Colloids like-charge

Crocker J C and Grier D G 1996 When like charges attract the effects of geometrical confinement on long-range colloidal interactions Phys. Rev. Lett. 77 1897-900... 

But time is money. The waste industry, therefore, breaks the colloid artificially to remove the particulate solid from the water. They employ one of two methods. Firstly, they add to the water an inorganic polymer such as silicate. The colloid s thermodynamic stability depends on the surface of its particles, each of which has a slight excess charge. As like charges repel (in consequence of Coulomb s law ... 

A charged object immersed in an electrolyte solution attracts ions of opposite charge and repels ions of like charge, thereby creating an electrical double layer. Thus, the resistance of two colloidal ions to coagulation is due primarily to repulsion of the interpenetrating electrical double layers. 

The use of membranes for separating particles of colloidal dimensions is termed dialysis. The most commonly used membranes are prepared from regenerated cellulose products such as collodion (a partially evaporated solution of cellulose nitrate in alcohol plus ether), Cellophane and Visking. Membranes with various, approximately known, pore sizes can be obtained commercially (usually in the form of sausage skins or thimbles ). However, particle size and pore size cannot be properly correlated, since the permeability of a membrane is also affected by factors such as electrical repulsion when. the membrane and particles are of like charge, and particle adsorption on the filter which can lead to a blocking of the pores. 

In this discussion of colloid stability we will explore the reasons why colloidal dispersions can have different degrees of kinetic stability and how these are influenced, and can therefore be modified, by solution and surface properties. Encounters between species in a dispersion can occur frequently due to any of Brownian motion, sedimentation, or stirring. The stability of the dispersion depends upon how the species interact when this happens. The main cause of repulsive forces is the electrostatic repulsion between like charged objects. The main cause of attractive forces is the van der Waals forces between objects. 

The DLVO theory [1,2], which describes the interaction in colloidal dispersions, is widely used now when studying behavior of colloidal systems. According to the theory, the pair interaction potential of a couple of macroscopic particles is calculated on the basis of additivity of the repulsive and attractive components. For truly electrostatic systems, a repulsive part is due to the electrostatic interaction of likely charged macroscopic objects. If colloidal particles are immersed into an electrolyte solution, this repulsive, Coulombic interaction is shielded by counterions, which are forming the diffuse layer around particles. A significant interaction occurs only when two double layers are sufficiently overlapping during approach of those particles. 

Dispersion-flocculation processes are generally controlled by double layer swelling, adsorbed hydrolyzed Fe or Al, and chemical bridging (tactoid formation) (Stumm and O Melia, 1968). Once dispersed, clay colloids are kept dispersed by repulsive double layers (Van Olphen, 1971). The force of repulsion is related to the thickness of the double layer (see Chapter 4). This dimension is represented by the ions concentrated near the oppositely charged colloid surface. Any colloid that has a net negative or a net positive charge repulses a like-charged colloid. 

The particles in a colloid usually have an electric charge. These like-charged particles repel each other, so they do not collect into larger particles that would settle out. If you added acid to milk, the acid would neutralize the charge, and the particles would coagulate and settle to the bottom of the container. 

---