The role of hydrogen bonds in condensed monolayers

Abstract

From the behaviour of monolayers of compounds containing the —CO.NH— linkage it is concluded that intermolecular hydrogen bonding can play a major role in determining the properties of condensed monolayers. Such effects, which are well marked in the ureas, amides, acetanilides, unsubstituted and α -amino acids, tend to bring about condensation and solidification, and a marked increase in the half-expansion temperature. A detailed examination of the acetamides has been made in both the expanded and condensed regions by combined force area and surface potential methods. Comparison of the condensed films with those of the analogous acetates, where no such intermolecular hydrogen bonding is possible, shows several striking differences. With the acetamides and ureas the hydrogen bonding has been shown to be quite sensitive to the pH of the substrate, very acid substrates leading to complete liquefaction. The hydrogen bond distances can be calculated and the values so obtained are found to agree quite well with those in the crystal for the same or similar head groups. The free energy difference between the —CO.NH— group when forming hydrogen bonds to water (as in the expanded films), and when cross-linked (as in the low temperature form), is calculated for the acetamides to be about 840 cal./g.mol. Values of the same order can be calculated for the other systems discussed. The importance of such measurements in determining the part played by the hydrogen bond in the protein molecule is pointed out.