Understanding and control of crystallographic polymorphism and crystal habit of
organic as well as inorganic compounds is scientifically and technologically important to
a number of industries. To date, however, the experimental control of polymorphs
(crystalline solids with different arrangements of the same constituents) is difficult. Since
a polymorph is determined at the nucleation of a crystal, methods that lead to an advanced
understanding of early crystal formation pathways and mechanisms are highly desirable.
We have successfully worked with a pharmaceutical compound, acetaminophen
(ACM) as our model system, on arrays of self-assembled monolayers (SAMs) from alkanethiols
and -silanes, with different terminal (omega) functional groups, on various substrates
(gold, silicon oxide/silicon). In the first two years of this project, we have established that
(i) both solvent and substrate work together to control crystal polymorph, and that (ii) on
hydrophobic SAMs, pure solvent systems such as ethanol, water, and 1,4-dioxane yield the
thermodynamically stable, monoclinic polymorph form I, while mixtures of water and 1,4-
dioxane produce the less favored orthorhombic form II. In the third year of this project, we
have studied the early formation stages of these form I and II crystallization events by
means of time-resolved in situ wide-angle x-ray scattering (WAXS) at Cornell’s High
Energy Synchrotron Source (CHESS). Using seeded crystallization events of form II
crystals and moving the x-ray beam vertically through the elongated film sample we
verified that crystallization originates at the substrate-solution interface. Studying
spontaneous crystallization events in a simple droplet of form I we identified unusual shifts
along scattering vector, q, of isolated scattering peaks at the earliest time points of crystal
nucleation and growth, pointing to the possible existence of structural transformations at
these early stages.
By providing first insights into the earliest formation stages of the crystallization of
ACM on SAMs as revealed by in situ synchrotron x-ray experiments in the third year of
the project, we are on target with achieving the goal of the original project brief. These
first results are quite exciting and warrant further in-depth studies of these phenomena.
With that knowledge, we are now in the course of collecting new data that we hope will
become relevant for the development of molecular dynamics simulations. Furthermore,
since the methodologies worked out in our studies to date have been effective, we consider
to extend our work to other model compounds.