Xanthan hydrodynamic size

In a quasielastic light scattering study of macromolecular solutions, it is essential to establish a clear relationship between the experimental conditions and the type of relaxation phenomena manifest in the light scattering data. To accomplish this, one must perform experiments over a range of scattering angles and solution concentrations. One of our objectives is to determine the translational diffusion coefficient (D ) for xanthan, and to relate this parameter to a hydrodynamic size. 

The HPAM molecule is a flexible chain structure sometimes known as a random coil in polymer chemistry. There is essentially no permanent secondary structure in polyacrylamide which affords it some degree of rigidity in the way that the helical structure acts in xanthan. Like xanthan, HPAM is a polyelectrolyte, and as such it will interact quite strongly with ions in solution. However, since the polyacrylamide chain is flexible, it may respond much more to the ionic strength of the aqueous solvent, and its solution properties are much more sensitive to salt/hardness than are those of xanthan. This is illustrated schematically in Figure 2.11, in which the effect of ionic strength on the hydrodynamic size of the molecule is shown. The effects of ions on the solution properties of polyacrylamide are discussed in more detail in Chapter 3. 

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