2 replies to this topic

[xmb09129]

Hi folks

 

I was hoping for some help on how to determine the effect soap would have on the density and viscosity of glycerol, which it is soluble in, at around 60^oC? 

 

It does not need to be exact values as it is simply for a preliminary design step and it is only present in around 7% by mass.

 

Is it possible for me to find the physical properties of liquid soap and use those to determine an estimated value of density and viscosity change or is this to big an assumption to use?

 

Hopefully somebody can help me out with this

 

Regards

[MrShorty]

Is it possible for me to find the physical properties of liquid soap and use those to determine an estimated value of density and viscosity change or is this to big an assumption to use?

A lot depends on how accurate you want this estimate to be. In either case, without some mixture information, about the only choice you have for these estimates is to assume that density/viscosity of the mixture is a straight line interpolation between the pure component properties.

 

For density, this means assuming simple additivity of the volumes:

 

Vmix=V1+V2 (Vi is the total volume of i)

Vi=mi*V0i (V0i is the specific volume of pure i)

Vmix=m1*V01+m2*V02

If we assume that m1+m2=1, then we can substitute weight fractions (xi) in for mi and the resulting Vmix will become V0mix

V0mix=x1*V01+x2*V02

 

This kind of "ideal" density is usually pretty accurate. I checked Perry's data for water ethanol and, at 50 wt%, the ideal density is about 4% different from the real density.

 

Viscosity is harder to justify, because viscosities of liquid mixtures can do all kinds of things. Just about any good estimate of the viscosity of the mixture is going to require some empirical data from the mixture - especially for compounds like soap and glycerol where you can have a lot of hydrogen bonding that can strongly affect viscosity. For a detailed discussion on estimating liquid mixture viscosities, I will refer you to chapter 9 of The Properties of Liquids and Gases by Poling, Prausnitz and O'Connel (if making these kind of estimates is going to be important in your career, this is probably a good text to add to your library). They discuss a few different methods for estimating mixture viscosities. (UNIFAC-VISCO perhaps of most interest to you because it is based on group contributions rather than empirical mixture data -- abstract here: http://www.sciencedi...009250994800650).

[shan]

The physical properties of a liquid solution are not simply determined by the mathematic means of each individual component. Please notice the difference of "solution" and "mixture". You may not have 107 liter of soap/glycerol solution by adding 7 liter soap to 100 liter glycerol.