Synthesis, characterization and physicochemical properties of glycosyl-modified polysiloxane

Journal of Molecular Liquids 266 (2018) 90–98

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Synthesis, characterization and physicochemical properties of glycosyl-modified polysiloxane Yanyun Bai a, Huijun Liu b, Xiaoyuan Ma a, Xiumei Tai a, Wanxu Wang a, Zhiping Du a, Guoyong Wang a,⁎ a b

China Research Institute of Daily Chemical Industry, Taiyuan 030001, PR China Department of Chemistry, Shanxi Datong University, Datong, Shanxi 037000, PR China

a r t i c l e

i n f o

Article history: Received 3 May 2018 Received in revised form 12 June 2018 Accepted 12 June 2018 Available online 18 June 2018 Keywords: Glycosyl-modified polysiloxane Biosurfactants Surface activities Aggregation behaviors

a b s t r a c t Four glycosyl-modified polysiloxane (GPSO) biosurfactants were designed and synthesized via two-step reactions using green materials. The physicochemical properties: surface activity and aggregation behaviors in aqueous solution were systematically investigated. Investigation of the surface tension indicates that GPSO have relatively low critical aggregation concentrations (CAC) and surface tensions at CAC (γCAC) compared with hydrocarbon surfactant. Adsorption at the air-water interface and the formation of micelles in aqueous are spontaneity and the tendencies of adsorption are favored over the process of micellization. Dynamic surface tension results indicate that D-glucoheptono-1, 4-lactone modified polysiloxanes (GHL@Si) have good diffusion ability than that of glucono-δ-lactone modified polysiloxanes (GL@Si). Transmission electron microscopy (TEM) and Dynamic light scattering (DLS) results indicate that part of biosurfactants can form spherical aggregates. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Compared with the carbon chain polymers, polymers with polysiloxane chain possess remarkable properties such as flexibility, low surface energy, biocompatibility and low toxicity, which can be used as bio-materials raw material. Modified polysiloxanes polymers are widely used in the fields of coating, adhesives, impact-resistance plastic, etc. [1, 2]. Amphiphilic polysiloxane surfactants, as a kind of modified polysiloxanes polymers, have been used as emulsifiers and spreading agents, wetting agents, anti-foaming agents and agricultural adjuvant due to its good emulsifying wettability properties and surface activity both in aqueous and organic solvent [3, 4]. Compared with conventional carbon chain surfactants, surfactants structured with polysiloxanes chains, are effective in reducing the surface tension in both aqueous and nonaqueous media due to a lower intermolecular cohesive force and the greater flexibility of polysiloxanes chains [5–7]. And such surfactants have been extensively used as emulsifiers, spreading agents, wetting agents and antifoaming agents [8]. In the past few years, many kinds of polysiloxane surfactants have been synthesized by hydrosilylation reaction. Dae-won Chung synthesized series of polydimethylsiloxane grafted with polyethyleneoxide (PDMS-g-PEO) by two-step reactions and the surface properties of PDMS-g-PEO were investigated by comparing the each effect caused by EO content [9]. M.M.A. EL-Sukkary successfully synthesized three ⁎ Corresponding author at: China Research Institute of Daily Chemical Industry, 34 Wenyuan Street, Taiyuan, Shanxi Province 030001, PR China. E-mail address: [email protected] (G. Wang).

https://doi.org/10.1016/j.molliq.2018.06.052 0167-7322/© 2018 Elsevier B.V. All rights reserved.

series of amino-grafted polysiloxane surfactants. And their surface activities, aggregation behavior in aqueous solution, interfacial tension, foaming power, emulsification power, surface parameters, micellization and adsorption in liquid/air interfaces thermodynamics have been investigated [10]. Polysiloxane grafted polyamide-amine surfactants also have been prepared by grafting the single epoxy terminated polydimethylsiloxane onto the dendritic polyamide-amine. Furthermore, the stability, surface activity and emulsifying ability of polysiloxane grafted polyamide-amine surfactants were studied [11]. Polyglycerol modified polysiloxane surfactants [12] and butynediol-ethoxylate modified polysiloxanes [13, 14] have also been synthesized and their physicochemical properties have been studied systematically. With the increase of surfactants usage and deterioration the ecosystem, green and environmental friendly surfactants which are made of natural renewable materials, have aroused the world's attention and become succedaneums of conventional surfactants in the last decade [15]. Green surfactants structured with carbon chains, such as alkyl polyglycoside (APG), have been widely used in detergent, cosmetics, and agriculture surfactants [16]. As a kind of green biologic carbohydrate material, lactones (such as glucono-δ-lactone) can be used to prepare carbohydrate-based surfactants. However, researches and application polysiloxanes surfactants made from renewable raw material are relatively scarce [17, 18]. In this work, green biologic ingredients glucono-δ-lactone (GL) and D-glucoheptono-1,4-lactone (GHL) were used to synthesize glycosylmodified polysiloxanes green biosurfactants. The structures of biosurfactants were confirmed by FT-IR, 1HNMR, 29Si NMR and GPC. The effects of structure of biosurfactants on the surface properties and

Y. Bai et al. / Journal of Molecular Liquids 266 (2018) 90–98

91

Fig. 1. Synthetic route of GPSO.

aggregation behavior in aqueous solution were investigated by means of surface tension measurements, dynamic light scattering (DLS) and transmission electron microscopy (TEM). 2. Materials and methods 2.1. Materials Octamethylcyclotetrasiloxane (D4) was purchased from Shenzhen Osbang New Material Co. Ltd. (China), 3-aminopropyl-methyldiethoxysilane (APMDS) was obtained from Shanghai Molbase Biotechnology Co. Ltd. (China). Hexamethyldisiloxane (HMDS) was obtained from Hangzhou Guibao Chemical Co. Ltd. (China), Tetramethylammonium hydroxide (TMAH) and glucono-δ-lactone (GL) was purchased from Aladdin (China), D-glucoheptono-1, 4lactone (GHL) was obtained from Sigma-Aldric. D4 and APMDS were purified by reduced pressure distillation before used, and other chemical reagents were used as received. 2.2. Synthesis of GPSO The synthesis procedure of GPSO involves two steps. Initially the intermediate aminopropyl polysiloxanes (APSO) is formed by polymerization of D4 with APMDS and HMDS as shown in Fig. 1 (S. 1). The

reaction effective concentration of catalyzer TMAH was 1%. TMAH was used in the form of tetramethylammonium hydroxide silicate salt (1%), which was carried out at 80 °C by D4 and under a pressure reducing condition. The reactions mixture of APSO, APMDS, D4 and catalyzer was stirred for 9 h at 90 °C under nitrogen atmosphere. The catalyzer was inactivated at 140 °C after polymerization. The purification of APSO was performed at 160 °C under a pressure reducing condition to eliminate unreacted materials and low weight oligomers. Two APSO products (APSO-1 and APSO-2) with different feeding ratio (listed in Table 1) were obtained. APSO was then reacted with lactone (GL or GHL) using methanol as solvent at refluxes temperature for 12 h as shown in Fig. 1 (S. 2). The reactants were taken out in a 1:1 M ratio (lactone and amine content). Four GPSO were obtained after distillation and evaporation of the solvent: GL modified APSO-1 (GL@Si-1), GHL modified

Table 1 Effect of the feeding ratio of n (D4): n (APMDS) on the structure of APSO. APSO

Feed ratio D4: APMDS

Mn (expected)

APSO-1 APSO-2

1:2 1:4

628 862

GPC result

Amine content (mmol/g)

Mn

Mw

Expected

Actual

851 936

886 1074

2.89 4.32

3.76 5.06

92

Y. Bai et al. / Journal of Molecular Liquids 266 (2018) 90–98

2.3.2. Nuclear magnetic resonance (1HNMR) spectrum The nuclear magnetic resonance (NMR) (1H and 29Si) spectrum was performed on a Bruker nuclear magnetic resonance instrument (400 MHz) using Deuterochloroform (CDCl3) as solvent.

2.3.3. Molecular weight of APSO The molecular weight of APSO was determined by GPC. THF was used as eluent and the flow rate was 1 mL/min. The concentration of APSO was 0.3%.

2.3.4. The surfactivity of GPSO The surface tension of GPSO, which was correlated to the force raising the ring from the surface of the air-liquid interface, was measured using the Wilhelmy plate method using Sigma 700 surface tension meter (Biolin, Sweden) with the continuous method at 25 ± 0.1 °C. The platinum ring was washed with alcohol and distilled water and flame-dried prior to each measurement. All the solution samples were prepared with distilled water. Fig. 2. FT-IR spectra of GL, APSO-1 and GL@Si-1.

APSO-1 (GHL@Si-1), GL modified APSO-2(GL@Si-2) and GHL modified APSO-2 (GHL@Si-2).

2.3.5. Dynamic surface tensions measurement The dynamic surface tensions of GPSO were carried out on a Krüss BP100 bubble-pressure tensiometer (Krüss Company, Germany) at 25 ± 0.1 °C. The effective surface ages were running in ranging from 10 to 200,000 ms.

2.3. Characterization and methods 2.3.1. Fourier transform infrared analysis spectroscopy (FT-IR) The FT-IR spectra analysis (Vertex70, Bruker) was carried out to characterize the APSO and GPSO in the mid-IR region ranging from 500 cm−1 to 4000 cm−1. APSO samples were dropped directly onto a KBr plate and smeared evenly to characterize the structure. GPSO were mixed with pre-dried KBr and the mixture was pressed into disks to characterize the structure of surfactants.

2.3.6. Dynamic light scattering measurement A Zeta Plus Particle Size Analyzer (Brookhaven, USA) apparatus was used to investigate the effective diameter and size distribution of surfactants aggregates. The concentrations of solutions were 500 mg/L and 1000 mg/L. Filtration method was used to eliminate dust particles present in the solution. All the analyses were performed with a scattering angle of 90° at 25 ± 0.1 °C.

Fig. 3. 1H NMR spectra of APSO-1 and GHL@Si-1.

Y. Bai et al. / Journal of Molecular Liquids 266 (2018) 90–98

2.3.7. Transmission electron microscopy (TEM) TEM is a kind of direct-vision method to show the aggregation behavior when the concentrations of GPSO surfactant solutions were above the CAC. The concentrations were 1 g/L. Several drops of solutions were dropped net and negative stained with 1.5% phosphotungstic acid on a copper. Samples were dried on the carbon-coated copper grids and TEM images were recorded on a JEM-1011 type microscopy (JEOL) operated at 100 kV.

3. Result and discussion 3.1. Synthesis and characterization of GPSO As shown in Fig. 1 (S. 1), two different polysiloxanes with aminopropyl intermediates (APSO-1 and APSO-2) were synthesized by polymerization of D4 with APMDS and HMDS. Feeding ratio, amine content (determined as reported earlier) [19] and molecular weights (obtained by GPC) [20] for APSO-1 and APSO-2 were list in Table 1. As listed in Table 1, the amine and molecular weights of APSO have an increasing trend with the increasing of feeding APMDS. The chemical structure of APSO was confirmed by FT-IR, 1H NMR, 29Si NMR spectra results. Fig. 2 shows the FT-IR of APSO-1 (take APSO-1 as the example but not limit to) synthesized in the laboratory. The adsorption peaks near

GL@Si-1 GHL@Si-1

surface tension/(mN• m-1)

60

3380 and 1620 cm−1 are attributed to stretching and bending vibrations of\\NH2 groups. Stretching and bending vibrations of alkanes group are displayed by adsorption bands close to 2930 cm−1 and 1450 cm−1. Peaks in the spectral regions near 1260 cm−1 and 800 cm−1, which is the stretching and bending vibrations of Si\\C, signify the presence of Si\\C groups. The adsorption peaks near 1090 and 1020 cm−1 are attributed to the symmetrical and asymmetrical stretching vibrations of Si\\O\\Si groups. FT-IR analysis shows that D4 and APMDS are successfully copolymerized. Furthermore, synthesized APSO were then characterized by 1H NMR spectrum in CDCl3. For simplicity of analysis, the positions in chemical structure of APSO are numbered (a, b, c, and so on) corresponding to chemical shifts as shown in Fig. 3. The presence of characteristic signal (showing a triplet) at chemical shift around δ = 0.08 ppm is cumulative influence of the terminal methyl protons at the ends. The resonance signals at around δ = 2.66 ppm, 1.46 ppm, 0.49 ppm are due to the presence of\\CH groups attached on the side of APSO. The signal found at around δ = 1.23 ppm is assigned to the \\NH2. 29Si NMR spectrum was also used to characterize the structure of APSO. As shown in Fig. 4, three group chemical shifts were presented. The clustered signals at around δ = −(18–22) ppm, δ = −(12–14) ppm, δ = 7.4 ppm correspond to different Si nuclei as numbered (a, b, c) in Fig. 4. The above results of FT-IR, 1H NMR, and 29Si NMR spectra can corroborate that APSO were synthesized successfully as designed. A series of four glycols-modified polysiloxane biosurfactants were synthesized by means of amidation reaction in this study as shown in Fig. 1 (S. 2). APSO and lactone (GL or GHL) were stirred at reflux temperature for 12 h using methanol as solvent. The feeding molar ratio of APSO and Lactone was 1:1 (\\NH2: lactone). White powder GPSO was obtained after evaporation of the solvent, dried under reduced pressure and gently crushed. The chemical structures of GPSO were confirmed by FT-IR, 1H NMR, spectra results. Fig. 2 shows the FT-IR of GL modified APSO-1 (GL@Si-1) (take as the example but not limit to) synthesized in the laboratory. The disappearance of ester peak of GL at 1730 cm−1 and the formation of amide bond peak at 1650 and 1540 cm−1 prove that GL was successfully attached to APSO. Fig. 3 shows 1H NMR spectrum of GHL@Si-1. The new peaks located between δ = 3.60 and 4.30 ppm were attributed to protons of the GHL. Also, the signal δ = 2.66 ppm (d′) peak for\\N\\CH2 shifted to 3.31 ppm and δ = 2.66 ppm disappeared. The result indicates that GHL was attached to APSO successfully without unreacted amino groups [18]. 3.2. Surface activity of GPSO The surface activity of the GPSO in aqueous solution was evaluated by means of equilibrium surface tension. The surface tensions of four GPSO (GL@Si-1, GL@Si-2, GHL@Si-1 and GHL@Si-2) in different

GL@Si-2 GHL@Si-2

60

surface tension/(mN• m-1)

Fig. 4. 29Si NMR spectra of APSO-1.

93

50

50

40

40

30

30

20

1E-3

0.01

0.1

Concentration/(mmol• L-1)

1E-3

0.01

Fig. 5. Surface tensions (mN/m) vs. the log (mass concentration) (mg/L) of GPSO solutions at 25 ± 0.1 °C.

0.1

-1

94

Y. Bai et al. / Journal of Molecular Liquids 266 (2018) 90–98

Table 2 Parameters of aggregation and adsorption of GPSO solutions at 25 °C. GPSO

CAC (mmol·L−1)

γCAC (mN·m−1)

ΓCAC (mol·cm−1)

Amin (Å2/molecule)

ΔG 0mic (kJ∙mol−1)

ΔG0ads (kJ∙mol−1)

Standard error (ǝγ/ǝlogC)

GL@Si-1 GHL@Si-1 GL@Si-2 GHL@Si-2

0.095 0.111 0.062 0.078

27.33 29.20 25.22 26.30

4.40 × 10−10 3.90 × 10−10 4.77 × 10−10 3.78 × 10−10

37.67 42.59 34.82 43.95

−32.90 −32.51 −33.95 −33.37

−43.21 −43.69 −43.92 −45.67

1.59 1.83 2.18 2.88

PD20H80 [24]

N1 g·L−1

N55

–

–

–

–

concentrations are showed in Fig. 5. The γ of surfactant solution exhibits a continuous decrease to certain value and remain unchanged with the increase of concentration for typical low-molar-mass molecules surfactants. As depicted in Fig. 5, γ decreases rapidly with the increasing of GPSO concentration at low concentration, which is consistent with typical surfactants. For classical surfactant systems, γ is constant once the micelles are formed. Whereas as observed in previous study, γ is reported to have a continuous decrease after micellization for modified polysiloxane surfactants and two transition points are formed in the plots [21, 22]. In fact, continuous decreasing of the γ with concentration is attributed to the continuous but slow adsorption of polymeric surfactant onto surface [22]. In our case, surfactants with polysiloxane groups also show such a behavior. The first transition point can be explained by the formation of premicellar aggregates as suggested in previous studies [23]. The second transition point means the critical aggregation concentrations (CACs) of surfactants and surface tensions at CAC (γCAC) can be determined. Table 2 lists CACs, γCAC and parameters of aggregation and adsorption of four surfactants. It is observed that four glycosyl-modified polysiloxane surfactants exhibit relatively low CAC values of 0.062–0.111 mmol·L−1 and low γCAC values of 25.22–29.20 mN·m−1. But conventional glycosyl-modified carbon carbon amphiphilic copolymers have higher CAC and γCAC (above 55 mN·m−1) [24]. These results imply that GPSO are agglomerated into aggregates in solution and can significantly reduce the surface tension of water. The good surface activity may be attributed to favorable orientation of the branched polysiloxane moiety lying along the air-water interface exposing the highly surface active methyl groups to the air or the lower mutual attraction and high chain flexibility characteristic of methylsiloxane polymers [9, 13]. Table 2 lists the CAC and γCAC values of for surfactants. It can be seen that GL modified polysiloxane surfactants have lower CAC and γCAC values than that of GHL modified polysiloxane surfactants. This result may be attributed to the increasing of hydrophilic content in sugar. Nevertheless, glycosyl-modified APSO-2 surfactants have lower CAC and γCAC values than that of glycosyl-modified APSO-1. It appears likely

that the surface activity of polymeric surfactants is affected by many factors, including, but not limited to, polydispersity index, molecular structure, molecular weight, and solubility [9, 25]. Gibbs' law is typically used to study the adsorption and aggregation properties of surfactants at equilibrium. The saturation surface excess concentration (Γmax) and minimum area at the air/water interface (Amin) can be obtained from the Gibbs adsorption isotherm equations (Eqs. (1) and (2)). While the standard free energies of aggregation (ΔG0mic) and adsorption (ΔG0ads) can be estimated from Eqs. (3) and (4) [26]. Γmax ¼ −

Amin ¼


 1 ∂γ 2:303nRT ∂lgC

1016 NA Γmax

ð1Þ

ð2Þ




 CAC 55:5

ð3Þ

 Y Cπ −6:022 Amin 55:5

ð4Þ

ΔG0mic ¼ RTln
ΔG0ads ¼ RTln

where k can be regarded as 1, R is the gas constant, T is the absolute temperature, NA is Avogadro's number, γ0 is the surface tension of pure water, Π (=γ0 − γ) is the surface pressure in the region of surface saturation, and CΠ is the molar concentration of the surfactant at a surface pressure Π (mN·m−1). All the parameters listed in Table 2 are obtained through Eqs. (1)–(4). The Amin values of GHL@Si is large than GL@Si, suggesting that GHL@ Si pack more loosely at the air-water interface. Amin is determined by the occupied area of hydrophilic groups and higher content of hydrophilic groups makes the surfactants more prone to stretching and occupy more area thus increases the Amin. In the contrary, the Amin varies inversely with Γmax. The values of ΔG0mic and ΔG0ads are negative, which implies the spontaneity of adsorption at the air-water interface

Fig. 6. Dynamic surface tensions versus surface age: a. GL@Si-1 solutions in the concentration of 10, 50, 100, 500 and 1000 mg/L; b. Four biosurfactant solutions in the concentration of 1000 mg/L.

Y. Bai et al. / Journal of Molecular Liquids 266 (2018) 90–98 Table 3 Dynamic surface tensions parameters of GPSO (1000 mg·L−1) at 25 °C. GPSO

n

t*(s)

R1/2 (mN·m−1·s−1)

Standard error (n)

GL@Si-1 GL@Si-2 GHL@Si-1 GHL@Si-2

0.75 0.79 0.70 0.62

5.61 5.24 7.26 8.47

4.05 4.16 3.28 2.74

0.011 0.005 0.005 0.002

P3–4 (210 mg/L) [7]

0.91

0.80

27.50

95

study, glycosyl groups is the main facts that influences the surfactant mobility. Dynamic surface tensions versus surface age mainly conform to Rosen model [32] and the process can be divided into the following four regimes: (I) induction region; (II) rapidly reduction region; (III) meso-equilibrium region; (IV) equilibrium region. Eqs. (5) and (6) can be used to calculate Rosen models: lg½ðγ0 −γt Þ=ðγt −γm Þ ¼ nlgt−nlgt

and the formation of micelles in aqueous solution. Furthermore, value of ΔG0ads is larger than ΔG0mic, suggesting that tendencies of adsorption are favored over the process of micellization. Moreover, GPSO have greater propensity to migrate to interface rather than aggregate into micelles structure [27–29]. The ΔG0mic value of GL@Si-2 and GHL@Si-2 are larger than that of GL@Si-1 and GL@Si-1. It can be explained as follow: GL@Si-2 (or GHL@Si-2) have larger molecular weight than that of GL@Si-1 (or GHL@Si-1) and is more likely to aggregate in the solution. APSO-2 has more methylene and provides more driving force to form aggregates [13, 25]. But the ΔG0ads presents uncertain regulations due to affect by many actors (including, but not limited to the molecular structure, molecular weight, polydispersity index and solubility) [12, 14].

3.3. Dynamic surface tension of GPSO solutions Dynamic surface tension is a widely used method to study the surfactant adsorption kinetics at the air-water interface. In the study, dynamic surface tension measurements were performed using the maximum bubble pressure method. Fig. 6a shows the plot of dynamic surface tension measurements versus surface age for GL@Si-1 solutions in the concentration of blow and above CAC. The reduction rate of surface tension is observed to increase with the increases of the concentration of GL@Si-1 aqueous solutions. Moreover, the adsorption rate is more rapid as the concentration is above the CAC than that of below. Fig. 6b shows the plot of dynamic surface tension measurements versus surface age for four kinds of surfactant solutions in the concentration of 1000 mg/L (above CAC). The values of dynamic surface tension were still large than that of equilibrium surface tension as the surface age was 200 s, which imply that the adsorption of surfactant molecule still has not attained equilibrium. Different with conventional lower molecular weight surfactants, GPSO surfactants have substantially larger molecular structure and high molecular weight. More time is required to attain adsorption equilibrium at the air-water interface [13, 25]. The reduction rate of surface tension for GL modified polysiloxane surfactants are faster than that of GHL modified. The plot of dynamic surface tensions versus surface age for GL@Si-1 and GL@ Si-2 with different polysiloxane backbone has little difference in both the reduction rate of surface tension and the time need to attained adsorption equilibrium, which indicates that the length of polysiloxane backbone has little significant effect on dynamic adsorption at the airwater interface. As reported in the previous study, the mobility has important influence in the process of adsorption. The polysiloxane backbone could diffuse fast and absorbed at the air-water interface effectively. As a matter of fact, the length of polysiloxane backbone has little significant effect on surfactant mobility [14, 30, 31]. In this

ð5Þ

R1=2 ¼ ðγ0 −γm Þ=2t

ð6Þ

Here, γ0 is the surface tension of water; γt is the dynamic surface tension of surfactant solution in meso-equilibrium region; γm is the equilibrium surface tension of surfactant solution; t is the adsorption time; t* and n are constant; R1/2 is the rate of descent after induction region. Here, we keep (γ0 − γt) / (γt − γm) = k. All the parameters, which can obtained from Eqs. (5)–(6) and the curves of lgk versus lgt, are listed in Table 3. The value of n is positive correlation with diffusion barrier. That is to say, diffusion barrier increases and diffusion rate of surfactant molecules decreases with the increasing of n. It can be seen from Table 3, GHLmodified polysiloxane surfactants have faster diffusion rate in solution than GL-modified polysiloxane surfactants. Also, the value of n is smaller than that of butynediol-ethoxylate modified polysiloxane surfactants [7], which indicate that glycosyl modified polysiloxane have faster diffusion rate than polyglycerol modified polysiloxane surfactants. The value of t* has a negative correlation with adsorption barrier. Surfactant molecules are more prone to absorb on the surface of solution as the value t* increases. From Table 3, It can be seen that glycosyl modified polysiloxane is easier to absorb onto surface than butynediol-ethoxylate modified polysiloxane surfactants. Also, it can be seen GL-modified polysiloxane surfactants more difficult to absorb on the surface than GHL-modified polysiloxane in meso-equilibrium region. It can be explained as GL-modified polysiloxane surfactants have smaller Amin and pack more densely at the air-water interface and it is more difficult to absorb on the surface for surfactants molecular (As listed in Table 2). GHL-modified polysiloxane surfactants also have smaller value of R1/2, which means higher dynamic surface activity. The same result was reported in form research [7]. ΓðtÞ ¼ 2C0

rffiffiffiffi Z pffi rffiffiffiffiffi pffiffiffiffiffiffiffiffiffi t Dt D Cs d t−τ −2 π 0 π

ð7Þ

rffiffiffiffiffiffi Dt π

ð8Þ

γðtÞt→0 ¼ γ0 −2nRTC0

γðtÞt→∞ ¼ γeq þ

nRTΓ2eq C0

rffiffiffiffiffiffiffiffi π 4Dt

ð9Þ

For the surfactant solutions, the diffusion-controlled adsorption model is used to illustrate the diffusion process by Word-Tordai equation (showed as Eq. (7)) [33, 34]. The values of the diffusion coefficients of short-time (Ds) and long-time (Dl) can be calculated by Eqs. (8) and (9). All the parameters were reported in previous paper [35].

Table 4 Diffusion coefficients of GPSO (1000 mg·L−1) at 25 °C. GPSO

Ds/(m2·s−1)

Standard error qffiffiffiffi ð2nRTC o Dt πÞ

Dl/(m2·s−1)

Standard error nRTΓ2 pffiffiffiffiffiffi π Þ ð C 0 eq 4Dt

GL@Si-1 GL@Si-2 GHL@Si-1 GHL@Si-2

1.03 × 10−11 1.55 × 10−11 1.52 × 10−11 1.87 × 10−11

0.271 0.289 0.217 0.202

5.28 × 10−13 7.70 × 10−13 1.60 × 10−12 1.40 × 10−12

0.095 0.094 0.095 0.094

96

Y. Bai et al. / Journal of Molecular Liquids 266 (2018) 90–98

Fig. 7. Intensity-weighted particle size distribution profiles of GL@Si-1 and GHL@Si-1 surfactants in aqueous solution at 25 °C.

Eqs. (8) and (9) show the linear relation of γ(t) against t1/2 and t−1/2, which can be used to calculating the apparent diffusion coefficient from the slope of plots. The values of the diffusion coefficients of Ds and longtime Dl of GPSO were calculated by fitting linear slopes and listed in Table 4. It also can be seen the value of Ds is large than that of Dl, which means adsorption is mainly controlled by diffusion for shorttime adsorption process. For long-time adsorption process, molecules pack more densely at the air-water and the value of diffusion coefficients is relatively small, which agree well with Table 2 and 3. Also, Ds for GHL-modified polysiloxane surfactants is large than GL-modified. That means GHL-modified polysiloxane surfactants have good diffusion ability. In addition, the large ratios of Ds/Dl imply that the process of adsorption is controlled by mixed diffusion-kinetic adsorption [33–35].

3.4. Aggregation behaviors of GPSO in aqueous solution To minimize the interfacial energy, surfactants can self-assemble into ordered architecture when the concentrations of surfactant solutions are above the CAC. The solutions of GL@Si-2 and GHL@Si-2 are colourless and transparent. The solutions of GL@Si-1 and GHL@Si-1 are blue and transparent, which indicates that they may self-assemble into aggregation of nanoparticles. Dynamic light scattering (DLS) and negatively stained transition electron micrographs (TEM) are frequently-used instrument to investigate the size distribution and morphology of aggregate. Particle sizes and size distribution of GL@Si-1 and GHL@Si-1 surfactants in aqueous solution were investigated by DLS. Fig. 7 shows the

Fig. 8. Negatively stained transmission electron micrographs of GPSO:(a) GL@Si-1, 500 mg/L; (b) GL@Si-1, 1000 mg/L; (c) GHL@Si-1, 500 mg/L; (d) GHL@Si-1, 1000 mg/L.

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characteristic intensity-weighted distribution profiles of different surfactants at 500 mg/L and 1000 mg/L. At the concentration of 500 mg/L, two different size distributions in the range of 34–320 nm for GL@Si-1 and 40–400 nm are observed to show the existence of wide distribution aggregates. For higher concentrations (1000 mg/L), a single peak in the range of 260–280 nm for GL@Si-1 and 270–400 nm for GHL@Si-1 is observed. Aggregates show narrow size distribution. In comparison with conventional surfactants aggregates, these aggregates are much larger in size. Because of the difference of micelles aggregation numbers, the aggregates have an inhomogeneous distribution. Also, the aggregates exhibit an increasing trend to form larger aggregates with the increasing of surfactant concentration. Similar results have been reported previously for other polymer surfactants [13, 20]. TEM images in Fig. 8 were analyzed to confirm the results obtained from DLS studies. All the aggregates are spherical for both of two surfactants in different concentrations. The aggregates tend to have smallest surface to minimize the energy. So spherical aggregates are formed. The diameters of the aggregates from 50 nm to 400 nm have a good consistent with those obtained from DLS measurements. Because of the effect of hydrogen bonds and van der Waals forces among the hydrophilic, smaller aggregates are further assembled into larger and more complex aggregates [36, 37]. 4. Conclusion A series of friendly biosurfactants glycosyl-modified polysiloxane with different glycosyl (GL and GHL) and polysiloxane backbone lengths were synthesized and characterized. The main conclusions in this work are listed as follows: 1) The surface tensions of four GPSO (GL@Si-1, GL@Si-2, GHL@Si-1 and GHL@Si-2) in different concentrations show that four glycosylmodified polysiloxane biosurfactants exhibit high surface activity. 2) The process of adsorption at the air-water interface and the formation of micelles in aqueous is spontaneous. Furthermore, tendencies of adsorption are favored over the process of micellization. 3) Dynamic surface tension measurements results indicate that the adsorption rate of GPSO is slow and more time is required to attain adsorption equilibrium at the air-water interface. Furthermore, GHL modified polysiloxane surfactants have higher propensity of adsorption at the air-water interface than that of GL modified polysiloxane surfactants. 4) TEM and DLS studies show that the aggregates are spherical. Aggregates have a wide distribution and two different size distributions are presented at lower concentration. This work will be useful in the design of novel friendly biosurfactants glycosyl-modified polysiloxane. It also provides theoretical direction in the potential application in agricultural adjutants and home care products. Acknowledgments This project is funded by the National Key R & D plan (Grant No. 2017YFB0308704). We would also like to express our gratitude to Guojin Li of China Research Institute of Daily Chemical Industry for TEM observations. Conflicts of interest There are no conflicts to declare. References [1] G.O. Yahaya, B.J. Brisdon, M. Maxwell, R. England, Preparation and properties of functionalized polyorganosiloxane, J. Appl. Polym. Sci. 82 (2001) 808–817.

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