Advancing and receding water contact angles (qa and qr respectively) on SAM modified gold surfaces are presented in table 4.1. Also included are water contact angles on ‘bare’ gold surfaces which were identically treated to SAM modified surfaces except with omission of any thiol or disulfide in the solvent.
|
Sample |
Advancing angle (qa) / ° |
Receding angle (qr) / ° |
|
58.2 (1.7) |
22.8 (2.2) |
|
|
52.1 (0.9) |
18.5 (1.4) |
|
|
rac 184 |
49.5 (1.7) |
12.5 (1.1) |
|
(R) (-) 184 |
47.8 (5.1) |
11.7 (1.1) |
|
104.7 (0.4) |
76.5 (1.3) |
|
|
102.3 (1.4) |
57.4 (4.1) |
|
|
105.4 (1.8) |
78.5 (2.5) |
|
|
107.7 (1.1) |
77.3 (2.2) |
|
|
105.2 (1.9) |
67.1 (5.4) |
|
|
106.7 (2.4) |
78.9 (2.5) |
|
|
99.0 (0.8) |
62.9 (2.9) |
|
|
92.3 (0.7) |
50.2 (4.2) |
|
|
90.4 (0.9) |
54.3 (4.6) |
|
|
92.9 (2.3) |
59.2 (2.5) |
|
|
Bare gold (THF) |
78.5 (1.7) |
0 |
|
Bare gold (EtOH) |
77.8 (1.1) |
16.8 (3.4) |
Table 4.1
Water contact angles for self assembled monolayer surfaces on Au. Standard deviations are given in parentheses. A value of 0 indicates a low and ill defined angle. Samples of 180 and 181 used for these experiments were prepared using the method described in section 3.1.6.
The values for modified surfaces are significantly different from the bare gold samples indicating that SAM formation was successful. The observed angles fall into classes which parallel the gross structure of the molecules comprising the SAM. The amine and pyridine functionalized surfaces, 180, 181 and 184, exhibit lower advancing contact angles than the porphyrin monolayers. Monolayers terminated by polar functional groups have previously been reported to give rise to low qa.19 The mono- and di-thiol porphyrins, 227 - 229 and 223 - 225, all display qa in the range 102 - 108° consistent with a relatively hydrophobic surface, a result unsurprising given the alkyl substituents at the porphyrin periphery. qa of the strapped porphyrins, 234 - 237, are lower, and this result is of significance in that it indicates that monolayers of these compounds, prepared as described, are not identical to their dithiol analogues. The absence of any significant differences between contact angles of monolayers of the cis and trans isomers 237 and 236 does not enable any conclusions to be drawn about the structures of these monolayers.
All monolayers exhibit large contact angle hysteresis (qa - qr) which has been ascribed to a variety of causes including disorder, surface roughness and heterogeneity.434 Recently hysteresis was modelled thermodynamically as an adsorption/desorption process435 without the need to specify a mechanism for the hysteresis. It has been proposed that hysteresis on macroscopically smooth surfaces is lowest when the surfaces are either crystalline and rigid, or fluid-like and disordered, whereas rigid amorphous surfaces give rise to a large hysteresis.436,437 The reason437 that has been advanced for this observation is that a rigid crystalline surface cannot be penetrated or rearranged by contact with the probe fluid thus the dewetting process is simply the reverse of wetting. A highly mobile surface can rapidly reach equilibrium with the probe fluid as it wets or dewets so although the fluid may penetrate or rearrange the surface this is a reversible process on the time scale of the experiment and thus there is little hysteresis. In contrast a rigid disordered layer cannot rapidly reach equilibrium with the probe fluid thus rendering the wetting process irreversible and leading to hysteresis.
A possible cause for disorder and contact angle hysteresis in these monolayers is a mismatch of the size of the headgroups and alkyl thiol chains which prevents a dense two dimensional packing of the molecules thus allowing water to penetrate into the layer.372
There appears to be slightly larger hysteresis on monolayers of Zn porphyrins 235, 228, and 224 than their free-base or Ni analogues. Possible reasons could be interaction of water with the Zn centre, or a greater degree of disorder due to solvent coordination to the Zn porphyrin during the assembly process.