Various experimental methods are presented to evaluate the physical properties of microcapsules. The membrane thickness can be obtained either by means of optical and electron microscopy or from the mean diameter, density, and weight of microcapsules. For microcapsules of very thin membrane the use of a certain type of cell elastomer and the application of Laplace's law permit of determination of the membrane tension. The permeability of microcapsules is conveniently known from the rate of solute entry into the microcapsules after mixing a microcapsule suspension and a test solute solution. It is possible to evaluate the equivalent pore size of very thin microcapsule membranes on the basis of the experimentally determined percentage of ruptured microcapsules in solutions of different concentrations of those solutes which can permeate the membranes with the help of nonequilibrium thermodynamics. The total pore volume of microcapsule membrane can be estimated from the zeta potential of microcapsules containing a polyelectrolyte solution as a function of membrane thickness.
