Contact angle

  • Category:
    Physics
  • Document type:
    Assignment
  • Level:
    Masters
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    2
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    1390

Contact Angle Experiments on Self-assembled Monolayers

contact angle

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. Self-assembled 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 self-assembled 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 10-30°. Water

reduces its contact with a hydrophobic surface resulting in large contact angles, sometimes in exceeding 90°.

contact angle 1contact angle 2

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 pre-treatment 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 1-dodecanethiol (methyl terminated thiol) and the second one was 11-mercaptoundecanoic acid (carboxyl terminated thiol).

Mole fractions of 1-dodecanethiol (methyl terminated thiol) with respect to 11-mercaptoundecanoic acid (carboxyl terminated thiol)

1-dodecanethiol (methyl terminated thiol)

11-mercaptoundecanoic acid (carboxyl terminated thiol)

0

0

0

Table1: table1 shows the Mole fractions of 1-dodecanethiol with respect to 11-mercaptoundecanoic acid and amount in ml of both 1-dodecanethiol 11-mercaptoundecanoic acid that are mixed together to form self-assembled monolayers on silver.

Mole fractions of 1-dodecanethiol (methyl terminated thiol) with respect to 11-mercaptoundecanoic 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 1-dodecanethiol with respect to 11-mercaptoundecanoic 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.

contact angle 3Calculating the sample variance can be obtained via using the following formula:

…………(1)

Where M: is the mean

S2: 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 1-dodecanethiol (methyl terminated thiol) with respect to 11-mercaptoundecanoic acid (carboxyl terminated thiol)

Advancing

Contact angle measurements /degree

the sample mean (average)

the sample Variance (S2) 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 1-dodecanethiol with respect to 11-mercaptoundecanoic acid.

In terms of receding contact angles:

Mole fractions of 1-dodecanethiol (methyl terminated thiol) with respect to 11-mercaptoundecanoic acid (carboxyl terminated thiol)

Receding

Angle measurements

the sample mean (average)

the sample Variance (S2) 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

62.62542

Table 4: shows the sample mean, sample variance and sample standard deviation for the receding contact angle measurements in five different mole fractions of 1-dodecanethiol with respect to 11-mercaptoundecanoic acid.

contact angle 4

Figure 2: shows a plot of the average of both (advancing and receding) contact angles Vs the mole fraction of 1-dodecanethiol (methyl terminated thiol) with respect to 11-mercaptoundecanoic 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 1-dodecanethiol with respect to 11-mercaptoundecanoic acid increases from 0-1, the mean advancing contact angle also increases from 29.4o to 73.1o. This demonstrates that SAMs made on silver substrate at different mole fractions of 1-dodecanethiol with respect to 11-mercaptoundecanoic 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 0-100%.

Figure 1 shows that the average of advancing and receding contact angles of the monolayers increase almost linearly as the mole fraction increases from 0-100%. 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 11-mercaptoundecanoic SAMs are likelyto be assembled on the silver surface. Include a journal reference. (not a website)

contact angle 5

Figure 3: 11-mercaptoundecanoic 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 29.4o. However, addition of alkenthiol solutions (1-dodecanethiol
with respect to 11-mercaptoundecanoic 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 co-adsorption 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 self-assembled monolayers of octadecylphosphonic acid on titania. Colloids and Surfaces A: Physicochem. Eng. Aspects, pp. 1-8.

Love, J. C. et al., 2005. Self-Assembled 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 Self-Assembled Monolayers Investigated by Noncontact Atomic Force Microscopy. Langmuir, 22(1), pp. 116-126.

Wang, Z. M., 2010. Toward Functional Nanomaterials. New York: Springer Science & Business Media.