Rheology of Xanthan Gum Solutions

12 Rheology of Xanthan G u m Solutions

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P. J. WHITCOMB General Mills Chemicals, Inc., 2010 E. Hennepin Ave., Minneapolis, MN 55413 B. J. EK and C. W. MACOSKO Dept. of Chemical Engineering and Material Science, University of Minnesota, Minneapolis, MN 55455

Xanthan gum is a high molecular weight p o l y s a c charide produced by fermentation with the bacterium "Xanthomonas campestris". Adding ½% of t h i s biopolymer increases water's v i s c o s i t y by a factor of 100,000 at low shear r a t e s ; yet at high shear r a t e s , the factor i s reduced to 10. This remarkable shear t h i n n i n g ability ( p s e u d o p l a s t i c i t y ) can be used to great advantage. In fact the main use of xanthan gum is rheology c o n t r o l . In the past most data on xanthan gum rheology has been taken over a l i m i t e d shear rate range and i s relative, not absolute. Figure 1 presents t y p i c a l shear stress v s . shear rate data. Only two decades of shear rate are covered and the shear rate i s given as rpm, r e l a t i v e u n i t s . The use of arithmetic scales for t h i s p l o t make it difficult to resolve s o l u t i o n properties. Figure 2 presents a typical v i s c o s i t y v s . shear rate p l o t . The use of l o g scales aids i n i n t e r p r e t a t i o n of the flow curves. However shear rate covers only two decades and i s again reported as rpm. While r e l a t i v e data i s useful for comparisons under s p e c i f i e d c o n d i t i o n s , there are many advantages to having absolute data, where the units have p h y s i c a l s i g n i f i c a n c e . Absolute data i s independent of the instrument or geometry used to gather the data. R e l a t i v e data i s not. This means that a very broad shear rate range can be covered by compiling r e s u l t s , obtained i n absolute u n i t s , from several instruments. Shear rate overlap between instruments insures the integrity of the data, since a systematic error due to a p a r t i c u l a r instrument will be detected. With absol u t e data, c o n s t i t u t i v e or e m p i r i c a l r e l a t i o n s h i p s can be used to model the dependence of v i s c o s i t y on shear r a t e . Such models are e s s e n t i a l to p r e d i c t flow 160

In Extracellular Microbial Polysaccharides; Sandford, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

Rheology

of Xanthan

Gum

Solutions


WHITCOMB ET AL.


162

EXTRACELLULAR MICROBIAL POLYSACCHARIDES

behavior, design equipment, and i n t e r p r e t rheology i n terms o f molecular s t r u c t u r e . I t i s t h e purpose o f t h i s work t o c o l l e c t a b s o l u t e data and e v a l u a t e i t i n t e r m s o f an a p p r o p r i a t e model.


Description A commercial p r o d u c t , G A L A X Y X B X a n t h a n gum ( l o t D 5 3 5 3 A ) , was u s e d f o rt h i s s t u d y . The c o m m e r c i a l p r o d u c t was p u r i f i e d f u r t h e r u s i n g a m o d i f i c a t i o n o f a p r o c e d u r e b y J e a n e s (l) : T h e gum i s h y d r a t e d i n a water-ethanol mixture, c e n t r i f u g e d , p r e c i p i t a t e d and washed. T h e w e t gum i s t h e n d r i e d a n d g r o u n d , A schematic o f t h e p u r i f i c a t i o n procedure i s given i n F i g u r e 3. A n a l y s i s o f t h e commercial and p u r i f i e d gums a r e g i v e n i n T a b l e I , T h e v i s c o s i t y o f t h e p u r i f i e d gum i s s l i g h t l y l e s s t h a n t h a t o f t h e c o m m e r c i a l g u m , s e e F i g u r e k. The r e m a i n d e r o f t h i s paper d e a l s o n l y w i t h t h e p u r i f i e d gum, TABLE

1

ANALYSIS Commercial Product S o l u t i o n O.D, kOO NM Water

- Wt. %

Nitrogen Protein Ash

- Wt, % MFB -

Wt, % MFB

- Wt, % MFB

Sodium

- Wt. % MFB

Phosphorus

- W t , % MFB

Purified Gum

.07

.19 10,6

1 3 .0

0.67

0 .69

3.8

3 .9

13.8

7 .9

k.h

1 .9

0.32

0 .23

A l l s o l u t i o n s used i n t h i s study were prepared f r o m gum h y d r a t e d i n d i s t i l l e d w a t e r . The s o l u t i o n s w e r e p r e p a r e d b y s p r i n k l i n g t h e g r o u n d x a n t h a n gum onto t h e s i d e s o f a v o r t e x formed i n a high speed blender. A l l gum c o n c e n t r a t i o n s a r e r e p o r t e d o n a moisture free basis.


WHITCOMB E T A L .

Rheology

of Xanthan

Gum

Solutions

95% eTWAMOU

IL Downloaded by UCSF LIB CKM RSCS MGMT on August 26, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0045.ch012

moisi

Figure 3.

Purification procedure

4+

••

•

4470 Ppri COflHEftciAL XAHIHOH

•

4470 PPM PUfUHfcP )j

2 +

S

ot

•—I—

-4

-z

H— Ο

UOC, 5H&AP- £ATe, -

t

Figure 10.

»

Xanthan gum rheology


*

h

12.

WHITCOMB ET AL.

Rheology

of Xanthan

Gum

Solutions

extrapolated t o zero concentration. This i s i l l u s t r a t e d i n F i g u r e 11 a n d y i e l d s an v i s c o s i t y o f 35.70 d e c i l i t e r s / g m .


Discussion

and

169

determination intrinsic

Conclusions

There appears t o be a c r i t i c a l concentration b e t w e e n 2 , 0 0 0 a n d 1 0 , 0 0 0 ppm a b o v e w h i c h a y i e l d s t r e s s exists. I d e n t i f y i n g and measuring y i e l d stress i s important for p r e d i c t i n g t h e long term s t a b i l i t y o f suspensions, Concentrations below t h e c r i t i c a l value w i l l have a Newtonian r e g i o n a t very l o w shear r a t e s , This r e g i o n i s c h a r a c t e r i z e d by t h e zero shear rate v i s c o s i t y , r)o. A l l c o n c e n t r a t i o n s s h o u l d have a Newtonian r e g i o n a t very high shear r a t e s . Without t h i s r e g i o n t h e s o l u t i o n v i s c o s i t i e s w i l l go b e l o w that o f their solvent, water. This region i s charac t e r i z e d by t h e i n f i n i t e shear r a t e v i s c o s i t y , . Many m a t h e m a t i c a l e x p r e s s i o n s have been proposed to model t h e p s e u d o p l a s t i c behavior e x h i b i t e d by xanthan. The most w i d e l y used i s t h e power l a w o f O s t w a l d (6) . η = Κ f ~ n

1

The p o w e r l a w i s a p p e a l i n g b e c a u s e o f i t s s i m p l i c i t y , t h e r e a r e o n l y t w o a d j u s t a b l e p a r a m e t e r s n a n d Κ. K, t h e v i s c o s i t y a t 1 s e c "" * m e a s u r e s c o n s i s t e n c y a n d n , t h e f l o w i n d e x , m e a s u r e s p s e u d o p l a s t i c i t y . Many e m p i r i c a l and a n a l y t i c a l s o l u t i o n s f o r complex flows have been worked o u t u s i n g t h e power law. Examples b e i n g l a m i n a r a n d t u r b u l e n t p i p e f l o w , (7.) a n n u l a r f l o w (8_) , f l o w t h r o u g h p o r o u s m e d i a (9.) » m i x i n g c h a r a c t e r i s t i c s a n d h e a t t r a n s f e r p r o b l e m s (10_) t o c i t e a few. The m a i n d i s a d v a n t a g e o f t h e power l a w i si t s f a i l u r e i n t h e r e g i o n s o f v e r y l o w s h e a r , rio o r τ , a n d very high shear, . More s o p h i s t i c a t e d models c S n account f o r t h e Newtonian regions i n high and l o w shear rate regions. However, f o r t h e xanthan c o n c e n t r a t i o n s s t u d i e d t h e l o g data i s l i n e a r f o r s e v e r a l decades o f shear r a t e . T a b l e I I I shows t h e power l a w f i t o f o u r xanthan data. The h i g h d e t e r m i n a t i o n i n d e x and t h e broad range o f shear r a t e f i t f o r each c o n c e n t r a t i o n v e r i f y t h e u t i l i t y o f t h i s model, 1


170


TABLE I I I POWERLAW

CONSTANTS

Κ CONC. (DYNE*SEC) η PPM /CM DIMENSIONLESS 2


10,000

35

2,5xKf

2

2.5xlO

1

6.25xlO

3.U

.39

l.OxlO""

1,000

0 98

Λ9

U.TlxlO

0.23

.6k

h.71x10"

kkj

1

- 1

U.OxlO* 2,5xl0

.98

+ 3

.23

2,000

f

DETERMINATION INDEX

1

γ RANGE, SEC"" MAX. MIN.

+ 2

2

+ 2

.97 .99 ,99

Examination of Table I I I reveals Κ increases with increasing concentration while η decreases. T h i s means t h a t t h e h i g h e r t h e c o n c e n t r a t i o n o f xanthan t h e thicker the s o l u t i o n . However, h i g h e r c o n c e n t r a t i o n s a r e more p s e u d o p l a s t i c , have a l o w e r η v a l u e , so a t h i g h shear r a t e s a l l c o n c e n t r a t i o n s , a t l e a s t t h o s e o f 1 0 , 0 0 0 ppm and l e s s , a p p r o a c h t h e v i s c o s i t y o f w a t e r . The power l a w f i t f o r t h e 1 0 , 0 0 0 a n d 1 , 0 0 0 ppm s o l u t i o n s a r e shown i n F i g u r e 12. Note t h e d i f f e r e n c e i n Κ and η f o r the two c o n c e n t r a t i o n s , see T a b l e I I I . Power law d a t a i s v e r y v a l u a b l e i n e v a l u a t i n g s o l u t i o n p r o p e r t i e s and solving p r a c t i c a l problems. A n o t h e r type o f t h e o r y has been used t o e x p l o r e intrinsic viscosity. Assuming the conformation of a x a n t h a n m o l e c u l e c a n be a p p r o x i m a t e d by a c y l i n d r i c a l rod, i t i s possible to estimate i t s characteristic length. U s i n g t h e t h e o r y o f K h a l i k a n d B i r d (11.) r o d l e n g t h c a n be d e t e r m i n e d by t h e f o l l o w i n g e x p r e s s i o n : T.3

=

[η]

(US)

L [η]MW D Ν -

(MW)

( I n (L/D)

)

Rod l e n g t h Intrinsic viscosity Molecular weight Rod d i a m e t e r A v o g a d r o number

T h i s e x p r e s s i o n can be s o l v e d by i t e r a t i o n i f r o d l e n g t h i s t h e o n l y unknown. The m o l e c u l a r w e i g h t o f x a n t h a n h a s e s t i m a t e d t o b e i n t h e r a n g e o f l . U t o 3.6 χ 10 ( D i n t z i s et a l (12)) , The i n t r i n s i c viscosity was m e a s u r e d t o be 35.7 d e c i l i t e r s / g m . We h a v e 6



12.

WHiTCOMB E T A L .

ο

1

1

1

'

Rheology of Xanthan Gum Solutions

4 — ι — ι — ι — ι — \ — ι — Η .ά>5 .ΟΙΟ Λ ~ '' ' J. ~

171

1 — ι — I — ι — ι — I — " — I .οΊ* *™ Jill

Figure 11.

Intrinsic viscosity

Figure 12.

Power lawfitof data


172


v

v

estimated the diameter of a xanthan r o d t o be i n t h e r a n g e o f l 6 t o kOA the length calculation i s quite i n s e n s i t i v e t o diameter, 9

TABLE


CALCULATED MOLECULAR WEIGHT GM/MOLE

IV LENGTH

DIAMETER 1

[η] ML/GM

LENGTH MICRONS

l.k

χ

10

6

16

3570

0,73

l.U

χ

10

6

ho

3570

0.68

3.6 χ

10

6

16

3570

1.01

3.6 χ

10

6

ho

3570

0.96

Table IV gives the r e s u l t s of the r o d length calculation. I t would appear that a xanthan " r o d " has a l e n g t h b e t w e e n 0.7 a n d 1.0 m i c r o n s . This i s i n g o o d a g r e e m e n t w i t h H o l z w a r t h s (l_3) membrane c h r o m a tography measurements. He f o u n d t h a t e s s e n t i a l l y a l l x a n t h a n p a r t i c l e s c a n p a s s t h r o u g h a membrane w i t h 1.0 m i c r o n p o r e s b u t a r e b l o c k e d by a membrane w i t h 0.8 micron pores. f

Nomenclature D Κ L M MWn Ν O.D. R Τ β t η Ποη

οο-

[η]

Rod d i a m e t e r , (microns) Power l a w c o n s t a n t , i n t e r c e p t Rod l e n g t h (microns) Torque (gm-cm) Molecular Weight Power l a w c o n s t a n t , s l o p e A v o g a d r o number Optical Density Cone r a d i u s (cm) T e m p e r a t u r e (°C) Cone a n g l e (radians) Shear r a t e (sec ) Viscosity (poise) Zero shear r a t e v i s c o s i t y (poise) (poise) I n f i n i t e shear rate v i s c o s i t y Intrinsic viscosity (deciliters/gm) - 1


WHITCOMB E T A L .

12.

τ τ oo

Rheology of Xanthan Gum Solutions

173

2

1

2

Shear s t r e s s (dyne/cm ) Yield stress (dyne/cm ) Angular speed (radians/sec , 1 2

y

Literature Cited


(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

Jeanes, A., P i t t s l e y , J.E., Senti, F.R., J. Applied Polymer S c i , (1961), 5, p. 519-526. Van Wazer, J.R., et al, "Viscosity and Flow Measurement", p. 113-116, Wiley, Ν.Υ. 1963. Macosko, C.W., S t a r i t a , J., S.P.E. Journal,(1971), 27, p. 38-42. Middleman, S., "The Flow of High Polymers", p.28, Wiley, New York, 1968. Willey, S.J. Ph.D. Thesis, University of Minnesota, 1976. Ostwald, W., Kolloid-Zeitschrift, (1925), 36, p. 99-117. Metzner, A . B . , Reed, J.C., A.I.Ch.E. J., (1955), 1, p. 434-440. Mishra, P., Mishra, I., A.I.Ch.E. J.,(1976), 22, p. 617-619. Sheffield, R.E., Metzner, A . B . , A.I.Ch.E. J., (1976), 22, p. 736-744. Wilkinson, W.L., "Non-Newtonian Fluids", Pergamon, New York, 1960. Abdel-Khalik, S . K . , B i r d , R.Β., Biopolymers, (1975), 14, p. 1915-1932. D i n t z i s , E . R . , Babcock, G.Ε., Tobin, R., Carbohydrate Research. (1970), 13, p. 257-267. Holzwarth, G . , "Polysaccharide from Xanthomonas Campestris: Rheology, Solution Conforma t i o n , And Flow Through of Petroleum Chemistry, A.C.S., New York Meeting, April 4-9, 1976.


Rheology of Xanthan Gum Solutions

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