Porosity of Polyacrylamide Gradient Gels

In 1962 Ornstein and Davis were the first to suggest a formula to estimate roughly the average pore diameter of homogeneous PA gels:

where pav (nm) is the average pore diameter in nanometres and %T is the total acrylamide concentration (g acrylamide + g Bis in 100 mL).

Based on the Ogston model which describes dex-tran gels as assembled from arbitrarily arranged gel rods, Raymond and Nakamichi related the average pore diameter of PA gels to the total polymer concentration (T) as follows:

where K is the factor resulting from the angle in which the gel rods are linked together (1.5), d (nm) is the diameter of a PA gel rod (0.5), p (g cm-1) is the density of gel rod (1.2). This results in:

The largest pore diameter in a PA gel of a certain concentration is, however, much larger than the average pore diameter (Figure 1). Moreover, the largest pore diameter deviates increasingly from the average pore diameter with decreasing gel concentration. The pores therefore are statistically distributed, but the standard deviations of the average pore radii and the distribution function (Gaussian or logarithmic distribution) are unknown.

The generally held assumption of a random mesh-work of cross-linked individual PA rods could not be confirmed by electron microscope images. They revealed sponge-like structures in the submicron range. Such structures are in accordance with the mode in which gels polymerize. PA molecules first arrange as high molecular aggregates that are in the sol state and not interconnected. Thereafter, cross-linkage to a three-dimensional gel occurs: this is indicated by an abrupt start of gelation.

Figure 1 Plot of average pore radius (rav (nm)) against PA gel concentration (T(%)). Triangles, average pore radii calculated as suggested by Ornstein and Davis (1962). Squares, average pore radii calculated as suggested by Raymond and Nakamichi (1962). Circles, maximum pore radii as marked by native proteins of known radius: 1, thyroglobulin; 2, ferritin; 3, catalase; 4, lactate dehydrogenase; 5, bovine serum albumin; 6, ovalbumin. Reproduced with permission from Rothe and Maurer (1986).

Figure 1 Plot of average pore radius (rav (nm)) against PA gel concentration (T(%)). Triangles, average pore radii calculated as suggested by Ornstein and Davis (1962). Squares, average pore radii calculated as suggested by Raymond and Nakamichi (1962). Circles, maximum pore radii as marked by native proteins of known radius: 1, thyroglobulin; 2, ferritin; 3, catalase; 4, lactate dehydrogenase; 5, bovine serum albumin; 6, ovalbumin. Reproduced with permission from Rothe and Maurer (1986).

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