STRUCTURED NUCLEI OF 4-(OCTADECYLOXY)BENZOIC ACID MONOLAYER FOR INDUCED NUCLEATION OF 4-HYDROXYBENZOIC ACID MONOHYDRATE AS DETERMINED BY GRAZING-INCIDENCE X-RAY-DIFFRACTION ON THE AQUEOUS-SOLUTION

A monolayer of 4-(octadecyloxy)benzoic acid at the air-solution interf ace has been used to induce oriented nucleation of three-dimensional ( 3-D) crystals of 4-hydroxybenzoic acid (HBA) monohydrate. The two-dime nsional (2-D) crystalline structure of this monolayer in a self-assemb led uncompressed state, both on a water subphase and on a solution con taining HBA, was determined by grazing incidence X-ray diffraction (GI D) using synchrotron radiation. Crystalline domains of large coherence length were formed on water subphase at 14 degrees C, and the fractio n of the monolayer in the ordered state was much increased on HBA solu tion. The agreement between the observed and calculated GID data was f ound to be sensitive to the conformation and orientation of the monola yer molecules. These results demonstrate the ordered binding of the HB A solute to the amphiphile benzoic acid head groups to form hydrogen-b onded cyclic dimers between the two carboxylic acid moieties. The GID method was complemented by other surface sensitive techniques such as specular X-ray reflectivity, surface pressure-area isotherms, and nonl inear optics. A resulting model arrangement of the monolayer, based on the molecular packing, has been proposed for the induced nucleation o f the 3-D crystals of 4-hydroxybenzoic acid monohydrate attached to th e monolayer.