Aggregation Behavior of âLinearâ Trisiloxane Surfactant with Different

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B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials

Aggregation Behavior of “Linear” Trisiloxane Surfactant with Different Terminal Groups (CH , ClCH , and CF ) in Aqueous Solution 3-

2-

3-

Jinglin Tan, Meihong Xiao, and Qinghua Hu J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b01245 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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The Journal of Physical Chemistry

Aggregation

Behavior

of

“Linear”

Trisiloxane

Surfactant with Different Terminal Groups (CH3-, ClCH2-, and CF3-) in Aqueous Solution Jinglin Tan a b*, Meihong Xiao c, and Qinghua Hu a b a

School of Chemical and Environmental Engineering, Jiujiang University,

Jiujiang 332005, China b

Jiangxi Province Engineering Research Center of Ecological Chemical Industry,

Jiujiang 332005, China c

University Hospital, Jiujiang University, Jiujiang 332005, China

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

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The Journal of Physical Chemistry 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

Absrtract Novel “linear” trisiloxane surfactants with different terminal groups (CH3-, ClCH2-, CF3-) and two polyether hydrophilic groups were successfully synthesized and confirmed using 1H NMR,

13C

NMR,

29Si

NMR, and FT-IR spectroscopy. The

aggregation and adsorption behavior of the “linear” trisiloxane surfactants in aqueous solution were studied by surface tension, dynamic light scattering (DLS), transmission electron microscopy (FF-TEM), and TEM. Owing to the introduction of two polyether hydrophilic groups in the terminal positions of trisiloxane hydrophobic part, “linear” trisiloxane surfactants (Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8) tend to lie flat in the air/water interface and result in an increasing the surface tension at the CMC (γCMC) and single trisiloxane surfactant molecule at the air/water interface (Amin) values. Difference in the intermolecular forces and molecular volumes (CH3- < ClCH2- < CF3-), the γCMC values decrease following the order, Me-Si3-EO8 > Cl-Si3-EO8 > F-Si3-EO8, and the adsorption efﬁciency (pC20), surface pressure at the CMC (πCMC), CMC/C20, and Amin values increase following the order, Me-Si3-EO8 < Cl-Si3-EO8 < F-Si3-EO8. As comparison, fluorinated trisiloxane surfactant (F-Si3-EO8) has greater surface activity attributed to the terminal CF3- group. The TEM and FF-TEM results illustrated that all the investigated “linear” trisiloxane surfactants can form non-uniform size spherical aggregates.


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Introduction Fluorinated surfactants, one kind of amphiphilic molecules with at least one fluorine atoms coupled to the hydrophobic group, show lower surface tension and critical micelle concentration compared with hydrocarbon and siloxane surfactants. The extraordinary physicochemical properties of fluorinated surfactants were attributed to 1) the lower polarizability of fluorine compared with hydrogen and 2) the larger molecular volume of a fluorocarbon chain.1-6 However, the aggregation behavior of fluorinated surfactants is less often reported owing to the poor solubility, bioconcentration and bioaccumulation.

6-8

Even if so, fluorinated materials, such as

fluoroalkyl sulfonates, 9-10 fluorosilicone material, 11-13 and so on, are of great interest. Siloxane surfactants, consisting of methylated siloxane/polysiloxane as hydrophobic group attached one or more hydrophilic groups, have been attracting great attention in both the academic and industrial fields owing to their unique properties, such as, excellent surface activity, less toxicity, and so forth. Typically, trisiloxane surfactants with a molecular structure MDM, M is Me3SiO1/2- and D is Me(R)SiO1/2-, have been under the spotlight. 14-17 The R is the hydrophilic groups (cationic, 18-21 nonionic, 22-25 and zwitterion 26) grafted onto trisiloxane hydrophobic group through spacer (-(CH2)3-, and -CH2SCH2-) by quaternarization,18-19 hydrosilylation, 22 and thiol-ene reaction. 2728

In comparison with hydrocarbon surfactants, permethylated trisiloxane surfactants provided lower surface tension (~20 mN∙m-1) resulting from the shorter and wider trisiloxane hydrophobic group which can adopt various conformations and present the adsorbed layer at the air/water interface to be dominated by methyl groups. Therefore, the evidences shown that the Si-O-Si backbone had only a minor effect on the hydrophobicity of siloxane sheltered by the methyl groups and served as a ﬂexible



framework on which to attach multiple organic groups.

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6,14,15, 29

However, a few

investigations of non permethylated siloxane surfactant were reported, meanwhile, an increasing number of investigation about the aggregation behavior of permethylated siloxane surfactants with different hydrophilic groups, such as, pyrrolidinium, quaternary ammonium,

21, 34

polyether,

17, 22,23, 30

sulfobetaine,

26

glucosamide,

18

25, 30-31

and so forth, were investigated. 18, 20,22-26, 30-36 It illustrated that the molecular structures of hydrophilic groups play a crucial role in the aggregation behavior of siloxane surfactants. Despite trisiloxane surfactants having two Me3SiO1/2-, more effectively than many linear hydrocarbon surfactants. It was certain that the molecular structures of siloxane hydrophobic tails have a major effect on their physiochemical properties. Tan et. al 3738

demonstrated that the CMC and γCMC values can be decreased as increasing the

branched Me3SiO1/2- groups. In addition, the introduction of longer alkyl groups into the siloxane hydrophobic groups can result in remarkably increasing the γCMC values, and the Amin values decrease about 3.5 Å with each increasing methylene group. Different with trisiloxane surfactant of unique “T” or “umbrella” molecular architecture, three “linear” trisiloxane surfactants coupled to different terminal groups (CH3-, ClCH2-, CF3-) and two polyether hydrophilic tails were synthesized. Their aggregation behavior in aqueous solution were investigated by surface tension, DLS, FF-TEM and TEM. Our aim is to investigate the effect of the terminal groups (CH3-, ClCH2-, CF3-) and two polyether hydrophilic groups on the their micellization in aqueous solution, which help us better understand the role of the molecular structures of siloxane hydrophobic groups in affecting the aggregation behavior of trisiloxane surfactants.


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Experimental Section Materials γ- Chloropropyldichloromethylsilane, propyldichloromethylsilane were provided by Jiangxi Chenguang New Materials Co., Ltd. Poly(ethylene glycol) 400 methyl ether thiol was obtained from Changsha Flying-Brid Bio-Tech Co., Ltd. Dimethylvinylchlorosilane, 3,3,3-trifluoropropyldichloromethylsilane, methanol, and isopropanol were purchased from Tokyo Chemical Industry Co., Ltd. All reagents were used as received. The structures of the three “linear” trisiloxane surfactants used in this work are shown in Figure 1.

Apparatus and Procedures 1H NMR,

13C

NMR,

19F

NMR, and

29Si

NMR spectra

were recorded using a Bruker AV 300 spectrometer. The samples were prepared in chloroform-d (CDCl3) without internal standard. FT-IR was carried out on a VERTEX 70 FT-IR spectrometer and the sample was dispersed in anhydrous KBr pellets. Surface tension was determined by the ring method using a model BZY-1 tensiometer (Shanghai Fangrui Instrument Co., Ltd.). The temperature (25 ± 0.1 °C) was controlled by a thermostatic bath (DC-5070, Shanghai Hengping Instrument Co., Ltd.). All measurements were repeated at least twice until the values were reproducible. Calibration was performed using a liquid with known values of surface tension. Dynamic light scattering (DLS) was conducted to study the size distribution of aggregation using a Dynapro Titan system (Wyatt Technology, Santa, Barbara, CA). The scattering angle was 90o. The solutions were pre-equilibrated for 12 h and filtrated using 0.20 μm Teflon filter before the measurements. The morphologies of aggregates in aqueous solution were recorded on a JEM-1011 TEM (JEOL, Japan) at 100 kV. A drop of “linear” trisiloxane surfactant solution was



loaded onto a carbon coated grid, following by stained with phosphotungstic acid solution (2 wt%), and then the specimens were dried at room temperature. FF-TEM observation was performed with a JEM-100CX II transmission electron microscope operating at 100 kV. Samples were immersed rapidly into the liquid ethane cooled by the liquid nitrogen. The samples were fractured and replicated in the freezeetching apparatus (Balzers BAF-400D).

Preparation of 3,3,3-trifluoropropyl di(dimethylvinylsiloxy)methylsilane. It was synthesized by the modified process of our work.

18

Chlorodimethylvinylsilane (0.3

mol, 36.19 g) and 3,3,3-trifluoropropyldichloromethylsilane (0.05 mol, 10.55 g) was mixed into a flask and then isopropanol (0.45 mol, 27.0 g) was added. After that, water (0.45 mol, 8.1 g) was slowly added into the resulting reaction. Finally, the organic layer was separated by separating funnel and washed until it was neutral. The product was obtained by distilling in vacuum and confirmed by 1H NMR, 13CNMR, 29Si NMR, 19F NMR, and FT-IR. 1H NMR (CDCl3): δ (ppm) = 0.10~0.19 (SiCH3, 15 H), 0.70~0.74 (SiCH2CH2, 2 H), 2.01~2.09 (SiCH2CH2, 2 H), 5.74~5.80 and 6.10~6.19 (CH2=CH, 4 H), 5.97~6.01 (CH2=CH, 2 H); 13C NMR (CDCl3): δ (ppm) = 139.2 (CH2=CH), 131.4 (CH2=CH), 125.8(SiCH2CH2CF3), 28.5, 28.1, 27.7, 27.3 (CH2CH2CF3), 9.1 (CH2CH2CF3), 0.9 ((SiCH3)2), 0.1 (SiCH3);

29Si

NMR (CDCl3): δ (ppm) = -2.98

((SiCH3)2), -22.97 (SiCH3); 19F NMR (CDCl3): δ (ppm) = -68.8; FT-IR (KBr, cm-1): 3054 v (CH2=CH), 2961 v (CH3), 2905 v (CH2), 1261 v (Si-CH3), 1065 ν (Si-O-Si), 837 v (Si-CH3). Synthesis of propyldi(dimethylvinylsiloxy)methylsilane, and γ-chloropropyldi(dimethylvinylsiloxy)methylsilane. The two siloxanes were synthesized and characterized by analogous procedure to γ-chloropropyldi(dimethylvinylsiloxy)-


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methylsilane using propyldichloromethylsilane, γ-chloropropyldichloromethylsilane to displace 3,3,3-trifluoropropyldichloromethylsilane, respectively. Propyldi(dimethylvinylsiloxy)methylsilane 1H NMR (CDCl3): δ (ppm) = 0.03~0.11 (SiCH3, 15 H), 0.35~0.36 (SiCH2CH2, 2 H), 0.76~0.81 (SiCH2CH2CH3, 3 H), 1.15~1.27 (SiCH2CH2, 2 H), 5.54~5.61 and 5.92~6.03 (CH2=CH, 4 H), 5.75~5.81 (CH2=CH, 2 H);

13C

NMR (CDCl3): δ (ppm) = 139.1 (CH2=CH), 131.4 (CH2=CH),

19.90 (CH2CH2CH3), 17.6 (CH2CH2CH3), 16.2 (CH2CH2CH3), 0.9 ((SiCH3)2), 0.1(SiCH3);

29Si

NMR (CDCl3): δ (ppm) = -4.43 ((SiCH3)2), -22.85 (SiCH3); FT-IR

(KBr, cm-1): 3052 v (CH2=CH), 2959 v (CH3), 2873 v (CH2), 1256 v (Si-CH3), 1043 ν (Si-O-Si), 838 v (Si-CH3). γ-Chloropropyldi(dimethylvinylsiloxy)methylsilane 1H NMR (CDCl3): δ (ppm) = 0.05~0.19 (SiCH3, 18 H), 0.57~0.62 (SiCH2CH2CH2, 2 H), 1.74~1.84 (SiCH2CH2CH2, 2 H), 3.47~3.54 (SiCH2CH2CH2, 2 H), 5.68~5.79 and 6.07~6.18 (CH2=CH, 4 H), 5.90~5.98 (CH2=CH, 2 H);

13C

NMR (CDCl3): δ (ppm) = 139.2 (CH2=CH), 131.5

(CH2=CH), 47.3 (CH2CH2CH2Cl), 26.5 (CH2CH2CH2Cl), 14.3 (CH2CH2CH2Cl), 0.9 ((SiCH3)3), 0.2 (SiCH3); 29Si NMR (CDCl3): δ (ppm) =-3.69 ((SiCH3)2), -21.73(SiCH3); FT-IR (KBr, cm-1): 3051 v (CH2=CH), 2958 v (CH3), 2901 v (CH2), 1258 v (Si-CH3), 1044 ν (Si-O-Si), 838 v (Si-CH3). Preparation of F-Si3-EO8, Cl-Si3-EO8, and Me-Si3-EO8 Three “linear” trisiloxane surfactants were synthesized and confirmed by the modified process of our work. 37 F-Si3-EO8 3,3,3-trifluoropropyldi(dimethylvinylsiloxy)methylsilane (0.69 g, 2.0 mmol), poly(ethylene glycol) 400 methyl ether thiol (1.68 g, 4.2 mmol), 2,2dimethoxy-2-phenylacetophenone (42 mg, 0.16 mmol), and THF (10 mL) were added into a sealed flask. Then, the resulting mixture was irradiated with UV light for 3.5 h



under N2 atmosphere. The solvent was removed by distilling in vacuum, and the resultant residue purified by column chromatography (silica, 20% ethyl acetate/ 80% hexane). The pure F-Si3-EO8 was confirmed by 1H NMR, 13C NMR, 29Si NMR, and FT-IR. 1H NMR (CDCl3): δ (ppm) = 0.00~0.05 (SiCH3, 15H), 0.55~0.59 (CH2CH2CF3, 2H), 0.77~0.81 (SiCH2CH2S, 4H), 1.88~1.95 (CH2CH2CF3, 2H), 2.45~2.49 (SiCH2CH2S, 4H), 2.58~2.61(SCH2CH2O, 4H), 3.25 (OCH3, 6H), 3.41~3.43 (SCH2CH2O, 4H), 3.51~3.53 (OCH2CH2O, ~48H); 13C NMR (CDCl3): δ (ppm) = 0.00 (SiCH3) 0.93 (Si(CH3)2), 8.96(CH2CH2CF3), 18.69 (SiCH2CH2S), 26.99 (SiCH2CH2S), 31.03 (CH2CH2CF3), 38.14 (SCH2CH2O 58.75 (OCH3), 69.37, 70.01, 70.23, 70.30, 70.33 (OCH2CH2O), 71.66 (SCH2CH2O);

29Si

NMR (CDCl3): δ (ppm) = -2.92

((SiCH3)2), -21.85(SiCH3); FT-IR (KBr, cm-1): 2962 v (CH3), 2871 v (CH2), 1260 v (SiCH3), 1110 ν (Si-O-Si), 1065 ν (C-O-C), 842 v (Si-CH3), 795 v (Si-CH3).

Cl-Si3-EO8 and Me-Si3-EO8 were synthesized and characterized by analogous procedure to Si3-F-EO8. Cl-Si3-EO8 1H NMR (CDCl3): δ (ppm) = 0.08~0.05 (SiCH3, 15H), 0.44~0.49 (CH2CH2CH2Cl, 2H), 0.77~0.81 (SiCH2CH2S, 4H), 1.63~1.67 (CH2CH2CH2Cl, 2H), 2.46~2.50 (SiCH2CH2S, 4H), 2.59~2.62(SCH2CH2O, 4H), 3.26 (OCH3, 6H), 3.37~3.40 (CH2CH2CH2Cl, 2H), 3.42~3.44 (SCH2CH2O, 4H), 3.52~3.54 (OCH2CH2O, ~48H);

13C

NMR (CDCl3): δ (ppm) = -0.55 (SiCH3) 0.00 (Si(CH3)2), 14.78

(CH2CH2CH2Cl), 18.77 (SiCH2CH2S), 27.02 (SiCH2CH2S), 30.95 (CH2CH2CH2Cl), 38.10 (SCH2CH2O), 47.32 (CH2CH2CH2Cl), 58.72 (OCH3), 69.33, 69.98, 70.20, 70.27, 70.30 (OCH2CH2O), 71.63 (SCH2CH2O);

29Si

NMR (CDCl3): δ (ppm) = -3.05

((SiCH3)2), -22.53 (SiCH3); FT-IR (KBr, cm-1): 2961 v (CH3), 2874 v (CH2), 1257 v (Si-CH3), 1106 ν (Si-O-Si), 1045 ν (C-O-C), 842 v (Si-CH3), 792 v (Si-CH3).


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Me-Si3-EO8 1H NMR (CDCl3): δ (ppm) = 0.00~0.06 (SiCH3, 15H), 0.43~0.51 (CH2CH2CH2Cl, 2H), 0.80~0.82 (SiCH2CH2S, 4H), 0.87~0.91 (CH2CH2CH3, 3H), 1.62~1.68 (CH2CH2CH3, 2H), 2.45~2.49 (SiCH2CH2S, 4H), 2.58~2.63 (SCH2CH2O, 4H), 3.32 (OCH3, 6H), 3.47~3.50 (SCH2CH2O, 4H), 3.57~3.59 (OCH2CH2O, 48H); 13C

NMR (CDCl3): δ (ppm) = -0.60 (SiCH3) 0.00 (Si(CH3)2), 14.82 (CH2CH2CH2Cl),

16.32 (CH2CH2CH3), 19.02 (SiCH2CH2S), 30.52 (SiCH2CH2S), 32.63 (CH2CH2CH3), 38.10 (SCH2CH2O), 59.03 (OCH3), 69.64, 70.40, 70.52, 70.56 70.63 (OCH2CH2O), 71.94 (SCH2CH2O);

29Si

NMR (CDCl3): δ (ppm) =-3.43 ((SiCH3)2), -22.21(SiCH3);

FT-IR (KBr, cm-1): 2960 v (CH3), 2876 v (CH2), 1260 v (Si-CH3), 1118 ν (Si-O-Si), 1085 ν (C-O-C), 843 v (Si-CH3), 795 v (Si-CH3).

Results and Discussion Surface tension properties Surface tension of the polyether based “linear” trisiloxane surfactants, Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8, were measured to evaluate their surface activities, and the surface tension isotherms of Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8 were shown in Figure 2. As seen in Figure 2, the surface tension of Me-Si3-EO8, Cl-Si3-EO8, and FSi3-EO8 decrease initially with an increasing their concentration, and then reach a plateau region implying the micelles have been formed. From the surface tension isotherms, accordingly, the critical micelle concentrations (CMC, concentration of the break point) were obtained by extrapolation and listed in Table 1. Notably, the surface tension isotherms display unusual two transition points. The first transition point can be known as pre-micellization which has also been reported by Du et, al., and the second transition point with higher concentration is commonly considered as CMC.



19,20,30,35

Page 10 of 25

As shown in Table 1, the surface tension at CMC (γCMC) value of Me-Si3-EO8

was 32.8 mN∙m-1, which was higher than the γCMC values of permethylated siloxane surfactants, such as bis(butynediol ethoxylate) terminated polysiloxane (25~27 mN∙m1),

32

permethylated trisiloxne surfactants (∼20 mN∙m-1). It was believed that

permethylated trisiloxane surfactants with unique “T” or “umbrella” shape can effectively reduce surface tensions of water to ~20 mN∙m-1 resulting from the wider and shorter trisiloxane portion oriented at the air/water interface with an “umbrella” conformation and packed closely with methyl groups.

6,16,17

However, Me-Si3-EO8,

which possesses three CH3Si(R)O1/2- units (D) grafting with two polyethylene glycol chain (hydrophilic chains), cannot form an “umbrella” conformation similar to trisiloxane with “T” shape (Figure 3). Consequently, Me-Si3-EO8 promote films with a greater proportion of -CH2- groups (higher surface energy) caused by the propyl and two CH2CH2SCH2CH2 (spacer), inevitably, result in a higher γCMC values. Meanwhile, the γCMC and CMC values decrease in the following order, Me-Si3-EO8 > Cl-Si3-EO8 > F-Si3-EO8. The replacement of the terminal H atom of propyl groups for Cl atom obviously increases the hydrophobicity of Cl-Si3-EO8, and directly decreases the γCMC and CMC values compared with Me-Si3-EO8. It can be explained that chlorine is more electronegative and smaller polarizability than hydrogen, as a consequence, the total dispersion interaction is lower for the interaction between chlorine atoms. Chloropropyl group is expected to have weaker attractive intermolecular forces than propyl group. Moreover, the greater molecular volume of perfluoromethyl moiety compared with CH3- and ClCH2- makes 3,3,3-trifluoropropyl groups of F-Si3-EO8 more hydrophobic. Hence, F-Si3-EO8 provides the lower γCMC and CMC values attributed to the weaker intermolecular force and the larger molecular volume of perfluoromethyl terminal group. 2-4,6


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From the plots of surface tension versus concentration, several additional parameters, the maximum surface excess (Γmax), minimum surface area per surfactant molecule (Amin), the adsorption efficiency (pC20), and surface pressure at CMC (πCMC) can also be obtained by eq 1-4 38-39 and listed in Table 1. 1

Γ𝑚𝑎𝑥 = ― 2.303𝑅𝑇 𝐴𝑚𝑖𝑛 =

(

∂𝛾 ∂𝑙𝑜𝑔𝐶

)

(1)

1016

(2)

𝑁𝐴Γ𝑚𝑎𝑥

(3)

𝑝𝐶20 = ― log 𝐶20

(4)

𝜋𝐶𝑀𝐶 = 𝛾0 ― 𝛾𝐶𝑀𝐶

where γ is the surface tension, R is the ideal gas constant, T is the absolute temperature, NA is Avogadro’s number, C20 is the concentration required to reduce the surface tension of pure water by 20 mN∙m-1, γ0 is the surface tension of pure water. Generally, a higher value of Γmax or lower value of Αmin suggested a denser arrangement of trisiloxane surfactant molecules at the air/water interface. 37-38 As shown in Table 1, the Amin values of Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8 were 77.6, 86.1, and 95.5 Å2, respectively, which were higher than that of polyether based trisiloxane surfactant, such as, Si3EO8OH (32.6 Å2), Si3EO10-OH (39.3 Å2), and Si3EO7.5-OMe (68.0 Å2). 22 The introduction of two polyether hydrophilic groups in the terminal position of trisiloxane hydrophobic part tends to lie flat in the air/water interface and results in an increasing Amin value. As illustrated in Figure 3, the Si-O-Si chain of trisiloxane surfactants with peculiar molecular “T” shape serves as a flexible framework and adopts a variety of configurations to present methyl with their best advantage and forms denser adsorbed films at air/water interface. Me-Si3-EO8 has flexible Si-O-Si chain and adopts configurations their best advantage. However, the adsorbed layer at air/water interface was similar to the long chain hydrocarbon surfactants attributed to the introduction of



Page 12 of 25

propyl and CH2CH2SCH2CH2 groups. It was noted that the adsorbed layer was dominated by a greater proportion of higher-surface-energy methylene groups owing to the propyl and CH2CH2SCH2CH2 groups which was the main factor to be higher γCMC and Amin values compared with the trisiloxane surfactants with “umbrella” shape. 6, 16

As comparison, the Amin value of Cl-Si3-EO8 is larger than that of Me-Si3-EO8 owing to the bigger size of chlorine atom (ClCH2-) in comparison to hydrogen atom (CH3-). In addition, the greater molecular volume of CF3 group compared with CH3 and ClCH2- makes the Amin values of F-Si3-EO8 to be higher. Moreover, the larger are the values of pC20 and πCMC for F-Si3-EO8, the higher are the adsorption efficiency and effectiveness, which was caused by the more hydrophobicity and weaker van der Waals interaction of 3,3,3-trifluoropropyl. Furthermore, the CMC/C20 values can be used to evaluate the tendency of micellization compared with their adsorption. Generally, a larger CMC/C20 values indicated surfactant can adsorb more easily at the air/water interfaces compared to their preference to form aggregates. As seen in Table 1, the CMC/C20 values of Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8 were 27.9, 55.5, and 67.0, respectively. It was worth notice that the CMC/C20 values increase following the order, Me-Si3-EO8 < Cl-Si3-EO8 < FSi3-EO8, owing to the increasing in the size of the hydrophobic groups (CH3, ClCH2, and CF3). It also indicated that F-Si3-EO8 adsorbed at the air-water interface more easily relative to its preference to form aggregates than do Me-Si3-EO8, and Cl-Si3-EO8. In order to further understand the effect of terminal group (CH3-, ClCH2-, and CF3-) on the micellization of polyether based “linear” trisiloxane surfactants, the standard Gibbs energy change ( Gm ) and the standard free energy of adsorption ( Gads ) at the 0

0

air/water interface can be obtained from the eq 5-6 38-39 and listed in Table 1.


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∆𝐺0𝑚 = 𝑅𝑇ln

( ) 𝐶𝑀𝐶

(5)

55.5

𝜋𝐶𝑀𝐶

∆𝐺0𝑎𝑑𝑠 = ∆𝐺0𝑚 ― 𝛤𝑚𝑎𝑥

(6)

As shown in Table 1, the values of Gm , and Gads of the investigated “linear” 0

0

trisiloxane surfactants are all negative, indicating the micellization process is spontaneous. The values of Gads are higher than that of 0

Gm0 , revealing the

adsorption process of Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8 is comparatively stronger than their micellization process.

Aggregation properties The hydrodynamic diameters distribution of the aggregates for Me-Si3-EO8, Cl-Si3EO8, and F-Si3-EO8 in aqueous solution were performed by DLS and their ultrastructure were observed by TEM and FF-TEM. As shown in Figure 4, the aggregates were shown to have monomodal for Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8. The aggregates of MeSi3-EO8, Cl-Si3-EO8, and F-Si3-EO8 have average diameters of 38.2, 108.9, and 132.1 nm, respectively. Generally, in aqueous solution, the aggregates of surfactants were formed mostly by hydrophobic interactions. Meanwhile, compared with hydrocarbon chains of hydrocarbon surfactants and trisiloxane surfactants with “T” shape, the “linear” trisiloxane groups were bulkier and stiffer which caused the formation of larger aggregates with lower curvature. Owing to the aggregates diameter increased with the increasing the hydrophobicity of trisiloxane groups which can reveal a stronger ability to facilitate the formation of larger aggregates, the sizes of aggregates follow the order, Me-Si3-EO8 < Cl-Si3-EO8 < F-Si3-EO8. 19,20,30,35 The TEM micrographs in Figures 5 (a-c) clearly show the existence of aggregates with non-uniform size. Meanwhile, the size of aggregates observed by TEM were smaller



Page 14 of 25

than the results obtained by DLS which were the swollen state in aqueous solutions. It was caused by the loss of water when the samples were observed by TEM.

19,20,30,35

However, the size distribution corroborated the observations from the DLS and FFTEM. FF-TEM is a powerful method to characterize the aggregates microstructure selfassembled by the polyether based “linear” trisiloxane surfactants. Figure 5(d-f) shows a typical FF-TEM image of Me-Si3-EO8 (Figure 5-d), Cl-Si3-EO8 (Figure 5-e), and FSi3-EO8 (Figure 5-f) above the CMC. It is clear that irregular aggregates and some larger cluster aggregates formed by the fusion of independent irregular aggregates were observed. 40 The TEM and FF-TEM results demonstrated that Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8 can form non-uniform size of spheroidal aggregates in aqueous solutions at 10-3 mol∙L-1.

Conclusions Three novel polyether based “linear” trisiloxane surfactants, different terminal groups (CH3-, ClCH2-, CF3-) located at the medium position of trisiloxane and the hydrophilic tails coupled to the terminal positions of trisiloxane, were successfully synthesized. The micellization behavior of the polyether based “linear” trisiloxane surfactants in aqueous solution were investigated by surface tension, DLS, TEM, and FF-TEM. The results shown that the aggregation behavior was influenced by the position of hydrophilic groups and terminal groups (CH3-, ClCH2-, CF3-). Owing to a greater proportion of higher surface energy -CH2- groups caused by the introduction of two CH2CH2SCH2CH2 groups, polyether based “linear” trisiloxane surfactants (Me-Si3EO8, Cl-Si3-EO8, and F-Si3-EO8) result in the larger γCMC and Amin values compared with the trisloxane surfactant with unique “T” shape. The γCMC values decrease following the order, Me-Si3-EO8 > Cl-Si3-EO8 > F-Si3-EO8, with an increasing the


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intermolecular force and molecular volume of terminal groups (CH3- < ClCH2- < CF3-), while, the the ΠCMC, pC20, CMC/C20, and Amin values increase following the order, MeSi3-EO8 < Cl-Si3-EO8 < F-Si3-EO8. In comparison to Me-Si3-EO8 and Cl-Si3-EO8, fluorinated trisiloxane surfactant (F-Si3-EO8) has greater surface activity attributed to the terminal CF3- group. All the investigated polyether based “linear” trisiloxane surfactants can form non-uniform size spherical aggregates in aqueous solution and confirmed by TEM and FF-TEM.

Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21563016, and 21862009).

Compliance with ethical standards Conflict of interest We have no conflict of interest.



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Figure Captains Figure 1 Chemical structures of Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8 Figure 2 Surface tension of Me-Si3-EO8 (▲), Cl-Si3-EO8 (●), and F-Si3-EO8 (◼) in aqueous solutions as a function of their concentrations at 25 o C Figure 3 Schematic comparison of the adsorption for trisiloxane surfactants, Me-Si3EO8 and hydrocarbon surfactants at air/water interface Figure 4 Size distributions of Me-Si3-EO8 (▲), Cl-Si3-EO8 (●), and F-Si3-EO8 (◼) in aqueous solution, concentration is 10-3 M Figure 5 TEM images of aggregates of Me-Si3-EO8 (a), Cl-Si3-EO8 (b), and F-Si3-EO8 (c), FF-TEM images of aggregates of Me-Si3-EO8 (d), Cl-Si3-EO8 (e), and F-Si3-EO8 (f), concentration is 10-3 M



Figure 1

Figure 2


Page 22 of 25

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Figure 3

Figure 4



Page 24 of 25

Figure 5

Table 1 CMC and Adsorption Parameters of Me-Si3-EO8, Cl-Si3-EO8, and F-Si3-EO8 in Aqueous Solutions at 25 o C Γmax

Αmin

CMC

∆G0m

∆G0ads

(μmol∙m-2)

(Å2)

/C20

kJ/mol

kJ/mol

5.24

2.14

77.6

27.9

-31.6

-50.2

41.8

5.67

1.93

86.1

55.5

-32.3

-54.0

45.8

5.84

1.74

95.5

67.0

-32.9

-59.2

CMC/

γCMC

Πcmc

10-4 M

(mN∙m-1)

(mN∙m-1)

Me-Si3-EO8

1.61

32.8

39.7

Cl-Si3-EO8

1.19

30.7

F-Si3-EO8

0.97

26.7

pC20


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