**4.1 Organic PCMs**

Among the organic materials used in latent heat storage, paraffin remains the most commonly used compared to fatty acids, polyethylene glycol (PEG) or benzene derivatives, which nevertheless have good characteristics (congruent fusion, excellent stability, good latent heat, and low corrosion). These fatty acids and their derivatives have excellent thermal characteristics. Nevertheless, their high cost is a barrier to their use. In the rest of this chapter, paraffin selected due to their high latent heat compared to other potential "choices" are mainly described. Paraffin is linear chain aliphatic hydrocarbons of the general formula CnH2n + 2. Due to their molecular structure and complex structural and thermodynamic behavior, they have been the topic of a many number of studies for more than half a century. These compounds, which are widely distributed in nature in several forms, are found in gasoline and industrial solvents for some carbon atoms (n) between 4 and 10, or in diesel and fuel for 10 ≤ n ≤ 28. At T = 295°K, the paraffin are gaseous (1 ≤ n ≤ 5), liquid (5 < n ≤ 15), or solid (n > 15) form. On an industrial scale, these compounds are obtained either by cracking and isomerization or by fractional distillation of petroleum.

### *4.1.1 Chains conformation*

The most stable conformation, for which the potential energy is minimal, corresponds to a zigzag arrangement or trans conformation. The symmetry of the molecule depends on the parity of the carbon number. Thus, even n-alkanes have an inversion center while odd-numbered ones have a plane of symmetry perpendicular to the chain. These molecules are likely to have conformational defects, varying according to chain length and temperature. These defects, illustrated in **Figure 1**, result from a sudden increase in the concentration of non-planar conformers among molecules. The first two defects, left terminal type defect and torsion type defect are the most frequent. Conformers with two left bonds are much less numerous, even if their concentration increases near the fusion. The concentration of defects increases significantly with temperature within each crystal phase. It is all the more marked when the temperature is close to the melting point of the materials.

#### *4.1.2 Intermolecular interactions*

Since molecules are composed only of carbon and hydrogen atoms, crystal cohesion is ensured through Van der Waals' forces (London, Keesom, Debye), and more specifically London's interactions.

#### *4.1.3 Stacking of molecules*

The general laws of compact molecular stacking were established by Kitajgorodskij in 1957 [21]. These laws obey two rules, maximum stacking, and free energy. During the trend towards maximum stacking, molecules minimize their energy by adopting compact arrangements that generate maximum contact. The molecular arrangements observed are those with the highest symmetry compatible with the structure of the molecules. In the case of aliphatic chains, the molecules

**73**

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

*4.1.4 Structural analysis*

**Figure 1.**

in the **Figure 2**, where *a,*

solid-solid transitions before melting.

<sup>→</sup> *b* <sup>→</sup> and *c*

A crystallographic group describes each structure:

• orthorhombic structure: *a* ≠ *b* ≠ *c*,α = β = γ = 90 °, noted β;

• monoclinic structure: *a* ≠ *b* ≠ *c*,α = β = 90 °, β ≠ 90 °, noted δ.

• triclinic structure: *a* ≠ *b* ≠ *c*,α ≠ β ≠ γ ≠ 90 °, noted γ;

are arranged in successive layers, in which the chains are parallel to each other. Therefore, the problem of stacking is at two levels, stacking within a layer and stacking these layers together. During his work, Kitajgorodskij also highlights three

*Conformation defects, (a) left terminal type defect, (b) torsion type defect, and (c) two left connections defect.*

The phase changes required for energy storage correspond to first-order transitions. For practical reasons, only solid/solid and solid/liquid transitions can be used. Solid/solid transitions are rarely used because they are too slow, or the amount of energy involved is relatively small. Crystal energy results from two types of intermolecular interactions, those of methylene groups and terminal methyl groups. The values of these forces vary with the length of the chains, leading to differences in the stability of the crystal structure. Thus, n-alkanes crystallize under different structures and are polymorphic, that is, they are likely to undergo one or more

Each structure is part of an elementary mesh whose parameters are defined

between the Oxyz reference axes. The reproduction of this mesh a large number of times allows describing the whole crystal. Each mesh contains one or more molecules and has certain symmetry elements (symmetry axis, inversion center, mirror).

In the aliphatic chains of n-alkanes, each carbon atom is bound to two other carbon atoms and two hydrogen atoms, or one carbon atom and three hydrogen atoms at the end of the chains. The distance between the two carbon atoms is 0.153 nm, and the carbon-carbon binding angle is 114°; the latter value is higher than that of a perfect tetrahedron (109°8′). The most stable molecular arrangement in the solid state, which corresponds to the minimum in potential energy, is the one where the chains

<sup>→</sup>, are the basis vectors, and α, β, and γ, the angles

types of crystal arrangement: orthorhombic, monoclinic and triclinic.

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

#### **Figure 1.**

*Textile Industry and Environment*

hydrates and their eutectics.

*4.1.1 Chains conformation*

*4.1.2 Intermolecular interactions*

specifically London's interactions.

*4.1.3 Stacking of molecules*

**4.1 Organic PCMs**

**4. Classification of phase change materials**

Among the fusible materials likely to be suitable for heat storage by phase change, there are two main families, that is, (i) organic materials, and (ii) salt

Among the organic materials used in latent heat storage, paraffin remains the most commonly used compared to fatty acids, polyethylene glycol (PEG) or benzene derivatives, which nevertheless have good characteristics (congruent fusion, excellent stability, good latent heat, and low corrosion). These fatty acids and their derivatives have excellent thermal characteristics. Nevertheless, their high cost is a barrier to their use. In the rest of this chapter, paraffin selected due to their high latent heat compared to other potential "choices" are mainly described. Paraffin is linear chain aliphatic hydrocarbons of the general formula CnH2n + 2. Due to their molecular structure and complex structural and thermodynamic behavior, they have been the topic of a many number of studies for more than half a century. These compounds, which are widely distributed in nature in several forms, are found in gasoline and industrial solvents for some carbon atoms (n) between 4 and 10, or in diesel and fuel for 10 ≤ n ≤ 28. At T = 295°K, the paraffin are gaseous (1 ≤ n ≤ 5), liquid (5 < n ≤ 15), or solid (n > 15) form. On an industrial scale, these compounds are obtained either

by cracking and isomerization or by fractional distillation of petroleum.

The most stable conformation, for which the potential energy is minimal, corresponds to a zigzag arrangement or trans conformation. The symmetry of the molecule depends on the parity of the carbon number. Thus, even n-alkanes have an inversion center while odd-numbered ones have a plane of symmetry perpendicular to the chain. These molecules are likely to have conformational defects, varying according to chain length and temperature. These defects, illustrated in **Figure 1**, result from a sudden increase in the concentration of non-planar conformers among molecules. The first two defects, left terminal type defect and torsion type defect are the most frequent. Conformers with two left bonds are much less numerous, even if their concentration increases near the fusion. The concentration of defects increases significantly with temperature within each crystal phase. It is all the more

marked when the temperature is close to the melting point of the materials.

The general laws of compact molecular stacking were established by

Kitajgorodskij in 1957 [21]. These laws obey two rules, maximum stacking, and free energy. During the trend towards maximum stacking, molecules minimize their energy by adopting compact arrangements that generate maximum contact. The molecular arrangements observed are those with the highest symmetry compatible with the structure of the molecules. In the case of aliphatic chains, the molecules

Since molecules are composed only of carbon and hydrogen atoms, crystal cohesion is ensured through Van der Waals' forces (London, Keesom, Debye), and more

**72**

*Conformation defects, (a) left terminal type defect, (b) torsion type defect, and (c) two left connections defect.*

are arranged in successive layers, in which the chains are parallel to each other. Therefore, the problem of stacking is at two levels, stacking within a layer and stacking these layers together. During his work, Kitajgorodskij also highlights three types of crystal arrangement: orthorhombic, monoclinic and triclinic.

### *4.1.4 Structural analysis*

The phase changes required for energy storage correspond to first-order transitions. For practical reasons, only solid/solid and solid/liquid transitions can be used. Solid/solid transitions are rarely used because they are too slow, or the amount of energy involved is relatively small. Crystal energy results from two types of intermolecular interactions, those of methylene groups and terminal methyl groups. The values of these forces vary with the length of the chains, leading to differences in the stability of the crystal structure. Thus, n-alkanes crystallize under different structures and are polymorphic, that is, they are likely to undergo one or more solid-solid transitions before melting.

Each structure is part of an elementary mesh whose parameters are defined in the **Figure 2**, where *a,* <sup>→</sup> *b* <sup>→</sup> and *c* <sup>→</sup>, are the basis vectors, and α, β, and γ, the angles between the Oxyz reference axes. The reproduction of this mesh a large number of times allows describing the whole crystal. Each mesh contains one or more molecules and has certain symmetry elements (symmetry axis, inversion center, mirror). A crystallographic group describes each structure:


In the aliphatic chains of n-alkanes, each carbon atom is bound to two other carbon atoms and two hydrogen atoms, or one carbon atom and three hydrogen atoms at the end of the chains. The distance between the two carbon atoms is 0.153 nm, and the carbon-carbon binding angle is 114°; the latter value is higher than that of a perfect tetrahedron (109°8′). The most stable molecular arrangement in the solid state, which corresponds to the minimum in potential energy, is the one where the chains

adopt a trans or zigzag conformation. The structure is different according to the parity of the number n of carbon atoms of the n-alkane. Indeed, the steric congestion conditions imply that the CH2▬CH3 bonds on either side of the stacking plane are aligned, regardless of the parity of the number of carbon in the n-alkane chain.

## *4.1.4.1 Low-temperature crystalline structures (ambient temperature)*

Historically, the structure of n-alkanes has been discovered by Miller, based on the structure of n-nonacosane (n-C29H60) [22]. Terminal methyl groups are arranged in planes forming parallel or inclined lamellar surfaces to the axis of the molecular chains. In the solid state, n-paraffin has a much more compact molecular arrangement than in the liquid state. Some paraffin compounds are polymorphic in a defined range of pressure and temperature; they crystallize in different forms. The crystalline phases can be distinguished, in which the chains of carbon atoms are in a perpendicular position concerning the planes of the terminal methyl groups, relative to the orthorhombic structure, noted β0 (n-C2p+1); and inclined for the planes of the terminal methyl groups, relative to triclinic and monoclinic noted γ0 (n-C2p) and δ0 (n-C2p), respectively [23].

The orthorhombic structure β0 (n-C2p+1) found in odd alkanes (n ≥ 5) includes four molecules per mesh, two layers of molecules that generate the periodicity in a direction perpendicular to the stacking plane of the chains, with c = 2\*L (L: length of a molecule), and the parallelism of the bonds on either side of the stacking plane of the molecules between the last group CH2 and the terminal methyl group. The orthorhombic structures of n-alkane mixtures are isomorphic to those of pure n-alkanes. However, they have irregularities at the end of the chain, caused by differences in the length of the molecules.

**75**

**Figure 3.**

*Triclinic structure of even n-alkanes.*

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

perpendicular plane.

anced position [24].

equal to √

\_\_

*melting temperature)*

which certainly reflects a second-order transition.

The series of even n-alkanes between hexane and hexacosane (6 ≤ n ≤ 26) adopts a triclinic structure (γ0 (n-C2p)) with a space group P and some patterns per mesh equal to 1. The series of even n-alkanes (n > 26) has a monoclinic structure with two molecules per cell. Besides, it can be noted that the axis of the molecule is inclined to the plane of the terminal methyls. The periodicity is in an oblique direction with regards to the plane by considering the alignment of the links on each side of this plane. Thus, even n-alkanes crystallize in triclinic or monoclinic structures (δ0 (n-C2p)) [23]. **Figure 3** shows this type of molecular arrangement in a triclinic structure along the axis of the chains, as well as the projection on the

*4.1.4.2 Structure of the high-temperature solid phases (slightly lower than the* 

High-temperature solid phases called "rotator phases" exist only in a few degrees between the transition temperature in the solid state and the melting temperature. These high-temperature phases maintain an organized structure. In these rotator phases noted, respectively, β-RI and α-RII, the disorder is caused by a 180° rotation of some chains around their axis, which allows them to take a bal-

The β0 (n-C2p+1) low-temperature phases of odd n-alkanes undergo a solid state

transition accompanied by a significant enthalpic effect that characterizes the appearance of a new orthorhombic phase, noted b. Then, this phase passes into a state called rotator noted β-RI, which has been highlighted by heating only in odd n-paraffin for 9 ≤ n ≤ 25. This phase has an orthorhombic structure with the space group Fmmm, where two layers of molecules generate the periodicity in the direction perpendicular to the stacking plane according to the sequence …ABAB… In the RI state of the β phase, the parameters (a, b) of the mesh continuously change without changing the space group: this causes an evolution as a function of temperature,

The rotator phase noted, α-RII, was highlighted in the case of even and odd paraffin for 22 ≤ n ≤ 26. The increasing temperature study of the β-RI phase in odd n-alkanes (tricosane and pentacosane) identified the α-RII phase. The transition from the b-RI phase to the α-RII phase occurs when the mesh parameter ratio b/a is

succession … ABCABC… for the phase α-RII. The transition from the β-RI phase to

the α-RII phase is a first-order transition, with a small enthalpy effect.

3. A change in the stacking sequence accompanies this evolution with the

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

*Textile Industry and Environment*

adopt a trans or zigzag conformation. The structure is different according to the parity of the number n of carbon atoms of the n-alkane. Indeed, the steric congestion conditions imply that the CH2▬CH3 bonds on either side of the stacking plane are aligned, regardless of the parity of the number of carbon in the n-alkane chain.

Historically, the structure of n-alkanes has been discovered by Miller, based on the structure of n-nonacosane (n-C29H60) [22]. Terminal methyl groups are arranged in planes forming parallel or inclined lamellar surfaces to the axis of the molecular chains. In the solid state, n-paraffin has a much more compact molecular arrangement than in the liquid state. Some paraffin compounds are polymorphic in a defined range of pressure and temperature; they crystallize in different forms. The crystalline phases can be distinguished, in which the chains of carbon atoms are in a perpendicular position concerning the planes of the terminal methyl groups, relative to the orthorhombic structure, noted β0 (n-C2p+1); and inclined for the planes of the terminal methyl groups, relative to triclinic and monoclinic noted γ0 (n-C2p)

The orthorhombic structure β0 (n-C2p+1) found in odd alkanes (n ≥ 5) includes four molecules per mesh, two layers of molecules that generate the periodicity in a direction perpendicular to the stacking plane of the chains, with c = 2\*L (L: length of a molecule), and the parallelism of the bonds on either side of the stacking plane of the molecules between the last group CH2 and the terminal methyl group. The orthorhombic structures of n-alkane mixtures are isomorphic to those of pure n-alkanes. However, they have irregularities at the end of the chain, caused by dif-

*4.1.4.1 Low-temperature crystalline structures (ambient temperature)*

**74**

**Figure 2.** *Lattice constants.*

and δ0 (n-C2p), respectively [23].

ferences in the length of the molecules.

The series of even n-alkanes between hexane and hexacosane (6 ≤ n ≤ 26) adopts a triclinic structure (γ0 (n-C2p)) with a space group P and some patterns per mesh equal to 1. The series of even n-alkanes (n > 26) has a monoclinic structure with two molecules per cell. Besides, it can be noted that the axis of the molecule is inclined to the plane of the terminal methyls. The periodicity is in an oblique direction with regards to the plane by considering the alignment of the links on each side of this plane. Thus, even n-alkanes crystallize in triclinic or monoclinic structures (δ0 (n-C2p)) [23]. **Figure 3** shows this type of molecular arrangement in a triclinic structure along the axis of the chains, as well as the projection on the perpendicular plane.
