M4 Self-Assembly

Topic

According to DLVO theory, the van der Waals potential—which is always attractive between two particles in this colloidal suspension—and the electrostatic potential—which is repulsive between those same two particles when they carry surface charges of the same sign—combine to form the total DLVO potential; this potential governs the net interaction between them at any given separation distance. If the attractive van der Waals potential dominates at a specific separation distance, the total DLVO potential becomes net attractive at that distance; conversely, if the repulsive electrostatic potential dominates, the total DLVO potential becomes net repulsive at that distance.

The range of this repulsive electrostatic potential—the distance over which it remains significant before decaying—is determined by the Debye length. This length measures how far the ionic screening (exerted by ions dissolved in the surrounding solution on each particle's surface charge) extends into the solution; the higher the concentration of these ions, the shorter the effective range of the repulsive electrostatic potential becomes. Thus, a higher concentration of dissolved ions results in a shorter Debye length and, consequently, a shorter range for the repulsive electrostatic potential. For this reason, this model—characterized by the total DLVO potential and the Debye length—serves as the theoretical foundation for another model in this set; the latter describes the loss of colloidal stability in this same suspension when the net DLVO potential balance ceases to be sufficiently repulsive. Conversely, when this net DLVO potential balance remains repulsive as the particles approach but becomes slightly attractive at a greater distance, the particles can spontaneously organize into ordered superlattices, rather than aggregating irreversibly or remaining completely randomly dispersed. The fact that this repulsive electrostatic potential—upon which the net DLVO potential balance depends—can be deliberately modified through surface functionalization (according to yet another model in this set) means that such functionalization effectively provides a way to control whether the particles organize into ordered superlattices, aggregate uncontrollably, or remain stable and dispersed.

ID:4459