CONTACT ANGLE AND DEPTH OF FREE-ENERGY MINIMUM IN THIN LIQUID FILMS . THEIR MEASUREMENT AND INTERPRETATION

The depth of the energy minimum corresponding to an equilibrium thin liquid film is determined by measuring the contact angle between the film and bulk solution. The significance of this angle and its relation to conventional contact angles is discussed and an experimental method for measuring it described. Results for solutions of sodium dodecyl sulfate containing NaCl alone or with LiCl, KCl, or (CH3)4NBr range up to 16° in contact angle and over 2 ergs/cm2 in depth of the minimum. These provide excellent support for previous qualitative observations of kinetic and equilibrium relations. It is also shown that the minima should lie close to the attractive energy curve, but the disagreement between the simple theory of van der Waals forces and the experimental results is very large. Various possible reasons for this discrepancy are briefly discussed and an analogy between the second black films and compounds is pointed out.