Monotectic mixtures

Monotectic mixtures arise when the individual components have similar melting points, molecular volumes and polymorphic forms. Figure 17.12(a) represents a possible phase diagram for monotectic mixtures. A typical monotectic solution occurs when SSS is mixed with SOS. Lutton (1955) determined that the a form was present and associated with limited solid solution formation, and contrasted with the a forms of other glyceride mixtures that formed continuous solid solutions (Rossell 1967). It was found, for this system, that tristearin incorporates about 50% of the SOS into a solid solution on the other hand, SOS incorporates very little SSS into a solid solution. 

PEGs and PEOs are semicrystalline, therefore having a tendency to form eutectic or monotectic mixtures with crystalline drugs. Poloxamers (Lutrol ) are also known to be excellent solubilizers, as well as plasticizers when used in mixtures with other polymers. 

Recent high-temperature DTA experiments yielded the expected region of two liquids in the metal-rich portion above 1950 °C28),but neither the exact composition of the monotectic (liquid 1) nor that of the coexisting liquid 2 is known. The DTA results of a successful experiment with a mixture of Mo and M02.06S3, of 18 at.% S composition, definitely showed the eutectic temperature to be 1620 °C, the monotectic temperature to be 1950 °C, and a third effect at 2050 °C, which may indicate a homogeneous liquid above that temperature. 

Table 17.4. Phase behavior of binary mixtures of triacylglycerides adapted from Rossell (1967). (M) represents monotectic phase behavior, (E) represents euteetie phase behavior and (P) represents peritectic phase behavior.

A molecular compound of p form, pf., was formed at the 1 1 concentration ratio of the binary mixtures of PPO-POP, and SSO-SOS, giving rise to two monotectic... 

Binary organic mixtures with PAHs can form monotectic systems. Table 2 lists the monotectic and eutectic point of a few monotectic forming PAH systems. Monotectic systems are characterized by monotectic, eutectic and upper consolute temperatures, though the upper consolute temperature is often not reported. The monotectic temperature, tM, is the temperature at monotectic composition and the upper consolute temperature is the highest melting temperature of the mixture system, i.e. the critical point where the two liquid phases having identical composition become indistinguishable. 

Rai and Pandey studied the phase behavior of succinonitrile (1) + pyrene (2) mixture system (Rai and Pandey, 2002), which is a typical monotectic system (Figure 5). The enthalpy of fusion of pyrene, 17.65 kj mole-i (Chickos and Acree, 1999), is much higher than that of succinonitrile, 3.7 kJ mole-i (Rai and Pandey, 2002). The monotectic point is 416.5 K (143.3°C) at Xi=0.025. The eutectic temperature is 328.5 K (55.4°C) at i i=0.744 and the upper consolute temperature, tc (465.2 K, 192.0°C), is 48.7 K above the monotectic point. When is between monotectic and eutectic composition, the two liquids, Li (rich in pyrene) and L2 (rich in succinonitrile) are mutually immiscible. However, if the temperature is above the consolute temperature, there is complete miscibility in liquid state, i.e. only one liquid phase exists. 

The phase behaviors of binary PAH-containing mixtures are comphcated. Most of these mixture systems are eutectic systems, which have a behavior like the anthracene + pyrene system. Fewer binary PAH-containing mixtures can form monotectic and solid solution systems, such as succinonitrile + pyrene system and phenanthrene + anthracene... 

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