Monomolecular Layer Properties of Stearic Acid · RMPH 2018 #C
Physics · fluid-mechanics
Problem
Determine properties of droplets of stearic acid solution in organic solvent. When spread onto water surface, these droplets form a monomolecular layer.
Stearic acid is a saturated fatty acid with 18 carbon atoms, chemical formula C17H35CO2H. The molecule has a hydrophilic "head" (carboxylic group COOH) that attaches to water through hydrogen bonds, and a hydrophobic "tail" (carbon-hydrogen chain) that repels water. On water surface, molecules align with heads down and tails up, forming a monomolecular layer.
[Figure: Lewis structure of stearic acid showing covalent bonds, and 3-D representation showing black circles for carbon atoms, white for hydrogen, red for oxygen]
A stearic acid molecule occupies an L×L parallelepiped. The molecular length is smaller than visible light wavelengths, making the monomolecular layer invisible unless molecules pile up or talcum particles are used to observe the cleared circular area.
Given data: molar mass M=284 kg/kmol, density ρ=941 kg/m³, molecule's sectional surface s=2×10−20 m², solution concentration c=0.02 kg/m³, solvent density ρsolvent=878 kg/m³.
Available apparatus: paper sheet with arbitrary units, marker, pipette, two test tubes (non-flat base), one containing stearic acid solution, one empty.
C.1.1. Describe briefly a method to determine the volume of a drop delivered by the pipette.
C.1.2. Using available objects, determine (in m³) the volume Vp of a solution drop.
C.1.3. Using the calibrated paper sheet from task A, determine other required quantities for subsequent analysis.