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I sometimes encounter explanations of phenomena where only entropy is quoted as a driver, for example, osmosis, diffusion, hydrophobic effect... But can entropy ever be the sole driver of a process? I encountered 'entropic force', is it a real force? Aren't energy changes always present in these processes?

If this the case, when it comes to concentration gradiënts, these gradients 'contain' potential energy but I can't figure out where it originates from. For example, a water molecule that is a part of the cage around a lipophilic molecule is missing out on potential hydrogen bonding and thus feels a net force which is opposed. This missing out on stable interactions, feeling a force which is opposed...is what 'creates' potential energy. I can't see these things in diffusion and osmosis for neutral solutes (when the solute has charge, the electrostatic force comes into play - electrochemical potential - this feels more natural to consider as potential energy) which seem just to be random mixing... but in the case of osmosis the gradiënt is able to do work and create a hydrostatic pressure difference which proves the potential energy there since it can be converted into another subtype? The semi-permeable membrane seems essential in these phenomena for work to be done (osmosis, transmembrane potential generation in biology)? Free diffusion in bulk solution without membranes could never do work and all potential energy would be dissipated as heat?

The role of charge in all of this... Osmosis is a colligative property, so the charge of the solute doesn't actually matter.

But when the membrane is only permeable to the solute, a chemical gradiënt can store electric potential energy over the membrane. The chemical gradiënt does work on the charged particles by pushing them against their electric potential. But what causes this push against the electric field, the difference in concentration? But this seems more of a probabilistic factor.The electric field over de membrane does negative work and the concentration gradiënt positive work?

Coming back to osmosis, the solutes gradiënt does positive work and the hydrostatic pressure performs negative work. So charge doesn't play a part after all?

What happens when equilibrium is reached in all of these cases? Zero work?

Thank you

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  • $\begingroup$ The instantaneous force acting on atoms always acts to minimise the potential energy. But if you average the force acting on atoms, it acts to minimise the free energy (i.e. you get an 'entropic force' contribution). And this is essentially because kinetic energy can drive the system into high potential energy states. $\endgroup$
    – lemon
    Commented Jul 15, 2018 at 20:40
  • $\begingroup$ My answer to a similar (although somewhat more specific) question: physics.stackexchange.com/a/650027/247642 $\endgroup$
    – Roger V.
    Commented Jul 21, 2021 at 8:24

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As I see it, the driver behind all the system processes you describe involves disequilibrium, i.e., pressure, temperature, concentration etc. differentials or gradients; and the achievement of equilibrium as a result of the process results in entropy production, that is, an overall increase in entropy (system plus surroundings) as occurs with all real (irreversible) processes. So in that sense, one can think of entropy production as a driver of processes.

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All these processes are due to random events. The atoms do not "feel" a concentration gradient, but random walks will cause diffusion to even out differences in concentration.

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