19F NMR Study of Ovalbumin Diffusion in Guar Filter Cake

Abstract

Abstract Enzymatic degradation is commonly used to degrade the filter cakes formed from hydraulic fracturing fluids (guar solution). In this highly concentrated filter cake, enzymatic degradation kinetics is diffusion-controlled. It becomes crucial to understand the diffusion process of enzymes in the polysaccharide gel layer. The transport of a protein that mimics the enzyme protein (i.e. ovalbumin) in concentrated polymer solutions (hydroxylpropyl guar), including both translational diffusion and rotational diffusion, has been studied by NMR. Translational diffusion is required to degrade the filter cake over long length scales, but rotational diffusion is necessary for the enzyme to bind to the guar chains to initiate degradation. The proton NMR signal for protein/polymer systems is too complicated to be applied in practice. A CF3 group, which is chemically tagged to protein by reacting with S-ethyl trifluorothioacetate, is introduced to differentiate the protein from polymer matrix and reduce the complexity. 19F pulsed field gradient NMR is used to measure the translational diffusion coefficient, while T1 and T2 relaxation of the 19F signal is measured to obtain the rotational diffusion coefficient. Translational and rotational diffusion coefficients are compared with Stokes Einstein (SE) equation. Both diffusion coefficients deviate from Stokes Einstein (SE) equation significantly. For translational diffusion, this deviation indicates that on the length scale of the protein size the protein molecule feels inhomogeneity of the matrix polymer. This results in a 67% decrease in protein mobility when the matrix polymer concentration is only 5wt%. The rotational diffusion coefficient is found to stay relatively constant when the tracer size is much smaller than the mesh size of polymer matrix, which is consistent with the SE equation. But when the protein size is comparable to the mesh size of polymer solution, the SE model fails. In this range, proteins only feel a fraction of the hydrodynamic interaction of the polymer matrix and the relationship between the rotational diffusion coefficient and macroscopic viscosity is found to obey a power law. Further, experimental results are compared with effective medium theory and confined diffusion model, from which we can see that proteins are confined by "dynamic" polymer chains.