Hemijska Industrija (Jan 2010)
Influence of substituent type on properties of starch derivates
Abstract
The subject of the study was investigation of influence of substituent type on the properties of starch derivates in diluted solutions. Three samples were prepared: two anionic (carboxymethyl starch, CMS) and one cationic starch (KS). Starch derivates were synthesized in two steps. The first step was preparation of alkali starch by the addition of sodium-hydroxide to the starch dispersed in ethanol or water. In the second step, the required amount of sodium monocloracetate or 3-chloro-2-hydroxypropyl-threemethylamonium chloride was added to the obtained alkali starch in order to prepare CMS or KS, respectively. The degree of substitution of carboxymethyl starch was determined by back titration method, and the degree of substitution of cationic starch was determined by potentiometric titration. The degrees of substitution of prepared samples were: 0.50 (assigned as CMS-0.50) and 0.70 (assigned as CMS-0.70) for carboxymethyl starch and 0.30 (assigned as KS-0.30) for cationic starch. The properties of starch derivatives in dilute solutions were investigated by the methods of static and dynamic light scattering. Aqueous solutions of sodium chloride of different concentrations were used as solvent. The values of the mass average molar mass, MW, radius of gyration, Rg, and second virial coefficient, A2, were determined for all samples together with hydrodynamic radius, Rh. Molar masses of the samples were: 5.06×106, 15.4×106 and 19.2×106 g/mol for CMS-0.50, CMS-0.70 and KS-0.30, respectively. The samples, CMS-0.70 and KS-0.30 had similar molar mass and hydrodynamic radius, but radius of gyration of KS-0.30 was smaller then radius of gyration of CMS-0.70 at all sodium chloride concentrations. Consequently, ρ value for KS-0.30 was smaller then for CMS-0.70, as a result of more compact architecture of KS-0.30 then of CMS-0.70. Kratky graph confirmed this result. For all samples, radius of gyration and hydrodynamic radius decreased with increasing of sodium chloride concentration, but decrease of the radius was greater for CMS-0.50 then for other two samples due to its significantly lower molar mass. On the other hand, change of both radius of gyration and hydrodynamic radius of CMS-0.70 and of KS-0.30 with increasing sodium chloride concentration were similar. It can be concluded that the decrease of both Rg and Rh with increasing sodium chloride concentration in water depends far more on molar mass than on degree of substitution.
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