J. Phys. Chem. B 2010, 114, 15857–15861
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Aggregation Behavior of Sodium Lignosulfonate in Water Solution Xueqing Qiu, Qian Kong, Mingsong Zhou,* and Dongjie Yang School of Chemistry and Chemical Engineering, Guangdong ProVincial Laboratory of Green Chemical Technology, South China UniVersity of Technology, Guangzhou, Guangdong, 510640, China ReceiVed: July 28, 2010; ReVised Manuscript ReceiVed: September 19, 2010
Lignosulfonate is a type of macromolecular surfactant widely used as interfacial additive in various industrial fields and it is produced during chemical pulping process. In this paper, we present a new effective method for measurement of the critical aggregation concentration (CAC) of sodium lignosulfonate (SL) in water solution, with which a value of 0.38 g L-1 was obtained. Through the determination of CAC and observation by DLS, the state and dynamics of the formation of the SL micelles were disclosed. The results showed that SL was the state of individual molecules when its mass concentration was less than CAC; the individual SL molecules started to aggregate above CAC and thus micelles formed and grew with increasing SL concentration. The SL solution was quickly frozen and the structures of SL molecules or micelles were observed by ESEM, revealing that the spherical micelles were the main form of SL in the solution. Based on the results, the spherical hollow vesicular structure is proposed as a model of the aggregated micelles of SL in the solution. 1. Introduction Nowadays, environmental concerns have stimulated interest in utilizing renewable resources in various industrial fields. Particularly, the utilization of plant-derived materials such as industrial residues has received increasing attention. Lignin is one of the most abundant biomacromolecules existing in the plant kingdom, and many functional groups such as aromatic ring, hydroxy, phenol hydroxyl, and carboxy are contained in its molecule, and so it is a promising renewable resource to obtain new materials for industry.1 Lignosulfonate is a sort of water-soluble lignin derivative, produced as a byproduct of the sulfite pulping process. During the sulfite pulping process, the native lignin is broken down by cutting randomly distributed R-O-4 ether bonds; the broken fragments are made water soluble by the introduction of sulfonic groups.2 The lignosulfonate molecule is thus composed of hydrophobic aromatic skeleton and hydrophilic sulfonic groups. The lypohydrophilic character endows lignosulfonate with marked interfacial activity and physicochemical properties such as wettability, adsorptivity, and dispersive ability, and so it can be used as effective interfacial additive in many industrial fields.3,4 The studies on the microscopic structure of individual lignosulfonate molecules in solution had been carried out by many researchers.5,6 Rezanowich and Goring considered that the lignosulfonate molecules form microgels in solution, and this standpoint was further elaborated by others. This microgel model explains the spherical structure of lignosulfonate in solution well, but does not fit with the dimension of lignosulfonate at interfaces. For this case, Goring proposed a disklike model instead of spherical, but it could not explain the known spheroidal structure of lignosulfonate in solution. Pla and Myrvold proposed a randomly branched polyelectrolyte model and viewed lignosulfonate as a randomly branched polyelectrolyte. The above three models were proposed on the basis of dilute solution with the lignosulfonate molecules thought of as * To whom correspondence should be addressed. E-mail: zhmsong1980@ yahoo.com.cn. Tel.: +86-20-87114722. Fax: +86-20-87114721.
separate entities. However, lignosulfonate is a type of macromolecular surfactant, and so the aggregation behavior in solution could not be ignored. Many surfactants are easy to aggregate in a very dilute solution when their concentration exceeds the critical micelle concentration (cmc), and so does lignosulfonate.7 Therefore, it is necessary to determine the critical aggregation concentration (CAC) of lignosulfonate in solution accurately, and the aggregation behavior of lignosulfonate in solution can thus be more accurately disclosed. Unfortunately, little work on this topic has been reported in the literature. In this study, we report a new effective method to measure the CAC of sodium lignosulfonate (SL) in solution accurately. Through the determination of CAC and the observation by ESEM, the different states of SL molecules in the solution with different mass concentrations are revealed, and the molecular processes involved in micelle formation and growth are elucidated. Based on the investigation, the models of SL molecules and micelles in the solution are presented. 2. Experimental Section 2.1. Materials. The SL is from the waste liquor of poplar sulfite process (supplied by Shiyan Paper-Making Corp., Jilin Province, China). The SL is dissolved in deionized water to obtain the SL solution, and the SL solution is ultrafiltered for purification. The SL solution is first ultrafiltered through the membrane with the cutoff molecular weight of 50 000, so the insoluble solid matter in SL is eliminated. Then the filtered SL solution is ultrafiltered through the membrane with the cutoff molecular weight of 10 000, and so the low molecular weight matter like sugar, hemicellulose, and inorganic salt is filtered. The residual SL solution is concentrated under low-temperature and low-pressure conditions, and the obtained SL sample is used in this studies. During the ultrafiltered process, the mass concentration of SL is kept at 10-20 wt %, and deionized water is used in the ultrafiltration procedure. The molecular weight and polydispersity of the obtained SL sample are measured using gel chromatography (GPC, type 1515 isocratic HPLP pump/Waters 2487 dual λ absorbance detector, Waters Corp., America). The sulfonic group (-SO3H) content
10.1021/jp107036m 2010 American Chemical Society Published on Web 11/15/2010
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TABLE 1: Molecular Weight, Sulfonic Group and Phenol Hydroxyl Group Contents of SL sample
Mw
Mn
Mw/Mn
-SO3H/wt %
Ph-OH/wt %
SL
20291
16451
1.23
9.56
0.94
of the obtained SL sample is measured by an automatic potentiometric titrator (Type 809 Titrando, Metrohm Corp., Switzerland). Before titration, the SL is ion-exchanged through anion exchange resin and then cation exchange resin to acidify the -SO3H groups. The phenolic hydroxyl (Ph-OH) content of the obtained SL sample is measured by means of nonaqueous conductometric titration using the automatic potentiometric titrator (Type 809 Titrando, Metrohm Corp. Switzerland). The measurements are performed in dimethylformamide medium, and tetrabutyl aqueous ammonia is used as titrant, the phydroxybenzoic acid is used as internal standard sample. The measured structural data of the obtained SL sample are given in Table 1. 2.2. Quantitative Determination of CAC of SL in the Solution Using UV Method. The CAC of SL in water solution was measured by the UV method. The SL has the UV characteristic absorbance in the wavelength of 280 nm. When the SL solution is very dilute, the SL molecules exist in solution individually, so the UV absorbance increases linearly upon increasing the mass concentration. When the SL concentration exceeds the CAC, the SL molecules begin to aggregate. The SL molecules inside the aggregated micelles have very weak absorbance, because the SL molecules in the external layer of the micelles mask most of the UV light, and so the increase of UV absorbance slows down upon the increasing mass concentration of SL. The CAC of AL is thus found out in the UV absorbance-mass concentration curve. The detailed methods are in the following. The purified SL was dissolved in the deionized water to prepare the solutions with different mass concentrations from 10 to 2500 mg L-1, and the differential values of every two close mass concentrations are less than 100 mg L-1. The obtained solutions were treated for 30 min by an ultrasonic instrument, and then set 24 h for equilibrium. The absorbance in 280 nm ultraviolet wavelength of the prepared series of SL solutions was measured by a UV spectrophotometer (type UV2450, Shimadzu Corp., Japan), and deionized water was used as reference sample in the reference cell. The obtained UV absorbance data were correlated to the mass concentrations of SL, and an inflection point was evidently observed in the curve. The mass concentration at the beginning of the inflection point was regarded as the CAC of SL in the solution. 2.3. Dynamic Light Scattering (DLS) Measurements of SL in the Solution. The particle size distribution of the unimolecules and aggregations of SL in solution was measured using a zeta potential and particle size analyzer (type ZetaPlus, Brookhaven Instruments Corp., USA). The PALS (phase analysis light scattering) technique is used in this instrument; the zeta potential, mobility, and particle size distribution of the colloid can be measured accurately. The range of the particle size measurement is 0.6-6000 nm. The purified SL was dissolved in the deionized water for certain mass concentration, the obtained solution was treated for 30 min by an ultrasonic instrument, then the solution was set 24 h for equilibrium. The SL solution sample of 4 mL was prepared by filtering with a disposable syringe filter unit into the polyester resin cuvette cell (10 mm length, 10 mm width, 50 mm height) with a cap. The measurement was performed for 5 min; three parallel measurements were performed for a sample, and the mean value was
adopted. During the measurements the cuvette was kept under isothermal conditions at 25 ( 0.02 °C, controlled by the instrument automatically. The scattering angle range was 90°. Data analysis was by cumulant and histogram methods. The Marquadt and the nonnegative least-squares (NNLS) analyses were applied in this work in order to obtain first the distribution of the decay rates, and then the diffusion coefficients or effective diameters.8 The analysis technique was based on the homodyne mode of operation. The correlator was operated in time-interval mode depending on the intensity of scattered light. 2.4. Transmission Electron Microscopy (TEM) Imaging of SL on Cooper Mesh Substrate. The purified SL was dissolved in deionized water to prepare 1.5 wt % solution, and the obtained solution was treated for 30 min by an ultrasonic instrument; then the solution was set 24 h for equilibrium. The obtained SL solution was dropped on the Cooper mesh substrate. After drying at room temperature, the substrates were examined using the TEM (type H-7500, Hitachi Corp., Japan). No additional sample treatment was performed, thus avoiding introduction of possible specimen coating artifacts. 2.5. Environment Scanning Electron Microscope (ESEM) Measurements of SL in Solution Using Quick-Freezing Method. The purified SL was dissolved in deionized water to prepare 1.5 wt % solution, and the obtained solution was treated for 30 min by an ultrasonic instrument; then the solution was set 24 h for equilibrium. The obtained SL solution was dropped on the stainless steel basal disk; the SL solution sample in the basal disk was quickly frozen to -10 °C under normal atmosphere condition, and then vacuumized to about 170 Pa by a pumped vacuum system. The SL solution was frozen quickly, so that the existing state of SL molecules and micelles in the solution was fixed and maintained in the ice. During the course of pumping to 170 Pa, the surface ice kept sublimating, and the SL molecules and micelles inside the ice began to appear. The emerged SL on the ice surface was examined using the ESEM (type QUANTA 200, FEI Corp., The Netherlands) under 170 Pa condition. The acceleration voltage was 20 kV. The capturing speed for the images must be quick, because the sublimation of the ice was continuous. 3. Results and Discussion 3.1. CAC Determination and Aggregation Behavior of SL in the Solution. SL belongs to macromolecular surfactants, composed of hydrophobic base and sulfonic hydrophilic groups. As surfactant, the molecules of SL spontaneously aggregate when exceeding a certain mass concentration in the solution. This specific concentration value is its CAC. In this paper, the UV method was employed to measure the CAC of SL in the solution, and the result is shown in Figure 1. As shown in Figure 1, the curve of UV absorbance versus SL concentration can be divided into three zones: zone A, zone B, and zone C. In zone A where the SL concentration was very low, the SL molecules existed in the solution individually, and the UV absorbance increased linearly with the SL concentration. When the concentration exceeded 0.38 g L-1, an inflection point appeared in the curve and the slope reduced as shown in zone B. In this stage (0.38-0.87 g L-1), the SL molecules started to aggregate, and the separate SL molecules and aggregates coexisted in the solution. During this stage, more aggregated micelles were formed and the formed micelles continuously grew. Because many SL molecules were wrapped in the formed micelles, these SL molecules inside the micelles had very weak UV absorbance, and so the increase of the UV absorbance slowed down. When the concentration exceeded 0.87 g L-1,
Aggregation Behavior of Sodium Lignosulfonate
J. Phys. Chem. B, Vol. 114, No. 48, 2010 15859 TABLE 2: Volume Percentage and Hydrodynamic Diameter Distributions of SL Micelles in the Solutions with Different Mass Concentrations volume percentage (%)
Figure 1. Variation of UV absorbance upon increasing SL mass concentration.
Figure 2. Size distribution of SL in the solutions with different mass concentrations.
the UV absorbance increased linearly again with the SL concentration, as shown in zone C with a much smaller slope of the curve than zone A. In this stage (more than 0.87 g L-1), the SL molecules existed in the solution mainly in the form of aggregated micelles; more and more new micelles were formed and the formed micelles kept growing upon further increase in the SL concentration. There was a balance established between the aggregation speed and the increased concentration, and so the UV absorbance increased linearly again. 3.2. Aggregation Behavior of SL in the Solution by DLS. The researching results in section 3.1 considered that smallsized new micelles were continuously formed and grown with increasing SL concentration. Now, we measured the size distributions of the SL micelles by DLS and studied the dynamic aggregation behavior of SL in the solution with the increasing SL concentration. The volume percentage and hydrodynamic diameter distributions of the SL micelles in the solutions with different mass concentrations are shown in Figure 2, with detailed values summarized in Table 2. As shown in Figure 2 and Table 2, there is a single peak in the volume percentage-hydrodynamic diameter curve at 0.2 g L-1. The hydrodynamic diameter distribution is about 14-22 nm. Since this mass concentration (0.2 g L-1) is below the CAC
mass concn (g L-1)
first peak
second peak
third peak
0.2 0.5 2.0 5.0
100 76 60 14
0 24 4 5
0 0 36 81
hydrodynamic dia distribn (nm) first peak second peak third peak 14-22 14-24 14-23 13-22
s 75-140 75-155 80-135
s s 300-600 400-700
value and it is in zone A, the SL in the solution existed in the form of separate molecules so that only one peak is present in the curve. At the mass concentration of 0.5 g L-1 (in zone B), there are two peaks in the curve. The hydrodynamic diameter distribution of the first peak remains to be about 14-24 nm, and the hydrodynamic diameter distribution of the second peak is about 75-140 nm. It is evident that the separate SL molecules began to aggregate into small-sized micelles, and the SL molecules and micelles coexisted in the solution. At 2.0 g L-1, there are three peaks in the curve. While the hydrodynamic diameter distribution of the first peak remains about 14-23 nm, those of the second peak and the third peak are about 75-155 and 300-600 nm, respectively. The first peak represents separate molecules, the second peak is the smallsized transitional micelles, and the third peak is the big-sized micelles. At 5.0 g L-1, there are also three peaks in the curve and the diameters of the first and second peaks are similar to those at 2.0 g L-1. However, the third peak increased to about 400-700 nm. From Table 2, it can be seen that when the SL concentration exceeded the CAC value, the small-sized SL micelles formed and the volume percentage of the first size range decreased. With the increase in the concentration, the small-sized SL micelles grew into big-sized micelles and so the third size range increased. The second size range still coexisted but its volume percentage reduced, while that of the third size range kept increasing. Another interesting observation is the change of the micelle size. While the hydrodynamic diameters in the first and second size range remained unchanged, that of the third size range kept increasing. The big-sized SL micelles grew because the newly increased SL molecules were continuously wrapped in the formed micelles. 3.3. TEM of SL Aggregates. For the purpose of observing the aggregation state of the SL molecules and micelles, the 1.5 wt % SL solution was prepared and dropped on a copper mesh substrate. After air drying, the dried SL molecules and micelles were observed by TEM. Their images are shown in Figure 3. As shown in Figure 3, the SL molecules mainly existed in solution in the form of aggregates when the SL concentration was 1.5 wt %. Most SL molecules were aggregated in the form of loose spheroidal micelles, and a small number of SL molecules scattered between the micelles. The diameters of the micelles are about 500 nm to 1 µm. The dried SL molecules on the substrate exhibited disklike gelatinous floccules. 3.4. ESEM of the SL Aggregates. The SL molecules at dried state were observed by TEM in section 3.3. Now we observe the actual state of SL molecules in the aqueous solution using a quick-freezing method by ESEM. The prepared 1.5 wt % SL solution was quickly frozen to -10 °C under the atmosphere condition, so that the actual state of SL molecules in the solution was fixed and maintained in ice. During the course of sublima-
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Figure 3. TEM micrography of SL on copper mesh substrate.
Figure 4. ESEM micrography of SL in ice.
Figure 5. Existing and growing model of SL in the solution with the increase of concentration.
tion, the state of SL molecules was revealed, and the quickly captured images are shown in Figure 4. As shown in Figure 4, the white or light-colored substrate is ice, marked with white arrows, and many spheroidal micelles scatter in the substrate, marked with black and red arrows. Different from the TEM, the structure of the SL micelles in ESEM is very tight, and very few SL molecules are present in the ice. The SL molecules in the ice mainly existed in the form of aggregated micelles with the diameters about 500 nm to 1 µm. It is very interesting to discover that the large-sized SL micelles appeared to be hollow because the middle of some SL micelles has deep color, as shown by the red arrow arrows in Figure 4. During the course of sublimation, the SL micelles in the ice began to appear. If the micelles are solid, the exposed micelles on the ice surface should have the same color in the ESEM images. If the micelles are hollow, the micelles should be collapsed in the middle, so that the center of the micelles exhibited deep color. However, only a part of the exposed micelles showed deep color in the center in Figure 4. This is because the surface ice has not sublimated enough to make the
micelles outstanding on the ice surface and thus the micelles with the hollow structure have not begun to collapse. On the basis of the ESEM observation, most of the formed SL micelles in the solution are presumed to be hollow. 3.5. State and Aggregation Model of SL in the Solution. On the basis of the above studies, the existing state and growth rule of SL in the solution with the increase of concentration are presumed and illustrated in Figure 5. In the first phase, the solution is very dilute and the SL molecules exist individually. In the second phase, the SL concentration exceeds its CAC, the individual SL molecules begin to aggregate, and some small newly produced micelles are formed. In the third phase, the small micelles keep being formed and growing up with increasing SL concentration. In the fourth phase, more and more spherical micelles are formed, the transitional micelles are also formed, and the equilibrium between the generating speed and the increasing SL concentration has been attained. The spherical hollow vesicular micelles are presumed to be the main form of SL in the solution. From Figure 1, in zones B and C, the UV absorbance increased slowly with the SL
Aggregation Behavior of Sodium Lignosulfonate concentration, indicating that most of the increased SL molecules are packaged in the micelles. The SL micelles in the solution may become solid. But from Figure 3, the TEM shows that the SL micelles are loose and there is much space inside the micelles. From Figure 4, the ESEM shows that many of the outstanding SL micelles on the ice surface have clearly collapsed in the center of the micelles. The SL micelles are thus not completely solid, but contain some secondary vesicular structures. Because the hydrophobic base and hydrophilic groups in SL molecules are not totally separated from one another and the hydrophilic groups are scattered in the whole molecule, the SL molecules cannot aggregate into stable hollow micelles by their hydrophobic sites as conventional linear molecule surfactants. The packed SL molecules inside the micelles tend to aggregate through hydrogen-bonding and intermolecular van der Waals forces.9,10 However the electrostatic repulsion forces between SL molecules make the aggregates very loose. Many compactdistributing small-sized vesicles are contained in the SL micelles, and the stable vesicular structures can sustain the SL molecules in the external layer and help the stability of the micelles. Based on the studies, the spherical hollow vesicular micelle structure is regarded as the major form of SL in water solution. 4. Conclusion In this work, we have presented a new method to measure the CAC of SL in the solution with which the state and dynamics of the formation of the SL micelles were revealed. Our investigation showed that the CAC value of SL in the solution was 0.38 g L-1. Before CAC, the SL molecules existed individually in the solution; when the concentration exceeded CAC, the individual SL molecules started to aggregate and thus the micelles were formed and grew with increasing SL
J. Phys. Chem. B, Vol. 114, No. 48, 2010 15861 concentration. The TEM and ESEM showed that the spherical micelles were the main form of SL in the solution, and the spherical micelles were presumed to contain many compactdistributing small-sized vesicles. Therefore, the spherical hollow vesicular structure was regarded as the major form of SL micelles. Acknowledgment. We are thankful for the financial support of the National Science Foundation for Distinguished Young Scholars of China (20925622), the National Natural Science Foundation of China (21006036), the National Natural Science Foundation of China (20976064), The National Basic Research Program of China (2010CB732205) and The Natural Science Foundation of Guangdong Province of China (8351064101000002). We also thank Professor Shiping Zhu of McMaster University Canada for his invaluable suggestion, helpful discussion, and revision of the manuscript. References and Notes (1) Dizhbite, T.; Zakis, G.; Kizima, A.; Lazareva, E.; Rossinskaya, G.; Jurkjane, V.; Telysheva, G.; Viesturs, U. Bioresour. Technol. 1999, 67, 221. (2) Milczarek, G. Langmuir 2009, 25, 10345. (3) Hornof, V.; Hombek, R. J. Appl. Polym. Sci. 1990, 41, 2391. (4) Qiu, X. Q.; Yan, M. F.; Yang, D. J.; Pang, Y. X.; Deng, Y. H. J. Colloid Interface Sci. 2009, 338, 151. (5) Myrvold, B. O. Ind. Crops Prod. 2008, 27, 214. (6) Vainio, U.; Lauten, R. A.; Serimaa, R. Langmuir 2008, 24, 7735. (7) Norgren, M.; Edlund, H.; Wa˚gberg, L. Langmuir 2002, 18, 2859. (8) Okubo, T.; Kiriyama, K.; Yamaoka, H.; Nemoto, N. Colloids Surf. A: Physicochem. Eng. Aspects 1995, 103, 47. (9) Norgren, M.; Edlund, H. Colloids Surf. A: Physicochem. Eng. Aspects 2001, 194, 239. (10) Gundersen, S. A.; Ese, M.; Sjo¨blom, J. Colloids Surf. A: Physicochem. Eng. Aspects 2001, 182, 199.
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