Effect of Alcohols on Aggregation Behaviors of Branched Block

ARTICLE pubs.acs.org/Langmuir

Effect of Alcohols on Aggregation Behaviors of Branched Block Polyether Tetronic 1107 at an Air/Liquid Surface Teng Liu, Guiying Xu,* Houjian Gong, Jinyu Pang, and Fang He Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China ABSTRACT: The aggregation behaviors of branched block polyether Tetronic 1107 (T1107) at an air/liquid surface was investigated in mixed solvents consisting of water and one of the following polar cosolvents: ethanol, n-propanol, ethylene glycol (EG), or glycerol (GLY). Surface tension measurements provide information about the effects of cosolvents on the critical micellization concentration (cmc), the standard Gibbs energy (ΔG°mic), the maximum surface excess concentration (Γmax), the minimum area per polyether molecule at the air/liquid surface (Amin), and the standard free energy of adsorption (ΔG°ads). The addition of ethanol and n-propanol to water disfavors the micellization and progressively increases the cmc of T1107, whereas the cmc decreases with the addition of EG and GLY. The values of ΔG°mic of T1107 are all negative in mixed solvents, and their absolute values become smaller as the ethanol or n-propanol content increases but become larger as the EG or GLY content increases. The cosolvents have a significant effect on the surface adsorption and cmc, and the order is as follows: n-propanol
water > ethanol
water > water > EG
water > GLY
water. The octanol/water partition coefficient (log P) of the cosolvent is used to correlate the effects, and it could capture the effect of cosolvents on the cmc qualitatively. The surface dilational rheological properties of T1107 in water and water
alcohol mixtures were also studied by surface dilational viscoelasticity and surface tension relaxation measurements. The dilational elasticity decreases monotonously in the presence of ethanol or n-propanol. With the increasing concentration of EG and GLY, the dilational elasticity of T1107 passes through a maximum that coincides with the change in Γmax.

1. INTRODUCTION Numerous studies have been carried out to investigate block polyethers poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), which have abundant aggregation behaviors not only in the bulk solution but also at the air/liquid surface.1
4 They have numerous applications in a variety of industrial, nanomaterial synthesis, and pharmaceutical fields.5,6 In the majority of applications, various solvents or mixed solvents are widely used. The addition of polar cosolvents (such as glycerol and ethanol) can tailor the solution properties for specific applications, for example, in drug-delivery formulations that employ PEO-PPO-PEO as excipients or carriers,7 or as templates in the synthesis of mesoporous materials.8 Therefore, an understanding of the effect of cosolvents on the aggregation behavior of polyethers is essential. There have been some investigations of the influence of a cosolvent on the aggregation behavior of PEO-PPO-PEO.9
11 Water is typically used as a solvent (being selective for PEO) for PEO-PPO-PEO block polyethers. The addition of cosolvents to water can influence the critical micelle concentration (cmc), the critical micelle temperature (cmt), and the structure of the micelles. In the case of short-chain alkanols (methanol and ethanol), they are good solvents for both PEO and PPO blocks of the polyether and hence delay the onset of micellization.10 This is reflected as an increase in the cmt, cmc, and cloud points of the polyethers. For medium-chain alkanols (1-butanol, 1-pentanol, and 1-hexanol), they promote the micellization of block r 2011 American Chemical Society

polyethers.12 This is explained in terms of a cooperative association of medium-chain alkanols with the PPO blocks of polyethers that expel water from the core of the micelles. The effects of polyhydroxy-alcohol on the micellization of PEO-PPO-PEO were also found.13
16 The effects of glycerol, propylene glycol, or glucose on the self-assembled microstructure are related to changes in the mean interfacial area occupied by the PEO blocks, its preference to locate in different microdomains, and its ability to modify the interfacial curvature by swelling different blocks of the polyether to different extents.14,15 Tetronics are X-shaped polyethers with four PPO-PEO arms bonded to a central ethylene diamine linker. Tetronics are relatively less studied although they are widely used as antifoaming agents, wetting agents, dispersants, thickeners, and emulsifiers for different industrial purposes.17,18 And they are also increasingly finding diverse applications in biomedical and pharmaceutical fields19,20 such as drug delivery, genetic immunization, and membrane biochemistry. Because of the branched structure, the aggregation behavior and micellar structure of Tetronics are different from those of linear counterparts.17,18,21 A series of block polyethers with branch structures have been investigated in our laboratory and shown many advantages in practical applications.22
27 Though the influencing factors in the Received: May 5, 2011 Revised: June 22, 2011 Published: June 23, 2011 9253

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Figure 1. Chemical structure of T1107.

aggregation behaviors of polyethers, such as the relative size of the PEO and PPO blocks,21,28 polyether concentration,29,30 pH,30
32 temperature,31,33 and the presence of salts,34
36 have been investigated, the influence of cosolvents on the aggregation behavior of polyethers with branch structure is still lacking. In this article, the effects of cosolvents on the aggregation behavior of Tetronic 1107 (T1107) are investigated. This study has been incited by fundamental interest (the polar cosolvents that we have chosen to study (ethanol, n-propanol, ethylene glycol, and glycerol) resemble the monomers of the polyether blocks) as well as by practical interest (cosolvents are less polar than water, they increase the solubility of hydrophobic solutes, and they are used in many pharmaceutical formulations). Moreover, special attention has been paid to films of T1107 at the air/liquid surface. The aggregation and adsorption properties of T1107 are studied in detail via the equilibrium surface tension, surface dilational viscoelasticity, and surface tension relaxation measurements.

2. EXPERIMENTAL SECTION 2.1. Materials. Branched block polyether Tetronic 1107 (T1107) was purchased from Sigma-Aldrich and used as received without further purification. The T1107 structure is shown in Figure 1. The average molecular weight (Mw) is 15 000 g 3 mol
1, and the polydispersity index (PDI) of T1107 is 1.443, which was measured by Waters 515 gel permeation chromatography (GPC). Analytical-grade ethanol, n-propanol, ethylene glycol (EG), and glycerol (GLY) were purchased from Sinopharm Chemical Reagent Co. Aqueous solutions containing T1107 were prepared by weighing. Water used in the experiments was triply distilled by a quartz water-purification system. 2.2. Methods. 2.2.1. Equilibrium Surface Tension Measurements. The surface tension was measured with a K12 processor tensiometer (Kr€uss Co., Germany; the precise degree of the measurement is 0.01 mN 3 m
1) using a Wilhelmy plate. The average values of the surface tension were obtained by measuring three times. All experiments in this study were performed at 25.0 ( 0.1 °C. 2.2.2. Surface Dilational Viscoelasticity Measurements. The dilational rheology gives a measure of the surface resistance to changes in area. The surface dilational modulus is defined as the ratio of a small change in surface tension to a change in surface area37 ε¼

dγ d ln A

ð1Þ

where ε is the dilational modulus, γ is the surface tension, and A is the area of the surface. ε can also be expressed as the summation of the elasticity and viscosity contributions ε ¼ εd þ iωηd

ð2Þ

where εd is the dilational elasticity and ωηd is the dilational viscosity component that represents a combination of internal relaxation processes and relaxation due to the transport of matter between the surface and the bulk. Phase angle θ is calculated according to ωηd tan θ ¼ ð3Þ εd where the θ value reflects the responding speed of the surface layer to the applied small-area perturbation. In the absence of relaxation processes,

Figure 2. Surface tension isotherms of the T1107 aqueous solution with different contents of ethanol. The inset is the magnified image of the curves in the circle. phase angle θ is equal to zero and the surface layer behaves as a purely elastic body. In this study, the surface dilational viscoelasticity meter JMP2000A (Powereach Ltd., Shanghai, China) was used to measure the parameters of dilational viscoelasticity, which has been described elsewhere.38
41 It includes a modified Langmuir trough with two symmetrically oscillating barriers for changing the interfacial area and a Wilhelmy plate for measuring the surface tension. The Langmuir trough was filled with the T1107 solution. Then, the adsorption film in its equilibrium state was expanded and compressed sinusoidally at small amplitude in the frequency range of 0.005
0.1 Hz. 2.2.3. Surface Tension Relaxation Measurements. Surface tension relaxation experiments are a reliable way to obtain surface dilational parameters. This technique uses small but fast axisymmetric area expansion or compression to disturb the monolayer equilibrium slightly, which causes an interfacial tension jump and then the surface tension will decay to equilibrium again. The relaxation behavior of surfactant layers provides deep insight into the composition and structure of adsorbed layers at liquid surfaces.1,38,42
45 The information includes the orientation and the configuration changes of molecules at the surface, the diffusion and adsorption of molecules from the bulk to the surface, and so on. For a real system, a number of relaxation processes may occur, and the decay curve would be expressed by the summation of a number of exponential functions46   n t ð4Þ ΔγðtÞ ¼ Ai exp
τi i¼1

∑

where 1/τi is the characteristic frequency of the ith process, Ai is the fractional contribution, which the relaxation process makes to restore equilibrium, and n is the total number of relaxation processes. In the surface tension relaxation measurements, the film was expanded about 15% in area by the sudden simultaneous movement of two parallel barriers, situated in the surface over 2 s, and then the film relaxed spontaneously. The dynamic surface tension was recorded and analyzed by a nonlinear curve-fitting method.

3. RESULTS AND DISCUSSION 3.1. Surface Activity of T1107 in the Presence of Cosolvents. The surface tension isotherms of T1107 aqueous solution

in the presence of ethanol at different concentrations (5, 10, 15, and 20 wt %) are shown in Figure 2. The surface tension values of the T1107 aqueous solution without ethanol decrease with increasing T1107 concentration, and there are two breaks in 9254

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Table 1. Surface Activity Parameters of the T1107 Aqueous Solution with Different Contents of Ethanol ethanol (wt %)

cmc (wt %)

ΔG°mic (kJ 3 mol
1)

Γmax (10
6 mol 3 m
2)

Amin (nm2)

γsolvent (mN 3 m
1)

γcmc (mN 3 m
1)

ΔG°ads (kJ 3 mol
1)

0

5.0


24.1

0.54

3.08

72.01

38.54


86.1

5

6.1


23.6

0.48

3.46

57.03

38.15


62.9

10

8.1


22.9

0.35

4.75

49.77

37.74


57.3

15

10


22.4

0.23

7.22

44.15

37.03


53.4

0.12

13.84

39.93

36.42

20

the isotherms. The second break at high concentration corresponds to the onset of micellization, which is considered to be the real critical micelle concentration (cmc) of T1107. According to the literature, the appearance of the two breaks is ascribed to the broad molecular weight distribution of polyethers,47 changes in the configuration of polyethers at the air/water surface,48 or the formation of unimolecular micelles or oligomers before the cmc is reached.49 The variation rule of the surface tension isotherms of the T1107 aqueous solution is similar to that of PEO-PPOPEO.50 The cmc increases with increasing concentration of ethanol (Table 1). Ethanol is usually considered to be a water structure breaker,51 and it seems to be quite capable of fitting fairly comfortably into the structure of bulk water. Ethanol clearly decreases the hydrophobic interaction, which is the driving force for micellization. Thus, it should be anticipated that increasing ethanol concentration will increase the cmc. According to the mass action model, the standard Gibbs energy of micellization per mole of monomer (ΔG°mic) is given by ΔG°mic ¼ RT lnðXcmc 0 Þ

ð5Þ

where Xcmc0 is the molar fraction of polyether at the cmc. It is observed that the values of ΔG°mic are negative (Table 1), implying that thermodynamically stable micelles are formed spontaneously. Furthermore, the values of ΔG°mic become less negative as the ethanol content in the mixed solvent system increases, indicating that micellization becomes less spontaneous at higher ethanol content. From the surface tension isotherms, some surface activity parameters can be obtained. The maximum surface excess concentration (Γmax) and the minimum area per molecule at the air/ liquid surface (Amin) are obtained via the following equations Γmax ¼


1 ∂γ ð2:303nRTÞ ∂ðlog CÞ

ð6Þ

1018 NΓmax

ð7Þ

Amin ¼

where R is the gas law constant, T is the absolute temperature, n is taken to be 1, N is Avogadro’s number, and ∂γ/∂(log C) is the slope between the two breaks in the surface tension isotherm. The values of Γmax and Amin are also shown in Table 1. Ethanol is considered to be a water structure breaker, and its presence looses the icelike structure of water, which results in a less-dense coiled packing of T1107. As a consequence, the Γmax value decreases and the Amin value increases upon increasing the amount of ethanol. The standard free energy of adsorption (ΔG°ads) at the air/ liquid surface is estimated from the relation52 Πcmc Γmax

ð8Þ

Πcmc ¼ γsolvent
γcmc

ð9Þ

ΔGοads ¼ ΔGοmic


where γsolvent is the surface tension of solvent and γcmc is the surface tension of the solution with polyether at the cmc. The values of ΔG°ads become less negative as the ethanol content increases, suggesting that adsorption is less favorable. Unlike water, which is a PEO-block-selective solvent, ethanol is known to be a good solvent for both the PEO and PPO blocks of polyethers.10 The ethanol
water mixture becomes a better solvent for T1107 compared to pure water. As a consequence, the T1107 molecules are more easily solubilized and their tendency to be adsorbed at the air/liquid surface decreases. Furthermore, the ΔG°ads values are more negative than their corresponding ΔG°mic values at all ethanol concentrations, suggesting that self-aggregation in the bulk is secondary and less spontaneous than adsorption. It is important to note that the phenomenon of two breaks gradually disappears as the ethanol content increases. The appearance of the two breaks is possibly ascribed to the broad molecular weight distribution of T1107 (PDI = 1.443). T1107 molecules with a smaller molecular weight are dissolved in the ethanol
water mixed solvent. The polydispersity of T1107 at the air/liquid surface is weakened, thus the first break disappears. When 20 wt % ethanol is added, two breaks disappear, meaning that the sign of micelle formation disappears. The surface tension of mixtures of water and 20 wt % ethanol without T1107 is 39.93 mN 3 m
1. Therefore, the addition of T1107 has a small effect on the surface tension, and few T1107 molecules are adsorbed on the air/liquid surface, which coincides with the results of a very small Γmax. T1107 molecules neither adsorb on the surface nor form micelles. It can be inferred that T1107 molecules mainly exist in the bulk. They can be considered to be well-solvated in an ethanol
water mixture or to exist as a unimolecular micelle. The surface tension isotherms of a T1107 aqueous solution in the presence of n-propanol at different concentrations (5, 10, 15, and 20 wt %) are shown in Figure 3. The addition of n-propanol to water has almost the same effects as ethanol, but the effect of npropanol is more pronounced. When 5 wt % n-propanol is added, the cmc is 8.6 wt %. Ethanol and n-propanol have similar structures, but the dielectric constant of n-propanol (20.5) is smaller than that of ethanol (24.3), indicating that the bulk phase of n-propanol
water mixtures will be a better solvent for T1107 molecules than will ethanol
water mixtures. Thus, T1107 molecules have less opportunity to form micelles. Compared to the addition of ethanol, the addition of n-propanol produces much lower surface tensions. For example, when 20 wt % npropanol is added, the surface tension is 28.83 mN 3 m
1. (For 20 wt % ethanol, it is 36.42 mN 3 m
1.) The limiting surface tension is solvent-dependent, being 8 mN 3 m
1 lower in n-propanol
water mixtures than in ethanol
water mixtures because of the inherent differences in γsolvent. The surface tension of mixtures of water and 20 wt % n-propanol without T1107 (γsolvent) is 28.93 mN 3 m
1. Therefore, T1107 has little influence on the surface tension, and the lower surface tension is mainly attributed 9255

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to the fact that n-propanol molecules are adsorbed on the air/ liquid surface. The surface tension isotherms of the T1107 aqueous solution in the presence of GLY at different concentrations (5, 10, 20, and 30 wt %) are shown in Figure 4a. The surface activity parameters of the T1107 aqueous solution with different contents of GLY are listed in Table 2a. Opposite to the results of ethanol and n-propanol, the addition of GLY shifts the cmc to lower concentrations. It indicates that the presence of GLY favors the micellization of T1107 compared to the case of pure water. Because each GLY molecule has three groups capable of forming

Figure 3. Surface tension isotherms of the T1107 aqueous solution with different contents of n-propanol.

hydrogen bonds, there is a high probability that all molecules are interlinked by a number of hydrogen bonds. GLY interacts favorably with water and strengthens the H-bond network of the mixed solvent. This may be the result of an increase in water structuring, which will produce an increase in hydrophobic interactions; consequently, the cmc decreases. From another point of view, it is found that the miscibility of GLY with water decreases in the presence of PEO-PPO-PEO (1 wt % EO37PO58EO37 decreases it by a factor of 10%).14 At the same time, the solvency conditions for PEO-PPO-PEO become worse in the GLY
water mixed solvent. This means that the GLY
water mixed solvent becomes a poor solvent for T1107 compared to pure water. T1107 molecules tend to self-assemble at lower concentrations to reduce the solvophobic interaction between it and the solvent. It is also observed that the Gibbs energies of micellization of T1107 are negative and absolute values become higher as the GLY content increases, indicating that the micellization becomes more spontaneous in the presence of GLY. The surface tension isotherms of the T1107 aqueous solution in the presence of EG at different concentrations (5, 10, 20, and 30 wt %) are shown in Figure 4b. The surface activity parameters of the T1107 aqueous solution with different contents of EG are listed in Table 2b. Obviously, EG and GLY have similar effects, but the effect of EG is less pronounced. Each EG molecule has two groups capable of forming hydrogen bonds with water and appears to produce fewer depressions of the cmc than GLY with three groups capable of hydrogen bonding to water.

Figure 4. Surface tension isotherms of the T1107 aqueous solution with different contents of (a) GLY and (b) EG. The inset is the magnified image of the curves in the circle.

Table 2. Surface Activity Parameters of the T1107 Aqueous Solution with Different Contents of (a) GLY and (b) EG GLY (wt %)

cmc (wt %)

ΔG°mic (kJ 3 mol
1)

(a) In GLY
Water Mixed Solvents Γmax (10
6 mol 3 m
2) Amin (nm2) γsolvent (mN 3 m
1)

γcmc (mN 3 m
1)

ΔG°ads (kJ 3 mol
1)

5

2.0


26.4

0.62

2.64

70.71

39.40


76.9

10

1.4


27.2

0.55

2.84

70.43

39.72


83.0

20

0.74


28.8

0.50

2.98

69.48

40.21


87.2

30

0.38


30.5

0.34

4.39

68.61

40.62


112.8

γcmc (mN 3 m
1)

ΔG°ads (kJ 3 mol
1)

EG (wt %)

cmc (wt %)

ΔG°mic (kJ 3 mol
1)

(b) In EG
Water Mixed Solvents Γmax (10
6 mol 3 m
2) Amin (nm2) γsolvent (mN 3 m
1)

5

3.4


25.0

0.63

2.64

70.42

39.01


74.9

10

2.4


25.9

0.60

2.77

68.91

39.27


75.3

20

1.0


28.1

0.56

2.97

63.01

40.08


69.0

30

0.53


29.6

0.51

3.26

59.45

40.36


67.0

9256

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Figure 5. Surface dilational viscoelasticity as a function of T1107 concentration at different dilational frequencies: (a) dilational modulus and (b) phase angle.

Although the cosolvents are all polar and soluble-water, they have different polarities. There are several parameters that can be used to represent the solvent polarity, such as the dielectric constant and the octanol/water partition coefficient (log P). The dielectric constant is a parameter defining a single physicochemical property, and reliable data are available. log P is a more complex parameter that comprises a number of solvent properties and can serve as a measure of the hydrophobic interactions in the system.53 According to their log P values, the cosolvents can be classified as PEO-resembling and PPO-resembling (with a lower log P than that of EG and a higher log P than that of propylene glycol). Because EG’s structure is closest to that of the PEO blocks and propylene glycol’s structure is closest to that of the PPO blocks, the log P value of propylene glycol is 1.41.54 The log P value of ethanol is
0.32 and that of n-propanol is 0.28, they are both more hydrophobic than the PEO blocks, and they have higher affinities than water for the hydrophobic PPO blocks. The addition of ethanol or n-propanol to water results in better solvent conditions for T1107 and leads to an increase in the cmc. Moreover, n-propanol is relatively more hydrophobic (has a less negative log P value) than ethanol and has a higher preference to mix with hydrophobic PPO blocks, but the formation of micelles is more difficult. GLY and EG affect the micellization in opposite ways compared to the cases of ethanol and n-propanol. Although EG’s structure is closest to that of the PEO blocks, EG is more hydrophilic than PEO. Note that the log P values of GLY (
2.55) and EG (
1.93) are more negative than that of PEO, indicating a strong affinity to water. GLY has the most negative log P value among all of the cosolvents examined here. The addition of GLY to aqueous solution leads to a pronounced dehydration of the PEO group.13 The higher cmc observed at the presence of GLY may result from the attempt to reduce the unfavorable interactions between the PPO blocks and the mixed solvent, as well as between the PEO blocks and the mixed solvent. In different solvent mixtures, the cmc increases following the order n-propanol
water > ethanol
water > water > EG
water > GLY
water. The values of log P of different cosolvents follow the order n-propanol > ethanol > water > EG > GLY. Thus, log P appears to capture the effect of cosolvents on the cmc qualitatively. The addition of ethanol and n-propanol to water progressively increases the cmc, but the addition of EG and GLY decreases the cmc. 3.2. Dilational Viscoelasticity. 3.2.1. Dependence of Surface Dilational Viscoelasticity on the T1107 Concentration. The variations of the dilational modulus and phase angle as a function of

the T1107 concentration at different dilational frequencies (from 0.005 to 0.1 Hz) are shown in Figure 5. It can be seen that the dilational modulus has a very low concentration dependence below 0.00005 wt %, which increases with increasing T1107 concentration up to 0.001 wt % and then decreases dramatically above it. The phase angle is at its minimum at 0.0001 wt % and then increases with increasing concentration. The surface dilational viscoelasticity is caused by the microscopic relaxation processes at the air/liquid surface and near the surface. Therefore, the dilational viscoelasticity can provide information about the molecular interaction and structure at the surface layer.45 For low-molecular-weight surfactant systems, the two types of relaxation processes are usually considered to be the exchange of molecules between the bulk solution and the surface and conformational changes in the surface layer.55 However, for macromolecules, the characteristic diffusional time is too large and the exchange between the surface layer and the bulk can be neglected, but the exchange between different regions of the surface layer has to be taken into account.42,56 There are two main regions of the surface layer: one is the proximal region (a relatively narrow concentration region contiguous to the gas phase), and the other is the distal region where tails and loops protrude into the bulk and the concentration of monomers is lower. At very low T1107 concentrations (below 0.00005 wt %), the adsorbed T1107 molecules are lying flat at the air/water surface and do not form long loops and tails. The dilational viscosity component is close to zero, and dilational elasticity increases slightly with increasing T1107 concentration. This results in the obvious decrease in the phase angle at an almost constant dilational modulus value. With further increases in the T1107 concentration (from 0.00005 to 0.001 wt %), the surface concentration increases and hinders the complete unfolding of T1107 molecules in the surface layer. Gradually, some loops and tails formed by PEO blocks appear in the distal region of the surface layer. This leads to a relaxation of the exchange of hydrophobic microdomains between the proximal and distal regions of the surface layer. At the same time, the interaction among hydrophobic microdomains is stronger, which results in the increase in the dilational modulus. A subsequent increase in the concentration (above 0.001 wt %) leads to the faster exchange of hydrophobic microdomains between the proximal and distal regions, a decrease in the dilational modulus, and an increase in the phase angle. To confirm our explanation above, the surface tension relaxation measurement has been carried out to detect the microscopic 9257

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relaxation processes in the surface layer. We can determine the characteristic relaxation time (τ) and the contributions of different relaxation process (A), which are connected to the dynamic characteristics of macromolecules and can provide the most important information about relaxation processes. The surface relaxation processes and their characteristic relaxation times at different T1107 concentrations are shown in Table 3. We can see clearly that there exist three relaxation processes at the surface. Process 1 is the fast relaxation process with a characteristic time value from several tens of seconds to several seconds with the increase of T1107 concentration. It is closely acquainted with the fast relaxation process involving the exchange of T1107 molecules between different regions in surface layer. It can be found that the characteristic relaxation time of the fast relaxation process (τ1) decreases with the increase in concentration. That is, the greater the concentration of the solution, the faster the exchange of the molecule between different regions. The slowest process may correspond to the arrangement of the whole surface layer. The middle relaxation process may attribute to the conformational changes in the T1107 chains at the surface. The contribution of the fast relaxation process (A1) is overall smaller than that of the slow relaxation process (A2 plus A3), indicating that the slow relaxation process is the major factor in the relaxation process of T1107 at the surface. It is important to note that the phenomena of the negative phase angle indicates a negative viscosity. The surface dilational viscosity is negative as obtained by the method of surface quasielastic light scattering (SQELS) for solutions of both conventional surfactants57,58 and polymer films,59,60 and there is no exact mechanism responsible for it. In other groups’ studies,38 the negative phase angles have been obtained for the adsorbed surface film at which the slow relaxation process dominates the dilational viscoelasticity, and positive phase angles appear when Table 3. Surface Relaxation Processes and Their Characteristic Relaxation Times at Different T1107 Concentrations process 1

process 2

process 3

C/wt % A1/mN 3 m
1 τ1/s A2/mN 3 m
1

τ2/s

A3/mN 3 m
1

τ3/s

0.0001

0.14

87.7

2.35

3975

2.36

4105

0.001

0.53

27.9

1.16

185.5

1.64

768.9

0.01

0.43

11.7

0.80

62.2

0.38

431.1

0.1 1

0.53 0.42

6.51 5.29

0.63

33.8

0.16 0.46

207.1 58.3

the fast exchange of molecules between the surface and the bulk solution dominates the dilational viscoelasticity. It is believed that the slow relaxation processes, which dominate the dilational viscoelasticity of the T1107 film, are responsible for this phenomenon in our employed experimental method. For a certain area change, the negative phase angle will appear when the oscillation frequency becomes high enough that the fast exchange of molecules cannot compensate for the surface concentration change caused by the area change. Therefore, the higher the oscillation frequency, the larger the possibility that the film has a negative phase angle. 3.2.2. Influence of Alcohols on the Surface Dilational Viscoelasticity of T1107 Solutions. To study the effect of alcohols on the dilational rheological properties of the T1107 solution, the dilational viscoelasticity of the 0.001 wt % T1107 system with different concentration of alcohols are obtained, as shown in Figure 6. The concentration of T1107 (0.001 wt %) is chosen because the dilational modulus is largest in the range of concentration studied (Figure 5). With increasing concentration of ethanol or n-propanol, the dilational elasticity of T1107 solutions decreases; this coincides with the change in Γmax. In the presence of ethanol or n-propanol, the mixtures become better solvents for T1107 and their tendency to adsorb at the surface decreases. Ethanol and n-propanol are considered to be water structure breakers, and their presence disrupts the hydrogen-bond network of water, resulting in a less-dense coiled packing of these molecules. With increasing concentration of EG or GLY, the dilational elasticity passes through a maximum, which is also the same as the change in Γmax. The effect of alcohols on the dilational elasticity of the T1107 solution is much more obvious than that on the dilational viscous component. The largest change in the dilational modulus is about 20 mN 3 m
1, whereas the largest change in the dilational viscous component is about 4 mN 3 m
1. As is known, dilational elasticity is caused by the energy change due to a departure from the equilibrium state of the molecules in the surface layer after perturbation, which is related to molecular interactions. The dilational viscous component reflects the summation of the various complex microscopic relaxation processes at and near the surface, such as the transport of molecules between different regions in the surface layer and the rearrangement of molecules at the surface. It can be inferred that the effect of alcohols on the dilational rheological properties is mainly through affecting the interaction between T1107 molecules, whereas the microscopic relaxation processes are influenced slightly by the added alcohols.

Figure 6. Surface dilational viscoelasticity of 0.001 wt % T1107 with different alcohols at the dilational frequency of 0.01 Hz: (a) dilational elasticity and (b) dilational viscous component. 9258

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4. CONCLUSIONS The effects of cosolvents such as ethanol, n-propanol, EG, and GLY on the aggregation behaviors of branched block polyether T1107 at air/liquid surface have been investigated as a function of the cosolvent content in the mixed solvent. The addition of ethanol or n-propanol to water disfavors the micellization, and the cmc shifts to higher concentration. Ethanol and npropanol are more hydrophobic than the PEO blocks, and they have a higher affinity for the hydrophobic PPO blocks than for water. The addition of ethanol or n-propanol to water results in better solvent conditions for T1107, decreases the hydrophobic interaction, and leads to an increase in the cmc. In the presence of ethanol or n-propanol, the dilational elasticity is monotonously reduced, which coincides with the change in Γmax. Because the mixtures become better solvents for T1107, their tendency to adsorb at the surface decreases. Ethanol and n-propanol are both considered to be water structure breakers, and their presence disrupt the hydrogen-bond network of water, which results in a less densely coiled packing of these molecules. GLY and EG affect the micellization in opposite ways compared with the cases of ethanol and n-propanol. GLY and EG are more hydrophilic than the PEO blocks, and GLY
water and EG
water mixed solvents become poor solvents for T1107 compared to pure water. T1107 molecules tend to self-assemble at lower concentrations. Furthermore, GLY and EG interact favorably with water and strengthen the H-bond network of the mixed solvent. They produce an increase in hydrophobic interactions; consequently, the cmc decreases. With increasing concentrations of EG and GLY, the dilational elasticity values of T1107 solutions pass through a maximum, which coincides with the change in Γmax from surface tension measurements. Thus, the micellization and dilational viscoelasticity can be changed by the variation of the cosolvent type and content. ’ AUTHOR INFORMATION Corresponding Author

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