# effect of overpressure on lowering viscosity of polysaccharide solution

it has been reported that agarose dissolved at 110C under slight over-pressure has a viscosity at least 10X smaller than dissolved at atmospheric pressure at 90C (ref:https://doi.org/10.1016/j.ceramint.2004.06.007). I have not independently verified this result (but I plan to).

The context of this result is ceramic but the solution viscosity result is independent of this context. I see this result as quite interesting with many other applications BUT I cannot find ANY other reference of similar results with agarose gel or any other polysaccharide for that matter, which surprise me.

Do I miss something?

I am looking for references to better understand this result and the physico-chemistry behind it. If somebody could provide some pointers it will be greatly appreciated

Thanks

JF

The pressure is irrelevant, it is only applied so the solution can reach 110°C. Water boils at 100°C at ambient pressure.

Agarose is a polysugar, and at 110°C in water, I expect it breaks down into smaller molecules, disugars, monosugars etc.

Solutions of smaller molecules have lower viscosity. https://en.wikipedia.org/wiki/Mark%E2%80%93Houwink_equation

For typical polymers, the scaling exponent $a$ of Mark and Houwink is 0.5-0.8, but that's for infinitely low concentrations. It goes up drastically for higher concentrations, reaching a typical 3.4 in the melt.

(That's a bit sloppy, Mark-Houwink predicts the intrinsic viscosity, while in an entangled melt the actual viscosity scales with $M^{\approx3.4}$.)

So, if you have a saturated solution, and the average agarose polymer molecule breaks just twice, a factor ten in viscosity decrease is very reasonable.

• Thanks this is useful as I was ignorant of the Mark-Houwink equation. On the other hand and on a qualitative level, breaking down polymer chains should impact gel properties which was not observed in the mentioned reference.Is it possible that other effect could be at play such that the quality of the unfolding of the chains.. which is a pure speculation on my side... Commented May 18, 2018 at 12:12
• I don't have access to that paper. What gel, and what properties?
– Karl
Commented May 18, 2018 at 21:50
• agarose used to gel alumina green body and the property is gel strength after cooling. The increased solubility and greatly reduced viscosity ( measured at 60C after dissolving agar at 110C ( vs 90C for the reference case) allow for a much better gel (defined as increased solid loading and greater mechanical strength after gelling). I find this result impressive (in my field) and I dont understand its root cause(s), but I am not a chemist... Commented May 19, 2018 at 9:59
• OK I think I got iy now and your answer was correct.Temperatures*time hydrolysis the agar molecule reducing the chain length and viscosity (ref:arxiv.org/pdf/1603.00778.pdf) . Thanks for pointing me in the right direction. I cannot vote for the answer because of a low reputaiton but it answered my question. Commented May 19, 2018 at 14:49