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What is the hydrodynamic diameter (DH) defined as?The hydrodynamic diameter (DH) is defined as the diameter of a hard sphere that diffuses at the same speed as the particle being measured. Only for a perfectly spherical particle, the particle size can be accurately defined as a linearly measured length. For all other shapes, particle sizes have to be measured via equivalent diameter techniques and the DH is one of these.

How is the hydrodynamic diameter (DH) calculated? What physical parameters can affect it?

The hydrodynamic diameter is measured using the Stokes-Einstein equation: The physical parameters that affect the diameter measured by DLS are:

• Sample viscosity
• Temperature
• Refractive index of the dispersant

Is the hydrodynamic diameter (DH) equal to the diameter measured via TEM?

The DH is typically larger than the diameter measured via TEM. In addition to the core of the particle, the DH takes surface structure and ions into account. The DH also includes waters of hydration. Several studies have also shown a gel layer consisting of silica chains to protrude from the surface of a silica nanoparticle when it is in contact with water which results in a higher size being reported.

Then, what is the comparison between the hydrodynamic diameter (DH) and the physical diameter (D) of a silica particle?

In our experience, the DH is approximately 5% larger than the physical diameter.

What does Polydispersity mean?

The term ‘polydispersity’ as per IUPAC defines the non-uniformity of a specific distribution.

In Dynamic Light Scattering, the mean size and the standard deviation from the mean can be obtained directly from the statistics of the distribution. Here, the absolute width of the distribution can be compared to the mean to calculate a relative polydispersity. For a theoretical Gaussian distribution, the overall polydispersity is equal to the relative polydispersity and measured as [width/mean]2.

How do you define a distribution as monodisperse vs polydisperse quantitatively?

A monodisperse distribution has a polydispersity index between 0.0 to 0.1. This is classified as a “narrow” distribution. A polydisperse sample has a PDI between 0.1 to 0.4 and is classified as a “broad” distribution.

What is the contact angle with water for hydrophobic and superhydrophobic silica?

The contact angle for superhydrophobic silica is approximately 150o whereas that for hydrophobic silica is approximately 100o.