Preparation and water sorption properties of novel SiO2-LiBr microcapsules for water-retaining pavement

Wenjing Li,Gilmore Wellio,Tiejun Lu,Changjun Zou,Yongliang Li,*

1 School of Chemical Engineering,University of Birmingham,Edgbaston,Birmingham B15 2TT,UK

2 College of Chemistry and Chemical Engineering,Southwest Petroleum University,Chengdu 610500,China

3 Chengdu Advanced Metal Materials Industry Technology Research Institute Co.,Ltd,Chengdu 610000,China

Keywords: Microcapsules Water vapor sorption Thermal performance Adsorption kinetics Water-retaining pavement

ABSTRACT Novel SiO2-LiBr microcapsules for water-retaining pavement were prepared and firstly characterized by scanning electron microscope (SEM),particle size analysis,and Fourier transform infrared spectroscopy(FT-IR).The water vapor sorption and desorption of the formulated microcapsules was then experimentally studied using dynamic vapor sorption (DVS),with the results fitted to three kinds of adsorption kinetics models.In addition,the specific surface area (SSA) was also calculated based on BET theory;and the thermal performance was investigated by laser flash analysis (LFA).Experimental results show a change of 103% in mass of the microcapsule sample under 90% relative humidity (RH) at 30 °C after water vapor sorption.The fitting of results indicates that the adsorption process is mainly governed by the intra-particle diffusion mechanism,followed by the pseudo-first-order adsorption process.In comparison with most conventional pavement materials,it is found that the SSA of the formulated microcapsules is much larger while the thermal conductivity is lower.The unique properties of the formulated SiO2-LiBr microcapsules have significant potential to take the edge off the urban heat island effect and reduce rutting when applied to water-retaining pavement materials.

1.Introduction

With the continuous progress of urbanization,the urban heat island effect has become a growing concern[1–3].The three main factors of the urban heat island effect are: the impact of urban underlying surface; the impact of artificial heat and air pollution;and the impact of atmospheric climate and environment (refer to Fig.1).Among these,the impact of urban underlying surface is particularly significant.Traditionally,pavements,plazas,parking lots and so on,are mainly constructed with compact materials such as asphalt or cement concrete [4–6].These pavement materials have high strength,good bearing capacity and durability.However,they have negative consequences on the city’s water cycle.Rainwater falls on the surface of the road,it is then discharged into the city’s drainage network and enters nearby rivers,which means the water cannot complete evaporation and as a result it decreases the heat dissipation.

In order to alleviate the urban heat island effect,many researchers have carried out a variety of research approaches,one of which is the development of water-retaining pavement [7,8].Waterretaining pavement is a kind of functional pavement,whose structure can preserve moisture and dissipate latent heat by evaporation of moisture[8,9]so as to restrain the temperature rise of pavement and then reduce the air temperature.Water-retaining pavement can be divided into water-retaining asphalt pavement,water-retaining cement concrete pavement and water-retaining pavement block/brick pavement according to the parent material.Jianget al.[10]have produced water-retentive asphalt concrete manufactured by combining water-retentive slurry into porous asphalt concrete and found that it could meaningfully reduce the surface temperature of pavements.Qinet al.[11]invented a new water-retaining pavement block for capturing rainwater and cooling pavement,and pavements made of interlocking water-retaining paver blocks can be used as a strategy to mitigate urban heat island.Baoet al.[12]also reported a novel drainable water-retaining pavement block for mitigating the urban heat island effect and simultaneously reducing surface runoff.Nevertheless,the water absorption and evaporation of these materials needs to be further improved because its water absorption rate is not high enough.

Fig.1.Schematic of the urban heat island effect and potential use of SiO2-LiBr microcapsules as water-retaining pavement materials.

The present work is therefore devoted to preparing SiO2-LiBr microcapsules with a high capacity for water vapor adsorption/desorption,which can be used as water-retaining pavement materials.Moreover,the formulated SiO2shell microcapsules have lower thermal conductivity,which as a result can reduce the temperature sensitivity and improve the high temperature rutting of pavement.High concentration LiBr solution is selected as core material in the preparation process as LiBr is an efficient vapor absorbent and air humidity regulator,which is stable,not easy to deteriorate and decompose in the atmosphere,and also has strong hygroscopicity [13,14].The encapsulation process can also overcome the corrosive nature of LiBr.In this study we investigated the adsorption and desorption characteristics and kinetics,thermal performance,and specific surface area of the formulated SiO2-LiBr microcapsules.It is shown that the SiO2-LiBr microcapsules can potentially play a significant role in the optimization of pavement materials for real applications.

2.Experimental

2.1.Materials

Tetraethyl orthosilicate (TEOS:C8H20O4Si,CAS:78-10-4),Methyltrimethoxysilane (MTMS:C4H12O3Si,CAS:1185-55-3) and Silica (SiO2,12 nm,CAS: 7631-86-9) were supplied by Sigma-Aldrich.Polysorbate 80 (C64H124O26,CAS: 9005-65-6) and Span?80 (C24H44O6,CAS: 1338-43-8) were purchased from Fluka.The rest of the materials were all sourced from Merck,including,lithium bromide (LiBr,CAS: 7550-35-8),Hydrochloric acid (HCl,CAS:7647-01-0),Hexane(CH3(CH2)4CH3,CAS:110-54-3)and Mineral oil (CAS: 8042-47-5).

2.2.Synthesis of SiO2-LiBr microcapsules

The synthesis of SiO2-LiBr microcapsules includes the formation of LiBr solution and silica precondensate.LiBr powder was dissolved in 0.1 mol·L-1HCl solution with 1% (mass) polysorbate 80 at room temperature to formulate LiBr solution with a concentration of 62% (mass) (which is close to its saturation concentration).During the silica precondensate preparation,the TEOS,MTMS and water were firstly mixed at a molar ratio of 1:1:1.3 and stirred at 450 r·min-1for 4.5 h at 100 °C.The vapor was led into the condenser.The condensation by-products,which mainly consisted of ethyl alcohol,were introduced into a beaker.The mixing process then continued for a further 16 h at room temperature to obtain silica precondensate.The reaction took place through a series of condensation reactions,which convert TEOS molecules into mineral-like solids by forming Si-O-Si bonds.The main reaction was as follows:

The synthesis procedure was as follows(see Fig.2):75 g of mineral oil,2 g of Span?80,6.4 g of silica precondensate and 0.1 g silica nanoparticles(as inoculating seed),were mixed and stirred with an IKA Overhead Stirrer at 300 r·min-1.During this period,2.5 ml of the 62 wt% LiBr solution was dripped slowly into mixture.The emulsion was then stirred for 15 mins at 20 °C,while the contact of the precondensate with the acid solution (LiBr solution) triggered the polymerization process.After that,the emulsion was heated upt to 50 °C and kept for 60 min.3 g of MTMS was then added into the emulsion to ensure the integrity of microencapsulation and prevent microencapsule agglomeration.The emulsion was stirred for a further 60 minutes and then cooled to room temperature.Finally,the microcapsules were obtained by washing carefully with hexane to obtain pale yellow particles upon filtration.

2.3.Characterization

The chemical functional groups of the resultant microcapsules[15–17]were characterized by Fourier transform infrared spectroscopy (Bruker Lumos FTIR Microscope,Germany).Spectra were collected in the range 600 to 4000 cm-1with 32 average scans for noise reduction.

Fig.2.Flow chart of SiO2-LiBr microcapsules synthesis.

The morphology of the microcapsules was observed using a scanning electron microscope (Hitachi TM3030 SEM,with highsensitive semiconductor and 4-segment BSE detector) at 5 kV.

The size distribution of microcapsules was measured by the Particle Size Analyzer (Malvern Mastersizer 2000),which has an automated sample dispersion unit for the measurement of wet samples controlled through Standard Operating Procedures.

Thermal conductivity of the resultant microcapsules is also a vital property which plays a significant role in pavement material application [18,19].Laser flash analysis ((LFA427,Netzsch,Germany) was used to measure the thermal diffusivity as well as the thermal conductivity of the SiO2-LiBr microcapsules [20,21].The uncertainty of thermal conductivity measurements was found to be in the range of ± 5%.

In laser flash method the thermal diffusion coefficient（α）could be obtained from the following equation [22,23]:

wheredrepresents the sample thickness andt50is half heating time(the time required for the surface temperature of the sample to rise to half of its maximum after receiving light pulses).

Thermal conductivity (λ) can be calculated using the following equation [24]:

whereTsignifies temperature,Cprepresents specific heat capacity,and ρ is the density of the sample.

The adsorption and desorption property of SiO2-LiBr microcapsules were characterized by a Dynamic Vapor Sorption device(DVS RLSM01501,UK) which could deeply study bulk and surface adsorption effects of water and organic vapor[25–27].The Surface Measurement System(SMS)of the DVS device gives excellent control of the vapor generation to within ± 0.1% relativity humidity(RH).The high temperature preheater could heat the sample of up to 200°C and test temperature could be controlled in the range of 5–60 °C with ± 0.1 °C.Moreover,the mass changes were recorded by a high resolution SMS microbalance with an accuracy of ± 1 μg.Before the sorption test,the samples were first dehydrated completely through preheating at 180 °C for 15 min and then cooled down to 25 °C for 15 min.During the tests,for each sample the RH was changed from 0% to 90% (adsorption process)and then to 0% (desorption process) with an equilibrium criterion of dm/dt< 0.02% per minute.The working principle of the DVS is illustrated in Fig.3.

2.4.Adsorption theory and kinetics

The physical adsorption of gas molecules on a solid surface can be explained by the Brunauer-Emmett-Teller (BET) theory,which serves as the basis for an important analysis technique for the measurement of the specific surface area of materials[28–30].The BET equation as well as the specific surface area (SSA) are given by:

Fig.3.The working principle diagram of DVS.

wherepandp0represent the equilibrium pressure and saturation pressure,respectively (Pa);Cis the BET constant,which is a function of the heat of sorption and heat of vaporization of the gas;VandVmdenote the total adsorption gas quantity and the monolayer adsorbed gas quantity,respectively (mol·g-1);NAsignifies the Avogadro constant which equals to 6.02×10-23mol-1; σ is the area of the adsorbate molecule.

Three types of equations,including Pseudo-first order model(Eq.(5)),Pseudo-second order model(Eq.(6))and intraparticle diffusion model(Eq.(7)),have been commonly used to represent the adsorption kinetics [31–33].

In above equations,qtrepresents the amount of adsorbed gas at the timet,whileqerepresents its value at equilibrium(mg·g-1).k1is the pseudo-first-order adsorption rate constant (min-1).k2is pseudo-second-order adsorption rate constant (g·mg-1·min-1).kiis the intra-particle diffusion constant (mg·g-1·min-1/2).Cis the constant related to the boundary layer thickness.

The pseudo-first-order kinetic model based on adsorption capacity assumes that the adsorbate reaching the adsorbent surface is controlled by diffusion,and there is only one binding site on the adsorbent surface.The pseudo-second-order kinetic model is based on the hypothesis that the adsorption rate is controlled by chemisorption,including electron transfer and sharing between adsorbent and adsorbate,and there are two binding sites on the adsorbent surface.Intraparticle diffusion is usually divided into three steps: ion diffusion to adsorbent surface,intraparticle diffusion,as well as adsorption and desorption between active sites and adsorbates.In order to determine the adsorption kinetics mechanism of microcapsules for water vapor,the pseudo-firstorder,pseudo-second-order and intraparticle diffusion kinetics models were applied and compared in this paper.

3.Results and Discussion

3.1.Morphological studies and chemical structure

The chemical structures of the prepared microcapsules were determined using FTIR as shown in Fig.4(a),while a visual image(Fig.4(b)) was captured by FTIR microscope.An FTIR absorbance spectrum was collected from 600 cm-1to 4000 cm-1and a number of characteristic vibrational peaks could be seen.Absorbance peaks appeared at around 3388 cm-1for the -OH stretching vibration,three peaks at 2960 cm-1,2917 cm-1and 2852 cm-1for the C-H stretching vibration.Moreover,the peak at 1631 cm-1appeared for C=C stretching,1460 cm-1and 1272 cm-1came out for the presence of alkyl groups.The 1019 cm-1and 921 cm-1showed for C-H external bending vibration.This indicated that TEOS hydrolyzed and condensed as planned in the reaction process,which meant a successful synthesis of the silica case.Moreover,the above results also implied that MTMS,which was added to introduce hydrophobicity on the capsule surface to prevent agglomeration between the capsules during and after the preparation of the capsules,successfully modified their surface.Meanwhile,it could be clearly seen from the visual image that microcapsules were polydisperse mostly spherical.Additionally,microcapsules had a core–shell structure with a yellowish center nucleus,which indicated that the preparation of microcapsules was successful.

Fig.5 showed the SEM micrographs of whole and crushed SiO2-LiBr microcapsules.As seen from the SEM micrographs,the prepared microcapsules were spherical in shape,and the surface of the microcapsules was rough due to a portion of silica particles.LiBr salt crystals can be seen inside the crushed microcapsules.It can also be seen that a small percentage of LiBr was not encapsulated by the silica.The microcapsule sizes ranged from 5 μm to 50 μm.The SEM image of a ruptured microcapsule with a diameter of 50 μm is shown in Fig.5(c),within which a multicore structure is observed.The largest core is about 30 μm in diameter with surounding smaller cores with diameters between 5 and 10 μm.The core–shell structure indicates that a large number of LiBr solution has been encapsulated to offer high water sorption properties.

Fig.4.The FTIR absorbance spectrum (a) and optical image (b) of SiO2-LiBr microcapsules.

The Malvern Mastersizer 2000 particle size analyzer was used to measure the microcapsule size distribution.Sunflower oil was selected as a carrier liquid to form stable microcapsule suspension for the measurements.The particle size analyzer used a He-Ne laser with a wavelength of 633 nm whilst the suspension was maintained at a stirring speed of 2000 rpm.The measured size distribution of the prepared SiO2-LiBr microcapsules is shown in Fig.6.The D4,3of three cycles were (229 ± 1)μm,(272 ± 1.0)μm,(228 ± 1) μm,respectively.The average D4,3of microcapsules was (243 ± 1)μm with the corresponding average SPAN values of(1.93 ± 0.01).It should be noted that there is still particle agglomeration in oil suspension,which can also be observed from SEM images,resulting in larger measured average diameters.

3.2.Thermal performance of microcapsules

Laser Flash technique is a fast,versatile and precise absolute method for thermal conductivity and diffusivity measurement[34,35].In the above calculation (Eqs.(1) and (2)) the density of the sample was measured at room temperature (25 °C) and was assumed to be a constant.TheCpvalue of the sample was measured by differential scanning calorimetry (DSC).The results of thermal diffusivity,specific heat capacity,density and thermal conductivity under the temperature range of 25–65 °C are shown in Fig.7.With the temperature changes from 25 to 65°C,the thermal diffusivity varies from 0.013 to 0.017 mm2·s-1.In addition,the specific heat capacity increased with temperature and did not exceed 2 J·g-1·K-1.Hence,results of thermal conductivity were very small and maximum value was 0.0126 W·m-1·K-1at 65 °C.It can be seen from Table 1 that the thermal conductivity of conventional pavement materials was much higher than that of the microcapsules prepared in this work.Therefore,our material has very low sensitivity to temperature and have great potential to optimize the performance of pavement materials,prevent rutting and reduce the ambient temperature.

Table 1Thermal conductivity of common pavement materials

3.3.Water vapor sorption and desorption

The mass change of water vapor sorption and desorption after preheating is shown in Fig.8.The sample was kept at a constant relative humidity (RH from 0% to 90% and returned from 90% to 0%)until the rate of change in mass by time(dm/dt)was less than 0.02% per minute over 10 min at 30 °C.Meanwhile,the computer recorded relative data including mass change,actual RH,target RH,every 20 s.It was apparent that the mass of the sample increased with time,which was mainly because the saturated aqueous solution layer was formed on the surface of LiBr absorbed water vapor by the microcapsules under certain humidity.The vapor pressure generated by the saturated aqueous solution was lower than that generated by pure water,hence the water in the air was constantly absorbed and the LiBr was continuously dissolved,leading to the improvement of water absorption.Moreover,water vapor sorption was enhanced with the increase in RH and the adsorption equilibrium time increases with the increase in relative humidity.It took only 75 mins to reach the state of equilibrium in the first stage (RH=0%),while this process took 359 mins at RH=90% to reach equilibrium between the measurement times of 930 to 1289 mins.

Fig.5.The SEM micrographs of (a,b) whole SiO2-LiBr microcapsules and (c) crushed SiO2-LiBr microcapsules.

Fig.6.Distribution of SiO2-LiBr microcapsules size by three test cycles.

Fig.7.Thermal diffusivity,specific heat capacity,density and thermal conductivity of microcapsules under 25–65 °C.

Fig.8.The mass change of water vapor sorption and desorption at 30 °C.

Comparison of water vapor sorption and desorption based on change in massviadifferent RH could see from Fig.9 and Table 2.It was noticeable that the trend of adsorption and desorption with RH was the same.Hysteresis of RH between sorption and desorption of change in mass (Hysteresis=|RHsorption-RHdesorption|)was tiny,all of which was smaller than 5% and most of which was around 1% along with changed RH.The maximum mass change (103.9%) occurred when the relative humidity was 90%,while minimum mass change was 12.4% which appeared at RH=10%.The RH of human settlements ranges from 40% to 70%,change in mass of LiBr microcapsules sorption was from 26.7% to 50.4%.Commonly,the RH in rainy days is higher than 80%,our microcapsules have excellent hygroscopicity and water absorption in rainy days.

Table 2DVS change in mass (preheat 180 °C and test temperature 30 °C)

Fig.10 shows the adsorption of microcapsules to equilibrium(dm/dt< 0.02% per minute) at different temperatures (30 °C,40°C,50°C) at RH=90%.It was obvious that the adsorption equilibrium time at the temperature of 30 microcapsules was the longest and the adsorption capacity was the largest,this adsorption equilibrium time was 360 min and the change in mass was over 100%.However,the adsorption capacity of microcapsules decreased with a rise in temperature,the change in mass was nearly 70% when the temperature was 40 °C and at 50 °C,the change in mass was 47%.At the same time,when the temperature was high,the microcapsules quickly reached adsorption equilibrium.Nevertheless,the absorption of water by microcapsules at the temperature of human habitation shows great potential for application.In conclusion,the lower the temperature,the greater the adsorption amount,and the longer the adsorption time within a certain temperature range.It could be seen that the application of our microcapsules in pavement materials is conducive to improving the water absorption and hygroscopicity of pavement materials,and it also has great potential in regulating urban heat island effect.

3.4.Specific surface area of microcapsules

Measurement of the specific surface area(SSA)of LiBr microcapusles plays an essential role for application in pavement materials.The Brunauer,Emmett,Teller (BET) multilayer adsorption model is not only be able to evaluate the quantity of a monolayer of adsorbate but can also be used to deduce the SSA by water vapor sorption.

We could fit Eq.(1)as a linear equation with one variable.P/P0represents the independent variable andP/P0/［1-P/P0）V］ as dependent variable.

The ordinate intercept a and slope b were determined by the following Eq.(8) and Eq.(9)

According to Eqs.(8)and(9),CandVmare obtained by deformation,which could be as in the following Equations:

Fig.9.Comparison of water vapor sorption and desorption based on change in mass via different RH.

Fig.10.The adsorption of microcapsules to equilibrium (dm/dt < 0.02%) with different temperatures (30 °C,40 °C,50 °C) at RH=90%.

Based on theory and practice,it shows that whenP/P0is in the range of 5% to 35%,the BET equation is consistent with the actual adsorption process.Hence,P/P0=10%,20%,30% were the selected points in this work and the results could be found in Fig.11.With a series of calculations,Vmwas equal to 227.5 cm3·g-1,BET constant C was 14.94,SSA result was 641.8403 m2·g-1and regressionsquared of line fitting was 99.929%.Surface area was also gained(632.126 m2·g-1) and regression coefficient was 99.9%.In general,the specific surface area of cement and concrete is around 350 m2·g-1.SSA of LiBr microcapsules was larger than general pavement materials.In theory,the larger SSA of the material,the faster the absorption rate,our microcapsules have good water sorption ability.

3.5.Kinetic analysis

The main method of adsorption kinetic analysis is to deform the kinetic equation and fit the relevant data linearly.The pseudo-first order model is based on the assumption that adsorption is controlled by diffusion steps and it is mainly a physical adsorption process.When the pseudo-first order kinetics(Eq.(5)) were correlated according to adsorption kinetics,twas taken as the abscissa and lg(qe-qt) as the ordinate,andk1andqewere obtained from slope and intercept calculations.The pseudo-second order kinetic model assumes that the adsorption rate is determined by the square value of the number of unoccupied adsorption vacancies on the adsorbent surface and the adsorption process is controlled by the chemical adsorption mechanism,this chemisorption involves electron sharing or electron transfer between adsorbents and adsorbates.Similarly,it tooktas thex-coordinate,t/qtas they-coordinate,and it could attain the slope and the intercept,in whichqeandk2were calculated,while the pseudo-second order equation (Eq.(6)) was analyzed.Equally,if you plott0.5on thex-coordinate,andqton they-coordinate,you will computekiandCfrom the slope and intercept,and thus determine whether the adsorption process is governed by the intra-particle diffusion (Eq.(7)).After a series of calculations and fitting,the data processing results of water vapor adsorption under 30 °C were shown in Fig.12,Fig.13,Fig.14 and Table 3,Table 4,Table 5.Based on the range of human living humidity and the range of urban humidity after rainfall,the range of humidity we fit was 40%–90%.

Table 3Results of pseudo-first-order kinetic model

Table 4Results of pseudo-second-order kinetic model

Table 5Results of intraparticle diffusion kinetics model

Fig.11.BET fitted linear graph for SSA.

Fig.12.Fitting the linear graph based on pseudo-first-order kinetic model.

Fig.13.Fitting the linear graph based on pseudo-second -order kinetic model.

Fig.14.Fitting the linear graph based on intraparticle diffusion kinetics model.

According to the results,the linear correlation coefficientRof the pseudo-first-order adsorption rate equation for SiO2-LiBr microcapsules adsorbing water vapor was greater than 0.9 except RH=60%,butR2is only above 0.85.Meanwhile,there was a certain gap between the calculated theoretical equilibrium adsorption capacity (qe) and the actual equilibrium adsorption capacity obtained by experiments.Moreover,the linear correlation coefficientsRfitted by pseudo-second-order adsorption rate equation was lower than 0.71,and the value ofR2does not reach 0.6,which showed that the experimental results were inconsistent with the pseudo-second-order adsorption and were not mainly controlled by chemical adsorption.

The Weber-Morris intra-particle diffusion model was also used to analyze the control steps in the process of water vapor adsorption by SiO2-LiBr microcapsules,and the adsorption mechanism was explored.Based on the fitting results,the adsorption process of water vapor by microcapsules at 30 °C was in good agreement with the intraparticle diffusion model,in which all ofR2were larger than 0.95 andRwere better than 0.97,especiallyR2of fitting results was larger than 0.99 at RH=90%.It could also be seen that the fitting lines at different relative humidity were all nearly straight lines without origin,which meant that intraparticle diffusion was the main rate-limiting step in the adsorption process,but it was not the only rate-controlling factor.Combining the analysis of pseudo-first-order adsorption and pseudo-second-order adsorption results,the adsorption process was mainly based on the intraparticle diffusion mechanism and supplemented by the pseudofirst-order adsorption and pseudo-second-order adsorption process.It included that particle diffusion to microcapsules surface,intraparticle diffusion,adsorption and desorption between active sites and water,and diffusion of water vapor to surface of microcapsules.Furthermore,the adsorption rate constantskiwere 5.68588,6.35153,6.35396,7.90432,9.07057 and 13.90784 mg·g-1-·min-1/2,respectively.It could be concluded that the adsorption process of microcapules for water vapor adsorption has endothermic properties.

4.Conclusions

This study proposes a novel kind of SiO2-LiBr microcapsules for adding into pavement materials as water-retaining pavement materials for mitigating the urban heat island effect.A laboratory study was aimed at manufacturing novel SiO2-LiBr microcapsules and investigating the characteristic properties of this type of microcapsule.To evaluate the performance of these microcapsules,the thermal performance,water vapor adsorption and desorption were researched at the same time.Based on above experimental data,adsorption kinetics were studied.Hence,the following conclusions could be obtained: The SiO2-LiBr microcapsules were spherical and contained multiple LiBr cores in the SiO2shell,which was discovered by SEM and FTIR microscopy.The average size of microcapsules was (243 ± 1) μm and the size of some submicrocapsules was around 10 μm.The value of thermal diffusivityand thermal conductivity were small,which meant microcapsules have very low sensitivity to temperature and have great potential to optimize the performance of pavement materials,prevent rutting and lowering the ambient temperature.And most importantly the maximum mass change (103.9%) occurred when the relative humidity was 90% at 30 °C.Meanwhile,hysteresis between sorption and desorption of change in mass was small,all of which was smaller than 5% and most of which was around 1% along with changed RH.They also exhibited good water absorption at human habitation temperatures.Moreover,SSA result was 641.8403 m2·g-1,which was larger than general pavement materials.Through the study of adsorption kinetics,our adsorption process was mainly based on the intra-particle diffusion mechanism and supplemented by the pseudo-first-order adsorption process.In conclusion,the prepared novel microcapsules have good water absorption and desorption properties,superior specific surface area and low thermal performance,which has great potential to weaken the urban heat island effect and reduce rutting after being applied to water-retaining pavement materials.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to acknowledge the financial support of The National Scholarship Foundation of China,China Scholarship Council ([2018]3101).