- Email: [email protected]
Studies on potassium-l -carrageenate gels by photon correlation spectroscopy
Notes to the Editor
Studies on potassiumL-carrageenate gels by photon correlation spectroscopy V. J. Morris and K. S. Fancey ARC Food Research Institute, Colney Lane, Norwich, NR4 7UA, UK (Received 16 September 1980; revised 3 December 1980)
The manipulation of the rheological properties of anionic polysaccharide gels, by controlling the ion content of the medium, is a topic of~onsiderable importance in the food industry 1. However, the mechanism of such ion effects, at the molecular level, is at present poorly understood. The present results form part of a research programme on the relative effects of different cations in z-carrageenate gels. This communication presents a preliminary account of the application of photon correlation spectroscopy (p.c.s.) to study molecular vibrational motion of potassium+ carrageenate gels and its interpretation in terms of the elastic properties of the system. Pure samples of potassium-t-carrageenate were prepared from commercial samples of t-carrageenan by ion exchange at 90°C. Gels were prepared in rectangular glass cuvettes: Appropriate concentrations of biopolymer were dispersed in double-distilled deionized water at 90°C, agitated to achieve a homogeneous dispersion, and allowed to cool to room temperature. P.c.s. measurements were made using single clipped homodyne detection at a wavelength of 633 nm. Above the sol-gel transition temperature the autocorrelation functions were found to be smoothly decaying multi-exponential functions. On cooling below this transition temperature, oscillatory correlation functions were observed (Figure la). The frequency of the oscillatory part
of the correlation function, at room temperature, was found to remain constant for up to two weeks. However, the amplitude of these oscillations varied erratically with time. Oscillatory correlation functions have been reported in a number of polymer and biopolymer gel systems including polyacrylamide 2"3, calcium alginate *'5, agarose 2"6,and collagen 2. Recent theoretical and experimental work, particularly on mechanically excited gels 2'3, attributes the effect to fluctuations of the density of the gel, and hence the refractive index, due to resonant oscillatory modes. We have tested for such resonant effects in potassium+carrageenate gels by using essentially the method proposed by Gelman et al. 2"3. By varying the frequency of the externally applied mechanical excitation we have observed the appearance of well-resolved resonant modes. Figures l b - l d illustrate a few typical results observed at resonant excitation frequencies. The resonant modes of vibration of a rectangular isotropic gel can be related to the elastic moduli 8. For gels such as carrageenates, which adhere strongly to the glass walls of the container, the resonant frequencies are expected to be of the forma: (#~t/2
where o9 is the angular frequency, p is the rigidity modulus and p the density of gel. k~ar is a wavenumber given byS;
where a, b and c are the dimensions of the gel sample and ~,/3 are integral and y integral or half-integral constants. The lowest order resonant mode corresponds to k0:½(Refs 3 and 8). We have interpreted the natural frequency of the unexcited gel as this lowest resonant mode and calculated # on the basis that p = l . Figure 2 shows a plot of calculated p values as a function of biopolymer concentration, c. The full line represents a least-squares fit to the eauation: p = k c " ( k = 6 0 _ 10P, m=2.3 +0.4).
~f
Figure 1 Photographs of the autocorrelation functions obtained from potassium-t-carrageenate gels. (a) unexcited gel, sample time (z)=2ms, (b) excitation frequency (f)= 100 Hz, z =0.5 ms, (c)f= 180 Hz, ~=0.5 ms, (d)f= 280 Hz, z =0.5 ms. The first four dots correspond to monitor channels and the remaining 48 represent the correlation function. The scattering angle (0) was 20° 0141-8130/81/0302134)2502.00 ©1981,IPC Business Press
In addition, we have included values of /~ measured conventionally using the modification of the Saunders and Ward method 9 suggested by Scott-Blair and Burnett 10, The data in Figures I and 2 suggest that the assignment of the oscillations of the correlation function to resonant vibrations of the gel is reasonable. The calculated values of p are of an acceptable order of magnitude and increase with increasing biopolymer concentration. There is some evidence of a systematic difference between the bulk rheological values and those calculated from the p.c.s. data. One plausible explanation is that the natural frequency is not, in fact, the lowest or k02½mode. However, we have been unable to excite mechanically resonant frequencies lower than the frequency observed for the unexcited gel. A second possibility is that in small gel samples, where the upper surface is not fiat, the shape of the meniscus and hence the influence of the container material may affect the measured resonant frequency. This is worthy of
Int. J. Biol. Macromol. 1981, Vol 3, June
213
N o t e s to the E d i t o r
further investigation as it c o u l d severely limit the usefulness of the t e c h n i q u e as an a b s o l u t e m e t h o d of e v a l u a t i n g elastic moduli. A t h i r d possibility, p a r t i c u l a r l y in view of the suggested structures for c a r r a g e e n a t e gels TM, is t h a t the gel structure is n o t isotropic a n d t h a t the technique preferentially selects a n d m o n i t o r s the m o t i o n of certain regions within the gel. These p r e h m i n a r y results are, however, e n c o u r a g i n g a n d suggest that, at least qualitatively, the t e c h n i q u e is p r o m i s i n g for the s t u d y of the elastic p r o p e r t i e s of gels. In a d d i t i o n , the e x t r e m e r a p i d i t y a n d n o n - i n v a s i v e n a t u r e of the m e t h o d a u g u r well for kinetic studies such as gelation, aging o r syneresis.
400
500
E :k
-
200-
References 1
•
j
2
o
3 4
IO0
5 6
o I~~°1 0
°
I
I C (wt
I 2
%1
Figure 2 Graph of rigidity modulus, #, vs. biopolymer concentration, c: o, values calculated from p.c.s, data; o, bulk values. ( ) Least squares fit of/~ = kc m to the p.c.s, data
214
Int. J. Biol. M a c r o m o l . 1981, Vol 3, June
7 8 9 10 11 12
Guisley, K. B. in 'Encyclopaedia of Chemical Technology' J. Wiley, New York, 1968, Vol. 17, p. 763 Brenner, S. L., Gelman, R. A. and Nossal, R. Macromolecules 1978, 11, 202 Gelman, R. A. and Nossal, R. Macromolecules 1979, 12, 311 Mackie, W., Sellen, D. B. and Sutcliffe, J. J. Polym. Sci. (Polym. Syrup.) 1977, 61, 191 Mackie, W., Sellen, D. B. and Sutcliffe, J. Polymer 1978, 19, 9 Wun, K. L, Feke, G. T. and Prins, W. Faraday Discussions Chem. Soc. 1974, 57, 146 Nossal, R and Brenner, S. L. Macromolecules 1978, 11, 207 Nossal, R. J. Appl. Phys. 1979, 50, 3105 Saunders, P. R. and Ward, A. C. in 'Proc. 2nd Int. Congress Rheol.' Butterworths, London, 1954, p. 284 Scott-Blair, G. W. and Burnett, J. Lab. Practice 1957, 6, 570 Robinson, G., Morris, E. R. and Rees, D. A. JCS Chem. Commun. 1980, 152 Morris, E. R., Rees, D. A. and Robinson, G. J. Mol. Biol. 1980, 138, 349
Studies on potassiumL-carrageenate gels by photon correlation spectroscopy V. J. Morris and K. S. Fancey ARC Food Research Institute, Colney Lane, Norwich, NR4 7UA, UK (Received 16 September 1980; revised 3 December 1980)
The manipulation of the rheological properties of anionic polysaccharide gels, by controlling the ion content of the medium, is a topic of~onsiderable importance in the food industry 1. However, the mechanism of such ion effects, at the molecular level, is at present poorly understood. The present results form part of a research programme on the relative effects of different cations in z-carrageenate gels. This communication presents a preliminary account of the application of photon correlation spectroscopy (p.c.s.) to study molecular vibrational motion of potassium+ carrageenate gels and its interpretation in terms of the elastic properties of the system. Pure samples of potassium-t-carrageenate were prepared from commercial samples of t-carrageenan by ion exchange at 90°C. Gels were prepared in rectangular glass cuvettes: Appropriate concentrations of biopolymer were dispersed in double-distilled deionized water at 90°C, agitated to achieve a homogeneous dispersion, and allowed to cool to room temperature. P.c.s. measurements were made using single clipped homodyne detection at a wavelength of 633 nm. Above the sol-gel transition temperature the autocorrelation functions were found to be smoothly decaying multi-exponential functions. On cooling below this transition temperature, oscillatory correlation functions were observed (Figure la). The frequency of the oscillatory part
of the correlation function, at room temperature, was found to remain constant for up to two weeks. However, the amplitude of these oscillations varied erratically with time. Oscillatory correlation functions have been reported in a number of polymer and biopolymer gel systems including polyacrylamide 2"3, calcium alginate *'5, agarose 2"6,and collagen 2. Recent theoretical and experimental work, particularly on mechanically excited gels 2'3, attributes the effect to fluctuations of the density of the gel, and hence the refractive index, due to resonant oscillatory modes. We have tested for such resonant effects in potassium+carrageenate gels by using essentially the method proposed by Gelman et al. 2"3. By varying the frequency of the externally applied mechanical excitation we have observed the appearance of well-resolved resonant modes. Figures l b - l d illustrate a few typical results observed at resonant excitation frequencies. The resonant modes of vibration of a rectangular isotropic gel can be related to the elastic moduli 8. For gels such as carrageenates, which adhere strongly to the glass walls of the container, the resonant frequencies are expected to be of the forma: (#~t/2
where o9 is the angular frequency, p is the rigidity modulus and p the density of gel. k~ar is a wavenumber given byS;
where a, b and c are the dimensions of the gel sample and ~,/3 are integral and y integral or half-integral constants. The lowest order resonant mode corresponds to k0:½(Refs 3 and 8). We have interpreted the natural frequency of the unexcited gel as this lowest resonant mode and calculated # on the basis that p = l . Figure 2 shows a plot of calculated p values as a function of biopolymer concentration, c. The full line represents a least-squares fit to the eauation: p = k c " ( k = 6 0 _ 10P, m=2.3 +0.4).
~f
Figure 1 Photographs of the autocorrelation functions obtained from potassium-t-carrageenate gels. (a) unexcited gel, sample time (z)=2ms, (b) excitation frequency (f)= 100 Hz, z =0.5 ms, (c)f= 180 Hz, ~=0.5 ms, (d)f= 280 Hz, z =0.5 ms. The first four dots correspond to monitor channels and the remaining 48 represent the correlation function. The scattering angle (0) was 20° 0141-8130/81/0302134)2502.00 ©1981,IPC Business Press
In addition, we have included values of /~ measured conventionally using the modification of the Saunders and Ward method 9 suggested by Scott-Blair and Burnett 10, The data in Figures I and 2 suggest that the assignment of the oscillations of the correlation function to resonant vibrations of the gel is reasonable. The calculated values of p are of an acceptable order of magnitude and increase with increasing biopolymer concentration. There is some evidence of a systematic difference between the bulk rheological values and those calculated from the p.c.s. data. One plausible explanation is that the natural frequency is not, in fact, the lowest or k02½mode. However, we have been unable to excite mechanically resonant frequencies lower than the frequency observed for the unexcited gel. A second possibility is that in small gel samples, where the upper surface is not fiat, the shape of the meniscus and hence the influence of the container material may affect the measured resonant frequency. This is worthy of
Int. J. Biol. Macromol. 1981, Vol 3, June
213
N o t e s to the E d i t o r
further investigation as it c o u l d severely limit the usefulness of the t e c h n i q u e as an a b s o l u t e m e t h o d of e v a l u a t i n g elastic moduli. A t h i r d possibility, p a r t i c u l a r l y in view of the suggested structures for c a r r a g e e n a t e gels TM, is t h a t the gel structure is n o t isotropic a n d t h a t the technique preferentially selects a n d m o n i t o r s the m o t i o n of certain regions within the gel. These p r e h m i n a r y results are, however, e n c o u r a g i n g a n d suggest that, at least qualitatively, the t e c h n i q u e is p r o m i s i n g for the s t u d y of the elastic p r o p e r t i e s of gels. In a d d i t i o n , the e x t r e m e r a p i d i t y a n d n o n - i n v a s i v e n a t u r e of the m e t h o d a u g u r well for kinetic studies such as gelation, aging o r syneresis.
400
500
E :k
-
200-
References 1
•
j
2
o
3 4
IO0
5 6
o I~~°1 0
°
I
I C (wt
I 2
%1
Figure 2 Graph of rigidity modulus, #, vs. biopolymer concentration, c: o, values calculated from p.c.s, data; o, bulk values. ( ) Least squares fit of/~ = kc m to the p.c.s, data
214
Int. J. Biol. M a c r o m o l . 1981, Vol 3, June
7 8 9 10 11 12
Guisley, K. B. in 'Encyclopaedia of Chemical Technology' J. Wiley, New York, 1968, Vol. 17, p. 763 Brenner, S. L., Gelman, R. A. and Nossal, R. Macromolecules 1978, 11, 202 Gelman, R. A. and Nossal, R. Macromolecules 1979, 12, 311 Mackie, W., Sellen, D. B. and Sutcliffe, J. J. Polym. Sci. (Polym. Syrup.) 1977, 61, 191 Mackie, W., Sellen, D. B. and Sutcliffe, J. Polymer 1978, 19, 9 Wun, K. L, Feke, G. T. and Prins, W. Faraday Discussions Chem. Soc. 1974, 57, 146 Nossal, R and Brenner, S. L. Macromolecules 1978, 11, 207 Nossal, R. J. Appl. Phys. 1979, 50, 3105 Saunders, P. R. and Ward, A. C. in 'Proc. 2nd Int. Congress Rheol.' Butterworths, London, 1954, p. 284 Scott-Blair, G. W. and Burnett, J. Lab. Practice 1957, 6, 570 Robinson, G., Morris, E. R. and Rees, D. A. JCS Chem. Commun. 1980, 152 Morris, E. R., Rees, D. A. and Robinson, G. J. Mol. Biol. 1980, 138, 349