Influence of soluble parts in gelatin/water gels on their network structure in an ultracentrifugal field

Abstract

If gels are investigated in an analytical ultracentrifuge by means of sedimentation equilibrium experiments, so called soluble parts which are originally not incorporated into the network influence the swelling pressure equilibria of the gels. For gelatidwater, it can be shown that an irreversible process takes place which is dependent on the rotational speed of the experiment. Experimental evidence is given that soluble parts which are partly associated and aggregated to the polymer network can act as a branching and/or crosslinking agent. A model for this process is given. In such a case an application of the results of the swelling theory, e. g. that of Flory and Huggins assuming a constant number of network junctions, cannot be applied anymore. A universal procedure is described how swelling pressure equilibria can be identified, although the crosslinking density is changed. It can be shown that the gradient of the soluble parts, respectively the additional structural changes of the gel by the soluble parts, is reproducible and with it the swelling pressure equilibria. Furthermore the radial distribution of dyed soluble parts inside the gel phase is presented as a function of time during a sedimentation equilibrium run with a gelatidwater gel. It can be shown that the back diffusion process of the soluble parts in the gel is very slow and does not lead to the sedimentation-diffusion equlibrium of free soluble parts in the considered time interval of 4 months. In combination with the detected fast sedimentation of the soluble component in the gel phase this gives further evidence for the association of the soluble parts to the gel network forming a gradient gel. The predicted formation of the anisotropic gel could be proved by a swelling experiment. a).

Analytical Ultracentrifuge Technologies for the Characterization of Biopolymer Gels and Microgels

“…If such an altered gel swells again at a lower rotational speed, this gel should not swell up uniformly in an excess of solvent after such an ultracentrifuge experiment. This feature could be verified when a sector shaped gel piece swelled up to a nearly rectangular shaped one (Colfen and Borchard, 1995). Hence, the crosslinking density at the meniscus gel/sol must be lower (leading to the uptake of more solvent) than that at the cell bottom, as it was predicted.…”

“…1994). The suggestion to stain the soluble component selectively with an UV -marker to detennine its sedimentation equilibrium concentration profile is difficult to realize, although possible (Colfen and Borchard, 1995). It becomes quite obvious that the ternary system ~ which considers the soluble components with their molar mass distribution as one single component -is difficult to handle experimentally due to the limitations of the present optical ultracentrifuge detection systems.…”

“…The influence of soluble parts on the swelling pressure-concentration curves e. g. the entire network structure of the gel was the subject of a further study (COlfen and Borchard, 1995). An alkaline treated gelatin of Mn = 68,000 g/mol containing soluble parts with a M ... of 2,900 glmol was investigated.…”

“…The interpretation of this phenomenon was that soluble parts, which partly associate or even aggregate, preferably at the higher gel concentrations of the deswollen geillear the cell bottom, form new crosslinks with neighbouring polymer network chains and hence leading to the formation of a gradient gel (Colfen and Borchard, 1995). If such an altered gel swells again at a lower rotational speed, this gel should not swell up uniformly in an excess of solvent after such an ultracentrifuge experiment.…”

“…It was pointed out that the additional crosslinking does not allow the investigation of the original gel structure anymore (COlfen and Borchard, 1995). As this is the aim of the ultracentrifuge experiments, future investigations of gels with soluble parts causing additional crosslinking have to be carried out at rotational speeds below the critical value without the sedimentation of the gel phase (this is the 'Gradient method')~ Under such conditions, the crosslinking of associated/aggregated soluble parts is unlikely due to the much smaller concentration gradients inside the gel, as we consider below.…”