Structural Characterization of Crystalline Ternary Inclusion Compounds at the Air−Water Interface

Crystalline ternary inclusion monolayers consisting of a two-dimensional hydrogen-bonded host
network of guanidinium (G) ions and organosulfonate (S) amphiphiles, and biphenylalkane guests, can be
generated at the air−water interface through synergistic structural enforcement by hydrogen bonding and
host−guest packing. Surface pressure−area isotherms of the 4‘-hexadecylbiphenyl-4-sulfonate (C16BPS)
amphiphile in the presence of G, with or without guest, are characterized by lift-off molecular areas expected
for the GS sheet based on single-crystal X-ray structures of homologous bulk crystals. Intercalation of
biphenylalkane guests (4-CnH2n+1−C6H4−C6H5, n = 1, 4, 6, 10, 16; denoted CnBP) between organosulfonate
hydrophobes, which define pocketlike cavities in the GS monolayer host, afford ternary inclusion monolayers
with a 1:1 host−guest stoichiometry. These inclusion monolayers are less compressible than the guest-free host, consistent with dense packing of the biphenylalkane moieties of the host and the biphenylalkane
guests. The inclusion monolayers are distinguished from the amorphous guest-free host and from selected
guanidinium-free mixed monolayers by structural characterization with grazing-angle incidence X-ray
diffraction (GIXD). The GIXD data for the ternary (G)C16BPS:C16BP and (G)C16BPS:C6BP inclusion
monolayers obtained upon compression are consistent with a rectangular unit cell. The dimensions of these
unit cells and refinement of the GIXD data suggest a “rotated shifted ribbon” GS hydrogen-bonding motif
similar to that observed in some bulk GS crystals, including (G)(ethylbiphenylsulfonate). GIXD reveals that
(G)C16BPS:C16BP and (G)C16BPS:C6BP are more crystalline than the corresponding guanidinium-free
mixed monolayers. The (G)C16BPS:C6BP inclusion monolayer is stable upon compression, even though
the alkyl−alkyl host−guest interactions are reduced due to the shorter hexyl substituents of the guest,
demonstrating an important reinforcing role for the hydrogen-bonded GS sheet. The structure of a C16BPS:tetracosane (C24) mixed monolayer is independent of G; the unit cell symmetry and dimensions suggest
a structure governed by alkyl−alkane interactions that prohibit formation of a GS network. These results
illustrate that the existence of ternary inclusion monolayers with an intact GS network requires guest
molecules that are structurally homologous with the hydrophobes of the host, in this case biphenylalkanes.
The observation of these inclusion compounds suggests an approach for introducing functional nonamphiphilic molecules to an air−water interface through inclusion in a well-defined host.