Biphenyl lyophilization

Table II. Retention of Biphenyl, Phenanthrene, and Pyrene in Aqueous Solutions during Vacuum Distillation and Lyophilization...

To explore the potential for loss of moderately volatile compounds during vacuum distillation, solutions of biphenyl (4 /zg/mL), phenanthrene (4 /zg/mL), and pyrene (0.14 /zg/mL) were freeze-dried or vacuum-distilled for varying times, and the amount of compound remaining in the flask was determined. The results (Table II) show that vacuum distillation resulted in greater losses than lyophilization and that the ease of loss varied considerably. Biphenyl was so readily lost that the mere degassing of its solution resulted in a loss of about two-thirds of the original compound (see Figure 2). 

These few experiments suggested that an intuitive concept of relative volatility is inadequate to describe the behavior of dilute hydrophobic solutes under vacuum in aqueous solution. For example, the classification used in Table I, where compounds having boiling points over 300 °C at 1 atm are described as poorly volatile, inadequately predicts the losses of phenanthrene (bp 340 °C) in Table II. Also, relative volatility offers no prediction of the rapid loss of biphenyl during degassing (Tables II and III, Figure 2) and the relatively slow further loss of this compound during subsequent lyophilization (Tables II and III). 

Table III. Loss of Biphenyl, Quinoline, and Quinaldic Acid from 100 mL of Aqueous Solution during Lyophilization...

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