## 4 Discussion  
Shown in Figure 9 is an illustration of (1) an OTMS molecule, (2) the conversion of its three methoxy groups to three hydroxyl groups and the release of three methanol molecules via hydrolysis, and (3) the condensation reaction between silanols (Si-OH) and the hydroxyl groups of the SiO\({}_{2}\) substrate to anchor the molecules to the substrate, as well as the homocondensation reaction [19,44] between silanols to polymerize the molecules to form the -Si-O-Si- network. The condensation and homocondensation reactions result in the formation of water molecules, which evaporate. The closely packed methylene chains terminated with the methyl groups of the OTMS SAMs provide hydrophobicity, which is responsible for the large water CAs observed (Figure 1b). As shown in Figure 9, the OTMS SAMs are characterized by the polymerization of silanols, which are responsible for the SAMs of organosilanes. In contrast, for the other two types of SAMs of alkanethiols and organophosphonic acids, there is no cross linking between the molecular headgroups. This distinctive difference was revealed by ToF-SIMS, thanks to its extremely high surface sensitivity and superb chemical selectivity. For example, for alkanethiol SAMs formed on gold, ions associated with both the alkanethiol molecule (whose molecular formula is represented by M) and gold were detected, including Au\({}_{2}\)M\({}^{-}\) and [Au\({}_{2}\)M\({}_{2}\)-H]\({}^{+}\)[30], M\({}_{2}\)Au\({}^{+}\)[31] and [M-H + Au\({}_{2}\)]\({}^{-}\)[32]. Even though alkanethiol molecules are covalently bonded to the substrate via the Au-S linkage [2], the detection of molecular or pseudomolecular ions by ToF-SIMS is attributed to the lack of cross linking of the molecular headgroups themselves. In the case of OPA SAMs, protonated and deprotonated molecular ions were detected [33,34]. The phosphonic acid headgroups can form mono-, bi- and tridentate linkages to the substrate via the condensation reaction [45,46], but there is no homocondensation reaction between the phosphonic acid headgroups with hydroxyl groups [44]. This is regarded as the reason for the fragmentation of protonated and deprotonated OPA molecular ions from their SAMs that are covalently bonded to the substrate [33,34]. In special cases, OPA molecules can even form SAMs that are weakly attached to the substrate via H-bonding when a solvent having a dielectric constant of \(\sim\)4 is used [23]. In this case, the OPA molecules in their SAMs are confirmed to have the same ion fragmentation pattern as that of free OPA molecules [34]. Our ToF-SIMS results on free OTMS molecules (denoted as M) showed the presence of molecular or pseudomolecular ions, such as [M + H]\({}^{+}\) (Figure 8), [M-H]\({}^{-}\) and [M-CH\({}_{3}\)]\({}^{-}\) (Figure 6). This is proof that the methoxy groups of the free OTMS molecules used for ToF-SIMS analysis (in vacuum) were not hydrolyzed. In contrast, for OTMS SAMs formed on a SiO\({}_{2}\) substrate, no molecular or pseudomolecular ions were detectable, which confirms  
Figure 9: An illustration showing an OTMS molecule and the hydrolysis of its methoxy groups, resulting in the formation of silanols (Si–OH) and the evaporation of methanol molecules. An OTMS monolayer is formed on a SiO\({}_{2}\) substrate via the condensation reaction between silanols and hydroxyl groups from the substrate to anchor the molecule to the substrate and homocondensation reaction between silanols to establish the polymerized –Si–O–Si– network. that the three methoxy groups for each molecule were hydrolyzed. Moreover, ToF-SIMS did not detect any ions with the alkyl group (C\({}_{18}\)H\({}_{37}\)) bonded with Si or SiO\({}_{n}\) (\(n\) = 1-3). This experimental observation reveals the most significant difference between organosilane SAMs and organophophonic acid SAMs in their ion fragmentation; that is, if the molecules are polymerized across their headgroups, there will be no generation of ions bonded to the headgroup.