MadSci Network: Physics |
No, in general you can't know the viscosity of a mixture from just knowing the pure components. This is actually the case for any mixture property (density, vapor pressure, refractive index, etc.), but for many properties you can make a good estimate based on pure-component numbers, especially for systems where the components are chemically similar. The situation for viscosity is worse than for most properties in that there is not even a theoretical "ideal" mixture behavior to use as an approximation. There have been various procedures developed to estimate mixture viscosities. The most common is the Grunberg equation, which is: ln(VS) = sum(Xi*ln(VSi)) + sum(sum(Xi*Xj*Gij)) which for a binary mixture reduces to: ln(VS) = X1*ln(VS1) + X2*ln(VS2) + X1*X2*G12 where VS is the viscosity, VSi is the viscosity of pure component i, Xi is the mole fraction of component i, and Gij is a parameter that has to be fitted to binary viscosity data between components i and j. For mixtures of similar nonpolar components (like simple hydrocarbons of similar molecular weight), it is often not a bad approximation to set the Gij to zero. Another common approximation, again mainly good for similar nonpolar components, is to use a mole-fraction average of the cube roots of the viscosities. I should mention that the large difference in viscosity between your two components probably means that the simple methods for "similar" fluids would be less likely to apply. So if you really need a good value you might have to find some binary data for your mixture (the place to look would be the book by D.S. Viswanath, "Data Book on the Viscosity of Liquids," Hemisphere Publishing, New York, 1989) or else take some yourself. If you have access to a good library, you can find more about estimating viscosities for mixtures in the book by Reid et al. "The Properties of Gases and Liquids", and in a journal review by W.D. Monnery et al., Canadian Journal of Chemical Engineering, vol. 73, pp. 3-40 (1995). Allan Harvey, Physical & Chemical Properties Division, NIST "Don't blame the government for what I say, or vice-versa."
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