Surfaces vapor pressure

Small-pore surface, vapor pressure of, 23 68-69 Smallpox, 3 136... 

Thermodynamics of Monolayer Solutions of Lecithin and Cholesterol Mixtures by the Surface Vapor Pressure Method... 

Thermodynamic parameters for the mixing of dimyristoyl lecithin (DML) and dioleoyl lecithin (DOL) with cholesterol (CHOL) in monolayers at the air-water interface were obtained by using equilibrium surface vapor pressures irv, a method first proposed by Adam and Jessop. Typically, irv was measured where the condensed film is in equilibrium with surface vapor (V < 0.1 0.001 dyne/cm) at 24.5°C this exceeded the transition temperature of gel liquid crystal for both DOL and DML. Surface solutions of DOL-CHOL and DML-CHOL are completely miscible over the entire range of mole fractions at these low surface pressures, but positive deviations from ideal solution behavior were observed. Activity coefficients of the components in the condensed surface solutions were greater than 1. The results indicate that at some elevated surface pressure, phase separation may occur. In studies of equilibrium spreading pressures with saturated aqueous solutions of DML, DOL, and CHOL only the phospholipid is present in the surface film. Thus at intermediate surface pressures, under equilibrium conditions (40 > tt > 0.1 dyne/cm), surface phase separation must occur. 

To study the interactions in mixed films of lecithin and cholesterol under conditions of homogeneity and equilibrium, the surface vapor pressure method, first explored by Adam and Jessop (4), was used. This method uses spread films in the transition region of the isotherms, where... 

Chemical Activities by the Surface Vapor Pressure Method. Surface pressure measurements in the transition region between the condensed and gaseous monolayer states of a single lipid component spread as a monolayer on water yield a value of ir which is independent of the surface area. This value—the surface vapor pressure, irv—is analogous to the vapor pressure of a liquid in equilibrium with its vapor. When a second lipid component is in the surface, the limits of miscibility in the condensed phase may be determined on the basis of the surface vapor pressure dependence on the mole fraction in the condensed phase (8). 

The dependence of the ttv values on the composition of the vapor and condensed states for DML-CHOL, DOL-CHOL, and DOL-DML mixtures is shown in Figure 6. The upper curve is the surface vapor pressure as a function of the mole fraction of the liquid-expanded film the lower curve is for the dependence of irv on the composition of the gaseous phase. Ideal mixing behavior is given by the linear dotted line which joins the 7ry° points for each of the pure compounds. In all cases there was complete miscibility of the components as represented by the continuous function of 7rv with x. In the cholesterol mixtures positive deviations from Raoult s law are observed for the mixture of lecithins, ideal mixing is observed. These results confirm those obtained with lipid mixtures—i.e., cholesterol mixed with liquid-expanded lipid films forms rion-ideal mixtures with positive deviations for mixtures of lipids which are in the same monolayer state, as in the case of the liquid-expanded DOL-DML mixtures, ideal mixing results (8). 

The distribution for the volume reaction case has significantly more large particles than any other. Thus it should be fairly easy to distinguish experimentally between a volume reaction and the other mechanisms. On the other hand, it would probably be difficult to distinguish between the surface reaction case and diffusion growth with zero surface vapor pressure. It would probably be impossible to distinguish between diffusion growth with a vapor pressure and a surface reaction. 

APis the bulb vapor pressure minus the surface vapor pressure his the depth of immersion of the vapor pressure bulb... 

The difference between the pressure measured in the vapor-pressure bulb and the controlled surface vapor pressure is AP and 0.030 in. Hg corresponds to a temperature difference of one millidegree. The immersion depth of the vapor-pressure bulb in liquid helium is /i. At corresponding depths in the liquid helium, there is little difference in the pressures indicated by the vacuum-jacketed and non- jacketed vapor-pressure thermometers. These results indicate that the effect of a cold spot on the vapor pressure thermometer at 4.2 K is negligible. Just above the point, however, effects of cold spots have been observed. This type of bulb eliminates the effect. 

Whether one is referring to a 2D or 3D case is not necessarily the same as the pore geometry and is embedded in whatever theory is being used. This could be a confusing point and herein it will be clearly stated as whether a 2D or 3D interface is being referred to. There could be intermediate cases between strictly a cyhndrical interface and a spherical interface and there could be, in principle at least, cases where m is > 2. Obviously for flat surfaces m=0 and P=Ps- Therefore P/ wiU always be used for the flat surface vapor pressure. (The notation P has been used occasionally in the literature for the vapor pressure over a pure liquid with possibly a curved interface. Therefore it will be avoided here.)... 

It is desirable for a VPI to provide inhibition rapidly and to have a lasting effect. Therefore, the compound should have a high volatility to saturate all of the accessible vapor space as quickly as possible, but at the same time it should not be too volatile, because it would be lost rapidly through any leaks in the package or container in which it is used. The optimum vapor pressure of VPI then would be just sufficient to maintain an inhibiting concentration on all exposed metal surfaces. Vapor pressures and other properties of some VPIs are given in Table 5.3. 

C. The Effect of Curvature on Vapor Pressure and Surface Tension... 

Here, r is positive and there is thus an increased vapor pressure. In the case of water, P/ is about 1.001 if r is 10" cm, 1.011 if r is 10" cm, and 1.114 if r is 10 cm or 100 A. The effect has been verified experimentally for several liquids [20], down to radii of the order of 0.1 m, and indirect measurements have verified the Kelvin equation for R values down to about 30 A [19]. The phenomenon provides a ready explanation for the ability of vapors to supersaturate. The formation of a new liquid phase begins with small clusters that may grow or aggregate into droplets. In the absence of dust or other foreign surfaces, there will be an activation energy for the formation of these small clusters corresponding to the increased free energy due to the curvature of the surface (see Section IX-2). 

Adsorption may occur from the vapor phase rather than from the solution phase. Thus Fig. Ill-16 shows the surface tension lowering when water was exposed for various hydrocarbon vapors is the saturation pressure, that is, the vapor pressure of the pure liquid hydrocarbon. The activity of the hydrocarbon is given by its vapor pressure, and the Gibbs equation takes the form... 

Water Vapor Pressure (atm) Surface Tension (dyn/cm) Water Vapor Pressure (atm) Surface Tension (dyn/cm)... 

An interesting consequence of covering a surface with a film is that the rate of evaporation of the substrate is reduced. Most of these studies have been carried out with films spread on aqueous substrates in such cases the activity of the water is practically unaffected because of the low solubility of the film material, and it is only the rate of evaporation and not the equilibrium vapor pressure that is affected. Barnes [273] has reviewed the general subject. 

Some mention should be made of perhaps the major topic of conversation among surface and colloid chemists during the period 1966-1973. Some initial observations were made by Shereshefsky and co-workers on the vapor pressure of water in small capillaries (anomalously low) [119] but especially by Fedyakin in 1962, followed closely by a series of papers by I>eijaguin and co-workers (see Ref. 120 for a detailed bibliography up to 1970-1971). 

Not all molecules striking a surface necessarily condense, and Z in Eq. VII-2 gives an upper limit to the rate of condensation and hence to the rate of evaporation. Alternatively, actual measurement of the evaporation rate gives, through Eq. VII-2, an effective vapor pressure Pe that may be less than the actual vapor pressure P. The ratio Pe/P is called the vaporization coefficient a. As a perhaps extreme example, a is only 8.3 X 10" for (111) surfaces of arsenic [11]. 

The Kelvin equation (Eq. HI-18), which gives the increase in vapor pressure for a curved surface and hence of small liquid drops, should also apply to crystals. Thus... 

A heat of immersion may refer to the immersion of a clean solid surface, qs.imm. or to the immersion of a solid having an adsorbed film on the surface. If the immersion of this last is into liquid adsorbate, we then report qsv.imm if tbe adsorbed film is in equilibrium with the saturated vapor pressure of the adsorbate (i.e., the vapor pressure of the liquid adsorbate P ), we will write It follows from these definitions... 

It is important to keep in mind that the phases are mutually in equilibrium. In particular, the designation is a reminder that the solid surface must be in equilibrium with the saturated vapor pressure and that there must therefore be an adsorbed film of film pressure (see Section X-3B). Thus... 

The microscopic contour of a meniscus or a drop is a matter that presents some mathematical problems even with the simplifying assumption of a uniform, rigid solid. Since bulk liquid is present, the system must be in equilibrium with the local vapor pressure so that an equilibrium adsorbed film must also be present. The likely picture for the case of a nonwetting drop on a flat surface is... 

The case of a vapor adsorbing on its own liquid surface should certainly correspond to mobile adsorption. Here, 6 is unity and P = the vapor pressure. The energy of adsorption is now that of condensation Qu, and it will be convenient to define the Langmuir constant for this case as thus, from Eq. xvn-39. 

The first term on the right is the common inverse cube law, the second is taken to be the empirically more important form for moderate film thickness (and also conforms to the polarization model, Section XVII-7C), and the last term allows for structural perturbation in the adsorbed film relative to bulk liquid adsorbate. In effect, the vapor pressure of a thin multilayer film is taken to be P and to relax toward P as the film thickens. The equation has been useful in relating adsorption isotherms to contact angle behavior (see Section X-7). Roy and Halsey [73] have used a similar equation earlier, Halsey [74] allowed for surface heterogeneity by assuming a distribution of Uq values in Eq. XVII-79. Dubinin s equation (Eq. XVII-75) has been mentioned another variant has been used by Bonnetain and co-workers [7S]. 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Extensive tables and equations are given in ref. 1 for viscosity, surface tension, thermal conductivity, molar density, vapor pressure, and second virial coefficient as functions of temperature. 

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