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Keywords:

  • evaporation;
  • surfactants;
  • monolayers;
  • natural convection;
  • free surface;
  • oleyl alcohol;
  • stearic acid;
  • stearyl alcohol;
  • Sherwood;
  • Rayleigh

Presented here is an experimental investigation of the effects of several surfactant monolayers on evaporation driven by natural convection in the air above a water surface. Experiments were performed in a controlled laboratory setting with tanks of heated water for the following cases: (1) a clean water surface, and for surfaces covered with monolayers of (2) oleyl alcohol, (3) stearic acid, and (4) stearyl alcohol. Evaporation rates were measured using a laser-based method, and the Sherwood and Rayleigh numbers, Sh and Ra, were computed from the data. Power law scalings of the form Sh = B·Ram were developed for each case which yielded, essentially, m = 1/3 for all four surface conditions. The oleyl alcohol and stearic acid conditions give essentially the same value for B as for the clean surface case. For stearyl alcohol, B is smaller than for all other surface conditions; this result is attributed to the ability of the stearyl alcohol monolayer to inhibit evaporation by blocking the passage of water molecules through the monolayer: the barrier effect. The surface temperature is measured in this work enabling a separation of the effect of surfactants on evaporation due to a reduction in surface temperature from their effect on evaporation due to a true barrier effect. This has not been accomplished heretofore. © 2012 American Institute of Chemical Engineers AIChE J, 59: 303–315, 2013