Surface pressure-molecular area isothermal curv

FIG. 23 Surface pressure vs. area/molecule isotherms at 300 K from molecular dynamics simulations of Karaborni et al. (Refs. 362-365). All are for hydrocarbon chains with carboxylate-like head groups, (a) (filled squares) A 20-carbon chain, (b) (filled circles) A 16-carbon chain with a square simulation box the curve is shifted 5 A to the right, (c) (open squares) A 16-carbon chain with a nonsquare box with dimensions in the ratio xly = (3/4) to fit a hexagonal lattice the curve is shifted 5 A to the right. (Reproduced with permission from Ref. 365. Copyright 1993 American Chemical Society.)... 

While the barrier confines the amphiphiles to a smaller area, the force exerted by the monolayer is continuously measured and a surface pressure (I7)-area (A) isotherm can be drawn at a constant temperature. This curve plots the surface pressure (force per unit length) versus the mean molecular area occupied by the amphiphiles at the air/water interface. Usually, a U-A isotherm shows four interesting regions [21]. An initial horizontal region where the mean molecular area is large and the interaction between molecules is small so the surface pressure is approximately constant. The first linear region deviates from the... 

Figure 17 shows the 11/A isotherms of racemic and enantiomeric films of the methyl esters of 7V-stearoyl-serine, -alanine, -tryptophan, and -tyrosine on clean water at 25°C. Although there appears to be little difference between the racemic and enantiomeric forms of the alanine surfactants, the N-stearoyl-tyrosine, -serine, and -tryptophan surfactants show clear enantiomeric discrimination in their WjA curves. This chiral molecular recognition is first evidenced in the lift-off areas of the curves for the racemic versus enantiomeric forms of the films (Table 2). As discussed previously, the lift-off area is the average molecular area at which a surface pressure above 0.1 dyn cm -1 is first registered. The packing order differences in these films, and hence their stereochemical differentiation, are apparently maintained throughout the compression/expansion cycles. 

The primary evidence for the conversion of gaseous monolayers at the air-water interface to other intermediate states lies in the abrupt changes found on the n-A isotherms of many film-forming compounds. So many of these isotherms have been reproduced in fine detail in a number of laboratories under a variety of conditions that they cannot possibly be rejected wholesale as artifacts. The sharp transitions from curves to plateaus, where the molecular area varies readily at constant surface pressure, may be related... 

Surface-pressure/surface-area isotherms provide valuable insight into the molecular packing of surfactants in monolayers. A steep slope in the n-A curve... 

Sears and Schulman (51) measured surface pressures and potentials vs. molecular area (20 to 110 sq. A. per molecule) for the alkali metal stearates over 0.5N solutions of LiOH, NaOH, and KOH. Like Adam and Miller, they detected a specific cation effect on the 7r-A and AV-A isotherms of the ionized monolayer at high pH the ir-A curves were expanded in the order of the crystalline sizes of the alkali metal cations K > Na > Li. The sequence is the reverse observed for the long-chain... 

Over a long period of time experimental results on amphiphilic monolayers were limited to surface pressure-area ( r-A) isotherms only. As described in sections 3.3 and 4, from tc[A) Isotherms, measured under various conditions, it is possible to obtain 2D-compressibilities, dilation moduli, thermal expansivities, and several thermodynamic characteristics, like the Gibbs and Helmholtz energy, the energy cmd entropy per unit area. In addition, from breaks in the r(A) curves phase transitions can in principle be localized. All this information has a phenomenological nature. For Instance, notions as common as liquid-expanded or liquid-condensed cannot be given a molecular Interpretation. To penetrate further into understanding monolayers at the molecular level a variety of additional experimental techniques is now available. We will discuss these in this section. 

If we further compress the surface area, the head-groups become dehydrated and the 7r-As isotherm is also linear with a steeper slope. The close-packed and dehydrated molecular area, A , can be obtained by extrapolating the slope of this (S) phase to zero pressure. However, in reality, it is not possible to obtain both A and Ah values simultaneously for many 7r-As isotherms, because the upper part of the isotherm curve is frequently not so linear. The reason is that the phase transition between S and (L-Con.) is usually not sharp, but smooth. In these circumstances, only one of these molecular areas can be determined. If we further compress the area very slowly, the surface pressure increase will stop at a specific surface pressure value, the collapse of the monolayer (C) occurs, and %... 

Bansal, studied the adsorption desorption isotherms of benzene, toluene and o-xylene on sugar charcoal associated with varying amounts of the carbon-oxygen surface groups and observed that the area of the hysteresis loop decreases as the molecular dimensions of the adsorbate increase from benzene to o-xylene (Table 2.7). The point of inception of the hysteresis loop was also found to shift to lower relative vapor pressures as we move up the series of hydrocarbons, which is due to an increase in the molecular size of the adsorbate. The point of inception of hysteresis loop was calculated using Cohn postulates and compared with the values read from the experimental curves (Table 2.8). It is seen that the two values agree closely for all the adsorbate-adsorbent systems. Higute, from thermodynamic considerations, also proposed that the critical radius for the inception of capillary condensation is equal to four times the molecular radius of the adsorbate. 

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