Lower critical solution temperature

The lower and upper critical solution temperature ( engl. lower and upper critical solution temperature, LCST and UCST abbreviated ) describes the temperature range in which several substances are fully miscible. Below the LCST and above the UCST are two or more substances completely miscible in all proportions concentration and thus form a single phase. Consequently there is above the LCST and UCST below a miscibility gap for certain compositions. This mixture may be represented by gaps temperature-composition diagram ( Figure 1). Partial Miscibility with LCST or UCST is very often observed when polymers are part of the mixtures.

Representation in the phase diagram

Figure 2 shows an idealized phase diagram of a mixture of components A and B with UCST miscibility gap. Phase separation processes can be illustrated by the phase diagram. In the example shown, the components A and B in the mixing ratio and the temperature T1 c1a form a homogeneous phase. The single phase is separated from the two-phase region by the binodal. The temperature is lowered to T2, is a phase separation, and a second phase of the composition is about the emergence C1b. In a further cooling to T3, the phase separation proceeds. A phase of the composition c2a c2b coexists with a second phase of the composition. The two phases are in thermodynamic equilibrium with each other. The composition of the coexisting phases can be graphically by means of a horizontal auxiliary line, the tie line, is determined.

Experimental determination

Whether a miscibility gap with LCST or UCST exists, can be determined by several methods. Widely used is the turbidimetry. Two distinct phases as a rule have different refractive indices. This leads to light scattering, when separation occurs, and manifests itself in a turbidity of the mixture. Using a turbidimeter the turbidity as a function of temperature can be measured. Thus, the direction of the phase separation ( LCST or UCST behavior ) and the cloud point ( engl. cloud point ) was observed. The cloud point depends on the heating and cooling rate and the composition of the mixture, and should not be confused with the LCST or UCST. In strictly binary mixtures is the UCST and the LCST at the maximum of the minimum of the binodal, ie, they can only be at a critical composition of the mixture observed. Because of this, the phase separation must be examined over a wide composition range, the determination is experimentally consuming. Therefore, the phase separation is generally examined only in composition ranges which are of interest for the experimenter.

The phase separation is accompanied by a change in the enthalpy is positive and negative in UCST transitions at LCST transitions. This enthalpy can be determined by differential scanning calorimetry.

Examples

Mixtures of low molecular weight liquids

Mixtures of n-Hexan/Anilin show a UCST of 60 ° C. The critical mole fraction is, as expected from the Flory -Huggins theory, at about 0.5, and the phase diagram is highly symmetrical. Phenol / water mixtures exhibit a UCST at 66 ° C. Examples of LCST behavior are the mixtures of water / triethylamine or water / N- isopropylpropionamid with LCSTs of 18 ° C and 28 ° C. N represents the Isopropylpropionamid Repitiereinheit the polymer is poly ( N-isopropylacrylamide), which also has a similar LCST. Both LCST and UCST shows the mixture of nicotine / water.

Polymer solutions

Due to the low entropy of mixing occur miscibility gaps in polymer solutions at relatively frequent. In the case of polymer solutions, the miscibility gaps are often very wide and the course of the binodal is flat, so that even small temperature changes can employ when a drastic phase separation. Therefore, it is one of polymers having LCST or UCST in a solvent to the class of thermoresponsive polymers. In organic solvents, both UCST and LCST are very frequently observed. Examples in aqueous solution are also known.

Polymer blends

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

Solid solutions

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