6

 

Monolayers at the air-water interface

 

 

         Stearic acid molecule

 

Chemical formula:               H₃C-(CH₂)₁₆-COOH

Model (grey - carbon, white - hydrogen, red - oxygen):

         Interaction of amphiphilic moleciles with water and with each other

 

Surface tension of water:                                72-73 mN/m

Surface tension of octadecane:                      28-29 mN/m

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Energy of hydrogen bond:                             4-7 kCal/mol

Energy of Van der Waals interaction per CH2-group inside the monolayer:                                                       2.0-2.2 kCal/mol

Energy of Van der Waals interaction between adjacent monolayers (per one molecule):                                                         1.9-2.1 kCal/mol

(For comparison, kT SYMBOL 187 \f "Symbol" 0.5 kCal/mol, energy of covalent bond is of about 50 kCal/mol.)

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Solubility of fatty acids in water (g/l, at 20°C):

         Valeric acid                (C₄H₉-COOH)          -        32.0

         Caproic acid               (C₅H₁₁-COOH)        -        9.7

         Enanthic acid             (C₆H₁₃-COOH)        -        2.4

         Caprylic acid              (C₇H₁₅-COOH)        -        0.68

         Pelargonic acid          (C₈H₁₇-COOH)        -        0.26

         Stearic acid                 (C₁₇H₃₅-COOH)      -        0.0029

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