Vicinal water monolayer

In Eqs. (27) and (28), p is the contribution of the substrate water molecules, p that of the adsorbate polar head, and p that of the hydrophobic moiety of the adsorbed molecules. Consistently, 8i, 82, and 83 are the effective local permittivities of the free surface of water and of the regions in the vicinity of the polar head and of the hydrophobic group, respectively. The models have been used in a number of papers on adsorbed monolayers and on short-chain substances soluble in water. " Vogel and Mobius have presented a similar but more simplified approach in which p is split into two components only. " Recently some improvements to the analysis using Eq. (27) have been proposed. " An alternative approach suggesting the possibility of finding the values of the orientation angle of the adsorbate molecules instead of local permittivities has been also proposed.""... 

It is well-known that free films of water stabilized by surfactants can exist as somewhat thicker primary films, or common black films, and thinner secondary films, or Newton black films. The thickness of the former decreases sharply upon addition of electrolyte, and for this reason its stability was attributed to the balance between the electrostatic double-layer repulsion and the van der Waals attraction. A decrease in its stability leads either to film rupture or to an abrupt thinning to a Newton black film, which consists of two surfactant monolayers separated by a very thin layer ofwater. The thickness of the Newton black film is almost independent of the concentration of electrolyte this suggests that another repulsive force than the double layer is involved in its stability. This repulsion is the result of the structuring of water in the vicinity of the surface. Extensive experimental measurements of the separation distance between neutral lipid bilayers in water as a function of applied pressure1 indicated that the hydration force has an exponential behavior, with a decay length between 1.5 and 3 A, and a preexponential factor that varies in a rather large range. 

In a subsequent work, Woessner [57] used deuterium TiS of D O adsorbed at different saturation levels on a hectorite sample to monitor water mobility in the vicinity of a clay surface. It is clear from the data that the first two monolayers of water on the clay sheets experience restricted motion. 

The surface light scattering method has been used to show that the low interfaciai tensions in the Winsor I and II systems (O/W microemulsion in equilibrium with excess oil and W/O microemulsion in equilibrium with excess water, respectively) are due to the large surface pressure of the surfactant monolayer coating the interface, which almost balances the bare oil/water interfaciai tension [36,37]. Schulman and Montagne [38] proposed early that the low interfaciai tensions in microemulsion systems should be associated with these large surface pressures tt, i.e., 7 = 70 - tc 0. In other models, the origin of the low interfaciai tensions was attributed to the vicinity of critical points [39,40]. 

The particular feature with ethylene oxide based surfactants is that their interaction with water is less favorable at higher temperatures. This leads to a decrease in the spontaneous monolayer curvature with temperature, explaining the transition from oil-in-water emulsions below the PIT to water-in-oil emulsion above the PIT. In the vicinity of the phase inversion temperature the energy barrier against co-alescence(lP) varies very strongly with temperature. For the system n-octane-Cj2E5-water the following approximate relation was obtained in terms of AT = T-Tj, where is the PIT (4) ... 

Although the influence of water on the PEG conformational stmcture is widely accepted, it is less clear to what extent the water stmcture is modified by the presence of PEG. The density depression illustrated in Fig. 3 would suggest that a modified water stmcture potentially exists several nanometers beyond the nominal bmsh length. This is comparable to recent neutron-scattering observations (Schwendel et al., 2002) on ethylene-oxide self-assembled monolayers. The general idea is that this effect be linked to the hydrophobic effect in the vicinity of the amphiphilic PEG layer. 

FIGURE 2.5 Intermolecular forces stabilizing a lipid monolayer at the air-water interface. The apolar chains interact through London forces. Hydrogen bonds stabilize the interaction of two vicinal carboxylic acid head groups as well as the interaction of water molecules with these polar groups. 

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