57Ecl. Quím., São Paulo, 29(2): 53-56, 2004

Volume 29, número 2, 2004www.scielo.br/eq

Thermal behavior of alginic acid and its sodium salt

J. P. Soares1, J. E. Santos1,2, G. O. Chierice3, E. T. G. Cavalheiro3

1 Departamento de Química - UFSCar - 13565-905 - São Carlos – SP - Brasil2 Departamento de Ciências Exatas e Natureza - UFCG - 58900-000 - Cajazeiras – PB - Brasil

3 Departamento de Química e Física Molecular - IQSC - USP 13560-970 - São Carlos – SP - Brasil

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Abstract: An evaluation of hydration and thermal decomposition of HAlg and its sodium salt isdescribed using thermogravimetry (TG) and differential scanning calorimetry (DSC). TG curves inN2 and air, were obtained for alginic acid showed two decomposition steps attributed to loss of waterand polymer decomposition respectively. The sodium alginate decomposed in three steps. The firstattributed to water loss, followed by the formation of a carbonaceous residue and finally the Na2CO3.DSC curves presented peaks in agreement with the TG data. In the IR alginic acid presented bands at1730 and 1631 cm-1, while sodium alginate presented a doublet at 1614 e 1431 cm-1, evidencing thepresence of salified carboxyl groups.

Keywords: alginic acid; thermogravimetry; differential scanning calorimetry._________________________________________________________________________________

IntroductionThe HAlg (Figure 1) is a copolymer

derived from the 1,4-linked-b-D-mannuronic (M)and a-L-guluronic (G) acids [1], containingcarboxylic groups in their structures that definethe adsorption capacity for metals [2,3].

Figure 1. Chemical structures of components of the alginicacid.

According to Haug et al [4] and Aspinall[5] in alginates both residues can be combined toform homopolymer blocks (M-block, G-block) orcopolymer blocks (MG-block) with several

possible polymers. Each seaweed can present aspecific proportion of M and G units, with aspecific alginic acid from each species.

The biological properties ofpolysaccharides, especially of the algínates, havebeen explored since many decades in countlessmedical and surgical applications. Thoseproperties, and the solubility in water, are due tothe presence of inorganic ions in the alginatestructure6.

Alginic acid form water-soluble salts withmonovalent cations but is precipitated uponacidification. Alginates of many bivalent cations,particularly of Ca2+, Sr2+ and Ba2+, are insoluble inwater and can be prepared when sodium ions ofNaAlg are replaced by di- and trivalent cations.This property is used in the isolation of alginicacid from algae [7,8].

Due to their physical and chemicalproperties HAlg and sodium alginate (NaAlg),have widely been used in food processing, medicaland pharmaceutical industries [9] such as drugcarrier [5], moreover, sodium alginate has been

58 Ecl. Quím., São Paulo, 29(2): 53-56, 2004

employed in the preparation of gels for the deliveryof biomolecules such as drugs, peptides andproteins [10].

Toxicological data showed that alginatesare safe when used in food. HAlg and itsderivatives acts as stabilizers and thickenersfacilitating the dissolution and improving viscosityof the ingredients preventing the formation ofcrystals that will prejudice the appearance andhomogeneity, mainly in frozen products [11].

Nakamura et al. [12] investigated thethermal properties of water insoluble alginate filmscontaining di and trivalent cations. The resultsindicated that the alginates form compactstructures when the ionic radii of the cation islower. Changes in the film structure during ionicexchange were studied on the basis of its glasstransition temperature (Tg) and heat capacity usingdifferential scanning calorimetry (DSC).

Hatakeyama et al. [13] determined thenon-freezing water contents of mono and divalentcation salts of some polyelectrolites. The numberof water molecules strongly bounded to the cationsinfluences remarkably the polyelectrolite structure.

Said et al. [14] investigated the thermaland electrical properties of silver, selenium anduranyl alginates using thermogravimetry anddifferential scanning calorimetry, concluding thatmetal oxalates were formed as decompositionintermediate.

Considering the importance of thispolysaccharide and its derivatives and the lackingin information about its thermal behavior, thepresent work describes the purification,characterization and the thermal decomposition ofthe alginic acid and sodium alginate.

The thermal analysis is also importantconsidering the fact that in industrial processes heatis commonly employed and the thermaldecomposition data yields information regardingthe stability and residues of decomposition. Watercontent is another important parameter that canbe extracted from thermal measurements.

ExperimentalReagents and solutions

All the reagents were of analytical gradeand used without further purification, except whenindicated.

Purification of the HAlgAlginic acid (Aldrich) was purified by

dissolving 5 g in 0.5 mol L-1 NaOH and stirringfor two h with a magnetic stirrer. After that 0.5mol L-1 HCl was added slowly until pH 2-3 andstirred for 24 h. The resulting gel suspension wasfiltered in a quantitative filter paper and washedwith ethanol. The resulting solid was then dried ina vacuum oven at 40 °C for 24 h. This purificationprocedure was performed since the commercialreagent presented sodium residue when submittedto thermogravimetric analysis and theconductimetric titration suggested that somecarboxyl groups were not free.

Complete dehydration of both acid formand sodium salt was achieved only whether thecompounds were used just after drying in avacuum oven.

Conductimetric titrationConductimetric titrations were performed

by suspending 300 mg of alginic acid in 30.0 mLof water. 10.0 mL of 0.0969 mol L-1 HCl solutionwere added to this solution. The excess of HClwas titrated with 0.505 mol L-1 NaOH in athermostated conductimetric cell at 25.0 ± 0.1 °Ckept with a MA 184/6 thermostatic bath (Marconi).

Thermal analysisThermogravimetry (TG) measurements

were performed in a TGA-2050 thermogravimetricmodulus coupled to a TGA-2100 thermal analyser(both from TA Instruments) using sample mass ofca. 7 mg and an alumina sample holder at heatingrates of 5, 10 and 20 °C min-1 under a dynamic airand nitrogen atmospheres flowing at 90 mL min-

1. Humidity and ash contents were determined bymeans of two determinations, with the help of thefirst derivative thermogravimetry (DTG) dataunder atmospheric pressure.

Differential scanning calorimetry (DSC)measurements were performed in a DSC-910modulus coupled to a TGA-2100 thermal analyser(both from TA Instruments) using sample mass of4,0 mg in a covered aluminum sample holder witha central pin hole at heating rates of 5, 10 and 20°C min-1 under dynamic air and nitrogenatmosphere, flowing at 90 mL min-1 underatmospheric pressure.

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Elemental analysis, IR and 1H NMR spectroscopyThe C and H contents in the samples were

determined in a EA1108-CHN-S elementalanalyser (Fisons). IR spectra were recorded in KBrpellets from 4000 - 400 cm-1 in a MB102 FT-IR(Bomen-Michelson). The 1H NMR spectra wererecorded in a BZH 200/52 spectrometer (Brüker).

Results and discussionCharacterization of the HAlg and NaAlg

The elemental analysis of the samples arein agreement with the general formula (C6H8O6)nfor HAlg and (C6H7O6Na)n, only after completedehydration. Dry material was obtained after 24 hunder vacuum at 40 °C and the elemental analysisbeing performed just after removing from the ovenand keeping in a dessicator under vacuum. If thecompounds are manipulated at atmospheric air,they rapidly re-hydrate by absorbing water fromthe environment.

IR spectra obtained for the HAlg andNaAlg in the 2000 – 800 cm-1 range, presented thestretching of the C=O group of the protonatedcarboxylic group at 1730 and 1631 cm-1. Whenthe proton is displaced by a sodium ion, the peaksare observed at 1614 and 1431 cm-1 and they areassigned to the symmetric and antisymmetric COO-

stretching vibrations of the free carboxyl group asobserved by Huang et al. [15].

Yeom [16] reported similar results whenmeasuring the alginate content in films using IR.According to Chandia [17], the alginates FT-IRspectra present bands around 800 e 700 cm-1,relative to mannuronic and guluronic acidsrespectively.

The main difference of the 1H NMRspectra of acidic form is the singlet signal at 5.04ppm assigned to carboxylic proton, whichdisappeared in the sodium salt spectra, accordingto the literature [17]. The spectra are alsoindependent on the amount of G and M monomerssince they are structural isomers.

Conductimetric titrations of excess HCladded to the alginic acid allowed to thedetermination of the number of acidic sites in thepolymeric matrix. This number was determined inthe range of 0.87 ± 0.07 (n = 3) and 0.99 ± 0.02mol (n = 3), for carboxylic groups per mol ofcommercial and purified alginic acid, respectively.

Thermal analysis of HAlg and NaAlgInitially, a study of the influence of

heating rate in the thermal behavior of HAlg andNaAlg was performed under both air and N2atmospheres. Since every synthetic or naturalpolymer containing hydrophilic groups usuallypresents strong interaction with water, thehumidity content may influences their propertiesand is an important characteristic to be determined[13].

Thermogravimetry is a technique that canyield information about humidity and ash contentsin a simple and fast way, but factors such asheating rate present a severe influence on theresolution of the TG curve [18,19] and thedefinition of this parameter is fundamental for athermogravimetric investigation.

Table 1 presents the percentage forhumidity and decomposition mass losses,temperature ranges and residues for the alginicacid and sodium alginate at 5.0; 10.0 and 20.0 °Cmin-1 heating rates in both air and N2 atmospheresas well as the related DSC peaks.

The TG curves presented in Figure 2suggest that, under heating, the alginic acidpresents initially a dehydration process followedby decomposition in two overlapping steps undernitrogen (Figure 2a), without residue at the endof the experiment. Similar behavior was observedin air (Figure 2b). The decomposition productaround 400 °C was characterized as acarbonaceous material in both cases.

Figure 2. TG (solid)/DTG(dashed) curves of alginic acid inN2 (a) and (b) under air flow of 90 mL min-1 in different heatingrates.

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Table 1. Thermal decomposition data of the HAlg and NaAlg under N2 and air atmosphere

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Figure 3 shows the TG curves for thesodium alginate. According to such a results, thesalt decomposed by dehydration followed bydegradation to Na2CO3 and a carbonized materialthat decomposes slowly from 600-750 °C in N2(Figure 3a). According to Newkirk [20] theNa2CO3 decomposition is dependent of the sampleholder and atmosphere used. In this work suchdecomposition appeared above 800 °C inagreement with the findings of those author usingPt crucible and N2 atmosphere. Under air (Figure3b) the dehydration occurs in a similar way, butthe decomposition of the carbonized material isfaster, around 600 °C, resulting in Na2CO3 thatdecomposes as described in N2. The presence ofNa2CO3 and carbonized residue was confirmed byheating the compound up to 550 °C, in an ovenunder the respective atmosphere and addition ofHCl to the degradation product. A vigorousliberation of CO2 was observed while a darkinsoluble residue remained in the test tube, in bothcases. If the residue percentage is correct for thewater content values close to the expected 26.75%for ½ Na2CO3 for each C6H7O6Na unit can befound.

Figure 3. TG (solid)/DTG(dashed) curves of sodium alginateunder N2 (a) and air (b) under flow of 90 mL min-1 in differentheating rates.

The presence of the metallic oxalatesinstead of carbonates as decompositionintermediates is a difference from the resultsobserved by Said et al. [15] for alginates of Ag+,Se4+ and UO2

2+, in relation to the sodium salt.As presented in Figures 2 and 3, the TG

general profiles for each compound did not changemarkedly with these heating rates. It is possible to

observe from the data in Table 1 that the humidityvalues changed depending on the heating ratewhich can be related with the influence of theheating rates on the resolution of the dehydrationstep [18,19], as observed in the DTG curves. Theash contents are not severely affected by theheating rate. Similar results were obtained in allthe scan rates, but at 20°C min-1 some overlap ofthe thermal events can occur. So it was concludedthat at 10°C min-1 a better compromise betweenresolution and time of analysis can be reached andthis was the heating rate chosen for the furtheranalysis.

Figures 4 and 5 show the DSC curves,showing the heating rate influence on the thermalbehavior of HAlg and NaAlg under both air andN2 atmospheres, respectively.

Under N2 (Figure 4a), HAlg decomposedshowing three overlapping peaks. The firstendothermic peak suggests the dehydrationprocess at around 80 °C. Then, an endothermicpeak close to 200 followed by an exothermic peakat ca. 240 °C, respectively, has appeared. Suchprocesses were attributed to the decomposition ofthe polymer resulting in the carbonized residue,in agreement with TG results.

Figure 4b shows the thermal behavior ofthe HAlg in air atmosphere. The endothermic peak,centered between 75 and 96 °C depending on theheating rate, may be attributed to the dehydrationprocess. The small endothermic process at 190 °Cfollowed by a strong exothermic peak at c.a. 300°C, correspond to the decomposition of thebiopolymer. The TG/DTG data are in agreementwith this assumption.

Figure 4 - DSC curves of alginic acid under N2 (a) and underair (b) flow of 90 mL min-1 in different heating rates.

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and 340-410 °C (depending on the heating rate)may be attributed to the decomposition of thebiopolymer and formation of the respectivecarbonate, which is in agreement with TG curves.

ConclusionAccording to the present results it can be

concluded that the heating rate do not presentedsignificant influence on the TG and DSC profilesfrom 5 – 20 °C min-1. This suggests that higherheating rates can be used with gain in experimentaltime.

The alginic acid decomposed in twosuccessive steps after dehydration under both airand nitrogen atmospheres, without residue in thesample holder at 700 °C. The sodium alginatedecomposes also in two steps after the dehydrationprocess. The decomposition leads to a Na2CO3residue, the decomposes above 750 °C undernitrogen, due probably to the presence ofcarbonaceous residue and is stable under airatmosphere where the carbonaceous residue istotally decomposed above 600 °C. thesedifferences in the Na2CO3 behavior have beenalready described by Newkirk [20].

Recebido em 30/04/04Aceito em 21/07/04

The DSC curves of NaAlg under N2 arepresented in Figure 5.a. The dehydration wasevidenced by an endothermic peak close to 100°C, then the decomposition of the biopolymer takesplace represented by an exothermic peak at c.a.240-260, depending on the heating rate. Finallythe decomposition of the carbonaceous materialoccurred above 300 °C.

Figure 5. DSC curves the sodium alginate under N2 (a) andunder air (b) flow of 90 mL min-1 in different heating rates.

Under air (Figure 5b), the NaAlgpresented a first endothermic peak at 100 °Cattributed to the water release. The second andthird exothermic peaks with a maximum at 250

J. P. Soares, J. E. Santos, G. O. Chierice, E. T. G. Cavalheiro. Comportamento térmico do ácido algínicoe do seu sal de sódio.

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Resumo: Uma avaliação da hidratação e de decomposição térmica do HAlg e seu sal de sódio foirealizada usando termogravimetria (TG) e a calorimetria exploratória diferencial (DSC). As curvas TGem N2 e ar, para o ácido algínico apresentaram duas etapas de perda de massa correspondendo à saídade água e posterior decomposição do polímero. Já para o alginato de sódio, a decomposição térmicaocorreu em três etapas. A primeira corresponde à liberação de água, seguida da formação de resíduocarbonizado e formação do carbonato de sódio. As curvas DSC concordam com os eventos observadosem TG. Na região do infravermelho, o ácido algínico apresentou bandas em 1730 e 1631 cm-1, enquantoo alginato de sódio apresentou duas bandas em 1614 e 1431 cm-1, evidenciando a presença de gruposcarboxílicos salificados.

Palavras-chave: Ácido algínico; termogravimetria; calorimetria exploratória diferencial.___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

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