S.M.Ali Dadfar

Carry out a Stokes' Law calculation as per ASTM E2490-09 (15). What is the material and its density? The definition of nanoparticles is < 100 nm so sub-micron may be a better description. You can thicken the continuous phase with cornflour or glycerol to increase the viscosity. This is probably really the only way to try to keep the material in suspension if it is large or dense. I assume (as you're talking about zeta potential) that you're talking of aqueous suspension. Otherwise, a suitable silicone oil could work.

Alan F Rawle Just FYI, in the pharmaceutical industry, nanoparticle generally refers to sub-micron. And many classically-trained colloid scientists in academia quite happily do likewise. Personally, I've never understood the arbitrary notion of 100nm as a cut-off size. I use the sub-micron idea with most of the people I deal with, though I usually prefer to call them what they are: colloidal.

S.M.Ali Dadfar Why are you trying to increase the stability? How does insufficient stability affect the performance of your product?

Sometimes it is advantageous not to make a dispersion too stable. I have solved many problems for clients by telling them to deliberately aggregate their dispersions! Without knowing exactly what you need to achieve, you may get inappropriate advice.

Dear Ali,

Nanoparticle types and sizes are so important factors for increasing stability. To be honest, nanoparticle size is so big and huge nanoparticles tend to precipitate. However, I think, you can increase stability by using a surface agent (like gum arabic, SDS,...) considering the previous situation. Furthermore, you may accommodate a mechanical homogenizer to keep homogenized the nanofluids, all the time (if possible).

It is not clear what your particles are but I will assume you already have some kind of stabilizer already. In my experience, a combination of steric and electrostatic stabilization provide a very robust colloidal stability even at relatively high electrolyte concentrations (like PBS). I would try something like HPMC and SLS or DOSS.

Niels Ryde Thank you so much for your useful comment. I already tried some surfactants to increase the stability of the particles, but none of them were successful.

John Francis Miller Thank you so much for your helpful response. I would like to use the particles for injection. Hence, I need long stability in the aqueous phase.

Alan F Rawle Thank you so much for your kindness. I will try to increase the viscosity by adding glycerol or PEG and hope will get stable particles.

Gökberk Yalçın Thank you so much for your nice comments. Unfortunately, I can't use homogenizer as I need long time stability (static)

S.M.Ali Dadfar Unless you have particles with exactly the same density as the liquid, you will always get some sedimentation. If your application requires after some long duration of static storage that you redisperse your material, this is where deliberately making the initial suspension as stable as possible may cause problems. How you deal with that depends on your application. If you need fully dispersed particles, then you will need to maximize stability. But that comes at the cost of "caked" sediments that can be very difficult to agitate sufficiently. However, if you only need to ensure the correct amount of dispersed material and can accept some aggregation, a less stable system can be very helpful. Examples include unit dose oral suspensions of pharmaceuticals.

I have solved a number of issues related to this by deliberately NOT trying to stabilize the formulation. Sometimes the worst thing you can do is increase the viscosity or reduce the particle size to slow down sedimentation. The slower the sedimentation, the more likely you will have redispersion issues for products stored under static conditions for extended periods (months/years).

If your sedimentation leads to material that is irreversibly lost to the bottom surface of the container, you can minimize the problem by using tall/thin storage vessels instead of short/wide ones.

John Francis Miller

Very minor comment on 'Unless you have particles with exactly the same density as the liquid, you will always get some sedimentation'. This isn't strictly true - gravitational sedimentation will be in competition with free Brownian motion as the sizes get smaller and smaller (there's a good table in Terry Allen's Particle Size Measurement text). Below a certain size then the particles will be in free suspension and will never settle (a Stokes' Law calculation may show settling times of days or weeks) - a true colloidal dispersion (yes, I use the term colloid too!) A good example was the gold sols (another word used!) made by Michael Faraday in the 1850's that were still stable into the 1940's when they were bombed out of existence (apparently).

I agree with the rest of your commentary in relation to densely packed sediments. Other problems can also arise through solubility as well as zeta potential issues.

Alan F Rawle You are right, of course. I should have clarified my comment was addressing large particles.

I had the same problem even my particles were in size around 80 nm, but they sediment . Another problem occurred when making nano-phytosome of bio-active compounds is the bitter taste which is completely unacceptable. If I encapsulated the nano-phytosome with chitosan it will be bigger, may be 50-150 nm larger. but when I added milk, no sedimentation occurred.

Hi S.M.Ali Dadfar ,

I think a good way to tackle your question is to evaluate the the mass-transfer Péclet number in your system, defined as the ratio of differential settling (advective transport) to Brownian motion (diffusive transport) of the particles.

This will lead to the appended expression for the Péclet number (Pe), where Dp is particle diameter, rhop and rhobf are density of your particle and continuous phases, kappaB is Boltzmann's coefficient (in units of J/K), T is temperature, and g is gravitational acceleration.

When Pe ≫ 1, diffusive transport is negligible relative to advective transport (i.e., your particles will rapidly sediment), whereas when Pe ≪ 1, advective transport is negligible relative to diffusive transport (i.e., your particles will remain dispersed for a long time).

I don't think the use of surfactants (if that is what you meant by additives) will help if your particles are too heavy to remain dispersed. As you can see, it is all a matter of particle size and the density difference between your two phases. These are the parameters I'd focus my attention on if I want to reduce the probability of sedimentation. In other words, either reduce your particle size, increase the density of your continuous phase, or reduce the density of your particles.

If due to application constraints you cannot do anything about the solvent, and if you cannot reduce the particle size, then perhaps you can consider a core-shell configuration for your particles (where the core is made of a lighter material) or a mesoporous structure.

Dear Omar Z. Sharaf,

What's about the effect of T (appears in Pe equation) on Pe and the stability?

Salam Aimen Zeiny ,

A high temperature increases Brownian diffusion (which has a more pronounced effect for nanoparticles relative to sub-micron/microparticles), which increases the collision frequency between particles. It also reduces fluid viscosity, further facilitating interparticle collisions. In terms of Pe, a higher temperature leads to a lower Pe.

Ultimately, an increase in collision frequency favors the clustering (aggregation and/or agglomeration) of particles, unless they are strongly protected against clustering, e.g., via a thick molecular layer (steric stabilization).

But if the particles are heavy and large enough, i.e., Pe >> 1, the effect of an increase in temperature will be negligible since gravitational force is dominant.

Perhaps more importantly for nanoparticles (especially when steric-protected using surfactants or polymers, which is usually the case), an increase in temperature can lead to a reduction in the affinity between the extended stabilizing segment of the surfactant/polymer and the dispersing medium (conformational transformations) and/or weakening the bond between the anchoring segment of the surfactant/polymer and the nanoparticle (partial or complete detachment). In other cases, it may even lead to decomposition of the surfactant/polymer.

Wa Aleikum Asalam,

I agree with you. From my experimental experience, mostly, high temperature leads to aggregates, although the aggregation rate differs from a nanofluid to another.

Regards,

As Alan noticed you have no nanoparticles yours are sufficiently large to cause settling

You May play with viscosity density

Titus Sobisch It depends on the industry. The pharmaceutical industry typically refers to anything less than a micron as a nanoparticle. This is also true of a lot of academic groups, especially those from the traditional colloid schools rather than other disciplines that have managed to confuse the situation.

(Yes, I'm from a traditional colloid school and worked in the pharmaceutical industry :) )

Not a good situation originated by wish to gain in the nanoparticles hype, but leading to confusion when discussing issues as here natural settling of particles almost micron!