# Gels: w/w to volume fraction

If we have a gel composite which is made up of 12%w/w gelatin + 12%w/w
agarose, would it be correct to say that the volume fraction of each
gel component is 50%?
Why would you think you could say that? And why would you want to? Specifying amounts of powder by volume is not very helpful.
Gelatin is a protein, agarose is a polysaccharide. They have very different densities.
bob
Bob, I am trying to build a computer simulation of the interfacial separation between the two components in the composite. The only information I have is about w/w, and I need to translate that to volume fraction in the composite for the modelling. But what I feared, and certainly confirmed by your comments, is that I am going to need more information to get an accurate model.
Wow, interesting problem. (Giving some context often helps.)
I vaguely recall... density of (hydrated) protein in general is around 1.3 g/mL, of carbohydrate, 1.6-1.7 (don?t remember exactly). But don?t use those numbers as anything more than a sense of magnitude, because (aside from being from my memory) they really vary, esp for proteins.
As to "interfacial separation between the two components in the composite", doesn't that also depend on how they interact with other?
Is there structural info on the gel available? (EM?)
bob
Thanks guys for helping me out here. My research background is in (macro) mechanical properties of soft foods, specifically cheese, bread dough and lipid systems e.g. butter. But gels and particularly microstructures are relatively new to me so I am sorry if I sound ignorant. To Uncle Al's comments - I agree that any predictions are as good as the input number (assuming everything else e.g. the maths is perfect). But in your example, if what you need in practice is just one half of the cycle, it doesn't matter what equation you use to fit your data - in fact even a polynomial will fit most data sufficiently well. Of course, a phenomenological model is never as good as a physical one, but if developing the latter is way too difficult then there is nothing wrong with using the former as long as one is pragmatic and recognises the restrictions (hey I am an engineer!). But I personally don't quite agree when someone says their model is good simply because it fits the data, because fitting is never sufficient proof and a good model is one whereby the fitted numbers can make accurate predictions of other sets of data under relatively similar conditions. So if I come up with a computer model I will need to makesure that it gives similar results to whatever that has been obtained experimentally before I can use it to 'replace' the experiments.
To Bob, other research suggests that how a gel composite fractures depends on the relative stiffness of the particle and the matrix and the size and shape of the particle as well. I would presume that the interfacial stress is also important. And I would like to develop a model to learn more about the influence of these parameters on the macro mechanical behaviour.
Coming back to the question I originally posed, I have managed to find some results in the literature which relate w/w to the actual phase volume. So my initial assumption is definitely wrong.

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