Effect of headgroups on the aggregation behavior of cationic silicone surfactants in aqueous solution

Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 146–153

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Effect of headgroups on the aggregation behavior of cationic silicone surfactants in aqueous solution Jinglin Tan, Depeng Ma, Shengyu Feng ∗ , Changqiao Zhang ∗ Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, China

h i g h l i g h t s

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 Cationic silicone surfactants with the same hydrophobic group and different headgroups were synthesized.  Cationic silicone surfactants have higher surface activity compared with common hydrocarbon surfactants.  The ˇ values for Si4 pyCl and Si4 mamOHCl increase with increasing the temperature in the investigated temperature range.

Aggregation behaviors of three novel cationic silicone surfactants are investigated. The figure of ˇ–T for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl shows that the values of ˇ for Si4 pyCl and Si4 mam-OHCl increase as the temperature increases.

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 30 August 2012 Received in revised form 22 October 2012 Accepted 24 October 2012 Available online 30 October 2012 Keywords: Cationic silicone surfactants Electrical conductivity Degree of counterion binding Entropy-driven Enthalpy-driven

a b s t r a c t

Three cationic silicone surfactants, 1-methyl-3-[tri-(trimethylsiloxy)]silylpropyl-imidazolium chloride (Si4 mimCl), (2-hydroxyethyl)-N,N-dimethyl-3-[tri-(trimethylsiloxy)]silylpropylammonium chloride (Si4 mam-OHCl), 1-methyl-1-[tri-(trimethylsiloxy)]silylpropylpyrrolidinium chloride (Si4 pyCl), with the same hydrophobic group and different headgroups were synthesized. Their aggregation behavior in aqueous solution was systematically investigated by surface tension, electrical conductivity, and steadystate fluorescence. Surface tension of water can be reduced almost to 20 mN m−1 with the addition of the cationic silicone surfactants. This result indicates that all the three surfactants exhibit remarkable surface activity. Because of the effect of the headgroups, the critical micelle concentrations (CMC) values increase following the order Si4 pyCl < Si4 mimCl < Si4 mam-OHCl, and Si4 pyCl packs more tightly at the air/water interface compared with Si4 mimCl and Si4 mam-OHCl. Electrical conductivity measurements show that all the three cationic silicone surfactants have low degree of counterion binding (ˇ) and the ˇ values for Si4 pyCl and Si4 mam-OHCl increase with increasing the temperature in the investigated 0 0 0 temperature range. Thermodynamic parameters (Hm , Sm , and Gm ) of micellization indicate that the micellization for Si4 mimCl in aqueous is enthalpy-driven, and that for both the Si4 pyCl and Si4 mam-OHCl entropy-driven. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Silicone surfactants consist of a permethylated siloxane group coupled to one or more polar groups, which can be nonionic,

∗ Corresponding authors. Tel.: +86 531 88364866; fax: +86 531 88564464. E-mail addresses: [email protected] (S. Feng), [email protected] (C. Zhang). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.10.041

cationic, anionic, or zwitterionic. They are widely used in industrial fields, such as agricultural adjuvant, polyurethane foam additives, paint additives, emulsifiers in cosmetics, and textile conditioning [1–6]. The characteristics of silicone surfactants are that they have surface activity in both aqueous and nonaqueous solutions [7]. This extraordinary quality is attributed to three properties of silicone surfactant: (i) its flexibility enables it to pack more compactly and efficiently at various interfaces,

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(ii) its greater voluminous and cross-sectional area of the siloxane unit bring about its low cohesive energy at the interface and (iii) its low glass transition temperature even for higher molecular weight make it easier to use at room temperature [8,9]. Because of the distinctive chemical and physical properties, especially their high surface activity, low toxicity and excellent association behavior, silicone surfactants have been widely investigated and used during the past decades [10–15]. Particularly, trisiloxane surfactants are under the spotlight, because they have abilities to decrease the surface tension of water to approximately 20 mN m−1 [16,17]. Aggregation and phase behavior of silicone surfactants with various structures, ABA-type or comb-type, has been reported [16,18–22]. Much research has focused on nonionic silicone surfactants, especially trisiloxane surfactants. In dilute aqueous solutions, silicone surfactants can also form spheroidal, threadlike or wormlike micelles, large multilamellar, and small single lamellar vesicles induced by sodium dodecyl sulfate [18,23]. Kuniada et al. investigated that the self-organized structures are highly related to the surfactant concentration and the ratio of ethylene oxide (EO) and dimethylsilicone unit [24]. It is found that its surface activity is sensitive to weight fraction of EO, the length of the EO chain and the size of the hydrophobic siloxane [19,25]. However, few investigations have considered the cationic surfactants prepared by hydrosilylation of functional siloxane, which may result in impurities of isomer. Steven et al. reported that a series of cationic silicone surfactants prepared by hydrosilylation, quaternization, and methylation, can decrease the surface tension of water to 20–26 mN m−1 [26]. They also studied the phase behavior of zwitterionic silicone sulfobetaine surfactants, found that purified surfactants can decrease the surface tension of water to 21 mN m−1 . However, the surfactants would decompose in both acidic and basic aqueous solutions attributed to the hydrolysis of siloxane bonds [27]. He et al. studied the aggregation behavior of cationic silicone surfactants in aqueous solution and first reported the vesicle formation by silicone surfactants. They demonstrated that the correlation between the microstructures of surfactants in dilute solutions and “superwetting” is consistent with that of nonionic silicone surfactant solutions [28,29]. Schmaucks et al. reported the surface activity of a family of quaternary ammonium based silicone surfactants: (Me3 SiO) (SiMe2 O)n SiR1 R2 (CH2 )3 N+ Me2 R3 X− (R1 , R2 = Me, Me3 SiO; n = 0, 1; R3 = alkenyl or alkyl; X = halogen). They concluded that their interfacial behaviors were strongly influenced by the number of silicon atoms [30]. To our knowledge, few definite relationships between the molecular structure and their aggregate behavior for cationic silicone surfactants have been found. Herein, in the present work, the cationic silicone surfactants with different headgroups, based on imidazolium (Si4 mimCl), and (2-hydroxyethyl)dimethylammonium (Si4 mam-OHCl), pyrrolidinium cations (Si4 pyCl), were synthesized, and their aggregation behaviors were investigated. A series of useful parameters obtained in the present study will be useful in

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understanding the role of the headgroups in affecting the aggregation behavior of cationic silicone surfactants and in the design of novel silicone surfactants. Additionally, the cationic silicone surfactants may have potential application in the fields related to colloid and interface science, such as silicone emulsions, skin care, textile industry and construction of porous materials [1,2]. 2. Materials and methods 2.1. Materials -Chloropropyltrichlorosilane was purchased from Shandong Qiquan Silicon Co., Ltd. 1-Methylimidazole and 1methylpyrrolidine are purchased from Aladdin Chemical Reagent Co., Ltd. N,N-Dimethylethanolamine, chlorotrimethylsilane, pyrene, methanol, isopropanol and diethyl ether were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Diethyl ether was distilled from sodium-benzophenone before use. The other regents were used as received. Triply distilled water was used to prepare all the solutions. The molecular structures of the cationic silicone surfactants used in this work are shown in Fig. 1. Their detailed synthesis process is presented in the Supporting Information. 2.2. Methods 2.2.1. Surface tension Surface tension measurements were carried out on a model BZY-1 tensiometer (Shanghai Hengping Instrument Co., Ltd., accuracy ± 0.1 mN m−1 ) employing the ring method and using a thermostatic bath (DC-0506, Shanghai Hengping Instrument Co., Ltd.). All measurements were taken at 25.0 ± 0.1 ◦ C and repeated until the values were reproducible. 2.2.2. Electrical conductivity Specific conductivity measurements on the aqueous solutions were performed using a low-frequency conductivity analyzer (model DDS-307, Shanghai Precision & Scientific Instrument Co., Ltd., accuracy ± 1%). 2.2.3. Fluorescence measurements Fluorescence measurements were carried out using a Hitachi fluorescence spectrophotometer F-7000 equipped with a thermostat cell holder at 25◦ C. The excitation wavelength was 335 nm. Emission spectra were detected from 350 to 500 nm with the slit widths fixed at 2.5 and 5 nm for emission and excitation. The concentration of pyrene was fixed at 1.0 × 10−6 M in all the measurements. 3. Results and discussion 3.1. Surface tension results The surface tensions of the aqueous solutions of the three cationic silicone surfactants, Si4 mimCl, Si4 pyCl, and Si4 mam-OHCl,

Fig. 1. Chemical structures of the three cationic silicone surfactants.

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J. Tan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 146–153 Table 2 Adsorption parameters of Si4 mimCl, Si4 pyCl, and Si4 mam-OHCl in aqueous solutions at 298 K.  CMC (mN m−1 ) CMC (mN m−1 )  max (␮mol m−2 ) Amin (Å2 ) Si4 mam-OHCl 21.2 20.4 Si4 mimCl 20.0 Si4 pyCl

Fig. 2. Surface tension of Si4 mimCl, Si4 pyCl, and Si4 mam-OHCl in aqueous solutions as a function of their concentrations at 25 ◦ C. Si4 mimCl (), Si4 pyCl (䊉), and Si4 mamOHCl (). The inset, (a) shows the regions near the break points related to the CMC in more detail.

were measured to evaluate their surface activities. Fig. 2 shows the plots of the surface tension () versus concentration (C) for the aqueous solutions of Si4 mimCl, Si4 pyCl and Si4 mam-OHCl at 25 ◦ C. It can be seen from Fig. 2, the surface tension decreases initially with increasing the concentration of the three cationic surfactants, suggesting all the surfactants are adsorbed at the air/solution interface. Then a plateau appears in the (–C) plot, indicating that the micelles are formed. The critical micelle concentrations (CMC) were regarded as the concentrations at the break point of the two linear portions of the –C plots [31], and the values of the CMC for the three silicone surfactants are listed in Table 1. It is worth noting that the –C plots for Si4 mimCl, Si4 pyCl, and Si4 mam-OHCl do not show any minima, indicating that silicone surfactants exhibit high purity. The structure of head groups is one of the most important factors to affect the surface activities and micellization behavior of surfactants [32–34]. As shown in Table 1, the CMC value follows the order Si4 pyCl < Si4 mimCl < Si4 mam-OHCl. This result may be due to the delocalized positive charge on the imidazolium ring [35,36] compared with Si4 mam-OHCl. Meanwhile, the CMC of Si4 pyCl is lower than that of Si4 mimCl. The reason might be the lower hydrophilicity of the pyrrolidinium cations caused by the introduction of methyl in comparison with the imidazolium cations. The surface tension at CMC ( CMC ) and surface pressure at CMC (CMC ) can also be used to evaluate the surface activities of surfactants. The surface pressure at CMC, CMC , is defined as Eq. (1) [37]. CMC = 0 − CMC

(1)

where  0 is the surface tension of pure water and  CMC is the surface tension at CMC. The values of  CMC and CMC were obtained and listed in Table 2. The  CMC were calculated to be 21.2, 20.4, and 20.0 mN m−1 for Si4 mam-OHCl, Si4 mimCl, and Si4 pyCl, respectively. The cationic silicone surfactants investigated in this paper have lower  CMC values than that of conventional cationic surfactants [32], i.e., hexadecylpyridinium chloride ( CMC = 42.0),

51.3 52.1 52.5

1.01 0.84 1.24

164.5 197.5 134.0

hexadecyltrimethylammonium chloride ( CMC = 41.0) [38] and imidazolium-based surfactants [35,39]. Correspondingly, their CMC values are larger compared with those of cationic hydrocarbon surfactants. This result could demonstrate that the cationic silicone surfactants possess much more remarkable surface activity. This is attributed to both the superiority of highly surface active methyl substituent and a flexible Si O Si which enables the methyl groups to orient in low energy configurations [26,30]. The maximum excess surface concentration ( max ) and the area occupied by a single amphiphile molecule at the air/water interface (Amin ), obtained from the Gibbs adsorption isotherm, can reflect the surface arrangement of surfactants at the air/liquid interface [34]. A greater value of  max or smaller values of Amin means a denser arrangement of surfactant molecules at the surface of the solution [35]. The maximum excess surface concentration,  max , is calculated using Gibbs adsorption isotherm equation (Eq. (2)) [39]. max = −

1 nRT



∂ ∂ ln C



(2)

where  is the surface tension, R is the ideal gas constant, T is the absolute temperature, C is the surfactant concentration, (∂/∂ ln C) is the slope of the linear fit of the data before the CMC in the surface tension plots. Moreover, the Gibbs equation can only be used in the pre-CMC region and the polynomials can be fitted to the pre-CMC curves to generate gradients, then surface excesses were calculated by the gradients. The value of n is theoretically dependent on the surfactant type and structure, as well as the presence of extra electrolytes and impurities in the aqueous solution [40,41]. However, in this paper, we took n = 2 assuming that the cationic silicone surfactants in the absence of any other solutes [42,43]. When  max is estimated, the value of Amin is obtained using Eq. (3) [39]. Amin =

1016 NA max

(3)

where NA is Avogadro’s number. The values of  max and Amin were obtained and are given in Table 2. From Table 2, it is clearly known that the Amin of cationic silicone surfactants are larger than those of cationic hydrocarbon surfactants, i.e., dodecylpyridinium chloride (Amin = 52.5) [37], 1-dodecyl-3-methylimidazolium chloride (Amin = 86.8) [58], indicating a looser arrangement of the cationic silicone surfactants molecules at the air/water interface [43]. The reason may be that the three bulky trimethylsilyoxyl groups causes a greater distance between the surfactant molecules, and thus results in the so-called “umbrella” conformation with the siloxanyl group oriented parallel to the water surface [26,30]. Moreover, the Amin and  max values vary with the change of head groups of silicone surfactants. The  max value of the Si4 pyCl is the largest compared with Si4

Table 1 Critial micelle concentration (CMC) of Si4 mimCl, Si4 pyCl, and Si4 mam-OHCl in aqueous solutions at 298 K. Surfactants

Si4 mam-OHCl Si4 mimCl Si4 pyCl

CMC/(mM) Determined from surface tension

Determined from electrical conductivity

Determined from fluorescence

11.4 ± 0.1 7.4 ± 0.1 5.7 ± 0.1

11.6 ± 0.1 7.5 ± 0.1 5.5 ± 0.1

10.8 ± 0.1 7.7 ± 0.1 5.8 ± 0.1

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mam-OHCl and Si4 mimCl. Correspondingly, the Amin value of Si4 mimCl is almost twice times higher than that of Si4 pyCl. The  CMC and Amin values for Si4 pyCl are much lower, which is similar with the observation by Kickelbick [20]. The reasonable reason may be that the headgroups with stronger charge density are more tightly bound by the surrounding water and thus less effective in reducing the charge repulsion and closer packing for Si4 pyCl [44,45]. The charge density of headgroups for silicone surfactants follows the order imidazolium < ammonium < pyrrolidinium (from Fig. 1). Therefore, the Amin value of Si4 pyCl (134.0 A˚ 2 ), is smaller compared with those of Si4 mam-OHCl (164.5 A˚ 2 ) and Si4 mimCl (197.5 A˚ 2 ). 3.2. CMC for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl at different temperatures Conductometry is useful for studying the association behavior of various systems [46]. In order to gain an insight into the aggregation behavior of Si4 mimCl, Si4 pyCl and Si4 mam-OHCl in aqueous solution, the electrical conductivity was measured as a function of the surfactant concentration at different temperatures. Fig. 3 depicts the electrical conductivity, , as a function of surfactant concentration for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl at different temperatures. As shown in Fig. 3, the electrical conductivity reveals a progressive increase with increasing temperature due to an increase in the thermal energy of the molecular entities. The CMC values were determined by the intersection of the two straight lines in –C plots and given in Table 3. It can be seen that the CMC values increase with increasing the temperature. This phenomenon is influenced by two aspects [47]: on one side, temperature increase would decrease the hydration of the head group of surfactants, which is beneficial to the micelle formation, on the other side; it accelerates the breakdown of the water structure surrounding the hydrophobic trimethylsilyl group, which is not conducive to the micelle formation. 3.3. Degree of counterion binding for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl at different temperatures The degree of counterion binding of micelle can be obtained from conductivity measurements using Eq. (4) [32]. ˇ =1−

˛1 ˛2

(4)

where ˛1 and ˛2 are the slopes of the straight lines before and after the CMC in the conductivity plots, respectively. The ˇ values for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl obtained from conductivity at different temperatures are summarized in Table 3. In Table 3, it is clearly seen that the degree of counterion binding of the three silicone surfactants are all pretty low, indicating the self-repulsion of the head group and the counterion is stronger than the attraction between the head groups and counterion. The ˇ values decrease with the increase of temperature for ordinary ionic surfactants, which can be attributed to that the motion Table 3 Critial micelle concentration (CMC) and degree of counterion binding (ˇ) of Si4 mimCl, Si4 pyCl, and Si4 mam-OHCl in aqueous solutions at different temperatures. T (◦ C)

15 20 25 30 35

Si4 mam-OHCl

Si4 mimCl

Si4 pyCl

CMC (mM)

ˇ

CMC (mM)

ˇ

CMC (mM)

ˇ

10.3 10.9 11.6 12.1 12.7

0.31 0.32 0.34 0.35 0.36

6.6 6.9 7.5 8.2 9.0

0.32 0.30 0.27 0.25 0.22

4.8 5.0 5.5 5.9 6.6

0.24 0.27 0.30 0.33 0.35

Fig. 3. Specific conductivity as a function of concentration at different temperatures: (a) Si4 mam-OHCl, (b) Si4 mimCl, and (c) Si4 pyCl.

of surfactant molecules accelerated with temperature increase [35,48]. The curves of ˇ–T for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl are shown in Fig. 4. The ˇ values of Si4 mimCl decrease as the temperature increases in the whole studied temperature range, following the regular rule for ordinary ionic surfactants [47]. However, the values of ˇ for Si4 pyCl and Si4 mam-OHCl increase as the temperature increases. This result can be interpreted by the effect of headgroups. It can be seen from Fig. 1, the effective charge of the

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J. Tan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 146–153 0 ), and the standard enthalpy change (H 0 ), could be change (Sm m 0 can be obtained form Eq. (5) [35,39,51]. estimated. Gm 0 = (1 + ˇ)RT ln CMC Gm

(5)

where the CMC is the mole fraction of silicone surfactant at CMC, T is the absolute temperature, and R is the ideal gas constant. Once the Gibbs free energy is obtained, the enthalpy of micelle formation can be obtained by using the Gibbs–Helmboltz equation (Eq. (6))



0 Hm =

0 /T ) ∂(Gm ∂(1/T )



(6)

Then the entropy of micelle formation is determined by using Eq. (7) 0 = Sm

Fig. 4. Degree of counterion binding (ˇ) of the micellization as a function of temperature: Si4 mimCl (), Si4 pyCl (䊉), and Si4 mam-OHCl ().

quaternary ammonium nitrogen would be affected by the presence of the unsaturated bond on the imidazolium ring and partial negative charge on the oxygen atom for Si4 mimCl and Si4 mam-OHCl, respectively. Consequently, the charge density of headgroups should be increased in the order of imidazolium < (2hydroxyethyl)dimethylammonium < pyrrolidinium. What is more, the hydrophobic group of siloxanes possesses the partial negative charge on the oxygen atom, which may be expected to explain the fact that some attractive interaction exists between the nitrogen atom of one surfactant molecule with the oxygen atom of another surfactant molecule [35,46,49]. In other words, the attractive interaction would hinder the association of counterions with head group of surfactant. Clearly, the attractive interaction would be weakened as the temperature increases, and the association of counterions with headgroups would be enhanced compared with the Si4 mimCl. This may explain why the values of ˇ for Si4 pyCl and Si4 mam-OHCl increase with increasing temperature. 3.4. Thermodynamic analysis of micellization In order to further understand the effect of head group on the micellization of cationic silicone surfactants, the thermodynamic parameters of micellization were estimated by the mass action model [50]. According to the mass action model, thermodynamic parameters of micelle formation for silicone surfactants, such as 0 ), the standard entropy the standard Gibbs energy change (Gm

0 − G0 Hm m T

(7)

Therefore, the thermodynamic parameters for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl were obtained and are given in Table 4. 0 , G0 , and −TS 0 as a funcFig. 5 shows the plots of Hm m m tion of temperature for silicone surfactants Si4 mimCl, Si4 pyCl, and Si4 mam-OHCl. As temperature increases in the investigated 0 values of the three silicone surfactants are all range, the Gm negative, indicating a spontaneous micelle formation process. It 0 for Si pyCl and Si mamis worth noting that the value of Gm 4 4 0 , indicating that the OHCl are both mainly contributed by −TSm micellization process for Si4 pyCl and Si4 mam-OHCl in aqueous solution both is entropy-driven. On the other side, the value of 0 for Si mimCl is mainly contributed by H 0 , indicating that Gm 4 m the micellization process for Si4 mimCl in aqueous solution is enthalpy-driven. It is well known that the micellization for ionic surfactant is mainly contributed by the hydrophobic interactions and electrostatic interactions [52]. As the results shown in Table 4 and Fig. 5, 0 values for Si pyCl and Si mam-OHCl are positive within the Hm 4 4 the temperature investigated and unfavorable for micellization. This phenomenon could be expounded by the attractive interaction between the positive charged N atoms and negative charged oxygen atom of Si O Si (as mentioned above). Meanwhile, the stronger attractive interaction between the positive charged N atoms and negative charged oxygen atom may be one reason for the 0 values for Si pyCl. S 0 values for Si mimCl larger positive Hm 4 4 m and Si4 pyCl decrease with increasing temperature, which could be due to the breakage of hydrogen bonds in water with increasing temperature [53,54]. Because of the “melting” of the “iceberg structure” of water molecules surrounding siloxane chains moieties, the hydrogen bonds in water are gradually destroyed with increasing temperature. Herein, it can be proved that the headgroups play an

Table 4 Thermodynamic parameters of micelle formation for Si4 mimCl, Si4 pyCl, and Si4 mam-OHCl in aqueous solutions at different temperatures. Surfactants

T (◦ C)

0 Gm (kJ mol−1 )

0 Hm (kJ mol−1 )

0 −TSm (kJ mol−1 )

Si4 mam-OHCl

15 20 25 30 35

−26.95 −27.53 −28.08 −28.71 −29.29

5.20 6.01 6.80 7.56 8.30

−32.15 −33.54 −34.88 −36.27 −37.59

Si4 mimCl

15 20 25 30 35

−28.64 −28.42 −28.11 −27.71 −27.32

−40.48 −44.39 −48.17 −51.82 −55.35

11.84 15.97 20.06 24.11 28.03

Si4 pyCl

15 20 25 30 35

−27.75 −28.81 −29.74 −30.63 −31.35

36.16 29.88 23.81 17.94 12.27

−63.91 −58.68 −53.55 −48.57 −43.62

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Fig. 6. I1 /I3 of pyrene as a function of concentration of Si4 mimCl (), Si4 pyCl (䊉), and Si4 mam-OHCl ()in aqueous solution at 25 ◦ C.

fluorescence emission spectrum consisting of five bands due to its strongly polarity dependent florescence intensities. When pyrene molecules are solubilized in the inner hydrophobic region of the micelles, the intensity ratio of the first to the third band (I1 /I3 ) behaves a sudden change. Therefore, the I1 /I3 ratio can be served to a measure of the polarity of the environment [55,20,56]. The plots of I1 /I3 ratio as a function of silicone surfactant concentration are illustrated in Fig. 6. As shown in Fig. 6, the ratios I1 /I3 keep constant initially and then decreases sharply until CMC are reached. After CMC, the I1 /I3 ratios are almost constant with further increase in the concentration of silicone surfactants. The CMC values derived from the I1 /I3 data are listed in Table 1 and are in accordance with the values obtained from tensiometry and conductometry. The I1 /I3 for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl is 1.29, 1.27, and 1.06, respectively. That result can be explained by the following way: The I1 /I3 value for Si4 mimCl is larger compared with Si4 pyCl which may be caused by the – interaction between the imidazolium and the pyrene, consequently, the pyrene molecules tend to stay near the head group region and the environment is water world. While, intermolecular hydrogen bonds of headgroup for Si4 mam-OHCl cause the pyrene to be solubilized in the palisade layer near the polar headgroups in micelles, and therefore, Si4 mam-OHCl has the lowest values of I1 /I3 [57,58]. 4. Conclusions

Fig. 5. Thermodynamic parameters of micellization as a function of temperature for (a) Si4 mimCl, (b) Si4 pyCl, and (c) Si4 mam-OHCl. Squares, circles, triangles corre0 0 0 , Hm , and −TSm , respectively. spond to Gm

important part in the micelle formation by cationic silicone surfactants. 3.5. Fluorescence investigations The aggregation behavior and the microenvironment of the aggregates for Si4 mimCl, Si4 pyCl and Si4 mam-OHCl in the aqueous solutions were investigated with steady-state fluorescence spectroscopy using pyrene as a probe. Pyrene shows a distinct

Aggregation behavior of silicone surfactants has attracted the most attention in the colloid and interface fields, especially trisiloxane surfactants [16,17,19]. However, few investigations have considered the cationic surfactantsns [29,30]. To expand the varieties of cationic silicone surfactants, we synthesized three cationic silicone surfactants with the same hydrophobic groups but different headgroups and investigated their aggregation behaviors. They have excellent surface activity compared with common hydrocarbon surfactants [34,35] due to the superiority properties of siloxane. Because of the different structure of the head groups, the CMC values increase following the order Si4 pyCl < Si4 mimCl < Si4 mam-OHCl, and the ˇ values for Si4 pyCl and Si4 mam-OHCl increase as temperature increases in the investigated temperature range. As comparison, Si4 pyCl packs more compactly at the air–water interface. Concerning the driving force of the micellization, Si4 mimCl is an enthalpy-driven process, while the process for Si4 pyCl and Si4 mam-OHCl is entropy-driven. These results indicate that properties of headgroups of the cationic

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silicone surfactants can significantly affect their surface activity and micellization process. Understanding the aggregation behaviors of the cationic silicone surfactants may help to design cationic silicone surfactants with new structures and find potential applications in colloid and interface science.

Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 20874057) and the Key Natural Science Foundation of Shandong province of China (ZR2011BZ001).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2012.10.041.

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