Preparation of a new lignin-based anionic/cationic surfactant and its solution behaviour

Abstract

The lignin-based polyoxyethylene (SL–PEG) was synthesized by grafting sodium lignosulphonate (SL) with polyethylene glycol (PEG) long chains. The obtained SL–PEG was formulated with cetyltrimethylammonium bromide (CTAB) at different mass ratios to prepare different lignin-based cationic/anionic surfactants (CA-SLs). The solution behaviour of CA-SL with different mass ratios of CTAB/SL–PEG was studied. The particle size of the CA-SL aggregates was highest when the zeta potential is zero and then becomes small when the zeta potential is positive. It is concluded that the electrical properties and hydrophobic properties of the CA-SL molecules are the fundamental reasons which induce the aggregation behaviour to change. Besides, the results about the Critical Aggregation Concentration (CAC) and surface tension indicate that CA-SL has a stronger ability to lower the surface tension at the air/water interface compared with SL–PEG, but a weaker one than CTAB. However, the CACs of CA-SLs inside the solution are lower than those of both SL–PEG and CTAB. In conclusion, the CA-SL exhibits more obvious physical and chemical properties of polymeric surfactants than SL–PEG whether on the solution surface or inside the solution. The successful preparation of water-soluble lignin-based anionic/cationic surfactants may be a useful method in the efficient utilization of SL.

---

Introduction

Aqueous mixtures of cationic surfactants and anionic surfactants have been studied extensively for the last decades due to numerous applications of these systems in industrial areas. The cationic/anionic surfactant is obtained relying on the electrostatic interaction between cationic surfactant and anionic surfactant. The cationic/anionic surfactant can exhibit low interfacial tension at an oil/water interfacial and its properties are superior to primary cationic surfactants and anionic surfactants. Nowadays, more and more studies are focused on the preparation of the cationic/anionic surfactants and their action mechanism in solution. Wang¹ found that the mixtures of the cationic surfactant cetyltrimethylammonium bromide (CTAB) with the anionic surfactant sodium dodecyl sulfate (SDS) can form more stable coatings in fused silica capillaries than CTAB alone. The synergistic effect between CTAB and SDS were studied by Sohrabi,² and the results show that the surface tension of the compound system reduces which are markedly affected by the temperature. Tajik³ prepared the SDS/CTAB/polyethyleneglycol (PEG) compound system, and he disclosed the surface activity and foaming property of SDS/CTAB/PEG are related to the concentration and PEG content. Zhao⁴ prepared a cationic/anionic surfactant using cationic CTAB and anionic sodium alcohol ether sulphate (AES), and the obtained product can make the interfacial tension decrease to ultra low value, about 10⁻⁴ mN m⁻¹ in the oil field application. These studies indicate that the formulation of cationic surfactant and anionic surfactant can improve their surface activity prior to their individual.

The polymeric anionic/cationic surfactant compounded by oppositely charged polyelectrolytes and surfactants have been extensively studied in solution and at interfaces also. In general, the compounded anionic/cationic surfactants exhibit various complex phase behaviours in solution. Fechner⁵ prepared cationic polyelectrolyte–SDS compounds exhibiting low surface tension and Critical Aggregation Concentration (CAC) and revealed that the mole ratio of the cationic polyelectrolyte to SDS and the content of cationic groups are the most important factors. Sajid Ali⁶ studied the interactions between two homologous cationic Gemini surfactants and their corresponding monomeric counterparts with neutral polymer polyvinylpyrrolidone and found that the surfactant with more hydrophobicity shows stronger interaction comparing to that with less hydrophobicity.⁶ The system by compounding SDS with cationic polyelectrolyte PDADMAC is found to have even stronger ability to lower the interfacial tension in oil/water interface.⁷ The surface tension of the compound system of SDS and polymine was studied by Penfold,⁸ and they found that the adsorption pattern of the compounds shows a strong dependence upon the solution pH, the molecular weight and structure of polymine. The phase separation phenomena and aggregates were examined in the chitosan and SDS compound system prepared by Nizri,⁹ and the CAC tends to decrease with increasing SDS addition amount. Guillot¹⁰ prepared the compound system of carboxymethylcellulose and dodecyltrimethylammonium bromide, and found the aggregates form in air/water surface at extremely low concentration, above the CAC. Kogej¹¹ suggested that the electrostatic attraction and the hydrophobic association between the chain and the micelle core play important roles in the strong interactions between polystyrenesulfonate and cetylpyridinium compound system. The above studies indicate that the compound system of the polyelectrolyte with cationic or anionic charge and oppositely charged surfactants can produce new cationic/anionic compounds, which exhibit stronger physical and chemical performance.

As a polymeric surfactant, SL is byproduct of the sulfite pulping process, which contains many functional groups such as aromatic ring, sulfonic group and carboxyl group.¹² Because of the spherical structure and random distributed groups in the molecule, SL has very weaker physical and chemical performance, for example, the surface tension of SL aqueous solution is about 60–70 mN m⁻¹,^13,14 which is much higher than traditional surfactant. The weaker physical and chemical performance of SL in solution severely limit its performance in industry because the majority of the utilization of SL is applied in solution, so SL is generally considered as a lower-end product.^15–17 In order to enhance the surface activity and application performance, to introduce hydrophobic group into SL molecule by chemical methods is commonly tried. In the presence of pyridine catalyst, a bio-based surfactant was synthesized via the alkylation of sodium SL with 1-bromododecane in alkalic isopropanol–water solution by Zhou,¹⁸ and the surface tension of the obtained alkylated sodium SL at 1% aqueous solution is reduced to 28.2 mN m⁻¹. The organosilicon monomer with low polymerization degree is used to improve the hydrophobicity of SL by Telysheva,¹⁹ and the surface tension of the modified products in aqueous exhibit marked decrease. The halohydrocarbon was engrafted in the SL molecule by means of alkylation reaction by Cen²⁰ and the surface tension of the obtained product in aqueous is obviously reduced. The methods engrafting hydrophobic groups into SL molecules by chemical modification are generally carried out in solvents or heterogeneous medium, so the reaction efficiency is low and the purification cost is expensive, which bring much difficulty for industrial application. The CAC of SL in aqueous is determined by Qiu,¹² and the effect of pH on the aggregation state of SL in aqueous solution is studied by Yan,¹³ the results show that the electrostatic interaction plays important roles in the aggregation of SL in aqueous solution. Due to the charge neutralization, mixing SL and cationic surfactant will lead to large quantities of aggregates. Therefore, it is difficult to prepare SL-cationic surfactant compounds via mixing SL and cationic surfactant directly.

Investigations have shown that the compound cationic/anionic surfactants exhibit excellent physical and chemical performance. However, the preparation technology is hard, because of the difficulties in process controlling.^21,22 Due to the strong electrostatic interaction between the oppositely charged head groups of the surfactant molecules, the sedimentation is easy to occur in compounding process which results in failure. To solve this problem, the ethoxy chain is often introduced into the surfactants, so that the water-solubility of the produced compounds is improved, which ensures the stability of the inborn compounds in solution. Another way is to introduce non-ionic polyoxyethylene ether surfactants into the compounds, which can improve the water-solubility of the produced compounds in solution also. For example, the anionic AES surfactant with three ethoxy units is used to compound with CTAB, and the stable compounds are obtained by Zhao.⁴ The stability is markedly improved by adding polyoxyethylene into the anionic/cationic surfactant system by Yan.²³ In addition, the modifying of PEG is studied extensively^24,25 and modified PEG is widely used in controlled drug release system.^26,27

In this study, the long hydrophobic chains are attempted to introduce into the SL molecules to decrease the electrostatic interaction between cationic CTAB and the anionic groups in SL molecule and to improve the hydrophobicity and surface activity in aqueous of SL. In order to solve the sedimentation caused by mixing SL with the cationic surfactant directly, some hydrophilic long chains of polyoxyethylene ether are introduced in SL molecule firstly, and then the modified SL is compounded with cationic surfactant to prepare the polyelectrolyte–surfactant compounds. The obtained polymeric anionic/cationic surfactant contains many long hydrophobic chains and long hydrophilic polyoxyethylene ether chains simultaneously.

Experimental section

Materials

SL was derived from the by-product of wood sulfite pulping. Borontrifluoride–diethyl ether complex (BF₃–Et₂O) was 47% BF₃ and 53% Et₂O. PEG, CTAB and all the chemicals and solvents were analytical grade. Milli-Q water was used for the preparation of all solutions.

Preparation of SL–PEG

100 g PEG was put in a reactor flask equipped with a temperature-controlling electric heating device, a motor stirrer, a thermometer, a peristaltic pump and a reflux condenser. Then 0.8 g BF₃–Et₂O was added into the flask. Subsequently, a certain amount of epichlorohydrin was added dropwise at a rate which was sufficient to maintain the reaction temperature at 55–60 °C. After dropping off, the reaction was carried on for two more hours at 55 °C. Then the excess epichlorohydrin was removed by distillation and the PEG–chlorohydrin intermediate was generated. A certain amount of LS was dissolved in water to prepare a 30% (w/w) SL solution in another reactor flask. The PEG–chlorohydrin intermediate was dropped into the SL solution and reacted at 80 °C for 2 h. The sodium hydroxide solution was used to control the reaction pH at 10–11. After cooling, the pH value of the solution was adjusted to 7 with hydrochloride acid and the water-soluble SL–PEG copolymer was obtained.

Preparation of CA-SL

A certain amount of SL–PEG was dissolved in water to prepare a 30% (w/w) SL–PEG solution. Then the different mass of 30% (w/w) CTAB solution was added to prepare the lignin-based cationic and anionic surfactant CA-SLs with different molecular structures.

Characterization

Surface charge measurement. Samples were ion-exchanged through anion exchange resin and cation exchange resin to remove the low molecule mass organic salts and other impurities. Surface charge was determined by PCD-03 particle charged detector (BTG Mütek GmbH, Herrsching, Germany) with CTAB as a cation standard solution. The concentration of CTAB was corrected by sodium polyethylene sulphonate (0.001 N)

Molecular weight and functional group quantitative determination. Aqueous gel permeation chromatography (GPC) measurements were conducted with TSK gel Super Multipore PW-N (15 cm × 6 mm). The effluent was monitored at 280 nm with a Waters 2487 UV detector (Waters Co., Milford, MA, USA). Calibration standard was polystyrene sulfonates with M_w range from 2 to 100 kDa. A 0.10 M NaNO₃ solution was the eluent (0.50 mL min⁻¹). All samples were prepared using double-distilled water (d.d.w.) and filtered by a 0.22 μm filter. The phenolic group content, carboxyl group content and sulfonic group content of samples were measured by an automatic potentiometric titrator (Type 809 Titrando, Metrohm Corp., Switzerland). Before titration, the samples were ion-exchanged through anion exchange resin and then cation exchange resin.

Zeta potential measurement. The zeta potential of the CA-SL solution was measured by zeta potential and particle size analyzer (type ZetaPlus, Brookhaven Instruments Corp., USA). CA-SL solutions at different ratios were filtered through a 0.45 μm mfilter membrane before analysis to remove any dust in solution and then equilibrated for 12 h before zeta potential data collection.

Particle size distribution measurement. The particle size distribution of the unimoleculars and aggregations of CA-SL in solution was measured using a zeta potential and particle size analyzer (type ZetaPlus, Brookhaven Instruments Corp., USA). The measurement was performed for 5 min, and 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 298 K, controlled by the instrument automatically. The scattering angle range was 90°.

Surface tension measurement. The surface tension was measured using a Wilhelmy plate with DCAT21 tensiometer (Dataphysics Company, Germany). Experimental errors inherent in the measurement were ±0.03 mN m⁻¹. The surface tension was determined as an average value measured three times at 298 K.

AFM measurement. Surface morphology and surface roughness was observed by means of AFM (Park XE-100, ParkSYSTEMS Corp., Gyeonggi-Do, Korea) by the tapping mode.

CAC measurement. The CAC was determined by fluorescent spectrometry. The 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 SL–PEG. The excitation spectra of SL–PEG 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 behaviour of SL–PEG. 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⁻¹ and then diluted in acetone to obtain a concentration of 2 × 10⁻⁵ mol L⁻¹ 0.25 mL of this solution was added to a 50 mL volumetric flask, letting the acetone evaporate naturally. The SL–PEG solution was then prepared in a volumetric flask, keeping the concentration of pyrene at 1 × 10⁻⁷ mol L⁻¹. 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.

Results and discussions

Characterization of SL and SL–PEG

The anionic/cationic surfactant CA-SL was obtained by mixing CTAB and modified SL by PEG (SL–PEG). The content of main function groups, the mass average molecular weights (M_w) and the surface charge of SL and SL–PEG have important impact on CA-SL molecular structure. The quantitative determination results are shown in Table 1.

Table 1 Quantitative determination characterization of SL and SL–PEG^a

Samples	Surface charge (mmol g^−1)	Mw	Mn	Polydispersity Mw/Mn	Sulfonic (mmol g^−1)	Carboxyl (mmol g^−1)	Phenolic hydroxyl (mmol g^−1)
a Relative error: Mw, 5%; surface charge, 3%; sulfonic, 5%; carboxyl, 5%; phenolichydroxyl, 5%.
SL	1.73	13 021	5191	2.51	1.33	0.82	0.99
SL–PEG	0.96	24 902	12 088	2.06	1.07	0.65	0.63

---

As shown in Table 1, the M_w of SL–PEG increased a lot because of the introduction of PEG chains in SL molecule comparing to SL. Simultaneously, the mass percentage of the function groups of SL–PEG decreased as the introduction of non-ionic long chain. For example, the contents of sulfonic group, carboxyl, phenolic hydroxyl and the surface charge of SL–PEG reduced.

Zeta potential of CA-SL in solution

The formation of CA-SL depended on the electrostatic interaction between the anionic groups of SL–PEG and the cationic groups of CTAB. Various CA-SLs were prepared based on the different CTAB/SL–PEG mass ratio, which have different molecular structure. The electric properties of the different CA-SLs were determined by the zeta potential in solution, and the results are shown in Fig. 1.

Fig. 1 Zeta potentials of CA-SLs with different CTAB/SL–PEG ratios in solution.

From Fig. 1, the zeta potential values of the CA-SL aggregates in solution changes from negative to positive with the increase of the CTAB/SL–PEG mass ratios. The isoelectric point occurs at the CTAB/SL–PEG mass ratio of 0.3. Therefore, the CA-SLs are negatively charged when CTAB/SL–PEG < 0.3, and positively charged when CTAB/SL–PEG > 0.3. The CA-SL is electrically neutral and dissolves in water depending on PEG branches when CTAB/SL–PEG = 0.3.

So, the electrical properties of the formulated anionic/cationic surfactants CA-SLs are varied by changing the mass ratio of CTAB/SL–PEG. In this study, the CA-SLs are prepared at the CTAB/SL–PEG mass ratio of 1/10, 2/10, 3/10, 4/10, 5/10.

Average particle size of CA-SL in solution

As one type of polymer surfactant, the CA-SLs are easy to aggregate in solution. Except for the effect on the electrical properties of CA-SLs, the CTAB/SL–PEG mass ratio also has important influence on the average particle size of CA-SL aggregates in solution. The average particle sizes of CA-SL aggregates in solution are determined by DLS method, and the results are shown in Fig. 2.

Fig. 2 Average particle sizes of CA-SLs with different CTAB/SL–PEG ratios in solution.

As shown in Fig. 2, with the increase of the CTAB/SL–PEG mass ratio, a sharp drop from about 95 nm to about 30 nm of the diameters of the CA-SL aggregates can be observed before isoelectric point (CTAB/SL–PEG < 0.3). The diameters of the CA-SL aggregates increase to the maximum value about 130 nm at the isoelectric point (CTAB/SL–PEG = 0.3), then decrease continuously to about 25 nm after isoelectric point (CTAB/SL–PEG > 0.3).

There are two reasons for such a change about the diameter of CA-SL aggregates influenced by CTAB/SL–PEG mass ratio. Firstly, with the increasing CTAB/SL–PEG mass ratio, the negative charge amount of CA-SL molecule decrease continuously, so the CA-SL molecules in solution contract and the electrostatic repulsion between CA-SL molecules is weaker, which induce the decrease of the diameter of CA-SL aggregates. Secondly, the increasing CTAB/SL–PEG mass ratio makes the number of the hydrophobic long chain increase, so the hydrophobic property of the CA-SL molecule is strengthened and the Van der Waals' attraction force among CA-SL molecules is improved, which cause the CA-SL aggregates contract and turn smaller. However, the aggregation of CA-SL molecules in solution is also intensified by the enhanced hydrophobicity, which leads to the increase of the diameter of CA-SL aggregate. Common function in above both side, cause the decrease of the diameter of CA-SL aggregate with increasing CTAB/SL–PEG ratio before the isoelectric point. However, the electrostatic repulsion among CA-SL molecules disappear at the isoelectric point, so the suddenly intensified Van der Waals' attraction force among CA-SL molecules result in the most severe aggregation. Upon exceeding the isoelectric point with increasing CTAB/SL–PEG ratio, the electrostatic repulsion between CA-SL molecules appear again because the CA-SL molecules are positively charged, which cause the disaggregation of big CA-SL aggregates and the decreasing diameter of CA-SL aggregates.

Particles size distribution of CA-SL in solution

In order to make further reveals that the influence of the CTAB/SL–PEG mass ratio on the aggregation behaviour of CA-SL in solution, the CA-SLs with different SL–PEG/CTAB ratios are prepared, and their existing states in solution are examined by DLS. The results are shown in Fig. 3.

Fig. 3 Particle size distribution of CA-SLs with different CTAB/SL–PEG ratios in solution (a) SL–PEG; (b) CTAB/SL–PEG = 0.1; (c) CTAB/SL–PEG = 0.2; (d) CTAB/SL–PEG = 0.3; (e) CTAB/SL–PEG = 0.4; (f) CTAB/SL–PEG = 0.5.

From Fig. 3a, there are three peaks in the DLS curve. The first peak (about 2 nm) is regarded as the single molecules, which is very weak. The second peak (about 15–30 nm) and the third peak (about 150–400 nm) are regarded as the aggregates, which is dominant. Therefore, the SL–PEG mainly exists in solution by aggregate state, but the aggregates are expanded and widely distributed.

When CTAB/SL–PEG = 1/10 (Fig. 3b), the peak of separate CA-SL molecules disappears. The two peaks of the aggregates in the ranges of 15–30 nm and 150–400 nm also move to the ranges of 10–15 nm and 70–100 nm, and the peak intensity of the small aggregates also exceeds that of the big aggregates, which indicate that the introduction of CTAB into the SL–PEG molecule makes the particle size of the aggregates decrease. Keep increasing the amount of CTAB (CTAB/SL–PEG = 2/10, as shown in Fig. 3c), the two peaks of CA-SL aggregates continue moving to the low ranges of 8–12 nm and 35–50 nm, which corresponds with the regular pattern in Fig. 3b.

When CTAB/SL–PEG = 3/10 (Fig. 3d), the CA-SL molecules are at isoelectric point. The aggregation is severely intensified because of the disappearance of the electrostatic repulsion between CA-SL molecules and the small aggregates, thus the peak of the small aggregates almost disappears and the peak of the big aggregates grows and moves to the high ranges of 70–220 nm. The most severe aggregation state of CA-SL molecules in solution is achieved.

When CTAB/SL–PEG > 3/10 (Fig. 3e and f), the CA-SL molecules are positively charged. As the disaggregation of big CA-SL aggregates, the peak of aggregates moves to smaller ranges.

AFM analysis of CA-SL

In order to directly observe the aggregation behaviour of CA-SLs in solution at different CTAB/SL–PEG mass ratios, the surface morphology and microstructure characteristics of the CA-SLs are observed by AFM. The results are shown in Fig. 4.

Fig. 4 The AFM of CA-SLs with different CTAB/SL–PEG ratios (a) SL–PEG; (b) CTAB/SL–PEG = 0.1; (c) CTAB/SL–PEG = 0.2; (d) CTAB/SL–PEG = 0.3; (e) CTAB/SL–PEG = 0.4; (f) CTAB/SL–PEG = 0.5.

The topographic images undergo significant changes as illustrated in Fig. 4. The aggregates of SL–PEG are widely distributed and the big aggregates appear to be collapsed state (Fig. 4a). With the addition of CTAB, the number of CA-SL aggregates increases obviously and the particle size distribution turns more uniform (Fig. 4b). Keep increasing the amounts of CTAB, the number of CA-SL aggregates keeps increasing and the shape of the aggregates becomes uniform and sharp (Fig. 4c). When the isoelectric point is reached, many big aggregates similar to yurt appear and there are some scattered small aggregates also (Fig. 4d). Upon exceeding the isoelectric point with further increasing CTAB amounts, the big aggregates similar to yurt disaggregate to be uniform and sharp aggregates again (Fig. 4e and f). The quantitative analysis of the spectroscopy parameters in Fig. 4 is shown in Table 2.

Table 2 AFM analysis parameters of CA-SLs with different CTAB/SL–PEG ratios^a,b

M	Charge	q^v (nm)	H1 (nm)	H2 (nm)
a Relative error: q^v, 5%; H1, 5%; H2, 5%.b M: mass ratio of CTAB/SL–PEG; q^v: Rms roughness; H1: Z-axis height; H2: typical peak height.
0	Negative	0.531	16.0	12.0
1/10	Negative	0.335	6.0	2.3
2/10	Negative	0.438	2.0	1.7
3/10	Neutral	4.319	20.0	19.6
4/10	Positive	3.295	16.0	14.8
5/10	Positive	1.969	7.5	4.7

---

Table 2 demonstrates that the Rms roughness of CA-SL aggregates decrease first then reach the biggest at the isoelectric point, then keep decreasing with further increasing CTAB/SL–PEG mass ratios, so do the Z-axis height and the typical peak height. These changes all attribute to the changes of the aggregate structure caused by the charge amounts and hydrophobic property of CA-SLs with different CTAB/SL–PEG mass ratios, which correspond to the results of DLS.

Surface tension of CA-SL in air/water interface

As one type of polymeric surfactants, CA-SL has the ability to reduce the surface tension in air/water interface because of their hydrophobic long chains. The surface tensions of 1 g L⁻¹ SL–PEG solution with different CTAB amounts are measured, and the measured values are compared with SL–PEG and CTAB. The results are shown in Fig. 5.

Fig. 5 Surface tension of CA-SLs with different CTAB/SL–PEG ratios in solution (a) relative error: surface tension, 0.3–0.5%.

As shown in Fig. 5, the surface tension value of 1 g L⁻¹ SL–PEG is 63 mN m⁻¹. With the addition of CTAB (CTAB/SL–PEG = 1/10), the surface tension value of the obtained CA-SL reduces to 42.5 mN m⁻¹. By further addition of the CTAB amounts, the surface tension values of the obtained CA-SLs tend to be constant, approximately 39 mN m⁻¹. These indicate that the CA-SLs have stronger ability reducing the surface tension in air/water interface than SL–PEG, but a little inferior than CTAB.

The above phenomenon is attributed to the hydrophobic property of the surfactant molecules and their preferred orientation in air/water interface. Comparing with SL–PEG, many long hydrophobic chains of CTAB are introduced into the CA-SL molecules, so the CA-SL molecules are more hydrophobic and have stronger aggregate capacity in air/water interface, thus cause lower surface tension.²⁸ However, the regular degree of the polymeric surfactants arraying in air/water interface is inferior to the small-molecule surfactants like CTAB with classic chemical structure, thus the abilities of CA-SLs to reduce surface tension is worse than CTAB.

CAC of CA-SL inside the solution

Except for the disclosure of the aggregation ability of CA-SL in air/water interface, the aggregation ability of CA-SLs inside the solution is also studied. The CACs of the CA-SLs with different SL–PEG/CTAB ratios are determined by the fluorescent spectrometry (using pyrene as probe),²⁹ and the SL–PEG and CTAB are compared simultaneously. The results are shown in Table 3.

Table 3 CAC of CA-SLs with different CTAB/SL–PEG ratios in solution^a

Mass ratio of CTAB/SL–PEG	0	0.1	0.2	0.3	0.4	0.5	CTAB (without SL–PEG)
a Relative error: CAC, 1%.
CAC (g L^−1)	0.0150	0.0062	0.0058	0.0065	0.0062	0.0050	0.3000

---

As shown in Table 3, the CACs of SL–PEG and CTAB in solution are 0.0150 g L⁻¹ and 0.3000 g L⁻¹, respectively. The CACs of CA-SLs with different CTAB/SL–PEG mass ratios in solution are in the range of 0.0050–0.0065 g L⁻¹ in general, which are less than those of SL–PEG and CTAB.

CAC is related to the free energy of forming the first micelle in the vicinity of the polyion which reflects how strong the interaction is in a certain system. The surfactants trends to aggregate mutually inside the solution and their aggregation abilities can be characterized by the CAC values. The hydrophobic property of SL–PEG is markedly improved through the formulation with CTAB, thus the newborn CA-SLs have stronger aggregation ability inside the solution. Therefore, the CAC of CA-SL is lower than SL–PEG, and it can form aggregate at lower concentration. So the newborn lignin-based cationic/anionic surfactant has more typically physical and chemical properties of polymeric surfactants.

Aggregation models of CA-SLs in solution

In conclusion, the aggregation behaviour of CA-SL in solution is mainly affected by its charge property and hydrophobic property. With the increasing CTAB ratio in CA-SL molecule, the charge property of CA-SL molecule turns from negative to zero, then positive, while the hydrophobicity of CA-SL molecule continues increasing. The reduction of the total charge of CA-SL molecule reduces the electrostatic repulsion of internal aggregates which makes the particle size of aggregates decrease. The enhanced hydrophobicity of CA-SL molecule makes the aggregates more dense and their particle size decrease because of the Van der Waals force, while the aggregation among aggregates is intensified.

The CA-SLs with different charge properties can be obtained under different CTAB/SL–PEG mass ratio. The aggregation process of CA-SLs with different charge properties can be divided into the following four stages: SL–PEG, negatively charged CA-SL, zero charged CA-SL, positively charged CA-SL, and their aggregation states in different stages are illustrated in Fig. 6.

Fig. 6 Aggregation models of CA-SLs with different CTAB/SL–PEG ratios in solution. (a) SL–PEG; (b) CTAB/SL–PEG = 0.1; (c) CTAB/SL–PEG = 0.3; (d) CTAB/SL–PEG = 0.5. positive charge, negative charge, CTAB.

From Fig. 6, there are mainly four aggregation states of CA-SLs with increasing CTAB/SL–PEG mass ratios in solution. Firstly, as shown in Fig. 6a, without CTAB, the SL–PEG molecules are stretch, and the aggregates are expanded and widely distributed. Secondly, as shown in Fig. 6b, the CA-SL is negatively charged. Because the expanded CA-SL molecules are compressed, the aggregates turn more compact, smaller and more evenly distributed. Thirdly, as shown in Fig. 6c, the CA-SL is not charged. The CA-SL molecules are compressed seriously and many small aggregates grow and become bigger aggregates for the secondary aggregation. Fourthly, as shown in Fig. 6d, the CA-SL is positively charged. The big aggregates disaggregate into small aggregates, and the newly generated aggregates turn smaller and uniformly distributed again.

Conclusions

The lignin-based cationic/anionic surfactants of CA-SLs with different molecular structure are prepared. The charge properties of the obtained CA-SLs turn from negative to zero and then positive with increasing the CTAB/SL–PEG mass ratio. The aggregates change along with the zeta potential. During the course, the charge properties, the electrostatic repulsive force and the hydrophobic van der Waals force play an important role. The CA-SLs have stronger ability to lower the surface tension in air/water interface comparing with SL–PEG, but weaker than CTAB. The CACs of the CA-SLs inside the solution are much lower than those of SL–PEG and CTAB. These indicate that the compound CA-SLs exhibit more typical physical and chemical properties of polymeric surfactants, whether in air/water interface or in solution internal. The aggregation process of CA-SLs with different charge properties can be divided into the following four stages: SL–PEG, negatively charged CA-SL, zero charged CA-SL, positively charged CA-SL.

Acknowledgements

We gratefully acknowledge the financial supports from the National Basic Research Program of China (973 Program, 2012CB215302), the National Natural Science Foundation of China (21476092), the special major science and technology of Guangdong Province (2012A080105012).

Notes and references

1. C. Wang and C. A. Lucy, Electrophoresis, 2004, 25, 825–832.
2. B. Sohrabi, H. Gharibi, B. Tajik, S. Javadian and M. Hashemianzadeh, J. Phys. Chem. B, 2008, 112, 14869–14876.
3. B. Tajik, B. Sohrabi, R. Amani and M. Hashemianzadeh, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2013, 436, 890–897.
4. H. N. Zhao, X. H. Cheng, O. D. Zhao, J. B. Huang, C. J. Liu and B. Zhao, Acta Phys.-Chim. Sin., 2014, 30, 693–698.
5. M. Fechner and J. Koetz, Langmuir, 2013, 29, 7600–7606.
6. M. S. Ali, M. Suhail, G. Ghosh, M. Kamil and K. Din, Colloids Surf., A, 2009, 350, 51–56.
7. I. M. Tucker, J. T. Petkov, C. Jones, J. Penfold, R. K. Thoma, S. E. Rogers, A. E. Terry, R. K. Heena and I. Grillo, Langmuir, 2012, 28, 14974–14982.
8. J. Penfold, I. Tucker, R. K. Thomas and J. Zhang, Langmuir, 2005, 21, 10061–10073.
9. G. Nizri, S. Lagerge, A. Kamyshny, D. T. Major and S. Magdassi, J. Colloid Interface Sci., 2008, 320, 74–81.
10. S. Guillot, M. Elsanti, S. Désert and D. Langevin, Langmuir, 2003, 19, 230–237.
11. K. Kogej, Adv. Colloid Interface Sci., 2010, 158, 68–83.
12. X. Qiu, Q. Kong, M. Zhou and D. Yang, J. Phys. Chem. B, 2010, 114, 15857–15861.
13. M. Yan, D. Yang, Y. Deng, P. Chen, H. Zhou and X. Qiu, Colloids Surf., A, 2010, 371, 50–58.
14. R. Li, S. F. Aghamiri, D. Yang, P. Chen and X. Qiu, J. Dispersion Sci. Technol., 2013, 34, 709–715.
15. D. Areskogh, J. Li, G. Gellerstedt and G. Henriksson, Ind. Crops Prod., 2010, 32, 458–466.
16. Y. Ge, Z. Li, Y. Pang and X. Qiu, Int. J. Biol. Macromol., 2013, 52, 300–304.
17. H. Zhou, D. Yang, X. Qiu, X. Wu and Y. Li, Appl. Microbiol. Biotechnol., 2013, 97, 10309–10320.
18. B. Zhou, C. Ha, L. Deng, J. Mo, C. Sun and M. Shen, Acta Polym. Sin., 2013, 1363–1368.
19. G. Telysheva, T. Dizhbite, E. Paegle, A. Shapatin and I. Demidov, J. Appl. Polym. Sci., 2001, 82, 1013–1020.
20. X. Y. Cen, C. Y. Ha, Z. Z. Hu, J. Q. Mo, M. M. Shen and B. W. Zhou, C N Patent CN102580610-A, [P], 2012.
21. S. Hellebust, A. M. Blokhus and S. Nilsson, Colloids Surf., A, 2004, 243, 133–138.
22. K. Thuresson, S. Nilsson and B. Lindman, Langmuir, 1996, 12, 530–537.
23. P. Yan, L. Chen, C. Wang, J. X. Xiao, B. Y. Zhu and G. X. Zhao, Colloids Surf., A, 2005, 259, 55–61.
24. H. Yang, K. Shin, G. Tae and S. K. Satija, Soft Matter, 2009, 14, 2731–2737.
25. W. Deng, J. Chen, A. Kulkarni and D. H. Thompson, Soft Matter, 2012, 21, 5843–5846.
26. A. Zhang, Z. Zhang, F. Shi, J. Ding, C. Xiao, X. Zhuang, C. He, L. Chen and X. Chen, Soft Matter, 2013, 7, 2224–2233.
27. S. S. Payyappilly, S. Dhara and S. Chattopadhyay, Soft Matter, 2014, 13, 2150–2159.
28. X. Qiu, M. Yan, D. Yang, Y. Pang and Y. Deng, J. Colloid Interface Sci., 2009, 338, 151–155.
29. L. Gan, M. Zhou, D. Yang and X. Qiu, Holzforschung, 2013, 67, 379–385.

---