Chemistry Extended Essay - Viscosity of Xanthan Gum solutions

Xanthan gum (C35H49O29) is a long chain polysaccharide derived from the digestion of corn sugar by the bacteria Xanthomonas campestris. It is commonly used as a thickener and stabiliser in food products, from salad dressings to gluten-free bread. It is readily soluble in water, forming highly viscous gels even at low concentrations. It is also a very stable compound that does not spontaneously react over a wide pH range or with any salts. The viscosity of xanthan gum solutions is almost unchanged even when the temperature is raised; allowing comparison of reactions over a wider range of temperatures then would otherwise be possible.

The effect of the presence of xanthan gum in a solution in which a reaction is occurring will be measured through the rates of said reactions. A range of concentrations of xanthan gum will be used to fully evaluate their effects on the reaction rates.

2.        Research Question

How does the introduction of xanthan gum into a solution and the resulting increased viscosity affect the rates of reactions occurring within that solution?

3.        Background Information

3.1        Xanthan Gum in Solution

Xanthan gum solution varies from soft gel to pseudoplastic, depending on concentration.  The viscosity, the concentration value at which transition from soft gel to pseudoplastic behaviour occurs and the degree of pseudoplasticity are all directly related to gum concentration.

Four different concentrations of xanthan gum solution were chosen to be tested in this investigation. The percentage of xanthan gum for these solutions, by mass are:

Solution 1, composed of pure, distilled H2O was also tested as a control, to evaluate how effective xanthan gum is as a treatment to a solution in which the reaction must be slowed.

The solution concentrations were chosen because at approximately 0.4% xanthan gum transitions from acting as a soft gel to acting as a pseudoplastic. A pseudoplastic is a substance that varies in viscosity depending on the shear rates affecting it at the time. In other words if the concentration exceeds a certain threshold, slightly above 0.4%, it starts acting as a non-Newtonian fluid and the viscosity of the solution changes when it is subject to any kind of kinetic force. For this reason, solution concentrations both above and below the pseudoplastic threshold were chosen to best evaluate the cumulative effectiveness of xanthan gum solutions on rates of reaction.

Another factor that must be considered is the effect of salts on the viscosity of solutions. When salts are present in a xanthan gum solution, the viscosity of the solution increases according to the concentration of salt present. However, the viscosity values increase in a proportionally equal manner for each salt concentration, allowing comparative results to be obtained regardless of this change.        

3.2        Electrolytic Refinement

Although there are many forms of electrolytic refinement that could be measured, the refinement copper was chosen because it is a cheap, abundant, and highly conductive metal.

The purification uses an electrolyte of copper(II) sulphate solution, impure copper anodes, and strips of high purity copper for the cathodes.

The diagram shows a simplified view of the cell.

At the cathode, copper(II) ions are deposited as copper.

At the anode, copper goes into solution as copper(II) ions.

For every copper ion that is deposited at the cathode, in principle another one goes into solution at the anode. The concentration of the solution should stay the same.

Simplistically what is occurring is a transfer of copper from the anode to the cathode. The cathode increases in mass as more and more pure copper is deposited; the anode gradually disappears as the copper atoms that comprise it lose electrons and bond with the sulphate ions contained in the solution.

In the context of the experiment that will be performed to measure the effect of viscosity on these reactions, the solution in the reaction will be replaced with solutions containing varying concentration gradients of xanthan gum. The salt solution is maintained at the same concentration, but the water that would normally be used to dilute the concentration to an appropriate level is replaced with a xanthan gum solution.

3.3        Na-H2O Ionisation Reactions

These reactions involve sodium, a group I metal, reacting with water in a violent exothermic reaction. The variable to be measured is the temperature, which will peak faster and at a higher value if the rate of reaction is higher. Since water’s reaction with sodium is so violent, xanthan gum’s pseudoplasticity at higher concentrations must be taken into account as the movement of the sodium in the water during the reaction may cause the viscosity to fall as the sodium displaces the solution kinetically. However, if the total mass of sodium reacted is relatively small, then the amount of solution displaced and the amount of total kinetic disturbance across the solution will be minimal.

Another factor that must be taken into consideration is that, as water is one of the reactants, the frequency of collisions with the Na atoms will be decreased when a percentage of the water within the solution has been replaced with xanthan gum. The testing solutions will have water content, by mass of:

Therefore, within the ionisation reaction for sodium, the peak temperature for the reaction can be expected to reflect the values above before viscosity is taken into account.

4.        Method

4.1        Viscosity Measurements

Usually a Couette instrument would be used to measure the viscosity of a solution. Due to the lack of a Couette instrument, an alternative method had to be found.

1. The xanthan gum solution to be tested was prepared and the temperature was measured.
2. A standard burette used for titration was placed vertically in a clamp stand.
3. 30 ml of the solution were poured into the top of the closed burette and allowed to settle.
4. The tap at the base of the burette was opened and the time taken for the burette to empty completely was measured.
5. Steps 1-4 were repeated for all 5 solutions to be tested.

A comparative analysis of the times taken for the burettes to empty will yield values relative to each other for the viscosity of the solutions.

4.2        Rate of reaction measurement: Electrolytic refinement

1. 100 ml of a solution of 0.1M CuSO4- was mixed with 100ml of the solution to be tested in a 250ml beaker.
2. Two small sheets of copper of roughly equal size were weighed, and the mass was recorded.
3. The two copper sheets were placed in the beaker filled with CuSO4- solution in such a way that the two sheets did not touch directly
4. A 12V power supply was connected to the two copper sheets to form a circuit in such a way that one copper sheet became the cathode and the other became the anode.
5. The power supply was set to produce 12V of direct current and then switched on.
6. After 15 minutes the power supply was switched off.
7. The copper sheets were then removed from the solution, dried off, and the mass was once again recorded.

The electrolytic refinement experiment was repeated for all 5 test solutions, and each solution was given an equal amount of time to react. The difference in the final mass values of the cathode and anode should yield information about how viscosity affects the electrochemical refinement process

4.3        Rate of reaction measurement: Neutralisation reactions

1. 100 ml of the solution to be tested was placed in a 250ml beaker
2. A Vernier temperature probe was placed in the solution and the temperature of the solution was recorded
3. A precisely measured quantity of Sodium, either 1g or 2g, of a cubic shape, was placed in the solution and allowed to react completely.
4. The Vernier probe was consulted to find the maximum temperature of the solution
5. The maximum temperature was recorded.

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5.        Data

5.1        Relationship between xanthan gum concentrations and time taken for burette to empty, in order to model viscosity

+/- .5 s for time values

+/- .002% for xanthan gum concentrations

5.2        Comparison of mass change of the cathode in an electrolytic purification reaction involving a copper anode and a copper cathode in a copper sulphate solution

Weights of cathode and anode before and after electrolytic reaction:

+/- 0.01 grams for all weight values

Cathode and anode mass changes during electrolytic refinement experiment:

5.3        Peak temperatures achieved during Na/H2O ionisation reaction

+/- .5 degrees C

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6.        Analysis

The variations in rates of reaction occurring within the solutions are due to the speed at which inter-molecular collisions occur. A number of assumptions must be made in order for the collision theory to hold true as tested:

6.1        The Homogeneity Assumption

Consider a chemical reaction occurring in a small cubic container with sides of length L. If the mixture is homogeneous throughout the container at all times, a condition which can be obtained by constant stirring, the state of the mixture at any point in time is fully described by the total number of particles of each molecular species in the container. Let nI be the total number of particles of species I in the container; then the concentration of the species, denoted by [I], is equal to . 

For a reversible bimolecular reaction, , the forward rate of reaction (i.e. the number of molecular events leading to successful reaction per unit volume per unit time) is k1[A][B], whereas the backward rate of reaction is k−1[C]. The rate equation (RE) for species C is then given by the familiar classical equation:

A crucial assumption implicit in REs is that the local concentration at each point in space inside the container must equal the global concentration at all times. In other words, all concentration gradients must be zero and, as a result, a non-spatial model is enough to describe the kinetics. This is called the homogeneity assumption.

6.2        Sodium and Water Reaction

When Na(s) was reacted with increasingly concentrated solutions of xanthan gum, the reactions occurred at reducing rates due to the increased density of the solution. The results of this experiment were in a narrower range than the others attempted and this is consistent with Fick's first law of diffusion. 

Fick's first law relates the diffusive  to the concentration field, by postulating that the flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient.

In one (spatial) dimension, this is

where

• J is the diffusion flux in dimensions of [() length−2 time-1], example . J measures the amount of substance that will flow through a small area during a small time interval.

•  is the diffusion coefficient or  in dimensions of [length2 time−1], example 

•  (for ideal mixtures) is the concentration in dimensions of [(amount of substance) length−3], example 

•  is the original position of the element or compound being diffused in solution

In the Sodium and water reaction experiment attempted, Fick’s first law would have caused the separation of the NaOH- ions being formed, and in doing so would have caused differences in temperature that would be narrower in range as heat energy diffused much faster through a smaller area, or the small amount of solution used, due to the J constant being smaller for lower concentrate solutions.

6.3        Hydrogen Bonding in Water

Any solution that contains water exhibits hydrogen bonding between the water molecules. When the net percentage of water within the solution is reduced, such as in the solutions used for testing, the hydrogen bonds will be weaker and the orientation of the water molecules may differ due to the interference of the xanthan gum molecules present.

If the orientation of the molecules within the solution was affected by the presence of xanthan gum molecules then this may have slowed the rate of reaction, as collision theory states that molecules must collide not only with the right amount of kinetic energy but also with the correct orientation in order for a reaction to occur. Usually H2O molecules line up in a uniform fashion, as they are polar molecules and the slightly negative O atom attracts the slightly positive H atoms. The complex xanthan gum molecule could disrupt this alignment.

In a study executed by the University of Surrey, it was found that the presence of organic molecules can have a major effect on the Gibbs free energy, enthalpy and other heat values of the solution. During the study the Gibbs free energy, the enthalpy, and the entropy of mixing of ethanol with the polymer polyvinylpyrrolidone (PVP) and with its low molecular weight analog, N-ethylpyrrolidone (NEP) were calculated. The calculation of the free energy of mixing was achieved with a thermodynamic model for hydrogen-bonded polymer systems. This model, based on the use of an association model, gives the free energy of mixing as a function of the Flory−Huggins interaction parameter, the composition of the mixture, and the association equilibrium constants. The self-association of the ethanol molecules was described by two equilibrium constants, one for the formation of dimers and one for the formation of multimers. The equilibrium constants of inter-association of PVP or NEP with ethanol were determined from the quantitative analysis of NEP/ethanol and PVP/ethanol FTIR spectra at different temperatures and compositions. The values of the equilibrium constants were then used to calculate the theoretical Gibbs free energy of mixing as a

function of the composition. The enthalpic and entropic contributions to mixing were compared for the NEP/ethanol and PVP/ethanol mixtures.

The values found the enthalpic contributions were much greater in the presence of an organic polar solvent, in this case ethanol, than when in the presence of a non-organic polar solvent such as polyvinylpyrrolidone. This would point towards a lower enthalpic contribution, and thus a lower peak temperature would be expected for solutions containing an organic polar solvent such as xanthan gum.

7.        Conclusion

7.1        Xanthan gum solution viscosity testing

Unsurprisingly, a xanthan gum solution takes longer than a standard water solution to empty through a thin burette tap. This is due to the thickness of the solution being increased by the presence of xanthan gum within the solution. The greatest difference between two values occurred between a concentration by mass of 0.6% and 0.8%. This is in accordance with Zantz et al.’s research in “Xanthan Gum solutions at low shear rates” which states that a xanthan gum solution’s viscosity does not increase in a linear fashion when related to its concentration gradient, but in an exponential fashion.

7.2        Electrolytic refinement testing

The results of the refinement testing points towards a tendency for the xanthan gum solutions to slow refinement, indicating that the presence of xanthan gum, and the resulting increase in viscosity, reduces the rate of reaction within the solution. The value for the mass change of the anode and cathode for the highest concentration gradient of xanthan gum, 0.8%, was approximately half the value for the mass change occurring in unadulterated water. The other factors that could have caused this cannot account for such a large difference (approximately 50%) unless the viscosity factor played a part in slowing the reaction.

3. Na/H2O ionisation reaction

The differences between the peak temperatures obtained in unadulterated water and those obtained in one of the treated solutions are significant. The values of the peak temperatures showed a steady reduction as the concentration gradient of xanthan gum rose, totalling a reduction of over 10% of the initial value across the concentration spectrum. Although the presence of xanthan gum could have reduced the diffusion constant for the solution in which the temperature was measured, this change is no more than a 4.8% reduction in diffusion, as per Fick’s first law, and could not account for a 10% or greater change in the peak temperature values. The trend shows that the presence of xanthan gum, and the increased viscosity which occurs as a result, reduces the rate of ionisation for sodium atoms in water.

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Bibliography

Neuss, Geoffrey. Chemistry Course Companion. OUP 2007, page 113.

Field, Simon Quellon. http://sci-toys.com/ingredients/xanthan_gum.html (26 September 2009)

Field, Simon Quellon, Personal Correspondence with, (26 September 2009)

wiseGEEK Corp. http://www.wisegeek.com/what-is-xanthan-gum.html (26 September 2009)

Jungbunzlauer AG. http://www.jungbunzlauer.com/products-applications/products/xanthan-gum.html (26 September 2009)

Zantz, Joel L and Knapp, Steven. Viscosity of xanthan gum solutions at low shear rates. Rutgers College of Pharmacy, 1982

Hall, Nina. “The New Chemistry”. Cambridge University Press, 2001.

De Roek et al., “Defeating the Homogeneity Assumption”, Open University Press 2004

Wilson, Bill. “Fick’s First Law Module”. CNX Corp. 2004

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