Silica Physical Properties

Silica Nanoparticle Size Control

Spherical silica nanoparticles can be produced across a broad size range, from tens of nanometers to micron-scale particles. Final particle diameter depends on synthesis conditions including reactant concentrations, catalyst concentration, solvent composition, temperature, and the way particle nucleation and growth are controlled.

For Stöber-type silica synthesis, changing the relative concentrations of water, ammonia, and tetraethyl orthosilicate (TEOS) can alter particle growth and final diameter. Careful control over these conditions enables silica particles with narrow size distributions across very different particle sizes.

The ability to produce uniform silica spheres across a broad size range also makes silica useful for measurement and calibration applications. Precisely characterized silica nanoparticles have been investigated as size standards for particle-measurement instrumentation.

Structure, Porosity & Density

Colloidal silica is generally amorphous, meaning its silicon and oxygen atoms do not exhibit the long-range crystalline order found in many other inorganic materials. Instead, silica forms an interconnected siloxane network containing Si–O–Si bonds along with residual silanol groups.

The extent of condensation within this network influences silica density and porosity. More completely condensed silica generally contains fewer residual hydroxyl groups and less internal free volume, while intentionally porous silica can be engineered with substantially greater internal surface area.

Silica Porosity

Silica porosity can be controlled during synthesis and through post-synthetic processing. Changes in condensation, hydrolysis, dissolution, thermal treatment, templating, and etching can alter the amount and organization of internal pore space.

Solid silica nanoparticles have relatively limited internal porosity compared with intentionally templated mesoporous silica nanoparticles. Mesoporous and hollow silica structures can contain substantially larger internal voids and surface areas, allowing molecules to access the particle interior.

For mesoporous silica, pore size and pore architecture can be selected around the molecule being loaded or separated. Use the Mesoporous Silica Selection Guide to compare available architectures.

Silica Nanoparticle Surface Chemistry

Silica surfaces contain silanol groups that provide a versatile starting point for chemical modification. Silane chemistry can be used to introduce amines, thiols, carboxyl-containing linkers, aldehydes, hydrophobic groups, and other functionality depending on the particle and application.

Surface chemistry influences particle charge, colloidal stability, solvent compatibility, molecular adsorption, conjugation, and integration into downstream materials.

Silanol-Terminated Silica

Bare silica nanoparticles contain surface hydroxyl groups and generally exhibit good colloidal stability in water and alcohols under appropriate conditions. The native silica surface can also be modified to introduce alternative functional groups or change compatibility with the surrounding matrix.

Silanol groups produce a negative surface charge under many neutral and basic aqueous conditions. Representative zeta potential measurements of 80 nm silica colloids show an isoelectric region near pH 2, with increasingly negative charge as the pH increases above this region.

Amine-Terminated Silica

Amine-functionalized silica provides reactive primary amine groups for binding and conjugation. These groups can participate in coupling reactions with carboxyl-containing molecules and can react with dyes or other molecules containing appropriate amine-reactive functionality.

Surface amines can become protonated under acidic conditions, producing a positively charged particle surface. Representative zeta potential measurements of 120 nm amine-functionalized silica show an isoelectric region near pH 7.5.

Based on reagent input and the estimated surface area available during functionalization, the amine-functionalized silica process has been estimated to introduce a maximum of approximately 2.5 amine groups/nm². Literature measurements of related functionalized silica systems have reported surface densities on the order of approximately two amine groups/nm². The number of amines accessible for conjugation can be lower because orientation, packing, and incorporation of amine groups below the outer particle surface affect accessibility.

Current amine-functionalized solid silica products are supplied in ethanol to help preserve surface functionality. Because the formulation conditions can be near the isoelectric region of the amine-functionalized surface, some particle sizes may flocculate or settle when surface charge is low. This behavior is distinct from gravitational settling of larger particles and can change after transfer into a different solution environment.

Silica Nanoparticle Colloidal Stability

Colloidal stability describes the ability of silica nanoparticles to remain dispersed rather than aggregating or flocculating. Stability depends on particle size, surface chemistry, solvent, pH, ionic strength, concentration, and the molecules or materials present in the surrounding environment.

For charge-stabilized silica particles, pH and ionic strength influence the electrical double layer and therefore particle-particle repulsion. Amine-functionalized particles can behave differently from native silanol surfaces because their charge changes in the opposite direction as pH changes.

Zeta potential can help characterize this electrostatic behavior, but it should not be interpreted using a single universal stability threshold. The value depends on the particle surface and the conditions under which it is measured.

Silica Solvent Compatibility

Native silica surfaces are generally most compatible with polar solvents. The original compatibility guidance for these silica materials is summarized below:

Surface modification can change these compatibility relationships. Appropriate silanes can be used to produce more hydrophobic or otherwise application-specific silica surfaces when integration into a different solvent or matrix is required.

Silica Dissolution & Aqueous Stability

Silica has low but non-negligible solubility in water. Dissolution depends on particle size, surface area, degree of condensation, pH, temperature, concentration, and surrounding solution chemistry.

Dissolution can increase under strongly basic or acidic conditions, including conditions above approximately pH 8 or below approximately pH 3. At sufficiently dilute concentrations, silica can dissolve from the particle surface until the surrounding solution approaches equilibrium.

This effect can be particularly important for thin silica shells on other nanoparticle cores because relatively little silica is present. Under unfavorable solution conditions and low particle concentrations, substantial shell dissolution can occur over relatively short time periods.

Silica dissolution can also affect surface functionality when functional silanes are incorporated into portions of the silica network that are lost over time. For applications requiring different aqueous stability, surface chemistry or silica-shell composition can be evaluated through Custom Nanoparticle Development.

Silica Nanoparticle Concentration

Silica nanoparticle concentration can be expressed in several ways, including silica mass concentration, particle number concentration, silica mass percent, or optical density. The appropriate unit depends on whether an experiment is controlled by total material mass, number of particles, optical response, or another property.

The table below retains the reference conversions for 10 mg/mL solid silica nanosphere dispersions across a range of particle sizes.

Values are provided as technical reference data rather than a list of currently stocked product configurations. Particle-number calculations depend on particle size and assumed particle density, while optical density depends on particle dimensions, size distribution, formulation, and measurement conditions. Refer to the individual product page and batch-specific Certificate of Analysis for current configurations and measured values.

Choosing Silica Nanoparticle Physical Properties

The appropriate combination of particle size, porosity, surface chemistry, and formulation depends on what the silica particle must do in the final system. Particle diameter influences particle number, settling, external surface area, and scattering, while pore architecture controls internal surface area and molecular accessibility. Surface chemistry determines charge, solvent compatibility, conjugation options, and interactions with surrounding materials.

For size-controlled nonporous particles, explore Solid Silica Nanospheres. For applications that require internal pore volume or high internal surface area, explore Mesoporous Silica and the Mesoporous Silica Selection Guide.

For projects requiring a different particle size, surface chemistry, pore structure, silica shell, solvent, or formulation than the standard portfolio, see Custom Nanoparticle Development.

Custom Silica Architectures

Silica chemistry can also be adapted to create structures beyond conventional solid and mesoporous silica particles. These architectures are typically developed around specific application requirements rather than offered as standard catalog formats.

Aluminosilicate

Aluminosilicate materials incorporate aluminum into the silica network. Partial conversion of a silica surface to aluminosilicate can be used to modify the physical and chemical behavior of the silica layer, including its resistance to dissolution in aqueous environments.

Aluminosilicate can also be incorporated into shells around other nanoparticle cores when a more robust silica-like interface or additional surface functionality is required.

Studded & Composite Silica

Other nanoparticles or functional materials can be incorporated into or attached to silica structures to create composite architectures. For example, fluorescent components can be combined with silica-coated particles to add optical tracking or imaging functionality while retaining the surface chemistry of the silica platform.

For these and other non-standard structures, see Custom Nanoparticle Development.

Selected Literature

1. Kimoto, S. et al. “Characterization of Nanosized Silica Size Standards.” Aerosol Science and Technology, 51(8), 936–945 (2017).
2. Schiestel, T.; Brunner, H.; Tovar, G. E. M. “Controlled Surface Functionalization of Silica Nanospheres by Covalent Conjugation Reactions and Preparation of High Density Streptavidin Nanoparticles.” Journal of Nanoscience and Nanotechnology, 4(5), 504–511 (2004).
3. Liu, S.; Han, M.-Y. “Silica-Coated Metal Nanoparticles.” Chemistry – An Asian Journal, 5, 36–45 (2010).