Novel succinic acid based polymeric surfactants: Synthesis and performance investigation

Journal of Molecular Liquids 231 (2017) 72–79

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Novel succinic acid based polymeric surfactants: Synthesis and performance investigation Xiaoyi Yang, Xiaodan Ren, Ping Li ⁎, Chaohua Guo, Jianbo Li, Quanhong Li China Research Institute of Daily Chemical Industry, Taiyuan 030001, PR China

a r t i c l e

i n f o

Article history: Received 29 November 2016 Received in revised form 20 January 2017 Accepted 29 January 2017 Available online 2 February 2017 Keywords: Succinic acid-based polymer Sulphur trioxide Surface active Dispersion BaSO4

a b s t r a c t Three novel succinic acid based polymeric surfactants, MAPEG-OSO3Na (MPEG = 1000, 2000, 4000), were designed and synthesized by reaction of maleic anhydride (MA) and polyethylene glycols (PEG) of different molecular weight. Their surface activities, adsorption, spreading performances, aggregation behaviors and dispersion in aqueous solution were investigated by static/dynamic surface tension measurements, contact angle techniques, transmission electron microscope (TEM) and particle size distribution (PSD) at 25 °C. Surface tension measurement for all three surfactants are about 30 mN/m. From the results of static surface tension measurements, we could estimate the CAC/C20 ratio, adsorption efficiency (pC20), maximum surface excess concentration (Γmax) and minimum surface area per molecule (Amin) at air/liquid interface. The dynamic surface tension results indicated that adsorption process of aqueous solutions at air/liquid interface is mixed diffusion-kinetic adsorption mechanism. TEM analysis of MAPEG-OSO3Na solutions revealed that surfactant molecules can self-assemble into spherical micelles. PSD and TEM results showed that the application prospect of these surfactants is dispersant in aqueous solution. © 2017 Published by Elsevier B.V.

1. Introduction In recent years, surfactants have been widely applied as wetting agents [1,2], emulsifiers [3,4] and leather finishing agents [5] for the purpose of reducing surface tension. Increasing requirement of energy industry leads most researchers to focus their eyes on the exploitation of polymeric surfactants. Polymeric surfactants are a kind of substance that have characteristic molecular structure consisting of hydrophobic chain together with hydrophilic portion [6]. In comparison to traditional surfactants, they exhibit excellent properties such as dispersion, cohesion, thickening and emulsification. Due to their unusual properties, they are widely used as dispersers [7], flocculants [8,9], rheology modifiers [10]. It is known that PEG has a number of benign characteristic, for example, PEG is miscible with water and is biocompatible that can be utilized in tissue culture media. Also PEG has been found to be stable to acid, base and high temperature systems [15]. Furthermore, MA not only has high reactivity so that it can occur addition reaction, esterification and polymerization reaction, but also there is no byproduct produced in reaction process, which corresponding to the demands of green chemistry [16]. Sulfate polymeric surfactants containing the structure of PEG and MA are widely used in the field of washing industry, textile ⁎ Corresponding author: China Research Institute of Daily Chemical Industry, 34 Wenyuan Street, Taiyuan, Shanxi Province 030001, PR China. E-mail address: [email protected] (P. Li).

http://dx.doi.org/10.1016/j.molliq.2017.01.105 0167-7322/© 2017 Published by Elsevier B.V.

industry, coating technology and other fields. For instance, Rizvi et al. [11] synthesized the polymeric alkenoxy amino acid surfactants with sulfate head group, derived from leucinol, iso-leucinol, and valinol. They found that the sulfate surfactants have lower CMC and are very useful at low pH conditions due to improved solubility in acidic media. The sulfate polymers provide equally good chiral separation of the analytes investigated. Quan and co-workers [12] prepared the polystyrene-supported PEG-bound sulfonic acid, which widely used as catalyst due to the polystyrene linker can increase the mass transfer ratio of product from the reactants. Shahab [13] synthesized a polymerized surfactants with a sulfate headgroup, namely, poly(sodium undecylenic sulfate). The polymerized surfactants can be effectively as pseudostationary phases over a wide range of concentrations, and used for the separation of polycyclic aromatic hydrocarbons (16 PAHs). Zhang [14] obtained two environmentally friendly succinic acid mono-fluoroalkyl sulfonate surfactants and discovered that the Krafft points are below 0 °C and the surfactants have good thermostability. However, these surfactants seldom used in the dispersion of inorganic pigment and the sulfating agents in the literatures mentioned above are chlorosulfonic acid and concentrated sulfuric acid, in addition, the solvent is dichloroethane. In the process of sulfating reaction, waste acid and chlorine-containing compounds can cause some economic and environmental problems. Therefore, a new sulfating method is that the sulfating agent is changed to gas sulphur trioxide (SO3). There are no water and waste acid to produce in the reaction process, and the dosage of the SO3 can close to the theory of consumption, which are helpful for

X. Yang et al. / Journal of Molecular Liquids 231 (2017) 72–79

environmental protection [17,18]. On the basis of these characteristics, we envision a new surfactant, containing two sulfate groups and long chains (\\CH2CH2O\\), that can not only further enhance solubility, but also provide steric effect and electrostatic repulsive force for dispersion stabilization. In this paper, we make full use of advantages of MA and PEG to prepare an environmentally friendly and biocompatible succinate sulfate surfactants (MAPEG-OSO3Na, see Scheme 1) using gas sulphur trioxide as sulfating agent. The physicochemical performances, including surface tension, adsorption and spreading ability, were investigated systemically by various measurements. 2. Materials and methods 2.1. Materials MA was purified by crystallization from chloroform three times. Absolute ethanol, sodium hydroxide, ethyl acetate and PEGs (number average molecular weight M = 1000, 2000, 4000) were purchased by Tianjin Kermel Chemical Reagent Co., Ltd. (China). P-toluenesulphonic acid (PTSA) was supplied by Tianjin Guangfu Research Institute of Fine Chemical Industry. Fuming sulfuric acid (65%) were supplied by Tianjin Shentai Chemical reagent Co., Ltd. (China). All of other reagents were analytical grade and used without further purification except MA. The deionized water with a resistivity of 18.25 MΩ·cm was prepared by a UPD-II ultrapure water purifier.

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(SO3). Finally, the esterified product was neutralized by 30 wt% aqueous sodium hydroxide to pH = 8 gradually. An example of the typical procedure which involves MA and PEG1000 is as follows: MA (22.54 g, 0.23 mol), PEG (493.20 g, 0.49 mol), and PTSA (10.31 g) were mixed and put into round-bottom flask in the presence of nitrogen. The reaction was to continue until the acid value was constant essentially (about 6 h). After esterification reaction, catalyst was neutralized by alkali and filtrated with absolute ethanol. Liquid SO3 was collected by the condensation of SO3 vapor evaporating from fuming sulfuric acid. In the process of sulfate reaction, SO3 vapor was mixed with nitrogen which has a constant flow rate of 0.11 m3/h. Then the mixed gas was passed into a three-neck round-bottom flask containing MAPEG (165.05 g) obtained from first step at 80 °C. The esterified products (MAPEG-OSO3H) were neutralized by aqueous sodium hydroxide to pH = 8, filtered out inorganic salts with absolute ethanol and purified by extracting the unreacted MAPEG with ethyl acetate. The final product (yield is about 60%), presenting a viscous but pourable light yellow liquid at 60 °C and waxy solid at room temperature, was obtained after removing solvent by reduced pressure distillation. 2.3. Characterization

2.2. Synthesis of MAPEG-OSO3Na polymer

2.3.1. Fourier transform infrared analysis spectroscopy (FT-IR) Fourier transform infrared (FT-IR) spectra were recorded using a Bruker Vertex-70 spectrometer at room temperature. A small amount of sample was smeared onto the KBr tablets to detect the structure information of products. The wavelength range of FT-IR spectra was measured from 500 cm−1 to 4000 cm−1.

The disodium sulfates of succinic acid diesters were prepared through three steps reaction in accordance with Scheme 1. MAPEGOSO3Na was synthesized by esterification reaction of PEG and MA. Then sulfate groups were introduced by reacting with sulphur trioxide

2.3.2. Nuclear magnetic resonance (1H NMR) spectrum The 1H-nuclear magnetic resonance (1H NMR) spectra were obtained with a Varian INOVA-400 MHz spectrometer, using deuterochloroform (CDCl3) as the solvent.

Step 1 O CH C 2 H OCH2CH2 mOH

+

O CH C O

catalyst H OCH2CH2 m O C CH CH C O CH2CH2O O

m

H

O

Step 2 H OCH2CH2 mO C CH CH C O CH2CH2O mH + 2 SO3 (g) O O HO3 S OCH2 CH2 mO C CH CH C O CH2CH2O m SO3H O O

Step 3 HO3S OCH2CH2 mO C CH CH C O CH2CH2O m SO3H + NaOH O O NaO3S OCH2CH2 mO C CH CH C O CH2CH2O mSO3Na O O Scheme 1. The synthetic route of MAPEG-OSO3Na.

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2.3.3. The surface activity of MAPEG-OSO3Na The surface tension of aqueous surfactant solution was determined using an interfacial tensiometer with the continuous method at 25 ± 0.1 °C. The surface tensions were measured by adding high concentration solution into 50 mL water under stirring constantly. 2.3.4. Dynamic surface tension The dynamic surface tension was performed by a KRÜSS BP100 bubble pressure tensiometer (Krüss Company, Germany, accuracy ± 0.01 mN·m− 1) at 25 °C. In order to the values of measurement were more accurate, it is necessary to determine the internal diameter of capillary using ultrapure water. The effective surface ages were set ranging from 0.01 to 200 s. 2.3.5. Dynamic contact angle The spreading ability of polymeric surfactants was measured by the contact angle of droplet on hydrophobic substrate using a drop shape analyzer DAS-100 (Krüss Company, Germany, accuracy ± 0.1°) at 25.0 ± 0.1 °C. The paraffin film was chosen as solid substrates. The volume of solution was 5 μL that used for the measurements of dynamic contact angle. In order to ensure minimal error, each experiment was repeated at least three times at an environmental humidity of (50 ± 5)%, and contact angles were the average values of each set of measurements. 2.3.6. Particle size distribution (PSD) To investigate the granularity of BaSO4 suspensions, the particle size distribution was measured using a Mastersizer 2000 (Malvern Company, UK, accuracy: superior to 1%) under the condition of pump speed was 2000 r/min, ultrasonic intensity was 20 W/cm2, and the time of ultrasonic was 2 min. The process of sample preparation is as follows: a certain amount of polymeric surfactants was added into a beaker with 50 mL water. After dissolution completely, the solution was adjusted to pH = 8 with 0.1 mol/L NaOH solution. Then BaSO4 (1.0 g) particles were added and the whole system was emulsified 8 min (10,000 r/min) by high shear emulsifying machine. Subsequently, the suspensions were poured into 100 mL cylinder with plug and quantified to 100 mL with distilled water. Finally the suspensions were shaken well and taken a bit to analyze the particle size distribution. 2.3.7. Transmission electron microscopy (TEM) The dispersion and structures of polymeric surfactants aggregates in aqueous solutions were studied using transmission electron microscopy, JEM-1011 (JEOL Co., Japan), at an acceleration voltage of 100 kV by negative staining. A droplet of sample solution was placed on a coated grid and then staining reagent (2 wt% phosphotungstic acid). In the process of film preparation, the excess liquid was removed carefully by filter paper, and the grid was dried at room temperature.

Fig. 1. FT-IR spectra of MAPEG1000-OSO3Na (a), MAPEG2000-OSO3Na (b) and MAPEG4000-OSO3Na (c).

wiped by ethanol, the residue ethanol solvent may lead to the result [19]. As depicted in Fig. 2(1), the signals at δ = 3.593 ppm (172 H, \\CH2CH2O\\) was the main chemical shift of PEG. Under the current measurement conditions, the ethylene glycol penultimate end groups cannot be distinguished from those of internal ethylene glycol segment, so that the chemical shifts of hydrogen atoms are overlapped. The chemical shift of methylene protons are at δ = 7.087 ppm (2 H, methylidyne). The protons adjacent to the sulfate groups shift to δ = 4.140 ppm (4 H,\\CH2OSO3\\). In Fig. 2(2), the signals at δ = 3.551 ppm (356 H,\\CH2CH2O\\) was the main chemical shift of PEG. The chemical shift of methylene protons are at δ = 7.055 ppm (2 H, methylidyne), which can be seen from its magnification figure. The protons adjacent to the sulfate groups shift to δ = 4.524 ppm (4 H,\\CH2OSO3\\). From Fig. 2(3), we can see the signals at δ = 3.604 ppm (716 H, \\CH2CH2O\\) was the main chemical shift of PEG. The chemical shifts of methylene protons and the protons adjacent to the sulfate groups are so weak that we can hardly see them. Because the chemical shifts of hydrogen atoms of EO units are piled up, the number of hydrogen atoms of characteristic groups can be measured through integral quantities and vice-versa. Thus the integral quantities of methylene protons and the protons adjacent to the sulfate groups are too small to see. The 1H

3. Results and discussion 3.1. Structure characterization Figs. 1 and 2 show the FT-IR and 1H NMR spectra of these surfactants, respectively. From Fig. 1, the presence of adsorption peak at 1639 cm−1 for C _C stretching vibration and at 1724 cm−1 for C _O stretching vibration of ester group indicates the formation of esterified products. The adsorption peak of the stretching vibration peak of the hydroxyl group (OH) at 3425 cm−1 weakens and even disappears basically, while the characteristic adsorption peaks of the sulfate group at 1247 cm− 1 and 844 cm−1 are presenting, indicating the formation of MAPEG-OSO3Na through insertion of SO3 into OH. The presence of weak hydroxyl peak in Fig. 1(a) and (b) is mainly due to the high water absorptivity MAPEG-OSO3Na. In addition, since potassium bromide tablet was

Fig. 2. 1H NMR spectra of MAPEG1000-OSO3Na (1), MAPEG2000-OSO3Na (2) and MAPEG4000-OSO3Na (3).

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NMR spectra of three MAPEG-OSO3Na surfactants give the expected peaks. According to the FT-IR and 1H NMR spectra, it could be deduced that the end product of MAPEG-OSO3Na was obtained successfully. 3.2. Surface activity of MAPEG-OSO3Na solutions The surface tension of surfactant aqueous solutions was measured to evaluate the surface activity at 25 °C. Fig. 3 displays the plots of surface tension (γ) versus log concentration (log c) of three MAPEG-OSO3Na aqueous solutions. It is noteworthy that the γ value initially decreases sharply with increasing concentration, and then reached a plateau, indicating that aggregates had formed, this inflection point corresponding to the critical aggregation concentration (CAC)·The results are in agreement with classic surfactant behavior [20,21]. As depicted in Fig. 3, the CAC and surface tension at the CAC for three surfactants were obtained by determining the intersection point of the steeply downward sloping portion and plateau portion and are listed in Table 1. It can be seen from Fig. 3 that MAPEG-OSO3Na in solution reduces the surface tension at low concentration. The values of γCAC were 35.48, 38.78 and 25.80 mN/m for MAPEG1000-OSO3Na, MAPEG2000OSO3Na and MAPEG4000-OSO3Na, respectively. The γCAC values of MAPEG1000-OSO3Na and MAPEG2000-OSO3Na are essentially the same because there is only a small difference in the molecular weight. While the γCAC value of sulfosuccinate surfactant is around 50 mN/m [22]. Barry and Wilson [23] gave an explanation that the effect of the polar group on CAC have a limit. The CAC values for MAPEG1000OSO3Na, MAPEG2000-OSO3Na and MAPEG4000-OSO3Na were 69.18, 55.12 and 21.20 mmol/L, respectively. The CAC values showed an decrease with increasing space steric effect of oxyethylene groups. Compared with sulfosuccinate surfactant, the smaller CAC and γCAC can be attributed to hydrogen bonds between long chains and electron delocalization of sulfate ions [24]. The surface excess concentration, Γmax (in mol/cm2) and the minimum area per molecule, Amin (in nm2) at the air/water interface, could reflect the arrangement of surfactant molecule at the air/water interface, and were calculated by the Gibbs adsorption equation [25]: Γ max ¼ −


 1 ∂γ 2:303nRT ∂lgC T

ð1Þ

16

Amin ¼

10 NA Γ max

pC20 ¼ − lgC20

Fig. 3. Surface tension for three MAPEG-OSO3Na polymers at 25 °C.

ð2Þ ð3Þ

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Table 1 Physicochemical parameters of surfactant in aqueous solution at 25 °C. Surfactant

γCAC mN/m

CAC mol/L

1010 Γmax mol/cm2

Amin Å2

pC20

CAC/C20

MAPEG1000-OSO3Na MAPEG2000-OSO3Na MAPEG4000-OSO3Na

35.48 38.78 25.80

6.918 × 10−2 5.512 × 10−2 2.120 × 10−2

1.45 1.09 1.20

114.6 153.1 138.0

1.35 1.38 0.03

2.32 1.50 2.43

where n is the Gibbs prefatory (n = 3), R = 8.314 J·mol−1·K−1, T is the absolute temperature, NA is the Avogadro's constant, and ∂γ/∂lgC is the slope below the CAC. The Amin values for as-prepared three surfactants are summarized in Table 1. The Amin does not change substantially within either series of surfactants. The Amin values for these surfactants are around 150 Å2, which is higher than those obtained for sulfosuccinate with similar structure [26]. In general, the size of hydrophilic head is the critical factor in determining surface area occupied at the water-air interface [27]. when a second hydrophilic head is present, Amin increases. The Γmax value decreases as Amin increases [28]. The parameters such as pC20 and CAC/C20 were used to estimate the adsorption and aggregation properties. Here, the parameter C20 represents the surfactant concentration when surface tension was reduced by 20 mN/m. The values of pC20 is refer to the adsorption efficiency at air/liquid interface, and the ratio of CAC/C20 present the effectiveness that can be correlated with structural factors with regarding to the adsorption and micellization process [29]. The larger the pC20 value is, the more efficiently adsorbed at air/liquid interface and the greater the surface tension decreases. As depicted in Table 1, the pC20 for MAPEG2000-OSO3Na is 1.38, which is larger than 1.35 and 0.03 for MAPEG1000-OSO3Na and MAPEG4000-OSO3Na, indicating that adsorption ability at the air-water interface is stronger. Furthermore, the values of CAC/C20 decrease in the order MAPEG4000-OSO3Na N MAPEG1000-OSO3Na N MAPEG2000-OSO3Na. The larger ratio means that the surfactant molecules in aqueous solution are more inclined to adsorption than micellization.

3.3. Dynamic surface tension DST is an important property of surfactant solutions and the adsorption kinetics has been studied on the basis of DST data [30]. Fig. 4 shows the DST values at different concentrations of surfactant solutions. It indicates that the surface tension decrease faster with the surfactant concentration is increased.

Fig. 4. Dynamic surface tension with surface age for MAPEG1000-OSO3Na at different concentrations.

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To investigate the adsorption rate at air/liquid interface, the DST was measured with the maximum bubble pressure method. It is known that surfactant molecules adsorption onto the fresh surface with two steps: firstly, the surface molecules migrate from the bulk solution to subsurface; secondly, the adsorption equilibrium between molecules on the surface and subsurface. The diffusion-controlled adsorption model is used to investigate the diffusion process of surfactant molecules by following Word-Tordai equation [31]: rffiffiffiffi rffiffiffiffiffi pffiffiffiffiffiffiffiffiffi Dt D ptffi ΓðtÞ ¼ 2C0 ∫ Cs d t−τ −2 π0 π

ð4Þ

Where Γ(t) is the surface excess concentration at time t, D is apparent diffusion coefficient, C0 is the bulk concentration, Cs is the concentration at subsurface when t = 0, τ refers to the dummy variable. The former part refers to the molecules migration from bulk phase to subsurface, and the latter denotes that molecules diffusion from subsurface back to bulk phase with the increase of subsurface concentration. Due to the integral of back diffusion is hard to calculate, this equation cannot be solved. A asymptotic method for solving this Word-Tordai equation is put forward [32,33]. γðtÞt→0

rffiffiffiffiffiffi Dt ¼ γ0 −2nRTC0 π

γðtÞt→∞ ¼ γeq þ

nRTΓ2eq C0

rffiffiffiffiffiffiffiffi π 4Dt

ð5Þ

ð6Þ

Where γ(t) and γeq refer to the surface tension at time and at infinite time respectively, Гeq is the equilibrium surface excess concentration. Eqs. (5) and (6) show the linear relation of γ(t) against t1/2(short time adsorption) and t−1/2 (long time adsorption), and are used for calculating the apparent diffusion coefficient from the slope of data plots. The adsorption rate at the air-water interface of each MAPEG-OSO3Na is investigated at a fixed concentration of 1 mmol/L. The variation of the dynamic surface tension versus surface age for MAPEG-OSO3Na is shown in Fig. 5a. Compared with conventional surfactants, the time of arrival in equilibrium surface for MAPEG-OSO3Na increases, indicating the difficulty of adsorption from the interior of solution to the air-water interface. It is known that the increase in the overall size of the surfactant molecules results in the decelerated rate of the adsorption [34]. The values of the diffusion coefficients at short-time (Ds) and long-time (Dl) of three surfactants are listed in Table 2. We can see that both Ds and Dl increase when the hydrophilic chain length is increased. One possible interpretation of this is the more flexible polyoxyethylene chains and the increased repulsion between the polar groups [35]. In

Table 2 Apparent diffusion coefficients of MAPEG-OSO3Na solutions at 1 mmol/L. Surfactants

Ds (m2/s)

Dl (m2/s)

Ds/Dl

MAPEG1000-OSO3Na MAPEG2000-OSO3Na MAPEG4000-OSO3Na

1.32 × 10−14 1.92 × 10−14 2.25 × 10−13

7.50 × 10−14 8.51 × 10−14 7.82 × 10−12

0.18 0.23 0.03

addition, ratio of Ds/Dl is far less than 1 that means the adsorption process is controlled by mixed diffusion-kinetic adsorption mechanism [36]. 3.4. Dynamic contact angle The contact angle (θ) is used to characterize spreading ability of aqueous droplets over hydrophobic surfaces [37–39]. The dynamic spreading behavior of MAPEG1000-OSO3Na aqueous solution with different concentrations was investigated on paraffin film and the results are shown in Fig. 6. From Fig. 6, it is easy to find that the contact angle generally decreases gradually from an initial value (θ0), which is measured immediately as soon as the droplet drops on the paraffin film. The value of contact angle decreases slowly, which takes a period of time to reach equilibrium. Furthermore, it indicated that the dynamic spreading behaviors are influenced by the concentration of surfactants. Increasing concentration decreases the value of θ0 and θ. The dynamic spreading behaviors of MAPEG1000-OSO3Na, MAPEG2000-OSO3Na and MAPEG4000-OSO3Na with same concentration were studied on paraffin film in Fig. 7, in which θ of MAPEG4000OSO3Na reaches a lower value in a short time. This result is in agreement with previous works reporting the spreading behaviors and the dynamic surface tension of surfactants [40]. 3.5. Aggregation behavior of MAPEG-OSO3Na in aqueous solutions To confirm the formation properties of the aggregates from the three surfactants in water, the negative-stained TEM measurements were conducted. TEM is an intuitive method in studying the aggregation behavior of surfactant aqueous solution. Fig. 8 shows the TEM images that surfactant molecules can self-assemble into spherical assemblies with a wide range of diameters from 200 to 1000 nm at a concentration of 100 mmol/L. TEM images revealed that the aggregates are bigger than conventional polymeric micelles usually with a diameter of less than 100 nm. This result conforms to some reports that the formation of larger complex micelles is due to the further aggregation of simple micelles, which is induced by hydrogen bonding or van der Waals interactions among the hydrophilic portion [41,42].

Fig. 5. Dynamic surface tension of three MAPEG-OSO3Na aqueous solutions as a function of “a” the surface age (s), “b” short time (t1/2), “c” long time (t−1/2) at 1.00 mmol/L.

X. Yang et al. / Journal of Molecular Liquids 231 (2017) 72–79

Fig. 6. Variation of dynamic contact angle of MAPEG1000-OSO3Na aqueous solutions with different concentrations on parafilm surface at 25 °C.

Fig. 7. Dynamic contact angle for three MAPEG-OSO3Na aqueous solutions as a function of time at a concentration of 116 mmol/L.

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Fig. 9. Particle size distribution of BaSO4 suspension in the absence (a) and in the presence of MAPEG1000-OSO3Na (b), MAPEG2000-OSO3Na (c) and MAPEG4000-OSO3Na (d) at w = 4%.

suspensions, the particle distribution was wide. Based on statistics data, d(0.1), d(0.5) and d(0.9) were 1.113 μm, 3.380 μm and 9.301 μm, respectively. In comparison, when equal amount of three MAPEGOSO3Na surfactants was added (w = 4%) respectively, the distribution were narrow and the data were as follows: the d(0.1), d(0.5) and d(0.9) of MAPEG1000-OSO3Na were 0.977 μm, 2.489 μm and 5.890 μm, respectively; the d(0.1), d(0.5) and d(0.9) of MAPEG2000OSO3Na were 0.946 μm, 2.125 μm and 4.276 μm, respectively and the d(0.1), d(0.5) and d(0.9) of MAPEG4000-OSO3Na were 1.017 μm, 2.419 μm and 5.378 μm, respectively. The mechanism of dispersion is that electrostatic repulsion and spatial steric hindrance play a role in reducing the particle size. From the structure of MAPEG-OSO3Na, we can know that sulfate groups can adsorb on surface of barium sulfate particles which providing electrostatic repulsion, and long chain can provide spatial steric hindrance. As shown in Fig. 9, MAPEG2000-OSO3Na surfactants have a better dispersion than MAPEG4000-OSO3Na. Previous studies reported that the ratio of hydrophobic portion and hydrophilic portion has great impact on dispersion. If the hydrophilic chain is too long, surfactant is easy to fall off from the paint surface and at the same time the hydrophilic chain prone to tangles [43,44].

3.6. Particle size distribution

3.7. Morphology analysis of BaSO4 suspensions

The dispersive capacity of three MAPEG-OSO3Na surfactants was investigated in water and compared to that in the absence of MAPEGOSO3Na. The data are shown in Fig. 9. Without surfactants in BaSO4

In order to further investigate the dispersion ability of MAPEGOSO3Na, TEM was used to illustrate intuitively their dispersion effect after adding dispersants.

Fig. 8. Negative-stained TEM images of (a) MAPEG1000-OSO3Na, (b) MAPEG2000-OSO3Na and (c) MAPEG4000-OSO3Na.

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Fig. 10. The TEM images of BaSO4 suspension obtained in the absence (a) and in the presence of (b) MAPEG1000-OSO3Na, (c) MAPEG2000-OSO3Na, (d) MAPEG4000-OSO3Na.

The process of sample preparation is similar to the measurement of particle size distribution. From Fig. 10(a), it can be seen clearly that the BaSO4 particles aggregate closely together in the absence of MAPEGOSO3Na, which present random state and uneven size distribution. After adding the polymeric surfactants, aggregation situation improved obviously. And compared with other two surfactants, MAPEG2000OSO3Na has a better dispersion. It is consistent with above mentioned results of particle size distribution by the influence of electrostatic repulsion and space steric effect, which are important factors of stabilization of pigment particles with polymeric dispersants [45,46]. 4. Conclusion Three novel polymeric surfactants, MAPEG-OSO3Na, have been synthesized using three steps and characterized by FT-IR and 1H NMR. Not only physic-chemical performances but also dispersion properties were investigated. It was found that the surfactants, MAPEG-OSO3Na, exhibit excellent surface activities with a greater efficiency in decreasing the surface tension of water. The adsorption models for these surfactants are confirmed to be diffusion-kinetic adsorption model. TEM images show that the polymeric surfactant can self-assemble into spherical micelles at a concentration above the CAC. The narrow particle size distribution and TEM images of BaSO4 suspensions indicate that dispersion of MAPEG2000OSO3Na surfactants is more better than other two surfactants. Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgment We express our gratitude to Shanxi Province Science Foundation for Youths (no. 2015021052) for their support of this research. We also thank Peng Song and Guojin Li of China Research Institute of Daily

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