupling of diffusion from viscosity similar to that observed during the cooling of orthoterphenyl OTP below T A, where Arrhenius behavior is lost. Further, the decoupling effect grows stronger as temperature decreases (and viscosity increases). The liquid is cresol and the ~2nm inhomogeneities are cresol-soluble asymmetric derivatized tetrasiloxy-based (POSS) molecules. The decoupling is the phenomenon predicted by Onsager in discussing the approach to a liquid-liquid phase separation with decreasing temperature. In the present case the observations support the notion of a polyamorphic transition in fragile liquids that is hidden below the glass transition. A similar decoupling can be expected as a globular protein is dissolved in dilute aqueous solutions or in protic ionic liquids. IntroductionOne of the most intensively researched aspects of glassforming liquids is the heterogeneity that develops in their dynamic properties as temperature is decreased from above the melting point to deep in the supercooled liquid state 1-6 7,8 . It is found that, except for the case of polymer liquids, the behavior at high temperatures is simple: fluctuations, such as those explored by neutron scattering through the self-intermediate scattering function, relax exponentially on time scales that follow the simple Arrhenius equation. Then at temperatures that are usually, but not necessarily, below the melting point, (i) a shoulder develops in the relaxation function, (ii) the longer time component of the relaxation function becomes non-exponential and (iii) the temperature dependence of the relaxation time (both average and most probable) departs from Arrhenius behavior

Effects of 2 nm size added heterogeneity on non-exponential dielectric response, and the dynamic heterogeneity view of molecular liquids

“…Moreover, in that study, η increased when POSS was added, i.e., an increase in the POSS content had the same effect on η as a decrease in T. The increase was interpreted in terms of a "blocking effect" of POSS on the flow lines of m-cresol (see the illustration in Ref. 31), when η of the mixture is already very low. Since the propylamine group of the POSS they used 31 can form hydrogen bonds with the OH group of m-cresol, the flow lines of the m-cresol molecule would not be as simply blocked by the 2 nm POSS molecule as they would be blocked by an inert, 2 nm size spherical particle.…”

“…31), when η of the mixture is already very low. Since the propylamine group of the POSS they used 31 can form hydrogen bonds with the OH group of m-cresol, the flow lines of the m-cresol molecule would not be as simply blocked by the 2 nm POSS molecule as they would be blocked by an inert, 2 nm size spherical particle. In contrast, τ m in our study decreased when the POSS content was increased which shows that η did not increase, i.e., a POSS molecule did not act to block the "flow lines" of the DGEBA molecules.…”

Medium decoupling of dynamics at temperatures ∼100 K above glass-transition temperature: A case study with (acetamide + lithium bromide/nitrate) melts

“…123 At lower temperatures where heterogeneities are longer-lived than τ r , molecular aspects get prominence and a departure from the SED description ensues. Results obtained from experimental studies of reactive and non-reactive dynamics in ionic liquids, 52,124,125 and those from very recent measurements after isothermal introduction of ∼2 nm structural inhomogeneity in an otherwise homogeneous liquid 126 suggest that both the static and dynamic heterogeneities could contribute to the observed decoupling between solute rotation and viscosity in these melts. In the absence of LiNO 3 (f LiBr = 1.0) the extent of decoupling becomes even stronger as one finds p = 0.50 at this composition (see Fig.…”

Density, conductivity, viscosity, and excess properties of (pyrrolidinium nitrate-based Protic Ionic Liquid+propylene carbonate) binary mixture

“…The research concerning PILs has generally focused on surfactant self-assembly [26][27][28][29][30][31][32], and their physical properties [23,24,[33][34]. Due to their excellent catalytic effects and other advantages such as easy preparation, cheap cost, and low toxicity, theirs physicochemical properties have also attracted researcher's attention [35][36][37][38]. For example, recently, Nakamoto et al have reported that PILs are processing candidates as electrolyte for electrochemical storage and conversion devices [39,40].…”