Up to this point, the systems under consideration have been homogeneous solutions. For the biological scientist, these solutions represent an extremely simplified set of conditions, those of a system that, while containing a number of constituents, contains them in a single homogeneous phase. (A phase is considered to be a state in which there is both chemical and physical uniformity.) This simplification has been valuable because through it general concepts have been developed and introduced. However, in a biological system, such a presumption is not realistic. The nature of biology is that of heterogeneous systems. Many of the processes in cells and living organisms involve the transfer of chemical species from one phase to another. For example, the movement of ions across a membrane such as the cell membrane or an intracellular organelle is often treated as transport between two phases, one inside and the other outside. The nature of the equilibria that can exist between phases will be the focus of this section. When different phases come in contact with each other, an interface between them occurs. This interface is a surface, and the properties of a surface are different from those of either of the phases responsible for creating it. The thermodynamic treatment of the properties of surfaces will be the subsequent focus of this section.
KeywordsSurface Tension Phase Equilibrium Activity Coefficient Osmotic Pressure Bulk Phase
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