While the size the nanoparticles is decreased to some extent on increasing the concentration of the surfactant means, what is the nature of / the role of surfactant on the nanoparticles? Also what happen to the surface tension of nanoparticles? Please explain!! Thanking you in advance!!!
Surfactants controls the growth of nanoparticles while synthesis as well as prevents agglomeration.
As you increase the concentration of surfactant, during the synthesis process it caps the developed nanoparticles thus it's further growth is prevented. As the nanoparticles gets capped by surfactant its surface energy get decreased thus it prevents interaction with another nanoparticles hence prevents agglomeration.
All the best.
Surfactants adsorb on the surface of the nanoparticles, they adsorb with that part of the molecule which matches the surface tension of the nanoparticle to the best degree (let's say: the "polar" side of the surfactatnt); then, the other side of the surfactant is available for the matrix (water, solvent, polymer, whatever you have) to adsorb on that surface.
That means: now, the apparent surface of the nanoparticle is not anymore the nanoparticle surface, but the surface formed by the other side of the surfactant (let's say: the unpolar side).
Therefore, as much surfactant is needed as the surface of the nanoparticles (if fully dispersed) requires or offers for adsorption. Hence, if you start with a too low amount of surfactant, not all of the nanoparticles surface can be covered, so many of the nanoparticles will still be agglomerated.
With addition of more (and finally enough) surfactant, all of the available nanoparticle surface can be covered, and then you have complete dispersion.
furthermore, you asked "Also what happen to the surface tension of nanoparticles? "
The surface tension of the nanoparticle remains unchanged, but during the adsorption of the surfactant, the *interfacial tension* (in the interface between nanoparticle surface and surrounding medium) has changed, and for the dispersion matrix, it "appears" that the nanoparticle surface tension has changed, but in fact: the new surface tension is created by the surfactant's (unpolar) molecule side which after adsorption forms a new surface.
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