Hydrophilic-to-hydrophobic volume ratios as structural determinant in small-length scale amphiphilic crystalline systems: Silver salts of phenylacetylene nitriles with pendant oligo(ethylene oxide) chains

A series of 13 silver salts of phenylacetylene nitriles with pendant oligo(ethylene oxide) side chains have been crystallized, and their structures have been determined by single-crystal X-ray methods. All the organic molecules are amphiphilic in that they contain either a hydrophobic rigid linear or 3-fold planar phenylacetylene backbone to which are attached four to six rather plastic hydrophilic oligo(ethylene oxide) side chains. Six of the crystal structures have one-dimensional coordination networks based on Ag-N and Ag-O bonding, while the remainder are two-dimensional. Considering the hydrophilic and hydrophobic portions of these crystals as separate components of the crystal structure, five of the crystal structures contain parallel columns of hydrophobic moieties within a matrix of hydrophilic groups, four contain layers of hydrophilic and hydrophobic portions stacked one on top of the other, two have perforated lamellar structures and two are bicontinuous. Volume ratios of hydrophilic to hydrophobic portions prove a strong determinant as to which crystal topology is adopted for a given crystal composition. Such volume ratios can be calculated from knowledge of the molecular structure alone. A comparison is made to similar columnar and lamellar structures found in block copolymers and liquid crystalline systems. Finally, a similar breakdown of crystal topology as a function of volume ratios is found for aromatic-oligo(ethylene oxide) molecules in the Cambridge Structural Database.