A materials engineer must select a liquid coolant for a compact electronic device
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A materials engineer must select a liquid coolant for a compact electronic device. The coolant must satisfy three criteria at the operating temperature of 80 °C:
1. Remain in the liquid phase at 80 °C and 1 atm.
2. Flow easily through a network of narrow microchannels (low viscosity).
3. Minimize evaporative losses (high surface tension).
Four candidate liquids are listed below together with selected physical data measured at 25 °C:
Liquid W: 1-propanol, boiling point 97 °C, viscosity 2.2 mPa·s, surface tension 23 mN·m⁻¹
Liquid X: propylene glycol, boiling point 187 °C, viscosity 52 mPa·s, surface tension 37 mN·m⁻¹
Liquid Y: dimethyl ether (CH3OCH3), boiling point −25 °C, viscosity 0.18 mPa·s, surface tension 12 mN·m⁻¹
Liquid Z: perfluorohexane (C6F14), boiling point 56 °C, viscosity 0.64 mPa·s, surface tension 12 mN·m⁻¹
Assume that increasing the temperature from 25 °C to 80 °C lowers viscosity and surface tension for each liquid by approximately 40 % and raises the vapor pressure sufficiently to lower the boiling point by no more than 5 °C.
(a) Identify which of the four liquids fail to meet criterion 1 at 80 °C. Support your answer with evidence from the data.
(b) Among the liquids that satisfy criterion 1, determine which liquid best meets criteria 2 and 3 simultaneously at 80 °C. Show your calculations or reasoning using the estimated property changes.
(c) Explain, with reference to intermolecular forces and molecular structure, why the liquid you selected in part (b) attains a balance of low viscosity and relatively high surface tension compared with the other acceptable liquids.
(d) Suggest one structural modification to the molecule you selected that could further reduce evaporative losses without substantially increasing viscosity, and justify your suggestion based on intermolecular interactions.