International Journal of Biological Macromolecules 42 (2008) 429–435 Contents lists available at ScienceDirect International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac Single and multi-step carboxymethylation of water yam (Dioscorea alata) starch: Synthesis and characterization O.S. Lawal a,b,∗ , M.D. Lechner c , W.M. Kulicke b a Department of Chemical Sciences, Olabisi Onabanjo University, P.M.B 2002, Ago-Iwoye, Ogun State, Nigeria b Institute of Technical and Macromolecular Chemistry, University of Hamburg, Bundestrasse 45, D-20146, Hamburg, Germany c Institute of Physical Chemistry, University of Osnabr¨ uck, Babarastrasse 7, 49069, Osnabr¨ uck, Germany article info Article history: Received 26 November 2007 Received in revised form 3 February 2008 Accepted 20 February 2008 Available online 2 March 2008 Keywords: Water yam starch Carboxymethylation Characterization abstract Water yam (Dioscorea alata) starch was carboxymethylated by single and multi-step processes for nine times successively. Optimal degree of substitution (DS) of 0.98 and reaction efficiency (RE) of 70.5% were obtained in the single-step carboxymethylation at 40 ◦ C, 3 h in isopropanol–water reaction mixture ratio 1:0.16. The ratio of sodium hydroxide and sodium monochloroacetate moles to anhydroglucose unit moles for the optimal DS and RE were 1.62 and 1.39, respectively. The DS increased progressively as the steps of carboxymethylation increased from 2 to 9 and an optimal DS of 2.24 was obtained. Initial increases in carboxymethylation step increased the RE progressively up to 82.1% after the seventh carboxymethylation step but declined with further increases in the carboxymethylation step. The wide angle X-ray diffrac- tometry and the DSC revealed remarkable changes after carboxymethylation because starch crystallinity reduced significantly. Thermogram of native starch showed a characteristic three-step decomposition with 13.16%, 61.54% and 24.79% weight losses progressively while carboxymethyl derivative showed four decomposition stages with 9.86%, 36.57%, 3.04% and 23.07% weight losses progressively. Studies revealed that carboxymethylation improved thermal stability of the native starch. IR spectrometry revealed that carboxymethyl starch showed new bands at  = 1600, 1426 and 1324 cm −1 . The broad band 13 C NMR spec- tra of the ultrasonically degraded carboxymethyl starch showed a peak at ı = 180 ppm which was assigned to carbonyl carbon. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Functionalized starches are strategic in the developments and applications of new biomaterials. This is because starches are cheaper than most other biomaterial resources and are eas- ily compatible in the formulation of new products. Native and functionalized starches have diverse applications in food, phar- maceuticals, textile, paper, plastics as well as in biomedical engineering industries. Among the starch derivatives, car- boxymethylated starches are very important because they cover a wide range of industrial applications in the aforementioned indust- ries [1]. Carboxymethyl starches are obtained by reacting starch with monochloroacetic acid or its sodium salt following activation of the starch with aqueous NaOH in slurry of an aqueous organic solvent. In carboxymethylation, the first step is an alkalization where the ∗ Corresponding author. Tel.: +234 7033268164. E-mail address: laidelawal2@yahoo.com (O.S. Lawal). hydroxyl groups of the starch molecules are activated and changed into the more reactive alkoxide form (St–O − ). St–OH + NaOH  St–ONa + H 2 O (1) This is followed by etherification in the second step: St–ONa + Cl–CH 2 –CO–ONa → St–O–CH 2 –COONa + NaCl (2) A side reaction also occurs which competes with the produc- tion process of carboxymethyl starch. In this side reaction, sodium gylocate is produced at the expense of the starch derivative. NaOH + ClCH 2 COONa → HOCH 2 COONa + NaCl (3) In the literature, different types of reaction media have been investigated for the carboxymethylation of starch. These include the following aqueous organic solvents: isopropanol [1–3], methanol [1–4], ethanol [5], 1-butanol, 2-butanol, tert-butanol [6], acetone [7] and cyclohexane [8]. Influence of other reaction conditions has also been reported and such factors include the reagent concentra- tion, duration of reaction and temperature [1,9]. 0141-8130/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ijbiomac.2008.02.006