Green Glucamine-Based Trisiloxane Surfactant: Surface Activity

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A Green Glucamine-based Trisiloxane Surfactant: Surface Activity, Aggregate Behavior, and Superspreading on Hydrophobic Surfaces Jinxing Li, Yanyun Bai, Wanxu Wang, Xiumei Tai, and Guoyong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06282 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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ACS Sustainable Chemistry & Engineering

A Green Glucamine-Based Trisiloxane Surfactant: Surface Activity, Aggregate Behavior, and Superspreading on Hydrophobic Surfaces

Jinxing Li, Yanyun Bai, Wanxu Wang, Xiumei Tai, Guoyong Wang* China Research Institute of Daily Chemical Industry, Taiyuan 030001, P. R. China

*Corresponding Author Guoyong Wang China Research Institute of Daily Chemical Industry, 34 Wenyuan Street, Taiyuan, Shanxi Province, 030001 P.R. China E-mail:

[email protected]

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: A novel green glucamine-based trisiloxane surfactant (HAG) composed of a glucamine headgroup and trisiloxane tail was prepared by a completely “green” synthesis. Its chemical constitution, functional groups, and surroundings were characterized by Fourier-transform infrared spectroscopy, mass spectrometry, and 1H and 13C-NMR spectroscopy. The physicochemical properties of the target, including its surface activity, aggregation, and wetting behavior were evaluated systematically. The findings reveal that HAG solution has a comparatively low surface tension (γ = 19.04 mN/m) and can spread rapidly on a parafilm surface. Furthermore, it can spontaneously assemble into vesicles.

Keyword: siloxane surfactant, sugar surfactant, surface tension, spreading, low surface tension

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INTRODUCTION Surfactants play an important role in modern industry and house-hold products1. With the development of society, more attention is being paid to ecology and efficiency of a surfactant. Therefore, ecological effects must be taken into account in the exploitation of new surfactant-based products. Previous studies have shown that siloxane surfactants have highly efficient properties2-3. During the past decades, modified substances of siloxane, such as polyethers4, acetylene diols5-6, fluoroalkyl and polyether co-modified derivatives7, and polyglycerols8, have been studied extensively and their natures and functions have been explored. Besides, a number of groups have reported the effects of configuration9-10 and composition11 on the properties of siloxane surfactants12-13. Although the aforementioned surfactants show some favorable performances in certain domains, they do not align with the strategy of eco-sustainable development. In particular, siloxane surfactants modified by polyether have been studied extensively14. However, their use in some personal care products is restricted because of presence of the carcinogenic 1, 4-dioxane moiety in the raw materials. Furthermore, almost all polyethers are derived from fossil materials, which are sourced unsustainably. Hence, it is urgent to develop novel siloxane surfactants that can be synthesized in a cost-effective and environmentally friendly manner. The revolution with regards to alkyl polyglucosides (APG)15-16 provides a fresh prospect for the sustainable development of siloxane surfactants. Newly developed techniques for manufacturing siloxane surfactants using environmentally friendly glucoses17 could displace unverified substances. Some previous studies have quantified the environmental

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impacts of using APGs18-19. Because of the introduction of siloxane, the ability of the surfactant to reduce the surface tension is greatly enhanced. In fact, siloxane surfactants, even the ones modified by saccharide groups, have been known for a long time. Further, numerous synthetic routes have been reported10 to produce the corresponding siloxane products. The pairing between carbohydrates and siloxane moiety was accomplished via hydrosilylation between an allyl-modified carbohydrate and a siloxane moiety furnishing the Si-H bond and with amino-modified siloxane moiety and lactones (or acids) for forging amide bonds. To overcome issues related to incompatible conditions of the coupling reaction and to avoid side reactions, it is necessary to perform the time and material-consuming steps of protection and deprotection of the hydroxyl groups of the carbohydrate. A new brief way was exploited by our team20-21 and others22-23 to shorten the cycle of production: carbohydratemodified siloxane was reacted with amino-modified glucoside. However, these reactions require a lot of solvent. In an effort to develop highly effective siloxane surfactants, a trisiloxane surfactant based on N-methyl-D-glucamine (HAG) was designed. Glucamine, a safer and environmentally friendly hydrophilic group, is obtained from glucose. The synthetic processes of HAG are economic and no waste is generated during the reactions. Further, its surface activity, aggregate behavior, and superspreading on a hydrophobic surface were investigated. As a glucamine-based siloxane surfactant, it has a relatively high surface activity, and it is apt to biodegrade and is therefore environmentally friendly.

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EXPERIMENTAL SECTIONS AND METHODOLOGIES Materials. 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) was obtained from Alfa

Aesar. Allyl glycidyl ether and N-methyl-D-glucamine were purchased from Aladdin. The catalyst, chloroplatinic acid heptahydrate was supplied by Nanjing Chemical reagent Co., Ltd. Ethanol was furnished by Tianjin Kermel Chemical Reagent Co., Ltd. Bromophenol blue was obtained from Beijing Chemical Reagent Co., Ltd. Sephadex G-25 medium was purchased from Beijing Solarbio Science & Technology Co., Ltd. Deionized water was used in all the procedures. Synthesis of HAG. Allyl glycidyl ether (11.41 g, 0.1 mol) including chloroplatinic

acid heptahydrate (20 ppm) was first heated in a three-necked flask at 60 °C to activate allyl glycidyl ether. Then, HMTS (22.25 g, 0.1 mol) was added dropwise to the reaction mixture with vigorous vibration. After 30 min, the solution was heated to 110 °C for 5 h. The intermediate was distilled from the crude mixture under high vacuum. Immediately, the obtained product was heated with N-methyl-D-glucamine (19.52 g, 0.1 mol) in absolute ethanol at 30 °C until the solid dissolved absolutely. The twice process would be operated at 80 °C. After 6 h, the solution was cooled to the room temperature and the mixture was filtered to remove the residual substances. The solvent was evaporated using a rotary evaporator and the product was stored in a vacuum drying oven for 24 h to obtain a viscous product. The yield exceeded 96%. Characterization. The structure of product was characterized by Fourier transform

infrared spectroscopy (FT-IR; Bruker vertex-70 spectrometer) and NMR spectrometry (Varian INOVA-400 MHz spectrometer). For recording FT-IR spectrum, a drop of the

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product was ground with KBr and pressed to obtain a KBr pellet. For 1H-NMR analyses, the compound was dissolved in DMSO. Ultraflex MALDI-TOF/TOF mass spectrometry (TOF-MS) supplied by Bruker was performed to characterize the m/z of product. Equilibrium surface tension and critical micellar concentration (CMC). The Krüss

K12 (Krüss Company, Germany) processor Tensiometer was used to test the equilibrium surface tension by the DuNoüy method at room temperature (25 °C). To avoid the hydrolysis of the surfactant, each HAG aqueous solution was equilibrated only for 3 h. Hitachi F-4600 fluorescence spectrophotometer was employed to determine the CMC using a pyrene probe. Dynamic surface tension. The apparatus for measuring the dynamic surface tension

was equipped with a Krüss BP100 bubble-pressure tensionmeter (Krüss Company, Germany, accuracy ± 0.01 mN/M). The data were obtained by recording the maximum pressure required to blow a bubble in the solution from the top of a capillary tube at room temperature (25 °C). The period of measurement was managed congruously from 10 to 200000 ms. Contact angle. The Krüss DSA 25 instrument (Krüss Company, Germany) was

used to study the wettability. Parafilm was used as the support to test the wetting ability of the aqueous solution of HAG at 25 °C and environmental humidity. Diameter of the aggregate. In order to determine the diameter distribution of the

aggregate, dynamic light scattering (DLS) was conducted to measure the effective diameter of the aggregates in the aqueous solution of the surfactant using a Zeta Plus

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particle analyzer (Brookhaven, USA). Transmission electron microscopy (TEM). The aggregate structure was observed via

transmission electron microscopy (TEM) using a JEM-1011 transmission electron microscope (Joel Company, Japan) at an acceleration voltage of 100 KV. The specimen was cast on a carbon-coated copper grid and stained negatively with 1.5 wt.% phosphotungstic acid and allowed to stand for 48 h at 25 °C before observation. X-ray Diffraction (XRD). As a powerful method, XRD was used to elucidate the

long-range structure of the ordered aggregates, and the process for preparing the XRD sample is as follows: A drop of the aqueous solution of the surfactant was cast on the surface of a preprocessed glass substrate and dried naturally at room temperature. The specimen was mounted in a vacuum chamber for 15 min before the test. An X-ray diffractometer (Rigaku model D/MAX 2500) equipped with a Cu anode operated at 40.0 kV and 30 mA was used to determine the thickness of the vesicle. The wavelength of the X-ray beam generated by the Cu anode was 1.5406 Å, and the scanning angle (θ) was changed from 0.5° to 8°, with a step width of 0.01°. Bromophenol blue encapsulation experiment. In an effort to demonstrate the hollow

structure of the aggregate formed by the surfactant, dye encapsulation experiment was carried out. A 1.5 wt.% HAG aqueous solution containing 5 × 10-4 mol/L bromophenol blue was prepared and incubated for 4 h at room temperature. Then, a quantitative volume (0.5 mL) of the HAG solution was added to a chromatography column packed with Sephadex G-25. By detecting the absorbance of the dye in the eluent at 595 nm using a UV/VIS spectrophotometer, the concentration of bromophenol blue was

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determined. Primary biodegradation experiment. Primary biodegradation experiment was

performed by referring to the China National Standard GB/T 15818-2006. A HAG solution was prepared at 1.0 g/L. The 5.0 mL domestication strains fluid was interfused into a bottle which contains a constant volume (500 mL) of the culture medium solution and a quantitative amount of the HAG solution. One week later, the surfactant content of each bottle was detected separately in a specific way offered by the criterion.

RESULTS AND DISCUSSION Synthesis of a Glucamine-based Trisiloxane Surfactant. In comparison with the

universal pathways of synthetic routes, there are three advantages in the preparation steps of the glucamine-based trisiloxane surfactant. First, both the steps are economic, which implies that no side products are generated during the reaction. An excellent aspect of this route is that none of the processes generate waste and pollution. The siloxane surfactant, HAG was synthesized via a two-step reaction shown in Fig. 1. Allyl glycidyl ether was first grafted onto HMTS by hydrosilylation. Subsequently, ringopening reaction was carried out to connect the previous adduct and N-methyl-Dglucamine. The pure product obtained after distillation was a colorless and transparent solid. Thus, we implemented a class of technological parameters of synthesis to develop a promising route for HAG. Therefore, this production process is suitable to be applied on an industrial scale. The synthetic route of HAG embodies low-waste and virtually emission-free technologies.

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Me Me

Me Si

Me O

Si H

Me O

Si

Me Me

O

+

H2Pt2Cl4

O

Me Me

Me Si

Me O

Si

Me O

Si

Me Me

O

O

Me Me Me

Me

Me

Me

Si

Si

Si

O

O

Me Me

+

Me

H N

OH

Me

OH OH

OH

Me

Me

Me

Si

Si

Si

O

O

Me Me

ethanol O

OH

O

OH N Me

O

OH HO OH HO HO

Fig. 1 Synthetic route of HAG

The chemical structure of the target molecule and the functional groups were characterized by FT-IR and NMR spectroscopy and the molecule weight was verified by TOF-MS. The FT-IR spectra of the main reactants are shown in the Fig. 2. The spectra of the main raw materials are labeled as A, B, C, and D. FT-IR spectroscopy was used to monitor the synthetic process. The four FT-IR spectra revealed the progression of the synthesis of the product. Spectrum A shows a sharp strong peak at 2150 cm-1, corresponding to the stretching vibration of the Si-H bond. Spectrum B shows the C=C and epoxy functional groups at 1647 and 3060 cm-1, respectively. The characteristic absorption bands for Si-H and C=C gradually disappeared in spectrum C, when the hydrosilylation reaction occurred. Subsequently, the open-ring reaction was carried out and the epoxy functional group was converted into –OH, which resulted in a peak at 9



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3369 cm-1 in spectrum D. A Si-H

B C=C

O

C O

D

-OH

4000

3500

3000

2500

2000

(cm-1)

1500

1000

500

Fig. 2 FT-IR spectra of the main substances: A) HMTS; B) allyl glycidyl ether; C) AGE-HMTS; D) HAG

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Fig. S1 shows the 1H-NMR and 13C-NMR spectra of the intermediate and product. In the line A, the shifts of epoxy ring was shown at g (δ:3.1) and h (δ:2.8, 2.6). When N-methyl-D-glucamine was connected to the epoxy ring, a new signal of the epoxy ring disappeared and the –OH signal appeared at 4.6 in the upfield. 13C shifts are shown in spectra C and D. All homologous chemical shifts are located at the corresponding positions. The mass spectrum shows a strong peak for the principal ion at m/z 532.3, which corresponds to the mass of the final product (calcd. for HAG ： m/z 531.3(100%), 532.3(36%), 533.3(16%)). In summary, we successfully obtained some key chemical information for the structure of HAG. Based on the above analyses, this compound is the expected target. Surface activity. The equilibrium surface tension measurement directly revealed the

ability of the HAG to minimize the surface energy of the air-water interface. Previously, researchers24-26 have reported a series of different methodologies for determining the CMC of homologous surfactants. Fig. 3 exhibits the variation in the equilibrium surface tension with the concentration of the surfactant solution. The surface tension (γ) decreased with increasing concentration of HAG in the aqueous solution. This tendency implies that the quantity in unit volume of the surfactant molecules at the air-water interface increased continually with increasing concentration of the surfactant in the bulk. When the concentration was increased beyond the CMC of the sample, the surface tension in a plateau stage also increased with the concentration, which indicates that the

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surfactant adsorption at the air-water interface was saturated. The minimum value of surface tension (γmin) was found to be 19.04 mN/m with the CMC of HAG being 0.25 mmol/L; thus, HAG possesses a more favorable surface activity than that of APG (higher than 27 mN/m) reported previously27-28. This result is because the branched trisiloxane portion lies flat on the water surface, exposing the highly surface active methyl groups to air. The difference between the equilibrium surface tension values of HAG and APG suggest that HAG potentially has some application prospect than APG. 70

A 60

50

(mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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B

40

30

20

10 0.01

0.1

1

c(mmol/L) Fig. 3 Equilibrium surface tension of HAG solution

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Unlike the previously reported siloxane surfactants, HAG merely shows one inflection point because of its low molecular weight. The measurement of the equilibrium surface tension provided a way to calculate a series of parameters with respect to the sample, including the saturation adsorption (max), minimum area per molecule (

), and the standard free energy of aggregation and adsorption (

and

). A and B symbolize the aggregation behavior of the surfactant at different concentrations. The surfactant molecules are enriched at the surface of the solution at a comparatively low concentration, and monomers begin to form specific ordered aggregates following the increase of the surfactant concentration in the bulk phase. The corresponding equations are listed below. 1

𝛤𝑚𝑎𝑥 = ― 2.303𝑅𝑇

(∂log∂𝛾 𝑐)

1016 𝐴𝛤𝑚𝑎𝑥

𝐴𝑠𝑚 = 𝑁

(1) (2)

(𝐶𝑀𝐶 55.5 )

(3)

( ) ―6.022∏𝐴𝑠𝑚 55.5

(4)

∏𝐶𝑀𝐶 = 𝛾0 ― 𝛾𝐶𝑀𝐶

(5)

∆𝐺0𝑚𝑖𝑐 = 𝑅𝑇ln ∆G0𝑎𝑑𝑠 = RTln

𝐶𝛱

where, R is the gas constant (R = 8.314 J/(mol·K) ), T is the absolute temperature (K), (

) is the slope of the surface tension ( ) versus log (concentration) (logc) plot

and NA is the Avogadro constant (NA = 6.02 × 1023). The saturation adsorption (max) is 5.65 × 10-10 mol/cm2 and minimum area occupied by a single molecule (

) is 29.38 Å2. The saturation adsorption value reveals

the maximum concentration of the surfactant at the air-water interface. With accurate calculation, we found that the standard Gibbs free energy of micellization and

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adsorption are both negative (∆G0𝑚𝑖𝑐 = −30.51 kJ/mol, ∆𝐺0𝑎𝑑𝑠 = −39.88 kJ/mol), which imply that both the micellization and adsorption of the surfactant are spontaneous. Apart from the DuNoüy ring method, the fluorescence probe technique is a popular way to determine the CMC since it was first reported by Kalyanasundaram and Thomas29. Pyrene was used as a fluorescence probe in a series of HAG aqueous solutions with different concentrations to determine the CMC of HAG. A known amount of pyrene was added into each vial with a different concentration of the surfactant to obtain a final concentration of 5.0 × 10-3 mmol/L. Fig. S2 shows the result of the fluorophotometric determination of CMC. The CMC determined by the fluorescence method (0.36 mmol/L) slightly exceeded the value determined by the DuNoüy method. Adsorption and Diffusion Kinetics. The adsorption and diffusion kinetics at air-

water interface was verified by studying the dynamic surface tension30. We adopted the bubble pressure method to determine the relationship between the amount of surfactant molecules and the surface tension at the gas-liquid interface at different times. The results of a series of solutions with different HAG concentrations are exhibited in Fig. 4. When the concentration is lower than the CMC (0.25 mmol/L), the diffusion process is discussed using a diffusion-controlled model, and the Word-Tordai equation31 was introduced in. Each curve in the figure is very smooth, even when the concentration is very low. This result indicates that the membrane formed by HAG at the air-water interface is relatively stable than those of others. (𝑡) = 2𝑐0

𝐷𝑒𝑓𝑓𝑡 𝜋

―2


𝑡

∫0 𝑐𝑠𝑑( 𝑡 ― 𝜏)

14


(6)

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where,

is the excess surfactant per unit volume at the interface at time t,

represents the bulk concentration, Deff stands for the effective apparent diffusion is a dummy variable.

coefficient, Cs is the subsurface concentration, and 80

70

60

(mN/m（

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

40 Water 0.09 mmol/L 0.17 mmol/L 0.87 mmol/L 1.73 mmol/L 3.76 mmol/L 7.52 mmol/L

30

20 0.01

0.1

1

10

100

Surface Age（ s（

1000

Fig. 4 Dynamic surface tension results for HAG solutions at different concentrations at room temperature

In order to intuitively reflect the magnitude of the diffusion rate, a new method was introduced in this work. γ(t)𝑡→0 = 𝛾0 ― 2𝑛𝑅𝑇𝑐0 2

γ𝑡→∞ = 𝛾𝑒𝑞 + nRT𝑒𝑞 𝑐


𝜋 4𝐷𝑡

(7) (8)

where, γ(t) represents the surface tension at the time t and Γ eq stands for the equilibrium surface with excess concentration. γ(t)t→0 and γ(t)t→∞ represent the shorttime and long-time adsorption mechanisms, respectively. According to these equations, the slope of the curve can be used to calculate the apparent diffusion coefficient. The short-term adsorption (Ds) and long-term adsorption32 (Dl) are shown in Fig. S3 and S4, respectively. We obtained the corresponding diffusion coefficient by 15



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calculating the slope of each curve. This processing mode distinctly dissected the relationship between the concentration and diffusion rate at different modes. The relevant values are listed in Table 1. The value of Ds/Dl indicates a phenomenon that short-term absorption of HAG dominated the entire diffusion course, and it is exacerbated in the lower concentration regime. Table 1. Diffusion coefficients of HAG aqueous solutions at room temperature Concentration （mmol/L）

Ds（m2/s）

Dl (m2/s)

Ds/Dl

0.09 0.17 0.87 1.73 3.76 7.52

3.47E-9 1.40E-9 8.25E-11 3.49E-11 2.07E-11 7.94E-12

3.33E-11 2.96E-11 6.53E-12 3.66E-12 2.14E-12 8.07E-13

104.19 47.34 12.64 9.53 9.68 9.84

Aggregation behavior. When the concentration of the sample was higher than the

CMC, and when it was continually increased to several times the CMC, the aqueous solution of the sample became blue and turbid, which indicates that the aggregates formed by the surfactant might be vesicles. Therefore, the formation and diameter of the aggregates were analyzed by TEM and DLS, respectively. The distribution of the aggregate size is shown in Fig. 5.

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A

93

0.3wt%

62 31

Intensity (%)

0

0.5wt%

93 62 31 0

2.0wt%

93 62 31 0 0

200

400

600

800

1000

1200

1400

d(nm)

B

99

0.5wt%

66 33 0

Intensity(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


99

1d after

66 33 0 99

2d after

66 33 0 0

200

400

d(nm)

600

800

1000

Fig. 5 Size distribution of aggregates in aqueous solutions A)size distribution with different concentration, B)size distribution with time Illustration shows solutions in natural (left) and polarized (right) conditions.

As an auxiliary method, DLS plays a significant role in confirming the existence of nanoscale aggregates. However, when the concentration is high enough, the results provided by DLS are the sum of the particle sizes of several influxes of aggregates. Therefore, the intensity size distribution given by DLS often overestimates larger aggregates33. As shown in Fig. 5A, the size distribution of aggregates is mainly located in the range of 100–1500 nm. We exploited the relationship between the size 17



distribution and the surfactant concentration in a specific concentration range. With a decrease in the concentration, the second peak shifted to the left side, and the two peaks would merge at the first position. As shown in Fig. 5B, we also found that the size of the aggregate does not change with time over two days. The peaks located at 100 nm and 1500 nm represent single vesicles and aggregated vesicles, respectively. The formation of vesicles was observed by TEM (Fig. 6). In the TEM micrographs, typically spherical vesicles with 80 and 200 nm diameters are observed. The white region in the micrograph is the wall of the vesicle. The scale of the electron micrographs indicates that the results obtained by DLS cannot completely account for the particle size distribution.

Fig. 6 TEM micrographs of a 1.5 wt.% HAG aqueous solution at different magnifications

The result from dye encapsulation experiment is a positive evidence to verify the existence of vesicles34. Based on previous works35-36, we used bromophenol blue as the 18


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cargo to accomplish the task. The dye-encapsulated HAG was separated from the dyefree ones by size selection using a gel column. We detected the absorbance of in the eluted bromophenol blue at 595 nm. The result shows that the absorption spectrum contains two peaks. The encapsulation efficiency (EF) was calculated by the following formula: EF = 𝐶𝑑𝑦𝑒 𝐶𝑡, 𝑑𝑦𝑒 where, Cdye and Ct,

dye

(9)

represent the encapsulated dye and total dye concentration,

respectively. EF for HAG was found to be 1.80%. In order to confirm the structure of the aggregate, molecular packing parameter (P) of the surfactant developed by Israelachvili37 was adopted to evaluate the formation of vesicles. The method of packing parameter has been used as an intuitive tool for determining the relationship between the surfactant molecules and aggregates over the past 20 years. According to the laws, we can forecast the type of aggregates assembled by surfactants in a specific continuous phase by calculating the value of P38: (i) 0＜ P ≤ 1/3 represents spherical micelles, (ii) 1/3 ＜

P ≤ 1/2 stands for cylindrical

aggregates, (iii) 1/2＜ P ≤1 represents a bilayer. 𝑉0

P = 𝑎𝑙0

(10)

where, V0 and l0 denote the volume of the surfactant and length of the hydrophobic tail of the surfactant, respectively. a is the area occupied by the surfactant headgroup at the interface of the hydrophobic core and hydrophilic media39-40. However, the evaluation of P becomes complicated because of the differences between the siloxane moiety and straight-chain alkane. In this study, the accurate molecular volume and hydrophobic

19



length data could not be obtained. However, the branched siloxane tails are more bulky and hydrophobic than a hydrocarbon tail, thus rendering the vesicle formation favorable24, 41-42. We have verified the hypothesis for vesicles in a previous work mentioned above. XRD43, as an efficient tool, was used to determine the thickness of the wall, using the Bragg’s equation: (11)

nλ = 2dsin 𝜃

where, λ is the wavelength of the incident light (λ = 1.5406 Å), d is the distance between the scattering planes, and θ is the angle of incidence to the scattering plane. 1000

800

600

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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d

400

𝑑= 200

𝜆 2 sin 𝜃

0 0

1

2

3

4

2()

5

6

7

8

Fig. 7 Small angle X-ray diffraction measurement of a film with vesicle

From Fig. 7, we obtained the 2θ of the strong sharp peak and calculated the thickness of the wall of the vesicle using to the Bragg’s formula. The d value (33.9 Å) is larger than the sum of the size of two hydrophobic moieties (14.4 Å), which indicated that arrangement mode of the vesicle walls is the hydrated bilayer42. Wettability on a hydrophobic surface. The wettability on low energy surfaces has

always been a subject of great concern, whether it concerns commercial products or 20


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products used in daily life such as in pesticide spraying44, printing, painting, and cosmetics45. Wettability experiment usually uses the contact angle as the primary data, which reveals a degree of wetting for interaction between a solid and liquid. The most extensively used technique of contact angle measurement was reported by Bigelow et al.46 in 1946, which is a direct measurement of the tangent angle at the three-phase contact point on a sessile drop profile. As a significant criterion for interpreting the wetting ability, dynamic contact angle can intuitively reflect the wettability of surfactants. We determined the angle between the gas-liquid interface and liquid-solid interface via a height method on a parafilm surface. It is well known that the smaller the angle is, the better the wettability is. 120

100

80

Contact Theta()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 0

20

40

Age（ s（

60

80

100

Fig. 8 Contact angles on parafilm for water and aqueous HAG solutions with different concentrations (Illustration of contact angle with different concentration) ■ Water, ● 0.05 g/L (0.09 mmol/L), ◄0.09 g/L (0.17 mmol/L), ▲0.15 g/L (0.28 mmol/L),▼0.26 g/L (0.49 mmol/L), ►0.46 g/L (0.87 mmol/L), ◆0.92 g/L (1.73 mmol/L), ★2.00 g/L (3.76 mmol/L)

As shown in Fig. 8, pure water did not spread on parafilm (illustration A). When the concentration of the surfactant in the aqueous solution was fairly low, the solution could not wet the surface of the parafilm. When the concentration was increased to 0.87

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mmol/L, the solution could completely spread on the parafilm. Further, the period of the wetting process was shortened with continually increasing surfactant concentration. The illustration exhibits the circumstances when the solutions with different concentrations spread on parafilm. When the concentration of 3.76 mmol/L, the liquid drop spread on parafilm instantly, because the dynamic surface tension of HAG at this concentration is lower than the surface energy of the parafilm surface (26 mN/m). Therefore, HAG could completely wet the parafilm surface instantly. In contrast, this effect has never been demonstrated for any kind of carbohydrate surfactant. Considering the surface tension results of HAG in Fig. 3, the HAG solution cannot spread on parafilm at the CMC. Ideally, the shape of a liquid droplet on a hydrophobic surface is determined by the surface tension of the liquid. In practice, we found that the corresponding value of the dynamic surface tension to be higher than 26 mN/m, which implies that the surfactant concentration at the interface is possibly not saturated. Although HAG could lower the equilibrium surface tension to less than 20 mN/m at the CMC, this solution could not spread on parafilm instantanously. Obviously, the wettability of HAG is not only affected by the equilibrium surface tension, but also by the dynamic surface tension. Therefore, the precondition of spreading on parafilm is that the surfactant molecules can reduce the surface tension to the interface of the binary phase immediately.

CONCLUSION

This article reported a promising new more environment-friendly siloxane surfactant

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(HAG) with its aqueous solution possessing more favorable wettability and lower surface tension. The study of surface activities also indicated that HAG has excellent surface properties (γmin = 19.04 mN/m, and CMC = 0.25 mmol/L), and the wetting experiment results showed that the HAG aqueous solution can completely spread on parafilm when the concentration reaches 0.87 mmol/L, which agrees with the result of the equilibrium surface tension. Further, we demonstrated the relationship between the surface tension and time using the diffusion-adsorption theory. The dynamic surface tension studies were carried out to investigate the kinetics of adsorption at the air-water interface. The results of surface properties has verified that HAG pocess an excellent efficiency in surface tension reduction. By calculating the packing parameter of HAG, the hypothesis of the formation of vesicles was verified. The formation of vesicles from HAG was also verified by TEM and dye encapsulation experiment. Furthermore, as an eco-friendly siloxane surfactant, the vesicle formed by HAG can be utilized in for drug encapsulation and delivery and as a microreactor. Subsequently, we demonstrated the good wettability of HAG solutions on a low energy surface (Supporting information Fig. S5). Above all, HAG could be applied in agricultural adjuvants, home care products, or coatings. Ultimately, primary biodegradation experiments indicate that more than 99% HAG can be degraded in a week.

ACKNOWLEDGMENTS

We are grateful for the financial support by the National Key R & D Plan (Grant NO. 2017YFB0308704) and we also thank the sponsorship from the JALA Research Found

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Projects (JALA 2016). This work was also supported by the National Natural Science Found of China (21872088). ASSOCIATED COTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website CONTENTS Fig. S1 1H-NMR (A&B) and 13C-NMR (C&D) spectra of the intermediate and product Fig. S2 I1/I3 ratio for pyrene as a function of the concentration of HAG Fig. S3 Short-term adsorption of the HAG aqueous solutions Fig. S4 Long-term adsorption of the HAG aqueous solutions Fig. S5 Spreading effect on low energy leaf at different moment AUTHOR INFORMATION Corresponding Authors

*E-mails: [email protected] Notes

The authors declare no conflicts of interest. REFERENCES 1.

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Graphical Abstract:

An economic synthesis routine was adopted to prepare the green glucamine-based trisiloxane surfactant. Measurement results revealed that the trisiloxane surfactant can serve as a superspreading agent on low energy surface.

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Green Glucamine-Based Trisiloxane Surfactant: Surface Activity

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