Flocculation

Flocculationisthe process in which particles in a colloid aggregate into larger clumps. Often, the term is used for a reversible aggregation of particles in which the forces holding the particles together are weak and the colloid can be redispersed by agitation. The stability of a lyophobic colloidal dispersion depends on the existence of a layer of electric charge on the surface of the particles. Around this are attracted electrolyte ions of opposite charge, which form a mobile ionic atmosphere. The result is an electrical double layer on the particle, consisting of an inner shell of fixed charges with an outer mobile atmosphere. The potential energy between two particles depends on a repulsive interaction between double layers on adjacent particles and an attractive interaction due to van der Waals' forces between the particles.

At large separations, the repulsive forces dominate, and this accounts for the overall stability of the colloid. As the particles become closer together, the potential energy increases to a maximum and then falls sharply at very close separations, where the van der Waals' forces dominate. This potential-energy minimum corresponds to coagulation and is irreversible. If the 'ionic strength of the solution is high, the ionic atmosphere around the particles is dense and the potential-energy curve shows a shallow minimum at larger separation of particles. This corresponds to flocculation of the particles. Ions with a high charge are particularly effective for causing flocculation and coagulation.

Dipole and dipole-dipole interaction

Dipole is a pair of separated opposite electric charges. The dipole moment (symbol u) is the product of the positive charge and the distance between the charges. Dipole moments are often stated in debyes. The SI unit is the coulomb meter. In a diatomic molecule, such as HC1, the dipole moment is a measure of the polar nature of the bond; i.e. the extent to which the average electron charge is displaced towards one atom (in the case of HC1, the electrons are attracted towards the more electronegative chlorine atom). In a polyatomic molecule, the dipole moment is the vector sum of the dipole moments of the individual bonds. In a symmetrical molecule, such as tetrachloromethane (CC1₄), there is no overall dipole moment, although the individual C-Cl bonds are polar.

Dipole-dipole interaction is the interaction of two systems, such as atoms or molecules, by their dipole moments. The energy of dipole-dipole interaction depends on the relative orientation and the strength of the dipoles and how far apart they are. A water molecule has a permanent dipole moment, thus causing a dipole-dipole interaction if two water molecules are near each other. Although isolated atoms do not have permanent dipole moments, a dipole moment can be induced by the presence of another atom near it, thus leading to induced dipole-dipole interactions. Dipole-dipole interactions are responsible for van der Waals' forces and surface tension in liquids.

Texts from scientific articles

<== попередня сторінка	\|	наступна сторінка ==>
Colloids	\|	Journal of Electroanalytical Chemistry

Не знайшли потрібну інформацію? Скористайтесь пошуком google:

Генерація сторінки за: 0.005 сек.