Synthesis, Surface Activities, and Aggregation Behaviors of Butynediol

Article pubs.acs.org/JPCB

Synthesis, Surface Activities, and Aggregation Behaviors of Butynediol-ethoxylate Modified Polysiloxanes Zhiping Du,*,‡,†,§ Jieqiong Qin,†,§ Wanxu Wang,† Yanyan Zhu,† and Guoyong Wang*,† ‡

Institute of Resources and Environment Engineering, Shanxi University, No. 92 Wucheng Road, Taiyuan 030006, P. R. China China Research Institute of Daily Chemical Industry, 34 Wenyuan Street, Taiyuan 030001, P. R. China

†

ABSTRACT: Five different butynediol-ethoxylate modified polysiloxanes (PSi-EO) were designed and synthesized via two-step reactions: the preparation of low-hydrogen containing silicone oil (LPMHS) by acid-catalyzed polymerization and the following hydrosilylation reaction with 1,4-bis(2-hydroxyethoxy)-2-butyne. The chemical composition of each product was confirmed by FT-IR, 1H NMR, and 29Si NMR. The surface activities and aggregation behaviors of PSi-EO surfactants in aqueous solution were studied systematically using surface tension, dynamic light scattering (DLS), transmission electron microscopy (TEM), and contact-angle methodologies. Relatively low critical aggregation concentration (15−34 mg·L−1) and surface tension (∼25 mN·m−1) were measured for PSi-EO aqueous solution. The rate of surface tension reduction increased both with increasing PSi-EO concentration and with increases in the proportion of hydrophilic moieties within the synthesized compounds. Furthermore, DLS and TEM studies revealed that PSi-EO self-assembled in aqueous solution to form spherical aggregates. Contact-angle measurements conducted upon low-energy paraffin film surfaces demonstrated that PSi-EO exhibited efficient spreading at concentrations above the critical aggregation concentration.

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solvent-free hydrosilylation.10 Surfactants based upon oligomeric or polymeric molecules have yet to be studied in detail, however.11 Despite the considerable attention lavished upon polysiloxane surfactants within both the academic and industrial sectors, to date only limited studies, related to the synthesis and surface activity of such molecules, have been conducted.12−16 It is also necessary to investigate the aggregation behavior of such compounds to develop a better understanding of the influence of siloxane chain structure on polysiloxane surfactant properties. In this study, five butynediol-ethoxylate modified polysiloxanes with different composition (PSi-EO) were synthesized, and their properties were investigated. The differing effects of siloxane chain and 1,4-bis(2-hydroxyethoxy)-2-butyne (BEO) units on the surface activity and aggregation behavior were elucidated using various techniques, including measurements of surface tension, dynamic light scattering (DLS), transmission electron microscopy, and contact angle.

INTRODUCTION Surfactants are amphiphilic compounds that are able to adsorb onto the surfaces or interfaces of system and reduce the surfaces or interfaces tension.1,2 As a unique class of surfactants, siloxane surfactants are effective in reducing the surface tension in both aqueous and nonaqueous media. Such properties have been attributed to a lower intermolecular cohesive force and the greater flexibility of siloxane chains in comparison with canonical surfactants.3 Siloxane surfactants have been extensively used as emulsifiers and spreading agents in household and personal care products; wetting agents, antifoaming agents, and flow promoters in coating and paint formulations and also as polyurethane foam additives and agricultural adjuvants.4 On the contrary, acetylenic diol-based nonionic surfactants, which contain carbon−carbon triple bond and two adjacent hydroxyl groups, have been widely used in the formulations of inks and coatings due to their efficiency in defoaming, surface wetting, and low hydrophilic lipophilic balance (HLB) number.5−9 It is believed that the high electron density and hydrophilicity of the molecule are responsible for these notable properties and applications of acetylenic diol-based surfactants. It would therefore be extremely beneficial to combine siloxane and acetylenic diol to develop new acetylenic-diolmodified siloxane surfactants that may possess remarkable properties of low surface tension, good defoaming, and excellent surface wetting and have a wide variety of potential applications in industry. In a previous study from our group, a butynediol-ethoxylate-based trisiloxane was prepared through © XXXX American Chemical Society

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EXPERIMENTAL SECTION Materials. Polymethylhydrosiloxane with a Si−H content of 1.5% (1.5 mol of Si−H groups contained in per 100 g of PMHS), octamethylcyclotetrasiloxane (D4), hexamethyldisiloxReceived: August 5, 2015 Revised: September 29, 2015

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DOI: 10.1021/acs.jpcb.5b07618 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Scheme 1. Synthetic Routes of Low-Hydrogen Containing Silicone Oil (LPMHS) and Butynediol-Ethoxylate-Modified Polysiloxanes (PSi-EO)

Table 1. Composition, Molecular Weight, and Si−H Content of Each LPMHS Sample based on 29Si NMR spectra sample

feed composition m/n

expected Mn (g/mol)

expected Si−H content (%)

composition m/n

Mn (g/mol)

Si−H content (%)

L8-2 L6-4 L5-5 L4-6 L2-8

8/2 6/4 5/5 4/6 2/8

874 846 832 818 790

0.23 0.47 0.60 0.73 1.01

9.0/1.7 6.1/3.5 4.9/4.3 3.9/5.5 1.6/7.2

935 832 785 776 711

0.19 0.42 0.55 0.70 1.01

measured by a Krüss K12 tensiometer (Krüss Company, Germany) using the Wihelmy plate method at 25 ± 0.1 °C. For each concentration, the surface tension value is an average value measured three times to minimize data errors. Dynamic surface tension measurement was carried out on a Krüss BP100 bubble-pressure tensiometer (Krüss Company, Germany), with the range of effective surface ages running from 10 to 200 000 ms. Temperature was maintained at 25 ± 0.1 °C. To investigate the effective diameter and size distribution of the PSi-EO aggregates in aqueous solution, DLS was performed with a Zeta Plus Particle Size Analyzer instrument (Brookhaven, USA). The scattering angle was set to 90° for all experiments. Sample solutions were pre-equilibrated for 5 h, and each experiment lasted 3 min at 25 ± 0.1 °C. The micromorphology of PSi-EO aggregates in aqueous solutions was studied using negatively stained transmission electron micrographs obtained with a JEM-1011 transmission electron microscope (Jeol Company, Japan) at 100 kV. Samples were stained with 2 wt % phosphotungstic acid on a carboncoated copper grid before TEM scanning. Spreading of PSi-EO aqueous solutions on hydrophobic substrates was measured by the sessile drop method using a drop shape analyzer (Krüss Company, Germany) at 25 ± 0.1 °C. Paraffin film was chosen as the solid substrates and showed a contact angle of 106 ± 0.2° for pure water. Experimental temperature was controlled accurately using a K20 thermostatic, water-circulating bath (Haake Company, Germany) within which both heating and cooling systems were incorporated.

ane (MM), hydrogen hexachloroplatinate (IV) hydrate (CPA), and 1,4-bis(2-hydroxyethoxy)-2-butyne (BEO) were purchased from Aldrich. Sulfuric acid, sodium hydrogen carbonate, and isopropanol were obtained from Tianjin Kemiou Chemical Reagent (China). All chemicals were used as received. Synthesis of Low-Hydrogen Containing Silicone Oil (LPMHS). In a 250 mL three-necked flask, 59.2 g (0.2 mol) of D4, 12 g (0.2 mol repeating unit) of PMHS, 16.2 g (0.1 mol) of MM, and 2 mL of sulfuric acid were mixed. This mixture was heated to 60 °C and stirred for 5 h. Sodium hydrogen carbonate was then used to neutralize the acid in the reaction mixture. Then, LPMHS was obtained by filtering the solids and evaporating the low boiling matter (yield 98.3%). Synthesis of PSi-EO. PSi-EO was synthesized through a hydrosilylation reaction between BEO and LPMHS catalyzed by Speier’s reagent, which was prepared as described by Sacarescu and colleagues.17 BEO (17.4 g) and LPMHS (52.6 g; Si−H content of 0.19%) were mixed in the presence of Speier’s reagent in a 250 mL, four-necked flask equipped with reflux condenser, thermometer, and nitrogen inlet/outlet. The reaction was subjected to reflux at 130 °C for 2 h. After low boiling matter had been separated from the mixture, the product was obtained with an total yield of 97.2%. Structural Analyses. The chemical structures of LPMHS and PSi-EO were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR). FT-IR spectra were recorded on a Bruker Vertex-70 spectrometer. 1H NMR and 29Si NMR spectra were performed using a Varian INOVA-400 Hz spectrometer in CDCl3 solvent without the internal standard. Measurements of the Properties of PSi-EO in Aqueous Solution. The equilibrium surface tension was B


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Figure 1. 29Si NMR spectrum of L8-2 (LPMHS with m/n = 8/2).

Figure 3. 1H NMR spectra of (1) L8-2, (2) BEO, and (3) P8-2.

Table 2. Composition, Molecular Weight, and HLB of Different PSi-EO

Figure 2. FT-IR spectra of (1) L8-2, (2) BEO, and (3) P8-2.

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composition of PSi-EO

RESULTS AND DISCUSSION Synthesis and Characterization of LPMHS. LPMHS was synthesized by acid-catalyzed polymerization of PMHS, D4, and MM (Scheme 1). In LPMHS, the proportion of dimethylsiloxy groups (m) to methylhydrosiloxy groups (n) can be regulated by the ratio of D4/PMHS in feed, given that the dosage of MM is maintained at a constant level.12 We obtained five samples of LPMHS with different compositions according to this method (Table 1). FT-IR, 1H NMR, and 29Si NMR were used to characterize their structures. As has been previously reported, 1H NMR spectra of LPMHS afford the researcher only the ratio of m/n; however, 29 Si NMR can be used to quantify both the absolute ratio, m/n, and the molecular weight of LPMHS.13,18−20 L8-2 (LPMHS with m/n = 8/2) is used to provide an example of the application our 29Si NMR analytical methodology. As one can see in Figure 1, the clustered signals at around −37 ppm (peaks a), −18 to −22 ppm (peaks b), and 7 to 10 ppm (peaks c) correspond to Si nuclei within methylhydrosiloxy groups, dimethylsiloxy groups, and trimethylsiloxy groups, respectively. The ratio of dimethylsiloxy groups to methylhydrosiloxy groups, m/n, can be calculated exactly using the integral area values of peaks b and a. The molecular weight is determined by the peaks c, which are assigned to the terminal silicon nuclei. The molar ratio of m/n, molecular weight, and Si−H content were almost identical to the target values in every case (Table 1).

sample

m

n

DP(m+n)

HLBa

Mn (g/mol)

P8-2 P6-4 P5-5 P4-6 P2-8

9.0 6.1 4.9 3.9 1.6

1.7 3.5 4.3 5.5 7.2

10.7 9.6 9.2 9.4 8.8

1.2 2.2 2.5 2.8 3.2

1226 1432 1531 1738 1965

a HLB: hydrophilic lipophilic balance (HLB = E/5 from Griffin equation, where E is the molecular weight ratio of ethylene oxide).15

Synthesis and Characterization of PSi-EO. BEO was incorporated into LPMHS by hydrosilylation reaction with Speier’s reagent (Scheme 1). The molar ratio of Si−H bonds in BEO to LPMHS was held at 1 to 1 to allow the reaction to continue to completion. Five sets of PSi-EO with the required compositions were synthesized and characterized by FT-IR and 1 H NMR. Once again, P8-2 (PSi-EO with m/n = 8/2) is presented as the exemplar of the methodology we have applied to every sample. Figure 2 shows the FT-IR spectra of L8-2, BEO, and the final product P8-2. The absorption band at 2150 cm−1 in L8-2 is characteristic of Si−H stretching vibration and is not observed in P8-2, which indicates that the hydrosilylation reaction has approached completion. In Figure 2(3), the absorption peak at 3400 cm−1 is assigned to the OH group. The absorption peaks at 1050 and 1090 cm−1 demonstrate the presence of C−O−C C


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Figure 4. Surface tensions of PSi-EO aqueous solutions at 25 °C as a function of their concentrations: ■, P8-2; red ●, P6-4; blue ▲, P5-5; pink ▼, P4-6; green ◆, P2-8.

Figure 5. Dynamic surface tensions with surface age for P5-5 aqueous solutions at 25 °C: ■, 5 mg·L−1; red ●, 10 mg·L−1; green ▲, 25 mg· L−1; blue ▼, 50 mg·L−1; and light blue ◆, 100 mg·L−1.

group and Si−O−Si group, respectively. Finally, the absorption peak at 1650 cm−1 is indicative of the presence of the unsaturated, CC functional group. Figure 3 illustrates the 1H NMR spectra of L8-2, BEO, and the product P8-2. The peak at 4.7 ppm corresponds to the active hydrogen atom (Si−H) for L8-2 and is not visible in the spectrum of P8-2. The peak of new formative hydrogen atom (CC−H) at 6.2 ppm appears in P8-2, as expected. These results imply that the hydrosilylation reaction has essentially reached completion. Our interpretation of the FT-IR and 1H NMR spectra strongly supports the conclusion that the intended PSi-EO compounds have been obtained. The composition, molecular weight, and HLB for each PSi-EO are summarized in Table 2. All values are in agreement with those that would be expected for the intended compounds. Surface Activity of PSi-EO. The equilibrium surface tension is the most versatile method used to evaluate the surface activity of surfactants. Figure 4 shows the surface tension (γ) plotted against the concentration of PSi-EO in aqueous solutions at 25 °C. It displays that the surface tension decreases progressively with increasing surfactants concentration and shows two transition points in the curves. The first transition point may be explained by the formation of premicellar aggregates, as suggested in previous studies.21−23 The critical aggregation concentrations (CACs) and surface tensions at CAC (γCAC) are determined from the second transition point, and their values are listed in Table 3. It is observed that all five surfactants exhibit relatively low CAC values of 15−34 mg·L−1 and low γCAC values of ∼25 mN·m−1. These results imply that PSi-EO promotes the formation of micelles in aqueous solution and significantly reduces the surface tension of water, which may be attributed to the branched polysiloxane moiety lying along the air−water

Figure 6. Dynamic surface tensions with surface age for PSi-EO aqueous solutions of 100 mg·L−1 at 25 °C: ■, P8-2; red ●, P6-4; green ▲, P5-5; blue ▼, P4-6; and light blue ◆, P2-8.

interface, exposing the highly surface active methyl groups to the air. From P8-2 to P2-8, where the degree of polymerization (m +n) has been retained as close to a constant of 10 as possible, the CAC values decrease with each increase in hydrophilic content (n/(m+n)), except for compound P2-8, whereas the γCAC values do not differ substantially. 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 solubility12,13,24 Here the CAC values in mg·L−1 can be transformed into values in mol·L−1 according to the molecular weight of PSi-EO.

Table 3. Aggregation and Adsorption Parameters for PSi-EO Aqueous Solutions at 25 °C PSi-EO

γCAC (mN·m−1)

CAC (mg·L−1)

Amin (Å2/molecule)

ΔG0mic (kJ·mol−1)

ΔG0ads (kJ·mol−1)

P8-2 P6-4 P5-5 P4-6 P2-8

24.78 25.22 24.24 24.72 24.77

33.56 23.11 18.43 15.97 20.83

61.98 52.40 57.28 67.80 132.89

−36.00 −37.31 −38.04 −38.70 −38.35

−53.66 −52.10 −54.54 −58.04 −76.21

D

Γmax (mol·cm−2) 2.68 3.17 2.90 2.45 1.25

× × × × ×

10−10 10−10 10−10 10−10 10−10


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Figure 7. Intensity-weighted size distributions recorded for PSi-EO aqueous solutions: (a) P8-2, 0.5 wt %; (b) P8-2, 1.0 wt %; (c) P6-4, 0.5 wt %; (d) P6-4, 1.0 wt %; (e) P5-5, 0.5 wt %; (f) P5-5, 1.0 wt %; (g) P4-6, 0.5 wt %; (h) P4-6, 1.0 wt %; (i) P2-8, 0.5 wt %; and (j) P2-8, 1.0 wt %. Each curve has been fitted with a corresponding Gaussian distribution.

⎛C ⎞ 0 ΔGads = RT ln⎜ Π ⎟ − 6.022ΠA min ⎝ 55.5 ⎠

Gibbs’ law is typically used to study the adsorption and aggregation properties of surfactants at equilibrium. The saturation surface excess concentration, Γmax, and the minimum area, Amin, at the air−water interface may be obtained from the Gibbs adsorption isotherm equations (eqs 1 and 2). While the standard free energies of aggregation and adsorption can be estimated from eqs 3 and 4.25 Γmax = −

⎛ ∂γ ⎞ 1 ⎜ ⎟ 2.303kRT ⎝ ∂ log c ⎠

T

where k can be regarded as 3 (the expected value for multiheaded surfactants at the air−water interface), R is the gas constant, T is the absolute temperature, NA is Avogadro’s number, Π (= γ0 − γ) is the surface pressure in the region of surface saturation where γ0 is the surface tension of pure water, and CΠ is the molar concentration of the surfactant at a surface pressure Π (mN·m−1). The parameters obtained through these equations are summarized in Table 3. The Amin values for our PSi-EO samples increase from P6-4 to P2-8, suggesting that PSi-EO surfactants pack more loosely at the air−water interface with increasing hydrophilic content. A possible explanation may be that the higher content of hydrophilic groups makes the surfactants more prone to stretching and thus increases the Amin

(1)

1016 NA Γmax

(2)

⎛ CAC ⎞ 0 ⎟ ΔGmic = RT ln⎜ ⎝ 55.5 ⎠

(3)

A min =

(4)

E


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Figure 9. Contact angles versus time for P5-5 aqueous droplets on Parafilm at 25 °C: ■, 10 mg·L−1; red ●, 50 mg·L−1; blue ▲, 100 mg· L−1; pink ▼, 500 mg·L−1; and green ◆, 1000 mg·L−1.

Figure 10. Contact angles versus time for PSi-EO aqueous droplets of 500 mg·L−1 on Parafilm at 25 °C: ■, P8-2; red ●, P6-4; blue ▲, P5-5; pink ▼, P4-6; and ◆, P2-8.

for P5-5 (PSi-EO with m/n = 5/5) aqueous solutions below and above the CAC are shown in Figure 5. An increase in the rate of surface tension reduction is observed when the concentration of P5-5 aqueous solutions increases. The time required to attain adsorption equilibrium decreases correspondingly, according to increasing surfactant concentration, and the equilibrium is more rapid above the CAC than below it. As shown in Figure 6, the rate of surface tension reduction increases and the time required to attain adsorption equilibrium decreases markedly between the P8-2 and P2-8 surfactants in aqueous solution, indicating the higher propensity for adsorption at the air−water interface exhibited by PSi-EO solutions on increasing hydrophilic content. The time required to attain adsorption equilibrium of PSi-EO aqueous solutions is longer than that typical for lower molecular weight surfactants as PSi-EO has a substantially larger molecular structure. These results illustrate the effects of changes in the hydrophilic or hydrophobic content, molecular structure, and molecular weight on dynamic behavior, as previously reported.29−32 Aggregation Behaviors of PSi-EO in Aqueous Solution. Typically, surfactants can self-assemble at concentrations above the CAC to form ordered architecture that minimize the interfacial energy. DLS measurements were conducted and negatively stained transition electron micrographs were recorded and analyzed to investigate the aggregate formation of PSi-EO aqueous solution.

Figure 8. Negatively stained transmission electron micrographs of spherical aggregates formed in PSi-EO aqueous solutions: (a) P8-2, 0.5 wt %; (b) P8-2, 1.0 wt %; (c) P6-4, 0.5 wt %; (d) P6-4, 1.0 wt %; (e) P5-5, 0.5 wt %; (f) P5-5, 1.0 wt %; (g) P4-6, 0.5 wt %; (h) P4-6, 1.0 wt %; (i) P2-8, 0.5 wt %; and (j) P2-8, 1.0 wt %.

values. Correspondingly, the Amin varies inversely with Γmax. The standard free energies of micellization ΔG0mic and that of adsorption ΔG0ads of PSi-EO are both negative values, implying that the formation of micelles in aqueous solution and adsorption at the air−water interface will occur spontaneously. Furthermore, the negative ΔG0ads value is larger than the corresponding ΔG0mic value, suggesting that the adsorption may well be the dominant event rather than the aggregation. These results are consistent with other siloxane surfactants reports.10,26−28 Dynamic Surface Tension Measurements for PSi-EO Solutions. To investigate the surfactant adsorption kinetics at the air−water interface, dynamic surface tension measurements were performed using the maximum bubble pressure method. The variations of the dynamic surface tension versus surface age F


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in the rate of surface tension reduction and a decrease in the time required to attain adsorption equilibrium between P8-2 and P2-8. Hence, this work will be useful in allowing the design of novel siloxane surfactants that may find their potential application in agricultural adjuvants, home care products, or coatings.

As shown in Figure 7, the hydrodynamic size distributions of aggregates formed by PSi-EO in aqueous solution were calculated from measurements of DLS. The intensity-weighted distribution curves of PSi-EO aqueous solutions at 0.5 and 1.0 wt % show monomodel or bimodel functions. The aggregates formed have average hydrodynamic diameters of ∼64.7 nm (0.5 wt %) and 111.7 nm (1.0 wt %) for P8-2, 161.3 nm (0.5 wt %) and 91.8 nm (1.0 wt %) for P6-4, 174.9 nm (0.5 wt %) and 571.0 nm (1.0 wt %) for P5-5, and 149.1 nm (0.5 wt %) and 172.7 nm (1.0 wt %) for P4-6, respectively. For P2-8, notably, two peaks are recorded with diameters of ∼113.0 and 454.9 nm at 0.5 wt %, whereas only a single peak is observed with a diameter of ∼104.8 nm at 1.0 wt %. In comparison with conventional surfactants micelles, these aggregates are much larger in size. Similar results have previously been reported for other polymer surfactants.33 As a reliable and versatile method, TEM confirmed the presence of spherical aggregates in PSi-EO aqueous solutions (Figure 8). The diameters of the aggregates lie in the range 50 to 600 nm, consistent with those derived from DLS measurements. The large, complex aggregates may well be formed by a further assembly of smaller aggregates, due to the effect of hydrogen bonds or van der Waals forces among the hydrophilic shells.34,35 Dynamic Spreading Measurements for PSi-EO Solutions. To characterize the spreading ability of PSi-EO aqueous solutions, we measured the contact angle by forming one droplet of surfactant aqueous solutions on a paraffin film at 25 °C. Figure 9 shows the contact angles of P5-5 aqueous droplets at different concentrations. A decrease in initial contact angle is observed when the surfactant concentration increases from 10 to 1000 mg·L−1 due to the fast adsorption kinetics of PSi-EO molecules at the interface.36 Droplets spread slowly and appear to exhibit a spreading delay at the lowest concentration. These results reveal that increasing concentration of PSi-EO solutions enhances spreading and allows equilibrium to be achieved much more quickly. As shown in Figure 10, the contact angles for five surfactants droplets decrease gradually from the initial value (θ0) to an equilibrium value (θeq), following a classical wetting model where the droplets spread until the forces at the three-phase contact line balance.37 These five surfactants all reveal partial wetting (θ0 < 90° and θeq < 90°) at 500 mg·L−1 and can be used in agricultural adjuvant, coating and similar roles.

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AUTHOR INFORMATION

Corresponding Authors

*Z.D.: Phone: +86-13703582149. Fax: +86-351-4040802. Email: [email protected]. *G.W.: Phone: +86-351-4084691. Fax: +86-351-4040802. Email: [email protected]. Author Contributions §

Z.D. and J.Q. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is funded by the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (Grant No 2014BAE03B03), National Natural Science Found of China (Grant No. 21103228) and Natural Science Found of Shanxi Province (Grant No 2014011014-1). We would also like to express our gratitude to Guojin Li of China Research Institute of Daily Chemical Industry for TEM observations.

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CONCLUSIONS Butynediol-ethoxylate-modified polysiloxanes (PSi-EO) with different compositions were successfully synthesized and characterized by FT-IR and 1H and 29Si NMR. We found that the chemical combination of both siloxane and acetylenic diol in one molecule was highly beneficial. Such surfactants exhibit powerful surface activities, unique aggregation behaviors, and outstanding properties for spreading. PSi-EO surfactants proved capable of reducing the water surface tension to ∼25 mN·m−1 while exhibiting very low CAC values. They also show a remarkable tendency to self-assemble into spherical aggregates (50−600 nm) and partial wetting on paraffin film above the CAC. Furthermore, the surface activities of PSi-EO surfactants can be tuned by changing the content of hydrophilic groups. Decreases in the CAC values between P8-2 and P4-6 and approximate γCAC values were observed with increasing hydrophilic content. Measurements of dynamic surface tension at the air−water interface revealed an increase G


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