B. Kellner, D. Cadenhead
Mar 1, 1978
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Abstract
The monolayer characteristics of a series of homologous hydroxyhexadecanoic acids have been determined in order to obtain information on the effect of attaching a secondary hydrophilic group to various positions along the alkane chain of a simple fatty acid. Evidence is described for an immersion mechanism for 2-hydroxyhexadecanoic acid (2HHA) and 3-hydroxyhexadecanoic acid (3HHA), whereby the methylene groups separating the primary and secondary polar groups are additionally immersed in the water as a result of the combined strength of the two polar groups. These two molecules thus act as monopolar substances. In contrast, 9-hydroxyhexadecanoic acid (9HHA) and 16-hydroxyhexadecanoic acid (16HHA) act as bipolar substances, exhibiting gaseous behavior at high areas/molecule. Both show a point of inflection in their respective pressure-area and potential-area isotherms at the point at which the weaker polar group (the hydroxyl) is expelled from the interface. 9HHA exhibits a stable condensed region, whereas 16HHA does not. These differences are explained in terms of the respective transition states of the monomolecular films as well as the stability of the solid forms of the two molecules.