2017-11-21 08:36:29 UTC
"HYDROGELS are networked structures of polymer chains crosslinked to each other and surrounded by an aqueous solution. The polymer chains contain acidic or basic groups bound to them. The acidic groups on the chains deprotonate at high pH, whereas the basic groups protonate at low pH. In the presence of an aqueous solution, the polymer chains absorb water and the association, dissociation and binding of various ions to polymer chains causes the hydrogel to swell. The swelling and shrinking properties of hydrogels are currently being exploited in a number of applications including control of microfluidic flow, muscle-like actuators, filtration/separation, and drug delivery. The structure and properties of hydrogels are similar to many biological tissues such as cartilage and the corneal stroma in the eye." http://silver.neep.wisc.edu/~mandm/faculty_pages/crone/PDF/JMEMS_01038850_publver_02.pdf
By regularly changing the pH of the system, the experimentalist is able to extract unlimited amount of work from pH-sensitive polymers:
"When the pH is lowered (that is, on raising the chemical potential, μ, of the protons present) at the isothermal condition of 37°C, these matrices can exert forces, f, sufficient to lift weights that are a thousand times their dry weight." http://www.google.com/patents/US5520672
A. KATCHALSKY, POLYELECTROLYTES AND THEIR BIOLOGICAL INTERACTIONS, p. 15, Figure 4: "Polyacid gel in sodium hydroxide solution: expanded. Polyacid gel in acid solution: contracted; weight is lifted." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1367611/pdf/biophysj00645-0017.pdf
Consider Figure 4 in Katchalsky's article. The following four-step isothermal cycle, if carried out quasi-statically (reversibly), clearly violates the second law of thermodynamics:
1. The polymer is initially stretched. The experimentalist adds hydrogen ions (H+) to the system. The force of contraction increases.
2. The polymers contracts and lifts a weight.
3. The experimentalist removes the same amount of H+ from the system. The force of contraction decreases.
4. The experimentalist stretches the polymer and restores the initial state of the system.
The net work extracted from the cycle is positive unless the following is the case:
The experimentalist, as he decreases and then increases the pH of the system (steps 1 and 3), does (loses; wastes) more work than the work he gains from weight-lifting.
However electrochemists know that, if both adding hydrogen ions to the system and then removing them are performed quasi-statically, the net work involved is virtually zero (the experimentalist gains work if the hydrogen ions are transported from a high to a low concentration and then loses the same amount of work in the backward transport). That is, the net work involved in steps 1 and 3 is zero, and the net work extracted from steps 2 and 4 is positive, in violation of the second law of thermodynamics.