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Name: Brian
Status: other
Grade: other
Location: FL
Country: USA
Date: Summer 2011


Question:
When I hydrate sodium polyacrylate crystals: 4 grams or (1 teaspoon) with 8 ounces or(1 cup) of tap water it expands and creates "hydrated plant gel". If I put that hydrated "plant gel" into a clear plastic beaker, with an open top, and expose it to direct sunlight for 12-14 hours in florida July conditions of humidity and temperature, it becomes a clear liquid having very high tension properties and NO lumps of "hydrated plant gel" exist in the beaker. Why does the "plant gel" become a liquid? Is it a part or whole of the emmited solar rays? I performed an oven experiment at 105 degrees and got no similar results so I do not think it is heat.



Replies:
Dear Brian,

Since you have access to all these materials and conditions, it is the perfect opportunity to do some experimentation. In addition to some of the potential variables you have mentioned, you could also replace the tap water with distilled water.

--Jim Tokuhisa


Very astute observations! Sodium polyacrylate (CH2-CH(CO2Na) or simplified to Na-PAA) is remarkably resistant to thermolysis. The degradative mechanism of Na-PAA is threefold: shearing stress, oxidation, and photolysis (mediated by UV radiation), which you observed. The net effect is literally a ruptured molecule and a loss of viscosity. The latter can be quantified with a capillary viscometer. It would be interesting to observe the effect a blacklight would exert on the Na-PAA. Most of the radiation from a blacklight is imperceptible to the human eye, but it does emit both UV and visible light.

Dr. Tim Durham Instructor, Office of Curriculum and Instruction University Colloquium Department of Biological Sciences Florida Gulf Coast University


I'm not sure I have all the details of what and how you did your experiment, but one important variable is very important and you don't mention it. HUMIDITY: The gel exposed to Florida heat is also likely exposed to air that has a high relative humidity. As the relative humidity increases -- approaching 100%, water will not evaporate, no matter how long you leave the beaker exposed. The humidity of the oven is likely very low -- maybe less than 10%. Under those conditions, water will evaporate rapidly. Under the ambient conditions it is also possible that at night the temperature will drop below the dew point of the exposed gel. Under those conditions water will condense on the gel, and dilute the gel solution. Look up the terms "relative humidity" and "dew point" if they are unfamiliar. They measure different things about the amount of water in the atmosphere.

Vince Calder



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