Maximum additive concentration

Table 6.4 summarizes the concentrations of a range of endogenous (i.e., nondietary) simple phenols, including a-tocopherol, and ascorbate in plasma from healthy individuals. The total simple phenol and ascorbate concentration is between 159 and 380 pM. The maximum additional concentration that is likely to be achieved from dietary sources, 3 to 22 pM, is marginal by comparison adding only between 0.3 and 5% if it is assumed, quite reasonably, that the typical mean intake is taken over three equal meals. Many people consume a much smaller quantity of dietary PPT and even those consuming double the... 

The Effect of Temperature on the Maximum Additive Concentrations of Griseofulvin... 

The maximum additive concentration (MAC) is defined as the maximum amount of solubilisate, at a given concentration of surfactant, that produces a clear solution. Different amounts of solubilisates, in ascending order, are added to a series of vials containing the known concentration of surfactant and mixed until equilibrium is reached. The maximum concentration of solubilisate that forms a clear solution is then determined visually. This same procedure can be repeated for the different concentrations of surfactant in a known amount of solubilisate in order to determine the optimum concentration of surfactant (Figure 4.24). Based on this information, one can construct a ternary phase diagram that describes the effects of three constituents (i.e., solubilisate, surfactant, and water) on the micelle system. Note that unwanted phase transitions can be avoided by ignoring the formulation compositions near the boundary. In general, the MAC increases with an increase in temperature. This may be due to the combination of the increase of solubilisate solubility in the aqueous phase and the micellar phase rather than an increased solubilization by the micelles alone. 

The concentration of solubilisate that can be incorporated into a given system with the maintenance of a single isotropic solution (saturation concentration or maximum additive concentration, MAC) is obtained using the same procedures for measurement of solubility of any compound in a given solvent [30]. Since solubilisation is temperature sensitive, adequate temperature control is essential. 

In solubilization studies (15), different definitions and concentration units are used. As several quantities could change through a solubilization experiment, the initial, final and reference states have to be defined precisely. Usually, mainly one point of the solubilization isotherm is studied, i.e. the maximum additive concentration (MAC). This concentration is obtained when the solution is in equilibrium with the pure solute. Another easily accessible region is the diluted region of the isotherm where the number of moles of solute per mole of surfactant is low (of the order of 1% maximum). 

Furthermore, the means of obtaining the published maximum additive concentration, over and above variations from experimental techniques, can lead to misinterpretation of apparently comparable data. Results obtained for a given surfactant concentration need not necessarily correspond to those at a second surfactant concentration, as can be shown by the change in slope of a differential solubility curve with increasing concentration of surfactant (see Fig. 5.1). 

Figure 5.1 The variation in the maximum additive concentration of solubilizate (ethylbenzene) with concentration of surfactant (potassium dodecanoate, KC12). From Heller and Klevens [25] with permission.

In discussing solubilization in aqueous systems one has, in the simplest systems, two variables surfactant and solubilizate. How these affect each other and the maximum additive concentration is of paramount importance. 

Table 5.9 Maximum additive concentrations of long-chain alkyl compounds in 1 % aqueous solutions of CioH2iO(CH2CH20),oCHj at 27°C (from Nakagawa and Tori [152])...

Table 5.10 Maximum additive concentrations of a range of solubilizates in O.In solutions of dodecylamine hydrochloride (C12HCI), sodium oleate ...

Table 5.11 Maximum additive concentrations of polycyclic hydrocarbons in 0.5 m potassium laurate at 25° C (from Klevens [16])...

Table 5.12 Maximum additive concentrations in sodium lauryl sulphate for polycyclic aromatic hydrocarbons and steroid hormones [154]...

Table 5.14 The effect of temperature on the maximum additive concentration of DMAB in a range of surfactants. From Kolthoff and Stricks [137].

Table 5.15 The effect of temperature on the maximum additive concentrations of griseofulvin, hexoestrol and glutethimide in bile salts. From Bates et al [22].

McBain and Green [175] showed that the solubility of Orange OT in potassium laurate and pc assium oleate increased with the addition of benzene, toluene, and hexane at concentrations equivalent to 75 % of their respective maximum additive concentrations for the particular surfactant-solvent systems under examination. They further showed by comparison of the solubilities of Orange OT in the surfactant + additive solutions and in the respective additives that the calculated solubilities did not agree with the solubilities measured. From these results they concluded that the solubilizing properties of the mixed micelles were not an additive function of the components. 

The synergistic effect on the solubilization of n-heptane and n-alkanols has been shown to increase with the alcohol chain length over the range octanol to dodecanol [179] and changing the polar group on an octyl radical showed the synergistic powers in the order octyl mercaptan > octylamine > octanol [180]. In both cases this is in opposite order to the maximum additive concentrations of the additives in the surfactant-solvent systems. 

If the micelles are regarded as a phase, then adding an excess solubilizate phase means there are three phases (the third is the intermicellar bulk phase). The total number of components is three (solvent, surfactant, and solubilizate), so the presence of three phases makes the system divariant. That would mean that surfactant concentration would be constant at constant temperature and pressure— but, in fact, the maximum additive concentration (MAC) changes with total surfactant concentration. Even if it were postulated that the increase in the MAC with surfactant concentration above the CMC is due to an increase in the total micellar phase, the concentration of solubilizate in the micellar phase should still remain constant, because the concentration is an intensive property of the system and is therefore homogeneous throughout the micellar phase. 

A brief comment is in order on the maximum additive concentration. The above discussion pertains when the solubilizate concentration is less than its solubility. When solubility is exceeded, an excess solubilizate phase appears in the system, which then has two phases and is trivariant. In this case, the system is fixed by specifying the total surfactant concentration at constant temperature and pressure. This prediction from the phase rule is supported by the observed changes of the phenothiazine MAC with the concentration of sodium dodecyl sulfate (Fig. 9.1). ... 

TABLE 6.2. Maximum Additive Concentrations (MACs) Solubilized in 1% Aqueous Solutious of CioH2i(OE)ioCH3 Nonionic Snrfactant at ITC... 

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