Stability of the Monolayers

In most cases, the packing of the spacers is mainly controlled by van der Waals [1-8] and aromatic, e.g., jr-jr, [61-63] interactions. There are few examples of hydrogen bonds, (e.g., due to the presence of amide moieties [64]) or of dipole-dipole interactions (e.g., due to sulphone groups) between adjacent spacers [65]. Covalent bonds between atoms of adjacent spacers are only occasionally reported, as in the case of the formation of C-C bonds between alkyl chains, induced by electron irradiation [66]. Disorder in the spacer ensemble may be due to the head and tail groups in the case of bulky groups, the density of the molecules on the substrate is low. As a result, the forces between adjacent spacers are weak, disorder arises in the spacer moieties, and consequently in the SAM as a whole. 

Particular attention has been devoted to the influence of the temperature on the structure and degree of order of monolayers, an increase of temperature leading to a lower order. The thermal stability of alkanethiols is, in fact, quite low thiol 

The stability of thiol monolayers in atmosphere is fair for short time lengths they are sensitive to the presence of traces of ozone at the level present in the common environment, which causes oxidation of S head groups, with consequent desorption. Hence, it is strongly advised to store the system in solution [71-73]. 

As for the stability of the molecules to use, the product of the -SH group oxidation depends on the experimental conditimis different species containing oxygen have been reported, all of them weakly adsorbed on metal surfaces. In addition, thiols are easily oxidatively destroyed by OH radicals [74]. 

---

The question may then be raised as to whether insoluble monolayers may really be treated in terms of equilibrium thermodynamics. In general, this problem has been approached by considering (i) the equilibrium spreading pressure of the monolayer in the presence of the bulk crystalline surfactant, and (ii) the stability of the monolayer film as spread from solution. These quantities are obtained experimentally and are necessary in any consideration of film thermodynamic properties. In both cases, time is clearly a practical variable. 

As a cationic polymer and a cationic amphiphile, poly(allyl amine hydrochloride) (PAA) and octadecylamine (ODA) shown in Fig. 6 were used, respectively. The stability of the monolayers of the anionic amphiphiles was increased by polyion-complexation with PAA added in the aqueous subphase in comparison with Ca2+ salt formation. Ion complexation (1 1) of each anionic amphiphile with ODA was also performed at the air-water interface by spreading a chloroform solution of a 1 1 surfactant mixture. 

While the stability of the monolayer Pt alloy catalyst concept was initially unclear and therefore threatened to make the monolayer catalyst concept a questionable longer term solution, a very recent discovery seems to lend support to the claim that Pt monolayer catalyst could be made into stable catalyst structures Zhang et al. [94] reported the stabilizing effect of Au clusters when deposited on top of Pt catalysts. The presence of Au clusters resulted in a stable ORR and surface area profile of the catalysts over the course of about 30,000 potential cycles. X-ray absorption studies provided evidence that the presence of the Au clusters modified the Pt oxidation potentials in such a way as to shift the Pt surface oxidation towards higher electrode potentials. 

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between GfJ, the surface tension of pure water, and G, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constructed. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stability of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

Hexadecyl and octadecyl alcohol have been extensively studied and shown to be highly effective in evaporation retardation. Scattering powdered samples of commercial-grade alcohols by boat on lake surfaces or the continuous addition of alcohol slurries from floating dispensers are two of the methods that have been employed to apply these monolayers. Wind conditions and the activity of aquatic birds have a considerable effect on the stability of the monolayer and therefore on the rate at which the monolayer chemicals must be reapplied. Rates of application rarely exceed 0.5 lb acre- day-1, however, so that the cost of the materials used is not excessive. 

Surface rheology Viscoelasticity of the monolayer differentiation between fluid and solid phases. Surface elasticity and viscosity in the transversal and longitudinal mode wave damping characteristics. Relaxation processes in monolayers. Mechanical stability of the monolayer. Interpretation often complicated because several molecular processes may be involved and because viscous and elastic components may both contribute. 

Mechanical stability. Chemisorption to the smface, intermoleeular interactions and crosslinking between adjacent compounds—if possible—all contribute to the resulting stability of the monolayer film. Lateral force microscopy investigations revealed that the mechanical stability towards lateral forces on the nanometre scale is likely to be determined by the defect density and the domain size on a nano- to micrometre scale [163. 173],... 

Polydiacetylenes with appropriate amphiphilic structures can be used to fabricate thin films in this way, e.g., heptadeca-4,6-diyne-l-ol and the corresponding add. It is often found that the stability of the monolayers is a function of the stmcture of the molecules. It has also been found advantageous to use the neutralized form of acid derivatives, and diacetylene monocarboxylic acids generally form much more stable films if the cadmium salt is used initially, where the Cd + ion is then present in the aqueous phase as a counterion. 

Geerken et al. [25] investigated the chemical and thermal stability of OTS and a perfluorinated equivalent (perfluorinated octyltrichlorosilane) in liquids as a function of time (up to 200 h), temperature (50°C-80°C), and pH (2-13). They evalnated the stability of the monolayers (not stiction) by measuring water and hexadecane contact angles after exposure to harsh conditions. They reported that perflnorinated monolayers were more stable than OTS in all the conditions they investigated. 

Multiple scanning experiments on this and other Fc-peptide cystamine mono-layers shows only small decreases in the signal intensity, indicating the stability of the monolayers to scanning. For a SAM of 40, over a period of 60 s, the peak current decreased by —10%, indicating that loss of the Fc label is minimal (Fig. 9). Some... 

A large portion of the work that has been done in our group on the surface-enhancement of fluorescence from LB monolayers has been in conjunction with surface-enhanced Raman studies. In theses studies, SEF has been helpful in providing information about the aggregation, and orientation of dye molecules, as well as information about the thermal stability of the monolayers used. A separate study reported the effects of different variables pertaining to the fabrication of LB films. In particular. 

Before deposition of the material to form multilayers, the stability of the monolayer must be considered. The solid monolayer phase of most LB materials is formed at surface pressures above the equilibrium spreading pressure (ESP) of the material the ESP being defined as the pressure at which the monolayer and stable solid or liquid phases are in thermodynamic equilibrium. Consequently, the deposition of most LB materials occurs from a metastable state. This often presents no practical problems, since the approach to thermodynamic equilibrium is extremely slow. However, for some materials a loss of pressure or area is observed below the collapse pressure 11. These materials approach thermodynamic... 

LB film and essentially increases the stability of the monolayer at the air-water interface. 

Figure 16.22 Ferrocene covalently linked to an alkanethiol adsorbed to the surface of a gold electrode. The ferrocene-substitnted alkylthiol is diluted in other alkanetfaiols to minimise the interactions between ferrocenes and favour a homogenous coverage of the electrode. The stability of the monolayer is due to the strong adsorption of the sulphur atom at the surface of the gold electrode.

---