Salts Dissociate and Shield Other Charges

In this chapter we explore how ions shield charged objects. First we define what we mean by added salt. When you buy a bottle of some charged molecule, such as DNA, it will be electroneutral, so it will already have a stoichiometric complement of counterions. So you may get Na+-DNA or Mg -DNA, for example. With no additional salt, such solutions remain electroneutral. This is called the no salt condition. We are interested in conditions when additional salt is 

The Poisson-Boltzmann Model for the Distribution of Mobile Salt Ions Around Charged Objects 

In the absence of electrolyte, two negatively charged P particles repel each other. But if a salt such as NaCl is added to the solution, the counterions in-teiv ene to weaken the charge repulsions between the two negative P particles, often to the point that other weak attractions can prevail and cause the P particles to associate. In this w-ay charged colloids can be induced to aggregate by the addition of salts. 

River deltas are interesting cases of electrostatic shielding, for example where the Mississippi River meets the ocean. In the upstream fresh river water, fine silt particles are suspended, owing to soil erosion from the surrounding land. Silt particles eire charged colloids. The fresh water carries the silt downstream toward the ocean. Where the fresh water meets the salty seawater, the high salt concentration causes the colloidal silt to aggregate and precipitate to form the macroscopic landforms known as deltas. 

The simplest model of charge shielding and colloidal stability against aggregation was developed around 1910 independently by L-G Gouy (18.34-1926), a French physicist, and DL Chapman (1869-1958), a British chemist. They combined Poisson s equation of electrostatics with the Boltzmann distribution law. 

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