Thermal degradation of polysaccharide


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Thermal degradation of polysaccharide

  1. 1. IX Radiation Physics & Protection Conference, 15-19 November 2008, Nasr City - Cairo, Egypt Modifications of Viscoelastic Properties of Polysaccharides by Gamma Irradiation F. Mihai1, V. Tripadus1, M. R. Nemtanu2 and D. C. Negut1 1 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 2 National Institute for Laser, Plasma and Radiation Physics, Bucharest, Romania E-mail: ABSTRACT The aim of this work was to establish the effect of gamma irradiation on the viscoelastic properties of the sodium alginate. Aqueous suspensions of sodium alginate at different concentrations (0.25 – 4%) were irradiated using a 60Co gamma-ray source (10, 25 and 50 kGy). The monitored rheological parameters showed the non-Newtonian behavior of the samples is kept by gamma irradiation. The decrease tendency of the apparent viscosity by irradiation samples and with decrease of the concentration as well has been noticed. Key Words: Sodium alginate/ Rheological parameters/ Irradiation dose INTRODUCTION Polysaccharides are complex compounds of carbohydrates that play a crucial role in many biological processes like determination of sanguine human blood group types or immune response of the organism in tumor growth. At the same time, they are widely used in the food industry as functional ingredients, the most important application of these compounds being related to the rheological control of aqueous solutions (1, 2). Due to their different primary chemical structures that determine the shapes in aqueous systems, polysaccharides can exhibit large differences of solubility. The knowledge of the polysaccharide molecular shapes and dynamics is essential for the understanding and control of rheological properties in food applications (2), mainly many polysaccharides perform as simple thickeners. At that rate, the polysaccharide molecules exist as fluctuating disordered chains, random coils. All polysaccharides exhibit the same behavior of viscosity as a function of concentration. A polysaccharide solution is a complex system whose rheological properties are correlated to the polysaccharide type, its concentration and temperature of the system as well (4, 5). Polymers may be degraded by acidic hydrolysis or enzymatic treatment. The polysaccharide degradation by gamma or ultraviolet irradiation is free of initiators, and, on the other hand, the obtained products are chemically pure. Therefore, they are simpler and more environmentally friendly than conventional ones (6). The present paper reveals our last results concerning the effect of gamma irradiation on the rheological properties of sodium alginate. Thus, we carried out the rheological parameter measurements on control and irradiated sodium alginate aqueous solution of different concentrations. A modern theoretical rheological model was applied in order to describe the flow behavior of the studied samples. 261
  2. 2. IX Radiation Physics & Protection Conference, 15-19 November 2008, Nasr City - Cairo, Egypt MATERIALS and METHODS 1. Sodium alginate. Chemical structure Alginates are salts of the long-chain carbohydrate biopolymer alginic acid (7). Alginic acid is a copolymer derived from the 1,4 linked-b-D-mannuronic (M) and α-L-guluronic (G) acids (8), containing carboxylic groups in their structures which define the adsorption capacity for metals (9, 10). They are linear polymers (Fig.1) which contain homopolymeric COOH O O OH HO OH HO O O COOH D - mannuronic acid L - guluronic acid n (a) (b) Fig. 1. Chemical structures of (a) alginate monomers and (b) conformational alginate chain. sequences of D-mannuronate and L-galuronate with regions in which the two sugars alternate. Alginic acid itself is insoluble, but its salts are hydrocolloids. The hydration of polysaccharides leads to a physical system that comprises two components: the solvent and biopolymer. A fraction of the water molecules could be more or less bonded to the polymer chains while another fraction is composed from more free water molecules. Alginates are generally acid stable and heat resistant. Sodium alginate has been employed in the preparation of gels for delivery of biomolecules such drugs, peptides and proteins (11). 262
  3. 3. IX Radiation Physics & Protection Conference, 15-19 November 2008, Nasr City - Cairo, Egypt 2. Sample preparation and irradiation Sodium alginate (Alfa Aesar A. Johnson Mathey Company) used in our experiments was dissolved at different concentrations (0.25 – 4%) in distilled water by mechanical stirring at room temperature. Irradiations were carried out using a SVST Co – 60B gamma irradiator at dose rate of 31 kGy/h. The absorbed doses were measured with an ECB dosimetric system by oscillometric method using a radelkisz OK/303 oscillotitreztor. The uncertainty for absorbed dose measurements was about 3% (1σ). All irradiation dose values are expressed as absorbed dose in water. 3. Reoviscosimetric measurement technique The rheoviscosimetric tests of sodium alginate aqueous suspensions samples were performed with a rotational HAAKE VT® 550 viscometer at different shear rates (0 – 2,164 s-1) and 25o C. The obtained data were analyzed with RheoWin v.3.5 software. The reported data are the mean of two different measurements with standard deviation below 10%. RESULTS and DISCUSSION The studied control samples indicated a slight non-Newtonian behavior, while the shear stress dependence of shear rate indicates the pseudoplastic character of the alginate suspensions (Fig. 2). This aspect was kept for the irradiated samples, even at high irradiation dose, but with a trend towards Newtonian behavior. Fig. 2. Shear stress evolution with shear rate, at different alginate concentrations of aqueous solutions. In order to fit the experimental data and thus to describe the rheological behavior of the studied samples, we chose one of the most used and simplest rheological models in the known and applied ones for biopolymer rheo-analysis, the Ostwald de Waele model (eq. 1): 263
  4. 4. IX Radiation Physics & Protection Conference, 15-19 November 2008, Nasr City - Cairo, Egypt τ = k ⋅ γ& or η a = k ⋅ γ&−1 n n (1) where: τ [Pa] is the shear stress, γ& [s-1] – shear rate, ηa [mPa·s] – apparent viscosity, k [mPa·sn] – consistency index, and n – flow index. This model appeared to be suitable for describing the sample flow behavior as proved by the level of the determination coefficient that had values ranging from 0.8845 to 0.9996. The pseudoplastic character proved by the investigated control samples is in agreement with the literature reports (3), Fig. 3. Apparent viscosity of aqueous alginate solutions vs. concentration. and the apparent viscosity (25o C, γ& = 250 s-1) is an increasing function of the alginate solution concentration (Fig. 3). This dependence is a linear function of concentration having a breaking point connected with the transition point concentration, from dilute solution behavior (low degree of coil molecule overlap) to concentrated solution behavior (total interpenetration of random coil molecules) (2). In our case this break point may be considered to be 0.5%. Above this point, the slope of the curve increased due to high degree interaction between polysaccharides coils. The apparent viscosity (25o C, γ& = 250 s-1) of the alginate solutions showed exponential decreasing evolution as the increase of the irradiation dose (Fig.4) for all studied concentrations. This kind of behavior, generally for polymers, indicates the fragmentation of the macromolecular structure to lower molecular weight ones mainly at 10 kGy and 25 kGy. 264
  5. 5. IX Radiation Physics & Protection Conference, 15-19 November 2008, Nasr City - Cairo, Egypt Fig. 4. Apparent viscosity of aqueous alginate solutions of different concentrations before and after irradiation. The consistency index is one of the typical parameters used in the Ostwald de Waele model. In our study, this index showed a decrease of its value with the increase of irradiation dose (Fig. 5) in a same manner for all concentrations. The consistency index can be obviously correlated with the apparent viscosity that also decreased by irradiation as we described above. Fig. 5. Evolution of the consistency index with concentration and irradiation dose. 265
  6. 6. IX Radiation Physics & Protection Conference, 15-19 November 2008, Nasr City - Cairo, Egypt The viscoelastic properties of 1% and 4% irradiated at 10 and 25 kGy suggest the macromolecular chain of sodium alginate is broken by irradiation as the most polymers suffer. However, at 50 kGy the sample viscosity decreased in comparison to the control sample, but it is slightly higher than samples irradiated at 25 kGy. Similar aspect was reported recently by Mollah et al. (6) for sodium alginate solution of 3% exposed to gamma radiation. For lower concentrations (0.25%, 0.50%) of sodium alginate, a drastic reduction of the apparent viscosity value appeared from lower irradiation doses (e.g., 10 kGy). CONCLUSION The investigation of the rheological behavior of the irradiated aqueous sodium alginate solutions revealed the influence of irradiation on the rheological parameters. The apparent viscosity values for all studied concentrations decreased by irradiation. This aspect suggests a depolymerization phenomenon of the aqueous sodium alginate solutions. Our study contributes to the knowledge of the viscoelastic properties of irradiated sodium alginate as aqueous solution, with application for food, agriculture and medical products. REFERENCES (1) H. Douglas Goff; Pure&Appl. Chem.; 67, 1801 (1995). (2) Iain C.M. Dea; Pure & Appl. Chem.; 61, 1315 (1989). (3) W. Sabra and W.D. Deckwer; “Alginate – A Polysaccharide of Industrial Interest and Diverse Biological Functions, in “Polysaccharides: Structural Diversity and Functional Versatility”, S. Dumitru (ed.), 2nd Ed., Marcel Dekker, New York (2005). (4) J.P. Soares, J.E. Santos, G.O. Chierice and E.T.G. Cavalheir; Eclet. Quim.; 29, 57 (2004). (5) J.M. Wasikiewicz, F. Yoshii, N. Nagasawa, R. A. Wach and H. Mitomo; Radiat.Phys. Chem.; 73, 287 (2005). (6) M.Z.I. Mollah, M.A. Khan and R.A. Khan; Radiat. Phys. Chem.; doi:10.1016/j.radphyschem.2008.08.002. (7) H.A Kang, M.S. Shin and J.W. Yang; Polym. Bull.; 47, 429 (2002). (8) X. Liu, L. Qian, T. Shu and Z. Tong; Polym.; 44, 407 (2003). (9) C. Jeon, J.Y. Park and Y.J. Yoo; Water Res.; 36, 1814 (2002). (10) T.A. Davis, B. Volesky and A. Mucci; Water Res.; 37, 4311 (2003). (11) T.W. Wong, L.W. Chan, S.B. Kho and P.W.S. Heng; J. Controlled Release; 84, 99 (2002). 266