2 Water as a solvent and emulsions Flashcards
Which physical properties of water depend on the hydrogen bonds?
Hydrogen bonds are strong, directional bonds that hold the molecules in place. The ice/liquid water structure depends on the hydrogen bonds. In ice, the hydrogen bonds are stable, while in liquid they break and reform. In turn, water’s structured nature determines high values of Cohesive Energy Density (CED). Other properties include low viscosity (easy solute movement), cohesion/surface tension (wetting phenomena), high heat capacity (better T control), high dielectric constant (solvation), and dissociation: acids & bases, pH.
What is the microscopic structure of liquid water? Is the space occupied by a random continuum of water molecules?
No, because water is a structured liquid with strong, directional hydrogen bonds. The molecules do not fill space efficiently and cavities are present. Liquid water is represented as an equilibrium between such “structured water” (ice-type clusters of low entropy and low density, 4 neighbors per molecule) and “unstructured water” (having higher entropy and density, and many more neighbors per molecule).
Which mechanism accounts for the dissolution, at least in part, of organic apolar molecules in water?
Any solvation of a solute requires the creation of a cavity (cage formation) around which water molecules arrange themselves with the sole objective of retaining most of the stability of the original hydrogen-bonding network. For this:
1) an empty cage is formed in which one or more organic molecules will be hosted. This process requires energy to break H-bonds and, therefore, is a negative enthalpic contribution.
2) Once the cage is occupied by the solute, water molecules around the hydrophobic surfaces of the cage adopt a more tightly ordered arrangement. This restricted conformational freedom of the water molecules favors well-ordered and tight H-bonds. Positive entropic contribution.
3) The guest organic molecules are subjected to a positive pressure inside the cage by surrounding water.
4) If two reacting organic molecules enter the same cage, reactions will be favored that decrease overall molecule volume.
5) If the reaction can produce two different isomers, the smaller one will be favored.
Quote an example of a reaction that runs better in water than in a classic organic solvent.
Diels-Adler reaction is a classic.
SEE REACTION
Provide an example of how specific heat capacity of water can be exploited in chemical engineering.
In all liquid cooling systems.
Lewis acids in water offer new catalysts at the benefit of chemical synthesis. Discuss this topic.
Hydrolysis at different pH can generate different species; therefore, we can adjust our reaction depending on what we need. For example, highly hydrolyzable metal salts that are also Lewis acids like TiCl4 and AlCl3 will dissociate in water and react with water to generate several species (aqua ion, mononuclear species, and more complex polynuclear species), but each species will exist in a precise pH range in which it displays its characteristic Lewis acidity, while also being dependent on the metal concentration. Therefore, choosing the right pH and the right catalyst can give very specific reactions and allows for selectivity.
What are the necessary structural characteristics for a molecule that behaves as a surfactant?
Surfactants contain both hydrophobic groups (tails) and hydrophilic groups (heads). Therefore, a surfactant contains both a water-insoluble (or oil soluble) and a water-soluble component. Depending on the composition of the head, surfactants can be nonionic, anionic, cationic, or amphoteric. When one of these molecules with amphiphilic structure is dissolved in aqueous medium, some of the surfactant molecules are expelled to the surface with their hydrophobic groups oriented to minimize contact with the water molecules.
Design your own surfactant molecule.
Linear alcohol ethoxylate (non-ionic surfactant) very much in use in laundry detergents.
VER IMAGEN
What is a micelle?
A micelle is a mechanism for overall energy reduction in which molecules with amphiphilic structures form aggregates in aqueous mediums where the surface is already saturated with these molecules. Depending on the solvent’s nature (polar/nonpolar), the monomers can arrange themselves in spherical/cylindrical/etc. micelles or in reverse/inverted micelles, trapping water inside.
How do we determine the critical micellar concentration of a surfactant?
The CMC can be determined by slowly adding the monomers and, through any experimental technique, measuring if the concentration of the monomer keeps increasing or not. For example, adding small amounts of the surfactant to water and measuring the concentration via electrochemical spectroscopy. At one point, no matter how much more we add, no more micelles will form and we will have determined the CMC.
Is micelle formation dependent on temperature?
For surfactants, the Krafft temperature (Tk) is the minimum temperature for micelles to form.
Discuss the mechanism of the ester formation from an alcohol and a carboxylic acid in water in the presence of micelles.
(Full mechanism: slide 44, 2nd ppt)
In a hydrophobic environment (the internal part of the micelle will act as the reactor), the ester is synthesized from the carboxylic acid and the alcohol by substitution of the H in the carboxylic tail with the R’ of the alcohol, generating water as a by product. The ester (product) will be hydrophilic and is spontaneously ejected from inside the micelle to the surrounding water.
What is the difference between nanoemulsions and microemulsions?
Nanoemulsions are formed by mechanical shear (to rupture large droplets into smaller ones), while microemulsions are formed by self-assembly. Microemulsions form spontaneously and are thermodynamically stable. Nanoemulsions, on the other hand, are only kinetically stable and do not form spontaneously. A microemulsion is much more stable than having two separate phases, while with nanoemulsions the two separate phases are the most stable configurations; which is why the energy input to generate nanoemulsions is very high. Nanoemulsions use specific high-energy devices (like ultrasound generators or high-pressure homogenizers) in order to increase the water/oil interfacial area and generate submicronic droplets.
Additionally, the nanostructures in microemulsions are strongly affected or even broken-up by conditions of stress (like temperature changes or dilutions) or environmental changes, while the nanoemulsion droplets remain stable in these conditions.
Why are nanoemulsions the most exploited media to be used as drug carriers?
In nanomedicine, very small size droplets of nanoemulsions favor better drug absorption and targeting, opening opportunities for drugs to be designed more precisely with better bioavailability and accurate dosing, rendering in minimum side effects. As well, nanoemulsions have the advantages of kinetic stability and easy penetration of barriers/barrier crossing. Due to the very fine particle size and less surface tension between the oil and water molecules, they barely have the tendency to agglomerate or precipitate. Additionally, nanoemulsions can be administered by almost all available routes (parenteral, ocular, nasal, oral, topical, aerosolization to the lungs) and can easily incorporate lipophilic bioactive compounds and stabilize bioactive compounds that tend to undergo hydrolysis. Finally, nanoemulsions are biodegradable and can be produced on a large scale using ultrasonic processors.
