What is the change in entropy for a colloid settling out of solution?

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Discussion Overview

The discussion revolves around the change in entropy associated with colloids settling out of solution, exploring the thermodynamic implications of this process in both closed and open systems. Participants examine the factors influencing entropy changes during sedimentation, including the role of external forces like gravity and the conditions under which colloids remain suspended.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that if the settling occurs spontaneously, it must involve an increase in entropy, despite a reduction in possible microstates.
  • Others suggest that the initial agitation of the fluid is crucial for keeping the colloids suspended, and when the fluid settles, the kinetic energy is converted, potentially increasing the temperature and thus the accessible microstates of the water molecules.
  • There is a mention of Gibbs or Helmholtz free energy minimization in non-equilibrium closed systems, indicating a complex relationship between energy states and entropy.
  • Some participants argue that the colloid system is not closed due to the influence of gravity, which is an external force affecting sedimentation, and they highlight the possibility of phase separations occurring through other mechanisms.
  • One participant notes that the motion of settling colloid particles leads to frictional heating in the liquid, contributing to an increase in total entropy.
  • There is clarification regarding the terminology of closed versus isolated systems, with participants acknowledging that sedimentation indicates the system is not isolated from the environment.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the conditions under which entropy changes occur during colloid sedimentation, and the discussion remains unresolved with respect to the implications of these factors.

Contextual Notes

Participants highlight limitations in their assumptions about system isolation and the definitions of closed versus insulated systems, which may affect their arguments regarding entropy changes.

Timtam
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If it occurs spontaneously then it must increase entropy but the possible micro states reduce so what else is occurring to increase entropy
 
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Let's assume a closed system.

I think you have to ask yourself: why were the colloids originally suspended? I think because the fluid was agitated. The fluid then settles down, and the colloids then settle out of suspension. Where did that KE of the fluid go to? (...and the PE of the colloids.) It raised the temperature of your system, so now the water molecules have more accessible microstates.
 
Timtam said:
If it occurs spontaneously then it must increase entropy...
A closed system which isn‘t in an equilibrium state “evolves” in such a way to minimize – depending on the constraints – its Gibbs or Helmholtz free energy.
 
Timtam said:
Summary:: If it occurs spontaneously then it must increase entropy but the possible micro states reduce so what else is occurring to increase entropy

If it occurs spontaneously then it must increase entropy but the possible micro states reduce so what else is occurring to increase entropy

Settling occurs because of gravity- an external force- the colloid system is not closed. Take away gravity and while sedimentation no longer occurs, phase separations can occur by other mechanisms- an interesting one is entropy-driven phase transitions and self-assembly:

https://arxiv.org/abs/cond-mat/0402585
https://www.physics.upenn.edu/yodhlab/docs/yodh_Phys295_Entropic_Forces.pdf
 
Andy Resnick said:
Settling occurs because of gravity- an external force- the colloid system is not closed.
Your comment made me realize that I should have said closed and insulated. I believe you are trying to say the system is not isolated.
 
When the colloid particles settle on the bottom of the container, their motion causes frictional heating in the liquid. This increases the total entropy.
 

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