Contact angle
 Category:Physics
 Document type:Assignment
 Level:Masters
 Page:2
 Words:1390
Contact Angle Experiments on Selfassembled Monolayers
Introduction:
Surfaces in the environment are likely to have a coat of films which is partially composed of organic compounds. Different materials have different water adsorption capabilities.
The contact angle on SAMs are broadly used for assessment of features such as hydrophobicity, wettability, adhesion and firmly the surface tension of solids. Selfassembled monolayers (SAMs) have since attracted interest because of their wide range of applications – from optoelectronic devices to biosensors, biocompatibility to corrosion resistance, control of wettability etc. The organic molecules of selfassembled monolayers are molecular assemblies formed spontaneously on surfaces by adsorption. They are systematized into more or less huge ordered domains. Nonetheless, molecules that form the monolayer don’t interact strongly with the substrate in other cases. Figure 1 below shows how contact angle is measured between liquid surfaces a solid interface. Hydrophilicsurfaces have small contact angles and therefore, water films are easily formed on theses surfaces. On the other hand, hydrophobic surfaces have larger contact angle which inhibit water adsorption.
The contact angle is the convenient way to classify materials by their ability to absorb liquid droplet makes with the surface it is sitting on, as illustrated in Figure 1. Droplets of water
spread on hydrophilic surfaces resulting in a small contact angle, occasionally 1030°. Water
reduces its contact with a hydrophobic surface resulting in large contact angles, sometimes in exceeding 90°.
Figure 1: Measurement of contact angle
In this lab, we investigated the influence of parameters in controlling the SAM surface properties of Alkenthiol monolayers on silver metals; the nature of the surface pretreatment by use of mixed solutions of Alkenthiol at different mole fractions. The obtained monolayers were characterized by contact angle measurements.
Results and Discussions:
Forming SAMs on silver in this experiment was done via using two different types of Alkenthiol solutions. The first one was 1dodecanethiol (methyl terminated thiol) and the second one was 11mercaptoundecanoic acid (carboxyl terminated thiol).
Mole fractions of 1dodecanethiol (methyl terminated thiol) with respect to 11mercaptoundecanoic acid (carboxyl terminated thiol) 
1dodecanethiol (methyl terminated thiol) 
11mercaptoundecanoic acid (carboxyl terminated thiol) 
0 
0 

0 
Table1: table1 shows the Mole fractions of 1dodecanethiol with respect to 11mercaptoundecanoic acid and amount in ml of both 1dodecanethiol 11mercaptoundecanoic acid that are mixed together to form selfassembled monolayers on silver.
Mole fractions of 1dodecanethiol (methyl terminated thiol) with respect to 11mercaptoundecanoic acid (carboxyl terminated thiol) 
Advancing Contact angle measurements /degree 
Receding Angle measurements 
0 
17.947660 34.121964 36.118786 
15.188132 15.335848 20.892345 
46.134308 47.608795 38.133171 
39.494255 40.152318 40.648592 

59.834656 54.991814 54.071808 
35.190718 40.987343 53.943771 

75.048592 67.552490 64.934486 
46.179921 53.099842 53.072345 

68.992345 73.358284 76.881325 
65.860121 62.804622 59.211530 
Table2: table 2 shows both advancing and receding contact angles in five different mole fraction of 1dodecanethiol with respect to 11mercaptoundecanoic acid.
ANALYSIS OF RESULTS:
Variance and standarddeviation are considered good measurements of variability as they are very significant parts in statistical applications as well as analysis.
Calculating the sample variance can be obtained via using the following formula:
…………(1)
Where M: is the mean
S^{2: } is the Variance
N: is the number of scores
And the sample standard deviation can be obtained via calculating the square root of the Variance.in this experiment, both of the variance and standarddeviation have been calculated via using Excel as it shown in the following two tables.
In terms of advancing contact angles:
Mole fractions of 1dodecanethiol (methyl terminated thiol) with respect to 11mercaptoundecanoic acid (carboxyl terminated thiol) 
Advancing Contact angle measurements /degree 
the sample mean (average) 
the sample Variance (S^{2}) via using Excel 
The sample standard deviation via using Excel 
0 
17.947660 34.121964 36.118786 
29.39614 

46.134308 47.608795 38.133171 
43.95876 

59.834656 54.991814 54.071808 
56.29943 

75.048592 67.552490 64.934486 
69.17852 

68.992345 73.358284 76.881325 
73.07732 
Table 3: shows the sample mean, sample variance and sample standard deviation for the advancing contact angle measurements in five different mole fractions of 1dodecanethiol with respect to 11mercaptoundecanoic acid.
In terms of receding contact angles:
Mole fractions of 1dodecanethiol (methyl terminated thiol) with respect to 11mercaptoundecanoic acid (carboxyl terminated thiol) 
Receding Angle measurements 
the sample mean (average) 
the sample Variance (S^{2}) via using Excel 
The sample standard deviation via using Excel 

0 
15.188132 15.335848 20.892345 
17.13878 

39.494255 40.152318 40.648592 
40.09838 

35.190718 40.987343 53.943771 
43.373944 

46.179921 53.099842 53.072345 
50.78404 

65.860121 62.804622 59.211530 

Table 4: shows the sample mean, sample variance and sample standard deviation for the receding contact angle measurements in five different mole fractions of 1dodecanethiol with respect to 11mercaptoundecanoic acid.
Figure 2: shows a plot of the average of both (advancing and receding) contact angles Vs the mole fraction of 1dodecanethiol (methyl terminated thiol) with respect to 11mercaptoundecanoic acid (carboxyl terminated thiol.
Briefly Comment on table 3 and 4 and figure1:
The results in table 3 indicate that as the mole fractions of 1dodecanethiol with respect to 11mercaptoundecanoic acid increases from 01, the mean advancing contact angle also increases from 29.4^{o} to 73.1^{o}. This demonstrates that SAMs made on silver substrate at different mole fractions of 1dodecanethiol with respect to 11mercaptoundecanoic acid affects its surface properties due to adsorption of the solution and thus, controls its wettability depending on the mole fractions of the solution (Wang, 2010). Increasing the mole ratio decreases the wettability as observed with increasing contact angle of the monolayers. A similar observation is made in Table 4, where the receding angle increases as mole fraction increases from 0100%.
Figure 1 shows that the average of advancing and receding contact angles of the monolayers increase almost linearly as the mole fraction increases from 0100%. There is significant hysteresis which varies for each mole fraction. The essence of this figure is that changes in advancing and receding angles are determined by the molar fraction and hydrophobicities of mercaptoundecanoic acid.
Questions:
Question1:
Question2:
I am going to do q1 and q2.
Question 3:Comment on the hydrophobicity/hydrophilicity of the monolayers.
Why are they changing?
The hydrophobicity of the monolayers increases with increase in the mole ratio. There is gradual changes in the contact angle of the mixed SAMs with changes in the fraction of the SAMs on the silver surface (Schollmeyer, et al., 2006). This is attributed to the changes in the properties of the SAMs surface with different mole fractions of alkanethiols.
Question 4:
I am going to do it.
Question 5: Why is a Ph 11.0 solution used? Hint: to do with hydrogen bonding.
The SAMs are quite stable at pH 11. This is because the resulting union of hydrogen bonds within the alkanethiols are sufficiently strong and stable (Kanta, et al., 2006).
Question 6: Draw a diagram of how the 11mercaptoundecanoic SAMs are likelyto be assembled on the silver surface. Include a journal reference. (not a website)
Figure 3: 11mercaptoundecanoic SAMs assembled on the silver surface
(Love, et al., 2005)
Conclusion:
SAM clearly provides a model for studying wettability. The results obtained from this experiment idicate that SEMs of mole fractions of methyl terminated thiol and carboxyl terminated thiol can be deposited onto silver. The average advancing contact angle of SAMs formed on silver substrate without addition of Alkenthiol solutions is 29.4^{o}. However, addition of alkenthiol solutions (1dodecanethiol
with respect to 11mercaptoundecanoic acid) increases the contact angle with increasing mole fraction. Increase in contact angle with increase in the mole fraction of Alkenthiol increases the hydrophobicity of the monolayers due to the spatial gradients of wettability between the Alkenthiol monolayers and the silver surface. The properties of SAM surfaces can be controlled coadsorption of a mixture of alkenthiols which possess different functional groups. The fraction of an alkanethiol on a mixed SAM determines its mole fraction in the mixed SAM solution. The fraction of the alkanethiol adsorbed highly depends on the terminal functional group and its chain length. Thus, preparation of SAMs with a mixed solution of different alkanethiols enables us to change surface properties and produce a variety of model surfaces.
References:
Kanta, A., Sedev, R. & Ralston, J., 2006. The formation and stability of selfassembled monolayers of octadecylphosphonic acid on titania. Colloids and Surfaces A: Physicochem. Eng. Aspects, pp. 18.
Love, J. C. et al., 2005. SelfAssembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chemical Reviews, 105(4), p. 1103−1169.
Schollmeyer, H., Daillant, J. & Guenoun, P., 2006. Nanoscale Wettability of SelfAssembled Monolayers Investigated by Noncontact Atomic Force Microscopy. Langmuir, 22(1), pp. 116126.
Wang, Z. M., 2010. Toward Functional Nanomaterials. New York: Springer Science & Business Media.