Types of phase transition

There are different types of phase transitions. From the MCAT point of view, you should have a good grasp of phase transitions. Let s look at the basic terminology. 

The temperature at which a substance changes from solid state to its liquid state is called its melting point. 

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The general form of a phase diagram. Any point on the diagram corresponds to a specific temperature and pressure. Lines trace conditions under which phase changes occur, and the blue arrows show six types of phase transitions. 

A different type of phase transition is known in which there is a discontinuity in the second derivative of free energy. Such transitions are known as second-order transitions. From thermodynamics we know that the change in volume with pressure at constant temperature is the coefficient of compressibility, /3, and the change in volume with temperature at constant pressure is the coefficient of thermal expansion, a. The thermodynamic relationships can be shown as follows ... 

For certain types of phase transitions, it is possible to study the process by following changes in these variables. 

This section is concludedby discussing another type of phase transition, known as solvation, which refers to the dissolution of a gaseous substance in a liquid solvent. We already met this concept when we were deriving the enthalpy of reaction 2.1 (see figure 2.1 and sections 2.4 and 2.5) the solution enthalpy of gaseous O2 in water, AS]n7/(2), can also be called the solvation enthalpy of O2 in water. Note that when the solvent is water, the word solvation is often replaced by hydration. 

So far in the derivation we have made no assumption as to the nature of Phases A or B thus. Equation (8.30) is applicable to all types of phase transitions. When both A and B are condensed phases, the third term on the right side of Equation (8.30) is small compared with the others and the equation reduces to... 

A second, less common kind of pressure-induced phase transition (but in one instance at least, of very great importance) is that in which there is no change in primary coordination number and, to a good approximation, no change in nearest-neighbour bond lengths but, nevertheless, a substantial decrease in volume. We consider next, two examples of this latter type of phase transition. 

AMsopropylacrylamide (NIPA) gel is a typical example of gels which shows a thermoshrinking type of phase transition in aqueous solutions [2]. As shown in Fig. 1, an increase in the temperature causes this particular gel to shrink by one order of magnitude. 

Thermal treatment. Heating of the material may cause desorption of weakly bound species from the surface and can therefore be used to clean surfaces. A positive side effect is that annealing reduces the number of surface defects since it increases the diffusion rates of surface and bulk atoms. There can also be some unwanted side effects surface melting and other types of phase transitions may occur well below the bulk melting point, leading to other than the desired surface structure. 

Another type of phase transition is called a lambda transition, because a graph of heat capacity versus temperature for this type of transition resembles the Greek letter X, as shown in Fig. 4. This type of transition is usually associated with a change from an ordered state to a state with some disorder (order-disorder... 

A tolerance factor [9,10] can be used to determine the phase transition in AB03 perovskite oxides, as given by t — (rA + > o )/V2(J b + ro), where rA, rB, and rQ are the ionic radii [11] of the A, B, and O ions, respectively. This indicates that the spatial margin relates to the type of phase transition. However, the atomistic explanation has not been given for the factor in order to distinguish between ferroelectric and antiferrodistortive phase transitions in AB03 perovskite oxides. 

The gradual conversion of the monomer crystal into the equivalent polymer crystal can be considered a special type of phase transition. In some cases the topochemical polymerization is accompanied by an additional structural phase transition. This behaviour is most often observed in monomer structures with a comparatively moderate reactivity where only a partial conversion can be achieved. Here, the side group packing is rearranged either spontaneously or by thermal annealing. This process... 

Thus, the transitions I II and II III are examples of two different types of phase transition without the multiplication of the unit cell volume - the so-called ferrodistortion phase transitions, one of which (I II) is connected with the unusual change of translational symmetry, but another (II III) only with an abrupt distortion of the crystal lattice parameters. 

In this subsection we shall discuss means to identify another type of phase transitions, i.e., changes for a fixed size but as a function of temperature. Theoretical studies devoted to these transitions have to include some means of describing temperature, e.g., by being based on molecular-dynamics simulations (cf. Section 2.2). However, when increasing the temperature in a such calculation, first of all the atoms will become less localized and, therefore, it may not be easy to identify temperatures for which the structure changes. 

We studied in the previous section several types of phase transition, namely, bound-virtual, bound-resonance, and so on. A characteristic of a phase transition is that two different solutions merge (a / 1), or coexist at the critical point (a = 1). Many-body and multiparameter Hamiltonians could present more complicated transitions, and we will call them multicritical points. 

Figure 4.7 Schematic P-T diagram showing the effect of composition on the types of phase transitions that can occur near the UCEP. (A) A constant-composition loop at a heavy component concentration less than the concentration at the UCEP. (B) A constant-composition loop at a heavy component concentration greater than the concentration at the UCEP. The dotted portion of the SLG line in (B) is not observable since the overall concentration is too high.

Ehrenfest3 refers to the type of phase transition described above as a first-order transition because there are discontinuities in the first partial derivatives of the Gibbs free energy at the transition point. For example,... 

Different intermolecular forces, like van der Waals forces (e.g., for acrylamide gel in acetoneAvater), hydrophobic interaction (e.g., NIPAAm in water), hydrogen bonding (ionic repulsive, e.g., IPN acryhc acid-acrylamide in water), and electrostatic interaction (ionic attractive, e.g., polyampholytes (cation-anion) in water) contribute to the various types of phase transitions in polymer gels. Their influence on to free enthalpy must be taken into account by additional terms to AG (or to X). Examples are ... 

One of the original uses of modern EGD was to aid in the interpretation of DTA data. Using simultaneous DTA-EGD. Ayres and Bens (20) found that if an EGD peak was absent, while the DTA curve contained an endothermic or exothermic peak, it could be concluded that some type of phase transition had occurred in the sample. If curve peaks were found over the same temperature region for both EGD and DTA curves, some type of decomposition reaction involving volatile reaction products had occurred. A comparison of EGD and DTA curves is given in Table 8.1. Note that fusion and solid t - ... 

In liquid-liquid systems, upon increase of concentration of the dispersed phase, at a certain concentration suddenly the dispersed and continuous liquids exchange roles. This is known as a phase inversion. Salager et al. [1983] and Minana-Perez et al. [1986] reported two types of phase transition in ionic emulsions — in the first, viscosity goes through a minimum, whereas in the second it goes through a maximum. The first type of transition (normal) is associated with a decrease of the interfacial tension coefficient and formation of a micro-emulsion. The second (catastrophic) transition is associated with an inversion of unstable structure to a stable one. 

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