*4.1.5 Thermodynamic analysis*

Alkanes are generally characterized by their transition enthalpy (solid 1/solid 2) and melting enthalpy, noted Htr, respectively, and ∆Hm, and by the temperatures at which these phase changes (Ttr and Tm) or state occur. We can also look at the global transition enthalpy, noted ∆Hg, which reflects the transition from solid state 1 to liquid (**Figure 4**).

### *4.1.5.1 Temperature and enthalpy of solid-solid transition*

The phase transition temperature changes according to the length of the carbon chain and alternately according to the parity of the molecule. This alternation is because the final crystal structure is different according to the parity of the alkane. Indeed, an odd n-alkane will pass from the orthorhombic phase to the rotator I phase, while the even n-alkane (for n ≥ 20) will pass from a triclinic or monoclinic phase to a rotator II phase. This alternation is also found on transition enthalpies from n = 20; indeed, it appears that the enthalpy is lower for odd n-alkanes than for peers, the difference can be attributed to the evolution for odd ones from the β-RI phase to the α-RII phase (**Figure 5**).

The notation ∆H\* corresponds to the summation of the transition enthalpy (β-RI/α-RII) and the increase of enthalpy in the β-RI phase. This increase in enthalpy is related to the calorific capacity of this relatively large phase. Thus, this enthalpy gain in the β-RI phase contributes to the even-odd alternation. Nevertheless, solid-solid transitions are generally not very energetic, so they are not always visible in calorimetry.

#### *4.1.5.2 Melting temperature and enthalpy*

According to the parity of the n-alkane, and according to the number of carbon atoms it contains, it can be observed an even-odd alternation on the two melting temperature evolutions. This disappears when the structures before fusion of the even and odd n-alkanes are identical, that is, for n > 20, as a consequence of the structural difference between the triclinic and rotator I phases.

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*4.1.7.1 Gibbs rule*

*Phase Change Materials for Textile Application DOI: http://dx.doi.org/10.5772/intechopen.85028*

*4.1.5.3 Global enthalpy*

**Figure 5.**

*4.1.6 Improvement of thermal conductivity*

*Transition enthalpy of odd and even n-alkanes.*

*4.1.7 Binary systems of n-alkanes*

power is improved, but it takes up twice as much space.

of thermodynamics on the possibility of forming solid solutions.

no miscibility in the solid state, but a eutectic.

The global enthalpy represents the enthalpy balance of all transformations from the solid state to fusion. Its variation is practically linear as a function of the number of carbon atoms [25]. It, therefore, seems that this balance is independent of the

Much research has attempted to compensate for the poor thermal conductivity of paraffin by adding fins or expanded metal to the material or by dispersing it in a porous conductive material such as natural expanded graphite [26]. This method makes it possible to obtain composites with high thermal conductivity, high paraffin mass content (65–95%) trapped by capillarity [27]. A more common solution is to divide paraffin by volume by encapsulating it in PE spheres or by dispersing it as an emulsion in water. However, such a solution has the effect of considerably reducing (50%) the effective volumetric heat capacity of the process. In other words, its

By combining alkanes in pairs, it is possible to develop systems of various structural types, some of which have very interesting thermal characteristics for energy storage [28]. The study of the behavior of binary mixtures of n-alkanes through phase diagrams or phase equilibrium has generated growing interest in the design of latent heat storage materials since it allows the prediction of the system's behavior during phase changes. Nevertheless, this behavior is governed by some general rules

The solid solution of two n-alkanes is favored if the Gibbs free energy of the mixed crystal is lower than that of each of the pure constituents; otherwise, there is

nature of the low-temperature phase and the parity of the n-alkane.

**Figure 4.** *Evolution of the enthalpy of a pure n-alkane as a function of temperature.*

*Phase Change Materials for Textile Application DOI: http://dx.doi.org/10.5772/intechopen.85028*

*Textile Industry and Environment*

*4.1.5 Thermodynamic analysis*

phase to the α-RII phase (**Figure 5**).

always visible in calorimetry.

*4.1.5.2 Melting temperature and enthalpy*

*4.1.5.1 Temperature and enthalpy of solid-solid transition*

liquid (**Figure 4**).

Alkanes are generally characterized by their transition enthalpy (solid 1/solid 2) and melting enthalpy, noted Htr, respectively, and ∆Hm, and by the temperatures at which these phase changes (Ttr and Tm) or state occur. We can also look at the global transition enthalpy, noted ∆Hg, which reflects the transition from solid state 1 to

The phase transition temperature changes according to the length of the carbon chain and alternately according to the parity of the molecule. This alternation is because the final crystal structure is different according to the parity of the alkane. Indeed, an odd n-alkane will pass from the orthorhombic phase to the rotator I phase, while the even n-alkane (for n ≥ 20) will pass from a triclinic or monoclinic phase to a rotator II phase. This alternation is also found on transition enthalpies from n = 20; indeed, it appears that the enthalpy is lower for odd n-alkanes than for peers, the difference can be attributed to the evolution for odd ones from the β-RI

The notation ∆H\* corresponds to the summation of the transition enthalpy (β-RI/α-RII) and the increase of enthalpy in the β-RI phase. This increase in enthalpy is related to the calorific capacity of this relatively large phase. Thus, this enthalpy gain in the β-RI phase contributes to the even-odd alternation.

Nevertheless, solid-solid transitions are generally not very energetic, so they are not

According to the parity of the n-alkane, and according to the number of carbon atoms it contains, it can be observed an even-odd alternation on the two melting temperature evolutions. This disappears when the structures before fusion of the even and odd n-alkanes are identical, that is, for n > 20, as a consequence of the

structural difference between the triclinic and rotator I phases.

*Evolution of the enthalpy of a pure n-alkane as a function of temperature.*

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**Figure 4.**

**Figure 5.** *Transition enthalpy of odd and even n-alkanes.*
