Debye-Huckel Theory - Polyampholytes like proteins or polyelectrolytes like DNA are called macroions because they are large and because, depending on the solution pH, they may carry a substantial net charge.

As shown in Figure 2.22a, each macroion collects about it a counterion atmosphere of oppositely charged small ions. These small ions, such as Mg²⁺ and Cl^-, are called counterions and they interact with macroions by charge-charge interactions. The counterion atmosphere shields macroions from each other, thus diminishing their ability to interact on the basis of charge.

The thickness of the counterion atmosphere depends on the concentration of counterions present in the solution. the greater the concentration of counterions, the thicker the counterion atmosphere, and the more effective is the electrostatic screening between individual macroions (See Figure 2.22b) The amount of counterions in solution is measured as the ionic strength, I.

,

where i is the number of different counterions in the solution, Mi is the molarity of each counterion 'i', and Zi is the charge of counterion 'i'.

Debye-Huckel theory attempts to measure the radius across which macroions can interact with each other:

,where K is a constant.

The larger 'r' is, the longer the distance two macroions can affect each other. Thus, as the ionic strength (concentration) increases, the smaller the radius with which macroions can influence each other. Another way to say this is that as 'r' gets smaller, macroions must be closer together before they can interact with each other, due to the shielding or shell of the counterions.