Pressure, surface

The surface pressure of a monolayer film, n, is defined as the difference between the surface tension of the pure supporting liquid, to, and that of the liquid with an adsorbed film, a  

The phenomenon of surface pressure has been studied since the late nineteenth century. Some consequences of insoluble monolayer formation were known (but not understood) as early as Biblical times and were of interest to the likes of Benjamin Franklin. For example, the pouring of oil on stormy waters was recognized as an effective measure to protect fragile ships in a storm. 

With a known amount of material on the surface, the n-A curve allows one to determine something about the physical nature of the film and some molecular characteristics of the adsorbed material. 

Apparent surface pressure resulting from the pushing  

FIGURE 8.12. The surface pressure [Eq. (8.12)] can be visualized as arising from the mutual pushing action of neighboring adsorbed molecules working against the puU of the surface tension of the hquid. 

The relationships between surface pressure and surface or interfacial area can be used to gauge hydrophilicity. This method has been used, for example, to quantify the hydrophilicity of PDMS modified with ethylene oxide or propylene groups.  

8 The thermal motions of the molocules arc neglected in this argument. It happened that all the oils used in the earlier researches formed films in which the thermal motions could be neglected, as the molecules had such a large attraction for one another laterally in the films that they were united into compact masses much too large to show independent thermal motion. For such coherent films Pockels s law that the surface tension does not drop appreciably till the critical point is reached, and then drops suddenly, is true. 

The connexion between the surface pressure of the film and the surface tension of the film-covered surface is also very simple. Surface tension is the free energy per unit area of the surface, or the work which must be done to increase the area of the surface by one square centimetre. If the floating barrier A is displaced a small distance dx to the right, then the work done on it by the surface pressure F is Fldx, where l is the length of the float. But if y is the surface tension of the clean water surface, and y that of the film-covered surface, an area Idx of free energy y has 

With less strictness the surface pressure can be compared with the three-dimensional pressure on isolated matter and, with due consideration of the effects of the underlying water molecules on the behaviour of the molecules in the film, the conception of surface pressure as the effect of the repulsive forces between the film molecules and the boundary A of the film has yielded a very large part of our present information as to the molecular structure of these films.1 

The other criticism is one of propriety in nomenclature, rather than of physics  

Devaux2 made numerous experiments between 1903 and 1914. Using a light powder sprinkled on the surface, which is a convenient way of rendering the movements of the oil visible, he confirmed most of the results of Pockels and Rayleigh. He found that the oils spread to a definite maximum extension, which is of course the same as that at which the first fall in surface tension appears. Calculating the thickness of the films, he found it of the same order as the then approximately known dimensions of molecules.8 He was the first to notice that the films may be solid, 

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A static bottom hole pressure survey (SBHP) is useful for determining the reservoir pressure near the well, undisturbed by the effects of production. This often cannot be achieved by simply correcting a surface pressure measurement, because the tubing contents may be unknown, or the tubing contains a compressible fluid whose density varies with pressure (which itself has an unknown profile). 

If the spreading is into a limited surface area, as in a laboratory experiment, the film front rather quickly reaches the boundaries of the trough. The film pressure at this stage is low, and the now essentially uniform film more slowly increases in v to the final equilibrium value. The rate of this second-stage process is mainly determined by the rate of release of material from the source, for example a crystal, and the surface concentration F [46]. Franses and co-workers [47] found that the rate of dissolution of hexadecanol particles sprinkled at the water surface controlled the increase in surface pressure here the slight solubility of hexadecanol in the bulk plays a role. 

Neumann has adapted the pendant drop experiment (see Section II-7) to measure the surface pressure of insoluble monolayers [70]. By varying the droplet volume with a motor-driven syringe, they measure the surface pressure as a function of area in both expansion and compression. In tests with octadecanol monolayers, they found excellent agreement between axisymmetric drop shape analysis and a conventional film balance. Unlike the Wilhelmy plate and film balance, the pendant drop experiment can be readily adapted to studies in a pressure cell [70]. In studies of the rate dependence of the molecular area at collapse, Neumann and co-workers found more consistent and reproducible results with the actual area at collapse rather than that determined by conventional extrapolation to zero surface pressure [71]. The collapse pressure and shape of the pressure-area isotherm change with the compression rate [72]. 

Fig. IV-21. Surface pressure versus area for monolayers of immiscible components a monolayer of pure cadmium arachidate (curve 1) and monolayers of mixed merocyanine dye, MC2, and cadmium arachidate of molar ratio r = 1 10 (curve 2) 1 5 (curve 3), 1 2 (curve 4), and pure MC2 (curve 5). The subphase is 2.5 x 0 M CdC, pH = 5.5 at 20°C. Curve 3a (O) was calculated from curves 1 and 5 using Eq. IV-44. (From Ref. [116].)...

An interesting early paper is that on the saponification of I -monostearin mono-layers, found to be independent of surface pressure [307]. 

Fig. XV-9. Fluorescence micrograph of the stripe patterns observed in a monolayer from a mixture of PA and SP-Bi-25 (20% by weight peptide) on a buffered saline subphase at 16 C and zero surface pressure. (From Ref. 55.)...

Fig. XV-14. Surface pressure-area isotherms at 298 K for a DPPC monolayer on phos-photungstic acid (10 Af) at the pH values shown with 10 A/ NaCl added. (From Ref. 123.)...

Wlien a surface is compressed by a force/= kL, the surface pressure it =JIL is the force per unit width L producing a decrease in length dl. (Note that L and / are not the same indeed they are orthogonal.) The work is then... 

Surface properties enter tlirough the Yoimg-Laplace equation of state for the surface pressure ... 

A particular surface pressure is tlien chosen, and plotted against the values of A at tliat pressure. From tlie... 

Spectral radiant energy flux d(j)/dk Surface pressure 7T... 

The primary site of action is postulated to be the Hpid matrix of cell membranes. The Hpid properties which are said to be altered vary from theory to theory and include enhancing membrane fluidity volume expansion melting of gel phases increasing membrane thickness, surface tension, and lateral surface pressure and encouraging the formation of polar dislocations (10,11). Most theories postulate that changes in the Hpids influence the activities of cmcial membrane proteins such as ion channels. The Hpid theories suffer from an important drawback at clinically used concentrations, the effects of inhalational anesthetics on Hpid bilayers are very small and essentially undetectable (6,12,13). 

A thorough description of the internal flow stmcture inside a swid atomizer requires information on velocity and pressure distributions. Unfortunately, this information is still not completely available as of this writing (1996). Useful iasights on the boundary layer flow through the swid chamber are available (9—11). Because of the existence of an air core, the flow stmcture iaside a swid atomizer is difficult to analyze because it iavolves the solution of a free-surface problem. If the location and surface pressure of the Hquid boundary are known, however, the equations of motion of the Hquid phase can be appHed to reveal the detailed distributions of the pressure and velocity. 

Langmuir-Blodgett was the first technique to provide a practical route for the constmction of ordered molecular assembhes. These monolayers, which provide design dexibiUty both at the individual molecular and at the material levels, are prepared at the water—air interface using a hiUy computerized trough (Fig. 1). Detailed discussions of troughs (4) and of surface pressure, 7T, and methods of surface pressure measurements are available (3,6). 

Fig. 1. A trough for deposition of monolayers on soHd substrates A, bath B, a moving barrier C, a motor D, a pressure-control device E, a surface pressure balance F, a motor with a gearbox that lowers and raises the substrate and G, a soHd substrate. The film material (S) has a hydrophobic tail and...

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

As the barrier moves, the molecules are compressed, the intermolecular distance decreases, the surface pressure increases, and a phase transition may be observed in the isotherm. These phase transitions, characterized by a break in the isotherm, may vary with the subphase pH, and temperature. The first-phase transition, in Figure 2, is assigned to a transition from the gas to the Hquid state, also known as the Hquid-expanded, LE, state. In the Hquid... 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

Leakage from Use adequate shaft sealing (mechanical or multimill ignited by pie gas purged lip or chevron seals). Harden spark or hot shafts in seal area surface., pressure tight flexible connections and clamps on mill inlets and outlets Provide adequate fixed fire protection where appropriate CCPS G-23 CCPS G-29 NFPA 13 NFPA 15 NFPA 16... 

At times when the surface pressure gradient is weak, resulting in light winds in the atmosphere s lowest layers, and there is a closed high-preSsure system aloft, there is potential for the buildup of air pollutant concentrations. This is especially true if the system is slow-moving so that light winds remain in the same vicinity for several days. With light winds there will be little dilution of pollutants at the source and not much advection of the polluted air away from source areas. 

Fig.4.53. Experimental and simulated PM (polarization modulated) IRRAS spectra of single monolayers of (A) PEG and (B) K(LK)7 at the air-water interface. The surface pressure was 20 mN m [4.281],...

The second part of computing building pressures involves the pressure coefficient for a particular spot on the building. The surface pressure coefficient, Cp, indicates the share of the wind kinetic energy that is transferred to the static pressure ... 

For a building with sharp corners, Cp is almost independent of the wind speed (i.e., Reynolds number) because the flow separation points normally occur at the sharp edges. This may not be the case for round buildings, w here the position of the separation point can be affected by the wind speed. For the most common case of the building with a rectangular shape, Cp values are normally between 0.6 and 0.8 for the upwind wall, and for the leeward wall 0,6 < C, < —0.4. Figure 7.99 and Table 7.32 show an example of the distribution of surface pressure coefficient values on the typical industrial building envelope. 

Values of Cp for simple building geometries may be obtained from the British Standards Institution or from Liddament. The following relationship between wind incident angle a, building side ratio, and average surface pressure coefficient is based on the database developed by Swami and Chandra ... 

FIGURE 7.99 Example of surface pressure coefficient values for a typical industrial building envelope. 

TABLE 7.32 Approximate Surface Pressure Coefficient Values for a Building with a Rooftop Vent... 

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