Hongying Hao ,Yadong Zhang *,Xiaoya Chen

1 School of Chemical Engineering and Energy,Zhengzhou University,Zhengzhou 450001,China

2 Medical School,HuangHe Science and Technology College,Zhengzhou 450063,China

1.Introduction

The sodium hydroxyethyl sulfonate(2-hydroxyethanesulfonic acid sodium salt,to abbreviate it into SHES,CASRN 1562-00-1)is an important organic intermediate[1–3].The chemical formula of SHES is C2H5NaO4S,and the relative molecular mass is 148.11 g·mol?1.SHES is easily dissolve in water,but hardly dissolve in ethanol,acetone or other organic solvents.Different researchers[4–9]have studied the synthesis and pharmacological effects of SHES.However,its thermostability has not been reported.Therefore,we have undertaken the investigation of the thermal decomposition process and the thermodynamic properties,such as specific heat capacity and thermal stability.These thermodynamic data may be helpful for further study and application of SHES.

2.Experimental

2.1.Materials and experiment apparatus

α-Al2O3was used as standard material in the thermal analysis process,provided by Shimadzu Company,Japan.Benzoic acid and ethanol used in the experiment were of AR grade with mass fraction purity of over 99.5%,purchased from Shanghai Chemical Reagent Company.SHES(mass fraction purity of 98%)was purchased from Aladdin Chemistry Co.Ltd.(Shanghai),China.The TG–DTA analyzer(type DTG-60,Shimadzu Company,Japan)was used to determine the TG–DTA curves of the sample.The differential scanning calorimetry(type DSC-60,Shimadzu Company in Japan)was used to determine the melting temperature and melting enthalpy of the sample.SPN-500 nitrogen generator(the HP analysis technology research institute in Beijing,China)provided nitrogen atmosphere for the experiment of thermal analysis.Thermo FT-IR200(Thermo Electron Corporation in America)was used to analyze the structure of the intermediate and final product.Elemental analyzer FLASHEA 1112(Thermo Electron Corporation in America)was used to determine the purity of the sample.

2.2.Puri fication of SHES

SHES was purified by recrystallized method three times at least in ethanol in order to obtain the content of more than 99%,and then placed in oven at383.15 K until it was dried.The dried sample was analyzed by elemental analyzer to determine the purity of sample.The elemental analysis results of SHES in Table 1 show that there was an excellent agreement between the theoretical and experimental values.

2.3.The determination procedure of melting temperature and melting enthalpy

In order to determine the melting temperature and melting enthalpy,the samples were cooled to room temperature when it was melted for the first time,and then reheated at a rate of 10 K·min?1until complete melting,or quenched to 20 K and reheated at 10 K·min?1to a temperature higher than the melting temperature.The method of melting temperature was used for SHES,which is described in detail in ISO 11357-3:2011[10].The melting temperature was obtained by multiple measurement and averaging.

Table 1Elemental analysis of SHES(C2H5NaO4S)

2.4.The speci fic heat measurement principle

The measuring methods of the specific heat capacity from DSC are described in the literature[11–15].The relationship between the specific heat capacity and heat flow rate at constant pressure can be expressed by Eq.(1)[16].wheremis the mass of the tested sample,g;Cpis the specific heat capacity,J·K?1·g?1,subscriptpindicates an isobaric process;Tis Kelvin temperature,K;Qis the quantity of heat.

whereCp(s)andCp(st)are the constant-pressure specific heat capacities of both a tested sample and a calibration material,α-Al2O3is used as the calibration material and the values ofCp(st)are extracted from ISO 11357-4[16];WsandWstare the mass of the tested sample and α-Al2O3,respectively;Dsis the vertical displacement between the empty crucible and the test sample on DSC thermal curves at a given temperature;Dstis the same asDswhich is between the empty crucible and α-Al2O3.

Fig.1.The TG curves of SHES at different rate under nitrogen atmosphere.β/K·min?1:

Fig.2.The TG–DTA curves of SHES at heating rate of 16 K·min?1 under nitrogen atmosphere.

3.Results and Discussion

3.1.Thermal decomposition behavior of sodium isethionate

TG and DTA curves of sodium isethionate sample are shown in Figs.1 and 2.The results indicate that the process of thermal decomposition has three stages(1–3 stages).The signature of them can be divided successively into the first endothermic peak,the second endothermic peak,and the third endothermic peak.The total mass loss(mass%)of decomposition was less than 51.98%as the maximum temperature was lower than 1123.15 K.The zero endothermic peak is a melting process between 463.14 K and 468.23 K so that the onset melting temperature of the SHES sample could be considered as 465.41 K,the melting temperature presented in literature was 465.15–467.15 K[20],so the experimental value of the melting point determined by DSC was reliable.The melting enthalpy was 25.69 kJ·mol?1calculated by TA-60WS workstation.The result is consistent with DSC in Fig.3.

Fig.3.The DSC curves of SHES under nitrogen atmosphere.

Table 2The basic results based on the TG–DTA curves

Fig.4.FT-IR spectrum of the decomposition products.(a):the decomposition product A at the first stage.(b):the decomposition product B at the second stage.(c):the decomposition product C at the third stage.

From Table 2,Figs.2 and 4,the results show that the thermal behavior is as follows:

In the first stage between 586.15 K and 618.07 K,the experimental data of mass loss is 15.56%,and there is an obvious endothermic peak.The infrared spectrum of the residue A is shown in Fig.4(a).FTIR data(νmax,cm?1):ν(O--H)=3436,β(O--H)=1386,ν(?CH2)=2924,ν(C--S)=627,and the characteristic peaks of sulfonate are 1123 and 967[12].The results demonstrate that the most possible group loss in this stage may be CH3CH2OH.On the other hand,the mass loss of the group CH3CH2OH in thermal decomposition process is 15.53%.Compared the experimental value with the theoretical value,the relative error is 0.19%.

In the second stage,there is an endothermic peak.The theoretical mass loss of the sample is 14.85%,and the mass loss of the group CH3CHO in thermal decomposition process is 15.39%.Compared experimental value with the theoretical value,the relative error of the experimental value is 3.64%.The infrared spectrum of the residue B is in Fig.4(b).FTIR data:the characteristic peaks of--SO3Na are 1192 and 1044 cm?1.

In the third stage,the most possible lose group is SO2because the lost mass in the stage is 21.41%in accordance with the lost mass(21.60%in theory)of SO2.The result demonstrates that the last residue substance may be Na2SO4.The infrared spectrum of the residue C is shown in Fig.4(c).FTIR data(cm?1):1126 and 621(SO42?)are in good agreement with the standard sodium sulfate.

3.2.The constant-pressure speci fic heat capacity of the sample

The accuracy of experimental results may be influenced by several material parameters.In order to eliminate the influence of the factors and obtain precise measurements,the specific heat capacity of α-Al2O3was determined firstly,and compared with the literature values of α-Al2O3[5,16].The results are shown in Table 3.The relative error between the measured values and the literature values were within 1%.The relationship between the specific heat capacity and temperature of α-Al2O3was obtained with the least squaremethod and represented by Eq.(3),where the linear correlation coefficient(R2)was 0.9959.

Table 3Experimental values and the literature values of specific heat capacity of α-Al2O3

Table 4Experimental values and the literature values of specific heat capacity of benzoic acid

In order to further verify the reliability of the method,the specific heat capacity of benzoic acid was measured by DSC[17],and the results are expressed in Table 4.The relationship between the specific heat capacity and temperature of benzoic acid could be expressed by Eq.(4).The linear correlation coefficient(R2)of Eq.(4)was 0.9998.

The constant-pressure specific heat capacity of SHES is expressed i

n Table 5 and Fig.5.The relationship between the specific heat capacity and temperature at solid phase state could be derived using the least square method and be represented by Eq.(5).The linear correlation coefficient(R2)of Eq.(4)was 0.9949.

Compared the values calculated by Eqs.(3)and(4)with the literature values[21]examined the relative deviationRDand average relative deviationARDare listed in Tables 3 and 4.And average relative deviation is defined as:

The results show thatARDis respectively 0.53 and 0.94 for α-Al2O3and benzoic acid and there are good agreement between the experimental and calculated results for the specific heat capacity.

Table 5Experimental values of specific heat capacity for SHES

Fig.5.The specific heat capacity of SHES.

4.Conclusions

From the thermal analysis of the differential thermal analysis(DTA)and thermogravimetric analysis(TGA),the decomposition process of SHES was divided into three stages:(1)the mass loss in the first stage and the second stage may be CH3CH2OH and CH3CHO;(2)the third stage may lose SO2.The residue sample of every stage was analyzed by FT-IR,and the results of FT-IR match well with the predicted structure.In addition,the thermodynamic data,the specific heat capacity,melting temperature,and melting enthalpy of SHES were determined using differential scanning calorimetry(DSC).The relationship between the specific heat capacity and temperature was obtained at the temperature range of 310.15–365.15 K.The equation wasCp=48.41118 ?0.43356T+0.00131T2?1.30619×10?6T3.The melting temperature and melting enthalpy were obtained to be 465.41 K and 25.69 kJ·mol?1,respectively.

Nomenclature

ARDThe average relative deviation

CpThe constant-pressure specific heat capacity,J·K?1·g?1

DThe signal value of DSC for sample,mW

mMass of the tested sample,kg

QQuantity of heat,J·mol?1

RDThe relative deviation

TAbsolute temperature,K

β Heating rate,K·s?1

λ Wavelength,nm

Subscripts

iSerial number

m Material

p Constant pressure

s Sample

st Standard sample

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