We began by watching a short video of an acidic solution and indicator in a semi-permeable membrane bag becoming dilute after it was placed in a beaker of pure water. The indicator turned pink in acidic solution, and yellow when neutral. However, two distinct phases were observed, the lower one pink and the upper yellow. This was unexpected; we expected the pink to become lighter shades over time. We didn’t come to a consensus on why this was.
We then discussed figure 7.1. We discussed how the external force term, such as gravity, affects a liquid element at equilibrium. We also discussed how osmotic pressure would affect the system that is not experiencing gravitational force, and whether the pressure would cause movement. Figure 7.3 was discussed, and how the depletion layers affect the entropy of the system when they overlap, in different concentrations of large molecule.
Figure 7.6 was analysed, as it caused some confusion. Parts a and b were straight forward, but we decided that part c was a situation that had no external pressure, and part d had an external pressure, the dotted line representing reverse osmosis. We can imagine there is a piston on the far right compressing the volume on the right side of the membrane. The solid line appears as thought it should be flat along the whole graph, but we attributed the dip in the pressure to the depletion layer; if there are no solute particles in this layer, then there is no cause for osmotic pressure, thus the pressure drops. It was also determined that the direction of the applied pressure can be deduced from the gradient of the pressure line on the graph.
The exercise from Your Turn 7D was briefly discussed, and why the energy goes down as the radius decreases, despite the charge being confined to a smaller area. This is due to the larger surface area to volume ratio of the smaller drops.
Figure 7.8 was discussed, and the effects of the depletion layer compared to the electromagnetic effects. We discussed why the hydrophobic effect should be called the hydrophilic effect, as it is the hydrophilicity of water which drives the phenomenon. Also, after looking at figure 7.14, we discussed how the solubility of hydrophobic molecules can decrease as temperature increases, due to the increase in the entropy of the ‘sheepdog’ solvent molecules. This principle allowed Walter Kauzmann to make predictions on the structure of proteins, before structural data was available.
Finally we discussed how the hydrogen bond gave water an unusually high melting and boiling point compared to other small covalent molecules. The hydrogen bonds decrease the rotational freedom of the water molecules, which decreases their entropy, so it takes more thermal energy to disrupt this order. Oxygen – hydrogen bonds have the highest potential for forming hydrogen bonds, as oxygen is a very electronegative atom.
We decided not to have a assignment this week, and we will read chapter 8 for our meeting next week.