Swelling of a cluster phase in Langmuir monolayers containing semi-fluorinated phosphonic acids

Langmuir monolayers of semi-fluorinated nonadecylphosphonic acid (F8H11PO), hexadecylphosphonic acid (H16PO), and their mixtures were investigated by Brewster angle microscopy (BAM), atomic force microscopy (AFM) and surface-pressure measurements. Nanometre-scale two-dimensional clusters were observed by AFM in a spread monolayer of pure F8H11PO transferred to mica. Two different organized arrangements of clusters were observed. AFM and BAM observations showed that the mixture exhibits a solid phase over a large range of mole fraction and surface pressure, sometimes in coexistence with clusters. With increasing mole fraction of H16PO, the lateral shape of these clusters remains the same while their organization and their height change.