Analysis of Contact Angle Experiments on Self-Assembled Monolayers

This assignment focuses on contact angle experiments performed on self-assembled monolayers (SAMs) to investigate surface energy, wetting properties, and the impact of surface roughness. The experiment involves measuring the angle between a liquid droplet and a surface coated with alkanethiols using a CCD camera and optics. The results, including advancing and receding contact angles, are presented in a table and analyzed to determine the mean angle, variance, and standard deviation for different samples. The hydrophobicity/hydrophilicity of the monolayers is discussed, along with theoretical contact angle calculations using the Young Equation. The suitability of pH for the reactions and the effect of surface roughness on wetting are also considered. The study concludes by highlighting the factors affecting SAM formation and the variations in contact angles based on surface properties. The document includes references to relevant research papers.

Contact Angle Experiments on Self-
Assembled Monolayers
Results and Discussion
Measurements of the angle between the liquid droplet and the surface was done and recorded. This
experiment was performed using various liquids. The metal surface was coated with self-assembled
single molecule layers of alkanethiols. Investigation on how the surface of the metal can affect the angle,
surface energy and wetting properties was done as well. The angles were measured using CCD camera
and optics. The results were recorded as shown in Table 1 below showing advancing and receding
contact angle versus mole fraction of 11-mercaptoundecanoic acid in1-dodecane.
Sample Mole Fraction Advancing Angle Receding Angle Droplet
volume
Mercaptoundecanoic
acid
0
0.5
0.25
1
25.93
52.47
42.39
87.52
32.15
52.47
41.43
87.53
16
25
32
38
The graph below shows the advancing and receding contact angle.
Calculations and Explanations
The mean angle in this experiment is the average contact angle in all the contact angle measurements. It
is calculated as;
Sample 1, continuous;
1

= (52.27 + 50.03 + 54.79)/3
= 52.36°
Sample 2, Dodecanethiol;
= (25.93+52.47+42.39+87.52+32.15+52.47+41.43+87.53)/8
= 52.74°
Sample 3, Nano;
= (20.13+ 18.89+32.58)
= 49,88°
Sample 4, PDMS;
= (95.03+ 88.04+ 95.03)/3
= 92.70°
Comparing the mean angles of the four samples, it is seen that only samples 1 and 4 have mean angles
closer to the distribution in the particular sample. This means samples 1 and 4 have good Sam material.
The variance and the closely-related standard deviation are measures of how spread out a distribution
is. In other words, they are measures of variability. The formula (in summation notation) for the variance
in for the contact angles is;
The Variance is defined as a measure of how the scores are distributed and it measures the variability.
the formula of Variance is basically the summation of the squire of the difference between a number, x,
and the mean per number of scores, or in this case angles. The formula is given as;
Where N is the number of angles, u is the mean. For samples;
Sample 1, Continuous
Variance = (50.03-52.36) ^2/3
= 1.81
Sample 2, Dodecanethiol;
= (52.74 - 42.39) ^2/8
= 13.39
Sample 3, Nano;
2

= 99.29
Sample 4, PDMS;
=1.81
Comparing the four variances of the samples 1,2,3 and four, it can be deduced that samples 1 and 4
have smallest value of their variance which is 1.81. this means that materials used in making samples 1
and 4 are suitable to be used to form SAMs.
Just like the variance, the standard deviation measures variability in a distribution. Its formula is known
as the squire root of Variance;
S= √σ
Sample 1, Continuous
= √ 1.81
= 1.36
Sample 2, Dodecanethiol
= √13.39
= 3.7
Sample 3, Nano
= √99.29
=9.96
Sample 4, PDMS
= √ 1.81
= 1.36
Comparing the standard deviations among the three material show us that sample 1 and sample have a
similar value and smaller value than sample. Sample 1 and 3 contains a nano material. The material
which has small standard deviation is best suited to be used to form self-assembled monolayers.
Hydrophobicity/Hydrophilicity of The Monolayers
Hydrophobic interactions or Hydrophobicity is important during hydrolysis. This interactions help in
displacement of water molecules of water molecules where hydroxyl ion separates from oxygen atom
thus allowing formation of adhesive bonds between surface and the monolayer (Lucas F. M. da Silva,
2011). Therefore, the spores which are attached to the metal surface floor of silver lose their adhesive
property and in turn the monolayer droplets will slide and not be seen as drops.
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Hydrophilicity is also important during the first stage of adhesion. During this stage the spores attach
rapidly to the metal surface floor and then spins off the surface rotating through its apical papilla by
behaving as if it is elastic. Displacement of water molecules within the substratum of the liquid
monolayer is thus helped by this process of hydrophobicity or hydrophilicity.
Theoretical Contact Angle Calculations
The Young Equation relates the contact angle that a liquid droplet with the silver metal surface to the
three interfacial tensions (surface energies) involved γsv = γsl + γlvcosθ (Gianangelo Bracco,
2013).The formula is;
For sample 1 surface, Dodecanethiol
= 0.25 (1+cos41.91) ^2 + 0.5(1+cos52.47) ^2
= 0.3078 + 0.6753
Therefore, θ = 1.58°
For sample surface 2, mercaptoundecanoic acid
= 0.25(1+cos95.03) ^2 + 0.5(1+cos88.04) ^2
= 1.78°
Suitability of the pH for the Reactions
On the idea of the current researches and research completed, all of the SAMs of alkanethiols studied
had been determined to be strong at pH eleven or underneath in aqueous answers, regardless of
capability at their terminal. The SAMs of mercaptoundecanoic acid and 1-dodecanethiol showed
exquisite stability at pH eleven and 11.5. Many reactions on SAMs have been performed underneath
acidic or simple conditions. in particular, hydrolysis concerned surprisingly simple situations (pH 12 and
above), despite the fact that the reaction time changed into relatively brief (typically much less than 1
hour). However, our effects suggest that the desorption of alkanethiols from gold ought to be taken into
consideration severely whilst SAM-primarily based reactions are performed below simple situations due
to the fact the desorption of thiols would intrude with the desired reactions.
Nicely-orientated bio-conjugates on gold electrode surfaces will indirectly have an effect on the
molecular popularity of antigens to surface-sure antibodies for this reason improving the detection
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overall performance of electrochemical immunosensors. The diagram below shows the change of self-
assembled monolayers (SAMs) on the silver surface with eleven-mercaptoundecanoic acid (eleven-
MUA).
figure
1. Diagram
of
formation of SAM on the silver surface
(American Chemical Society. Division of Polymer Chemistry, 2009).
Effect of Roughness of the Surface
The surface energy is determined the usage of the touch perspective dimension. consequently, the
micro or nanometre level floor roughness effect of the wetting of the surface by using the probe liquid
and have an impact on the calculated surface strength. The connection between roughness and
wettability turned into described already in 1936 through Wenzel who stated that adding floor
roughness will beautify the wettability as a result of the chemistry of the surface. as an instance, if the
floor is chemically hydrophobic, it will become even more hydrophobic when surface roughness is
added. that is the reason why the same fabric may be hydrophobic or first-rate-hydrophobic depending
on the surface topology.
Conclusion
As it can be seen, there are various factors which affect the formation of the self-
assembled monolayers. They include as discussed, roughness of the surface, type
of material used to paint the surface and thickness of the surface. The angle formed
between liquid drop monolayer and the surface varies and depends on the named
factors.
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References
American Chemical Society. Division of Polymer Chemistry, 2009. Polymer Materials. Papers Presented
at theConference, 41(2), p. 146.
Gianangelo Bracco, B. H., 2013. Surface Science Techniques. 2nd ed. London: Springer Science & Business
Media.
Lucas F. M. da Silva, A. Ö. R. D. A., 2011. Handbook of Adhesion Technology. 2nd ed. New York: Springer
Science & Business Media.
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