Omar Z. Sharaf

The stabilization (resistance to delamination) of a dispersed system can be thermodynamic and kinetic. Thermodynamic is measured by a decrease in the Gibbs energy of the system. The main contribution is made by the Gibbs surface energy. It is equal to the change in surface tension at the interface multiplied by the change in surface. Kinetic depends on the viscosity of the medium, the density of the nanoparticles ... and is measured by the settling time of the particles. Surfactants (stabilizer) reduce surface tension. High molecular weight surfactants can reduce the surface tension and viscosity of the medium.

Dispersion - reduction in size and interfacial surface of particles. A mill can disperse, but not a surfactant (reagent). Therefore, dispersion and stabilization are different processes. However, low molecular weight surfactants are classified as dispersing agents, for example, upon receipt of critical microemulsions. In these systems, dispersion and stabilization are difficult to separate.

Dear Dr. Omar Z. Sharaf,

According to my understanding, the key difference between dispersant and surfactant is that the dispersant improves the separation of particles in a suspension whereas the surfactant is a substance that can lower the surface tension between two phases of matter. A dispersant is a form of surfactant. But all surfactants are not dispersants.

Dear all, it is like the difference between suspension and emulusion. The stabilizer maintains mainly the drops against coellecence and setteling by Gravity. On the other hand, the dispersant role is essentially the formation of droplets. My Regards

Omar Z. Sharaf

The stages in making a stable dispersion of any powder in water are:

• Wetting
• Separation (the key step)
• (Possible) Stabilization

Additives to the system are called admixtures in general terms.

A surfactant (= surface active agent) is simply a wetting agent that will lower the surface tension and allow the powder intimate contact with the liquid. If the powder wets in the liquid then no surfactant is needed. A change of liquid can also lower the surface tension. 'Thin' liquids such as hexane or heptane should wet non-polar solids well and there are few options for surfactants here. Usual problem here is that the powder has been prepared from a polar liquid and thus the solid will clump in hexane or heptane as it is hydrophillic not hydrophobic (like prefers like is a good rule). No surfactant will help here - a polar continuous phase needs to be selected.

A stabilizer is used if, on separation (e.g. by sonication), the particles rapidly re-agglomerate. Placing the optimum level of charge on the particles (phosphates are the classic one for inorganic oxides) or a polymer attached to the particles (30 - 50 kDa PEG or PEI) confers charge and steric stabilization respectively. There's always an optimum level of a charge (mistakenly, IMHO, called electrostatic) stabilization. Too much ionic additive is as bad as too little and precipitates the solid by destruction of the double layer - this precipitate is much denser than the precipitate formed with too little charged additive.

Some more information in these webinars:

• Dispersion and nanotechnology

https://www.brainshark.com/malvern/vu?pi=zHSznNgprz5chz0

• PST & BDAS - An acronym approach to laser diffraction method development https://www.brainshark.com/malvern/vu?pi=zFhzA6idVz5chz0

BTW, the only 'dispersing agent' is energy.

A pigment dispersion is stabilized by dispersing agents in order to prevent the formation of uncontrolled flocculates. The resultant suspension is stabilized due to the adsorption of binder species or molecules at the pigment surface.

The spontaneous wetting process (on wetting solid surfaces) is driven by minimization of the free surface energy. Forced wetting processes (in non-wetting conditions) require the application of external force, and spontaneous de-wetting will take place when the force is removed. Thermodynamic condition for wetting requires the work of liquid/solid adhesion to be as high as possible and, for unlimited wetting, at least more than a half of the work of cohesion (Wk) is required. Sometimes an agent could both function as a stabilizer and as a dispersant. A good example is aniline. Go through this document for details please. Article Aniline as a Dispersant and Stabilizer for the Preparation o...