Critical solution temperature

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The lower and upper critical solution temperature (Engl. Lower and upper critical solution temperature , abbreviated LCST and UCST ) describes the temperature range in which several substances completely miscible are. Below the LCST and above the UCST, two or more substances are completely miscible in all concentration ratios and thus form a single phase . Consequently, there exists above the LCST and below the UCST a miscibility gapfor certain compositions. These miscibility gaps can be represented by temperature-composition diagrams (Figure 1). Partial miscibility with LCST or UCST is observed particularly frequently when polymers are part of the mixtures.

Fig. 1: Examples of phase diagrams with LCST and UCST. left: LCST> UCST; right: LCST <UCST.

Representation in the phase diagram

Figure 2 shows an idealized phase diagram of a mixture of components A and B with a UCST miscibility gap. Phase separation processes can be illustrated using the phase diagram. In the example shown, components A and B form a homogeneous phase at the mixing ratio c1a and the temperature T 1 . The single-phase range is separated from the two-phase range by the binodals . If the temperature is lowered to T 2 , phase separation sets in and a second phase of composition c1b is about to arise. With a further cooling to T 3, the phase separation continues. One phase of the composition c2a coexists with a second phase of the composition c2b. The two phases are in thermodynamic equilibrium with each other. The composition of the coexisting phases can be determined graphically by means of a horizontal auxiliary line, the conode .

Fig. 2: Idealized phase diagram of a binary mixture of components A and B. The UCST represents the maximum of the phase separation limit (binodal).

Experimental determination

Several methods can be used to determine whether there is a miscibility gap with LCST or UCST. Turbidimetry is widespread . Two different phases usually have different indices of refraction . This leads to light scattering as soon as phase separation occurs and manifests itself in a turbidity of the mixture. With the help of a turbidimeter , the turbidity can be measured as a function of the temperature. The direction of the phase separation (LCST or UCST behavior) and the cloud point are determined. The cloud point depends on the heating or cooling rate and the composition of the substance mixture and must not be confused with LCST or UCST. In strictly binary mixtures of substances, the UCST is at the maximum and the LCST at the minimum of the binodals, ie they can only be observed when the composition of the mixture is critical. Since the phase separation has to be investigated over a wide range of compositions for this, the determination is experimentally complex. That is why the phase separation is usually only investigated in areas of composition that are of interest to the experimenter.

The phase separation is accompanied by a change in enthalpy , which is positive for LCST transitions and negative for UCST transitions. These enthalpy changes can be determined by dynamic differential calorimetry .

Examples

Mixtures of low molecular weight liquids

Mixtures of n- hexane / aniline show a UCST of 60 ° C. The critical mole fraction, as expected from the Flory-Huggins theory , is around 0.5 and the phase diagram is highly symmetrical. Phenol / water mixtures have a UCST of 66 ° C. Examples of LCST behavior are the mixtures of water / triethylamine or water / N -isopropylpropionamide with LCSTs of 18 ° C or 28 ° C. N -isopropylpropionamide is the repeat unit of the polymer poly ( N -isopropyl acrylamide), which also has a similar LCST. Both LCST and UCST show the nicotine / water mixture.

Polymer solutions

Due to the low entropy of mixing , miscibility gaps occur relatively frequently in polymer solutions. In the case of polymer solutions, the miscibility gaps are often very wide and the course of the binodals is flat, so that a drastic phase separation can occur even with small temperature changes. That is why polymers with LCST or UCST in a solvent belong to the class of thermoresponsive polymers . Both UCST and LCST are very common in organic solvents. Examples in aqueous solution are also known.

Polymer blends

The combinatorial entropy of mixing for polymer blends is even lower than that of polymer solutions. That is why most polymer blends are immiscible. Partial miscibility with LCST is observed, for example, with polymethyl methacrylate / polycarbonate mixtures. UCST behavior is known for polystyrene / poly (4-methylstyrene) mixtures.

Fixed solutions

Solid solutions such as alloys or metal / metal hydride mixtures can have miscibility gaps. The palladium / palladium hydride system, for example, has a UCST at 300 ° C.

further reading

  • R. Koningsveld, WH Stockmayer, E. Nies, Polymer Phase Diagrams , Oxford University Press, Oxford 2001 .

Individual evidence

  1. ↑ Entry on upper critical solution temperature . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.UT07280 .
  2. ^ YC Bae, SM Lambert, DS Soane, JM Prausnitz, Cloud-Point Curves of Polymer Solutions from Thermooptical Measurements , Macromolecules, 1991, Volume 24, pp. 4403-4407.
  3. ↑ Donald B. Keyes, Joel H. Hildebrand, A Study of the System Aniline-Hexane. , Journal of the American Chemical Society, 1917, Volume 39, pp. 2126-2137.
  4. ↑ PJ Sinko, Martin's Physical Pharmacy and Pharmaceutical Sciences , 5th ed., Lippincott Williams & Wilkins, 2005, p 51st
  5. ↑ K. Stephan et al., Thermodynamics: Fundamentals and technical applications , Volume II, Springer Verlag, ISBN 3-540-64481-4 , p. 94. Restricted preview in the Google book search.
  6. ↑ B. Geukens, F. Meersman, Erik Nies, Phase Behavior of N- (Isopropyl) propionamide in Aqueous Solution and Changes in Hydration Observed by FTIR Spectroscopy , Journal of Physical Chemistry B, 2008, Volume 112, pp. 4474-4477.
  7. ↑ a b Peter W. Atkins, Physikalische Chemie , Third, corrected edition, Wiley-VCH, Weinheim, 2001.
  8. ↑ Ronald Koningsveld, Walter H. Stockmayer, Erik Nies, Polymer Phase Diagrams , Oxford University Press, Oxford, 2001.
  9. ↑ Christian Wohlfarth, Upper Critical (UCST) and Lower Critical (LCST) Solution Temperatures of Binary Polymer Solutions , Polymer Handbook, 87th ed., CRC press, 2006, chapter 13, pp. 19-34, ISBN 978-0849304873 .
  10. ↑ Vladimir Aseyev, Heikki Tenhu, Francoise M. Winnik, Non-ionic Thermoresponsive Polymers in Water , Advances Polymer Science, 2010, Volume 242, pp. 29-89.
  11. ↑ Jan Seuring, Seema Agarwal, Polymers with Upper Critical Solution Temperature in Aqueous Solution , Macromolecular Rapid Communications, 2012, Volume 33, pp. 1898-1920.
  12. ↑ T. Kyu, JM Saldanha, Phase separation by spinodal decomposition in polycarbonate / poly (methyl methacrylate) blends , Macromolecules, 1988, Volume 21, pp. 1021-1026.
  13. ↑ LL Chang, EM Woo, Morphology, Phase Diagrams, and UCST Behavior in Blends of Polystyrene with Poly (4-methylstyrene) , Macromolecular Chemistry and Physics, 2001, Volume 202, pp. 636-644.