Colligative effects

The first three colligative effects follow rather straightforwardly from (7.65), as described in Sidebar 7.12. The remainder of this section will be devoted to describing osmotic pressure and related dialysis phenomena. 

Saturated salt solutions and sulfuric acid solutions establish relative humidity by reducing the vapor pressure above an aqueous solution (a colligative effect). Saturated salt solutions at controlled temperature maintain a constant relative humidity as long as excess salt and bulk solution are present. As water is added or removed from the solution, moisture from the head-space will either condense or evaporate (as appropriate), with subsequent dissolution or precipitation of salt to maintain the equilibrium vapor pressure. Because the degree of vapor pressure depression is dependent on the number of species in solution and, further, since the solubility of most salts is somewhat dependent on temperature, the relative humidity generated is also temperature dependent. Hence, use of the same salt at different temperatures can result in different relative humidities. Refs. can be consulted for specific saturated salt solutions that result in defined relative humidities as a function of... 

We note that the linear term is now proportional to the total number of polymer solute molecules, xB/m = NB/N, which for polymers is typically very low. Thus, the colligative effect from polymeric solutions is typically very small. Furthermore, if x < i, then Eq. (22) and Eq. (19) are completely similar. However, if x > i, the AT vs xB plot would exhibit a downward curvature (Fig. 34). This could lead to a decrease of freezing point-lowering effect. 

An electrolyte in solution dissociates into two (in the case of NaCl) or three (in the case of CaCh) particles, and therefore the colligative effects of such solutions are multiplied by the number of dissociated ions formed per molecule. However, because of incomplete electrolyte dissociation and associations between the solute and solvent molecules, many solutions do not behave in the ideal case, and a 1-molal solution may give an osmotic pressure lower than theoretically expected. The osmotic activity coefficient is a factor used to correct for the deviation from the "ideal behavior of the system ... 

Enolates may form supramolecular species such as dimers, tetramers, or hexamers, and these species are often in equilibrium (Scheme 3.1). Enolates may also form mixed aggregates with added salts or with secondary or tertiary amines. The existence of such species has been proven in the solid state by X-ray crystallography, and colligative effects and NMR studies have confirmed their existence in solution (reviews [5,6,8,12-14] see also [15-18]). Interestingly, dimers are even found in crystals of tetrabutylammonium malonates and cyanoacetates [19], indicating that a metal is not necessary for supramolecular organization ... 

In samples with no free water, interfacial water freezes at about -40°C [61]. Lee and Lee [179] found a thermodynamic dependence between the lowering of the freezing point of bound water and the binding strength between water molecules and the surface of microstructures in sludges and the colligative effects of solutes in the aqueous phase ... 

The next section treats another colligative effect. Adding solutes to solvents can lead to osmotic pressures. 

Colligative properties are those properties of a solution of a non-volatile solute which in the limit of infinite dilution, depend only upon the number of solute species present in unit volume of the solution and not upon the nature or size of rtose species. Thus the colligative properties of polymer solutions enable Af to be measured for linear and branched homopolymers and copolymers with equal ease. The four important colligative effects are the osmotic pressure of the solution, the lowering of solvent vapour pressure, the elevation of solvent boiling point and the depression of... 

TABLE 3.4 The relative magnitudes of the colligative effects for polymers dissolved in benzene at a concentration of 10gdm The values are given in practical units and have been calculated assuming ideal solution behaviour... 

Phalen, R. F., Oldham, M. J., Mannix, R. C., and Schum, G. M., Cigarette Smoke Deposition in the Tracheobronchial Tree Evidence for Colligative Effect, AerosolSci Tech, 20, 215-226(1994). 

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