Three-step colloidal gelation revealed by time-resolved x-ray photon correlation spectroscopy

Abstract

The gelation of PEGylated gold nanoparticles dispersed in a glycerol-water mixture is probed in-situ by X-ray photon correlation spectroscopy. Following the evolution of structure and dynamics over 104 s, a three-step gelation process is found. First, a simultaneous increase of the Ornstein-Zernike length ξ and slowdown of dynamics is characterized by an anomalous q-dependence of the relaxation times of τ∼q-6 and strongly stretched intermediate scattering functions. After the structure of the gel network has been established, evidenced by a constant ξ, the dynamics show aging during second gelation step accompanied by a change towards ballistic dynamics with τ∼q-1 and compressed correlation functions. In the third step, aging continues after the arrest of particle motion. Our observations suggest that gelation is driven by percolation and stress release as evidenced by anisotropic dynamics once gelation sets in.

Coherent X-ray Scattering Reveals Nanoscale Fluctuations in Hydrated Proteins

“…The corresponding KWW exponents as a function of temperature are shown in Figure e. We observe that the KWW exponent exhibits a minimum from an average value of α ≈ 1.5 to α < 1 at T = 227 K. Similar transitions have been attributed to the emergence of dynamical heterogeneities upon approaching the glass transition temperature, − although here we observe that the exponent values return to α ≈ 1.5 below T = 210 K. This behavior can also be seen directly from the line shape of the g 2 functions, which appear distinctly more stretched at T ≈ 230 K. The observed minimum was reproduced over several beamtimes with similar experimental conditions, indicated by the different symbols in Figure , parts d and e (see Methods).…”

Coherent X-ray Scattering Reveals Nanoscale Fluctuations in Hydrated Proteins

“…The corresponding KWW exponents as a function of temperature are shown in Figure 2e. We observe that the KWW exponent exhibits a minimum from an average value of α ≈ 1.5 to α < 1 at T = 227 K. Similar transitions have been attributed to the emergence of dynamical heterogeneities upon approaching the glass transition temperature [53][54][55], although here we observe that the exponent values return to α ≈ 1.5 below T = 210 K. This behaviour can also be seen directly from the lineshape of the g 2 functions, which appear distinctly more stretched at T ≈ 230 K. The observed minimum was reproduced over several beamtimes with similar experimental conditions, indicated by the different symbols in Figure 2d and Figure 2e (see Experimental Methods section).…”

In situ aggregation and early states of gelation of gold nanoparticle dispersions

“…, we found ξ > 1000 nm for the gel phases formed by particles with longer ligands. 29 This is supported by further parameters, e.g. , the KWW exponent that decreases in this regimes (see ESI†).…”

“…We interpret this as formation of aggregates with still mobile particles showing diffusive dynamics, but with slow aging. Compared to the study on 5k PEG ligands, 29 about a factor of three less water is needed for 2k PEG considering the particle concentration, the larger number of water per PEG ligand, and different coating density. This may result in more PEG–PEG attractive interactions and entanglement leading to network formation and thus gelation for the 5k PEG ligands whereas for the 2k PEG ligands it results in agglomeration with very slow aging.…”

“…12,19–22 Thanks to the attractive interaction, the particles may form amorphous network structures which is characterized structurally by formation of aggregates accompanied by a slow down of the structural dynamics by orders of magnitude. 15,18,23–29…”