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"I shall be obliged if you can find space for the accompanying translation of an interesting letter which I have received from a German lady, who with very homely appliances has arrived at valuable results respecting the behavior of contaminated water surfaces. The earlier part of Miss Pockels' letter covers nearly the same ground as some of my own recent work, and in the main harmonizes with it. The later sections seem to me very suggestive, raising, if they do not fully answer, many important questions. I hope soon to find opportunity for repeating some of Miss Pockels' experiments." RAYLEIGH

My Lord, - Will you kindly excuse my venturing to trouble you with a German letter on a scientific subject? Having heard of the fruitful researches carried on by you last year on the hitherto little understood properties of water surfaces, I thought it might interest you to know of my own observations on the subject. For various reasons I am not in a position to publish them in scientific journals, and I therefore adopt this means of communicating to you the most important of them.

First, I will describe a simple method, which I have employed for several years, for increasing or diminishing the surface of a liquid in any proportion, by which its purity may be altered at pleasure.

A rectangular tin trough, 70 cm long, 5 cm wide, 2 cm high, is filled with water to the brim, and a strip of tin about 1 1/2 cm wide laid across it perpendicular to its length, so that the under side of the strip is in contact with the surface of the water, and divides it into two halves. By shifting this partition to the right or the left, the surface on either side can be lengthened or shortened in any proportion, and the amount of the displacement may be read off on a scale held along the from of the trough. ...."

Yours faithfully, (Signed) Agnes Pockels Nature, 43, 437 - 439, 1891.

n.b. This paper followed one by Prof. Van der Waals. Nice company, eh?

See also:

Surfactants from Lectures for AP225.


Extended Reading

  • de Gennes (1993)
    • Chapter 1. Liquid crystals: Main types and properties
  • de Gennes (2004)
    • Chapter 8. Surfactants
      • A surfactant is a molecule with a hydrophillic part (eager to mix with water) and a hydrophobic part (abhors water). The hydrophobic part is usually formed by an aliphatic chain CH<math>_3</math>(CH<math>_2</math>)<math>_n</math>. The hydrophillic part can be an ion (the polar head). This polar head likes liquids with high dielectric constants (e.g. water). p. 191
      • When we add surfactant to an oil/water system, only a very small amount is present at the boundary. The rest is dispersed in the two liquids. It is of interest to determine how much of a surfactant is present in each of the liquids (i.e. in oil and water). We use the HLB (hydrophilic lipophilic balance), but this system is far from perfect. p. 192-193
      • The size of a micelle (an aggregate of surfactant molecules with all the polar heads on the outside and aliphatic tails on the inside) is dependent on the area per polar head. and the volume per chain. p. 194
      • Below the critical micelle concentration (cmc<\i>), surfactant molecules will exist in the form of isolated molecules. Above the <i>cmc, surfactant molecules will form micelles. Higher concentrations of surfactant may produce cylinders or bilayers. p. 196
      • Surface tension decreases with increasing surfactant concentration up to the cmc. p. 199
      • "Surfactants that are highly hydrophobic (i.e., with a very low cmc) cannot by good foaming agents." p 200
  • Holmberg
    • Chapter 1. Introduction to surfactants
    • Chapter 3. Phase behavior of concentrated surfactant solutions
    • Chapter 11. Novel surfactants
    • Chapter 12. Surface active polymers
  • McBain
    • Chapter 17. Soap and other colloidal electrolytes
  • Tanford
    • Chapter 6. Micelles
    • Chapter 11. Biological lipids
    • Chapter 13. Proteins
    • Chapter 15. Serum lipoproteins
    • Chapter 16. Biological membranes
    • Chapter 17. Membrane proteins