IMPROVING THE HYDROPHOBICITY OF KITCHENWARE THROUGH THE
COVALENT BONDING OF PHOSPHONIC ACIDS
Emily Chen, Marcus Elias, Jonathan Lin, Nathaniel Okun, Olabade Omole, Matthew Piccolella
Suraj Shukla, Dominique Voso, Jonathan Wu, Peter Xiong, Tania Yu
Adviser: Dr. Michael Avaltroni
Assistant: Liz Day
ABSTRACT
In this experiment, phosphonic acids with various carbon chains were covalently bonded
to three common kitchen surfaces: glass, tile, and aluminum. The main goals of these trials were
to examine which alkyl groups correspond to the greatest increase in surface hydrophobicity, to
test the durability of these coatings, and to study different application methods. As expected, the
collected data show a general increase in hydrophobicity with the long, more ordered carbon
chains. In addition, the group used an oven, heat gun, and iron to heat each surface and
determine an optimal method for application. The group observed that placing the samples in the
120°C oven for eighteen hours proved to be the most consistent and effective method. The group
also examined the consequences of wear methods such as water rinsing and soap-water rubbing.
The results for these tests varied depending on whether more disordered multi-layer was
removed or more physically-bonded phosphonic acids. Lastly, the group found that applying two
coatings increases the covalent bonding of the phosphonic acids. Overall, the group collected and
analyzed data concerning the application, durability, and effects of various phosphonic acids on
common kitchen surfaces.
INTRODUCTION
The purpose of this experiment was to find a more advantageous replacement for
Teflon®, the most popular non-stick surface for kitchenware. Teflon®, or
polytetrafluoroethylene (PTFE), was discovered in 1938 by Dr. Roy Plunkett at the DuPontTM
research laboratories in New Jersey. While working on refrigerants, Plunkett discovered that a
compressed sample of tetrafluoroethylene had polymerized to form the waxy, hydrophobic
substance that is now marketed as Teflon®1. While Teflon® is extremely hydrophobic, it is also
slightly carcinogenic2. Fluorides present in Teflon® can potentially seep into food, and thus harm
consumers. In its search for an alternative to Teflon, the team turned to self-assembled
monolayers of phosphonic acids (SAMPs). SAMPs were developed by surface chemist Dr.
William A. Zisman at the Naval Research Lab in the 1950s. It was actually Zisman who notified
DuPont of the significance of PTFE’s high hydrophobicity, which led to its wide-scale marketing
as Teflon3. The self-assembly method developed by Zisman and his colleagues is very flexible
and has come to be applied in many different fields, including biology, electronics,
electrochemistry, and household goods (such as Rain-X®)4.
[2-1]
Figure 1: Polytetrafluoroethylene
PTFE is a polymer than forms from the alternate bonding of C to F in long chains
Hypothesis
If phosphonic acids with longer carbon chain tail groups are applied to chosen surfaces,
then the resulting hydrophobicity of those substances will increase because of their ability to
layer in a more organized fashion than those with shorter carbon chain tail groups.
If the phosphonic acids covalently bond to the oxide surfaces, then washing the samples
with plain water will increase hydrophobicity. The team believed that, if the phosphonic acid
solutions were covalently bonded correctly, the wash would remove any surface residue left from
manufacturing and help achieve the desired SAMP surface.
If consistent, moderate heat bonds phosphonic acid solutions the best, then the samples
baked in the conventional oven will display the highest hydrophobicity. The oven's temperature
(120oC) and method of administration (consistent, non-direct contact) pose no risk of
decomposing the phosphonic acids or blowing off the still-liquid coatings, in contrast to the heat
gun or iron.
If the ratio of hydroxyl groups to µ-oxo on an oxide surface determines the surface's
ability to bond to phosphonic acids, then aluminum will display the greatest hydrophobicity
improvement after being treated with the phosphonic acid solutions. Hydroxyl groups are more
reactive than the relatively inert µ-oxo. Out of the three materials, aluminum has the highest
hydroxyl to µ-oxide ratio, so it would theoretically be better able to bond the phosphonic acids.
Theory and Application of Self-Assembled Monolayers
Self-assembled monolayers, commonly known as SAMs, have special properties that
allow them to attach to oxide surfaces. SAMPs (self-assembled monolayers of phosphonic acids)
can covalently bond to hydroxyl groups and form a stronger surface coating than conventional
resins that bond Teflon and other coatings to surfaces. Because the phosphonic acids form strong
covalent bonds with the surface, the stability of the bonds to the surface is ensured#.
Conventionally, since high levels of heat are used to bond the SAMPs to a surface, the
same amount of high energy is needed to remove the SAMPs. Thus, the SAMPs form an
extremely durable coat that cannot be broken with daily kitchen practices. Despite modern
surface science applications, there are still obstacles to creating an optimal surface. Most oxide
surfaces possess two different kinds of surface bound oxygen: hydroxyl and µ-oxo groups (Fig
2). However, many molecules (including some SAMPs), are only capable of bonding to the
highly reactive hydroxyl group (OH) and minimally bond to the µ-oxo group2. With recent
[2-2]
developments in surface science, it has been found that phosphonic acid is able to effectively
bond to both the hydroxyl and µ-oxo groups on oxide surfaces.
Figure 2: Sample Oxide Surface
An example of an oxide surface containing a µ-oxo bridged oxygen (left) and a hydroxyl group
(right)
One molecule in particular, octadecylphosphonic acid (ODPA), has a central phosphate
bonded to two hydroxyl (OH) groups and double bonded to oxygen along with an eighteen
carbon chain length (Fig. 1). Because of the nature of the molecule (and the rest of the similar
molecules), the phosphonic acid is able to make covalent bonds while the long hydrocarbon
chain contributes to the hydrophobic nature of the molecule. In this study, phosphonic acid
molecules with different hydrocarbon chain tail lengths were tested on different surfaces.
Varying from six carbons to eighteen carbons, the molecules have different properties and once
bonded, serve as hydrophobic and anti-wear coatings for kitchenware surfaces – glass, tile, and
aluminum.
Figure 3: Octadecylphosphonic Acid
ODPA has a carbon chain of eighteen with a phosphonate group at the head that can readily bind
to oxide surfaces.
[2-3]
Phosphonic acids bond to oxide surfaces through many steps. First, phosphonic acid is
applied to the oxide surface and then the phosphonic acid donates a hydrogen atom (proton) to
the hydroxyl group of the oxide surface and forms a negative charge. Next, through the
application of heat and dehydration synthesis reaction, water is removed from the oxide surface
and a positive charge is formed. Because of the negative charge on the phosphonic acid and the
positive charge on the oxide surface, the phosphonic acid bonds to the oxide surface and donates
another hydrogen atom (proton) to the µ-oxo oxygen group. Lastly, the phosphonic acid bonds to
the newly formed hydroxyl group and a cascade of similar reactions follow, forming the
hydrophobic coating on the oxide surface (Fig. 2).
Figure 4: Bonding of ODPA to an Oxide Surface
A sample mechanism by which the octadecylphosphonic acid bonds covalently to the oxide
surface under heating conditions by bridging oxygens and hydroxyls to the surface
Surfaces of Interest
The three surfaces that were tested were glass, ceramic tile, and aluminum. These three
materials were selected because they are all oxide surfaces. An oxide surface is a material which
is protected on its surface by a thin coating of oxygen. This oxygen typically takes on the form of
surface hydroxyl groups and µ-oxides, the latter of which phosphonic acid has the rare ability to
bond to when heated5.
Glass is an indispensable material in the modern kitchen. The type of glass that is most
often used in the home, such as in Pyrex baking dishes, measuring cups, and drinking glasses, is
soda-lime glass2. Soda-lime glass is composed, by weight, of approximately 74% silica (SiO2)
and 16% sodium oxide (Na2O) with additional amounts of calcium oxide (CaO), potassium oxide
(K2O), magnesium oxide (MgO), and aluminum oxide (Al2O3) for stability3. Glass is an
amorphous solid, which is also classified as an oxide surface. An oxide surface is a material,
which is protected on its surface by a thin coating of oxygen. This oxygen typically takes on the
form of surface hydroxyl groups and the µ oxo groups, the latter of which phosphonic acid has
the rare ability to bond to when heated4.
Ceramic tile is another multipurpose material that can be found in most kitchens. For
example, porcelain can be found in wall and floor tiling, bake ware, and dinnerware place
settings. Ceramic is an inorganic crystalline material that is also highly heat and corrosionresistant, properties that contribute to its versatility and practicality in the kitchen5. The main
[2-4]
component of porcelain is the relatively inert clay mineral kaolinite (kaolin, Al2Si2O5
(OH)4)6. Porcelain, like soda-lime glass, is an oxide surface, which means that its surface
hydroxyl groups allow it to bond to phosphonic acid. While porcelain itself already displays
excellent abrasion resistance and hydrophobicity, its chemical composition still allows for an
increase in hydrophobicity.
Aluminum, one of the most abundant metals on the planet, is possibly one of the most
important materials in non-stick cookware. It is advantageous to use in the kitchen because it is
lightweight, corrosion-resistant, and non-toxic7. Non-stick pots and pans are most often made of
anodized aluminum - aluminum whose natural oxide layer is thickened by the process of
passivation8. Aluminum already passivates naturally, so industrial passivation just further
enhances its corrosion and wear resistance. Due to its high reactivity and status as an oxide
surface, aluminum is an ideal surface to bond with phosphonic acid4.
MATERIALS AND METHODS
Preparation of Phosphonic Acids and the Surfaces
The group tested seven different phosphonic acids (each with a different R-group) in
order to determine which was the most effective in improving the hydrophobicity of aluminum,
glass, and tile. The acids utilized in the experiment were hexylphosphonic acid (HPA),
octylphosphonic acid (OPA), decylphosphonic acid (DPA), dodecylphosphonic acid (DDPA),
tetradecylphosphonic acid (TDPA), hexadecylphosphonic acid (HDPA) and
octadecylphosphonic acid (ODPA).
The group first removed all potential contamination from each of the surfaces used. The
samples of aluminum were cleansed by rubbing their surfaces with sandpaper to remove any
residue. The group then cut the aluminum sheets into squares with dimensions one by one
centimeter. The team then used a hot plate to warm 250mL of 95% ethanol in a 450mL beaker
to about 65°C. The aluminum squares were immersed in the warm ethanol bath for three
minutes to remove any last vestiges of contamination. The group then dried the squares with
paper towels and set them aside for further use. This process was repeated with both glass and
tile, though the group did not sand either of these substances, as they were not covered with oil
during the production process.1
The group then prepared solutions of the phosphonic acids. To achieve this, phosphonic
acids (which were in powder form) were mixed into a solvent of 50% toluene and 50% ethanol.
The group calculated the amount of phosphonic acid necessary to ensure that the molarity of the
solutions prepared was 0.001M. A spray bottle was used to apply an even coat of the solution to
each surface. Excess liquid was then rolled off with a Mayer rod
Uncoated squares of glass, aluminum, and tile were prepared in the same manner as
controls. The hydrophobicity of the surfaces was measured using a Ramé-Hart® Contact Angle
goniometer. The instrument uses an infrared camera to take an image of a water droplet on a
surface. The angle of the water droplet to the surface determines hydrophobicity, where a
measure greater than 90o makes a surface be considered hydrophobic, which can be seen in
Figure 5.
[2-5]
Creating the Control Group
The team created a control group to serve as a comparison for future results. The
hydrophobicity of glass, aluminum, and tile without any chemical alterations was tested using
the Ramé-Hart® Contact Angle goniometer. This device acts as a powerful camera that enables
one to view and measure the contact angle of any liquid on a chosen surface.
Figure 5: Example Contact Angle Measurement
Once the goniometer had taken an accurate picture of the water droplet, a simple computergenerated protractor was used to measure the contact angle. This was done for both controls and
experimental groups.
Covalently Bonding the Phosphonic Acids to the Surfaces
The group first examined oven-based heating, where samples were placed in an oven set
at 120oC for twenty-four hours. The glass samples were laid out in the oven in an orderly
fashion, while the aluminum and glass samples were separated on watch glasses. The group also
experimented with using a Varitemp® heat gun and a Toast Master® iron to heat samples. When
using the heat gun, the group blew surfaces with air heated to roughly 400oC for three minutes.
The group heated surfaces using the iron by applying maximum heat to each sample for five
minutes. After heating, the samples were left to cool. Once the samples cooled, the group tested
each substance’s hydrophobicity using a contact angle test in which the angle of a water droplet.
Wear Tests for Coated Samples
Wear tests were performed on the samples to test the effectiveness of the covalent bonds
in withstanding everyday use. The group first soaked samples in distilled water at room
temperature for five minutes. After pat drying the samples with a paper towel, the team measured
the contact angles. In addition, the group tested samples by rubbing them with a 50-50 mixture
of soap and water. The contact angle was then measured once more.
Corollary Experiments
After completing the main part of the experiment, the group repeated the most successful
tests with two coats of phosphonic acid instead of one. New samples were prepared, and then
heated with a coating of phosphonic acid using either the oven or iron-based heating method.
[2-6]
After this was completed, the group coated the samples with another coat of the same
phosphonic acid and heated them using the same method as previously utilized. The contact
angles were then measured.
RESULTS
Figure 6: Comparing Chain Lengths and Wear Methods – Tile with Oven Heating
This graph expresses the relationship between the average contact angle and the seven different
carbon chain length phosphonic acids—HPA(C6), OPA(C8), DPA(C10), DDPA(C12), TDPA(C14),
HDPA(C16), and ODPA(C18)—as well as the untreated control. It also shows the angle after
each of the two wear tests, water wash (Water) and soap scrub (Soap). The error bars represent
the standard deviation among the trials conducted.
Figure 6 displays the increase in contact angle on tile after a phosphonic acid coating was
added in all cases except for the acid with 8 carbons in its tail group. With the OPA, contact
angle decreased by 11.51° from the control. The least increase was found in the HPA, with an
improvement of 10.78°; the greatest increase, 28.49°, was found in the ODPA, which had an
average contact angle of 70.17°. The lower contact angles indicate lower hydrophobicity, and
higher angles indicate higher hydrophobicity.
[2-7]
Figure 7: Comparing Chain Lengths and Wear Methods – Aluminum with Oven Heating
This graph shows the correlation between the average contact angle in degrees and each of the
different carbon chain length phosphonic acids (C6-C18) as well as the control. The red and green
bars show the results after each of our wear tests, washing with water and with soap,
respectively, were applied.
Figure 7 shows the effects of the various SAMPs on aluminum, with an overall increase
in contact angle for every carbon chain tested compared to the control. Least effective was the
OPA, as the angle measurement rose by 4.88°. HPA, at an average of 92.96°, improved the
most—31.35°— but ODPA followed closely at 92.38° with an increase of 30.77°.
Figure 8: Comparing Chain Lengths and Wear Methods – Glass with Oven Heating
The graph shows the comparison between contact angles measured and various chain lengths of
SAMPs(C6-C18) on oven-heated glass before and after water and soap washes, represented by the
red and green bars, respectively.
[2-8]
In Figure 8, an increase also occurs for each coating on the glass slides. HDPA yielded
the lowest change of 22.41°, whereas ODPA showed the greatest change of 48.67°. The average
contact angle measured with ODPA coating was 72.78°, as compared to the 24.11° of the
control. Like aluminum, an increase in hydrophobicity occurred for glass samples with all
SAMPs tested.
Effects of Washing on Hydrophobicity
The comparison between the regularly coated tile and the surface after undergoing water
and soap washes is shown in Figure 6. For the control, OPA, DPA, and HDPA, contact angles
strictly decreased from the regular coating, to being washed with water, to being washed with
soap. Tile with DPA experienced the sharpest decline, from 60.00° to 33.78° after the soap wash.
DDPA, TDPA, and ODPA, on the other hand showed decreased contact angles after water, but
then increased after soap, though none were higher than angles from the original coatings. The
average contact angle measured on HPA, however, increased after washing with water, but
decreased after soap.
Figure 7 reveals the relationships between washes and hydrophobicity on aluminum. For
all carbon chain lengths and the control, contact angle decreased after washing with water.
However, only HDPA proceeded to show an increase in contact angle after the soap wash, to
such an extent that the measurement, 90.17°, was greater than the original measurement, 86.95°.
HPA experienced the most significant drop in contact angle, from its original 92.96° to 25.73
after washing with soap.
Figure 8 demonstrates that, on glass, the contact angles measured on every coating
increased after the water wash. Angles on HPA, OPA, TDPA, and HDPA continued to increase
after washing with soap, while the other chain lengths and control all experienced slight
decreases. HDPA experienced the greatest overall increase, from 46.52° to 65.39° after its soap
wash.
Effects of Heating Methods on Hydrophobicity
Figure 9 shows the relative effectiveness of the three methods used: oven, heat gun, and
ironing. Heat gun produced the smallest contact angles for all three surfaces: 56.39°, 43.30°, and
24.18° for tile, glass, and aluminum, respectively. The highest average contact angle produced
was the aluminum heated by the oven with 92.38°. The angles for tile and glass heated by oven
and iron were comparable to one another, ranging from 68.80° to 72.78°.
[2-9]
Figure 9: Comparing Surfaces and Heat Methods with a Constant C18 Chain Length
This graph compares the contact angles of washed and unwashed tile, glass, and aluminum
heated by three different methods, all coated with ODPA
Effects of Double Coating on Hydrophobicity
Figure 10: Comparing Surfaces and Chain Lengths with Double Coating
This graph shows the comparison of three different chain lengths (C14-C18) on the three surfaces,
tile, glass, and aluminum, to average contact angle measured.
In Figure 10, glass is shown to have the smallest average contact angles with double
coatings of TDPA, HDPA, and ODPA, ranging from 36.90° to 44.85°. Contact angles measured
on tile ranged from 67.04° to 72.04°, and aluminum was the highest, measuring between 79.37°
and 99.33°. When comparing to the single-coated surfaces, these results are on average higher in
the cases of aluminum and tile, but significantly lower for the glass slides.
[2-10]
Figure 11: Comparing Effects of Wear on Different Surfaces with a C18 Chain Length
This graph compares ODPA-coated tile, glass, and aluminum before and after soap
washes(Soap) to contact angles.
Figure 11 demonstrates that for all surfaces twice coated with ODPA, contact angle and
thus hydrophobicity increased after being washed with soap. Tile showed only a 2° increase after
the wash, aluminum 6.07°, and glass 7.35°. A statistical t-test was performed to check the
significance of these results by measuring the p-values for each surface observed. The p-values
are the probabilities of obtaining the results as extreme as the ones taken, so lower p-values lead
to a larger credence for the results. For tile, the p-value was found to be 0.1497, or approximately
15%. Such a high p-value shows that the results for tile were not statistically significant at the
5% significance level and therefore were not very credible. This is probably due to the low
number of trials (3) done for tile, leading to large variability in the results. On the other hand,
there were a moderate number of trials (9) conducted for glass, leading to a very low p-value of
0.0046, or about 0.5%. This p-value is statistically significant even at the 1% level, meaning that
the results for glass are largely credible. However, the results for aluminum show a high standard
deviation in a low number of trials, leading to an extremely high p-value of 0.578, or about 58%.
This confirms that the results for aluminum are statistically insignificant. However, the graph
overall notes a general increase in hydrophobicity on all surfaces after soap is applied.
[2-11]
Figure 12: Comparing Surfaces and Coating with an Iron and a Constant C18 Chain
Length. The graph illustrates the average contact angles of tile, glass, and aluminum with single
coating versus double coating of C18 phosphonic acid.
Figure 12 compares the hydrophobicity of single-coated surfaces of that of double-coated
surfaces. Overall, the contact angles measured on each surface were greater with two coatings
than with just one. Tile, on average, experienced an 11.44° increase from 72.43° to 83.87°; glass
increased by 7.95° from 68.80° to 76.75°; and aluminum rose by 2.22° from 76.51° to 78.73°.
DISCUSSION
In this lab, the group aimed to discover which carbon chain length (attached to a
phosphonic acid head) was the most effective at covalently bonding to glass, aluminum, and tile.
The group succeeded in this endeavor, and confirmed its hypothesis that the most covalent
bonding occurred when the phosphonic acid was bonded to ODPA (molecular formula
C18H39O3P). The group had predicted this would occur due to the fact that phosphonic acids
attached to a carbon chain bond in a disorderly (and therefore ineffective) manner to surfaces at
low carbon chain length. As one can observe, the bonding of a phosphonic acid to any surface
greatly increased that substance’s hydrophobicity. By bonding a phosphonic acid with a sixcarbon tail (C6H15O3P) to glass, the group observed an increase of over 250% of the contact
angle of water. While this demonstrates that the bonding of any phosphonic acid to a surface
increases its hydrophobicity, the group discovered that using ODPA was the most effective.
Bonding ODPA to glass increased the contact angle of water by more than 250%. It also
increased the contact angle on aluminum by almost 25%, and the contact angle on tile by 50%.
In contrast, HDPA increased the contact angles on glass, aluminum, and tile by only 156%, 24%,
and 18% respectively. Not only does this prove that C18H39O3P was the most effective
phosphonic acid at bonding to any surface, but it also suggests that glass is the most receptive
[2-12]
surface of the three to covalent bonding, a fact that helps explain the results when each substance
was washed with soap.
The effects of washing each substance with soap revealed the degree of covalent bonding,
as only covalently bonded material would remain on each substance after it was washed with
soap. Washing each substance with water actually served to increase the contact angle as it
simply washed away dirt, and “useless” layers of non-covalently bonded phosphonic acid. As
one can observe in Figure 9, washing tile in soap resulted in a decrease in contact. This decrease
was more dramatic with smaller carbon tails, suggesting that more covalent bonding occurred as
the chain length increased. Comparing Chain Lengths and Wear Methods: Glass with Oven
Heating reveals a much different trend for glass – washing the glass with soap often increased
the contact angle. This indicates that the phosphonic acid bonded covalently to glass, and
washing the glass with soap merely removed dirt. Comparing Chain Lengths and Wear Methods:
Aluminum with Oven Heating demonstrates that washing aluminum with soap greatly decreased
the contact angle. Thus the group determined that glass is the most effective surface to bond
phosphonic acids to, especially in a kitchen-like environment.
Based on the data collected, it appears that some heating methods worked better than
others in increasing the hydrophobicity of the different surfaces. Oven heating at 120°C for 18
hours appeared to be the most consistent method of producing higher contact angles between the
surface and the water; however, ironing remains a highly viable and convenient heating method
for potential consumers of SAMP substances. Although ironing had slightly lower contact angles
as those from heating by the oven, consumers could quickly apply extra or replacement coatings
easily in their own homes with an iron.
The heat gun was the least effective heating method. Contact angles for the heat gun on
tile and glass were about 20° less than the contact angles for the iron and the oven methods. This
relative ineffectiveness can be explained by a multitude of factors. The heat gun works at a very
high temperature and even though it is only applied for a short period of time, it is enough to
decompose and wipe off much of the coating. It also was inconsistent in its performance,
especially for the aluminum. This could be due to the fact that the aluminum was constantly
blown away by the force of the heat gun, potentially compromising some of the coating and its
effectiveness.
The iron was an effective heating method because it applied heat directly to the surface of
the materials, which made for even heating coverage. Although direct heating worked well in
that it heated evenly and quickly, it may have also rubbed off some of the SAMP coating since it
slid over the surfaces of the sprayed SAMPs, which could easily have removed some of the
molecules. Moreover, the iron was also not cleaned or wiped down properly before each use on
different types of coatings (ODPA, DPA, etc.), which also could have confounded the results,
since the rubbed off layers left on the iron may have led to slight contamination and disorder on
the surface of the materials; therefore, affecting the hydrophobicity of the surface.
[2-13]
The oven applied heat over a longer period of time than the other two methods, and
essentially heated each surface evenly. While the oven proved to be the most consistent heating
method, it may not be commercially viable for the average consumer, or even for industries. If
the SAMPs are meant to be sprayed on and heated in an oven by the consumer, then the product
would never seem time, cost, and energy efficient. Heating the SAMPs for 18 hours in an
ordinary oven at temperatures up to 120°C is both inconvenient and costly for a consumer. In a
factory system, the same problems persist regarding time and money. In particular, the aluminum
responded quite well to the oven heating method, with contact angles that were considerably
higher than those of both tile and the glass.
While the iron and oven may .have been equally effective at increasing the
hydrophobicity of the surfaces, the iron is considerably more commercially feasible in that is
more cost, time, and energy efficient than the other methods.
CONCLUSION
As hypothesized, the phosphonic acids with longer alkyl chains were generally more
hydrophobic than those with shorter chains. ODPA, the acid with the longest chain, consistently
demonstrated high hydrophobicity in all tests. This was probably due to the previously derived
finding that a longer carbon tail on a phosphonic acid decreases disorder after it is bonded to a
surface. The data also demonstrates that glass was the most receptive to the covalent bonding of
all types of phosphonic acids. This contradicts the group’s original hypothesis that aluminum
would be the easiest surface for phosphonic acids to bond to because of its high proportion of
hydroxyl groups on its surface. This suggests that other factors must be considered in choosing
the best surface for the acids to bond to – however, this experiment does not reveal which
variables are significant. Nevertheless, glass demonstrated the greatest affinity for covalent
bonding with the phosphonic acids, as the washes generally increased hydrophobicity whereas
the washes generally decreased the hydrophobicity of the aluminum samples. While small
variations in hydrophobicity for each substance might be attributed to random error, the extreme
difference between the recorded hydrophobicity after the wear tests (of over 50o) demonstrates
that phosphonic acids clearly bonded most covalently to glass. The ceramic tile remained
hydrophobic after enduring each wear test, but also showed some reduction in hydrophobicity
following the washes. This suggests a weaker, but significant covalent nature of the bonds
between it and each phosphonic acid.
The group also determined that the oven was the most consistent method to heat samples.
While most of the contact angles measured from substances heated by the oven were more
hydrophobic than their counterparts, the twenty-four hours necessary to create the bonds detracts
from its usefulness in an industrial context. Ironing usually produced slightly less hydrophobic
results than the oven, but required only a small fraction of the time to create the covalent bond
[2-14]
necessary for hydrophobicity. The group found the heat gun as an ineffective heating method as
it produced the least hydrophobic contact angles on every surface.
Possible Sources of Error
The main source of error in all experimentation was human error in applying the SAMP.
Application of SAMP was not uniform due to uneven spreading of the solution. This caused
“holes” on the samples or samples covered in less SAMP than others--leading to inconsistent
data. Other potential problems were the rubbing of the iron on the surfaces when heating, which
could have removed part of the phosphonic acid layer. Application of heat may not have been
uniform as well, perhaps causing uneven evaporation of the solvent and bonding of SAMPs. In
addition, contamination present on the iron or on the Mayer Rod could have rubbed off onto the
surfaces, slightly affecting our results. Errors may also have been made in gathering data, as the
contact angles measured may not have been identified correctly and the method of evaluating
each angle could have varied from person to person. With aluminum, the surface would
sometimes tilt upward at one end, distorting the shape of the water droplet and contact angle. The
volume of the water droplet in each goniometer test was also not exact, which could possibly
have resulted in skewed contact angle results.
Future Research
In the future, more variables should be tested including more heating methods of SAMP, and
application of SAMP onto different surfaces. For example, instead of spraying on SAMP to all
samples, some samples could be dipped in the SAMP solution or SAMP could be poured onto
other samples. Solvents other than the 50-50 ethanol-toluene mixture could be tested to see this
effect on how strongly the SAMP bonds to the sample. The samples could also be tested to see if
the different time periods help the SAMP solution to bond to the samples more strongly, as well
as find the minimum heating period that allows for strong covalent bonds. While the iron is
economically efficient, it would be ideal if some heating method could be developed that is
equally as quick and more effective in order to compete with current industry standards with
Teflon. If these main issues can be solved, phosphonic acids have immense potential to become
industrially viable coatings in the kitchen and on other surfaces found in daily life.
[2-15]
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[2-16]
APPENDIX A
This appendix features each of the pictures that was taken using the goniometer. The top axis
represents each of the varying carbon chain lengths. The left axis features the material as well as
which stress was applied, either regular, in which no stress was applied, wash, in which each tile
was soaked in water for 5 minutes, or soap, in which each data sample was scrubbed 3 times
using a soap and water wash. The numbers represent either Trial 1, 2, or 3.
C6
C8
C10
C12
TILE
Regular1
TILE
Regular2
TILE
Regular3
TILE
Soap1
TILE
Soap2
TILE
Soap3
TILE
Wash1
TILE
Wash2
TILE
Wash3
ALUMINUM
Regular1
ALUMINUM
Regular2
ALUMINUM
Regular3
ALUMINUM
Soap1
ALUMINUM
Soap2
ALUMINUM
Soap3
ALUMINUM
Wash1
[2-17]
C14
C16
C18
ALUMINUM
Wash2
ALUMINUM
Wash3
GLASS
Regular1
GLASS
Regular2
GLASS
Regular3
GLASS
Regular4
GLASS
Regular5
GLASS
Regular6
GLASS
Regular7
GLASS
Regular8
GLASS
Regular9
GLASS
Wash1
GLASS
Wash2
GLASS
Wash3
GLASS
Wash4
GLASS
Wash5
[2-18]
GLASS
Wash6
GLASS
Wash7
GLASS
Wash8
GLASS
Wash9
GLASS
Soap1
GLASS
Soap2
GLASS
Soap3
GLASS
Soap4
GLASS
Soap5
GLASS
Soap6
GLASS
Soap7
GLASS
Soap8
GLASS
Soap9
[2-19]