Biomacromolecules 2001, 2, 1256-1259
1256
Polymeric Surfactants Based on Inulin, a Polysaccharide Extracted from Chicory. 1. Synthesis and Interfacial Properties C. V. Stevens,*,† A. Meriggi,† M. Peristeropoulou,† P. P. Christov,† K. Booten,‡ B. Levecke,‡ A. Vandamme,‡ N. Pittevils,‡ and T. F. Tadros§ Department of Organic Chemistry, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Coupure links 653, B-9000 Ghent, Belgium; Product Development Department, ORAFTI, Aandorenstraat 1, B-3300 Tienen, Belgium; and 89 Nash Grove Lane, Workingham, Berkshire, RG40 4HE, U.K. Received June 13, 2001; Revised Manuscript Received September 3, 2001
Inulin, the polydisperse reserve polysaccharide from chicory, has been modified by carbamoylation in organic solvents. The reaction of inulin with a range of alkyl isocyanates resulted, after crystalization, in a variety of carbamoylated inulins from which the interfacial properties were determined. The medium and long chain carbamoylated inulins showed a good to very good reduction of the interfacial tension which makes these biopolymers interesting in the field of biodegradable surface active agents. Introduction Polymeric surfactants attracted considerable attention in recent years as dispersants for solids in liquids and as emulsifiers (liquid in liquid dispersions).1 The effectiveness of these molecules as dispersants and emulsifiers is based on the role of stabilization of particles and droplets against flocculation and/or coalescence by a mechanism referred to as steric stabilization.2 The adsorption of these molecules at the solid/liquid and liquid/liquid interface is usually very strong (multipoint attachment), and hence, the molecules cannot be displaced from the particles or droplets during close approach in a suspension or emulsion. Most of the copolymeric surfactants consist of a block (A-B and A-BA) or graft structure (BAn) whereby the B chain represents the anchor portion of the molecule (that is strongly adsorbed to the particle or droplet surface) whereas the A chains remain dangling in solution and are strongly solvated by the molecules of the medium. These A chains are referred to as the stabilizing part of the molecules. Several block and graft copolymers are available in the literature and they are based on synthetic chains such as polystyrene and poly(methyl methacrylate) B chains or poly(ethylene oxide) A chains.3 In recent years, considerable attention was given to polymeric or oligomeric surfactants that are based on polysaccharide chains.4 Apart from being derived from renewable resources,5 these molecules are also environmentally preferable since they are biodegradable. In this paper, we will describe the synthesis and interfacial properties of a new class of surfactants that are based on polyfructose that is extracted from the roots of chicory (Cichorium intybus). Inulin is a polydisperse polysaccharide * Corresponding author: E-mail:
[email protected]; tel: +32-9264.59.57; fax: +32-9-264.62.43. † Ghent University. ‡ ORAFTI. § 89 Nash Grove Lane.
consisting mainly, if not exclusively, of β(2f1) fructosyl fructose units (Fm) with normally, but not necessarily, one glucopyranose unit at the reducing end (GFn).6,7 It is also known that the fructose molecules in the GFn form are all present in the furanose form. Only in the Fm forms are the ending and reducing fructoses in the pyranose form. To produce the required amphipathic character, the chains were modified by introduction of alkyl groups (C4-C18) on the polyfructose backbone through the isocyanates (Figure 1). In this respect, the alkyl groups represent the B chains (that are randomly distributed on the sugar backbone on the primary hydroxyl functions as well as on the secondary ones) which become strongly adsorbed on a hydrophobic surface such as carbon black or an oil droplet. The sugar chain will form the stabilizing part as this is water soluble. Materials and Methods Materials. Inulin (RAFTILINE HP) was supplied by Orafti and was used without modification, dried in a vacuum oven at 70 °C for 24 h. The main degree of polymerization was about 25 (as determinded by enzymatic hydrolysis followed by HPLC analysis8).9 The alkyl isocyanate was obtained from Sigma-Aldrich or Acros in Belgium and used as received. N-Methylpyrrolidone (NMP) and N,N-dimethylformamide (DMF) were obtained from Acros, and during use they were dried on 4 Å molecular sieves supplied by Acros. Dichloromethane was obtained from Acros and was dried by distillation over calcium hydride. IsoparM was obtained from Exxon and used as received. The NMR determination was performed on a JEOL PMX 270SI spectrometer in DMSO-d6 at 270 MHz for 1H NMR and at 68 MHz for 13C NMR. The IR spectra were recorded on a Perkin-Elmer Spectrum One. Synthesis. General Synthesis of the Inulin Carbamates. To 10 g of predried inulin (62 mmol fructose equivalents)
10.1021/bm015570l CCC: $20.00 © 2001 American Chemical Society Published on Web 10/06/2001
Polymeric Surfactants Based on Inulin
Biomacromolecules, Vol. 2, No. 4, 2001 1257
Figure 1. Structure of inulincarbamates.
that was weighed in a three necked flask and flushed with nitrogen was added 16 mL of N-methylpyrrolidone, and the mixture was heated to 80 °C in order to dissolve the inulin completely. Then, the amount of isocyanate that is required to obtain a certain degree of substitution was added using a syringe under a nitrogen atmosphere and the mixture was heated at 80 °C for 24 h. The reaction mixture was then poured into 300 mL of dry acetone under vigorous stirring, and the modified inulin crystallized immediately. The modified inulin was then filtered over a sintered glass filter and added to 150 mL of dichloromethane and stirred for 20 min in order to remove impurities. The modified inulin was filtered and added again to 150 mL of dichloromethane, stirred for 20 min and then filtered. The resulting powder was then air-dried. The modified inulin was characterized by 1H NMR. Interfacial Properties. Surface and interfacial tensions were measured using the Du Nouy ring method and Kru¨ss K8 tensiometer (Kru¨ss, Germany).10 Measurements were carried out at 20 °C. Before use, the ring and glass container were thoroughly cleaned by immersion in a 6% nitric acid solution, rinsing in de-ionized water, and finally the ring was flame dried in an ethanol burner. The reference for the interfacial tension water/air at 20 °C is 72.75 mN/m. Results and Discussion Synthesis and Characterization. The chemical modification of inulin was carried out in organic solvents using reactive reagents in order to avoid formation of secondary products in water (as is common in the case for materials used in the food industry). First, the solubility of inulin was investigated in different organic solvents, and it was found that dimethyl formamide (DMF) was suitable as solvent with an inulin solubility of 62.5 g/100 mL at a temperature of 80 °C. Therefore, the reactions were carried out in DMF. Unfortunately the resulting end product contained up to 3% of DMF, which was considered to cause too great a toxicity for the product. For that reason, DMF should be replaced by N-methylpyrrolidone (NMP), in which case any residual solvent remaining would be of much lower toxicity. In this
paper, the initial results obtained in DMF are described, although no differences were detected in characteristics when the reactions were performed in NMP. Our approach consisted of the reaction of inulin in a polar aprotic solvent with alkyl isocyanates resulting in the formation of inulin carbamates in almost quantitative yield. The inulin was dissolved in the appropriate solvent at 80 °C under a nitrogen atmosphere followed by the addition of the alkyl isocyanate. The degree of substitution (DS) that was envisaged depends on the amount of isocyanate that was added since the reaction proceeded nearly stoichiometrically after 24 h. After isolation of the products, the degree of substitution could be checked by NMR-spectroscopy at 270 MHz comparing the integration of the Me signals of the alkyl isocyanate (at ∼1.6 ppm, see Figure 1B) to a specific signal of the fructose unit integrating for 1 proton (at ∼5.8 ppm, see Figure 1B). It was shown that the ratio of the signal integration matched well with the equivalents of alkyl isocyanate added. Although the accuracy of the integration signals is not very good due to the broadness of the peaks of the inulin, the matching of the equivalents of isocyanate added to the reaction mixture and the DS determined on the basis of the NMR-measurements was always within 10%. A 1H NMR spectrum for one of the compounds is shown in Figure 1. However, special precautions had to be taken in order to exclude traces of water in the reaction mixture leading to the denaturation of the alkyl isocyanates. Therefore, the commercial inulin was predried in an oven at 70 °C before use in the reaction with the isocyanates. Also, the solvents were dried additionally on molecular sieves (4 Å) before use. Even with all these precautions, small amounts of dialkyl ureides could be detected in the reaction mixtures (
