Diffusion constant molecular weight relationship

CR, cryoscopic method DV, diffusion constant and intrinsic viscosity EB, ebullioscopic method EG, end-group titration IV, intrinsic viscosity-molecular weight relationship in other solvents LS, light scattering MV, melt viscosity-molecular weight relationship OS, osmotic pressure PR, analysis of polymerization rate SD, sedimentation and diffusion constants SE, sedimentation equilibrium (Archibald s method) SV, sedimentation constant and intrinsic viscosity [see Eq. (72)]. 

The relationship can also be cast in terms of distributions of molecular weights for polymer systems. This transformation is achieved by relating the decay constant to its corresponding diffusion coefficient D which in turn is related to molecular weight through a Mark-Houwink like relationship. Equation 3 shows the fundamental relationship between the autocorelation function and polymer molecular weight distributions. 

Capillaries in the Brain (the Blood-Brain Barrier). Capillaries in the brain are less permeable than capillaries in other tissues. This limited permeability, which is frequently called the blood-brain barrier, is essential for brain function. Reduced permeation provides a buffer that maintains a constant brain extracellular environment, even at times when blood chemistry is changing. The basis for this lower permeability is the relative paucity of pores in the brain endothelium. Therefore, molecules that move from blood to brain must diffuse through the endothelial cell membranes. As expected from this observation, the permeability of brain capillaries depends on the size and lipid solubility of the solute. In general, molecules that are larger than several hundred in molecular weight do not permeate into the brain. Empirical relationships between cerebrovascular permeability and the oil / water partition coefficient have been developed [26] (see Figure 5.27) ... 

A more appropriate application of a molecular weight parameter has been demonstrated in a QSAR study of the multidrug resistance CR of tumor cells [346], where the MW term stands for the dependence of CR values on diffusion rate constants. The relationship between MW and volume implies that /MW, corresponding to a linear dimension of size, i.e. being directly related to diffusion rate constants [347], should be better suited than log MW (n = 29 r = 0.871 s = 0.394) [346], which indeed is the case (eq. 56) [348]. 

---