2015 AIChE Annual Meeting Proceedings
(699f) Solvent Influence on Dodecanethiol Interactions Measured By Atomic Force Microscopy
Authors
Introduction:
The
control of nanoparticle dispersability is important for many applications, such
as size-selective nanoparticle precipitation, thin film deposition, and
nanoparticle composite formation. Current methods for the preparation of monodisperse
nanoparticles are costly, require a large quantity of solvent, or have a low
throughput [1-3]. Surfaces of nanoparticles are often
passivated with stabilizing ligands which prevent agglomeration by introducing
thermodynamic repulsive forces. Understanding
the forces between such ligands can provide insight into a better way to
control nanoparticle dispersion [3]. Recently, modelling efforts have shown that
traditional steric repulsion and van der Waals attractions are not sufficient
to characterize the interactions between nanoparticles in poor solvents [4]. In this study, the importance of solvation
forces on regulating the interactions of dodecanethiol ligands were quantified via
atomic force microscopy (AFM). We chose toluene, hexane, and ethanol to
represent good, mild and bad solvents, respectively to elucidate the forces
involved in the interactions of dodeceanthiol (C12SH) ligands.
Materials and Methods:
Silicon
nitride (Si3N4) AFM probes were coated with a 5 nm chromium
adhesive layer followed by a 40 nm gold layer. The gold-coated
cantilevers were then cleaned in absolute methanol and ethanol solution for 15
minutes each, respectively. Cantilevers were then immersed for 18 hours at room
temperature in 1 mM ethanolic solution of 1-dodecanethiol for self-assembled
monolayer (SAM) formation. Functionalized AFM cantilevers and substrates were
then loaded into the AFM and imaging and force measurements were conducted in
ethanol, hexane, and toluene. Areas of 2×2 μm2
were scanned at a rate of 1 Hz. Force-distance curves were collected with a
ramp size of 1 μm at 1 Hz
and with a 5 nN threshold force. Jump into contact and adhesion forces in the
approach and retraction curves were analyzed, respectively.
Results and Discussion:
As the ligand-decorated AFM
cantilever approaches or retracts from the opposing ligand surface, the ligands
interact with each other. Representative approach and retraction force-distance
curves are shown in Figure 1. As the ligand-decorated cantilever moves away
from the opposing ligand surface, adhesion forces can be measured. The mean
adhesion force was highest in ethanol (0.172 ± 0.073 nN) followed by toluene
then hexane with adhesion forces of 0.165 ± 0.100 nN and 0.147 ± 0.063 nN, respectively.
The high adhesion force between the hydrophobic C12SH layers in
ethanol is largely due to the unwillingness of hydrophobic molecules to
interact with hydrophilic solvents. Because ethanol is hydrophilic, the C12SH
will want to interact with other C12SH molecules and will therefore
not want to be separated. This increases the adhesion force or force required to
separate the two C12SH SAM layers. In comparison to hexane,
hydrophobic SAMs interacted with a higher force in toluene. This may be due to the
π bonds that form between toluene and the extended C12SH polymers.
In hexane, SAMs had the lowest retraction adhesion forces. This is mainly
because hexane is hydrophobic and C12SH will likely interact with
the solvent just as much as it will interact with the opposing C12SH
layer.
In the approach curve or as the SAM
layers come together, there is a jump into contact adhesion force that was
analyzed with respect to its magnitude, frequency, and jump distance. The
magnitude of these adhesion forces were 0.05 ± 0.02 nN for hexane, 0.08 ± 0.03
nN for ethanol, and 0.10 ± 0.03 nN for toluene. Although toluene seems to have
the strongest adhesion force, these events only made up 11% of the curves
whereas in hexane and ethanol, they were observed in 37 % and 60% of the registered
approach curves, respectively. Since toluene is expected to extend the ligands,
steric repulsion between the ligands will increase. Such increase in the
repulsive forces hinder the attractive forces between ligands and thus reduce
the probability of the occurrences of attractive forces. Ligands in hexane
interacted with each other intermediately with the lowest strength. This is
because the ligands are able to interact with the solvent as both are hydrophobic.
However, in the presence of ethanol (hydrophilic solvent), the hydrophobic
interactions will dominate the affinity of ligands and thus minimize the steric
repulsion between the ligands.
The molecules interacted together
in toluene at the longest distance (5.04 ± 3.58 nm). In comparison, molecules
needed to be closer to each other for the jump into contact interactions to
occur in ethanol and hexane with distances of 2.78 ± 1.11 nm and 2.90 ± 1.48
nm, respectively. These later distances were not statistically significantly
different. The longer distance registered in toluene suggests that ligands adopt
an extended conformation. It is important to note that jump distance does not
represent the actual length of the ligands but the distance where the ligands
start to attract each other. Currently, approach data is being fit to a model
of steric repulsion to quantify the conformational properties of the SAM
chains. All adhesion force and jump into contact data were statistically
different among solvents tested. With respect to jump distance, toluene was
statistically significantly different from the other treatment groups.
Conclusions:
When in hydrophobic or good solvents,
the C12SH ligands were extended whereas hydrophilic or bad solvents
caused the ligands to collapse. Ligands interacted more favorably with each
other when in hydrophilic solvents due to solvation excluding effects and vice
versa when they were in good hydrophobic solvents. As a result, the use of hydrophilic
solvents may be detrimental to dispersing nanoparticles.
Acknowledgements: This work
was supported by the, NIH Protein Biotechnology Training Program 24280305, a
NASA Space Grant, a WSU DRADS fellowship, and a Harold P. Curtis Scholarship.
Figure 1.
Representative force-distance curves of DDT SAMs in various solvents. a) Retraction curves with the adhesion forces as quantified form the
adhesion peaks b) Approach curves with jump into
contact peaks. The jump distances were quantified as the difference between the
distance of maximum peak value and the distance where there is no more
attractive forces between ligands.
References