1. 1 Fall, 2013 Ch 4. Solubility and Adsorption – Potential Energy Approach Instructor: J.-W. John Cheng Mech. Engr. Dept., Nat ’ l Chung Cheng Univ. Interfacial Physics and Thin-Film Processing

2. JCheng201311 2 Outline 4.1 Solubility of Like Molecules in a Medium 4.2 Solubility of Unlike Molecules in a 3rd Medium 4.3 Adsorption – Interaction between Solution and Solid Surface 4.4 Limitations of Potential Energy Approach Reference Israelachvili, J. N., Intermolecular and Surface Forces, 2nd ed., 1991, Chapter 9

3. JCheng201311 3 van der Waals between Unlike Molecules Recall van der Waals between like molecules Recall van der Waals between unlike molecules

4. 4 4.1 Hildebrand and Hanson Solubility Theories – A Free Energy Approach

5. JCheng201311 5 Scenario – Description & Analysis Assumption: a dilute solution with solute A and solvent B

6. JCheng201311 6 Discussion Q1. What is the reference state with potential = 0? Q2. Does above analysis also take into account the situation when there are more than one attachment of solvent B to a solute A, e.g., as shown in the 2D hexagonal compact lattice? A B B B B B B A2. A B B B B B B B B B A B B B B B B B B B + A B B A B B B B B B B B B B B B B B B B +

7. JCheng201311 7 Discussion  w=w as – w disp  0  like molecules tend to self associate But entropy increase in dispersion state against self associate Thus, simple potential energy analysis can not tell precisely whether good dissolution of A in B will occur One can only say When  w  0, dissolution is likely to occur When  w is much negative, self association is more likely to occur, i.e., solute A insoluble in solvent B contd

8. 8 4.2 Solubility of Unlike Molecules in a 3rd Medium

9. JCheng201311 9 Scenario – Description A dilute solution of two unlike solute molecules A & B in solvent C 1 2 3

10. JCheng201311 10 Potential Energy Analysis A, B dispersed AB associated 2 1

11. JCheng201311 11 Potential Energy Analysis A, B dispersed AA, BB associated contd 3 1

12. JCheng201311 12 Potential Energy Analysis AA, BB associated contd AB associated 2 3

13. JCheng201311 13 Potential Energy Analysis contd 1 2 3

14. JCheng201311 14 Discussion An interesting observation regarding transition from the state of dispersed (1) to AB associated (2) Take only d-component of  w 12 as an illustration situationcomment “ solute A in middle ” < 0favoring immiscibility “ solute B in middle ” < 0favoring immiscibility “ solvent C in middle ” > 0favoring dissolution

15. JCheng201311 15 Discussion Most favorable state: self associations of AA, BB, CC Although simple potential energy analysis can not tell precisely whether good solution in C will occur, It still provides us some selection guidelines of good solvent. Assume solutes A and B are given. Choosing solvent whose cohesion work lies in between those of A & B will increase the likelihood of dissolution The best choice is the solvent with cohesion work equal to the mid-value between those of A and B contd

16. 16 4.3 Adsorption of Solute Molecules on Solid Surface

17. JCheng201311 17 C C C B B B B B B B B B B B B C B B B B BB B B B BB B B B B B B B B B C C C B B B B B B B B B B B B BBB B B B BB B B B B B B B B B B B B B B Scenario – Description A dilute solution of solute B in solvent C and a solid surface A adsorption of Bassociation of B 1 23 4

18. JCheng201311 18 Solution to BB Association Desorption to BB association (state 1  3) A B A C C B B B B B B B C C C C C desorption BB association B C C B B

19. JCheng201311 19 Solution to Adsorption Desorption to adsorption (state 1  2) A B A C C B B B B B B B C C C C C desorption A C A C B B B B B B B B C C C C C adsorption

20. JCheng201311 20 Solution to Adsorption An interesting property of “solute in middle” “solvent in middle” “solid in middle” favoring adsorption favoring desorption favoring adsorption BUT…! contd

21. JCheng201311 21 Partial Wetting/Incomplete Adsorption Another point of view using Young ’ s equation Before solutes form a well-organized solid, a collection of solutes behave like a liquid Thus, Young ’ s equation is still valid vs. Note  ij is interfacial energy, not adhesion work

22. JCheng201311 22 Partial Wetting/Incomplete Adsorption contd An interesting property of “solute in middle” “solvent in middle” “solid in middle” favoring wetting favoring dewetting favoring partial wetting favoring film adsorption

23. JCheng201311 23

24. JCheng201311 24 Discussion C C C B B B B B B B B B B B B C B B B B BB B B B BB B B B B B B B B B contd C C C B B B B B B B B B B B B C B B B B BB B B B BB B B B B B B B B B C C C B B B B B B B B B B B B BBB B B B BB B B B B B B B B B B B B B B “solute in middle” adsorption of B “solvent in middle” desorption of B “solid in middle” partial wetting by B In summary

25. 25 4.4 Limitations of Potential Energy Approach

26. JCheng201311 26 Limitations Entropy effect was not taken into account Thus, potential energy approach cannot give precise conclusion regarding solubility and adsorption Its prediction can only be used as a guideline but it turns out to be a quite useful one Since it adopts the geometric mean assumption for interaction between two unlike molecules Potential energy approach can not be applied to interactions involving hydrogen or ionic bonding It also assumes pairwise additivity It does not include replacement effect.