Formation of uniform colloidal spheres based on lignosulfonate, a

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Formation of uniform colloidal spheres based on lignosulfonate, a renewable biomass resource recovered from pulping spent liquor Qianqian Tang, Mingsong Zhou, Yingxin Li, Xueqing Qiu, and Dongjie Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03756 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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ACS Sustainable Chemistry & Engineering

Formation of uniform colloidal spheres based on lignosulfonate, a renewable biomass resource recovered from pulping spent liquor Qianqian Tang,† Mingsong Zhou,*,‡ Yingxin Li,‡ Xueqing Qiu,*,‡ and Dongjie Yang‡ †

College of Chemistry and Chemical Engineering, Henan Key Laboratory of Function-Oriented

Porous Materials, Luoyang Normal University, 6 Jiqing Road, Yibin District, Luoyang, 471934, People’s Republic of China. ‡

School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and Paper

Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou, 510640, People’s Republic of China. E-mail: [email protected] (Xueqing Qiu); [email protected] (Mingsong Zhou)

ACS Paragon Plus Environment


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ABSTRACT: Effects of mass ratios on sodium lignosulfoante (NaLS) and cetyltrimethylammonium bromide (CTAB) mixing system were firstly investigated by zeta potential and surface tension measurements. Uniform colloidal spheres from NaLS/CTAB complex were then fabricated via electrostatic and hydrophobic self-assembly, and characterized by DLS, TEM, contact angle, elemental analysis, XPS and FTIR measurements. Results showed the stoichiometric mass ratio (SMR) of NaLS/CTAB system was 1:2.82, where the hydrophobicity was strongest and preparing colloidal spheres was feasible. Colloidal spheres were formed through gradual aggregation of NaLS/CTAB molecules at SMR, which was induced by continuously adding water into NaLS/CTAB/EtOH solutions. NaLS/CTAB molecules started to form spheres at critical water content of 58 vol%, and the formation process was completed at water content of 84 vol% when the initial concentration of NaLS/CTAB in EtOH was 1.0 mg mL-1. The sizes of NaLS/CTAB colloidal spheres could be well controlled by adjusting water-adding rates. This preparation of lignosulfonate-based nanoparticles is very simple, safe and low-cost, and these obtained nanoparticles have advantages of biodegradability and ultra-violet resistance. This study provides a green and valuable approach to value-added applications of lignosulfonate biomass recovered from pulping spent liquor, and is of great significance for both economic and environmental benefits.

KEYWORDS: Sodium lignosulfonate; cetyltrimethylammonium bromide; electrostatic and hydrophobic interactions; self-assembly; colloidal spheres


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INTRODUCTION Lignin is the second most abundant renewable biomass resource after cellulose and also the largest reservoir of aromatic polymer on earth.1-2 Besides as a component of wood, lignin is mostly present as a by-product from the pulp and paper industry.3 Currently, lignin has received worldwide attention due to the increasing crisis of oil resources and environmental pollution. Lignosulfonate (LS) is a typical derivative of lignin obtained through the sulfite pulping process.4-5 It is often recognized as anionic surfactant, which contains both hydrophilic and hydrophobic groups, such as sulfonic, carboxyl, phenolic hydroxyl, aromatic and aliphatic groups.6 Due to the favorable solubility and strong polyelectrolytic behavior,7-8 LS has been widely used in various adsorption and dispersion fields, such as dispersants of dye,9 concrete,10-12 pesticide,13 coal-water slurry,14-15 oil well,16 and graphene,17 as well as cleaning agents,18 auxiliary substances for paper coating,19 corrosion and scale inhibitors for recirculating cooling water,20 dopants for conductive polymers.21 Overall, these uses of LS are relatively ordinary and low value-added applications, and their market demand only accounts for a negligible fraction of the current worldwide LS production. Actually, the vast majority of LS is discarded as wastes or used for energy generation through combustion, causing serious resource waste and environmental crisis. Therefore, finding innovative and high value-added applications of LS is of great significance for both utilization of renewable resources and environmental protection. For the high value-added utilization of LS resources, new methods of modifying LS and potential applications of these LS-based polymers are encouraged. In view of the excellent performance and wide applications of nanomaterials in medicine, life sciences, agriculture, etc, it is considered to transform unordered LS aggregates into ordered colloids. To our knowledge, the introduction of LS into the nano area has rarely been reported before due to the complicated chemical structures, and is



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worthy of further investigations. With the aim of obtaining colloidal spheres from LS, it should firstly be treated with hydrophobic modifications.22 The incorporation of cationic surfactant with a positive charge and a long hydrocarbon chain could improve the hydrophobicity of LS system. Since LS is an anionic polyelectrolyte bearing negative charges.6 When cationic surfactant is added, it would adsorb and self-assemble at the surface of LS aggregates by electrostatic attractions, resulting in the shielding of hydrophilic groups and thus the increase of hydrophobicity. Despite the toxicity and irritation, the common cetyltrimethylammonium bromide (CTAB) is an excellent choice for cationic surfactant23 due to its simple structures, relatively low costs, accessibility, excellent product properties and biodegradability. Next, preparing colloidal spheres from LS/CTAB system is attempted. It is found that colloidal spheres could be obtained from the complex system in mixed EtOH/water solvent via self-assembly when the CTAB/NaLS mixing ratio (w/w) reaches the stoichiometric mass ratio (SMR). To the best of our knowledge, this should be the first time that the micellization of the renewable resource of LS is completed via electrostatic and hydrophobic self-assembly in nontoxic solvents through a basically green process, which provides some important enlightenment about how to transform complicated and unordered lignin-based aggregates into ordered colloidal spheres. Comparing to the formation of colloids from traditional amphiphilic block and grafted copolymers,24-26 the whole preparation method in this work has advantages of simpleness, safety and low costs without any sophisticated chemical modifications, toxic solvents or expensive reagents. Moreover, LS has excellent biodegradability and ultraviolet resistance,27 which would provide some inherent advantages when this LS-based nanomaterial is used in fields such as drug controlled release and pesticide microencapsulation. On account of these advantages, the nanomaterial prepared


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from LS in this method can be expected to have wide application prospects. This study not only gives a new approach to exploit the functionality of LS and other lignin derivatives, but also brings the high value-added application of lignin-based products into a new area. In this work, the impact of mass ratios on sodium lignosulfoante (NaLS) and CTAB (NaLS/CTAB) mixing system was firstly studied by means of zeta potential and surface tension measurements. On this basis, the nanometric colloidal particles from NaLS/CTAB complex at SMR were prepared via self-assembly. The structure and formation mechanism of NaLS/CTAB colloidal spheres were then investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), contact angle, elemental analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) measurements. Additionally, we also studied the feasibility for controlling the particle size by changing water-adding rates, which could establish the foundation for the exploitation of NaLS/CTAB colloidal spheres in different fields.

EXPERIMENTAL SECTION Materials. The commercial NaLS, separated from the sulfite pulping spent liquor of poplar wood, was supplied by Shixian Papermaking Co. Ltd. (Jilin, China). The raw NaLS samples were processed by filtration and ultrafiltration to remove the low-molecular-weight impurities, and the detailed pre-treatment process has been described in our previous work.7 Deionized water (resistivity≧18 MΩ·cm) was obtained from a Millipore water purification system and used for the experiments conducted in this work. CTAB, EtOH and other chemical reagents were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received without any further purification.



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Preparation method for colloidal spheres. Firstly, 60 wt% of NaLS aqueous solutions and 44.5 wt% of CTAB EtOH/H2O solutions were prepared, respectively. Then, the homogeneous NaLS/CTAB system with the mass ratio of solid CTAB and solid NaLS of 1:2.82 was obtained by mixing NaLS solutions and CTAB solutions at a mass ratio of 2.09:1, followed by stirring with a multiple-point magnetic heating plate (ZNCL-S-10D; Yuhua Co., Ltd. Gongyi, China) at a speed of 800 rpm for 10 min and putting aside for 1 h. Next, the sample was completely dissolved into EtOH (Very little water existed in the system, which could be negligible.) to prepare NaLS/CTAB/EtOH solutions with the concentration of 1.0 mg mL-1. To obtain stable colloidal suspensions, water was added dropwise into the stirred NaLS/CTAB/EtOH solutions until the water content reached approximately 90 vol%. The water-adding rate was adjusted to about 20 µL/s using a syringe pump. Characterizations. Detailed information regarding instruments and methods employed for characterization is given in the Supporting Information.

RESULTS AND DISCUSSION Zeta potential and surface tension of NaLS/CTAB mixing system. The formation of NaLS/CTAB mixing system depends on the electrostatic attractions between anionic groups of NaLS and cationic groups of CTAB. The zeta potential of NaLS/CTAB mixing system at different CTAB/NaLS ratios (w/w) is determined and the result is displayed in Figure 1a. It can be seen that the zeta potential of the mixing system changes from negative to positive with increasing CTAB/NaLS mass ratios, due to the gradual shielding of anionic hydrophilic groups of NaLS by cationic groups of CTAB and then the envelopment of aggregates by cationic groups of excessive CTAB molecules through hydrophobic interactions. The charge neutralization is achieved approximately at the CTAB/NaLS mixing ratio (w/w) of 1:2.82. This implies that SMR of the mixing


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system is 1:2.82, namely, the average 2.82 g of solid NaLS could be neutralized by 1 g of solid CTAB. Accordingly, the mixing system is negatively charged at the CTAB/NaLS mixing ratio (w/w) below SMR, and positively charged at the CTAB/NaLS mixing ratio (w/w) above SMR. At SMR, the surface negative charges of NaLS are completely neutralized by CTAB, from which it is concluded that the hydrophily is weakest and the hydrophobicity correspondingly is strongest for the mixing system at SMR. Additionally, the surface tension of NaLS/CTAB mixing system as a function of the CTAB/NaLS ratios (w/w) is also determined (Figure 1b). It is found that the surface tension decreases sharply with increasing CTAB/NaLS mixing ratios, due to the gradual introduction of CTAB hydrophobic chains into the NaLS molecules through electrostatic attractions, causing stronger aggregate capacity at air/water interface.23 A minimum is observed at SMR. This provides a visual demonstration of the strongest hydrophobicity of the mixing system at SMR. As the CTAB/NaLS mixing ratio continuously increases above SMR, the surface tension tends to remain unchanged because the excessive CTAB molecules are mainly employed to envelop the formed hydrophobe through hydrophobic interactions in solutions, rarely to aggregate at air/water interface. Formation of colloidal spheres. According to above results, SMR of the NaLS/CTAB mixing system is 1:2.82, which is just consistent with the surface charge measurement result. At SMR, the hydrophobicity of NaLS/CTAB system reaches the maximum, while the electrostatic repulsion and the hydrogen bonding weaken considerably due to the total charge shielding of NaLS by CTAB. According to Qian et al., the main driving force for the formation of colloidal spheres from acetylated lignin-based polymers is the hydrophobic interaction, but the presence of electrostatic repulsion and hydrogen bonding would prevent this process,22 from which we infer that preparing



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uniform colloidal spheres from NaLS/CTAB mixing system at SMR is feasible. Colloidal spheres from NaLS/CTAB complex at SMR of 1:2.82 are prepared via electrostatic and hydrophobic self-assembly. In the process, water is gradually added into 1.0 mg mL-1 of homogeneous solutions of NaLS/CTAB complex at SMR in EtOH. Since the NaLS/CTAB complex is water-insoluble, the solvent quality becomes progressively worse for NaLS/CTAB with the addition of water, which causes the self-assembly of NaLS/CTAB molecules through hydrophobic interactions. When the water content reaches a critical value, NaLS/CTAB starts to form colloids, which is reflected as a dramatic increase in the scattered light intensity. The water content at this critical point is defined as the critical water content (CWC).28 The CWC values are determined by analyzing the variation curves of the scattered light intensity of the samples versus the water contents. The inset of Figure 2 shows a typical plot of the scattered light intensity of NaLS/CTAB in EtOH/H2O media with the water contents. When the water content is low, the scattered light intensity is small and almost remains unchanged as the water contents increase. When the water content reaches CWC, the scattered light intensity increases sharply. In this case, the CWC is determined to be 58 vol%. CWC depends on both the initial concentration of NaLS/CTAB in EtOH and the molecular weight of NaLS/CTAB. For the NaLS/CTAB complex at SMR with specific molecular weight, the higher the initial concentration, the lower the CWC value (Figure 2). Specially speaking, when the initial concentration of NaLS/CTAB in EtOH increases from 0.5 to 3.0 mg mL-1, CWC decreases from 64 vol% to 50 vol%. A linear inverse proportion relationship is observed between CWC and the logarithm of the initial concentration (C0) of NaLS/CTAB in EtOH, as shown in Figure 2. CWC = − A log C0 + B

(1)


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Where A and B are two constants and need to be determined for a specific sample system. Here, A is 0.1783 and B is 0.5836. To obtain well-developed colloids, more water is continuously added into the solutions to ensure the aggregation process could be completed. Then, an excess of water is added to “quench” the structures formed in NaLS/CTAB suspensions. Figure 3a presents a typical TEM image of the colloids, which indicates the obtained colloidal spheres are relatively uniform in the size. The average size of the colloids is estimated to be about 500 nm, obtained from the statistics of 100 contiguous spheres in the TEM images. Figure 3b shows that the average size of spheres is approximately 539 nm with a low polydispersity index of 0.037, which is consistent with the TEM observations. The size of NaLS/CTAB colloidal spheres obtained through TEM is smaller than the DLS result, which is reasonable since the TEM image is obtained under dry conditions. Characterizations of colloidal spheres. The surface wettability of colloidal films could be estimated by the contact angle measurement. The contact angle of the solid film obtained from NaLS/CTAB complex at SMR in EtOH is 48.85°, while that of the colloidal film of NaLS/CTAB complex at SMR is 21.16°. Obviously, the surface of colloidal spheres is composed of more hydrophilic chains. This could be further proven by comparing the element ratio of O/C on the colloidal surface and in the NaLS/CTAB colloidal bulk. Since the hydrophilic groups of NaLS/CTAB are mainly carboxyl and phenolic hydroxyl groups, the more hydrophilic chains mean a higher O/C ratio. The O/C ratio of NaLS/CTAB colloidal bulk could be measured by elemental analysis, while the O/C ratio on the colloidal surface can be estimated by XPS. As Table S1 shows, the O/C ratio on the colloidal surface is 0.31, and the O/C ratio in the NaLS/CTAB colloidal bulk is 0.24. It provides a direct evidence that NaLS/CTAB colloids have a hydrophilic shell and a



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hydrophobic core. The solvents, such as water or EtOH, have a great effect on XPS and elemental analysis results, so they should be strictly removed from these experimental samples. As shown in Figure S1, both NaLS/CTAB and NaLS/CTAB colloidal spheres have sulfonic and phenolic hydroxyl adsorption peaks located at 1035 and 1218 cm-1, respectively. If there is water, the sample must have an adsorption peak at about 1630 cm-1. Clearly, there is no adsorption peak at 1630 cm-1 in both NaLS/CTAB colloidal spheres and NaLS/CTAB. EtOH has adsorption peaks at 3346 and 1050 cm-1, but both NaLS/CTAB colloidal spheres and NaLS/CTAB have no adsorption peaks at these locations. Therefore, FTIR analysis shows strong evidence that there is no water or EtOH in NaLS/CTAB and NaLS/CTAB colloidal spheres. Colloidal sphere formation mechanism. Uniform colloidal spheres are successfully obtained from NaLS/CTAB complex at SMR. However, when CTAB is insufficient and the mass ratio of CTAB and NaLS fails to reach SMR, the mixing system has worse abilities of colloidization. As shown in Figure 4, when the CTAB/NaLS mixing ratio (w/w) is 1:3.6, the CWC at which colloidal spheres start to form increases to 69 vol% from 64 vol% at SMR (The initial concentration of NaLS/CTAB in EtOH is 0.5 mg mL-1.), indicating the decreased ability to form colloids for the NaLS/CTAB mixing system with a lower additive amount of CTAB. Pure NaLS could be regarded as an extreme example-the additive amount of CTAB is 0, and NaLS could never form colloidal spheres. This phenomenon can be explained in the following two aspects. Firstly, the electrostatic repulsion caused by sulfonic acid, carboxyl and phenolic hydroxyl groups of NaLS prevents the formation of the dense packing in spherical nanoparticles. Secondly, the hydrogen bonding between NaLS and water caused by phenolic hydroxyl groups holds back the colloid formation. But the NaLS


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system still can not form colloids by adding salt to shield the electrostatic repulsion or urea to destroy the hydrogen bonding between NaLS and water, respectively. Therefore, there must be a lack of some intermolecular interaction in NaLS, neither electrostatic repulsion nor hydrogen bonding, which prevents the formation of colloids. For NaLS/CTAB complex at SMR, CWC is closely related to the initial concentration of NaLS/CTAB in EtOH. NaLS/CTAB with higher initial concentrations displays lower CWC, just as shown in Figure 2. This indicates the hydrophobic interactions play a critical role in NaLS/CTAB colloid formation. The hydrophobic effect might be related to π-π and Van der Walls interactions. In general, the NaLS aggregation can be classified into two levels. One is the π-π aggregation of aromatic groups in NaLS caused by π-π interactions (nonbonded orbital interaction). The other is the molecular aggregation of polymer chains caused by Van der Walls attractions.29-30 The π-π aggregation generally goes with energy splitting. Nevertheless, the molecular aggregation is not accompanied with energy transfer.30-31 Therefore, π-π aggregation does not always result in molecular aggregation, and the reverse is also true.30, 32 NaLS/CTAB has a certain amount of carbon chains, so the Van der Walls interaction may play a role in driving the colloid formation. Meanwhile, it is inferred that the π-π interaction would also influence the NaLS/CTAB colloid formation. In order to prove this viewpoint, FTIR spectra of NaLS and NaLS/CTAB are displayed in Figure 5. The NaLS/CTAB sample is obtained from NaLS/CTAB complex at SMR in pure EtOH. The FITR spectra of NaLS and NaLS/CTAB present obvious difference. The main aromatic peak shifts from 1514 cm-1 for NaLS to 1510 cm-1 for NaLS/CTAB. This variation in peak positions is ascribed to the aromatic groups being in a different environment (i.e., π-stacking). It actually provides a direct evidence that NaLS/CTAB has a stronger π-π



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interaction among aromatic groups than NaLS does. NaLS can not form colloidal spheres, but NaLS/CTAB can. This could be attributed to their different structure characteristics. Most of hydrophilic sulfonic acid and phenolic hydroxyl groups in NaLS have been shielded by the introduction of CTAB. Therefore, NaLS/CTAB has a stronger π-π interaction, a stronger Van der Walls interaction, a weaker electrostatic repulsion and a weaker hydrogen bonding comparing with NaLS. It is mainly the hydrophobic effect that leads to the formation of NaLS/CTAB colloids, and this hydrophobic effect is resulted from the strong π-π interaction and the strong Van der Walls interaction. Light scattering study can provide important information for the colloid formation process. Figure 6 shows the particle size (Dh) and the polydispersity index of structures formed in NaLS/CTAB suspensions with different v(H2O)/v(EtOH) ratios. In this experiment, the initial concentration of NaLS/CTAB in EtOH is 1.0 mg mL-1. When the v(H2O)/v(EtOH) ratios increase from 1.86 to 4.8 (namely the gradually increased water content), the corresponding Dh increases. At a lower water content, only a few NaLS/CTAB molecules with stronger hydrophobicity get together to form small-sized colloidal spheres like a nucleation process in most colloidal formation processes.33 With an increasing water content, more and more NaLS/CTAB molecules in solutions transfer to surfaces of the already formed colloidal spheres, which results in the gradual growing of the colloid size. It is noteworthy that when the v(H2O)/v(EtOH) ratio is above 1.86 (The water content has exceeded 67 vol%, which is above CWC (58 vol%) of the NaLS/CTAB system.), Dh continues to rise. This indicates that not only the amount of colloidal spheres increases, but the remaining NaLS/CTAB molecules with weaker hydrophobicity still aggregate on surfaces of the already formed colloidal spheres. At the v(H2O)/v(EtOH) ratio of 4.8 (a water content of 84 vol%), Dh reaches the maximum,


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and the colloid formation is completed. When the v(H2O)/v(EtOH) ratio is above 4.8, Dh begins to decrease with the increasing v(H2O)/v(EtOH) ratio, corresponding to the cluster collapse caused by hydrophobic interactions. This is due to the excess water additions, which “quench” the colloidal structures formed. For the polydispersity index, its changing tendency with increasing v(H2O)/v(EtOH) ratios in the whole range is just opposite to that of Dh. Up to now, only the colloid theory of block copolymers that have a structure similar with the low-molecular-weight amphiphilic molecules is widely accepted. The micellization process of block copolymers in selective solvents is generally regarded as a phase separation process.34 For an amphiphilic diblock copolymer in water, the hydrophilic blocks aggregate to form the shells of micelles, while the hydrophobic blocks form the cores of micelles. However, the structure of NaLS/CTAB is much different from that of block copolymers. Therefore, we bring forward a new and possible colloid formation mechanism of NaLS/CTAB according to our experiments and analysis. Firstly, it is worth mentioning that the electrostatic attraction is the prerequisite for NaLS/CTAB colloidal formation. Since it is possible to form NaLS/CTAB colloids only when NaLS is combined with CTAB through electrostatic interactions to strength hydrophobicity of the system. Compared to an structured amphiphilic block copolymer, NaLS/CTAB is randomly formed from various phenylpropanoid units, hydrophilic groups (mainly the carboxyl groups located in the interior of NaLS aggregates) and hydrophobic carbon chains. It is impossible for NaLS/CTAB to form a colloidal structure that is analogous to block copolymer micelles because the hydrophobic skeletons are not flexible enough to achieve a phase separation from the hydrophilic groups. The colloid formation process of NaLS/CTAB is represented in Figure 7. In state A, NaLS/CTAB



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is almost completely dissolved in EtOH, so only very small particles can be seen on the corresponding TEM images (Figure 7A). In state B, a few NaLS/CTAB molecules with stronger hydrophobicity get together to form local clusters with increasing water contents, and these local clusters could be seen clearly on the TEM figure (Figure 7B). As the water content further increases, more and more NaLS/CTAB molecules transfer from solutions to aggregates. In state C, most of NaLS/CTAB molecules start to form colloidal spheres at CWC of 58 vol%, and these colloidal spheres can also be seen clearly on the matching TEM images (Figure 7C). The colloidal spheres at this stage are still swollen due to the existence of EtOH. The already formed clusters in state B may become the cores of colloidal spheres. Unlike traditional polymer micelles obtained from block copolymers having shells composed of hydrophilic blocks and cores composed of hydrophobic blocks, the colloids obtained from NaLS/CTAB have shells composed of NaLS/CTAB fractions with less hydrophobicity and cores composed of NaLS/CTAB fractions with more hydrophobicity. With the continuously increasing water contents, the remaining NaLS/CTAB molecules with less hydrophobicity continue to assemble on surfaces of the already formed colloidal spheres, which results in the increased particle size. The colloid formation is completed at a water content of 84 vol%. In state D, when the water content exceeds 84 vol%, stable colloidal spheres could be obtained by adding excessive amounts of water to “quench” the NaLS/CTAB colloid structures, which would simultaneously cause the decreased particle sizes. Obviously, NaLS/CTAB colloidal spheres are obtained from the gradual hydrophobic aggregation of NaLS/CTAB molecules with different hydrophobicity in EtOH/H2O media, which is induced by the gradual addition of water. In order to further confirm the NaLS/CTAB colloid formation mechanism, colloidal spheres are prepared from the mixture of a more hydrophilic polymer (NaLS) and a more hydrophobic polymer


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(NaLS/CTAB complex at SMR). Although NaLS can not form colloidal spheres, a mixture of NaLS and NaLS/CTAB complex at SMR can form uniform colloidal spheres with a relatively larger size, as indicated in Figure S2. This provides a direct support for the NaLS/CTAB colloid formation mechanism, which can also be applied to other lignin-based polymers. Size control of colloidal spheres. Different particle sizes are required for different potential application fields of colloidal spheres.35 Generally, many factors could influence the size of colloidal spheres. However, adjusting the water-adding rate is the most accessible means to accomplish this purpose. Figure 8a reveals the relationships between Dh of NaLS/CTAB colloidal spheres and water-adding rates at a fixed initial concentration of NaLS/CTAB in EtOH of 0.5 mg mL-1. It is observed when the water-adding rate increases from 5 to 40 µL/s, the Dh decreases dramatically. As the water-adding rate continuously increases, the Dh of NaLS/CTAB colloidal spheres no longer markedly decreases. This is the result of competition between the thermodynamic and kinetic control during the self-assembly process of NaLS/CTAB molecules. At a slower water-adding rate, the colloid formation process is mainly controlled by the thermodynamic factor. The molecules within aggregates could rapidly and fully exchange with the single molecules in solutions until equilibrium achieved. In this stage, the remaining NaLS/CTAB single molecules in solutions continuously associate on the surface of cores already formed from molecules with the strongest hydrophobicity. Therefore, the obtained colloidal spheres have a relatively larger particle size. As the water-adding rate increases, the kinetic factor begins to become predominant. The lack of time would cause colloidal spheres to be kept in a “frozen” state. The exchange rate between molecules in aggregates and solutions significantly reduces and there is no enough time for the single molecules in solutions



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to transfer to surfaces of the already formed cores. In this case, the NaLS/CTAB single molecules in solutions would separately nucleate, which ultimately leads to the increased amounts of colloids but the decreased particle size. Correspondingly, the typical TEM images of NaLS/CTAB colloidal spheres obtained at two different water-adding rates are displayed in Figure 8b and c. It could be observed that the two samples own the similar spherical structure but quite different particle sizes. The size of colloidal spheres obtained at water-adding rate of 5 µL/s is obviously larger than that obtained at water-adding rate of 40 µL/s, which agrees well with the DLS result. Just by adjusting the water-adding rates, the size of NaLS/CTAB colloidal spheres could be easily controlled in a certain range. This method is simple to operate and cost-effective. Easily adjustable properties for the size of NaLS/CTAB colloidal spheres establish the foundation for improving their potential applications in different fields.

CONCLUSIONS The zeta potential and surface tension measurement results indicate that SMR of the NaLS/CTAB system is 1:2.82, where the hydrophobicity of the complex is strongest. Uniform colloidal spheres are firstly prepared from the NaLS/CTAB complex at SMR by the gradual electrostatic and hydrophobic aggregation of NaLS/CTAB molecules in the mixed EtOH/water media. Although NaLS/CTAB is water insoluble, the NaLS/CTAB colloidal spheres are water-dispersive. When water is continuously added into NaLS/CTAB/EtOH solutions, the polymeric chains start to form colloidal spheres at CWC of 58 vol% for an initial concentration of NaLS/CTAB in EtOH of 1.0 mg mL-1. The colloids continuously grow as the water content increases above CWC, and this process is completed at a water content of 84 vol%. An excess of water is added to “quench” the structures formed. The


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sizes of NaLS/CTAB colloidal spheres could vary by hundreds of namometers depending on water-adding rates. Unlike conventional polymer colloids obtained from block copolymers, colloids formed from NaLS/CTAB have cores consisting of NaLS/CTAB fractions with a stronger hydrophobility and shells consisting of NaLS/CTAB fractions with a stronger hydrophily. This work develops an easy, safe and economically viable strategy for the effective utilization of LS biomass waste. In the future, these obtained colloidal spheres can be used in a large range of applications, such as drug delivery and controlled release, pesticide microencapsulation, UV-blocking additive for thermoplastics, Pickering emulsions, coating formulations and antimicrobial materials. Future investigations towards various applications would be carried out.

ASSOCIATED CONTENT Supporting Information Detailed characterization methods; FTIR spectra of NaLS/CTAB and NaLS/CTAB colloidal spheres; TEM image of NaLS/CTAB colloidal spheres formed by adding NaLS aqueous solutions into NaLS/CTAB/EtOH solutions; Elmental distributions in NaLS/CTAB colloidal spheres and on their surface (PDF)

AUTHOR INFORMATION Corresponding Authors *(Xueqing Qiu) Tel: +86-020-87114722. E-mail: [email protected] *(Mingsong Zhou) Tel: +86-020-87114722. E-mail: [email protected] Notes The authors declare no competing financial interest.



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ACKNOWLEDGMENTS We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (21476092), the National Basic Research Program of China (973 Program) (2012CB215302), and Colleges and Universities in Henan Province Key Science and Research Project (16A530007).


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Figure Captions Figure 1 The zeta potential (a) and surface tension (b) of the mixing system with different CTAB/NaLS mixing ratios (w/w) of 1:15, 1:11, 1:9, 1:7, 1:5.5, 1:3.6, 1:2.82, 1:1.8 and 1:1. The NaLS concentration is fixed at 1.0 mg mL-1. Figure 2 Plot of the critical water content (CWC) vs logarithm of the initial concentration (C0) of NaLS/CTAB in EtOH. Inset: Scattered light intensity as a function of added water content in the NaLS/CTAB/EtOH solutions, the initial concentration of NaLS/CTAB in EtOH is 1.0 mg mL-1. Figure 3 Typical TEM image (a) and particle size distribution (b) of colloidal spheres obtained from the NaLS/CTAB dispersion Figure 4 Scattered light intensities of the complex with different CTAB/NaLS mixing ratios (w/w) of 1:2.82 and 1:3.6 as a function of water content in the NaLS/CTAB/EtOH solutions. The initial concentration of NaLS/CTAB in EtOH is 0.5 mg mL-1. Figure 5 FTIR spectra of NaLS and NaLS/CTAB Figure 6 Particle size (Dh) and polydispersity index of colloidal spheres as a function of H2O/EtOH ratio (v/v) (The initial concentration of NaLS/CTAB in EtOH is 1.0 mg mL-1) Figure 7 Schematic representation of the colloid formation process of the NaLS/CTAB in EtOH/H2O media and TEM images of the samples obtained from the dispersions with different water contents: (A) 0, (B) 0~58 vol%, (C)58~84 vol%, (D) > 84 vol%. The initial concentration of NaLS/CTAB in EtOH is 1.0 mg mL-1. Figure 8 Particle size (Dh) (a) of colloidal spheres as a function of water-adding rates and TEM images of colloidal spheres obtained at water-adding rates of 5 µL/s (b) and 40 µL/s (c)


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Figure 1 The zeta potential (a) and surface tension (b) of the mixing system with different CTAB/NaLS mixing ratios (w/w) of 1:15, 1:11, 1:9, 1:7, 1:5.5, 1:3.6, 1:2.82, 1:1.8 and 1:1. The NaLS concentration is fixed at 1.0 mg mL-1. 30

a

Zeta potential (mV)

20 10 0 -10 -20 -30 -40 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 m (CTAB):m (NaLS) 42.0 Surface tension (mN/m)

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b

41.4 40.8 40.2 39.6 39.0 38.4 37.8 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 m (CTAB):m (NaLS)



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Figure 2 Plot of the critical water content (CWC) vs logarithm of the initial concentration (C0) of NaLS/CTAB in EtOH. Inset: Scattered light intensity as a function of added water content in the NaLS/CTAB/EtOH solutions, the initial concentration of NaLS/CTAB in EtOH is 1.0 mg mL-1.


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Figure 3 Typical TEM image (a) and particle size distribution (b) of colloidal spheres obtained from the NaLS/CTAB dispersion



Figure 4 Scattered light intensities of the complex with different CTAB/NaLS mixing ratios (w/w) of 1:2.82 and 1:3.6 as a function of water content in the NaLS/CTAB/EtOH solutions. The initial concentration of NaLS/CTAB in EtOH is 0.5 mg mL-1. 500 Scattered light intensity (kcps)

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1:2.82 1:3.6

400 300 200 100 0 40

50 60 Water content (vol%)


70

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Figure 5 FTIR spectra of NaLS and NaLS/CTAB

NaLS/CTAB NaLS/CTAB

NaLS

1510

1514

NaLS

4000

3500

3000

2500

2000

1500

1000

500

1600

-1

Wavenumber (cm )


1550

1500

1450 -1

Wavenumber (cm )

1400


Figure 6 Particle size (Dh) and polydispersity index of colloidal spheres as a function of H2O/EtOH ratio (v/v) (The initial concentration of NaLS/CTAB in EtOH is 1.0 mg mL-1) 700 0.10 600

Dh (nm)

0.08 500 0.06 400 0.04

300

0.02

200 2

4

6 8 H2O/EtOH ratio

10

12


Polydispersity Index

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Figure 7 Schematic representation of the colloid formation process of the NaLS/CTAB in EtOH/H2O media and TEM images of the samples obtained from the dispersions with different water contents: (A) 0, (B) 0~58 vol%, (C)58~84 vol%, (D) > 84 vol%. The initial concentration of NaLS/CTAB in EtOH is 1.0 mg mL-1.



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Figure 8 Particle size (Dh) (a) of colloidal spheres as a function of water-adding rates and TEM images of colloidal spheres obtained at water-adding rates of 5 µL/s (b) and 40 µL/s (c)


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Table of Contents

Synopsis: Colloidal spheres based on renewable lignosulfonate were prepared via self-assembly, which was induced by continuously adding water into NaLS/CTAB/EtOH solutions.


Formation of uniform colloidal spheres based on lignosulfonate, a

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