Force-area

A second type of measurement that may be made on films, usually in conjunction with force-area measurements is that of the contact or surface potential. One essentially measures the Volta potential between the surface of the liquid and that of a metal probe. 

Gregg [52] (see also Ref. 48) has surveyed the types of force-area equations obtainable from adsorption isotherms. 

The units are force/area, that is, allowable stresses). The directions of each of these strengths are shown in Figure 2-16 obviousiy, the strengths result from independent application of the respective stresses, 1> 12-... 

The pascal is defined as the pressure exerted by a force of one newton acting on an area of one square meter (recall that pressure = force/area). 

Adhesiveness, defined as the work necessary to overcome the attractive forces between the surface of the sample and the surface of other materials with which the food comes into contact, e.g. tongue, teeth, palate, etc. (Szczesniak, 1963), is given on the texturometer curve by the negative force area, representing the work needed to pull out the plunger from the sample. This parameter s value may be considered an evaluation of stickiness of jelly. Fracturability, also called brittleness, is given by the measure (%) of the plunger path into the jelly when it breaks. 

Jarvis, N.L. and Zisman, W.A. "Surface Activity of Fluorinated Organic Compounds at Organic-Liquid/Air Interfaces Part II. Surface Tension vs Concentration Curves, Adsorption Isotherms, and Force-Area Isotherms for Partially Fluorinated Carboxylic Esters," Naval Research Labs Report 5364, Surface Chemistry Branch, Chemistry Division, October 8, 1959. 

Fig. 12 Force/area curves of stearamide films on 6n H2S04 at 25°C 1, natural racemate II, enantiomeric stearamide III, mixture of solutions of enantiomers on the surface in 1 1 ratio. Reprinted with permission from Arnett and Thompson, 1981. Copyright 1981 American Chemical Society.

Fig. 13 Force/area curves of dipalmitoylphosphatidyl choline monolayers spread on pure water at 25°C (solid line) and 45°C (dashed line). The compression rate is 7.2 A2/molecule per minute. The shape of the isotherms is identical for homochiral and heterochiral films.

Fig. 15 Force/area curves for monolayers of dipalmitoylphosphatidyl choline (solid line), dimyristoylphosphatidyl choline (dashed line), and dilauroylphosphatidyl choline (dotted line) at 25°C.

The methyl esters of stearoylalanine [1] and stearoylserine [2] were considered as quasi-racemate candidates because of their slight structural differences. No quasi-racemate behavior was observed, however, in their force-area isotherms although clear diastereomeric discrimination was seen for this combination (Verbiar, 1983). We have seen no indication of quasi-racemate behavior for any other mixed chiral monolayers. 

When compressed to surface pressures greater than their stability limits (see Table 10), diastereomeric mixtures of /V-(a-methylbenzyl)stearamides with both stearoylalanine and stearoylserine methyl esters provided clear evidence of chiral discrimination. Force-area isotherms at 35°C for homochiral and heterochiral pairs of N-(a-methylbenzyl)stearamide and stearoylalanine methyl ester show differences in both their lift-off and touchdown (the area per molecule where the surface pressure returns to zero on the expansion arm of the isotherm) areas per molecule (Fig. 32). In addition, monolayers of the heterochiral pair could be compressed to lower areas per molecule than monolayers of the homochiral pair. 

No discrimination in the pressure/area characteristics was seen for diastereomeric monolayer films spread from all possible mixtures of pure racemates (R- and S-) and their racemic mixtures (R-, S-) of stearoylalanine, stearoylserine, stearoyltyrosine, and stearoyltryptophan methyl esters. The one exception was heterochiral and homochiral mixtures of stearoylalanine methyl esters and stearoylserine methyl esters at 35°C. The force/area... 

Figures 40-43 compare the position of the bridging carbonyl group in each diastereomeric pair and the effects of stereochemistry on their force-area curves at 25°C. It is clear that there is a large effect of stereochemistry on the energetics of compression and expansion for a wide variety of ketodiacid surfactants all of the ketodiadds in this study showed a dependence of the shapes of their IT/A isotherms on their stereochemistry. Several facts are striking. In every case there is a sharp differentiation between the behavior of films cast from meso- and ( )-isomers. The isotherms for the...

An unusually extensive battery of experimental techniques was brought to bear on these comparisons of enantiomers with their racemic mixtures and of diastereomers with each other. A very sensitive Langmuir trough was constructed for the project, with temperature control from 15 to 40°C. In addition to the familiar force/area isotherms, which were used to compare all systems, measurements of surface potentials, surface shear viscosities, and dynamic suface tensions (for hysteresis only) were made on several systems with specially designed apparatus. Several microscopic techniques, epi-fluorescence optical microscopy, scanning tunneling microscopy, and electron microscopy, were applied to films of stearoylserine methyl ester, the most extensively investigated surfactant. 

Hysteresis was generally observed in the compression-expansion cycles of the force-area isotherms, indicating that the timescale for relaxation of the fully compressed film back to its expanded state was slower than the movement of the barrier of the Langmuir trough. Our studies, like many others, imply that monolayers are metastable and that reversible thermodynamics can only be applied to their analysis with caution. 

P is the pressure (force/area), p is the fluid density (mass/volume),... 

Movement of the Teflon sweeping barrier is effected through a worm screw assembly mounted beneath the aluminum base that supports the trough during operation. The screw is driven by a sealed reversible motor with a precision feedback controller to maintain constant speed and torque. Force-area isotherms are actually gener-... 

The first irrefutable observation of a discriminating interaction between enantiomeric surfactants in monolayers was most probably made by Filippus Johannes Zeelen (84) in a detailed study concerning the synthesis, monolayer behavior, and photochemistry of a series of A -stearoylamino acid derivatives that were employed to model the conformation and photochemical decomposition of proteins. Although Zeelen was able to demonstrate significant differences in the force-area curves obtained from racemic and optically active forms of several of these derivatives, publication of this work in 1956 was... 

Zeelen found the extent of chiral discrimination to be dependent on the type of monomolecular phase that was formed. Thus, racemic and optically active samples displayed identical force-area curves (Fig. 14) when both formed liquid-expanded films, but owed considerably different curves (Fig. 15) under conditions where both samples formed a more highly condensed monolayer. 

The monolayer behavior of A-stearoyltyrosine (Fig. 16) was more complex. Under conditions (0.0liV HCl, 22 C) where the racemic material formed a condensed film having a limiting molecular area of 39 2 A, the force-area curve of L-(+)-A-stearoyltyrosine exhibited a liquid-expanded film at large areas (ca. 100-45 per molecule) followed by a transition beginning at 16.5 dynes/cm surface pressure to a condensed phase having a smaller limiting molecular area of 34 2 A . However, both these latter samples exhibited only the liquid-expanded phase on distilled water alone. 

In her initial investigation, Lundquist studied the monolayer behavior of racemic and optically active forms of both tetracosan-2-ol and its acetate derivative on 0.0 lA aqueous HCl over a considerable range of temperature (77). In each case, it was possible to demonstrate chiral discrimination between pure enantiomers versus the racemic substance. Furthermore, the extent of enantiomer discrimination was significantly temperature dependent, being enhanced at lower temperatures and frequently disappearing at higher ones. Under favorable conditions of temperature, however, the appearance of the force-area curves could be very sensitive to the optical purity... 

The force-area curves for racemic and (5)-(+)-2-tetracosanyl acetate recorded with a barrier speed of 5 cm/min are shown in Figures 17 and 18, respectively. Again, both enantiomers showed identical monolayer behavior. The film balance behavior of the racemic acetate was indistinguishable from that of the pure enantiomers at temperatures above about 27°C however, below this temperature the force-area curves differed markedly even at low surface pressures, which indicates that racemic compound formation occurs at relatively large areas per molecule. 

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