Morphological Changes of Monolayers of Two Polymerizable Pyridine Amphiphiles upon Complexation with Cu(II) Ions at the Air-Water Interface

The monolayer behavior of two amphiphilic, diacetylenic units containing pyridine ligands at the air-water interface is studied by measuring the surface pressure-area isotherms and by Brewster angle microscopy (BAM). Both amphiphiles form stable monolayers at the air-water interface. The amphiphile containing an ester group shows a well-defined liquid-expanded (LE) to liquid-condensed (LC) phase transition, while the amphiphile with the amide group forms only a condensed monolayer film at 9.4 °C. For both amphiphiles, addition of CuCl2 to the subphase causes an increase of the surface pressure (Πc) at which the phase transition appears, suggesting the formation of a coordination complex at the air-water interface. Addition of Cu(ClO4)2 to the subphase instead of CuCl2 causes an even larger increase in Πc, indicating that more copper ions bind to the monolayer which results in a more charged monolayer. On a pure water subphase, Brewster angle microscopy of the monolayer of the ester-containing ligand shows the formation of spiral dendritic crystalline domains at the plateau in the isotherm near the solid state region. The formation of spiral crystalline domains indicates that the LC phase is L1'. The amide-containing ligand, however, forms two-dimensional crystalline domains directly after spreading at the air-water interface, which are pushed together upon compression. No chiral crystalline domains were observed for this amphiphile indicating that the ester and amide amphiphile have a different LC phase. Both amphiphiles spread uniformly when the subphase contains CuCl2, and upon compression crystalline domains are formed which grow when the area per molecule is reduced further, until a condensed monolayer film is formed. The shape of the crystalline domains on a Cu(II) ion containing subphase changes by replacing the Cl- counterion by a ClO4- anion. The size of the crystalline nuclei decreases when the Cu(II) concentration increases.