Monolayer lifetime

The time necessary for removing one monolayer during a SIMS experiment depends not only on the sputter yield, but also on the type of sample under study. We will make an estimate for two extremes. First, the surface of a metal contains about 1015 atoms/cm2. If we use an ion beam with a current density of 1 nA/cm2, then we need some 150 000 s - about 40 h - to remove one monolayer if the sputter yield is 1, and 4 h if the sputter rate is 10. However, if we are working with polymers we need significantly lower ion doses to remove a monolayer. It is believed [4] that one impact of a primary ion affects an area of about 10 nm2, which is equivalent to a circle of about 3.5 nm diameter. Hence if the sample consists, for example, of a monolayer film of polymer material, a dose of 10n ions/cm2 could in principle be sufficient to remove or alter all material on the surface. With a current density of 1 nA this takes about 1500 s or 25 min only. For adsorbates such as CO adsorbed on a metal surface, we estimate that the monolayer lifetime is at least a factor of 10 higher than that for polymer samples. Thus for static SIMS, one needs primary ion current densities on the order of 1 nA/cm2 or less, and one should be able to collect all spectra of one sample within a quarter of an hour. 

To increase monolayer lifetime and hence achieve SSIMS conditions, the sputter rate needs to be reduced (eqn (11.2)), and this can be realised by reducing the primary beam flux (eqn (11.1)). However, this also has the effect of reducing the secondary ion current. The choice of mass analyser is, therefore, most important for optimum spectrometer performance and to maximise the chemical information available from the material s surface. 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

For a molecule characterised by a AH value of 40 k.I mol 1 and undergoing facile surface diffusion, i.e. a A/ dir value close to zero, then each molecule will visit, during its surface lifetime (10 r s), approximately 107 surface sites. Since the surface concentration a is given by a = NtSUIf, then for a AH value of 40 kJ mol-1 and zsurf= 10-6 s at 295 K, the value of a is 109 molecules cm-2. These model calculations are illustrative but it is obvious that no conventional spectroscopic method is available that could monitor molecules present at a concentration 10-6 monolayers. These molecules may, however, contribute, if highly reactive, to the mechanism of a heterogeneously catalysed reaction we shall return to this important concept in discussing the role of transient states in catalytic reactions. 

Static SIMS, used for sub-monolayer elemental analysis. At the lowest current densities and hence the lowest rates of erosion, a monolayer on the surface has a lifetime of many hours. The surface is essentially unchanging during the experiment, but a vacuum system at a pressure of 10 10mbar is needed to allow adequate time to complete the analysis. In favourable cases, as little as 0.1% of a monolayer of material can be detected. 

Photoinduced excited states of the naphthalene derivatives included in the amphiphilic p-CD LB films were found to be stablized by measurements of the fluorescence lifetimes and the excimer formation of the naphthalene derivatives adsorbed by the CD monolayer occured mainly between the adjacent layers [29]. 

The monolayer strip with straight edges ax depicted in Fig. 1 can be taken as an idealised initial configuration of the top terrace. We are intesrested in calculating the typical time for. the top terrace to disappear (or most, part, of it, to be precise). This is actually a problem which I have not, been able to find an exact, answer. Qualitatively, however, an estimate for the lifetime of the strip can he made as follows. 

Upon irradiation of an IPCT band in degassed condition (/.ex > 365 nm), the color of both LB films changed from pale yellow to blue. The UV/vis absorption spectrum after irradiation of LB films with 120 monolayers is shown in Fig. 14, which is characteristic of 4,4 -bipyridinium radical cation monomer [61], Colored species photogenerated in mixed LB films of AV2+/AA or HV2+/AA systems decayed almost exponentially in the dark in vacuo with a lifetime of about 4 hr at 20°C [38,39], The lifetime of 4,4 -bipyridinium radical cations in LB films was almost the same as that in microcrystalline films [36], which indicated the microenvironment around photogenerated radical cations in LB films... 

Beyond their ideal ground-state electrochemistry, following photoexcitation using a laser pulse at 355 nm, emission is observed from the monolayers with an excited state lifetime, 6.2 ps, which exceeds that of the complex in solution, 1.4 gs. It appears... 

Data on emulsion film formation from insoluble surfactant monolayer are rather poor. It is known, however, that such films can be obtained when a bubble is blown at the surface of insoluble monolayers on an aqueous substrate [391,392]. Richter, Platikanov and Kretzschmar [393] have developed a technique for formation of black foam films which involves blowing a bubble at the interface of controlled monolayer (see Chapter 2). Experiments performed with monolayers from DL-Py-dipalmitoyl-lecithin on 510 3 mol dm 3 NaCl aqueous solution at 22°C gave two important results. Firstly, it was established that foam films, including black films, with a sufficiently long lifetime, formed only when the monolayer of lecithin molecules had penetrated into the bubble surface as well, i.e. there are monolayers at both film surfaces on the contrary a monolayer, however dense, formed only at one of the film surfaces could not stabilize it alone and the film ruptured at the instant of its formation. Secondly, relatively stable black films formed at rather high surface pressures of the monolayer at area less than 53A2 per molecule, i.e. the monolayer should be close-packed, which corresponds to the situation in black films stabilized with soluble surfactants. 

Thus, the foam bilayer can indeed be regarded as a system of two amphiphile monolayers adsorbed onto each other. In view of the strong effect of the concentration C of surfactant in the solution on the bilayer lifetime T, it is very convenient to use the t(C) dependence for experimental verification of the theory [399,402,403] of hole-mediated rupture of bilayers. 

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