Surface pressure attainment

Figure 4. The surface pressure attained after 40 minutes (U minutes) as a function of the initial subphase concentration for all the proteins studied at different ionic strengths (4)...

The concentration dependence of the surface pressure attained after 40 minutes ( 40) can vary substantially between proteins, which Figure 5 illustrates (4). The measurements have been made at the A/W-interface. A mildly produced soy protein isolate (kindly provided by Central Soya), a commercially available sodium caseinate (DMV, Holland) and an ultrafiltrated and spray-dried WPC were used. They were studied when dispersed in distilled water and in 0.2 M NaCl solution at pH 7 denoted as (0-7) and (0.2-7), respectively. Analysis of the proteins is given in (4). 

Figure 8. The surface pressure attained after 40 min., 402 function of the initial subphase concentration for the proteins soy protein, WPC, and caseinate adsorbing at the (, o) A/W- and (A,A) 0/W- interfaces closed symbols (0.2-7) and open symbols (0-7). (Reproduced with permission from Ref. 10. Copyright 1982 Blackwell Scientific Publications.)...

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. 

The differences in time-dependent adsorption behavior between 99% PVAC at 25° and 50°C demonstrate the influence of intra- and intermolecular hydrogen bonding in the adsorption process. The limiting surface pressure of the hydrophobic water-soluble polymer appears to be 33 mN/m, approximately 7 mN/m below that of commonly used surfactants. The rate of attainment of equilibrium surface pressure values is faster if there is uniformity of the hydrophobic segments among the repeating units of the macromolecule. 

The ultimate pressure attainable with adsorption pumps is determined in the first instance by those gases that prevail in the vessel at the beginning of the pumping process and are poorly or not at all adsorbed (e.g. neon or helium) at the zeolite surface. In atmospheric air, a few parts per million of these gases are present. Therefore, pressures < 10 mbar can be obtained. 

In Figure 2 the ir-A and AV-A plots for SODS on O.OIM NaCl sub-solutions having different pH values are shown. In all cases, phase transitions from liquid-expanded to liquid-condensed state are evident ( ). Acidification of the subsolution Increases the transition pressure but the transition is less pronounced at the lowest pH studied. This is also accompanied by an expansion of the condensed part of the curve. Small negative surface potentials are observed over most areas. The highest potential is obtained for film spread on the pH 2.2 subsolution. For small areas, the surface potential attains a positive value. This may be related to reorientation of the dipole moments of the molecules which occur once a threshold surface concentration is exceeded (O. Mlnglns and Pethlca (7) studied the monolayer properties of SODS on various sodium chloride solutions (0.1, 0.01 and O.OOIM) at 9.5 C, and they showed that the monolayer is only stable on the more concentrated salt solutions (0.1 and O.OIM). In this work, no noticeable... 

Figure 3.80. Surface pressure curves for stearic acid (CjyCOOH). Comparison between results of different authors, (a) Motomura et al. ), pH = 2 (HCl), spread from benzene, 25 0.1 °C, constant compression rate (cr), results independent of rate between 0.197 and 0.788 nm mol min (b) Menger et al. ). 0.03 M H2SO4, spread from hexane. 23.0°C, cr. 0.4 nm mol min data points by Mingotaud c.s. (c) Tomoaia-Cotisel et al., pH = 1-3 (no difference), spread from hexane after dissolution in ethanol, 22 + 2°C, no dependence on cr. (range not Indicated) (d) T. Murakata et al.4), 5x10 NaHCOg + 3 x lO M BaCl2. spread from chloroform, 17°C, cr. not given (e) Dorfler and Rettig pH = 2 (HCl). spread from benzene, 25°C, cr. 77.6 cm min (0 Halperin et al. ). pH = 2 (HCl), spread from chloroform-heptane mixture, 21°C, cr. 0.9 cm min (the lowest rate attainable on the Lauda apparatus).

Two experimental results indicate that there is an adsorption energy barrier related to the interfacial pressure. First, the presence of an energy barrier becomes evident only after an interfacial pressure of 0.1 mN m-1 is attained (Table II). In the second experiment, different compounds were spread at the air/water interface and the rate of adsorption of pepsin and lysozyme were measured under conditions where charge effects were minimized (MacRitchie and Alexander, 1963a). It was found that the rates of adsorption for these proteins were independent of the nature of the surface film and depended only on the surface pressure. 

The protein concentration (C) dependence on surface pressure (tt), that is, the surface pressure isotherm, showed sigmoidal behavior (Figure 14.2) (Nino et al., 2005). At low protein concentrations, the initial solutions caused only a small increase in tt. The surface pressure increased with C and tended to a plateau. This plateau commenced at the point where tt reached its maximum value over the range of protein concentrations. The behavior of adsorbed protein films can be interpreted in terms of monolayer coverage. At the lower C, as the tt value is close to zero, the adsorbed protein residues may be considered as a two-dimensional ideal gas. Proteins at higher C, but lower than that of the plateau, form a monolayer of irreversibly adsorbed molecules. As the plateau is attained, the monolayer is saturated by protein that is irreversibly adsorbed. The protein concentration at which the plateau is attained is the adsorption efficiency (AE). At higher C, the protein molecules may form multilayers beneath the primary monolayer, but these structures do not contribute significantly to the surface pressure. The maximum tt at the plateau is the superficial activity (SA). 

The surface pressures for small crystals are in general much larger than the equivalent pressures for liquid droplets because the surface tensions of crystalline solids are usually much greater than those of liquids. As an example, consider an NaCl nanocrystal with r, = 5 nm for the (100) face for which tr,- = 413 dyne cm (Tasker, 1979), The surface pressure A/ = 2(415)/(5)(10) dynes/cm or 1660 bars. Thus the internal pressures of nanometer-size solid particles in equilibrium with atmospheric pressure vapors may reach thousands of bars. However, small crystals formed under dynamic gas-phase conditions in which their shape Is controlled by heat and mass transfer may not have time to attain the equilibrium shape, so the use of (9.31) and (9.32) may not be quantitatively correct in practical applications. 

The efficiency of a surfactant in reducing surface tension can be measured by the same quantity that is used to measure the efficiency of adsorption at the liquid-gas interface (Chapter 2, Section HIE), pC20, the negative log of the bulk phase concentration necessary to reduce the surface tension by 20 dyn/cm (mN m-1). The effectiveness of a surfactant in reducing surface tension can be measured by the amount of reduction, or surface pressure, IIcmc, (= To Ycmc) attained at the critical micelle concentration, since reduction of the tension beyond the CMC is relatively insignificant (Figure 5-3). 

Surface pressure, measured by an apparatus already described (8, 9, 10), was 0.05 dyne/cm the surface area was 0.02 m2/mg. All isotherms were constructed by points after each area reached a constant surface pressure to guarantee that surface equilibrium was attained. All isotherms were obtained at various surface concentrations to ensure repro-... 

The growth of the bubble induces a micro-convective flow on the electrolyte that pushes each bubble from an ideal center of the surface ( active site ) in various radial directions (Figure 14.5). When each bubble attains a certain size, the buoyancy exceeds its adhesion and the bubble leaves the surface producing a drag flow. However, in the electrochemical cell, the radial and the azimuthal directions produces the opposite drag flow to the platinum surface displaying the so-called surface pressure effect. 

In another HFCVD experiment, the influence of gas pressure on diamond film coverage and crystal size was studied. As shown in Fig. 6, the crystal size and surface coverage attain a maximum at 1.3 kPa ( 10 torr). Crystal quality and phase purity are both optimized around a pressure value of 4 kPa (30 torr). At 665 Pa ( 5 torr) an amorphous film covers the substrate and ball-like particles form. 

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