Texture features of long-chain fatty acid monolayers at high pH of the aqueous subphase

The thermodynamic properties and the condensed phase textures of Langmuir monolayers of protonated and deprotonated long-chain fatty acids (stearic acid, arachidic acid, and behenic acid) are studied. Surface pressure–area (π–A) isotherms and Brewster angle microscopy (BAM) images are recorded in the regions of low and high pH values. In the range of pH 11–13, the π–A isotherms of these long-chain fatty acid monolayers show a plateau region indicating a two-phase coexistence region. Under these conditions, segmented domains with highly ordered textures are formed in the stearic acid arachidic acid monolayers, whereas the domains of behenic acid monolayers are irregularly shaped without an inner texture. The best developed domains of stearic acid and arachidic acid are circularly shaped and subdivided into six almost identical segments which do not meet exactly in the centre. The long-range orientational order of the condensed phase domains is discussed. Addition of neutral salt (NaCl) to the subphase improves the stability of the long-chain fatty acid monolayers at high pH values by a salting-out effect. A time-dependent pH drift due to atmospheric CO2 and desorption of monolayer material are characterised for realising optimal monolayer stability.