Aggregation Behavior of Polyether Based Siloxane Surfactants in

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Aggregation Behavior of Polyether Based Siloxane Surfactants in Aqueous Solutions: Effect of Alkyl Groups and Steric Hindrance Jinglin Tan, Xiaomei Xiong, Ziyan He, Fei Cao, and Desi Sun J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b10727 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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The Journal of Physical Chemistry

Aggregation Behavior of Polyether Based Siloxane Surfactants in Aqueous Solutions: Effect of Alkyl Groups and Steric Hindrance Jinglin Tan,* Xiaomei Xiong, Ziyan He, Fei Cao, and Desi Sun

School of Chemical and Environmental Engineering, Jiujiang University, Jiujiang Jiangxi 332005, China; Jiangxi Province Engineering Research Center of Ecological Chemical Industry .

*To whom the correspondence should be addressed. E-mail: Jinglin Tan, [email protected] Fax: +86-792-8314448 Tel: +86-792-8314448

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ABSTRACT: A series of polyether based siloxane surfactant with different branched chain and alkyl groups were synthesized by thiol-ene reaction and Piers-Rubinsztajn reaction. The effect of the siloxane structures (alkyl groups and branched chains) on the adsorption and aggregation behavior in aqueous solution was investigated by surface tension, fluorescence, dynamic light scattering (DLS), freeze-fracture transmission electron microscopy (TEM) and TEM. The molecular structures of siloxane can obviously influence their surface activities and thermodynamics. Replacing the methyl of trimethylsiloxyl groups with longer alkyl groups (ethyl, propyl, and butyl) and branching trimethylsiloxyl resulted in an obviously decreasing the values of critical micelle concentrations (CMC) and surface tension at CMC (γCMC). Dense surface films packed with CH3 groups result in the lower surface tensions can be disordered by longer alkyl groups or branched chain of siloxane hydrophobic groups. And the minimum surface area per surfactant molecule (Amin) values of Si3-PG, Et-Si3-PG, Pro- Si3-PG, But-Si3-PG successively decrease about 3.5 Å as increasing each -CH2- group. All polyether based siloxane surfactants can form non-uniform size of spheroidal aggregates in aqueous solution. Concerning the driving force, micellization process were spontaneous, however, less spontaneous compared with adsorption.


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INTRODUCTION In aqueous solution, surfactants as amphiphilic molecules can self-assemble into various kinds of aggregates, such as micelles, vesicles, bilayers, liquid crystal, and so on, which were influenced by the molecular structures of surfactants as well as some physicochemical parameters (temperature, concentration, etc.). 1-8 Siloxane surfactants, one kind of the surfactants, have attracted considerable interest during the past two decades owing to their outstanding properties. Typically, trisiloxane surfactants, considered as the types of MDM, where M is trimethylsiloxy group, D is -Si(CH3)R- group. R is the hydrophilic groups coupled to the Si-O-Si hydrophobic group with -(CH2)3- spacer, have received extensive attraction both at the academic and industrial fields. 9-15 Compared with hydrocarbon surfactant, trisiloxane surfactants had considerably higher surface activity attributed to the trimethylsiloxy group on Si-O-Si chain. Gentle et al.16 prepared a series of polyether based trisiloxane with different ethylene oxide (EO) chain length, and illustrated that the critical micelle concentration (CMC) and surface tension at CMC (γCMC) increases with increasing the EO chain length owing to the increasing EO chain length can increasingly penetrate and disrupt the closely packed surface monolayer, especially, polyether based trisiloxane with above 16 EO chain length. Moreover, the surface activities of poly(ethylene glycol) grafted polysiloxane were sensitive to the weight fraction of EO, the length of the EO chain, the size of the hydrophobic siloxane. And their self-organized structures are highly related to the surfactant concentration, the ratio of EO chain and Si(CH3)2 unit. 13-19 Meanwhile, the aggregation behavior of siloxane surfactants in aqueous solutions has been extensively attracted attention. Feng et al.20-21 illustrated that the molecular structures of cationic siloxane surfactants play a crucial role in their aggregation



behavior. The values of degree of counterion binding for (2-hydroxyethyl)- N, Ndimethyl-3-[tri(trimethylsiloxy)]silylpropylammonium chloride and 1-methyl-1-[tri(trimethylsiloxy)]silylpropylpyrrolidinium chloride increased with increasing the temperature, which were caused by the attractive interaction between the nitrogen atom of one surfactant molecule with the oxygen atom of another surfactant molecule. Du et al.22-23 demonstrated butynediol-ethoxylate based siloxane surfactants with shorter siloxane backbone show excellent surface activity, spreading and wetting ability and can form spherical aggregates with the diameters from 50 to 1000 nm, moreover, cationic trisiloxane surfactants can form vesicular aggregates with diameters from 20nm to 200nm. Eastoe et al.24 synthesized novel hydrocarbon surfactants with trimethylsilyl as end group, which can self-assemble into ellipsoidal micelles and have a lower surface tension (22.8 mN m-1). Zelisko et al.25 reported siloxane based phosphocholines can form bilayer vesicles and the area per lipid and lipid volume values were slightly larger than relevant phosphocholines owing to the trisiloxane moiety. In addition, it is worth noting that a growing number of siloxane based Gemini and Bola surfactants are under the spotlight.12, 26-29 To date, a larger number of efforts have been invested in the design and synthesis of permethylated siloxane/polysiloxane surfactants with different hydrophilic groups to investigate the relationship between molecular structures and aggregation behavior in aqueous solution. Despite no definitive relationships between the molecular structure of siloxane surfactants and aggregation behavior were found, it is believed that the lower surface tensions caused by siloxane/polysiloxane surfactants can be traced directly to the flexible Si-O-Si chain. Herein, in present work, polyether based siloxane surfactants with different hydrophobic siloxane groups (methy, ethyl, propyl, butyl, branched chain) were


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synthesized and their aggregation behavior in aqueous solution were investigated, which used to investigate the effect of the size of the siloxane groups, smaller or longer alkyl chains at the silicon atom on their aggregation behavior in aqueous solution. A series of useful parameters obtained in this paper will be useful in understanding the role of the molecular structures of siloxane surfactants in affecting the aggregation behavior and the design of novel siloxane surfactants.

EXPERIMENTAL SECTION Materials. γ-mercaptopropyldi(trimethylsiloxy)methylsilane, γmercaptopropyltri(trimethyl-siloxy)silane, propyldimethylsilane, and butyldimethylsilane were purchased from Jiangxi Yuren New Materials Co., Ltd. Polyoxyethylene -polyoxypropylene alkyl ether (CH2=CHCH2EO27PO6H) were purchased from Yangzhou Chenhua New Materials Co., Ltd. 2,2-Dimethoxy-2phenylacetophenone (DMPA), triethylsilane, diethylmethylsilane, tris(pentafluorophenyl)borane, ethyldimethylsilane, hexane, toluene, and methanol was obtained from J&K Scientific Ltd (Shanghai). All regents were used as received. Triply distilled water was used to prepare all the solutions. The molecular structures of the polyether siloxane surfactants used in this work were shown in Figure 1. Apparatus and Procedures. 1H NMR, 13C NMR, and 29Si NMR spectra were recorded using a Bruker AV 400 spectrometer in chloroform-d (CDCl3) without internal standard. FT-IR was recorded using a VERTEX 70 FT-IR spectrometer. Measurement was performed on samples dispersed in anhydrous KBr pellets. Surface tension was determined with a model BZY-1 tensiometer (Shanghai Fangrui Instrument Co., Ltd.,) using a platinum ring. The temperature (25 °C) was controlled



by a thermostatic bath. The uncertainty of the measurements was within ± 0.1 mN·m1.

All measurements were repeated until the values were reproducible. The dynamic light scattering (DLS) measurements were performed using a

Dynapro Titan system (Wyatt Technology, Santa, Barbara, CA) at a scattering angle of 90o The morphologies of aggregates in aqueous solution were observed by a JEM1011 TEM (JEOL, Japan) at 100 kV. The samples were prepared by dropping silicone surfactant solution on a carbon coated grid. Phosphotungstic acid solution (2 wt%) was used to stain the samples, and then the grids were dried at room temperature. Freeze-fracture transmission electron microscopy (FF-TEM) (Leica, BAF060) observations. The frozen samples were fractured and replicated in a BAF 400D (Germany) freeze-fracture device which had been cooled by liquid nitrogen. Synthesis of Polyether Based Methyldimethoxysilane. Polyoxyethylenepolyoxypropylene alkyl ether (7.81 g, 5 mmol), γ- mercaptopropylmethyldimethoxysilane (1.18 g, 6.5 mmol), 2,2-dimethoxy-2-phenylacetophenone (50 mg, 0.2 mmol), and methanol (10 mL) were added into a sealed flask. Then, the resulting mixture was irradiated with UV light for 2.5 h under N2 atmosphere. The excess silane and solvent were removed by distilling in vacuum and the resultant residue purified by Kugelrohr distillation at150 o C for 40 min. the pure product was confirmed by 1H NMR and FT-IR. 1 HNMR (CDCl3): δ (ppm) = 3.70~3.69 (CH2CH2O), 3.61~3.60 (SCH2CH2CH2), 3.59-3.53 (CHCH2), 3.52 (OCH3), 3.473.43 (CHCH2), 2.64~2.61 (SCH2CH2), 2.59~2.57 (SiCH2CH2CH2), 1.92~1.89 (SCH2CH2), 1.70~1.67 (SiCH2CH2), 1.21~1.19 (CHCH3), 0.76~0.69 (SiCH2CH2), 0.15~0.22 (SiCH3); FT-IR (KBr，cm-1): 2951, 2869, 1459, 1350, 1294, 1253, 1109, 950, 839.


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Preparation of Et-Si3-PG. A mixture of polyether based methyldimethoxysilane (3.48 g, 2 mmol), ethyldimethyl silane (0.71 g, 8 mmol), and dry hexane (20 ml) was added into a flask. Then, the mixture was stirred at room temperature for 4 min before the addition of tris(penta-fluorophenyl)borane toluene solution (150 μl, 15 mmol). After 5 min induction time, evolution of gas and heat from the reacting mixture occurred. 2 h later, the neutral alumina (2.0 g) was added, and then, the resulting solution was filtered and concentrated. Finally, the product was purified by Kugelrohr distillation at170 oC for 40 min and confirmed by 1H NMR, 13CNMR, 29Si NMR, and FT-IR. 1 HNMR (CDCl3): δ (ppm) = 3.65~3.63 (CH2CH2O), 3.57~3.55 (SCH2CH2CH2), 3.52-3.49 (CHCH2),3.42-3.39 (CHCH2), 2.59~2.56 (SCH2CH2), 2.50~2.54 (SiCH2CH2CH2), 2.06~2.05 (SCH2CH2), 1.83-1.87 (CH2CH3), 1.60~1.62 (SiCH2CH2), 1.12~1.17 (CHCH3), 0.74-0.75 (CH2CH3), 0.50~0.64 (SiCH2CH2), 0.06~0.13 (SiCH3); 13C NMR (CDCl3): δ (ppm) = 76.44, 75.74, 75.55 (CHCH2O), 71.41, 71.31, 71.14, 70.79, 69.10 (CH2CH2O), 59.59 (SCH2CH2CH2), 57.25 (CH2OH), 35.97 (SiCH2CH2CH2), 30.24 (SCH2CH2), 29.29 (SCH2CH2), 23.68 (SiCH2CH2), 19.19 (CHCH3), 17.03 (SiCH2CH3), 17.01 (SiCH2CH3), 0.23 (SiCH3); 29Si

NMR (CDCl3): δ (ppm) = 9.60 (Si(CH3)3), -22.88 (SiCH3); FT-IR (KBr, cm-1):

2951, 2873, 1457, 1255, 1109, 1035, 842. Preparation of Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, But-Si3-PG. Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, and But-Si3-PG were synthesized and characterized using the similar method as Et-Si3-PG using diethylmethylsilane, triethylsilane, propyldimethylsilane, and butyldimethylsilane to displace ethyldimethylsilane, respectively. Et2-Si3-PG.

1 HNMR

(CDCl3): δ (ppm) = 3.65~3.60 (CH2CH2O), 3.53~3.52

(SCH2CH2CH2), 3.50-3.47 (CHCH2),3.40-3.36 (CHCH2), 2.56~2.53 (SCH2CH2),



2.51~2.48 (SiCH2CH2CH2), 2.04~2.02 (SCH2CH2), 1.84-1.80 (CH2CH3), 1.60~1.57 (SiCH2CH2), 1.13~1.10 (CHCH3), 0.73-0.71 (CH2CH3), 0.62~0.60 (SiCH2CH2), 0.08~0.11 (SiCH3); 13C NMR (CDCl3): δ (ppm) = 75.78, 75.08, 74.88 (CHCH2O), 70.75, 70.65, 70.48, 70.09, 68.43 (CH2CH2O), 57.12 (SCH2CH2CH2), 54.79 (CH2OH), 35.29 (SiCH2CH2CH2), 29.70 (SCH2CH2), 28.65 (SCH2CH2), 23.01 (SiCH2CH2), 18.32 (CHCH3), 17.15 (SiCH2CH3) ,16.36 (SiCH2CH3), 0.58 (SiCH3); 29Si

NMR (CDCl3): δ (ppm) = 9.86 (Si(CH3)3), -20.26 (SiCH3); FT-IR (KBr, cm-1):

2979, 2867, 1453, 1253, 1149, 1053, 863. Et3-Si3-PG.

1 HNMR

(CDCl3): δ (ppm) = 3.73~3.68 (CH2CH2O), 3.64~3.62

(SCH2CH2CH2), 3.62-3.57 (CHCH2), 3.50-3.46 (CHCH2), 2.67~2.63 (SCH2CH2), 2.60~2.58 (SiCH2CH2CH2), 2.15~2.13 (SCH2CH2), 1.95-1.93 (CH2CH3), 1.69~1.67 (SiCH2CH2), 1.25~1.23 (CHCH3), 0.83-0.80 (CH2CH3), 0.72~0.71 (SiCH2CH2), 0.20~0.18 (SiCH3); 13C NMR (CDCl3): δ (ppm) = 74.88, 74.10 73.89 (CHCH2O), 69.76, 69.52, 69.10, 68.73, 67.49 (CH2CH2O), 57.89 (SCH2CH2CH2), 55.59 (CH2OH), 34.23 (SiCH2CH2CH2), 28.73 (SCH2CH2), 27.61 (SCH2CH2), 22.37 (SiCH2CH2), 16.20 (CHCH3), 17.51 (SiCH2CH3), 15.41 (SiCH2CH3), 0.11 (SiCH3); 29Si

NMR (CDCl3): δ (ppm) = -9.73 (Si(CH3)3), -22.77 (SiCH3); FT-IR (KBr, cm1):

2989, 2870, 1459, 1254, 1111, 1032, 841. Pro- Si3-PG. 1 HNMR (CDCl3): δ (ppm) = 3.58~3.56 (CH2CH2O), 3.49~3.47 (SCH2CH2CH2), 3.45-3.42 (CHCH2), 3.37-3.31 (CHCH2), 2.48~2.51 (SCH2CH2), 2.42~2.44 (SiCH2CH2CH2), 1.76~1.80 (SCH2CH2), 1.50~1.55 (SiCH2CH2), 1.28~1.33 (CH2CH2CH3), 1.06~1.08 (CHCH3), 0.88~0.919 (CH2CH2CH3), 0.45~0.49 (SiCH2CH2), 0.02~0.12 (SiCH3); 13C NMR (CDCl3): δ (ppm) = 75.66, 74.96, 74.76 (CHCH2O), 71.72, 70.53, 70.36,66,98, 68.32 (CH2CH2O), 58.76 (SCH2CH2CH2), 56.43 (CH2OH), 35.08 (SiCH2CH2CH2), 29.45 (SCH2CH2), 28.33 (SCH2CH2), 20.64


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(SiCH2CH2), 17.89(CH2CH2CH3), 17.01(CHCH3), 16.46 (CH2CH2CH3), 16.21(SiCH2CH2), 0.14 (SiCH3); 29Si NMR (CDCl3): δ (ppm) = 6.92 (Si(CH3), 67.52(Si(CH3)2; FT-IR (KBr, cm-1): 2956, 2870, 1459, 1252, 1110, 1030, 841, 774. But-Si3-PG. 1 HNMR (CDCl3): δ (ppm) = 3.58~3.56 (CH2CH2O), 3.48~3.46 (SCH2CH2CH2), 3.45-3.42 (CHCH2), 3.37-3.31 (CHCH2), 2.48~2.51 (SCH2CH2), 2.42~2.45 (SiCH2CH2CH2), 1.76~1.80 (SCH2CH2), 1.51~1.56 (SiCH2CH2), 1.24~1.26 (CH2 CH2CH2CH3), 1.06~1.09 (CHCH3), 0.81~0.93 (CH2CH2CH2CH3), 0.46~0.48 (SiCH2CH2), 0.00~0.02 (SiCH3); 13C NMR (CDCl3): δ (ppm) = 75.71, 75.02, 74.82 (CHCH2O), 70.83, 70.69, 70.60,70.42, 68.39 (CH2CH2O), 58.82 (SCH2CH2CH2), 56.49 (CH2OH), 35.14 (SiCH2CH2CH2), 29.54 (SCH2CH2), 28.38 (SCH2CH2), 26.16 (CH2CH2CH2CH3), 25.14 (CH2CH2CH2CH3), 23.70 (SiCH2CH2CH2), 17.79(SiCH2CH2), (CH2CH2CH3), 17.08(CHCH3), 13.96 (CH2CH2CH2CH3), 0.05 (SiCH3); 29Si NMR (CDCl3): δ (ppm) = 7.54 (Si(CH3), 67.02(Si(CH3)2; FT-IR (KBr, cm-1): 2978, 2869, 1459, 1252, 1111, 1036, 841, 788. Preparation of Si3-PG and Si4-PG. Si3-PG and Si4-PG were prepared and characterized using the same method as polyether based methyldimethoxysilane using γ-mercaptopropyldi(trimethylsiloxy)-methylsilane, γ mercaptopropyltri(trimethylsiloxy)silane in place of γ-mercaptopropyl methyldimethoxysilane, respectively. Si3-PG. 1 HNMR (CDCl3): δ (ppm) = 3.61~3.68 (CH2CH2O), 3.57~3.60 (SCH2CH2CH2), 3.52-3.55 (CHCH2),3.42-3.48 (CHCH2), 2.59~2.62 (SCH2CH2), 2.52~2.55 (SiCH2CH2CH2), 1.86~1.90 (SCH2CH2), 1.61~1.64 (SiCH2CH2), 1.16~1.19 (CHCH3), 0.57~0.60 (SiCH2CH2), 0.04~0.12 (SiCH3); 13C NMR (CDCl3): δ (ppm) = 74.02, 73.32, 73.12 (CHCH2O), 68.89, 68.72, 68.34, 67.97,66,67 (CH2CH2O), 57.12 (SCH2CH2CH2), 54.79 (CH2OH), 33.58 (SiCH2CH2CH2), 27.84



(SCH2CH2), 26.72 (SCH2CH2), 21.67 (SiCH2CH2), 15.37 (CHCH3)14.56(SiCH2CH2), 0.01, 0.54 (SiCH3); 29Si NMR (CDCl3): δ (ppm) = 6.40 (Si(CH3)3), -23.00 (SiCH3); FT-IR (KBr, cm-1): 2951, 2869, 1458, 1254, 1116, 1035, 844. Si4-PG. 1 HNMR (CDCl3): δ (ppm) = 3.61~3.66 (CH2CH2O), 3.52~3.57 (SCH2CH2CH2), 3.47-3.50 (CHCH2),3.39-3.42 (CHCH2), 2.52~2.57 (SCH2CH2), 2.49~2.50 (SiCH2CH2CH2), 1.80~1.85 (SCH2CH2), 1.59~1.61 (SiCH2CH2), 1.13~1.14 (CHCH3), 0.51~0.55 (SiCH2CH2), 0.02~0.11 (SiCH3); 13C NMR (CDCl3): δ (ppm) = 74.12, 73.42, 73.22 (CHCH2O), 68.99, 68.83, 68.06,66,79 (CH2CH2O), 57.21 (SCH2CH2CH2), 54.88 (CH2OH), 33.48 (SiCH2CH2CH2), 27.95 (SCH2CH2), 26.64 (SCH2CH2), 21.692 (SiCH2CH2), 15.48 (CHCH3),14.63(SiCH2CH2), 0.14 (SiCH3); 29Si NMR (CDCl3): δ (ppm) = 6.92 (Si(CH3)3), -67.51 (SiO3); FT-IR (KBr, cm-1): 2953, 2870, 1459, 1253, 1109, 1030, 844, 757.

RESULTS AND DISCUSSION Surface Tension Properties. Surface tension of the polyether based siloxane surfactants, Si3-PG, Et-Si3-PG, Et2-Si3-PG, Et3-Si3-PG, Pro-Si3-PG, But-Si3-PG, and Si4-PG, were measured to evaluate their surface activities. The curves of surface tension (γ) in aqueous solution of polyether based siloxane surfactants versus their concentration at 25 oC were shown in Figure 2. A gradually decrease in surface tension was observed with increase in concentrations for all investigated polyether based siloxane surfactants up to the plateau region. Accordingly, critical micelle concentrations (CMC, concentration of the break point) determined by extrapolation and listed in Table 1. As shown in Table 1, the molecular structure of siloxane had a considerable effect on CMC values. The CMC values of the investigated polyether based siloxane surfactants follow the order Si3-PG > Et-Si3-PG > Pro- Si3-PG > But-


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Si3-PG. It can be explained that the hydrophobicity of the siloxane groups increase as increasing hydrophobic -CH2- group to trimethylsiloxy group (M groups) which caused the CMC values decrease, namely, replacing methyl of trimethylsiloxyl groups with longer alkyl groups (ethyl, propyl, and butyl) resulted in a decreased the CMC values. Meanwhile, Si4-PG had three branched trimethylsiloxy groups (more hydrophobic) compared with Si3-PG resulting in the smaller CMC values. In comparison of Et-Si3-PG, Et2-Si3-PG and Et3-Si3-PG, they had less CMC values resulted

from

the

bulkier

hydrophobic

group

for

ethyldimethylsiloxyl,

diethylmethylsiloxyl and triethylsiloxyl, respectively. Generally, for siloxane surfactants, the conformations adopted at air/water interface with closely packed layer caused by the highly flexible Si-O-Si backbone, especially, permethylated siloxane surfactants can effectively reduce surface tensions of water (∼20 mN m-1) attributed to the favorable orientation of all methyl groups (lower surface energy) at air/water interface, namely, air/water interface is dominated by methyl groups. However, as seen in Table 1, the surface tension at CMC (γCMC) values of Si3-PG, Et-Si3-PG, Pro- Si3-PG, But-Si3-PG are 25.2, 27.9, 30.4, 37.9 mN m-1, respectively. Replacing methyl groups with ethyl, propyl, and butyl groups resulted in an obviously decreasing γCMC. Because of the efficient and dense packing layer at the air/water interface can be inhibited by each addition of a hydrophobic -CH2- group to the Si-CH3 groups compared with trimethylsiloxyl groups. Meanwhile, a comparison with Et-Si3-PG, the replacing CH3 groups with more ethyl groups, Et2-Si3-PG and Et3-Si3-PG can only reduce the surface tension of water to 39.8 and 41.6, respectively, which were lager than the γCMC values of polyether based siloxane surfactants, especially trisiloxne surfactants (∼20 mN m-1). Therefore, the surface tensions of siloxane surfactants can be affected directly by the molecular structure of siloxane, the lower surface tensions only provided by smaller



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siloxane groups and shorter alkyl chains on the Si-O-Si backbones caused by dense surface films packed with CH3- groups.16, 29-32 To elicit the adsorption behavior of the polyether based siloxane surfactant at air/water interface, the maximum surface excess (Γmax) and minimum surface area per polyether siloxane surfactant molecule (Amin), can be calculated from surface tension plots using eq 1-2 32 and listed in Table 2 1

Γ𝑚𝑎𝑥 = ― 2.303𝑅𝑇 𝐴𝑚𝑖𝑛 =

(

∂𝛾 ∂𝑙𝑜𝑔𝐶

)

(1)

1016

(2)

𝑁𝐴Γ𝑚𝑎𝑥

where γ is the surface tension, R is the ideal gas constant (8.314 J mol-1 K-1), T is the absolute temperature, NA is Avogadro’s number. A higher value of Γmax or lower values of Αmin indicated a denser arrangement of polyether based siloxane surfactant molecules at the surface of the solution.

27,32

As seen in Table 2, the Amin values are

63.4, 56.7, 49.6, and 42.8 Å2 for Si3-PG, Et-Si3-PG, Pro- Si3-PG, But-Si3-PG, respectively. As the number of -CH2- groups for the siloxane groups increases, Amin decreases and Γmax increases. It similar to the alkyloctaethyleneglycol ethers, the Amin values decreases by 6-7 Å with each addition of a hydrophobic -CH2- group to the alkyl chain. While, the Amin values (Si3-PG, Et-Si3-PG, Pro- Si3-PG, But-Si3-PG) decrease by about 3.5 Å for those polyether based siloxane surfactants (seen in Figure 3) as increasing each -CH2- group to the M groups.30,

33

One cannot directly extend this

rationale to the investigated polyether based siloxane surfactants due to their complex branched molecular architectures and the flexible Si-O-Si chain, however, the total hydrophobic effect of the siloxane surfactants is attributed to alkyl groups attached to Si-O-Si chain. The Si-O-Si chains serve as a flexible framework on which to attach multiple groups (methyl, ethyl, propyl, and butyl), and adopt a variety of configurations at the air/water interface to produce denser packed surface films. Meanwhile, the


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branched chain of siloxane groups cause Amin values to be higher owing to the increase in bulkiness of the hydrophobic groups. As the Amin values increases from 56.7 Å2 for Et-Si3-PG to 79.8 Å2 for Et2-Si3-PG and 84.3 Å2 for Et3-Si3-PG due to increasing in ethyl groups, which was similar with typical hydrocarbon surfactants promoted films with a greater proportion of higher-surface-energy -CH2- groups. In addition, the trimethylsiloxyl group is responsible for the distance between the siloxane surfactants molecules in the adsorption layer, the larger Αmin value of Si4-PG (72.2 Å2) compared with Si3-PG (63.4 Å2) mainly attributed to the steric hindrance of three trimethylsiloxy groups.14, 17, 33-34 Furthermore, the adsorption efficiency (pC20), and surface pressure at CMC (πCMC) can also be calculated using eq 3,4 27,33 and listed in Table 2. (3)

𝑝𝐶20 = ― log 𝐶20

(4)

𝜋𝐶𝑀𝐶 = 𝛾0 ― 𝛾𝐶𝑀𝐶

Where C20 means the concentration required to reduce the surface tension of pure water by 20 mN m-1, γ0 is the surface tension of pure water. In Table 2, the larger are the values of pC20 and πCMC, the higher are the adsorption efficiency and effectiveness of the investigated polyether based siloxane surfactants. The πCMC values follows the order Si4-PG > Si3-PG > Et-Si3-PG > Pro- Si3-PG > But-Si3-PG > Et2-Si3-PG >Et3-Si3PG. it can be explained that Si4-PG had three branched trimethylsiloxy groups compared with Si3-PG resulting in the larger πCMC. In comparison of Si3-PG, Pro- Si3PG, But-Si3-PG, Et2-Si3-PG, and Et3-Si3-PG had smaller πCMC values consequent caused by each addition of a CH2 group to the alkyl chain on silicon atom and the bulkier hydrophobic group for diethylmethylsiloxyl and triethylsiloxyl, respectively. Meanwhile, the tendency of micellization compared with the adsorption can be estimated by CMC/C20 values. A larger CMC/C20 values indicate that the surfactant can



Page 14 of 30

adsorb more easily at the air/liquid interfaces in comparison with their preference to form aggregates. As seen in Table 2, the CMC/C20 values of those polyether based siloxane surfactants smaller than that of polyether based polysiloxane surfactant (CMC/C20 = 522)18. However, the CMC/C20 values of Si3-PG, Et-Si3-PG, Pro- Si3-PG, But-Si3-PG decreases with increase in the length of the alkyl chain (methyl, ethyl, propyl, butyl) on silicon atom, in addition, the CMC/C20 values of Et-Si3-PG, Et2-Si3PG, and Et3-Si3-PG decreases with increase the branched ethyl groups for ethyldimethylsiloxyl, diethylmethylsiloxyl and triethylsiloxyl, respectively, Owing to the increase in the size of the hydrophobic groups. Si4-PG had the larger CMC/C20 values (66.1), indicating that Si4-PG adsorbed at the air/water interface more easily relative to its preference to form aggregates than do another investigated polyether based siloxane surfactants.18, 33 In order to further understand the effect of siloxane group on the micellization of polyether based siloxane surfactants. the standard Gibbs energy change ( Gm0 ) and the standard free energy of adsorption ( Gads ) for all investigated polyether based siloxane 0

surfactants at the air/water interface can be obtained from the eq 5-6

32,33

and

summarized in Table 2. ∆𝐺0𝑚 = 𝑅𝑇ln

( ) 𝐶𝑀𝐶

(5)

55.5

𝜋𝐶𝑀𝐶

∆𝐺0𝑎𝑑𝑠 = ∆𝐺0𝑚 ― 𝛤𝑚𝑎𝑥

(6)

0 0 As shown in Table 2, the values of Gm , and Gads are all negative, indicating the

0 0 micellization process is spontaneous. The values of Gads are large than that of Gm ,

which show that the adsorption process of Si3-PG, Et-Si3-PG, Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, But-Si3-PG, Si4-PG is fairly stronger than the process of their micellization.


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Namely, it may be concluded that the micellization is less spontaneous compared to adsorption. Fluorescence Investigations. The microenvironment of the aggregates for Si3PG, Et-Si3-PG, Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, But-Si3-PG, Si4-PG in the aqueous solutions were also investigated with steady-state fluorescence spectroscopy using pyrene as a probe. The intensity ratio of the first to the third band (I1/I3) are sensitive to the environmental polarity and usually served to a measure of the polarity of the environment.13,35-37 When the micellization of polyether siloxane surfactants occur, pyrene molecules would penetrate into the interior hydrophobic region of micelles from water. This can cause an abrupt change of the I1/I3 ratio and the concentration corresponding to the sudden change was considered as CMC. Figure 4a also exhibits the representative fluorescence spectrum of pyrene in aqueous solutions of Si4-PG at above and below the CMC. The plots of I1/I3 ratio as a function of concentration of all the investigated polyether siloxane surfactants are illustrated in Figure 4-b. And the CMC values obtained from the I1/I3 data (in Figure 4-b) were listed in Table 1 and are in good agreement with the values obtained from surface tension measurements. Comparison with Si3-PG, the value of I1/I3 for Si4-PG was smaller caused by three trimethylsiloxyl on hydrophobic groups. Meanwhile, owing to the introduction of ethyl group, Et-Si3-PG cannot be closer with each other and resulted in a less tightly packing of the palisade layer leading more water molecules to exist in the palisade layer of the Et-Si3-PG micelles, consequently, causing pyrene molecules to sense more polar microenvironment, and then, having the higher I1/I3 values, meanwhile, with a higher alkyl group (propyl, butyl) and branched chain in siloxane group, polyether based siloxane surfactants could have more hydrophobic solubilizing site for the pyrene probe, and hence a decrease in I1/I3 values.13,38-40



Aggregation Properties. The size distribution and morphology of the aggregates were investigated by DLS, TEM, and FF-TEM. Figure 5 shows the intensity hydrodynamic size distribution plots of Si3-PG, Et-Si3-PG, Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, But-Si3-PG, Si4-PG at 10-3 M, and the size distribution ranges from 9 nm to 900 nm. As shown in Figure 5, the aggregates were shown to have monomodal for Si3-PG, Et-Si3-PG, But-Si3-PG and bimodal distribution functions for Et2-Si3-PG, Et3Si3-PG, Pro- Si3-PG, Si4-PG. The aggregate size distribution shown in Figure 5 corroborated the observations from the TEM images. Figure 6 shows TEM images of the aggregations in Si3-PG, Et-Si3-PG, Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, But-Si3PG, Si4-PG aqueous solutions, indicating those polyether based siloxane surfactants can form non-uniform size of spheroidal, and the result confirms by FF-TEM observations.

CONCLUSIONS Siloxane surfactants with different hydrophobic siloxane group (larger and longer alkyl groups, branched chain) were successfully synthesized by thiol-ene and PiersRubinsztajn reaction. And their aggregation behavior in aqueous solution were investigated by surface tension, fluorescence, DLS, TEM and FF-TEM. The results indicated that the molecular structure of hydrophobic siloxane groups play a vital role in micellization process. Replacing the methyl in trimethylsiloxyl groups with ethyl, diethyl, triethyl, propyl, and butyl groups resulted in an obviously decreasing the values of CMC and γCMC. Especially, Et2-Si3-PG and Et3-Si3-PG can only reduce the surface tension of water to 39.8 and 41.6 mN m-1, respectively, caused by the efficient and dense packing layer at the air/water interface were inhibited by the more replaced ethyl groups. As the number of -CH2- groups for the siloxane groups increases, Amin decreases and


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Γmax increases, furthermore, the Amin values of Si3-PG, Et-Si3-PG, Pro- Si3-PG, But-Si3PG successively decrease about 3.5 Å as increasing each -CH2- group. Meanwhile, the branched chain of siloxane groups cause a higher Amin values owing to the increase in bulkiness of the hydrophobic groups. with a higher alkyl group (propyl, butyl) and branched chain in siloxane group, polyether based siloxane surfactants could have more hydrophobic solubilizing site for the pyrene probe, and hence a decrease in pyrene I1/I3 values. All polyeyher based siloxane surfactants can form non-uniform size of spheroidal aggregates in aqueous solution. Concerning the driving force, the micellization process of all the polyeyher based siloxane surfactants were spontaneous, however, micellization were less spontaneous compared with adsorption.

Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21563016, 21862009).

Conflict Of Interest We have no conflict of interest.



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(33) Soni, S. S.; Sastry, N. V.; Aswal, V. K.; Goya, P. S. Micellar Structure of Silicone Surfactants in Water from Surface Activity, SANS and Viscosity Studies. J. Phys. Chem. B 2002, 106, 2606-2617. (34) Nagarajan, R.; Wang, C. C. Theory of Surfactant Aggregation in Water/Ethylene Glycol Mixed Solvents. Langmuir 2000, 16, 5242-5251. (35) Ananthapadmanabhan, K. P.; Goddard, E. D. A Study of the Solution, Interfacial and Wetting Properties of Silicone Surfactants. Colloids Surf. A 1990, 44, 281-297. (36) Kuo, P. L.; Hou, S. S.; Teng, C. K.; Liang, W. J. Function and Performance of Silicone Copolymer VI. Synthesis and Novel Solution Behavior of Water-Soluble Polysiloxanes with Different Hydrophiles. Colloid Polym. Sci. 2001, 279, 286-291. (37) Srividhya, M.; Chandrasekar, K.; Baskar, G.; Reddy, B. S. R. Physico-Chemical Properties of Siloxane Surfactants in Water and Their Surface Energy Characteristics. Polymer 2007, 48, 1261-1268. (38) Singh, G.; Singh, G.; Kang, T. S. Micellization Behavior of Surface Active Ionic Liquids Having Aromatic Counterions in Aqueous Media. J. Phys. Chem. B 2016, 120, 1092-1105. (39) Zhang, P.; Xu, X. H.; Zhang, M. H.; Wang, J. B.; Bai, G. Y.; Yan, H. K. SelfAggregation of Amphiphilic Dendrimer in Aqueous Solution: The Effect of Headgroup and Hydrocarbon Chain Length. Langmuir 2015, 31, 7919-7925. (40) Barhoumi, Z.; Saini, M.; Amdouni, N.; Pal, A. Interaction between Amphiphilic Ionic Liquid 1-butyl-3-methylimidazolium Octyl Sulfate and Anionic Polymer of Sodium Polystyrene Sulfonate in Aqueous Medium. Chem. Phys. Lett. 2016, 661, 173178.


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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



Figure 1 Chemical Structures of the Polyether based Siloxane Surfactants


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Figure 2 Surface tension of Si3-PG (●), Et-Si3-PG (▲), Et2-Si3-PG (▶), Et3-Si3-PG (◀), Pro- Si3-PG (▼), But-Si3-PG (◆), Si4-PG (◼) in aqueous solutions as a function of their concentrations at 25 °C.

Figure 3 Amin of polyether based trisiloxane surfactant as a function of CH2 groups, n = 0 is Si3-PG, 2 is Et-Si3-PG, 4 is Pro- Si3-PG, 6 is But-Si3-PG



Figure 4 (a) representative pyrene fluorescence emission spectra Si4-PG in aqueous solution at the concentrations below and above the CMC, (b) I1/I3 ratio of pyrene as a function of concentration for Si3-PG, Et-Si3-PG, Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, But-Si3-PG, Si4-PG in aqueous solution at 298 K


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Figure 5 Size distributions of 10-3 M Si3-PG (a), Et-Si3-PG (b), Et2-Si3-PG (c), ProSi3-PG (d), But-Si3-PG (e), Et3-Si3-PG (f), Si4-PG (g) in aqueous solution.




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Figure 6 TEM images of aggregates of Si3-PG (a), Et-Si3-PG (b), Et2-Si3-PG (c), ProSi3-PG (d), But-Si3-PG (e), Et3-Si3-PG (f), Si4-PG (g)

Figure 7 FF-TEM images of aggregates of Si3-PG (a), Et-Si3-PG (b), Et2-Si3-PG (c), Pro- Si3-PG (d), But-Si3-PG (e), Et3-Si3-PG (f), Si4-PG (g)



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Table 1 CMC and I1/I3 of Si3-PG, Et-Si3-PG, Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, But-Si3-PG, Si4-PG in Aqueous Solutions at 25 °C Si3-PG

Et-Si3-PG

Pro- Si3-PG

But-Si3-PG

Et2-Si3-PG

Et3-Si3-PG

Si4-PG

12.5 12.6 1.32

8.12 8.13 1.55

7.24 7.24 1.19

6.46 6.45 1.20

7.08 7.07 1.45

6.61 6.61 1.35

9.33 9.33 1.25

CMC/10-5M CMC2/10-5M I1/I3 1 obtained

from surface tension, 2obtained from fluorescence

Table 2 Adsorption Parameters of Si3-PG, Et-Si3-PG, Et2-Si3-PG, Et3-Si3-PG, Pro- Si3-PG, But-Si3-PG, Si4-PG in Aqueous Solutions at 25 °C

Si3-PG Et-Si3-PG Pro- Si3-PG But-Si3-PG Et2-Si3-PG Et3-Si3-PG Si4-PG

γcmc (mN m-1)

Πcmc (mN m-1)

25.2 27.9 30.4 37.9 39.8 41.6 22.1

47.3 44.6 42.1 34.6 32.7 30.9 50.4

pC20 5.69 5.81 5.76 5.42 5.28 5.11 5.87

Γmax (μmol m-2) 2.78 2.42 2.16 1.99 1.98 1.86 2.46

Αmin (Å2) 59.7 68.6 76.9 83.5 83.6 89.3 67.5

TOC GRAPHIC


CMC /C20 61.7 52.4 41.7 16.9 13.5 8.5 69.2

∆G0m

∆G0ads

kJ/mol

kJ/mol

-32.2 -33.2 -33.5 -33.8 -33.6 -33.7 -32.9

-49.2 -51.7 -53.1 -51.2 -50.1 -50.4 -53.4

Aggregation Behavior of Polyether Based Siloxane Surfactants in

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