# Feasibility of Storing Latent Heat with Liquid Crystals. Proof of Concept at Lab Scale
## 1 Introduction
In this respect, compounds leading to stable mesophases due to the presence of intermolecular hydrogen bonds are expected to display both high clearing temperatures and enthalpies. Liquid crystals based on aromatic carboxylic acids are a significant and well investigated group that can form intermolecular hydrogen bonds [10; 11; 12]. These compounds, either alone or as mixtures, lead to supramolecular liquid crystals through the formation of two hydrogen bonds between the carboxylic groups of two different molecules, as shown in Figure 2.  
Figure 1: Clearing point enthalpy _vs._ clearing point temperature for various reported liquid crystals with properties in the desired range for latent thermal storage. In the family of liquid crystals containing the carboxylic acid group we can find the para-\(n\)-alkoxybenzoic acids (\(n\)-PhCOOH) [13], the 4\({}^{\prime}\)-\(n\)-alkoxybiphenyl-4-carboxylic acids (\(n\)-BPhCOOH) [14], the trans-para-\(n\)-alkoxycinnamic acids (\(n\)-CinnCOOH) [15] and the 6-\(n\)-alkoxy-2-naphthoic acids (\(n\)-NaphCOOH) [16]. The molecular structures of these compounds are shown in Figure 3.  
The preparation of all these compounds is already reported in the literature [13; 14; 15; 16], together with their phase transition temperatures and the kind of mesophase observed by polarized-light microscopy. The mesogenic behavior of each LC family included in Figure 3, in terms of transition temperatures variation with the alkyl chain length (\(n\)), is graphically displayed in Figure 4.  
Figure 4: Variation of phase transition temperatures with the alkoxy chain length for the four families of liquid crystals displayed in Figure 3.  
Figure 3: Molecular structures of different liquid crystal families with carboxylic acid as functional group. Figure 2: Intermolecular hydrogen bonds between two aromatic carboxylic acid molecules. As we can see, almost all compounds of these families have clearing temperatures above 140 \({}^{\circ}\)C. The biphenyl derivatives present the highest clearing temperatures, which vary from nearly 300 \({}^{\circ}\)C to 240 \({}^{\circ}\)C as alkoxy chain increases from \(n=1\) to \(n=18\). In contrast the phenyl derivatives show the lowest clearing temperatures so that only derivatives with \(n<10\) attain values higher than 140 \({}^{\circ}\)C. As for the energy associated to each transition, data are not available for all these LC families since they were prepared in the 50's and the measurement of enthalpies at that time was not a simple task. As Figure 4 displays, clearing enthalpy values were only available for the para-\(n\)-alkoxybenzoic acids [13]. In general, the values displayed by the LCs of this family are rather low (\(\sim\)10 kJ/kg) and only the derivatives with \(n\geqslant 14\), and as said above with clearing temperatures lower than 140 \({}^{\circ}\)C, have clearing enthalpies around 25 kJ/kg. In view of these data, it seems that this family is not the most appropriate for studying if liquid crystals can be used for thermal storage. Therefore, in order to prove the viability of LCs as phase change materials we decided to prepare some derivatives of 4\({}^{\prime}\)-\(n\)-alkoxybiphenyl-4-carboxylic acids because they have quite high clearing temperatures for all alkoxy chain lengths (see Figure 4). Other important reasons for choosing this series are the commercial availability of precursors and the simple synthetic route [14]. In this work we present the initial studies on the potential of our idea, which have been carried out on the 4\({}^{\prime}\)-decyloxybiphenyl-4-carboxylic acid (10-BPhCOOH). This compound has been characterized by polarized-light microscopy, differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. Additional properties like specific heat capacity (C\({}_{\text{p}}\)) and dynamic viscosity have also been obtained.