bcaFigure 2. AFM images of the (010) crystal surface of acetaminophen single crystals after partial
dissolution by a) water, b) acetone, and c) pyridine. The resulting surface texture provides information
about the dissolution mechanisms of acetaminophen and how the dissolution is related to its crystal
structure7. 60µm scans. Images courtesy T. Li, K.R. Morris, and K. Park, Purdue University(temperature, pH), and at an
appropriate rate for the drug to beeffectively absorbed. Dissolution rateand behavior have been shown tobe affected by surface area, temperature, crystal structure, pH and thenature of the solvents. 5 Thus, proper
characterization of these factors has
a very important role in a drug’soverall efficacy.

the imaging in air. Since the AFM
provides this information at thenanometer-to-micron scale, it canprovide information about the
dissolution mechanisms at the molecularscale. For these reasons, it isincreasingly being utilized bypharmaceutical researchers andmanufacturers.6–9

Traditionally, dissolution testing has
been carried out at the macroscale byconstant surface area or particulatedissolution methods to examine thebehavior of drug products underdifferent conditions. However, there
are several advantages of studying
the dissolution processes by AFM.
Dissolution is commonly tested at
37±0.5˚C, which is made possible
with AFM by heating the fluid
environment during experimentation.
Since the AFM can operate in airand fluid environments, the operator
has the choice of how to conduct theexperiment. It can be conducted in-situ,
i.e. by imaging the changing surface
structure in the dissolution medium
which provides information about thedynamics of the process.
Or it can be conducted ex-situ,
by viewing the resulting surface
topography at different stages of theetching process by removing thesample from solution and conducting

Dissolution Studies
The influence of different solvents on
acetaminophen single crystals wasinvestigated by AFM to gain a better
understanding of the dissolution
mechanisms at the molecular level.7
Partial dissolution tests were conductedon the (010) crystal surface with water,
dichloroethane, pyridine, acetone,
ethyl acetate, and acetic anhydride.
Dissolution patterns were produced
that are related to the interaction
between the solvent molecules and thecrystal structure.
As an example, Figure 2 showsparallelogram, square, and hexagonaldissolution patterns formed by water,acetone, and pyridine, respectively. In
many cases, the thickness of the steps
seen in these images is approximately
10 angstroms, which is consistent withthe dissolution of a layer of unit cells.
The dissolution process produced
etching patterns that are related to thecrystal structure, and the surface
interaction between the solvent andacetaminophen molecules in thediffusion layer. These results werecompared to computer simulationsof the same process to gain a better
understanding of how the resulting
surface texture provides information
about the dissolution mechanismsof acetaminophen. In the cases ofwater, acetic anhydride, and pyridine,
the etch pits were formed parallelto the a and c crystallographic axesof the crystal. However, the etchingpatterns formed by acetone,
dichloroethane, and ethyl acetate were
not clearly understood and did notmatch the structure expected from thecomputer simulations. It is believed thatin these cases, the etching process
was disrupted by the absorptionof solvent molecules on the crystal
surface, resulting in unexpectedetching patterns.
This hypothesis was further tested
by observing the etching patterns
produced by water with the tailormade additives, acetanilide and 4methyl acetanilide. 8 Drastically different
patterns resulted from small amounts ofthese additives. This effect is believed
to be caused by the absorption of
additive molecules that diffuse into thecrystal lattice and disrupt the original
supramolecular interaction network,changing the etch pits fromparallelograms (in pure water) torectangular, square, or circular forms.
In another example, AFM hasalso been applied to gain anunderstanding of the differences indissolution behavior of the (001)and (100) crystal surfaces of aspirin.9
Dissolution studies have shown a 50
percent larger dissolution rate for the(100) crystal plane with respect to the(001) surface. TappingMode images
show a smooth (001) surface with
7.3-angstrom molecular steps (Figure3). The (100) surface was muchrougher with 50- to 100-angstrom

abFigure 3.TappingMode images of aspirin crystal surface. a) (001) crystal surface showing 7.3Å monomolecular steps, b) (100) crystal surface irregularsteps ranging from 50 to 100Å9. 4 µm height (left) and Phase (right) images. Images courtesy C.J. Roberts, Univ. of Nottingham, UK.

steps. Thus, the (100) surface has agreater surface area, which may
contribute to the faster dissolution rate.
Further investigation of these surfaces
was conducted by the functionalizationof the AFM probes with –CH3
terminated groups and –COOH
terminated groups to producehydrophobic and hydrophilic sensors,
respectively. Using functionalizedprobes to study the chemicalinteractions with the surface iscommonly referred to as Chemical
Force Microscopy (CFM) 10. By using
amplitude-phase distance relationships
between the crystal planes and thefunctionalized probes, it was observedthat the hydrophobic –CH3 terminatedprobes had a strong attractive
relationship with the (001) surface,whereas the hydrophilic –COOH
terminated probe had a strong
attractive relationship with the (100)surface. Thus, the strong hydrophilic(100) surface would lead to easier
wetting in an aqueous environment
resulting in a faster dissolution rate.
The dynamics of the dissolution
process of aspirin (001) and (100)crystal planes were further tested byconducting real-time AFM dissolution
studies in a 0.05M HCl solution.6
By conducting the imaging during
dissolution, the dissolution mechanisms
and rate were studied. For the (001)plane, dissolution occurred by therecession of molecular step edges,

resulting in a dissolution rate of 0.45
nm/s (Figure 4). For the (100) plane,dissolution occurred by the crystal
terraces sinking at a rate of
2.93nm/s. Intrinsic dissolution rates
were determined to be 1.35 x 10-7
g s -1 cm-2 and 8.36 x 10 -7 g s-1 cm-2for the (001) and (100) planes,respectively. Thus, the (100) planedissolved at a rate six times greaterthan the (001) plane.
This difference in the dissolution rate
could be contributed partially to themolecular orientation of the crystal
faces. The (001) crystal face hasbenzene rings at the surface, which

are not easily removed in the 0.05M
HCl solution, whereas, the ester groups
are exposed as the terraces on the(001) face, which are more easily
removed, resulting in dissolution of thereceding step edges. The exposure ofmany ester groups at the surface of the(100) crystal face leaves all points on
the surface susceptible to removal
during dissolution, resulting in a fasterdissolution rate and a different
mechanism for material removal. Also,
as discussed in the previous example,
the increased surface roughness andthe hydrophilic nature of the (100)surface contribute to a faster dissolution
rate as well.

Figure 4. In situ dissolution of the (001) in 0.05M HCl by receding step edges. The white arrowshows the progression of the same crystal terrace in each images at times of a) 0, b) 66, c) 132,d) 197, e) 263, and f) 329 seconds.6 10µm scans. Images courtesy C. Roberts, University ofNottingham, UK.