Mechanism of solute solvent interaction
- 1. Mechanism of solute solvent interaction Vicky Lone
- 2. Introduction A solute dissolves in a solvent when it forms favourable interactions with the solvent. This dissolving process all depends upon the free energy changes of both solute and solvent. The free energy of solvation is a combination of several factors.
- 3. Stages The process can be considered in three stages: ● Removal of solute molecule ● Creation of cavity ● Insertion of solute
- 4. (i) A solute (drug) molecule is ‘removed’ from its crystal. ● The solute must separate out from the bulk solute. ● This is enthalpically unfavourable as solute-solute interactions are breaking but is entropically favourable.
- 5. Removal of solute molecule 🧊🧊🧊 🧊🧊🧊 🧊🧊🧊 Drug crystal 🧊🧊🧊 🧊 🧊 🧊🧊🧊 Drug crystal 🧊 Drug molecule
- 6. (ii) A cavity for the drug molecule is created in the solvent. ● A cavity must be created in the solvent. ● The creation of the cavity will be entropically and enthalpically unfavourable as the ordered structure of the solvent decreases and there are fewer solvent-solvent interactions.
- 7. Creation of cavity 💧💧💧 💧💧💧 💧💧💧 Solvent 💧💧💧 💧 💧 💧💧💧 Cavity in solvent Cavity
- 8. (iii) The solute (drug) molecule is inserted into this cavity. ● The solute must occupy the cavity created in the solvent. ● Placing the solute molecule in the solvent cavity requires a number of solute–solvent contacts; the larger the solute molecule, the more contacts are created. ● If the surface area of the solute molecule is A, and the solute–solvent interface increases by γ12 A, where γ12 is the interfacial tension between the solvent1 and the solute2 then it leads to favourable solute-solvent interactions. ● This is entropically favourable as the mixture is more disordered than when the solute and solvent are not mixed.
- 9. Insertion of solute 💧💧💧 💧 💧 💧💧💧 Solvent with cavity 🧊 Drug molecule 💧💧💧 💧 🧊 💧 💧💧💧 Drug in solvent
- 10. Reasons Reasons of solute-solvent interaction in different solvents ● Polar solvents ● Non polar solvents ● Semi polar solvents
- 11. Polar solvent ● Polar solvents dissolve ionic solutes and other polar substances. ● A difference in acidic and basic character of the constituents in the Lewis electron donor–acceptor sense also contributes to specific interactions in solutions. ● The solubility of a substance also depends on structural features such as the ratio of the polar to the nonpolar groups of the molecule. ● As the length of a nonpolar chain of an aliphatic alcohol increases, the solubility of the compound in water decreases.
- 12. Hydrogen bonding ● The ability of the solute to form hydrogen bonds is significant than the polarity. ● Water dissolves phenols, alcohols, aldehydes, ketones, amines, and other oxygen- and nitrogen containing compounds that can form hydrogen bonds with water.
- 13. Nonpolar solvants ● Nonpolar compounds, however, can dissolve nonpolar solutes with similar internal pressures through induced dipole interactions. ● The solute molecules are kept in solution by the weak van der Waals–London type of forces. ● Thus, oils and fats dissolve in carbon tetrachloride, benzene, and mineraloil. ● Alkaloidal bases and fatty acids also dissolve in nonpolar solvents.
- 14. Semipolar solvents ● Semipolar solvents, such as ketones and alcohols, can induce a certain degree of polarity in nonpolar solvent molecules, so that, for example, benzene, which is readily polarizable, becomes soluble in alcohol. ● Semipolar compounds can act as intermediate solvents to bring about miscibility of polar and nonpolar liquids.
- 15. ● Dissolution often occurs when the solute-solvent interactions are similar to the solvent-solvent interactions, signified by the term ‘Like dissolves Like’. ● Hence, polar solutes dissolve in polar solvents, whereas non-polar solutes dissolve in non-polar solvents. ● Dissimilar nature of solute and solvent makes solute insoluble in the solvent. ● Substances dissolve when solvent-solute attraction is greater than solvent-solvent attraction and solute-solute attraction.
- 16. Thank you