Solvent vapor pressure effects

Normally, Henry s constant for solute 2 in solvent 1 is determined experimentally at the solvent vapor pressure Pj. The effect of pressure on Henry s constant is given by... 

The effect of the bulk solution temperature lies primarily in its influence on the bubble content before collapse. With increasing temperature, in general, sonochemical reaction rates are slower. This reflects the dramatic influence which solvent vapor pressure has on the cavitation event the greater the solvent vapor pressure found within a bubble prior to collapse, the less effective the collapse. In fact, one can quantitate this relationship rather well (89). From simple hydrodynamic models of the cavitation process, Neppiras, for example, derives (26) the peak temperature generated during collapse of a gas-filled cavity as... 

Becaus-e of the similarity in the relations for osmotic pressure in dilute solutions and the equation for an ideal gas, van t Hoff proposed his bombardment theory in which osmotic pressure is considered in terms of collisions of solute molecules oil the semipeniieable membrane. This theoiy has a number of objections and has now been discarded. Other theories have also been put forward involving solvent bombardment on the semipermeable membrane, and vapor pressure effects. For example, osmotic pressure has been considered as the negative pressure which must be applied to the solvent to reduce its vapor pressure to that of the solution. It is, however, more profitable to interpret osmotic pressures using thermodynamic relations, such as the entropy of dilution,... 

A. Vacuum Line Filtration. For their work on the diammoniate of dibo-rane. Parry, Schultz, and Girardot24 devised a versatile vacuum line filtration apparatus which is useful when small quantities of solid are handled and when the solvent is sufficiently volatile to be distilled on the vacuum line. The filter is attached to the vacuum system through a standard taper joint which allows it to be rocked or inverted (Fig. 9.21). Prior to filtration, any volatile contents are frozen down and the apparatus is thoroughly evacuated (Fig. 9.21a). By inversion of the apparatus, the solution is then poured onto the frit, and the solvent vapor pressure is employed to effect a suction filtration by closing the stopcock in the equalizing arm and cooling the lower tube (Fig. 9.21b). The precipitate is washed by distillation of the solvent from the lower receiver into the upper portion of the apparatus (with the stopcock in the sidearm open) and repetition... 

One of the easy and effective approaches for quantifying the polymer volume fraction within films in situ is to use in situ spectroscopic ellipsometry (SE) [49,118, 119, 144], The measurements should be performed in a thermostated cell (Fig. 7) with full control over the solvent vapor atmosphere p/po, where po is the solvent vapor pressure at saturation and p is the actual pressure, which can be adjusted by a combination of the saturated vapor flow and dry nitrogen flow [118, 119], or by the difference between the temperature 7j of the polymer sample and the temperature 72 of the solvent vapor [49, 114, 144],... 

The costs of separating paraffin-olefin mixtures by extractive distillation are greatly affected by the solvent selectivity, the paraffin activity coefficient, and the solvent volatility they depend mostly upon solvent characteristics. For a solvent to be effective to separate key components A and B, the solvent must have a high value of y A/y B while it must also have a low y°V Further, the solvent vapor pressure should be lower, but not several orders of magnitude lower, than that of the less volatile hydrocarbon. 

A characteistic of an ionic solution is that any vapor pressure due to the dissolved electrolyte itself is effectively zero. The vapor pressure of the solvent in the solution therefore falls with increasing concentration of the electrolyte in the solution. Thus, the solvent vapor pressure in the solution will be less than the vapor pressure of the pure solvent because the nonvolatile ions block out part of the surface from which, in the pure solvent, solvent molecules would evaporate. 

The vapor pressure ratio measurements of separated isotopes are particularly useful because they directly measure the isotopic free energy ratios of the solvent. The solvent vapor pressure isotope effect between H2O and D2O is determined... 

If the liquid is pure solvent (.c = 0), this equation predicts that the partial pressure of the solvent vapor equals the vapor pressure of the solvent, as we would expect. Since the solute is nonvolatile, the solvent is the only component of the liquid solution that is also in the vapor. The pressure exerted by this vapor is referred to as the effective solvent vapor pressure ... 

Since x—and hence (1 - x)—is less than one, the effect of the solute is to lower the effective solvent vapor pressure. The vapor pressure lowering, defined as the difference between the vapor pressure of the pure component and the effective vapor pressure of the solvent, is... 

A solution of 5.000 g of a solute in 100.0 g of water is heated slowly at a constant pressure of 1.00 atm and is observed to boil at 100.421°C. Estimate the molecular weight of the solute, the effective solvent vapor pressure at 25°C and the solution freezing point at 1 atm. The necessary properties of water can be found in Table B.I. 

From Equation 6.5-2 the effective solvent vapor pressure at 25 C is determined from the vapor pressure of pure water at this temperature (found in Table B.3) as... 

Temperature is the first of the variables affecting selectivity. Increased temperature decreases retention time on the column, sharpens peaks, and produces a I chanp in selectivity. However, temperature is gener-I ally limited, by solvent vapor pressures, to an effective range of 20-60°C also important is the effect temper- ature has on the column packing. 

Solvent vapor pressure also has a significant effect on the cavitation phenomenon because the intensity of cavitation decreases as the vapor pressure of the solvent increases. This is because more vapor is enclosed in the microbubble, which cushions the collapse, leading to lower collapse temperatures and pressures. On the other hand, solvents with low vapor pressure tend not to diffuse into the growing microbubble thereby reducing the size of the bubble, which lessens the intensity of bubble collapse. Thus, a delicate balance of solvent properties must be achieved to attain the desired sonication conditions. 

Above the crossover pressure, the opposite eifect occurs. This behavior can be understood by considering two opposing effects of temperature on solubility (Chimowitz 2005). The vapor pressure of the solid solute always increases with temperature, while the density (or solvent power) of supercritical carbon dioxide decreases. Below the crossover pressure where the compressibility is larger, the density effect dominates, and the solubility decreases with increasing temperature. At pressures above the crossover pressure, the vapor-pressure effect dominates hence solubility increases with temperature. 

Vapor pressure at the flash point and the product of molecular weight and vapor pressure at the flash point were computed for 40 oxygenated solvents. Vapor pressure at the flash point varied between 7 and 58 mm Hg while the product of vapor pressure and molecular weight varied between 800 and 4200. Thus, some of the above methods of predicting flash points of mixed hydrocarbons appear unlikely to be effective for systems containing oxygenated solvents with their attendant non-ideal behavior. 

The region of pressure below the crossover pressure is known as the retrograde region. In this range of pressure, solubility decreases with an increase in temperature because the density of the SCF falls sharply. The decrease in density is sufficient to overcome any increases in solute vapor pressure that would normally lead to an increase in solubility. Above the crossover pressure, the decrease in solvent density is less sensitive to temperature and so solubility increases with temperature because the vapor pressure effect becomes dominant. 

Clearly, regime 1 is the simplest to analyze because it is kinetically controlled and therefore for all practical purposes can be treated as a homogeneous system. Even here, however, the solubility of A in phase 2 (the liquid), its partial pressure, and solvent vapor pressure can exert a significant influence. Thus, although the rate constant increases with increase in temperature, the solubility generally decreases, but the overall effect would still be positive because the activation energy is almost always higher than the heat of solution. On the other hand, the solubility can also increase with temperature (particularly for liquids), for example, chlorobenzene in aqueous sulfuric acid and esters such as ethyl p-nitrobenzoate and dichloroethyl oxalate in water. This serves only to supplement the effect of temperature on the rate constant. 

Now to determine the effect of solvent vapor pressure (or, equivalently, that of temperature), we write the Arrhenius equation for the two widely different temperatures Tj, and... 

Between 0 and 20 C, the lower the temperature, the faster the reaction. This result, consistent with increased cavitational effects due to a lower solvent vapor pressure, establishes that the reaction is not under mass transport control in this... 

Raoult s law permits the direct calculation of the effective molecular mass of a solute from data on the solvent vapor pressure of the solution, as shown by the following example. 

Regeneration is done by treatment of the bed of desiccant with heated gas — usually hot nitrogen. Adsorbed water is released from the desiccant internal surface because the attractive polar forces which bond water to the mineral are overpowered by the increased kinetic energy of the water molecules excited at the higher temperature. Said another way, the increase of temperature increases the solvent vapor pressure, making it more difficult for extra-molecular forces (polar bonding) to bind moisture to mineral. The gas is simply the agent which raises the temperature of the mineral, and has no additional effect on adsorption. 

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