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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 1 and 2, 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 1 with four n-dodecyl chains; compound 2 with two n-dodecyl chains in opposite positions). Significant differences exist in the π-A isotherms of the two calix[4]arene bisphosphonates 1 and 2 monolayers. In the case of derivative 1, 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 2 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 1 and 2 are not affected by interaction with counterions of different valency (Na + , Cd 2+ , Eu 3+ , Th 4+ ) present in the subphase. Both derivatives 1 and 2 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 1 and 2. The GIXD results show that only the monolayers of derivative 1 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.

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