2NaNO4 Contact Angle Experiments on Self-Assembled Monolayers - Lab Report Example

Name: Course: Instructor: Date: Experiment: 2NANO4 Contact Angle Experiments On Self-Assembled Monolayers AIM The main objective of this lab experiment was to measure the angle between a liquid droplet and a metal surface using a CCD camera and optics. The other objective was to investigate how self-assembled monolayer (SAM) of molecules of alkanethiols and functionalized alkanethiols can significantly modify the wetting behaviour and surface energy of metal surfaces coated with these monolayers. INTRODUCTION Attachment of a monolayer to a metal surface can have a significant effect on both surface energy and the wetting behaviour of such surfaces. These effects can be observed when a liquid droplet placed on the surface changes in terms of contact angle made with the surface. Control of wetting behaviour of both liquids and solids by measurement of contact angle has some important industrial applications, such as establishment of how clean semiconductor wafers are, penetration of pesticides when applied to plants, waterproofing materials among many other fields of applications. When monolayers spontaneously form on a surface, they are termed as self-assembled. When a surface is exposed to SAM organic molecules, stable monolayers are formed on materials such as copper, gold, silver, mercury, silica, alumina, gallium arsenide and silicon. For the formation of a stable monolayer, there must be a bond between the adsorbing molecules and the substrate. These adsorbing molecules are randomly attached on the metal surface until no active sites are left uncovered, then the process of adsorption ends and the monolayer is formed. The contact angle ( is a measure of the energy of interaction between the solid and the liquid surface. A strong interaction between liquid molecules and solid molecules causes wetting of the surface. As the liquid spreads on the solid surface, the drop flattens and the contact angle reduces. A weaker interaction of liquid molecules and solid surface molecules is expected to produce a larger contact angle. The three forces that result from surface tension are surface tensions for solid-vapour interfacial free energy (), solid-liquid interfacial free energy () and liquid-vapour interfacial free energy (). Figure 1 below shows the geometry of these three forces acting on a liquid droplet. (a) Receding contact angle (b) Advancing contact angle Figure 1: Graphical representation of the three forces resulting from surface tension on a liquid droplet. The relationship between the three forces and the Young’s contact angle at thermodynamic equilibrium is described by Young’s equation: = If any there is a change in any of these three forces, it will be reflected by a change in contact angle. Practically, two contact angles are normally measured; the advancing angle and the receding angle. The advancing contact angle is the maximum angle between the substrate and the three-phase line for a liquid droplet where increase in volume would result in expansion of the droplet across the surface. On the other hand, the receding contact angle refers to the minimum contact angle that occurs just before the droplet contracts across the surface. In this experiment, an investigation of how functionalization of macroscopic properties of a solid surface using self-assembled monolayer (SAM) of molecules modify such properties was performed. Two tests were carried out, and in each test, both the receding and advancing contact angles were measured. PROCEDURE Refer to the manual “2NANO4 Contact angle experiments on self-assembled monolayers”, Pages 41-47. ANALYSIS OF RESULTS The figure 1 below shows an image of a water droplet on a silver plate that was measured during the experiment. Figure 1: An image showing a liquid droplet during the experiment A small liquid droplet tends to be spherical in shape since the formation of the sphere minimizes surface area. When a droplet comes into contact with a solid surface, this shape changes due to surface tension and gravity. If the liquid does not completely wet the solid surface, the droplet meets the surface at the tree-phase line with an angle referred to as the Young’s contact angle (. When the advancing angle of the first image was measured, the results reported in table 1 below were obtained. Table 1(a): Measurement of advancing angle of the liquid droplet Mole Fraction Advancing Angle Average Variance Standard Deviation 0.00 93.862 11.384 3.3741 0.25 77.300 0.310 0.5571 0.50 76.675 35.668 5.9722 0.75 67.432 2.416 1.5544 1.00 56.087 12.154 3.4863 The graph of mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol against the average advancing angle for these results is shown below. Figure 2: Average advancing angle with respect to mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol. Table 1(b): Linear Regression of Advancing Angle Slope m -34.167 91.355 Y-intercept b Standard error for m -34.167 91.355 Standard error for b Coefficient of determination -34.167 91.355 Standard error for Y-estimate F-statistic -34.167 91.355 Degrees of freedom Regressional sum of squares -34.167 91.355 Residual sum of squares For the same image, the receding angle was measured as well and the results tabulated as shown in table 2. Table 2(a): Measurement of receding angle of the liquid droplet Mole Fraction Receding Angle Average Variance Standard Deviation 0.00 94.537 34.404 5.866 0.25 86.054 2.470 1.572 0.50 76.300 9.414 3.068 0.75 66.282 4.731 2.175 1.00 63.681 45.536 6.748 Figure 3: Average receding angle with respect to mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol Table (2b): Linear Regression of Receding Angle Slope m -32.594 93.668 Y-intercept b Standard error for m -32.594 93.668 Standard error for b Coefficient of determination -32.594 93.668 Standard error for Y-estimate F-statistic -32.594 93.668 Degrees of freedom Regressional sum of squares -32.594 93.668 Residual sum of squares From the graphs in figure 2 and 3, it can be observed that the mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol has a significant effect on the value of both advancing and receding contact angles measured. As the mole fraction of the SAMs increases, both contact angles were reducing. When this experiment was repeated for the second time, the following results were obtained. Table 3(a): Measurement of advancing angle of the liquid droplet Mole Fraction Advancing Angle Average Variance Standard Deviation 0.00 103.098 8.462 2.909 0.25 67.187 47.195 6.870 0.50 68.152 2.627 1.621 0.75 46.549 15.861 3.983 1.00 27.591 0.005 0.067 Figure 4: Average advancing angle with respect to mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol. Table 3(b): Linear Regression of Advancing Angle Slope m -68.661 96.846 Y-intercept b Standard error for m -68.661 96.846 Standard error for b Coefficient of determination -68.661 96.846 Standard error for Y-estimate F-statistic -68.661 96.846 Degrees of freedom Regressional sum of squares -68.661 96.84 Residual sum of squares Table 4(a): Measurement of receding angle of the liquid droplet Mole Fraction Receding Angle Average Biased Variance Biased Standard Deviation 0.00 97.014 28.784 5.365 0.25 66.535 190.412 13.799 0.50 63.335 5.031 2.243 0.75 42.319 0.119 0.344 1.00 23.881 14.236 3.773 Figure 5: Average receding angle with respect to mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol Table 4(b): Linear Regression of Receding Angle Slope m -68.193 92.713 Y-intercept b Standard error for m -68.193 92.713 Standard error for b Coefficient of determination -68.193 92.713 Standard error for Y-estimate F-statistic -68.193 92.713 Degrees of freedom Regressional sum of squares -68.193 92.713 Residual sum of squares From the results shown in figure 4 and 5, a similar trend as that observed in the first test is encountered. Both advancing and receding contact angles reduce with increase in mole fraction of SEMs. However, in the second test, the slope is steeper (-68.193) than the value obtained in the first test (-34.167 and -32.594). The study of self-assembling system where a metal is placed in an alkanethiol or functionalized alkanethiols is one of the most studied area of nanotechnology. This is for the reason that biological molecules can be attached to alkanethiols and likewise to metal electrodes to be used as transducers in biosensors. The thiol group has a sulfur atom that has a strong affinity with a number of metals, forming stable bonds on the metal lattice. Van der Waals interactions that occur between neighboring carbon chains in alkanethiols that have long unbranched carbon chains with more than 8 carbons results in highly ordered monolayers. When the outermost surface of a material is derivitised by a monolayer, its wetting, cohesive, adhesion and electrical properties are modified, depending on the terminal group of the derivitizing monolayer molecules. QUESTIONS Question 1 & 2: see results section above. Question 3. Mercaptoundecanoic acid on silver surface makes the surface to be very hydrophilic while dodecanethiol renders the surface very hydrophobic. The two substrates have different surface energies, thus, the changing contact angles. Question 4 Theoretical contact angle is given by: Where: and are mole fractions monolayer 1 and 2 respectively and are the contact angles of the monolayer 1 and 2 respectively For the first test: Theoretical advancing contact angle = (0.25cos 77.3 + 0.75cos 67.4) = 69.93o Theoretical receding contact angle = (0.25cos 86.0 + 66.3cos 67.4) = 89.7o Question 5. The COOH functional group termination SAMs, the alkanethiol used was mercaptohexadecanoic acid. This provides a hydrophilic surface. Question 6. Figure 6: 11-mercaptoundecanoic SAMs assembled on the Ag surface(Hsu, Reinhoudt and Huskens). Conclusion The wetting characteristics of self-assembled monolayer surface of molecules of alkanethiols and functionalized alkanethiols was examined by measurement of contact angles. It was found that the mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol can significantly modify the wetting behaviour and surface energy of silver surfaces coated with these monolayers. There was agreement of experimentally measured values of advancing and receding contact angles and theoretically calculated values. Reference Read More

In this experiment, an investigation of how functionalization of macroscopic properties of a solid surface using self-assembled monolayer (SAM) of molecules modify such properties was performed. Two tests were carried out, and in each test, both the receding and advancing contact angles were measured. PROCEDURE Refer to the manual “2NANO4 Contact angle experiments on self-assembled monolayers”, Pages 41-47. ANALYSIS OF RESULTS The figure 1 below shows an image of a water droplet on a silver plate that was measured during the experiment.

Figure 1: An image showing a liquid droplet during the experiment A small liquid droplet tends to be spherical in shape since the formation of the sphere minimizes surface area. When a droplet comes into contact with a solid surface, this shape changes due to surface tension and gravity. If the liquid does not completely wet the solid surface, the droplet meets the surface at the tree-phase line with an angle referred to as the Young’s contact angle (. When the advancing angle of the first image was measured, the results reported in table 1 below were obtained.

Table 1(a): Measurement of advancing angle of the liquid droplet Mole Fraction Advancing Angle Average Variance Standard Deviation 0.00 93.862 11.384 3.3741 0.25 77.300 0.310 0.5571 0.50 76.675 35.668 5.9722 0.75 67.432 2.416 1.5544 1.00 56.087 12.154 3.4863 The graph of mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol against the average advancing angle for these results is shown below. Figure 2: Average advancing angle with respect to mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol.

Table 1(b): Linear Regression of Advancing Angle Slope m -34.167 91.355 Y-intercept b Standard error for m -34.167 91.355 Standard error for b Coefficient of determination -34.167 91.355 Standard error for Y-estimate F-statistic -34.167 91.355 Degrees of freedom Regressional sum of squares -34.167 91.355 Residual sum of squares For the same image, the receding angle was measured as well and the results tabulated as shown in table 2. Table 2(a): Measurement of receding angle of the liquid droplet Mole Fraction Receding Angle Average Variance Standard Deviation 0.00 94.537 34.404 5.866 0.25 86.054 2.470 1.572 0.50 76.300 9.414 3.068 0.75 66.282 4.731 2.175 1.00 63.681 45.536 6.748 Figure 3: Average receding angle with respect to mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol Table (2b): Linear Regression of Receding Angle Slope m -32.594 93.668 Y-intercept b Standard error for m -32.594 93.668 Standard error for b Coefficient of determination -32.594 93.668 Standard error for Y-estimate F-statistic -32.594 93.668 Degrees of freedom Regressional sum of squares -32.594 93.668 Residual sum of squares From the graphs in figure 2 and 3, it can be observed that the mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol has a significant effect on the value of both advancing and receding contact angles measured.

As the mole fraction of the SAMs increases, both contact angles were reducing. When this experiment was repeated for the second time, the following results were obtained. Table 3(a): Measurement of advancing angle of the liquid droplet Mole Fraction Advancing Angle Average Variance Standard Deviation 0.00 103.098 8.462 2.909 0.25 67.187 47.195 6.870 0.50 68.152 2.627 1.621 0.75 46.549 15.861 3.983 1.00 27.591 0.005 0.067 Figure 4: Average advancing angle with respect to mole fraction of 11-mercaptoundecanoic acid in 1-dodecanethiol.

Table 3(b): Linear Regression of Advancing Angle Slope m -68.661 96.846 Y-intercept b Standard error for m -68.661 96.846 Standard error for b Coefficient of determination -68.661 96.846 Standard error for Y-estimate F-statistic -68.661 96.846 Degrees of freedom Regressional sum of squares -68.661 96.84 Residual sum of squares Table 4(a): Measurement of receding angle of the liquid droplet Mole Fraction Receding Angle Average Biased Variance Biased Standard Deviation 0.00 97.014 28.784 5.365 0.25 66.535 190.412 13.799 0.50 63.335 5.031 2.243 0.75 42.319 0.119 0.344 1.00 23.881 14.236 3.

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