Measurement of Proton Spin Diffusivity in Hydrated Cementitious Solids

NMR Characterization of Nanoscale Surface Patterning in Mixed Ligand Nanoparticles

Spontaneous phase separation in binary mixed ligand shells is a proposed strategy to create patchy nanoparticles. The surface anisotropy, providing directionality along with interfacial properties emerging from both ligands, is highly desirable for targeted drug delivery, catalysis, and other applications. However, characterization of phase separation on the nanoscale remains quite challenging. Here we have adapted solid-state ¹H spin diffusion NMR experiments designed to detect and quantify spatial heterogeneity in polymeric materials to nanoparticles (NPs) functionalized with mixed short ligands. Janus NPs and physical mixtures of homoligand 3.5 nm diameter ZrO₂ NPs, with aromatic (phenylphosphonic acid, PPA) and aliphatic (oleic acid, OA) ligands, were used to calibrate the ¹H spin diffusion experiments. The Janus NPs, prepared by a facile wax/water Pickering emulsion method, and mixed ligand NPs, produced by ligand exchange, both with 1:1 PPA:OA ligand compositions, display strikingly different solvent and particle-particle interactions. ¹H spin diffusion NMR experiments are most consistent with a lamellar surface pattern for the mixed ligand ZrO₂ NPs. Solid-state ¹H spin diffusion NMR is shown to be a valuable additional characterization tool for mixed ligand NPs, as it not only detects the presence of nanoscale phase separation but also allows measurement of the domain sizes and geometries of the surface phase separation.

Scaling analyses for hyperpolarization transfer across a spin-diffusion barrier and into bulk solid media

By analogy to heat and mass transfer film theory, a general approach is introduced for determining hyperpolarization transfer rates between dilute electron spins and a surrounding nuclear ensemble. These analyses provide new quantitative relationships for understanding, predicting, and optimizing the effectiveness of hyperpolarization protocols, such as Dynamic Nuclear Polarization (DNP) under magic-angle spinning conditions. An empirical DNP polarization-transfer coefficient is measured as a function of the bulk matrix 1H spin density and indicates the presence of two distinct kinetic regimes associated with different rate-limiting polarization transfer phenomena. Dimensional property relationships are derived and used to evaluate the competitive rates of spin polarization generation, propagation, and dissipation that govern hyperpolarization transfer between large coupled spin ensembles. The quantitative analyses agree closely with experimental measurements for the accumulation, propagation, and dissipation of hyperpolarization in solids and provide evidence for kinetically-limited transfer associated with a spin-diffusion barrier. The results and classical approach yield general design criteria for analyzing and optimizing polarization transfer processes involving complex interfaces and composite media for applications in materials science, physical chemistry and nuclear spintronics.

The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate

Despite use of blended cements containing significant amounts of aluminum for over 30 years, the structural nature of aluminum in the main hydration product, calcium aluminate silicate hydrate (C-A-S-H), remains elusive. Using first-principles calculations, we predict that aluminum is incorporated into the bridging sites of the linear silicate chains and that at high Ca:Si and H₂O ratios, the stable coordination number of aluminum is six. Specifically, we predict that silicate-bridging [AlO₂(OH)₄]^5- complexes are favored, stabilized by hydroxyl ligands and charge balancing calcium ions in the interlayer space. This structure is then confirmed experimentally by one- and two-dimensional dynamic nuclear polarization enhanced ²⁷Al and ²⁹Si solid-state NMR experiments. We notably assign a narrow ²⁷Al NMR signal at 5 ppm to the silicate-bridging [AlO₂(OH)₄]^5- sites and show that this signal correlates to ²⁹Si NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a "third aluminate hydrate" (TAH) phase. We therefore conclude that TAH does not exist. This resolves a long-standing dilemma about the location and nature of the six-fold-coordinated aluminum observed by ²⁷Al NMR in C-A-S-H samples.