Research Article: Osmotic Engine: Translating Osmotic Pressure into Macroscopic Mechanical Force via Poly(Acrylic Acid) Based Hydrogels

Date Published: May 30, 2017

Publisher: John Wiley and Sons Inc.

Author(s): Lukas Arens, Felix Weißenfeld, Christopher O. Klein, Karin Schlag, Manfred Wilhelm.


Poly(acrylic acid)‐based hydrogels can swell up to 100–1000 times their own weight in desalinated water due to osmotic forces. As the swelling is about a factor of 2–12 lower in seawater‐like saline solutions (4.3 wt% NaCl) than in deionized water, cyclic swelling, and shrinking can potentially be used to move a piston in an osmotic motor. Consequently, chemical energy is translated into mechanical energy. This conversion is driven by differences in chemical potential and by changes in entropy. This is special, as most thermodynamic engines rely instead on the conversion of heat into mechanical energy. To optimize the efficiency of this process, the degree of neutralization, the degree of crosslinking, and the particle size of the hydrogels are varied. Additionally, different osmotic engine prototypes are constructed. The maximum mean power of 0.23 W kg−1 dry hydrogel is found by using an external load of 6 kPa, a polymer with 1.7 mol% crosslinking, a degree of neutralization of 10 mol%, and a particle size of 370–670 µm. As this is achieved only in the first round of optimization, higher values of the maximum power average over one cycle seem realistic.

Partial Text

A charged polymeric network forms a hydrogel in water due to the difference in the osmotic pressure between the network and the surrounding solvent.1 Poly(acrylic acid)‐based hydrogels can swell up to 100–1000 times their own weight in desalinated water, while the swelling in saline solutions, e.g., 4.3 wt% NaCl having a similar chemical potential as seawater, is typically a factor of 2–12 lower.2 Such hydrogels are called superabsorbent materials and are employed in large quantities in sanitary products like diapers.3

In this publication, poly(acrylic acid)‐based hydrogels with different DN and different DC were evaluated with respect to their performance in an osmotic motor. The influence of the external load, respective pressure, was investigated as well as the effect of particle size on the resulting mean power production. The repeatability of the swelling and shrinking cycles of the polymeric hydrogel and the effect of drainage materials were also evaluated.

Polyelectrolyte hydrogels can translate chemical energy into mechanical energy driven by a chemical potential difference via the corresponding osmotic pressure difference in deionized water and salt water. The purpose of this publication is to demonstrate a proof of principle for this concept.

Synthesis: The hydrogels were synthesized by free‐radical polymerization of acrylic acid with N,N′‐methylenebisacrylamide as the crosslinking agent and sodium persulfate (Na2S2O8) as the initiator in an aqueous solution at room temperature for 15 h (see Figure6).

The authors declare no conflict of interest.




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