Partial solubility parameters membranes

Krause S (1987) Partial solubility parameter characterization of interpenetrating microphase membranes. In Lloyd DR (ed), Material science of synthetic membranes, ACS Symposium Series 269, Washington DC, p 351... 

Partial Solubility Parameter Characterization of Interpenetrating Microphase Membranes... 

The simplest criterion for a theoretical calculation of the miscibility of the hydrophobic portions of small molecules with the hydrophobic microphase of a membrane polymer appears to be a partial solubility parameter that is, calculation of the solubility parameter of only the hydrophobic portions of the molecules. This can be done using a group contribution approach in which each group (for example, a CHg-group) in the molecule contributes a certain attractive force and a certain volume to the molecule. In this way, a solubility parameter may be estimated for either a portion of a molecule or for a whole molecule. 

The solubility parameter has found previous use in membrane science. Casting solution components and composition have been selected using the Hansen solubility parameters (68-71). The total Hansen solubility parameter, which is equivalent to the Hildebrand parameter (.72), has been used to explain permeation and separation in reverse osmosis (23). Hansen s partial parameters have also been used to explain permeation and separation in pervaporatlon (61). The findings of these studies (61,73) plus those reported elsewhere in this volume (74) do lend credence to the use of 6, 6, and 6, for membrane material selection. 

I would like to thank the National Science Foundation for partial support of this work through Grant Number CPE81-15007. Special thanks to D. R. Lloyd for alerting me to some recent work on the use of solubility parameters in membrane research. 

Clearly, from Fig. 1, the solubility of a solute in an organic solvent correlates very well with the permeability of the Nitella membrane for that solute. But it is also clear that the correlation is only partial. Thus, of two solutes with the same partition coefficient the one with smaller molecular weight would seem to permeate faster. Solute size as well as hpid solubility are both important determinants of permeation rate. The particular solvent chosen, olive oil, seems however to be a very good model for the ability of the membrane barrier to discriminate between the various permeants, since the overall increase in permeability as the structure of the permeant is varied correlates closely with the increase in partition coefficient. Were the two parameters to be strictly linked all the data would fall on the line of unit slope in the figure, the line of identity. Later we shall see cases where the data do not support such a close similarity between certain membranes and model solvents. 

Eq. VI - 74 is the basic equation for liquid transport and it is the same as that for gas transpon (see eq. VI - 46) and vapour transpoit. However, due to the (high) interaction between organic liquids and polymer the permeability coefficient Pj is dependent on composition and temperature. Both solubility and diffusivity are concentration and temperature dependent as have been discussed in chapter V Eq. VI - 74 illustrates the important parameters involved, the permeabilitj coefficient is a membrane- or material-based parameter. Other parameters of interest are the effective membrane thickness I and the partial pressure difference Apj. The permeation rate is inversely proportional to the membrane thickness and proponional to the partial pressure difference across the membrane. In general eq. VI - 74 can be w ritten as... 

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