SAXS for Nanoparticles and Colloids
Nanoparticles and colloids are at the heart of many emerging technologies, from pharmaceuticals and energy storage to electronics, coatings, cosmetics, and food science. The field is increasingly focused on precise control of particle size, shape, surface chemistry, and self-assembly to create materials with enhanced functionality, while also improving sustainability, scalability, and manufacturing consistency.
Challenges
Colloidal stability is fragile.
Nanoparticle suspensions are prone to aggregation, causing loss of function, sedimentation, or in biopharmaceuticals, greater immunogenicity risk.
Self-assembly is hard to control.
Some nanoparticles self-assemble into ordered arrays and lattices, which is necessary to their function. This is controlled by interparticle forces, which can be inconsistent and difficult to reproduce.
Nanoscale parameters are interlinked and hypersensitive.
Size, surface-to-volume ratio, internal architecture, and dispersion/assembly state all simultaneously govern material properties, making it difficult to control any single parameter without affecting the others.
Complexity makes function harder to reach.
Core-shell particles, hybrid nanostructures, anisotropic colloids: as designs grow more sophisticated, translating structure into consistent, reproducible performance gets harder.
Solutions
USAXS, SAXS, and WAXS give nanoparticle and colloid scientists statistically robust measurements of particle size, shape, internal structure, and interparticle interactions in solution, matrices and powders. Spanning from atomic-scale crystallinity to micron-scale aggregates with time-resolved capability allows tracking of nucleation, growth, self-assembly, and destabilization kinetics under realistic conditions, linking nanoscale organization directly to colloidal stability and performance.
Information obtained
Particle size distribution
Important to control surface area, dispersion and stability of nanosuspensions, emulsions and other colloids, which are critical to product quality.
Particle shape/form factor
Particle shape is also important to control surface area, dispersion and stability but also reveals orientation and anisotropy, which influence rheology, conductivity and connectivity.
Particle concentration
By measuring the scattering intensity the concentration can be determined using known particle size distribution and the scattering contrast with the medium. This unlocks the mass per unit volume of the material, which is critical to its performance.
Core-shell structure and thickness
The shell layer can adapt the surface properties of the core particle and provide and a barrier to access, which controls, stability of the particle, surface activity and ingress/egress of molecules to and from the core.
Aggregation state and fractal dimension
SAXS can reveal the early onset of aggregation and the aggregation structure including its density through fractal dimensions. This gives insight into the propensity for the material to aggregate and its mechanism.
Ostwald ripening
This causes a reduction in surface area as smaller particles dissolve and redeposit on larger particles impacting stability.
Interparticle interactions
Knowing whether particles are attracted to or repelled from each other gives insight into whether they will aggregate and sediment or remain suspended and maintain a stable product.
Data example
In this example, gold nanoparticles were prepared as 20nm monomodal dispersions with different polydispersity and bimodal dispersions of 40 and 60nm at different ratios. Separately, 30nm gold dispersions were diluted from 1.4 x1011 particles per mL by factors of 5, 10 and 100. The data illustrates that compact laboratory SAXS systems can resolve diverse populations including multimodal systems with size ratios less than 2:1, and quantify their concentration.
Figure 1. SAXS scattering profiles and fits (in red) performed using the Expectation-Maximization optimization
algorithm. The insets show the associated size distributions corresponding to each fit and the transmission electron
microscopy (TEM) images of each sample.
Figure 2. Relative number concentration determined from the SAXS profile as a function of the relative dilution determined gravimetrically when preparing the dilutions.
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