Solubility curves, inverted

Calcium sulfate, which exists in sea water in ionic form, has a reverse or inverted solubility curve above about 37° C.—that is, solubility decreases with increasing temperature. 

Scale Prevention. The scale normally formed on heat transfer surfaces of sea water evaporators consists of calcium carbonate, magnesium hydroxide, and/or calcium sulfate. The first two form as a result of the breakdown of bicarbonate in sea water, which is initially saturated with calcium carbonate. Calcium sulfate scale forms purely as a result of its inverted solubility curve. Sea water is not saturated with calcium sulfate and an economically reasonable amount of fresh water can be recovered from sea water without exceeding saturation with calcium sulfate. However, at the start of this investigation, the solubility of calcium sulfate in sea water was not accurately enough known to tell whether 30, 50, or 80% of the water content could be removed at various temperatures without encountering calcium sulfate scale. 

Not all solubility curves are smooth, as can be seen in Figure 3.1b. A discontinuity in the solubility curve denotes a phase change. For example, the solid phase deposited from an aqueous solution of sodium sulphate below 32.4 °C will consist of the decahydrate, whereas the solid deposited above this temperature will consist of the anhydrous salt. The solubility of anhydrous sodium sulphate decreases with an increase in temperature. This negative solubility effect, or inverted solubility as it is sometimes called, is also exhibited by substances such as calcium sulphate (gypsum), calcium, barium and strontium acetates, calcium hydroxide, etc. These substances can cause trouble in certain types of crystallizer by causing a deposition of scale on heat-transfer surfaces. 

The general trend of a solubility curve can be predicted from Le Chatelier s Principle which, for the present purpose, can be stated when a system in equilibrium is subjected to a change in temperature or pressure, the system will adjust itself to a new equilibrium state in order to relieve the effect of the change. Most solutes dissolve in their near-saturated solutions with an absorption of heat (endothermic heat of solution) and an increase in temperature results in an increase in the solubility. An inverted solubility effect occurs when the solute dissolves in its near-saturated solution with an evolution of heat (exothermic heat of solution). 

The oral bioavailability of hypericum may be altered and improved by a combination of its constituents. A hypericum extract containing naphthodianthrones is inactive in a water suspension, but very effective when another constituent, procyanidin, is present. Procyanidin had the effect of increasing the water solubility of naphthodianthrones, and thus increasing their pharmacokinetic availability (Butterweck et ai. 1997). Further, the facilitative effect of procyanidin exhibited an inverted U curve. 

For lower consumed C02 the quantity of extracted lupanine decreases with temperature but this situation inverts for the lower temperatures with the increase of consumed C02. Thus, a retrograde solubility effect between the curves at 298 K and 313 K seems to exist. 

Thus, in most cases a fit could be obtained with the inverted phases with caroon dioxide solubility in a narrower range than the range required of the mixing parameter if the normal phases were used and the mixing parameter was varied. On the other hand, the use of the inverted j ases with carbon dioxide solubility as the only parameter available does not make use of the other features of the Multiflood simulator such as the trapped oil curve and the simulation of the effects of fingering in the way shown successfully before. 

Values of 0 and Xj/i, in Table 8.1, show that for systems 1 to 4 the entropy parameter is positive, as expected, but for poly(acrylic acid) in dioxan and polymethacrylonitrile in butanone, f is negative at the theta temperature. As /i = when T= 0, the enthalpy is also negative for these systems. This means that systems 5 and 6 exhibit an unusual decrease in solubility as temperature rises, and the cloud-point curve is now inverted as in area B. The corresponding critical temperature is located at the minimum of the miscibility curve and is known as the lower critical solution temperature (LCST). 

Figure 5.3. Phase diagram for several elastic-contractile model proteins, showing an inverted curvature to the binodal or coexistence line (when compared with petroleum-based polymers) that is equivalent to the T,-divide, with the value of T, determined as noted in Figure 5.IB. Solubility is also inverted with insolubility above and solubility below the binodal line, that is, solubility is lost on raising the temperature whereas solubility is achieved by raising the temperature of most petroleum-based polymers in their solvents. Note that addition of a CHj group lowers the T,-divide and removal of the CH2 group raises the T,-divide. For these and the additional reason of increased ordering on increasing the temperature, the phase transitions of elastic-contractile model proteins are called inverse temperature transitions. (The curve for poly[GVGVP] is adapted with permission from Manno et al. and Sciortino et al. ).

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