Viscosity of alginates

The viscosity of alginate solutions decreases with increasing temperature, but provided the temperature is not maintained at high levels for extended periods, the viscosity decrease is reversible. Partial depolymerization of the alginate occurs if solutions are exposed to excessive temperatures or to sufficiendy elevated temperatures for extended periods. 

The viscosity of alginate solutions is independent of pH in the range 5—10, but below pH 4.5, the viscosity increases until the pH reaches 3 when insoluble alginic acid precipitates. Propylene glycol alginate is soluble and stable at pH 2—3 but not above 6.5. 

A number of papers have been published on comparisons of the viscosity of algin at low concentrations with predictions from theoretical equations (55, 55, 55, 55, 211y 251). The specific and intrinsic viscosities (55, 54, 57, 100 209) of various samples have been determined in connection with molecular weight determinations and investigations of their properties in solution. 

R. L. Whistler, P. K. Chang, and G. N. Richards, Alkaline degradation of periodate-oxidized starch, J. Am. Chem. Soc., 81 (1959) 3133-3136 Compare I.-L. Andresen, T. J. Painter, and O. Smidsrod, Concerning the effect of periodate oxidation upon the intrinsic viscosity of alginate,... 

FIGURE 9.5 Inter-relationship effect of the collection distance and viscosity of alginate solution on the shape of Ca-alginate beads. (Reproduced from Lee, B.-B. et al., Chem. Eng. TechnoL, 36(10), 1627, 2013. With permission.)... 

The maxiinmn viscosity of alginate solutions is obtained at pH 3-3.5 because of the protonation of carboxylate groups in the alginate backbone forming hydrogen bonds. 

From a physical standpoint, at small velocities, the polymeric chains of the alginate have a random orientation, increasing the viscosity, while under a sufficient shear they align with the flow, and the viscosity is reduced. Different laws exist for the viscosity of alginate solutions the Carreau-Yasuda law is often used to describe the viscosity of semi-dilute alginate solutions. Similarly, at small velocities, red blood cells form stacks that considerably increase the viscosity. These stacks are dispersed at sufficiently high velocity. Usually blood viscosity is modeled by Cassons law, and an asymptotic value of 4.0 10 Pa.s for the viscosity is obtained when the cells are dispersed. 

Alginic acid and its calcium salt are insoluble in water, but its ammonium, sodium, potassium, and magnesium salts as well as its propylene glycol ester are readily soluble in cold and hot water in which they form viscous solutions. The viscosity of algin solutions depends on various factors including concentration, pH, degree of polymerization (DP),... 

The flow properties of sodium alginate solutions depend on concentration. A 2.5% medium viscosity sodium alginate solution is pseudoplastic, especially at the higher shear rates in the range of 10—10,000/s. 

Thickeners. Thickeners increase the viscosity of the polychloroprene latex adhesives. Amounts up to 1% of polyacrylates, methyl cellulose, alginates and polyurethane thickeners can be used. Particular attention should be paid to fluctuations in pH when thickener is added in the formulations. For low-pH (7-10) formulations, fumed silica or some silicates can be used. 

The viscosity of an emulsion can be of crucial importance for its stability, especially the viscosity of the external phase. A high viscosity reduces creaming and also lessens the tendency of particles to coalescence and produce phase separation. Examples of the widely used viscosity-imparting agents are alginates, bentonite, carboxymethylcellulose, polyvinyl pyrrolidone, hydroxypropylcellulose, and carbomer. 

A substance used in latex compounding to increase the viscosity of the mix. Examples are alginates, casein, certain clays and methyl cellulose. 

The viscosity of sodium alginate solutions is slighdy depressed by the addition of monovalent salts. As is frequendy the case with polyelectrolytes, the polymer in solution contracts as the ionic strength of the solution is increased. The maximum viscosity effect is obtained at about 0.1 N salt... 

Class A. Water-soluble synthetic and natural organic polymers, which increase the viscosity of the mixing water. They include cellulose ethers, pregelatinized starches, polyethylene oxides, alginates, carrageenans, polyacrylamide, carboxyvinyl polymers and polyvinyl alcohol. 

Investigations by X-ray methods have indicated that alginic acid possesses a fiber structure analogous to that of cellulose.86 An estimate of the molecular weight was obtained by Heen87 from a study of the viscosity of solutions of the acid in 2 JV sodium hydroxide. The values found, (14,100-15,400), corresponded to a degree of polymerization of about 80, but these were considered to be approximations only, as it was uncertain whether or not Staudinger s formula could be applied. 

The molecular weight of the sodium salt of alginic acid has been determined by measurements of osmotic pressure in sodium chloride solution.88 The n/C versus C curve was found to be independent of salt concentration for values between 0.2 N and 0.5 JV, and the slope of the curve was the same for a series of seven fractions with values of molecular weight between 48,000 and 186,000 (220-860 D-mannuronic acid units). Intrinsic viscosity measurements showed the coefficient a in the modified Staudinger s equation88 to be 1.0 and the approximate value of Km to be 15(10)-4. 

Even for a given charge with specific properties the operating variables are rather poorly defined and are closely related to the type of equipment Generally they include temperature, addition of binders and lubricants to modify viscosity and thixotropy, as for example additions of alginates, starch, kaohnitc and montmorillonitc... 

Salts of alginic acid which are capable of binding 200-300 times their own volume of water. They are added as swelling agents to explosive mixtures in order to improve their resistance to moisture, and to -> Slurries to increase viscosity. 

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