Discussion:
pH-Sensitive Polymers: Obvious Violators of the Second Law of Thermodynamics
Add Reply
Pentcho Valev
2017-11-21 08:33:21 UTC
Réponse
Permalink
Raw Message
"pH sensitive or pH responsive polymers are materials which will respond to the changes in the pH of the surrounding medium by varying their dimensions. Materials may swell, collapse..." https://en.wikipedia.org/wiki/PH-sensitive_polymers

"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

Loading Image...

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

Loading Image...

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.

Pentcho Valev
Pentcho Valev
2017-11-22 10:17:16 UTC
Réponse
Permalink
Raw Message
A rubber-band heat engine - does not violate the second law of thermodynamics, at least apparently. The system is regularly heated and cooled:

Loading Image...

An analogous pH-sensitive-polymer heat engine - almost obviously violates the second law of thermodynamics. Hydrogen ions are regularly added to and removed from the system:

http://www.gsjournal.net/old/valev/val3.gif

Pentcho Valev
Pentcho Valev
2017-11-23 07:12:15 UTC
Réponse
Permalink
Raw Message
A. Katchalsky, POLYELECTROLYTES AND THEIR BIOLOGICAL INTERACTIONS, pp. 13-15: "Let the polymolecule be a negatively charged polyacid in a stretched state and have a length L. Now let us add to the molecule a mineral acid to provide hydrogen ions to combine with the ionized carboxylate groups and transform them into undissociated carboxylic groups according to the reaction RCOO- + H+ = RCOOH. By means of this reaction, the electrostatic repulsion which kept the macromolecule in a highly stretched state vanishes and instead the Brownian motion and intramolecular attraction cause a coiling up of the polymeric chains. Upon coiling, the polymolecule contracts and lifts the attached weight through a distance ΔL. On lifting the weight, mechanical work f*ΔL was performed... [...] FIGURE 4: Polyacid gel in sodium hydroxide solution: expanded. Polyacid gel in acid solution: contracted; weight is lifted." http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1367611/pdf/biophysj00645-0017.pdf

Mineral acid (hydrogen ions, H+) is added to the system and "the polymolecule contracts and lifts the attached weight through a distance ΔL". Then added H+ can be removed and the macromolecule resumes its initial stretched state, ready to lift another weight. The net work involved in adding and removing hydrogen ions, if the process is carried out quasi-statically, is virtually zero, while the net work extracted from contracting and stretching is obviously positive. The system is cyclically lifting weights at the expense of heat absorbed from the surroundings, in violation of the second law of thermodynamics.

Pentcho Valev

Loading...