Non-exponential relaxation without dynamic heterogeneity in van der Waals liquids above the melting point

TL;DR

The study tests whether dynamic heterogeneity underpins non-exponential rotational relaxation in van der Waals liquids above their melting point by performing depolarized dynamic light scattering on optically anisotropic probes, both in bulk and diluted with an isotropic solvent. It finds that for single-component liquids (DEP, TBP, C13) the spectral relaxation shape remains non-exponential and largely unchanged upon dilution, indicating negligible dynamic heterogeneity in this regime. In contrast, probes with internal degrees of freedom (P13) or a molecular weight distribution (C14/C7) exhibit dilution-dependent relaxation shapes, revealing explicit heterogeneity effects. The results support a scenario where relaxation mechanisms change between $T_g$ and $T_m$, from heterogeneous near the glass transition to effectively homogeneous dynamics with intrinsic stretching at higher temperatures, challenging the notion that non-exponential relaxation above $T_m$ is solely a signature of dynamic heterogeneity.

Abstract

We investigate the influence of dynamic heterogeneity on the spectral shape of structural relaxation in van der Waals liquids above the melting point by means of depolarized dynamic light scattering. To this end, we study optically anisotropic probe molecules both in the bulk and when diluted in an optically isotropic solvent. Strikingly, the relaxation shape of the probe molecules in dilution is indistinguishable from that of the pure liquid composed of the probe molecules. By contrast, when explicit dynamic heterogeneity is introduced, e.g., through internal degrees of freedom or a distribution of probe molecule sizes, the relaxation shape becomes sensitive to the solvent concentration. These findings indicate that dynamic heterogeneity has a negligible influence on the rotational dynamics of single component van der Waals liquids above the melting point, despite the pronounced non-exponential character of their relaxation shape.

Figures (5)

• Figure 1: a) DDLS spectra of DEP, CCl$_4$ and mixtures of both liquids at a molar fraction of CCl$_4$ of $\Phi_{\mathrm{CCl4}}$. The inset shows the integrated depolarized scattering intensity $I_{\mathrm{tot}}$ normalized by the intensity scattered from pure CCl$_4$. The red line represents an ideal mixing law for the scattered intensity of the mixture. b) DDLS spectra of (a) normalized to the amplitude of the relaxation peak and shifted in frequency to collapse the low frequency flanks.
• Figure 2: Analogous analysis to Fig. \ref{['fig:DEP_dilution']} for probe molecules TBP (a-b) and C13 (c-d).
• Figure 3: DDLS spectra after subtraction of the CCl$_4$ contribution according to eq. \ref{['eq:subtraction']} for DEP (a), TBP (b) and C13 (c).
• Figure 4: DDLS spectra of P13 and mixtures of P13 and CCl$_4$ at 300 K. The spectra have been shifted in frequency and amplitude for comparison of the peak shapes.
• Figure 5: a) DDLS spectra of C14, binary mixture of C14 and C7 at a molar fraction of both constituents of 0.5 and tertiary mixtures of C14, C7 and CCl$_4$ (see text). b) Normalized DDLS spectra after subtraction of the CCl$_4$ contribution according to eq. \ref{['eq:subtraction']}.