Adsorption theory for polydisperse polymers.

Most polymers are polydisperse. We extend the self-consistent field polymer adsorption theory due to Scheutjens and Fleer to account for an arbitrary polymer molecular weight distribution with a cutoff chain length Nmax. In this paper, the treatment is restricted to homopolymers. For this case a very efficient scheme is proposed for which the enumerations to compute adsorption characteristics for a complete set of molecules {ni} is only a factor of 2 larger than for monodisperse polymers with a chain length Nmax. For polydisperse polymer, adsorption fractionation takes place, with the long chains preferentially adsorbing from dilute solutions. This fractionation depends on the volume/surface ratio and the polymer concentration; it leads to rounded adsorption isotherms when Nw/Nn>1. It is shown that the hystereses between the adsorption and desorption isotherms, which is often observed, can be explained from an equilibrium point of view. The full calculations corroborate the trends predicted by a simple model proposed by Cohen Stuart et al. In addition, in the numerical method many more details are retained

Power Law Behavior of Structural Properties of Protein Gels

Whey proteins are globular, heat-sensitive proteins. The gel structure, the formation of this structure, and the rheological properties of particulate whey protein isolate (WPI) gels have been investigated. On increasing the NaCl concentration, the permeability of the WPI gels increased, indicating a coarsening of the gel structure, confirmed by confocal scanning laser microscopy pictures. Only a part of the total amount of protein present contributed to the gel network at the gel point (the primary spatial structure). Large variations were observed in the amount of aggregated material at the gel point (and thus the primary spatial structure) as a function of NaCl concentration, due to differences in the kinetics of the denaturation/aggregation process. After the gel point more protein is incorporated in the gel network by “thickening” the strands in the gel and “decorating” the pores in the gel, apparently without changing the gross spatial structure. Power law behavior was found for the permeability dependence of aged gels on the amount of aggregated material at the gel point. For various salt concentrations the curves coincided to one master curve. This power law behavior is consistent with a primary spatial structure of fractal flocs with a fractal dimensionality of 2.4. The elastic modulus is remarkably related (via a power law) with the total amount of protein contributing to the gel network, in contrast to permeability