Monolayer stability

Traditional amphiphiles contain a hydrophilic head group and the hydrophobic hydrocarbon chain(s). The molecules are spread at molecular areas greater (-2-10 times) than that to which they will be compressed. The record of surface pressure (II) versus molecular area (A) at constant temperature as the barrier is moved forward to compress the monolayer is known as an isotherm, which is analogous to P-V isotherms for bulk substances. H-A isotherm data provide information on the molecular packing, the monolayer stability as de-... 

The monolayer stability limit is defined as the maximum pressure attainable in a film spread from solution before the monolayer collapses (Gaines, 1966). This limit may in some cases correspond directly to the ESP, suggesting that the mechanism of film collapse is a return to the bulk crystalline state, or may be at surface pressures higher than the ESP if the film is metastable with respect to the bulk phase. In either case, the monolayer stability limit must be known before such properties as work of compression, isothermal compressibility, or monolayer viscosity can be determined. 

In addition, it should be noted that none of the compression and expansion cycles for these films are coincident. The considerable hysteresis exhibited during the compression/expansion cycle is evidenced at every compression/expansion rate investigated, and is indicative of a stereoselective kinetic process that must occur upon film compression. Table 3 gives the monolayer stability limits of the amino acid methyl ester films as defined by... 

The stability limits of these monolayers are given in Table 6, and demonstrate the fact that the more highly expanded racemic film is also the more stable system over the temperature range studied. However, the general trend that can be observed in the 11/A isotherms and the monolayer stability limits is that, as the enantiomeric films become increasingly stable, their isotherms begin to take on the characteristics of the racemic system. Coupled with the observation of the temperature and compositional dependence of the... 

The latter point is illustrated by the surface shear viscosities of the homochiral and heterochiral films at surface pressures below the monolayer stability limits. Table 7 gives the surface shear viscosities at surface pressures of 2.5 and 5 dyn cm -1 in the temperature range given in Fig. 19 (20-40°C). Neither enantiomeric nor racemic films flow under these conditions at the lower temperature extreme, while at 30°C the racemic system is the more fluid, Newtonian film. However, in the 35-40°C temperature range, the racemic and enantiomeric film systems are both Newtonian in flow, and have surface shear viscosities that are independent of stereochemistry. These results are not surprising when one considers that (i) when the monolayer stability limit is below the surface pressure at which shear viscosity is measured, the film system does not flow, or flows in a non-Newtonian manner (ii) when the monolayer stability limit is above the surface pressure... 

Taken together, the equilibrium spreading pressures of films spread from the bulk surfactant, the dynamic properties of the films spread from solution, the shape of the Ylj A isotherms, the monolayer stability limits, and the dependence of all these properties on temperature indicate that the primary mechanism for enantiomeric discrimination in monolayers of SSME is the onset of a highly condensed phase during compression of the films. This condensed phase transition occurs at lower surface pressures for the R( —)- or S( + )-films than for their racemic mixture. 

Table 9 Monolayer stability limits of palmitic acid/stearoylserine methyl ester films at 25°C.

Since it has been shown that nonideal mixing occurs in the 2.5-15.0 dyn cm 1 range, the excess free energies of interaction were calculated for compressions of each pure component and their mixtures to each of these surface pressures. In addition, these surface pressures are below the ESPs and/or monolayer stability limits so that dynamic processes arising from reorganization, relaxation, or film loss do not contribute significantly to the work of compression. 

Moller etal. [462] have performed in situ STM observations of Ni electrodeposition on reconstructed Au(lll) electrodes. Ni nucleation proceeded in three distinct potential-dependent steps. The same group of researchers [463] has studied electrodeposition and electrodissolution of Ni on Au(lOO) electrodes. Pronounced differences were observed for the nucleation and submonolayer growth on the reconstructed and unreconstructed surfaces. On perfectly reconstructed Au(lOO), the formation of Ni islands started at overpotentials significantly higher (rj > 100 mV) than on unreconstructed surface (rj > 40 mV), where Ni monolayer islands were formed. Dissolution of the Ni film exhibited better monolayer stability in comparison to the multilayer deposit. 

An explanation for this gel formation is sought in the phase transition behavior of span 60. At the elevated temperature (60 °C) which exceeds the span 60 membrane phase transition temperature (50 °C) [154], it is assumed that span 60 surfactant molecules are self-assembled to form a liquid crystal phase. The liquid crystal phase stabilizes the water droplets within the oil. However, below the phase transition temperature the gel phase persists and it is likely that the monolayer stabilizing the water collapses and span 60 precipitates within the oil. The span 60 precipitate thus immobilizes the liquid oil to form a gel. Water channels are subsequently formed when the w/o droplets collapse. This explanation is plausible as the aqueous volume marker CF was identified within these elongated water channels and non-spherical aqueous droplets were formed within the gel [153]. These v/w/o systems have been further evaluated as immunological adjuvants. 

Protein monolayers behave differently from t)q5ical lipids under the same experimental conditions (Eigure 14.6). Eor protein monolayers, fits of the experimental data at tt lower than and higher than TTg require two exponential decays. The relaxation of the protein monolayer is therefore not a simple process. At tt < TTg the relaxation rate and the amplitude of the relaxation area depend on the surface pressure and the protein. The relaxation rate (quantified by means of the relaxation time, t, inverse rate constant) is higher at the highest surface. Protein monolayer stability was... 

Clegg, R. S., Reed, S. M., Hutchison, J. E. (1998). Self-assembled monolayers stabilized by three-dimensional networks of hydrogen bonds, J. Am. Chem. Soc., 120 2486. 

The Langmuir-Blodgett teehnique also makes it possible to monitor the monolayer stability. This is done by compressing the film to a eertain pressure that is held eonstant. The decrease in film area is measured as a ftmetion of time. An observed loss of film area may be as a result of rearrangements of the film molecules, dissolution of film molecules into a different state, and/or eollapse by nuele-ation (58) subsequently, solid bulk fragments start to grow. 

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