Compression isotherms and morphological characteristics of pure and mixed langmuir monolayers of C₈₀ Isoprenoid Tetraacids and a C₁₈ monoacid

Monolayers of indigenous isoprenoid tetraacids (TA) isolated from an oilfield naphthenate deposit sample, and mixed monolayers of TA and a synthetic monoacid (MA), have been investigated under different experimental conditions by means of the Langmuir balance equipped with a Brewster angle microscope (BAM). For aqueous subphases at pH 2.3 and 5.6, the surface pressure area isotherms of pure TA show a broad plateau region. This shape is ascribed to a phase transition in which two of the carboxylic groups in the folded conformation are lifted to an upright position, leading to tightly packed molecules in a bilayer-like arrangement and formation of TA domains at the water surface. Upon compression beyond the plateau region, the domains gradually fuse and a uniform morphology appears. At pH 7.0 and 8.0, the compression isotherms increase steeply toward the point of monolayer collapse without reaching any well-defined phase transition region. This behavior is associated with dissociation of the carboxylic groups, which due to electrostatic repulsion hinder the molecules to pack densely at the surface and further to adopt a bilayer-like arrangement. In presence of CaCl₂ in the subphase at pH 8.0, a Ca-TA network is formed at the water surface. This is clearly demonstrated by the morphology upon film decompression during which the network breaks up into fragments. Miscibility of mixed monolayers of TA and MA has been examined on the basis on excess area calculations. Negative deviations from ideality suggest interactions between TA and MA for all compositions. The match in length of the extended MA molecule and the longest TA chains allows MA to occupy the gap in between the TA chains to form mixed domains of high density. BAM images of high contrast and domain brightness support this assumption.