What is the use of surfactant in nanoparticle preparation?
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A surfactant lowers the surface tension at the interface between materials. Your nanoparticles can only grow bigger than a few tens of nanometers by successfully growing larger than their Kelvin barrier. This barrier is determined by the available surface energy and the radius of curvature of the particle. So when the particle is too small and doesn't have enough surface energy to exceed the Kelvin barrier, the low surface tension due to the high radius of curvature won't allow the particle to grow any larger. But if the particle successfully grows larger than the barrier, then the radius of curvature decreases, making a flatter particle with a larger radius, and the surface tension allows the particle to grow even larger into the mesoscopic domain.
The surfactant modulates the available surface energy of the particles so that the surface tension decreases, and the Kelvin barrier moves, allowing more particles to escape the aggregation process and generally lowering the mean particle size.