Langmuir 2003, 19, 10829-10834
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Adsorption of Modified Starches on Porous Glass Mehdi Shirazi,† Theo G. M. van de Ven,*,‡,§ and Gil Garnier†,‡,| Department of Chemical Engineering, Paprican, and Department of Chemistry, Pulp and Paper Research Centre, McGill University, Montreal, Canada H3A 2A7 Received June 16, 2003. In Final Form: September 26, 2003 The adsorption of cationic starch and nonionic hydroxyethyl ether starch on porous glass was investigated, for pore sizes in the range 7-222 nm. Well-dissolved cationic starch and the lower-molecular-weight hydroxyethyl ether starch both penetrate pores. Differences in the amylose/amylopectin ratio before and after adsorption indicate that it is the amylose fraction that penetrates pores. Freshly dissolved cationic starch is present as clusters in solution. These clusters, which are unable to penetrate pores, break up with time and shear. Adsorbed clusters do not desorb. Because the cluster size increases with starch concentration, the adsorption isotherms resemble low-affinity isotherms, despite the fact that adsorption is quasi-irreversible. Clustering is absent for hydroxyethyl ether starch, for which also low-affinity isotherms are observed. These isotherms are the result of a dynamic equilibrium between adsorption and desorption. Similarities and differences with starch adsorption on pulp fibers are discussed. Table 1. Properties of the Glass Substratesa
Introduction The adsorption of neutral and cationic starches on porous surfaces is important in a number of industrial processes, such as starch adsorption on pulp fibers and on textiles. Starch adsorption on pulp fibers was studied by us before,1 as well as by many others.2-8 Pulp fibers have a complex topography because they are rough, fibrillated, porous, and likely covered by a water-soluble layer of bound and adsorbed hemicelluloses. To study the effects of porosity in isolation, we looked at the adsorption of starches on smooth glasses having various well-defined porosities. In this paper, we discuss experimental findings on model porous glasses, with pores in the range 7-222 nm. As starches, we used cationic starch and hydroxyethyl ether starch, which is nonionic. To follow the expected preferential adsorption of amylose over amylopectin (the two components of starch), we measured the amylose/ amylopectin ratio before and after adsorption. After interpreting the adsorption data on glass, we will discuss the relevance of these findings to starch adsorption on pulp fibers at the end of the paper. * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemical Engineering, McGill University. ‡ Paprican, McGill University. § Department of Chemistry, McGill University. | Presently at Kimberly Clark, 2100 Winchester Road, Neenah, Wisconsin 54956. (1) Shirazi, M.; van de Ven, T. G. M.; Garnier, G. Adsorption of Modified Starches on Pulp Fibers. Langmuir, in press. (2) Wågberg, L.; O ¨ dberg, L. Polymer adsorption on cellulosic fibers. Nord. Pulp Pap. Res. J. 1989, 2, 135. (3) Hedborg, F. Adsorption of cationic starch on bleached softwood cellulosic fibers. Nord. Pulp Pap. Res. J. 1993, 2, 258. (4) Nedelcheva, M. P.; Stoilkov, G. V. Cationic starch adsorption by cellulose. J. Colloid Interface Sci. 1978, 66 (3), 475. (5) Wågberg, L.; Bjorklund, M. Adsorption of cationic potato starch on cellulosic fibers. Nord. Pulp Pap. Res. J. 1993, 4, 399. (6) Wågberg, L.; Kolar, K. Adsorption of cationic starch on fibers from mechanical pulp. Ber. Bunsen-Ges. Phys. Chem. 1996, 100 (6), 984. (7) Marton, J.; Marton, T. Wet end starch: adsorption of starch on cellulosic fibers. Tappi J. 1976, 59 (12), 121. (8) van de Steeg, H. G. M.; de Keizer, A.; Cohen Stuart, M. A.; Bijsterbosch, B. H. Adsorption of cationic potato starch on microcrystalline cellulose. Colloids Surf., A 1993, 70, 91.
glass
mean pore size (nm)
Aexternal (m2/g)
Atotala (m2/g)
pore volumea (cm3/g)
GF PG-75 PG-500 PG-2000
0 7 49 222
0.364 0.064 0.082 0.166
0.364 212.2 43.3 27.5
0 0.61 0.92 2.32
a
Provided by the manufacturer.
Experimental Section Materials. Nonporous glass fibers (GF, diameter 5 µm, length 3 mm) and three types of porous glasses were used as the substrates. The average pore sizes were 7, 49, and 222 nm (PG75, PG-500, PG-2000), determined from benzene adsorption/ desorption isotherms, using the Kelvin equation 9. The porous glass particles all had similar shapes with diameters between 75 and 120 µm. All the glass substrates were supplied by CPG, Inc. (Lincoln Park, NJ). The glass substrates were acid-washed and rinsed extensively before the experiments. The external and the total surface areas of the glass substrates are given in Table 1, whereas the scanning electron microscopy pictures of the glasses are shown in Figure 1. North American modified corn starch was used in all experiments. The specifications of the starches are given in Table 2. The degrees of substitution of the derivative groups and the molecular weights of amylose and amylopectin were measured by the manufacturer from the reaction conditions and by size exclusion chromatography, respectively. Methods. The starch solution (1%) was cooked daily in a microwave. It was stirred every 10 s until the temperature reached 98 °C (approximately 1 min total heating time). The solution was kept at this temperature for 30 min in an oven before adding cooled water and adjusted to 0.5% concentration. This solution was constantly stirred at low shear and was used as the stock solution. The standard setup consists of a beaker containing 1 g of glass particles suspended in 1 L of solution, stirred at 200 rmp, corresponding to an effective shear rate of about 50 s-1.9,10 Before a starch adsorption experiment, first an iodine solution (2 mg iodine + 20 mg KI) was added to the beaker and stirred for a few minutes. Iodine is a well-known starch (9) Alince, B.; van de Ven, T. G. M. Porosity of Swollen Pulp Fibers Evaluated by Polymer Adsorption. In The Fundamentals of Papermaking Materials, Transactions of the 11th Fundamental Research Symposium, Cambridge, U.K., Sept 1997; Baker, C. F., Ed.; Pira International: Surrey, U.K.; Vol. 2, pp 771-788. (10) Asselman, T.; Garnier, G. Adsorption of model wood polymers and colloids on bentonites. Colloids Surf., A 2000, 168, 175.
10.1021/la0350655 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/19/2003
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Table 2. Specifications of the Modified Starches type of starch cationic starch with quaternary ammonium group hydroxyethyl ether starch with ether group
DS
Mw (amylose)
Mw (amylopectin)
manufacturer
0.031 0.074
1.4 × 9 × 104
3× 3 × 107
Penford, IA Penford, IA
indicator; iodine forms a blue complex with starch (λ ∼ 585 nm), the intensity of which depends strongly on concentration.11
105
108
The hydrodynamic radius of polymers was measured using a dynamic light scattering apparatus with a vertically polarized 50-mW He-Ne laser (Spectra Physics). The scattering plane was perpendicular to the incident light polarization. The incident light wavelength was 632.8 nm. A commercial goniometer (Brookhaven Instruments BI-2030) was used with its original integrated optics to measure the scattered light at 90°. The temperature of the scattering cell was controlled at 25 °C.
Results and Discussion Properties of Starches. The apparent hydrodynamic radii of the cationic and the hydroxyethyl ether starch at different concentrations are shown in Figure 2. Both the cationic starch and the hydroxyethyl ether starch show a linear increase in the apparent radius with starch concentration, but the increase for cationic starch is much more pronounced. The apparent radius is obtained from the diffusion constant measured by dynamic light scattering and applying the Stokes-Einstein equation. However, the diffusion constant, D, is a function of concentration c:12
D ) D0(1 + kc)
Figure 1. (a) Glass fiber (GF), (b) porous glass particles (PG), and (c) magnification of the porous glass surface. In a typical experiment, starch was added to the beaker and the suspension was filtered continuously through a 200-mesh screen (74-µm openings), and the supernatant was sent to a spectrophotometer (Varian CARY IE UV/VIS) to measure the absorbency at 580 nm. The absorbency data were converted to concentrations using a calibration curve. The calibration curves for both starches were found to be linear in the range of our experiments. From the concentration in the supernatant, the amount on the glass can be readily determined. The amylose/ amylopectin ratio was measured at the end of an experiment, by treating part of the supernatant with I2 + KI and another part with acid-phenol, followed by measuring the absorbency at two wavelengths (490 and 625 nm), resulting in two equations with two unknowns (i.e., the amylose and amylopectin concentrations). More details of the experimental setup can be found in ref 1. (11) Banks, W.; Greenwood, C. T. Starch and its Components; Edinburgh University Press: Edinburgh, 1975.
(1)
The radius of (nonclustered) cationic starch molecules was estimated by extrapolating the data to zero concentration, which gives a value of 108 nm. This value is in good agreement with the value reported in the literature.13 The hydrodynamic radius of the hydroxyethyl ether was also extrapolated to zero concentration, and the estimated radius for the hydroxyethyl ether molecules was 47 nm, which is in the range of the reported size in the literature.14 In dynamic light scattering, the average size is mainly influenced by the largest molecules, which are amylopectin. The slope depends on the interaction energy, E, between two starch molecules, but also on qa (q being the magnitude of the scattering vector and a the starch radius).12 The extrapolated radii differ roughly by a factor of 2, and the starch-starch interaction is expected to be similar for both starches because they are similar chemically, their degree of substitution being very low. The slopes for the two starches differ, however, by 2 orders of magnitude, which is difficult to explain by differences in E and qa. Much more likely is the explanation that cationic starch forms clusters, which grow to sizes up to almost 3.5 µm. Further support of the clustering hypothesis is shown in Figure 3, which shows that the apparent hydrodynamic radius of cationic starch decreases with time, when subjected to shear. This decrease in size indicates the breakdown of starch clusters. The size of hydroxyethyl ether starch is not affected by shear, implying that it does not form clusters. Other evidence of cationic starch cluster formation follows from the adsorption kinetics of cationic starch on pulp fibers, which shows a maximum adsorption at intermediate times.1 This maximum is ascribed to the gradual displacement of clusters by single molecules. [The (12) van de Ven, T. G. M. Colloidal Hydrodynamics; Academic Press: New York, 1989; p 343. (13) van de Steeg, H. G. M. Ph.D. Thesis, University of Wageningen, The Netherlands, 1992. (14) Husband, J. C. The adsorption of starch derivatives onto kaolin. Colloids Surf., 1998, 131, 145.
Modified Starches on Porous Glass
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Figure 2. Apparent hydrodynamic radius of cationic starch and hydroxyethyl ether starch as a function of concentration.
Figure 4. Adsorption kinetics of 40 mg/g cationic starch clusters on porous glass (PG-500) for different cluster radii indicated in the figure.
Figure 3. Hydrodynamic radius of cationic and hydroxyethyl ether starch as a function of shearing time (0.5%).
possibility that cluster dispersion occurs at the surface of the fibers was rejected because this does not happen on glass (see the following).] The reason for the starch clustering is unknown. One possibility is that there are some intermolecular ionic bonds between cationic groups and anionic phosphate groups (which are naturally present at very low degree of substitution, DS). However, clusters do not reform after breakup, perhaps because intermolecular bonds are replaced by intramolecular ones. Adsorption of Cationic Starch. The adsorption of the clusters on porous glass (PG-500) was investigated. To study the effect of the cluster size, kinetic experiments were performed with cationic starch having different cluster sizes, obtained by subjecting the starch solution to various shearing times. The results are shown in Figure 4. As can be seen, the maximum adsorption increases with the cluster size. The increase with the cluster size was also observed for cationic starch adsorption on nonporous glass fibers, for which the maximum adsorption at equilibrium increased linearly with increasing cluster size (cf. Figure 5). The maximum adsorption of well-dispersed cationic amylopectin on nonporous fibers, Γmax, can be estimated as follows:15
Γmax ) kc
Mw A NA πa2
(2)
(15) van de Ven, T. G. M. A model for the adsorption of polyelectrolytes on pulp fibers: relation between fiber structure and polyelectrolyte properties. Nord. Pulp Pap. Res. J. 2000, 15 (5), 494.
Figure 5. Measured maximum adsorption capacity of cationic starch on glass fibers as a function of cluster size. ([) 40 mg/L (solution subjected to two different levels of shear) and (9) 100 mg/L. The cluster size was measured before each experiment.
Here, Mw/NA is the mass of a single polymer (NA is Avogadro’s number), A is the external surface area of the glass particles (m2/g), and a is the radius of a single molecule. The coverage factor (kc) is 0.9 for hexagonal close-packing molecules.16 The molecular weight and the hydrodynamic radius of the cationic amylopectin were measured as 3 × 108 g/mol and 108 nm, respectively. If the molecules maintain their spherical shapes upon adsorption, Γmax for the available surface area (0.364 m2/g for glass fiber, cf. Table 1) would be about 4.5 mg/g. For 73% amylopectin and 27% amylose, assuming amylose adsorption is about 1 mg/m2 and assuming no preferential adsorption on nonporous glass, the expected maximum adsorption is about 3.4 mg/g. The value obtained from experiment is 7.1 mg/g (cf. first data point in Figure 5), which is about twice as large. The difference is likely due to the fact that dynamic light scattering overestimates the size. The size of cationic starch obtained from intrinsic viscosity experiments was half the value obtained from dynamic light scattering,13 which would increase the estimate of the maximum adsorption by a factor of 4. Assuming starch clusters are as compact as individual (16) Dabros, T.; van de Ven, T. G. M. Collision-induced dispersion of droplets attached to solid particles. J. Colloid Interface Sci. 1994, 163 (1), 28.
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Figure 6. Kinetics of well-dispersed cationic starch adsorption on various types of porous glasses (starch addition 20 mg/g). The mean pore diameter is indicated in the figure.
starch molecules, we can estimate the apparent molecular weight of the clusters as follows:
( )
Mw,cluster acluster ) Mw,0 a0
n
(3)
where the exponent n depends on the compactness of an amylopectin molecule. For a similar compact globular branched polyelectrolyte, namely, poly(ethylene imine) (PEI), n )1/0.39 ) 2.6.9 Assuming the same value for amylopectin and knowing the molecular weight of the individual polymer and the radii of both the polymers and the clusters, the molecular weight of the clusters can be estimated. Using this approximation, the molecular weights of the clusters with 235- and 685-nm radii are calculated to be 2.3 × 109 and 3.7 × 1010, respectively, and the maximum adsorption capacity (Γmax) can be calculated as 10 and 25 mg/g. These estimations correspond to the experimental values of 14 and 36 mg/g (Figure 5). The fact that the estimated values are lower than the observed ones is again likely due to an overestimation of the cluster sizes by dynamic light scattering. We can, nevertheless, conclude that clusters form a monolayer on surfaces and that the maximum adsorption capacity is a (nearly) linear function of the cluster size. From these estimates, it follows that the clusters can contain up to about 100 molecules (in agreement with cluster sizes up to micrometers, cf. Figure 2). The fact that a pseudo-plateau is reached in Figure 4 is evidence for an irreversible cluster adsorption because preferential cluster desorption over the desorption of single molecules would result in a decrease in adsorption with time.1 Thus, the value at the plateau, Γ∞, equals Γmax. Although for nonclustered polyelectrolytes adsorption also increases with increasing concentration, it is difficult to ascribe an increase of a factor of 5 to causes other than clustering. The adsorption kinetics of well-dispersed cationic starch on the porous glasses is shown in Figure 6. The starch was diluted to 0.01% and subjected to high shear for 2 h to completely disperse the clusters. The initial adsorption rate (initial slope) was the highest for PG-2000 (pore size 222 nm) and decreased with decreasing pore size. The difference between the initial adsorption rates can be related to the collision frequency between polymers
and the porous glass particles (75-120 µm). The collision frequency Nij between the glass particles of concentration ni and the polymer molecules of concentrations nj can be calculated by17
Nij ) ksmninj
(4)
where ksm is the Smoluchowski rate constant. An analysis of the collision process shows that the collisions between starch molecules and glass particles are orthokinetic; that is, the initial adsorption rate is determined by shear18 and occurs on the external surface only. It can be concluded that the higher initial adsorption rate for PG-2000 is due to a higher collision rate (more particles in suspension). However, the equilibrium adsorption for PG-75 is higher because it has a higher (internal) available surface area. Adsorption of Hydroxyethyl Ether Starch. For hydroxyethyl ether starch, the initial adsorption on glass fibers is followed by polymer desorption from the surface (Figure 7). This behavior can be explained by the competition for adsorption between large and small molecules and the preferential desorption of larger molecules. This is opposite to the usual exchange of lowmolecular-weight polymers by high-molecular-weight ones. The reason is that in our system the desorption rate is determined by shear, and larger molecules experience larger shear forces. Hydrogen bonding was reported to be the main driving force for the adsorption of nonionic polymer on substrates with carboxylic or silanol groups.14,19 To be more precise, the driving force is due to differences in hydrogen bonding between polymer molecules and water and the increased entropy of the desorbed water. This bonding results in weaker bonds than those created by electrostatic attraction. This explains why adsorption of hydroxylethyl ether starch on glass is reversible, while that of cationic starch is (quasi-)irreversible. Hydroxyethyl ether starch adsorp(17) van de Ven, T. G. M. Particle Deposition on Pulp Fibers: The Influence of Added Chemicals. Nord. Pulp Pap. Res. J. 1993, 1 (8), 130-134. (18) Shirazi, M. Surface applications of yellowing inhibitors into paper. Ph.D. Thesis, Department of Chemical Engineering, McGill University, Montreal, Canada, 2001; pp 128-132. (19) Ishimaru, Y.; Lindstro¨m, T. Adsorption of water-soluble, nonionic polymer onto cellulosic fiber. J. Appl. Polym. Sci. 1984, 29, 1675.
Modified Starches on Porous Glass
Figure 7. Adsorption kinetics of hydroxyethyl ether starch (40 mg/g) on porous glass (PG-75) and glass fibers.
Figure 8. Adsorption isotherm of cationic starch clusters on porous glass substrates. Amylose and amylopectin showed a similar affinity for all the glass substrates.
tion in porous glass increases gradually as a result of polymer penetration into pores. Comparisons with Langmuir Adsorption Isotherms. The adsorption isotherms for the adsorption of cationic starch clusters on various porous glasses are shown in Figure 8. Because the amylose/amylopectin ratio did not change during adsorption, it follows that no pore penetration occurs, in which case amylose should adsorb preferentially. The fact that amylose penetration occurs for well-dispersed starch but not for clustered starch indicates that the larger starch clusters block the pores and prevent penetration. As discussed before, the adsorption of clusters of cationic starch on glass is irreversible, and the increase of Γmax with starch concentration is due to the increase of the cluster size with concentration (cf. Figure 2). Despite the fact that the shape of the isotherm is not due to a dynamic equilibrium between adsorption and desorption as described by Langmuir kinetics, the data can be surprisingly well-represented by a Langmuir plot, as shown in Figure 9. This indicates that Langmuir plots provide no information about the mechanisms of adsorption. The maximum adsorption correlates well with the external surface area of the glass (cf. Table 1), indicating adsorption on the outside surface only. The hydroxyethyl ether starch adsorption on nonporous glass fibers and porous glass is shown in Figure 10. The maximum adsorption on the glass fibers is less than 1 mg/g, and the adsorbed starch keeps its original amylose/ amylopectin ratio. Increasing the pore size increases the hydroxyethyl ether starch adsorption on the porous glasses. Contrary to the adsorption of cationic starch, the maximum adsorption is lowest for the nonporous glass.
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Figure 9. Langmuir plots for the adsorption of cationic starch clusters on porous and nonporous glass substrates.
Figure 10. Adsorption isotherm of hydroxyethyl ether starch on glass substrates. The amylose percentage on the adsorbed starch is shown in brackets.
The amylose/amylopectin ratio of the adsorbed starch increases with increasing glass pore size. The amylose contents on the adsorbed hydroxyethyl ether starch on PG-75, PG-500, and PG-2000 were 43 ( 4, 34 ( 2, and 28 ( 2%, respectively. This indicates that preferential amylose adsorption occurs in the small pores but not in the large pores (of 222 nm). For PEI adsorption in porous glass,15 it was concluded that the pore size must be at least 3 times the size of the molecule for complete penetration to occur. If the same is true for amylopectin, the critical radius for complete pore penetration is about 37 nm. The measured size for hydroxyethyl ether starch was 47 nm, but again dynamic light scattering probably overestimates the size. Also, amylopectin is more disklike13 than PEI, in which case the critical radius must be replaced by the critical minor semi-axis. Thus, pores of 222 nm appear to be sufficiently large for amylopectin penetration. The isotherms are low-affinity isotherms and are the result of a dynamic equilibrium between adsorption and desorption. That desorption occurs can be concluded from the overshoot observed in Figure 7. The data of Figure 10 are replotted as Langmuir plots in Figure 11. Differences in the apparent equilibrium constants are likely related to different desorption rates, reflecting the difficulty for amylose and amylopectin to desorb from the glass through the pores. Comparisons between Starch Adsorption on Pulp Fibers and That on Porous Glass. The adsorption of starches on pulp fibers1 is very similar to that on porous glass. For both, the amylose fraction of well-dissolved cationic starch and hydroxyethyl ether starch can pene-
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Figure 11. Langmuir plots for the adsorption of hydroxyethyl ether starch on glass substrates.
trate the porous substrate, indicating that the pore sizes in fibers are in a similar range as the pores of the porous glasses studied here, except for the penetration of hydroxyethyl ether starch in glass with 222-nm pores. Both amylose and amylopectin can penetrate these large pores, as evidenced by the observation that the amylose/ amylopectin ratio did not change upon adsorption. For
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pulp fibers, always a preferential adsorption of amylose was observed, similar to that for the glasses with 7- and 49-nm pores. Cationic starch clusters only adsorb on the external surface and prevent amylose penetration by blocking pores. Adsorption of hydroxyethyl ether starch is reversible on both surfaces, resulting in low-affinity adsorption isotherms. The main difference is the kinetics of cluster adsorption. On glass, the adsorption is quasiirreversible, whereas on pulp fibers starch clusters desorb, resulting in a replacement of clusters by single molecules,1 evidenced by an overshoot in the adsorption kinetics. This implies that adsorbed clusters do not break up in smaller clusters (under our experimental conditions), in which case the same overshoot should be seen with glass fibers, but rather desorb from pulp fibers as intact clusters. These results indicate that the affinity of cationic starch clusters to glass is much larger than the affinity to pulp fibers. Acknowledgment. The authors thank Dr. S. Middleton (Paprican) for valuable suggestions. Financial support from the NCE (Wood Pulps Network) is gratefully acknowledged. LA0350655
