Solution Rheology of Saline and Polysaccharide ...

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Mount Pleasant, MI 48859, USA. [email protected]. ABSTRACT. The problem of the characterization of the solution properties of water soluble polymers is ...
Proceedings of IMECE2006 2006 ASME International Mechanical Engineering Congress and Exposition November 5-10, 2006, Chicago, Illinois, USA

IMECE2006-15906

SOLUTION RHEOLOGY OF SALINE AND POLYSACCHARIDE SYSTEMS

Ekmagage Don N. Almeida Department of Physics, Central Michigan University, Mount Pleasant, MI 48859, USA. [email protected]

Leela Rakesh Department of Mathematics, Central Michigan University, Mount Pleasant, MI 48859,USA. [email protected]

Stanley Hirschi Department of Physics, Central Michigan University, Mount Pleasant, MI 48859, USA. [email protected]

Anja Mueller Department of Chemistry, Central Michigan University, Mount Pleasant, MI 48859, USA. [email protected]

ABSTRACT The problem of the characterization of the solution properties of water soluble polymers is long-standing. These polymers tend to form aggregated supramolecular gels that are resistant to molecular dispersion. These materials are being widely used in a variety of industrial applications. Their principle functions are as rheological modifiers, where they thicken or gel solutions in products such as hair-care, detergents, air fresheners and foods; as flocculants for particle separation as applied to water clarification, sewage, and effluent treatment, and as stabilizers to control the properties of concentrated suspension and emulsions, for example in paints, pesticides, dyes, and pharmaceutical industries. Therefore it is important to understand their rheological properties under various operating conditions such as stress, strain, temperature etc, which will induce gelation. The rheological properties of starch gels of high concentration (up to 86% starch) have been investigated before [1]. In this paper we have investigated experimentally the shear viscosity and viscoelasticity properties of saline and polysaccharide suspensions at various low concentrations and pH at different temperatures using controlled stress and strain rheometers (Vilastic-3 and AR 2000). The data were then fitted with the power law and Cross model for low and higher concentrations respectively. The present results show that the viscosity/elasticity does not significantly change for low concentrations at different pH values. The maximum viscosity/elasticity was obtained around pH 5-7.4 at higher concentrations.

1. INTRODUCTION Polysaccharides are a class of biopolymers. Cellulose, the most abundant polysaccharide, is the structural component of plant tissue. The structural integrity and mechanical strength of plant tissues is achieved by forming a hydrated, crosslinked 3-D network. Starch forms a less cross linked network, but with still a lot of hydrogen bonds. Starch is highly abundant in plants as well and is used as energy storage. Polysaccharides are used as thickeners due to their ability to modify flow properties of aqueous systems. They are used industrially to (i) stabilize aqueous systems and increase their viscosity, (ii) to produce gels, (iii) act as flocculants, binders, film formers, lubricants and friction reducers. Thickeners are also used in food industries that require a viscous liquid, or to physically bind water and provide humectancy in a fat-reduced product. They are also used as thermally reversible gels, dairy-based gel desserts, canned meats, and some confectionery applications. Rheological properties of polysaccharide dispersions in excess water are not only depend on polysaccharide concentration and salinity but also on the factors such as the granule size and pH [2, 3]. The viscosity of starch suspension is behaved as power law [4]. STARCH Starch is a complex polysaccharide important for human and animal nutrition. It occurs naturally as minute granules in numerous types of plants such as corn, wheat, rice, millet, barley, and potatoes. Starch is a carbohydrate that is partially

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linear (amylose) and branched (amylopectin). Amylose makes up 25% of the total starch. Both consist of α-D-glucose units linked via (1→4) bonds, whereas in amylopectin about one residue in every twenty or so is also linked (1→6), forming the branching.

3.3 PH METER A Thermo Orion pH meter, model 410, was used with refillable epoxy electrodes, (VWR model # 14002-780) for measuring the pH of the samples.

4. RESULTS AND DISCUSSION The soluble portion of starch was first measured at low concentrations to obtain the minimum concentration needed for gelation. The viscosity of low starch concentration can be described by a power law. Viscosity data for 1% and 3% concentration were fitted using power law model which is given by the expression of:

2. MATERIAL AND SAMPLE PREPARATION Starch Soluble S516-500 was obtained from the Fisher Scientific Company. It was diluted to 1%(w/w), 3%(w/w), 5%(w/w), 7%(w/w) and 9%(w/w) with saline solution (0.1M NaCl ).The pH was adjusted to 3.0, 5.0, 7.4, 9.0, 11.0 and 13.0 with diluted HCl or NaOH and heated up to 50ºC for 10-15 minutes to dissolve the starch. The samples were stored at room temperature.

σ = η * γ& c

3. EXPERIMENTAL PROCEDURE Data were taken using oscillatory flow at frequency 2Hz and shear rate in the range of 2 to 1000 s-1 at 25°C by using a Vilastic-3 rheometer. Samples were measured in an 1mm diameter tube, 6.27cm high. Steady state flow (temperature ramp) measurements were also made using an AR 2000 rheometer at temperatures between 10°C to 80°C at the shear rate of 25 s-1, with 4cm, 20 cone and plate geometry.

(1)

where σ is the shear stress, η the viscosity, γ& the shear rate and c the rate index. Table 1 shows values for η, c and the standard error for the 1% and 3% concentration. Viscosity variation at various pH for starch 1% (w/w) is given in figure 1. The data clearly shows pH does not influence viscosity at low concentration. Viscosity is very close to viscosity of water (0.01 poise)

3.1 VILASTIC- 3 RHEOMETER The Vilastic-3 rheometer provides extreme sensitivity and sample handling, required for measuring fundamental viscoelastic properties of biological, low viscosity fluids under controlled frequency, stress, strain, shear rate, time, and temperature. The Vilastic-3 instrument operates on the fundamental principle of oscillatory flow within a straight, cylindrical tube. The fluid in the measurement tube is forced into oscillatory flow, and the pressure gradient along and volume flow through the tube is measured. The shear stress σ at the tube wall is directly proportional to the pressure P. The shear strain γ and the shear rate γ& are directly proportional to the volume flow U. The instrument resolves both amplitude and phase at the tube wall of each of these fundamental parameters – shear stress, shear strain, shear rate – as well as the viscoelastic constants for the fluid.

pH3.0 pH5.0 pH7.4 pH9.0

viscosity (poise)

0.02500

pH11.0 pH13.0 Power law

3.2 AR 2000 RHEOMETER The AR 2000 is a highly versatile instrument, capable of operating in several control modes: flow, oscillation, creep and stress relaxation. Being a controlled-stress rheometer, (torque range, 0.1 micro N.m to 200 mN.m) the Mobius drive makes the AR2000 suitable for performing strain-controlled experiments. Due to the air bearing, the low inertia drag cup motor, and rotational mapping, the instrument has a high sensitivity and is capable of detecting tiny differences in viscosity. The Peltier temperature control allows very fast heating and cooling rates in the temperature range from -10°C up to 90°C. Cone/plate, plate/plate and concentric cylinder measuring geometries are available.

0.01000 1.000

10.00

100.0

1000

shear rate (1/sec)

Fig.1: Starch 1% (w/w) for different pH values at temperature 25°C, viscosity vs shear rate. Solid lines represent the power law fit of data.

Figure 2 shows the changes in viscosity at various pH for 3% (w/w) starch concentration. The data demonstrates again that viscosity changes are not significant at different pH’s. The power law was used because the solutions behaved almost Newtonian.

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σ = η * γ& c

η= Viscosity

c= Rate Index

Standard Error

pH/Con.

1%

3%

1%

3%

1%

3%

3.0

0.011

0.023

0.989

0.953

0.011

0.005

5.0

0.02

0.032

0.949

0.98

0.006

0.008

7.4

0.022

0.039

0.985

0.948

0.0593

0.005

9.0

0.022

0.028

0.987

0.951

0.133

0.008

11.0

0.019

0.036

0.998

0.980

0.269

0.297

13.0

0.018

0.022

1.004

1.001

0.005

0.009

Table 1: power law data for 1% and 3% starch concentration

pH3.0 pH5.0 pH7.4 pH9.0

0.05000

η being the viscosity, γ& the shear rate, a: zero rate viscosity, b: infinite-rate viscosity, c: consistency, and d: rate index. Table 2 shows values for a, b, c, d and the standard error for the 5%, 7% and 9% concentrations.

pH11.0 pH13.0 power law

viscosity (poise)

Figure 3 shows how the viscosity changes with 5% (w/w) starch concentration for various pH values. It was found that at pH 5.0 the viscosity is significantly increased.

pH 3.0 pH 5.0

pH 7.4 pH 11.0 pH 9.0 pH 13.0 Cross model

0.1000

0.01000 1.000

10.00

100.0

1000

shear rate (1/sec)

viscosity (poise)

Fig. 2: Starch 3% (w/w) for different pH values at temperature 25°C, viscosity vs shear rate. Solid lines represent the power law fit of data.

The viscosity is increased with increasing the starch concentrations above 3%, which can be evidenced from figures 3-5. At those concentrations, the suspension behave like shear thinning material, thus the cross model was used to fit the data. Also, we observed that the viscosity at higher concentrations is also dependant on pH. The viscosity changes with applied shear rate for concentration 5%, 7% and 9% were fitted with the Cross model;

η −b a−b

=

1 d 1 + (c * γ& )

0.03000 0.1000

1.000

10.00 shear rate (1/sec)

100.0

1000

Fig.3: Starch 5% (w/w) for different pH values at temperature 25°C, viscosity vs shear rate. Solid lines represent the Cross model fit of data.

(2)

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Concentration

pH

a (poise)

b(poise)

c (s)

d

Standard Error

3.0 5.0 7.4 9.0 11.0 13.0 3.0 5.0 7.4 9.0 11.0 13.0 3.0 5.0 7.4 9.0 11.0 13.0

0.042 0.088 0.044 0.048 0.045 0.04 0.33 1.47 1.07 0.42 0.221 0.125 0.326 1.47 1.07 0.419 0.221 0.087

0.037 0.063 4.01E-6 0.010 0.045 0.046 0.141 4.1E-6 5.2E-6 0.140 1.4E-5 0.082 0.141 4.1E-6 5.2E-6 0.140 1.4E-5 0.068

0.025 .001 3.9E-5 3.8E-4 0.038 0.367 5.2E-3 5.2E-3 5.0E-3 7.1E-3 6.9E-4 3.4E-3 5.2E-3 5.2E-3 5.0E-3 7.1E-3 6.9E-4 5.3E-3

0.993 0.864 0.532 0.646 1.275 2.551 0.793 0.762 0.761 0.704 0.354 0.839 0.793 0.762 0.761 0.704 0.354 0.405

54.18 6.58 20.10 22.19 250.6 3.5E-3 9.21 1.8E-3 3.7E-3 5.70 14.61 18.14 9.21 1.8E-3 3.7E-3 5.70 14.61 4.05

η −b

1 = a − b 1 + (c * γ& )d

5%

7%

9%

Table 2: Cross model data for 5%, 7% and 9% starch concentration

10.00

viscosity (poise)

As we can see from figure 4 that at 7%(w/w) starch, the viscosity was the highest at pH 5.0 and 7.4. Also pH 9.0, had an increased viscosity. This viscosity increase could result from the charge distribution or hydrophobicity of the molecule. Starch is a neutral molecule at neutral pH, which means that only at extreme pH’s there will be charges on the molecule. These charges would repel each other and thus increase solubility. This is why the viscosity increases at more neutral pH’s. At 9%, pH 5.0 (Fig.5) again has the highest viscosity, followed by pH 7.4 and 9.0 At this concentration, viscosity is considerably higher in general and the difference in pH is greater as well. At this concentration gel formation was observed except at pH 13.0. Gels formed faster at pH 5.0 and 7.4 and more slowly at the other pH’s. This data suggest that a viscosity of above 0.1 poise is needed for gel formation. At 9% and pH 5.0, gel formation is the fastest (10min.). At different pH’s and 7% concentration gel formation occurs above 0.1poise, but it is considerably slower.

pH 3.0 pH 5.0

pH 7.4 pH 11.0 pH 9.0 pH 13.0 Cross model

1.000

0.1000

0.01000 0.1000

1.000

10.00 shear rate (1/sec)

100.0

1000

Fig.4: Starch 7.0% (w/w) for different pH values at temperature 25°C, viscosity vs shear rate. Solid lines represent the Cross mode fit of data.

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10.00

pH 3.0 pH 5.0

When the data is graphed as a function of concentration, not pH, it confirms that at least 7% is needed for gel formation. Figures 7-a and 7-b show how viscosity and elasticity change with different starch solutions with the pH values. For low concentrations there are no significant changes in viscosity at different pH. But for higher concentrations viscosity and elasticity change with pH. The highest viscosity/elasticity again is formed at pH 5.0.

pH 7.4 pH 11.0 pH 9.0 pH 13.0 Cross model

viscosity (poise)

1.000

0.1000

pH 3 pH 5 pH 7.4

1.20 0.01000 0.1000

1.000

10.00 shear rate (1/sec)

100.0

Viscosity(Poise)

pH 9 1000

Fig. 5: Starch 9% (w/w) for different pH values at temperature 25°C, viscosity vs shear rate. Solid lines represent the Cross model fit of data.

1.00

0.40 0.20 0.00

Elasticity was measured as well. Figure 6-a summarizes viscosity changes (shear rate 50 s-1) at different pH for starch concentration from 1%(w/w) to 9 %(w/w). Figure 6-b summarizes elasticity changes for 7%(w/w) and 9%w/w) starch concentrations. PH dependencies of elasticity for low concentrations need further investigation. Both viscosity and elasticity have a maximum around pH 5.0, as expected.

0

3

4

5

6

7

8

9

10

pH 3 pH 5

Elasticity(Poise)

0.35

5% 7%

Viscosity(Poise)

2

0.40

3%

pH 7.4

0.30

pH 9

0.25

pH 11

0.20

pH 13

0.15 0.10 0.05

9%

0.00

1.00

b

6

7

8

9

10

Concentration( %) 0.10

Fig. 7: (a) Starch concentration %( w/w) at different pH s, viscosity vs. shear rate. (b) Starch concentration %( w/w) at different pH’s, elasticity vs. concentration at shear rate 50 (s-1).

0.01 2

a

4

6

8

10

12

14

pH

The viscosity of starch decreases with increasing temperature, according to the Arrhenius relationship [5];

7%

1

Elasticity(Poise)

1

Concentration(%)

1%

9%

0.1

η = Ae

−B

T

(3)

0.01

where T is the absolute temperature, and A and B are constants of the liquid. The viscosity changes with temperature data was obtained using AR2000 rheometer. At higher concentration the data could be modeled with Arrhenius equation. Table 3 shows values for A, B and the standard error for 7% and 9% starch concentration. Figure 8a and 8-b show how viscosity changes with increasing temperature. This data again shows that the viscosity has the maximum value at pH 5.0.

0.001

b

pH 13

0.60

a

10.00

pH 11

0.80

2

4

6

8

10

12

14

pH

Fig.6: (a) Starch concentration %( w/w) at different pH’s, viscosity -1 vs. pH’s at shear rate 50 ( s ) . (b) Starch concentration %( w/w) at different pH’s, elasticity vs. shear rate 50(s-1).

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pH 3.0 pH 5.0

viscosity (poise)

10.00

viscosity (poise)

1.000

pH 3.0 pH 5.0

pH 7.4 pH 11.0 pH 9.0 pH 13.0 Arrhenius model

pH 7.4 pH 11.0 pH 9.0 pH 13.0 Arrhenius model

1.000

0.1000

a b 0.05000 10.0

temperature (°C)

0.01000 10.0

100.0

temperature (°C)

100.0

Fig. 8: (a) 7% Starch concentration at different pH s , viscosity vs. temperature. (b) 9% Starch concentration at different pH s, viscosity vs. temperature at shear rate 25(1/sec). Solid lines represent the Arrhenius equation fit of data.

η = Ae

−B

T

A = viscosity coefficient(Poise)

B= Temperature coefficient(K)

Standard Error

pH/Con.

7%

9%

7%

9%

7%

9%

3.0

5.49E-4

1.13E-3

1841.9

1881.3

19.85

4.001

5.0

2.18E-3

2.78E-3

1609.8

2252.3

7.289

13.87

7.4

2.04E-3

1.01E-3

1645.5

2332.5

3.320

12.33

9.0

2.57E-3

1.17E-3

1538.2

1842.0

4.716

5.660

11.0

4.12E-3

3.72E-3

1306.7

1328.2

22.40

14.35

13.0

1.43E-4

3.65E-5

2170.4

2446.9

26.19

13.19

Table 3 : Arrhenius law data for 7% and 9% starch concentrations [2] W.A. Van Der Reijden, E.C.I. Veerman and A.V. Nieuw Amerongen,1994, “Rheological properties of commercially available polysaccharides with potential use in saliva substitutes” , Biorheology, 31,6, pp.631-642.

SUMMERY The rheology of starch at a near physiological salinity of 0.1M NaCl was measured at several concentration, pH and temperature. It was found that at 7% starch concentration was needed for gel formation. Gels form the most easily at 9% and pH 5.0. The viscosity has to be above 0.1Poise to form a gel.

[3] M. Laka, S. Chernyavskaya,2003, “Effect of electrolytes on the rheological properties of microcrystalline cellulose gels”, Applied Mechanics and Engineering, 8, pp.265-269. [4] I.D. Evans, D. R. Halsman, 1979, “Rheology of gelatinised starch suspensions”, J. Texture Stud., 10, pp.347370.

REFERENCES [1]G. Della Valle, P. Colonna, and A. Patria, 1996, ”Influence of amylose content on the viscose behavior of low hydrated molten starches”, J. Rheol., 40(3), May/June, pp.347-362.

[5] H.A. Barnes,J.F. Hutton, K. Walters,1989, ”An Introduction to Rheology”, Elsevier,

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