Influence of pH on the behavior of lignosulfonate macromolecules in aqueous solution

Colloids and Surfaces A: Physicochem. Eng. Aspects 371 (2010) 50–58

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Influence of pH on the behavior of lignosulfonate macromolecules in aqueous solution Mingfang Yan a , Dongjie Yang a , Yonghong Deng a , P. Chen b , Haifeng Zhou a , Xueqing Qiu a,∗ a b

School of Chemistry and Chemical Engineering, State Key Lab of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada

a r t i c l e

i n f o

Article history: Received 21 May 2010 Received in revised form 16 August 2010 Accepted 31 August 2010 Available online 15 September 2010 Keywords: Sodium lignosulfonate Solution behavior Aggregate

a b s t r a c t The solution behavior of purified sodium lignosulfonate (PSL) at different pH values were investigated by means of acid–base titration, surface tension, viscosity, fluorescence spectrometry, dynamic light scattering (DLS) and environmental scanning electron microscopy (ESEM) experiments. Fluorescence experiments showed that the critical aggregate concentration (CAC) of PSL was 0.05 g/L. DLS results indicated that the average dimension of PSL molecule and PSL aggregate was about 8 nm and 80 nm, respectively. The surface charge of PSL molecules and the aggregation degree of PSL in solution increased with the increasing of pH due to the ionization of sulfonic and phenolic hydroxyl groups. The size of PSL aggregates and reduced viscosity of PSL solution increased as pH values increased because expansion of the PSL cores. A model for the assembly behavior of PSL was first proposed to explain the influence of pH on the molecular configuration and aggregation behaviors of lignosulfonate in aqueous solutions. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Lignin is a natural biomacromolecule, and one of the main components of plant cell walls along with cellulose and hemicellulose. Lignin can be defined as an amorphous material containing three phenylpropanoid monomers: guaiacyl, syringyl and phydrophenyl, which are linked by a large number of interunit bonds that include several types of ether and carbon–carbon linkage [1,2]. Lignosulfonate and alkali lignin are typical derivatives of lignin, which are derived from different chemical pulping reactions. Lignosulfonate is produced through the sulfite pulping process as a by-product in the production of cellulose. Although the exact structure of lignosulfonate has not been elucidated, it is generally accepted that lignosulfonate not only contains hydrophobic groups, such as aromatic and aliphatic groups, but also contains many hydrophilic groups, such as sulfonic, carboxyl and phenolic hydroxyl groups. Lignosulfonate can be used in many fields because it exhibits good solubility in aqueous solution and possesses a certain degree of surface activity. Traditional use of lignosulfonate is mainly based on its dispersive, stabilizing, binding and complexing nature, where it can be used as dispersants of concrete [3–6], dyestuff [7] and coal–water slurry [8–10], industrial binders [11], agricultural chemicals [12] and floating chemicals [13,14]. In recent years,

∗ Corresponding author. Tel.: +86 20 87114722; fax: +86 20 87114721. E-mail address: [email protected] (X. Qiu). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.08.062

because the scarcity of petroleum is driving the inventive utilization of renewable resources (biomass) on earth, lignosulfonate as an important biomass resource attracts attention of researchers again. Many researchers found new applications areas for lignosulfonate. MartíN-Ortiz et al. [15] evaluated the efficiency of a zinc lignosulfonate (ZnLS) as Zn source for wheat and corn plants under hydroponic conditions. Their research provided evidence that ZnLS could be used as a Zn source to the roots of wheat and corn and seemed to be more efficient than ZnSO4 to correct Zn deficiency in both plants. Raghuraman et al. [16] showed that macromolecular lignosulfonate presented a multitude of substructures, it could interact with viral glycoproteins by hydrophobic, hydrogen and ionic interaction; thus lignosulfonate represents a large number of interesting structures with potential medicinal benefits. Although lignosulfonate is increasingly used today, only small percentages of lignosulfonate have been utilized in industry, and most of them are disposed as waste, likely causing environmental hazards. Therefore taking full advantage of lignosulfonate is important for both economic and environmental sides. One requirement for the development of new applications for lignosulfonate is a better understanding of its physicochemical properties. Lignosulfonate as a polyelectrolyte contains both hydrophobic and hydrophilic groups and dissolves easily in water, and its application and modification reactions are mostly carried out in aqueous solution. Therefore, understanding the solution behavior of lignosulfonate in aqueous solution is very beneficial to find out its application mechanisms, and then guide to the modifications in order to improve their application properties.

M. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 371 (2010) 50–58

In recent years, there have been many reports about the solution behavior of polyelectrolytes. While the research on the solution behavior of lignosulfonate was started decades ago, in recently there are fewer reports about it. In the past few years, the solution behavior of lignosulfonate in organic solvents and aqueous solutions was attracting attention again. Goring et al. [17,18] were the first to propose an oblate shape of the lignosulfonates macromolecules obtained from pulping on the basis of electron microscopy studies, while Kontturi [19] proposed that low molecular weight lignosulfonate macromolecules were spherical on the basis of the measurement of diffusion across a porous membrane. Moacanin et al. [20] and Olleman et al. [21] assumed a nonspherical shape of lignouslfonate on the basis of the relation between molecular weight and diffusion coefficient, and viscosity measurements. Vainin et al. [22] characterized purified sodium lignosulfonate by using Small-angle X-ray scattering (SAXS) and rheology, where the results revealed that lignosulfonate had characteristics of “soft” colloidal particles, and were an oblate spheroid shape with an axial ratio of at 3.5 that described the average dimensions of the lignosulfonate particles in saline solution and the self-association dependence on the temperature. Myrvold’s [23] research showed that lignosulfonate molecule formed a randomly branched polyelectrolyte; the molecule coils in solution to make a more ball-shaped molecule with the sulfonic groups enriched on the surface, and the lignosulfonate was not a microgel structure because of the dependence of the polyelectrolyte expansion on the molecular weight. Lignosulfonate is an amphiphilic molecule that may associate in a dynamic and thermodynamically driven process in aqueous solution when the concentration of lignosulfonate is above critical aggregation concentration (CAC). However, only few researches on the critical aggregation concentration (CAC) and the aggregation structure of lignosulfonate in aqueous solution are reported. Lignosulfonate contains sulfonic, carboxyl and phenolic hydroxyl groups; the ionization extent of these groups are affected by the pH values of solution, thus the solution behavior of lignosulfonate maybe change in different pH values solutions. So the aim of this paper was to determine the CAC of the purified sodium lignosulfonate (PSL) in solution; furthermore, investigated the influence of pH on the solution behavior of purified sodium lignosulfonate (PSL) by means of acid–base titration, electrophoresis apparatus, surface tensiometry, viscometry, fluorescence spectrometry and environmental scanning electron microscopy. In addition, we would also determine the size of the PSL molecule and PSL aggregate through dynamic light scattering technique.

2. Materials and methods

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Fig. 1. Relative molecular weight distribution of commercial SL and PSL at 298 K.

ular weight at 2500 Da. The obtained matter was then purified sodium lignosulfonate (PSL). The operation was under a pressure of 0.42 MPa and a temperature of <50 ◦ C. The molecular weight distribution of the commercial SL and PSL was determined using aqueous gel permeation chromatography (GPC) with Ultrahydragel 120 and Ultrahydragel 250 columns. The effluent was monitored at 280 nm with a Waters 2487 UV Absorbance Detector (Waters Corp., USA). The polystyrene sulfonic was used as the standard and the 0.10 mol/L NaNO3 solution with pH 8 as the eluent with a velocity of 0.50 ml/min [9]. The relative molecular weight distribution of commercial SL and purified SL (PSL) are shown in Fig. 1, and the mass average molecular weight (Mw ), the number average of molecular weight (Mn ), the polydispersity (Mw /Mn ) and the content of main function groups in lignosulfonate are given in Table 1. Analytical grade sodium chloride and sodium hydroxide were purchased from Sigma–Aldrich (Shanghai, China), and used without further purification. Double distilled water was used for the preparation of all solutions.

2.2. Surface potential measurements The acid–base titration curve of the PSL solution was determined by an automatic potentiometric titrator (809Titrando, Metrohm Corp., Switzerland). The zeta potential of the PSL solution was measured by a Brookhaven Laser Light Scattering spectrometer (IB-90Plus) at 298 K. First, PSL solutions at different pH values were filtered through a 0.22 ␮m filter membrane before analysis to remove any dust in solution and then equilibrated for 12 h before zeta potential data collection.

2.1. Materials Commercial sodium lignosulfonate was a by-product of the sulfite pulping process from Shixian Papermaking Co. Ltd.; it consisted of about 70 wt.% of sodium lignosulfonate (SL), about 11 wt.% of reductive substance with the remainder being sugar acids, low molar-mass molecules such as organics and inorganic salts as well as other impurities. Thus, before experiments, the commercial SL was purified by using filtration membrane, with a cut off molec-

2.3. Surface tension measurements The surface tension was measured using a Wilhelmy plate with Dynamic Contact Angle Meter from Dataphysics Instruments Co., Ltd. (German). Experimental errors inherent in the measurement were ±0.03 mN/m. The surface tension was determined as an average value measured three times at 298 K.

Table 1 The molecular weight of lignosulfonates and the content of main function groups in lignosulfonates. Sample

Commercial SL PSL

Molecular weight and polydispersity

Content of main function groups

Mw (Da)

Mn (Da)

Polydispersity Mw /Mn

Sulfonic (wt%)

Carboxyl (wt%)

Phenolic hydroxyl (wt%)

5300 13,000

1200 4900

4.4 2.7

9.04 11.34

5.81 6.57

1.73 1.86

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2.4. Viscosity measurements The viscosity of the solution was measured at 298 K using an Ubbelohde-type capillary viscometer. The time for the sample to flow from one level indicator to another, known as flow time, as well as the density of samples were measured. All of the experiments were repeated three times and the average values were taken, and the density of distilled water was 1 g/cm3 . The reduced viscosity (sp /C) was calculated as follows: sp = r − 1 r =

t 0 t0

(1) (2)

where sp is the specific viscosity, C is the concentration of PSL, r is the relative viscosity,  and 0 are the density of PSL solution and distilled water at 298 K, t and t0 are the flow time of PSL solution and distilled water. 2.5. Fluorescence measurements Fluorescence measurements were performed on a Fluorosens System (Gilden Photonics Ltd., England) equipped with a 150 W xenon Arc Lamp at 298 K. The slit width for both emission and excitation was selected to be 2.5 nm, and the integration time was 100 ms. Excitation spectra provide compelling evidence for ground-state interactions of PSL. The excitation spectra of PSL solutions with different concentrations were monitored in a range of 200–500 nm using an emission wavelength of 530 nm. Pyrene was used as the fluorescence probe to study the aggregation behavior of PSL. The sample solution for fluorescence measurement was prepared as follows: pyrene was first dissolved in acetone at a concentration of 1 × 10−3 mol/L and then diluted in acetone to obtain a concentration of 1 × 10−5 mol/L. 1 ml of this solution was added to a 100 ml volumetric flask, letting the acetone evaporate naturally. The PSL solution was then prepared in a volumetric flask, keeping the concentration of pyrene at 1 × 10−7 mol/L. Before measurement, the solution was ultrasonic dispersed for 10 min and then kept undisturbed for 12 h. The emission spectra of pyrene were recorded in a range of 350–500 nm using an excitation wavelength of 334 nm. 2.6. Dynamics light scattering (DLS) analysis DLS measurements were performed at 298 K using a Brookhaven Laser Light Scattering spectrometer (IB-90Plus) with a scattering angle of 90◦ . The light source is a power Solid State Lasers with a maximum power of 35 mW and a wavelength of 659 nm. The software for data analysis was provided by the supplier. The solutions were filtered through a 0.22 ␮m filter before analysis to remove any dust in solution. Samples were equilibrated for 12 h before data collection. Each measurement was repeated three times, and the average result was accepted as the final hydrodynamic diameters (Dh ) whenever all the values fluctuated within reasonable experimental errors. All solutions in our experiments were prepared in distilled water; the pH of solutions was adjusted by 1 mol/L NaOH and HCl and contained 10−3 mol/L NaCl to keep the ionic strength of the solutions.

Fig. 2. The acid–base titration curve of PSL solution at 298 K.

on a silicon flake at room temperature, and a Peltier electric cooling device made the temperature of the sample drop from room temperature to −15 ◦ C in one minute; after that, the frozen sample was put into the sample chamber, where the temperature was adjusted to dry the sample under vacuum conditions. At last, the morphology of PSL aggregates was examined. 3. Results and discussion 3.1. Surface charge of PSL Myrvold [23] thought that the hydrophobic coils of lignosulfonate in solution formed a more ball-shaped molecule and the sulfonic groups were distributed on the surface of molecule. The results in Table 1 show that besides sulfonic groups, the lignosulfonate also contains carboxyl and phenolic hydroxyl groups. How will these hydrophilic groups affect the behavior of lignosulfonate in aqueous solution? Whether there exists a hydrophobic microdomain in lignosulfonate molecule? How the pH values affect the ionization degree of hydrophilic group and thus affect the solution behavior of lignosulfonate in aqueous solution? In order to understand these problems the surface charge of PSL in solution at different pH, the acid–base titration curve and the zeta potential of PSL as a function of pH were investigated. The results are shown in Figs. 2 and 3. The research of Ratinac et al. [24] showed that the ionization equilibrium constants of sulfonic groups, carboxylate and phenolic hydroxyl were 1.5, 5.1 and 10.5, respectively. From Fig. 2 we can see that there have two end point of titration: one is pH = 5.1,

2.7. Environmental scanning electron microscopy (ESEM) The morphology of PSL aggregates was examined by environmental scanning electron microscopy (ESEM), which was performed on an XL 30 ESEMFEG scanning electron microscope (Micrion FEI PHILIPS). A drop of the PSL solution was deposited

Fig. 3. Zeta potential of PSL as a function of pH at 298 K.

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Fig. 4. The surface tension of PSL as a function of pH at 298 K.

Fig. 5. The reduced viscosity of PSL (300 g/L) as a function of pH at 298 K.

the other is pH = 8.3. According to Ratinac et al’s report, the first end point of titration should attribute to the sulfonic groups and the second end point of titration should belong to carboxyl groups. Thus combining the results in Figs. 2 and 3 we can see that when pH is <4, the zeta potential of PSL decreases with the increase of pH, which is mainly caused by the ionization of sulfonic groups. The zeta potential of PSL is invariable with the increase of pH when the pH varies from 4 to 10, while the results in Fig. 2 indicate that the carboxyl groups gradually ionize when pH is <7.7, which shows that the ionization of carboxyl groups has no attribution to the zeta potential of PSL. When the pH is >10, the zeta potential of PSL decreases as the pH increases; this is mainly caused by ionization of phenolic hydroxyl groups. These results show that the ionization of sulfonic groups and phenolic hydroxyl groups affect the zeta potential of the PSL, while the ionization of carboxyl groups does not change the zeta potential of PSL. These results indicate that the sulfonic groups and phenolic hydroxyl groups mainly distribute on the surface of the PSL molecules, and the carboxyl groups mainly location in the core of PSL molecules. From Fig. 3 we can also see that the zeta potential of PSL decreases as the concentration of PSL increases at same pH. This maybe because the zeta potentiometer just can determine the potential of PSL aggregates but not determine PSL molecules since the single PSL molecules are very small. While, with the concentration of PSL increases, more single PSL molecules assemble together, which result in the surface of PSL aggregate contains more ionizing groups; thus the zeta potential of PSL decreases as the concentration of PSL increases at same pH.

tion, although PSL is purified lignosulfonate, there are still some impurities in PSL, which will also affect the determined result. These reasons may cause there does not appear CAC during surface tension test. There are two breaks in surface tension isotherms: First the surface tension of PSL decreases slowly as the concentration of PSL increases when the concentration is below 1 g/L, then it decreases almost linearly as the concentration of PSL increases when its concentration is above 1 g/L. This maybe because the arrangement of lignosulfonates at the air–liquid interface changes with the concentration increases [26,27]. Meanwhile, the surface tension decreases with pH decreases; these results indicate that the pH maybe affects the assembly behavior of PSL, and furthermore affects the PSL arranged at the air/liquid interface. 3.3. Viscosity of PSL solution Viscosity is a basic and an important physical property among the various properties of polyelectrolyte solutions. Fig. 5 illustrates the plot of reduced viscosity as a function of pH of PSL solution at a concentration of 300 g/L. In general, the reduce viscosity increases with the pH value increases. This is due to the ionization of sulfonic, carboxyl and phenolic hydroxyl groups in the PSL molecules increases as the pH value increases; thus, the electrostatic repulsion in lignosulfonate molecules increases, which makes the hydrophobic chain of PSL molecules stretching in high pH value solution. These result in the reduced viscosity of lignosulfonates increases with the pH value increases. 3.4. Fluorescence spectrum

3.2. Surface tension of PSL in the aqueous solution The concentration dependence of the surface tension for the PSL as a function of pH is shown in Fig. 4. From Fig. 4 we can see that there is no flatform with the concentration of PSL increases. The PSL can decrease the surface tension of water obviously. These results are in good agreement with our previous experimental results [4,10,25]. These results imply that the distribution of hydrophilic groups on the surface of hydrophobic core of PSL is inhomogeneous; only in this case the area of the hydrophobic core which covered by fewer hydrophilic groups could escape from the liquid to adsorb at the air/liquid because of hydrophobic effect; thus the surface tension of water can be reduced. Although from Fig. 4 there is no apparently CAC for PSL, it does not mean that PSL cannot aggregate in solution. PSL has wide molecule weight distribution. Lignosulfonate with different molecule weights maybe exist different CACs in solution. In addi-

There is few report about the fluorescence spectrum of PSL, even though people realize the influence of fluorescence on the light scattering measurement [28]. It is well-known that the structure of PSL is very complex, and the guaiacyl, syringyl and phydrophenyl units have different absorbance maxima in their UV spectra. Moreover, molecular aggregation makes the absorbance and fluorescence spectra of PSL more complicated. Aromatic interactions are ubiquitous in nature. The (–( stacking of the aromatic rings has been reported to be responsible for lignin association phenomena in organic solvents [29]. The stacking of the aromatic rings results in two distinct aggregation modes by which PSL can be assembled, such as end-to-end and side-byside, analogous to the well-known H and J aggregation in organic chromophores [30]. According to the molecular excitation coupling theory, when the PSL molecules associate to form aggregates in the ground state, and the aromatic rings of PSL molecules are

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Fig. 7. Fluorescence emission spectra of pyrene in PSL solution as a function of PSL concentration at pH 7 and 298 K.

Fig. 6. (a) Fluorescence excitation spectra of PSL with different concentrations at 298 K, the emission wavelength is 530 nm; (b) Comparison of the excitation spectra of PSL with different concentrations at 298 K, the total intensity was normalized by the peak intensity at 280 nm.

close enough (e.g. distance <1 nm), they will exhibit perturbed absorption and excitation spectra. When the aromatic rings of PSL molecules are separated in the ground state with distance larger than 1 nm, the wavelengths of absorption and excitation spectra are invariant. Excitation spectra provide compelling evidence for groundstate interactions of aromatic molecules [31]. Fig. 6a shows the excitation spectra of PSL with different concentrations of PSL. The peak at 265 nm attribute to fluorescence half frequency interference, and the peak at 280 nm is the characteristic peak of benzene ring in PSL molecule. As shown in Fig. 6b, we can clearly notice the difference in the spectral data at different concentrations after the total intensity was normalized at 280 nm. The maximum absorbance peak is located at 280 nm when the concentration of PSL is low, which is similar to the UV–vis absorbance spectra. However, with concentration increase, the peak at 280 nm is way down, and then finally disappeared; but the shoulder peak at about 350 nm becomes stronger and stronger, and finally dominant. When concentration reaches 0.2 g/L, the maximum absorbance peak is located at about 350 nm. The peak at 280 nm disappears at concentration of 0.5 g/L. In the meantime, a new shoulder peak at 400 nm appears, and then becomes stronger and stronger at elevated concentrations. This phenomenon can be explained in this way. There exists a strong ground-state interaction of aromatic rings in PSL aqueous solutions. Molecular aggregation of PSL induces aromatic rings coupling with the neighboring aromatic rings, so that the excitation spectra are perturbed. The absorbance peak of PSL is about 280 nm when aromatic rings of PSL are separated enough without coupling interaction. When aromatic rings coupling with each other, energy level splitting occurs in PSL absorption and excitation spectra. The new shoulder peaks

at 350 nm and 400 nm in the PSL excitation spectra result from the energy level splitting. That is to say, the new shoulder peaks at 350 nm and 400 nm provide convincing evidence for molecular aggregation of PSL in aqueous solutions. Since the aromatic coupling requires that the separation distance between aromatic rings is <1 nm, so the concentration for aromatic coupling is higher than that for general molecular aggregation. Since this paper is focused on molecular aggregation, the detail of aromatic coupling of PSL will be described in other separated paper. The fluorescence of pyrene is known to be sensitive to changes in the microenvironment. When the hydrophobic microdomain is generated in an aqueous phase, pyrene molecules are preferentially located within or close to the hydrophobic microdomain. The photo physical characteristics of pyrene molecules in a hydrophobic surrounding differ noticeably from those in an aqueous phase. The intensity ratio of the third and the first major vibrational peaks in pyrene’s fluorescence spectrum (I3 /I1 ) is often used as a measure of the polarity of the microenvironment. The higher I3 /I1 value means lower polarity of microenvironment around pyrene [32,33]. In this paper, pyrene was used as a probe to test the polarity of the hydrophobic core of PSL aggregates. The results are shown in Figs. 7 and 8. Fig. 7 shows that the first and third peaks wavelengths of pyrene in PSL solution are 383 nm and 373 nm, and it is invariable with the increase of PSL concentration. Fig. 8 shows the I3 /I1 value of pyrene in PSL solution at different concentrations of PSL. We can see from Fig. 8 that the I3 /I1 value increases slightly with the PSL

Fig. 8. Intensity ratio I3 /I1 obtained from the fluorescence spectra of pyrene plotted versus the concentrations of PSL, measured at pH 7 and 298 K.

M. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 371 (2010) 50–58

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Fig. 9. The I3 /I1 value of pyrene in PSL solution as a function pH at 298 K. Fig. 10. The influence of concentration on the particles size distribution of PSL at pH 7 and 298 K.

concentration increases when the concentration is below 0.05 g/L. However, the I3 /I1 value increases significantly with PSL concentration increases when the PSL concentration is above 0.05 g/L. This demonstrates that there form hydrophobic core in PSL aggregates when the concentration of PSL is above 0.05 g/L, so these results indicate that the CAC of PSL is 0.05 g/L. Fig. 9 shows the effect of pH on the solution behavior of PSL molecules and PSL aggregations in solution. The I3 /I1 values change a little as the pH increases when the concentration of PSL is 0.04 g/L. This is because there is no hydrophobic microdomain in the PSL molecule core, the core PSL molecules just contain hydrophobic framework composed of benzene rings. Although the increase in pH increases the electrostatic repulsion in PSL molecule cores and just makes the core swell, it has a little affect on the polarity of PSL molecule core; thus, the I3 /I1 values have no change. When the concentration of PSL is 0.08 g/L, the I3 /I1 values decreasing as the pH value increases indicate that the polarity of hydrophobic cores of PSL aggregate increase with the pH value increases. The experimental results in Section 3.1 presented that the charge density of PSL molecules increases with the pH increase because of the ionization of hydrophilic groups in PSL molecules. There are many carboxyl and phenolic hydroxyl groups wrapped in PSL aggregates cores and they ionize gradually and result in the swelling of the core of aggregate when the pH increases. Thus, the increase in polarity of the core leads to the decrease of I3 /I1 values as the pH value increases.

we can get the diameter of the sphere.



V = 

d3 6



=

M NA

(3)

where  is the density of sphere, V is the volume of the sphere, d is the diameter of the sphere, M is the molecular weight of the PSL, which is 13,000 g/mol and NA is Avogadro’s number. Then from Eq. (3) we can get the diameter of PSL molecule to be about 3.44 nm. The calculated value and the experimental value of small particles have the same order of magnitude, which can support the speculation that the smaller particles are PSL molecules and the larger particles are aggregates of PSL in solution. The experimental value is larger than the calculated value indicating that PSL molecules in solution are not compact spheres, but are loose and softer particles. These results are in accordance with the research results of Vainin et al. [22] and Myrvold [23]. From Fig. 10 we can see that PSL exhibits bimodal size distributions even when concentration of PSL is blew 0.05 g/L. The size of PSL molecule nearly invariables as concentration of PSL increases, but the intensity of PSL molecule decreases along with the concentration of PSL increases. While the intensity and the size of PSL aggregates increase as the concentration of PSL increases. These results indicate that assemble of PSL is a gradually process; when the concentration of PSL is below 0.05 g/L, there are fewer PSL aggregates in PSL solution. When the concentration of PSL is above 0.05 g/L, there forms a lot of PSL aggregates in PSL solution, and

3.5. The size of PSL aggregates The results presented earlier in this paper have shown that the PSL molecules will aggregate when the concentration of PSL is above 0.05 g/L. In this section the DLS was used to determine the size of the aggregates, and the effect of pH on the assembly behavior of PSL was further illustrated, with the results are shown in Figs. 10 and 11. It can be observed from Figs. 10 and 11 that PSL exhibits bimodal size distributions at different PSL concentrations and different pH values. The smaller particles are about 8 nm and the larger ones are about 80 nm. The smaller particles maybe PSL molecules and the larger particles maybe PSL aggregates. In order to confirm the speculation, the diameter of PSL molecule was calculated from the molecular weight of PSL with two assumptions. We hypothesis that PSL molecules are compact sphere, and the density of the sphere is 1 g/cm3 , according to the follow equation

Fig. 11. The influence of pH on the particle size distribution of PSL at 298 K, the concentration of PSL is 0.10 g/L.

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Fig. 12. ESEM image of PSL at different concentrations and 298 K, the pH of solution is 7.

the assemble extent of PSL increase obviously, so the diameter of aggregates increases as the concentration of PSL increases. From Fig. 11 we can see that the diameter of PSL molecule and the PSL aggregate increase as the pH increase. Since as the pH value increase, the electrostatic repulsion in the core of PSL molecules and aggregates increases because of the ionization of carboxyl and phenolic hydroxyl groups; thus the hydrophobic core of the PSL molecules and aggregate expands, therefore the diameter of PSL molecules and aggregates increase as the pH value increases. The DLS results agree with the fluorescence experiment results and explain why the reduced viscosity increases with the pH value increases from another side.Afanasjev et al. [34] studied the morphology of lignosulfonate (LS) in solution through calculating the long chain branched parameters and the fractal dimension of LS. Their research indicated that the LS chain was random and did not conform to the three-dimensional network model. They thought LS should be classified as a polymer, which had a chain of medium stiffness, based on the thermodynamics compliance. In this paper the effect of PSL concentration and pH of solution on the morphology of PSL were studied by ESEM, with the results are shown in Figs. 12 and 13. The sample preparation process of ESEM measurement actually is the process of water drying, so in that process the hydration and rejection electric double layer is destroyed, thus lignosulfonate molecules form aggregates because of intermolecular forces. Therefore, as a matter of fact, what the ESEM observed is the morphology of PSL aggregates. From Figs. 12 and 13 we can see that the morphology of PSL aggregation is oblate. The PSL aggregates likely to agglomerate together and form clusters. The agglomeration extent of PSL aggregates increases as the concentration of PSL and the pH value of the PSL solution increases. 3.6. The assembly model of PSL in solution From all of the experimental results in this paper we can conclude that the surface of PSL molecules and PSL aggregates are

mainly covered by sulfonic groups as well as a few phenolic hydroxyl groups, and the carboxyl groups mainly location in the core of PSL molecules and aggregates. The zeta potential increases as the pH of solution increases. The CAC of PSL in solution is 0.05 g/L. The core of PSL molecules is formed by hydrophobic bone framework which cross-linked by three unit structures phenylpropanoid monomers: guaiacyl, syringyl and p-hydrophenyl; and the PSL aggregates are formed by aggregation of PSL molecules. Since there are many carboxyl and phenolic hydroxyl groups in the core of PSL aggregates, the ionization degree of these weakly ionized groups increase as the pH increases, so the core of aggregates swell because of electrostatic repulsion. Thus, the structure of the PSL aggregates and the assembly model of PSL in solution can be presented as shown in Chart 1. As the Chart 1(a) shows, the distribution of sulfonic groups on the surface of PSL molecule and aggregate is not uniform. The part of the hydrophobic core which covered by fewer sulfonic groups tends to adsorb at air/liquid interfaces because of hydrophobic effect, so PSL can decrease the surface tension of water. As shown in Chart 1(b), when the pH value of PSL solution increases, the core of the PSL expand because of electrostatic repulsion, and the number of hydrophobic core which adsorb at air/liquid interface in a unit area decreases as the pH value increases, thus the surface tension of PSL solution increases as the pH value increases. The cores of the PSL molecule and aggregate expand as the pH value of solution increases, and the hydrophobic chain of PSL stretches, which result in the reduced viscosity of PSL increase as the pH value increases. Because of the core, the PSL molecule and aggregate expand, more water molecules access to the loose hydrophobic core, thus the polarity of the hydrophobic core of PSL aggregates increases as the pH value increases, and I3 /I1 value of pyrene in PSL solution at the concentration of 0.08 g/L decreases as the pH value increases. Since the core of PSL molecule and aggregate expand, the diameter of the PSL particles increase as the pH value increases.

Fig. 13. ESEM image of PSL at different pH and 298 K, the concentration of PSL is 0.05 g/L.

M. Yan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 371 (2010) 50–58

(a)

Aggregate state

Monomeric state

57

Framework of the hydrophobic bone

increase of pH increase of pH

Increase of PSL concentration Formation of PSL aggregates

(b)

the monomeric state of PSL molecules

the aggregate state of PSL molecules

Hydrophobic chain

Sulfonic group

Carboxyl group

Phenolic hydroxyl group

Chart 1. (a) The schematic structures of PSL aggregate. (b) Schematic illustration of the effect of pH on the configuration of the aggregate and monomeric states of the PSL aqueous solution.

4. Conclusions In this study, the influence of the pH on the solution behavior of PSL in aqueous solutions was investigated. The acid–base titration and zeta IB-90Plus were used to determine the surface potential of PSL. The CAC of PSL in aqueous solution was ascertained fluorescence experiments. The size of PSL molecules and PSL aggregates in aqueous solution were determined by DLS, and the morphology of aggregate was observed by ESEM. The results indicate that the CAC of PSL in aqueous solution is 0.05 g/L; the average dimension of PSL molecule and PSL aggregates in aqueous solution is about 8 nm and 80 nm, respectively. The surface of PSL aggregates is mainly covered by sulfonic groups and a few of phenolic hydroxyl

groups and the core of PSL aggregates are generated by hydrophobic chains. The hydrophobic core is loose and contains many weakly ionized groups such as carboxyls and phenolic hydroxyl groups. The increasing of PSL concentration favors aggregation of PSL, while an increase in the pH of solution makes the aggregates cores expand because of the ionization of weakly hydrophilic groups. Thus the polarity of the PSL aggregate’s core and the reduced viscosity of PSL in solution increase with the pH values increase. Acknowledgments The authors would like to acknowledge the financial supports of the China Excellent Young Scientist Fund (20925622),

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