pH-controlled synthesis of sustainable lauric acid/SiO2 phase change material for scalable thermal energy storage

Lauric acid (LA) has been recommended as economic, eco-friendly, and commercially viable materials to be used as phase change materials (PCMs). Nevertheless, there is lack of optimized parameters to produce microencapsulated PCMs with good performance. In this study, different amounts of LA have been chosen as core materials while tetraethyl orthosilicate (TEOS) as the precursor solution to form silicon dioxide (SiO2) shell. The pH of precursor solution was kept at 2.5 for all composition of microencapsulated LA. The synthesized microencapsulated LA/SiO2 has been characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM). The SEM and TEM confirm the microencapsulation of LA with SiO2. Thermogravimetric analysis (TGA) revealed better thermal stability of microencapsulated LA/SiO2 compared to pure LA. PCM with 50% LA i.e. LAPC-6 exhibited the highest encapsulation efficiency (96.50%) and encapsulation ratio (96.15%) through Differential scanning calorimetry (DSC) as well as good thermal reliability even after 30th cycle of heating and cooling process.

the external environment of cement matrix in CBMs resulting in the initiation of corrosion on steel rebar and strength reduction, thereby serviceability of the building reduces 19 . Therefore, the microencapsulation techniques have been considered as a suitable solution to overcome the aforementioned issues. The microencapsulation techniques control the shifting rate of heat and decrease the reactivity of the PCMs with other materials 20 . There have been various encapsulation methods studied by researchers such as complex coacervation 21 , interfacial polycondensation 22,23 , in-situ polymerization 24,25 and sol-gel process [26][27][28][29] . However, sol-gel is a suitable method for the microencapsulation of PCMs owing to its simple operation under low-temperature condition 30,31 .
Moreover, a significant volume of research works has been focused on the synthesis and characterizations of microencapsulated PCMs using various types of shell/encapsulation materials such as silicon dioxide 32,33 , polystyrene 34 , melamine-formaldehyde 35 , urea-formaldehyde 36 , polymethyl methacrylate 37 , polycarbonate 38 , styrene-methyl methacrylate copolymer 39 , and rigid polyurethane 40 . Fang et al. 41 have synthesized flame retardant n-hexadecane with SiO 2 shell through sol-gel method where they have maintained the pH of precursor solution at 2-3 by adding hydrochloric acid. In other studies, authors have synthesized the palmitic acid-based PCMs through sol-gel process where encapsulation was carried out by controlling the pH of precursor solution 42 . Li et al. have produced a stable paraffin/silicon dioxide/expanded graphite composite PCMs through sol-gel process maintaining a pH value of 2 43 . In an another study, Zhang et al. 23 have synthesized PCMs via sol-gel process with n-octadecane as core material and SiO 2 as the shell material keeping a pH at 2-2.5.
From the literature search, it is found that the pH of the precursor solution is an important parameter to produce microencapsulated PCMs. Therefore, in the present study different amounts of core material i.e., LA has been taken to synthesize the microencapsulated PCMs by maintaining the 2.5 pH of the precursor solution using hydrochloric acid. Besides, it is very important to understand the actual mechanism involving in the formation of microencapsulated PCMs through sol-gel method. Lauric acid (LA) has been chosen as the core materials whereas tetraethyl orthosilicate (TEOS) as the encapsulation material i.e., precursor solution. The characterization of the synthesized microencapsulated PCMs have been carried by Fourier transformed-infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) while thermal properties by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Procedure for microencapsulation of LA. The microencapsulation of LA was carried out in two steps.

Materials and methods
The first step was the preparation of precursor solution which includes hydrolysis and condensation of TEOS. The second step was the preparation of emulsion solution for the encapsulation process.
Preparation of precursor solution. The precursor solution was prepared by mixing 10 ml TEOS, 10 ml EA and 20 ml distilled water in a beaker as shown in Table 1. The pH of this solution was reduced from 6.5 to 2.5 using HCl. Once the pH of precursor solution was attained up to 2.5, the solution was stirred at 400 rpm and 50 °C for 30 min until a transparent solution of TEOS was obtained. This step is called hydrolysis of TEOS, which is amphiphilic in nature and favourable to encapsulate the LA 44 .
Preparation of LA emulsion. Different amounts of LA i.e., 5,10,15,20,30, and 50 g were taken as PCMs as shown in Table 1. These amounts of LA were mixed with different amounts of SLS in 100 ml distilled water (Table 1) under vigorous agitation using a mechanical stirrer at 600 rpm and 65 °C for 2 h. The pH of the emulsion solution was measured and found to be 4-4.5. The mechanism for the microencapsulation of LA is described in Fig. 1. The precursor solution i.e. TEOS would start to hydrolyse in the presence of acid solution and form orthosilicic acid (Si(OH) 4 ). At the long term of stirring, further condensation reaction would occur. Alternatively, on another side, LA started to interact with SLS in an aqueous solution where SLS acted as an anionic surfactant carrying negatively charged ions. To form a stable emulsion solution, the hydrophilic head of the SLS molecules attach with the water molecules whereas the hydrophobic tails attach to the inner part of LA and separate away from water molecules. Thereafter the hydrolysed silica-sol solution was slowly added to the LA emulsion solution where the hydrophilic ends of hydrolysed silica slowly diffused and condensed onto the surface of LA. In the final step, polymerization reaction would take place resulting in microencapsulation of LA with SiO 2 .
Characterization. FT-IR spectroscopy (Perkin Elmer UATR Two, United States of America) was used to analyse the chemical composition of pure LA as well as microencapsulated LA. The spectra were recorded ranging from 500 cm −1 to 4000 cm −1 .
The XRD (D/MAX-2500, Rigaku, Japan) measurements were performed to analyse the phases present in the samples using 1.541 Å Cu-Kα radiation with 0.02° step width on continuous scanning mode. The XRD patterns were collected from 2θ = 5°-50° at 4°/min scan rate using 25 kV and 100 mA operating conditions.
The chemical states of the present element in the samples were analysed by XPS (Scienta Omicron R300, United States of America) with Al Kα X-ray as the source of radiation. All the collected XPS spectra were calibrated with the adventitious carbon (C1s) peak (peak position: 284.6 eV). CASA XPS software was used to calibrate and deconvolute the scans as well as to correct the background (Shirley method) of the XPS peak. The best peak fittings were achieved by suitable Gaussian/Lorentzian fit.
The morphology and chemical compositions of the samples were investigated by SEM (TESCAN MIRA3, Czech Republic) coupled with an energy-dispersive X-ray (EDX) spectrometer at 15 kV as well as TEM (JEOL, JEM 2010, Tokyo, Japan).
Thermal properties (melting/solidifying temperature and latent heat) and thermal stability (thermal cycle) of the microencapsulated PCMs were investigated by DSC (TA Instrument, Discovery DSC, New Castle, United States of America) instrument from 20 °C and 60 °C with a 10 °C/min heating/cooling rate. The weight loss of the samples was investigated by a TGA analyser (TA Instrument, Discovery TGA, New Castle, United States of America) at 10 °C/min heating rate starting from 20 to 650 °C.

Results and discussion
FT-IR analysis. The FT-IR analysis of bulk LA as well as microencapsulated LA/SiO 2 are shown in Fig. 2.
The bulk/pure LA has been chosen as reference materials. The FT-IR of pure LA shows four major peaks at 2914, 2847, 1696 and 937 cm −1 while two minor peaks at 1302 and 726 cm −1 as shown in Fig. 2a. The peaks at 2914 and 2847 cm −1 correspond to the stretching vibration of -CH 3 and -CH 2 group, respectively 45 . Alternatively, one intense peak at 1696 cm −1 exhibits the symmetrical C=O stretching vibration in LA 46 while another peak at 937 cm −1 reveals the out of plane bending vibration for -OH group 45 . Moreover, two weak peaks appeared at 1302 cm −1 and 726 cm −1 are attributed to the stretching vibration of O-C group and plane bending vibration of C-H bond 47 . These peaks i.e., 2914, 2847, 1696, 1302, 937 and 726 cm −1 are also found in microencapsulated LA with SiO 2 (Fig. 2b-g). www.nature.com/scientificreports/ An interesting observation can be found in microencapsulated LA/SiO 2 where one absorption peak at 1080 cm −1 is observed which corresponds to the asymmetric stretching vibration band of the Si-O-Si shell 23 . It depicts that the formation of SiO 2 has occurred during the encapsulation of LA. Our finding is well corroborated with Zhang et al. 45 works where the PCMs was encapsulated by a facile one-pot method with co-hydrolysis and co-condensation process of methyl-triethoxysilane (MTES).
The main absorption peak of SiO 2 at 1080 cm −1 in lower core-shell ratio i.e., LAPC-1 (Fig. 2b) and LAPC-2 ( Fig. 2c) is found to be highest in intensity and broadening revealing the thick and high amount of SiO 2 compared to other samples. As the core-shell ratio is increased, the intensity of SiO 2 absorption band is decreased confirming the thinning and lowering in amount of SiO 2 formed onto the LA surfaces. Initially, the pH of the emulsion solution is found to be in between 4 to 4.5. However, once the precursor solution is mixed with the emulsion solution, the polymerisation would occur (Fig. 1) resulting in encapsulation of LA with SiO 2 shell 48 . The slow condensation rate of SiO 2 forms smooth and more compact layers onto the LA surface attributed to the sufficient time i.e., 6 h given for the reaction. Consequently, the low core-shell ratio might exhibit thicker shell formation compared to the higher core-shell ratio.   Broadening in XRD peaks of microencapsulated LA/SiO 2 are observed attributed to the microencapsulation of core material i.e., LA with amorphous SiO 2 shell around 2θ = 20°-30° as shown in supplementary Fig. S1. It can be seen from Fig. S1 that as the core material amount is increased (Fig. S1b-g), amorphousity is decreased attributed either to the thinning of shell wall or decrease in the amount of SiO 2 onto the microencapsulated LA/ SiO 2 surfaces. Moreover, there is an interesting observation found in XRD peaks at (200), (300) and (500) where the intensity ratio of these peaks are increased as the core material is increased revealing the high amount of core materials in microencapsulated LA/SiO 2 .
It is observed by Wu et al. that the condensation rate is at its lowest point when the pH of the silica precursor is below 2.0 (isoelectric point) attributed to the neutral zeta (ζ) potential or zero surface charge 49 . At the neutral charge, the particles of the precursor solution tend to agglomerate, aggregate and flocculate. Thus, it is necessary to increase the pH of the precursor solution (pH > 2.0) to keep a stable solution with a good dispersion particle. In another study, pH at 2 is not recommended as the solution is prone to aggregate and flocculate 43 . For pH values above isoelectric point, the ζ potential values of silica become more negative 50 . Thus, it is suggested to keep the precursor solution pH greater than 2 where the charged particles are repelled to each other and avoid the agglomeration caused by the Van der Waals forces 48  The XPS spectra of C 1 s (Fig. 5) are fitted with three different peaks which are C-C, C=O and O=C=O for all samples (LA, LAPC-1, and LAPC-6) which are attributed to the core material. The binding energy (BE) of C-C, C=O and O=C=O peaks in the present study are well corroborated with other researchers works [54][55][56] . Figure 6 shows the XPS spectra of O 1 s with the presence of two peaks (Fig. 6a) of C-O-H (533.5 eV) and C-O/C=O (532.1 eV) for bulk LA whereas LAPC-1 (Fig. 6b) exhibits at 0.1 eV higher BE of C-O-H and C-O/ C=O peaks attributed the adsorption of SiO 2 onto the LA surface. There is broadening in C-O/C=O peaks observed for LAPC-1 (Fig. 6b) and LAPC-6 ( Fig. 6c) attributed to the change in the chemical moiety owing to the presence of SiO 2 . The BE of these peaks are well fitted and satisfied with the earlier studies 54,57,58 . Figure 7 shows the XPS spectra of Si 2p associated with the encapsulation of SiO 2 onto the LA surface. No peak of Si 2p is found in bulk LA owing to the absence of SiO 2 . The Si 2p peak is fitted with Si-O-Si and Si-OH in microencapsulated LA/SiO 2 samples. Both samples exhibited similar binding energy at 105.5 eV for Si-O-Si peak whereas Si-OH peak exhibits at 103.3 eV (Fig. 7a) and 103.0 eV (Fig. 7b) for LAPC-1 and LAPC-6, respectively. The intensity of Si-OH peak is higher in LAPC-1 compared to LAPC-6 owing to the formation of a thicker SiO 2 wall.
Morphology of the microencapsulated LA/SiO 2 . The morphology of LA and microencapsulated LA/ SiO 2 are shown in Fig. 8. It can be seen from Fig. 8a that LA exhibits irregular morphology with a rough surface. The particle of LA is around 200 µm in diameter and 500 µm in length. However, once the encapsulation has occurred, the morphology of the encapsulated PCMs has changed as shown in Fig. 8b-g. The morphology of LAPC-1 sample exhibited a dense structure of SiO 2 (Fig. 8b) where the proper encapsulation cannot be identified 43 . It can be seen that at low amount of core material, the surface is more pronounced www.nature.com/scientificreports/ to form dense and agglomerated (Fig. 8b-d) structure 59 owing to the significant amount of amorphous SiO 2 which cover the LA surface 60 . The encapsulation of SiO 2 on high amount of core materials i.e. LA leads to reduce the agglomeration 38 . The morphology of LAPC-4 (Fig. 8e) and LAPC-5 (Fig. 8f) samples are getting clear with the formation of globular particles even though the agglomerated structure still can be seen (Fig. 8e,f) 43,61,62 .
Once the amount of core materials i.e. LA is reached up to 50% i.e., LAPC-6, the SEM image has shown more globular and well-defined structure 47 (Fig. 8g inset). This result is well corroborated with TEM ( Fig. 9) where light white colour SiO 2 shell covered the core LA (black) uniformly. It can be seen from this Fig. that different  (Fig. 8g). It can be explained that a high amount of LA reduces the number of Si-O-Si bonds by replacing Si-OH with Si-CH 3 groups which decrease the connectivity of SiO 2 network and make less porous structure 63 . Once the amount of LA increases i.e., high surface area, only a few silica oligomers will deposit on the LA surface and led to increase the particle size of microencapsulated LA/SiO 2 . It can be seen from Fig. 8g that the particle size of microencapsulated LA/ SiO 2 is greater than 150 µm which is well corroborated with particles size measured by TEM. The condensation rate of SiO 2 is low in an acidic environment where it has significant time to encapsulate the LA 23 . At the low pH  Table 2. LA sample exhibited C and O attributed to the hydrocarbon. However, microencapsulated LA/SiO 2 contain C and O as well as different content of Si. At low core material amount, Si is highest but once the amount is increased, Si content is decreased, and C is increased. This result suggests that at the low amount of core material, the formation of Si is thicker whereas, at high amount of core material, the thinning of SiO 2 is occurred. Therefore, well-defined microencapsulation is observed in LAPC-6 sample (Figs. 8g, 9). There is no consistency in the amount of C and O due to the semiquantitative analysis of EDS 65 . The presence of Si in the microencapsulated LA/SiO 2 is well corroborated with the results of FT-IR, XRD and XPS where SiO 2 is observed.
Thermal performance of the microencapsulated LA with SiO 2 shell. Melting and solidifying characteristics of microencapsulated LA/SiO 2 are shown in Figs. 10 and 11, respectively, as well as the corresponding thermal data have been presented in Table 3. It can be seen from melting (Fig. 10) and solidifying (Fig. 11) curves that all samples exhibit different endothermic and exothermic peak temperatures. LA sample shows the highest melting i.e., 44.459 °C ( Fig. 10; Table 3) and solidifying temperature i.e., 38.487 °C (Fig. 11; Table 3) as well as 166.74 J/g melting latent heat and 159.54 J/g solidifying latent heat (Table 3). This result suggests that the latent heat of LA is high, thus, it can be considered as prudent PCMs for the application in thermal storage 62 . When the amount of core material is less, the melting and solidifying temperatures, as well as the corresponding latent heat values are lower compared to LA attributed either to the lowest amount of LA where shell i.e., SiO 2 act as an inert material or thickening of the shell wall. In this case, the movement of the LA molecules are limited and confined to a limited space by the shell 38,[66][67][68] or there is a possibility that microencapsulated LA/SiO 2 have many empty shells without core materials 69 . The core material is responsible for the storage and release of thermal energy rather than shell materials. However, once the core material i.e., LA is increased, the melting and solidi-     Table 3 shows that LAPC-6 sample exhibited 96.50% encapsulation ratio and 96.15% encapsulation efficiency which are the highest among all microencapsulated LA/SiO 2 . The comparison in the encapsulation ratio and efficiency of the PCMs with the recent reported studies [72][73][74][75] are presented in Table 4. It can be seen from this table that LAPC-6 has exhibited the highest encapsulation ratio and efficiency than earlier reported values.   shell hinders or delays the decomposition and resulting in the high thermal stability of LA 69 . The weight loss of LAPC-6 sample is greater than LAPC-1 suggesting that the decomposition and weight loss mainly caused by the evaporation of LA 61 . Alternatively, in the case of LAPC-1, thick layer (Fig. 8b) and high content ( Table 2) of SiO 2 hinder the degradation process, thus, lower weight loss is observed. The carbonaceous-silicate charred layer was formed onto the LA surface which can protect the core materials and slow down the volatile products transferred during the thermal decomposition 64 .
The other important parameter of the microencapsulated PCMs is the thermal reliability and consistency after being used for a certain period of time. There should be no significant difference in the thermal properties and stability after a certain duration that can affect the phase change temperature and latent heat capacity. Therefore, Fig. 13 shows the thermal reliability of LA and LAPC-6 after 30 cycles of heating and cooling process. The LA (without encapsulation) exhibits the melting and solidifying temperatures from 44.459 °C to 45.845 °C and 38.482 °C to 37.695 °C while latent heat changes from 166.74 J/g to 165.06 J/g and 159.54 J/g to 158.16 J/g from 1 to 30 cycles as shown in Fig. 13a, respectively. The melting and solidifying temperatures and latent heats of LA have decreased significantly. LAPC-6 sample shows the melting and solidifying temperatures change from 44.221 °C to 44.116 °C and 39.445 °C to 39.317 °C while the latent heat from 160.96 J/g to 160.75 J/g and 152.80 J/g to 152.71 J/g as shown in Fig. 13b, respectively. Negligible changes observed in melting and solidifying temperature and latent heat of LAPC-6 after 30 thermal cycles. Therefore, the microencapsulation of SiO 2 is proven to be advantageous and it helps to sustain the thermal characteristic of the PCMs after a long period of application.

Conclusions
In this study, LA has been chosen for microencapsulation with SiO 2 shell. The microencapsulation of LA with SiO 2 shell has been achieved successfully via sol-gel method using TEOS as precursor solution at 2.5 pH. Various kinds of analyses were conducted viz. FT-IR, XRD, XPS, SEM, TEM, DSC, and TGA to determine the properties of the microencapsulated LA/SiO 2 . FT-IR analysis has exhibited the presence of Si-O-Si peak at 1080 cm −1 for all microencapsulated LA while XRD analysis has shown the formation of hump at 2θ = 20°-30° associated with the amorphous SiO 2 shell. XPS analysis of microencapsulated samples has shown the Si-O-Si and Si-OH peaks. SEM and TEM images of LAPC-6 has shown the formation of the fine-globular shape of microencapsulated LA/ SiO 2 whereas the samples with low amount of LA have exhibited the agglomeration. TEM analysis has shown that the particle size of LAPC-6 is found to be around 160 µm. LAPC-6 has exhibited the highest encapsulation efficiency and ratio among all microencapsulated LA/SiO 2 with good thermal reliability.