Solution Chemistry and Solubility

• Solutions are complex systems that contain more than one chemical substance.

What is a Solution?

• The types of solutions are: gas in liquid, solid in solid, gas in solid, and liquid in liquid.
• The major component is called the solvent.
• The minor component is called the solute.
• Aqueous solutions have water as the solvent.
• Solutions are homogenous at the macroscopic scale, meaning they have the same composition throughout.
• Solute particles are evenly and randomly dispersed in the solvent at the molecular level.
• Solute and solvent molecules are in contact with each other.
• Molecular interaction must exist between the two types of molecules in a solution.

Molecular Formation of Solutions

• Ethanol and water are miscible in all proportions.
• For a process to be thermodynamically favorable, the Gibbs free energy change (ΔG) must be negative.
• Gibbs energy change depends on both enthalpy (H) and entropy (S) changes.
• The entropy of an ethanol-water solution is higher than that of either substance on its own.
• Some solution formations are entropic (involving ΔS) and some are enthalpic (involving ΔH).

Solubility

• Solubility isn't an all-or-nothing property
• A very small number of oil molecules presents the aqueous phase and vice versa.
• Molarity (M, mol/L) is a method to describe solubility and involves moles of solute per liter of solution
• Another measure is grams of solute per mass of solution.
• 1 mg of solute dissolved in 1 kg of solution equals 1 part per million (ppm) solute.
• Solubility is always reported at a particular temperature.
• A solution is saturated if no more solute can dissolve at the given temperature and unsaturated if it can.
• Hydrocarbons are not very soluble in water.
• Alcohols (hydrocarbons with an -O-H group attached) with up to 3 carbons are completely soluble.
• As the number of carbon atoms increases, the solubility of the compound in water decreases.
• Diols (compounds with 2 –O–H groups) are more soluble than similar alcohols.
• Common sugars are polyalcohols and are very soluble in water.

Hydrogen Bonding Interactions and Solubility

• Adding hydroxyl groups increases the solubility of a hydrocarbon in water.
• For a solute to dissolve in a liquid, the solute molecules must be dispersed in that liquid.
• Solubility depends on how many solute molecules can be present within a volume of solution before associating preferentially with each other.
• The stronger the interactions between the solute particles, the less favorable it is for the solute to dissolve in water.
• The stronger the interactions between solute and solvent molecules, the greater the likelihood that solubility will increase.
• Intermolecular interactions include hydrogen bonding and van der Waals interactions (LDFs and dipole-dipole).
• Alcohols have a dipole (unequal charge distribution), with a small negative charge on the oxygen(s) and small positive charges on the hydrogen.

Entropy and Solubility

• Oil molecules are primarily non-polar and interact with each other and with water molecules through London dispersion forces (LDFs).
• When oil molecules are dispersed in water, their interactions with water molecules include both LDFs and interactions between the water dipole and an induced dipole on the oil molecules.
• ΔH is approximately zero for many systems when estimating the enthalpy change associated with dispersing oily molecules in water.
• The energy required to separate the molecules in the solvent and solute approximately equals the energy released when the new solvent-solute interactions are formed.
• The entropy change associated with simply mixing molecules is positive.
• The change in entropy associated with dissolving oil molecules in water must be negative, thus making ΔG positive.
• As hydrocarbon molecules disperse in water, the water molecules rearrange to maximize the number of H-bonds they make.
• A cage-like structure forms around each hydrocarbon molecule, making it a more ordered arrangement.
• Increase in entropy leads to a negative value for –TAS, which causes the system to minimize the interactions between oil and water molecules and leads to the formation of separate oil and water phases.
• Entropy-driven separation of oil and water molecules is referred to as the hydrophobic effect.
• The insolubility of oil in water is controlled primarily by changes in entropy, so it is directly influenced by the temperature.
• Stabilization of water and hydrocarbons at low temperatures can be made using mixtures known as clathrates.

Solubility of Ionic Compounds

• Polar compounds tend to dissolve in water extend to ionic compounds.
• Table salt, or sodium chloride (NaCl), is soluble in water (360 g/L).
• NaCl is a salt crystal that is composed of Na+ and Cl– ions bound through electrostatic interactions.
• When NaCl dissolves in water, the electrostatic interactions within the crystal must be broken.
• During NaCl dissolving the intermolecular forces between separate molecules are disrupted.
• When a crystal of NaCl comes into contact with water, the water molecules interact with the Na+ and Cl- ions on the crystal's surface.
• The positive ends of water molecules interact with the chloride ions, while the negative end of the water molecules interact with the sodium ions.
• These water molecules form a dynamic cluster around the ion.
• Thermal motion moves the ion and its water shell into solution.
• The water shell is highly dynamic
• The ion-dipole interaction between ions and water molecules can be very strongly stabilizing (-ΔH).
• Solvation is the process by which solvent molecules interact with and stabilize solute molecules in solution
• When the solvent is water, it is known as hydration

Gibbs Energy and Solubility

• When calcium chloride dissolves in water, the interactions between ions are broken and new interactions between water molecules and ions are formed.
• Surroundings are the rest of the water molecules (the solution), the system is CaCl2 and the water molecules it interacts with .
• Enthalpy change for the solution of calcium chloride is around -80 kJ/mol
• Dissolving is exothermic and heat is transferred from the system to the surroundings.
• Solution temperatures decrease when NaCl is dissolved, so the solution (surroundings) loses energy to the ion-solvent interactions (system).
• Energy from the surroundings breaks up the NaCl lattice and allows ions to move into the solution.
• Enthalpy is not the critical factor determining whether a solution happens.
• It is critical to factor the entropy change for the solution.
• For a solution to form, the Gibbs energy change must be negative.
• When calcium chloride dissolves in water, ΔH is negative and AS is slightly negative
• When sodium chloride dissolves, ΔH is positive, but AS is positive enough to overcome the effect of ΔH.
• Entropy of mixing is the important factor

Polarity

• Biomolecules are termed amphipathic
• Biomolecules are very large compared to the molecules and often interact with themselves
• The intramolecular interactions of biological macromolecules, together with their interactions with water, are key factors in predicting their shapes.
• Oils or fats are also known as a triglycerides with hydrocarbon chain of a CH3CnH2n structure.
• The molecules will either form a standard micelle, spherical structure with the polar heads on the outside and the non-polar tails on the inside, or an inverted micelle arrangement, in which polar head groups (and water) are inside and the non-polar tails point outward.
• Lipid molecules have multiple hydrocarbon tails and carbon ring structures called sterols and structure creates a lipid bilayer.
• These ordered structures are possible only because dispersing the lipid molecules in water results in a substantial decrease in the disorder of the system.
• Many ordered structures associated with living systems, such as the structure of DNA and proteins, are the result of entropy-driven processes and biological systems do not violate the laws of thermodynamics

Solutions, Colloids & Emulsions

• Micelles are not molecules, but rather supramolecular assemblies composed of many distinct molecules.
• Solutions of macromolecular solutes are called colloids.
• Colloids of aggregates of molecules (like micelles), atoms (nanoparticles), or larger macromolecules (proteins, nucleic acids) and scatter light
• A salt or sugar solution is translucent, whereas a colloidal dispersion of micelles or cells is cloudy.
• Emulsions are when suspended particles are liquid.
• The system is known as a suspension when the particles in a solution maintain the structure of a solid.
• Emulsions are often unstable, and over time the two liquid phases separate.

Temperature and Solubility

• Gases have very small intermolecular attractions and not have very high solubility in water
• Solubility of most gases in water decreases as temperature rises
• Most gases have a slightly favorable (negative) enthalpy of solution and a slightly unfavorable (negative) entropy of solution.
• Increase in temperature is that the gas molecules have more kinetic energy and therefore more of them can escape from the solution, increasing their entropy as they go back to the gas phase increasing O2 levels.
• Manufacturing facilities expel warm water into the environment which causes thermal pollution

Solutions of Solids in Solids

• Alloys are when two or more elements, typically metals, are melted and mixed together so that their atoms can intersperse.
• Steel is an iron solvent with a carbon solute, and the carbon atoms do not replace the iron atoms, but fit in the spaces between them.
• Steel is an interstitial alloy.
• Steel is denser, harder, and less metallic than iron.

Formation of a Solution a Reaction

• There are still molecules of ethanol and water therefore it isn't a new reaction.
• When hydrogen chloride, HCI (a white, choking gas), is put in water it forms hydrochloric acid.
• Hydrochloric acid is a new chemical substance created from the process

HCI (g) + H2O → HCI (aq)

• It is implied that the HCl molecules are dissolved in water but aqueous indicates the molecules are no longer molecules.
• The covalent bond between H and Cl that is broken and a new covalent bond between H and O is formed.
• The new equation is HCI(g) + H2O H3O+ + CI-