Characteristic Features of Amphiphilic P-Functionalized Calixarene Monolayers at the Air/Water Interface

The combined application of surface pressure−area (π−A) isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD) offers a powerful possibility to characterize the amphiphilic P-functionalized calix[4]arene monolayers at the air/water interface. The studies are performed with two calix[4]arene bisphosphonates <BO>1</BO> and <BO>2</BO>, which are phosphorylated in the same way at the upper rim and differ only in the number of alkyl chains at the lower rim (compound <BO>1</BO> with four n-dodecyl chains; compound <BO>2</BO> with two n-dodecyl chains in opposite positions). Significant differences exist in the π−A isotherms of the two calix[4]arene bisphosphonates <BO>1</BO> and <BO>2 </BO>monolayers. In the case of derivative <BO>1</BO>, they resemble those of usual amphiphiles with the characteristic temperature dependence and a coexistence region between a fluid and condensed phase. The π−A isotherms of derivative <BO>2</BO> deviate considerably from those of typical amphiphilic monolayers having no temperature dependence and no “plateau” region. The characteristics of the π−A isotherms of both derivatives <BO>1</BO> and <BO>2</BO> are not affected by interaction with counterions of different valency (Na⁺, Cd²⁺, Eu³⁺, Th⁴⁺) present in the subphase. Both derivatives <BO>1</BO> and <BO>2</BO> form two-dimensional condensed phase domains with a specific shape which is more or less modified by different-valent counterions in the aqueous subphase. The analysis of the BAM results corroborates the general differences between the monolayer properties of derivative <BO>1</BO> and <BO>2</BO>. The GIXD results show that only the monolayers of derivative <BO>1 </BO>form a 2D lattice structure. The data indicate a rectangular unit cell, which is not changed by the complexation of thorium ions by the phosphonate groups.