Effect of the spacer length on the electrostatic interactions of cationic gemini surfactant micelles with trianionic curcumin

Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 80–86

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Effect of the spacer length on the electrostatic interactions of cationic gemini surfactant micelles with trianionic curcumin Dan Ke, Qianqian Yang, Mingling Yang, Yue Wu, Jinbing Li, Haibo Zhou, Xiaoyong Wang ∗ School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Interactions of gemini surfactant micelles with Cur3− are regulated by spacer length. • Cur3− is located in micelle palisade layer by electrostatic attraction. • Cur3− exhibits the smallest alkaline degradation in C12 C6 C12 Br2 micelle. • There is optimum charge matching of C12 C6 C12 Br2 micelle to Cur3− .

a r t i c l e

i n f o

Article history: Received 4 January 2013 Received in revised form 15 May 2013 Accepted 6 June 2013 Available online xxx Keywords: Gemini surfactant micelle Curcumin Stability Absorption Fluorescence

a b s t r a c t The absorption and fluorescence measurements have been used to study the interactions between cationic gemini surfactant micelles of alkanediyl-␣,␻-bis(dodecyldimethylammonium bromide) (C12 Cs C12 Br2 , where s = 2, 3, 4, 6, and 12, indicating the number of carbons in the spacer) with trianionic curcumin (Cur3− ) at pH 13. With increasing spacer length of gemini surfactant, the maximum intensities of absorption and fluorescence peaks of Cur3− give the highest values at s = 6, reflecting that the spacer length of gemini surfactant significantly regulates the electrostatic attractive interactions between C12 Cs C12 Br2 micelles and Cur3− . The maximum fluorescence intensity of Cur3− in C12 C6 C12 Br2 micelle shows increasing tendency at 0–100 mM sodium bromide concentration (CNaBr ), but decreases above CNaBr = 120 mM. This result is explained in terms of the enhanced electrostatic attraction of Cur3− with C12 C6 C12 Br2 micelle at low salt concentrations and the reduced electrostatic attraction at high salt amounts. The determined pKa1 values of curcumin in C12 Cs C12 Br2 micelles support the strongest electrostatic association between C12 C6 C12 Br2 micelle and anionic Cur− . The interaction mechanisms of C12 Cs C12 Br2 micelles with Cur3− have been further proposed at s = 2–4, s = 6, and s = 12, which is related to the matching of the positive charges in C12 Cs C12 Br2 micelles to the negative charges in Cur3− . © 2013 Elsevier B.V. All rights reserved.

1. Introduction The subject of interaction between surfactant and drug is extremely interesting both in fundamental science and in the application of drugs. It has been shown that surfactants can enhance drug solubility, maintain drug stability, control drug release and uptake, and improve bioavailability of drugs [1–3]. Although it has

∗ Corresponding author. Tel.: +86 21 64252012. E-mail address: [email protected] (X. Wang). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.06.017

been known that the surfactant structures such as headgroup and alkyl chain significantly influence drug functions, the mechanisms of surfactant–drug interaction are poorly understood and many unsolved questions about the relationship of surfactant structure with physicochemical properties and the bioactivities of drugs are still remained. As one natural polyphenolic compound isolated from the turmeric powder, curcumin has recently drawn increasing research attention owing to its numerous biological and pharmacological activities, including antioxidant, antitumor, anti-inflammatory, anticancer, and other desirable medicinal properties [4–6]. Nevertheless, one major challenge in using curcumin for treatment of

D. Ke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 80–86

diseases is the poor aqueous solubility at acidic and neutral pHs, which seriously limits its availability in biological systems [7,8]. Another main problem is that curcumin breaks down readily at neutral and basic pHs [9,10]. Upon changing pH, curcumin has three acid-base equilibria in aqueous solution. The pKa values for the dissociation of three acid protons of curcumin in water were previously determined as follows: pKa1 = 8.38 corresponding to the equilibrium Cur = Cur− + H+ due to the hydrogen of the enol group, pKa2 = 9.88 corresponding to the equilibrium Cur− = Cur2− + H+ , and pKa3 = 10.51 corresponding to the equilibrium Cur2− = Cur3− + H+ [11]. The pKa2 and pKa3 are attributed to the hydrogen of the phenolic groups of curcumin. Curcumin with high stability thus mainly remains neutral form under acidic condition, whereas the dissociations of three protons in the enol group and the phenolic parts of curcumin take place at basic pHs, where curcumin occurs fast degradation [9,12]. Various single-chain surfactant micelles have been designed to improve the stability and suppress the degradation of curcumin [13,14]. Tønnesen et al. studied the chemical stability of curcumin at pH 5 and 8 in the surfactant micelles of sodium dodecyl sulfate (SDS), Triton X-100 (TX-100), tetradecyltrimethylammonium bromide (TTAB), and cetylpyridinium bromide (CPB) [13]. At pH 5, they observed that these surfactant micelles offered protection against curcumin degradation in the order of SDS  TTAB > TX-100 > CBP. However, at the pHs above 9.2, Kee et al. demonstrated that cationic micelles such as dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (CTAB) instead of anionic micelle SDS can greatly stabilize the anionic curcumin against alkaline degradation [15,16]. These results indicate that the stabilizing effects of surfactant micelles on curcumin not only depend on the pH-related physicochemical properties of curcumin, but also have important relationship with the interaction between surfactant micelles and curcumin. Furthermore, it is usually considered that curcumin is located in the palisade layer of surfactant micelles [17,18], which can provide a more hydrophobic medium compared to water for the improved stabilization of curcumin. Thus, as one major composition of micellar palisade layer, the changes in the part of surfactant headgroup could markedly influence the properties of micelle-encapsulated curcumin. Gemini surfactants comprise two single-chain surfactant moieties joined by a hydrocarbon spacer group. Compared to the single-chain surfactant, this novel kind of surfactants has many unique properties that are superior to their single-chain counterparts, such as remarkably low critical micelle concentration, much higher surface activity, unusual aggregation morphologies, and better wetting, solubilizing, and foaming properties [19–21]. Most gemini surfactants so far investigated are alkanediyl-␣,␻bis(dodecyldimethylammonium bromide) compounds having the general structure [C12 H25 (CH3 )2 N(CH2 )s N(CH3 )2 C12 H25 ]Br2 , designated as C12 Cs C12 Br2 , where s indicates the number of carbons in the spacer. Many works have shown that the spacer plays a major role in the aggregation properties of gemini surfactants, which has been attributed to the conformational change of the surfactant molecule and to the change of the spacer location in the micelles [22]. Owing to the special architectures and properties of gemini surfactants, it is interesting to investigate the interactions between curcumin and gemini surfactants containing the spacer units of different length, which can promote our understanding of the interaction mechanism of surfactant micelles with curcumin and further help to design the effective encapsulation carrier of active molecules. In this paper, the interactions of curcumin with a series of cationic gemini surfactant micelles of C12 Cs C12 Br2 (s = 2, 3, 4, 6, and 12) have been spectroscopically studied at pH 13. We prepared 3 mM C12 Cs C12 Br2 micellar solutions above their critical micelle concentrations for encapsulating curcumin. The absorption and

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fluorescence spectroscopic properties of the trianionic curcumin (Cur3− ) as well as the lowest pKa (pKa1 ) values of curcumin in C12 Cs C12 Br2 micelles indicated that the electrostatic interactions of gemini surfactant micelles with Cur3− are significantly regulated by the spacer length of gemini surfactants. Further testing on the stability of Cur3− in five C12 Cs C12 Br2 micelles was also carried out. 2. Experimental 2.1. Materials Gemini surfactants alkanediyl-␣,␻-bis(dodecyldimethylammonium bromide) (C12 Cs C12 Br2 , s = 2, 3, 4, 6, and 12) were synthesized and purified according to the method of Menger and Littau [19]. Curcumin, pyrene and 1,6-diphenyl-1,3,5-hexatriene (DPH) were purchased from Sigma–Aldrich Chemical Company and used without further purification. All other chemical reagents used were of analytical grade and water was double distilled. 2.2. UV–vis absorption measurement The absorption spectra of curcumin were acquired using a Shimadzu UV-2450 spectrophotometer at 25 ◦ C by circulating water through the thermostated cuvette holder. Samples of 5 mg/L curcumin in water and micellar solutions of 3 mM C12 Cs C12 Br2 above their critical micelle concentrations were first prepared in double distilled water and then adjusted to pH 13 by concentrated sodium hydroxide before the absorption measurement. In the pH titration for the determination of the lowest pKa (pKa1 ) of curcumin, the pH of curcumin samples was adjusted with concentrated sodium hydroxide or hydrochloric acid. The curves of the ¯ of curcumin were plotted as a function absorption spectral mean () ¯ was calculated as [16] of pH. The value of 

=600 nm ¯ = 

=300 nm

OD() × 

=600 nm =300 nm

OD()

(1)

where OD is the optical density at a given wavelength . 2.3. Steady-state fluorescence measurement Steady-state fluorescence measurements were performed with an Edinburgh FLS900 spectrofluorophotometer at 25 ◦ C by circulating water through the thermostated cuvette holder. The fluorescence spectra of 5 mg/L Cur3− in water and micellar solutions of 3 mM C12 Cs C12 Br2 at pH 13 were taken from 500 to 800 nm with the excitation wavelength at 469 nm. The fluorescence spectra of Cur3− in selected C12 C6 C12 Br2 micelle were compared at 0–150 mM sodium bromide concentrations (CNaBr ). On the other hand, the micropolarity of C12 C6 C12 Br2 micelle at pH 13 was determined from measurement of pyrene polarity index (I1 /I3 ) at various CNaBr . I1 /I3 is the ratio of the intensities of the first and the third vibronic peaks in the fluorescence emission spectrum due to pyrene. Pyrene was excited at 337 nm and the emission spectra were scanned from 350 to 500 nm. 2.4. Fluorescence polarization technique An Edinburgh FLS900 spectrofluorophotometer with parallel and perpendicular polarizers was used to determine the fluorescence anisotropy (r) of Cur3− or 1,6-diphenyl-1,3,5-hexatriene (DPH) at pH 13. The fluorescence intensities were obtained at 0–0, 0–90, 90–0, and 90–90 angle settings of the excitation and emission

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polarization accessories at 25 ◦ C. The value of r was calculated according to [23] 4

I// − G × I⊥

(2)

I// + 2G × I⊥

where I// and I⊥ are the fluorescence intensities of the emitted light polarized parallel and perpendicular to the exciting light respectively, and G is the grating correction factor, which is the ratio of sensitivities of the instrument for vertically and horizontally polarized light. While Cur3− in C12 Cs C12 Br2 micelles was excited at 469 nm, the emission spectra were scanned from 500 to 600 nm. When the fluorescence anisotropy of DPH in micellar solution of C12 C6 C12 Br2 was measured at various CNaBr , the excitation wavelength was set at 360 nm and the emission spectra were measured from 350 to 500 nm. 3. Results and discussion 3.1. Absorption and fluorescence spectra of Cur3− in C12 Cs C12 Br2 micelles Fig. 1 depicts the UV–vis absorption spectra of freshly prepared Cur3− in water and micellar solutions of 3 mM C12 Cs C12 Br2 (s = 2, 3, 4, 6, 12) at pH 13. Different from the intense and round-shaped absorption band centered at 430 nm in water at acidic pH [24], the position of maximum absorption of Cur3− is seen to be red-shifted to 468 nm in water and about 475 nm in gemini surfactant micelles, respectively. While Cur3− presents higher maximum absorption intensities in C12 Cs C12 Br2 micelles than that in water, the absorption peak of Cur3− in C12 C6 C12 Br2 micelle has the highest intensity among five gemini surfactant micelles. Meanwhile, the steady-state fluorescence spectra of Cur3− in water and C12 Cs C12 Br2 micellar solutions at pH 13 are shown in Fig. 2. Similar to the changes of absorption curves of Cur3− , the fluorescence peak of Cur3− is markedly red-shifted respectively from 530 nm at acidic pH to 610 nm in water and 620 nm in C12 Cs C12 Br2 micelles at pH 13 [25]. Compared to the very weak fluorescence of Cur3− in water, the addition of C12 Cs C12 Br2 micelles can greatly improve the fluorescence intensities of Cur3− by 29–58 times. With increasing spacer length of gemini surfactant, the fluorescence maximum of Cur3− is observed to increase gradually (s = 2–4) up to a highest value (s = 6) and drop down again (s = 12), which is consistent with the changes of absorption curves of Cur3− in C12 Cs C12 Br2 micelles.

0.8

Water C12C2C12Br2

Absorbance

C12C3C12Br2

0.6

C12C4C12Br2 C12C6C12Br2 C12C12C12Br2

0.4

0.2

0.0 300

Fluorescence Intensity ( x 10 )

r=

35

350

400

450

500

550

600

Wavelength (nm) Fig. 1. UV–vis absorption spectra of Cur3− in micellar solutions of 3 mM C12 Cs C12 Br2 at pH 13.

Water C12C2C12Br2

30

C12C3C12Br2 C12C4C12Br2

25

C12C6C12Br2 C12C12C12Br2

20 15 10 5 0 500

550

600

650

700

750

800

Wavelength (nm) Fig. 2. Steady-state fluorescence spectra of Cur3− in micellar solutions of 3 mM C12 Cs C12 Br2 at pH 13.

At pH 13 above the pKa values of curcumin, three protons of curcumin are already dissociated from the enolic and phenolic OH groups and curcumin changes from a neutral molecule into a trivalent anionic species. Zsila et al. investigated the changes of the position of maximum absorption of Cur3− by addition of KOH to curcumin in ethanolic and aqueous solutions [26]. Their results indicated that the dissociation of p-hydroxy substituents other than the enolic OH of curcumin is almost entirely responsible for the large red-shift. The large red-shift of the spectra of curcumin at basic conditions was also reported in cationic micelles of singlechain surfactants [16]. It is here considered that the trianionic Cur3− can electrostatically associate with oppositely charged headgroups of C12 Cs C12 Br2 to form ion associations, which leads to the solubilization of Cur3− inside C12 Cs C12 Br2 micelles. The fact that more pronounced red-shift of maximum intensities in both absorption and fluorescence spectra of Cur3− in five C12 Cs C12 Br2 micelles than in water indicates the electrostatic attractive interaction between Cur3− and the micelle of C12 Cs C12 Br2 . The main driving force of association of Cur3− with C12 Cs C12 Br2 micelles at pH 13 is thought to be the electrostatic attractions between the negatively charged groups of Cur3− and quaternary ammonium headgroups of gemini surfactants. Additionally, there are the hydrophobic interactions between the alkyl chains of gemini surfactants and the aryl groups of Cur3− . Cur3− is therefore likely to be trapped in the palisade layer of C12 Cs C12 Br2 micelles [17,18]. The micellar palisade layer with relatively limited free water provides a nonpolar character compared to water, which may contribute greatly to the improved intensities of absorption and fluorescence maximum of Cur3− in C12 Cs C12 Br2 micelles. Moreover, the gemini surfactants of C12 Cs C12 Br2 have the same headgroups, and the same hydrocarbon chains, except for the spacer units with different spacer length. Therefore, the spacer length of gemini surfactants should be the principal factor that determines the interaction behaviors of Cur3− with C12 Cs C12 Br2 micelles. 3.2. Salt effect on fluorescence spectra of Cur3− in C12 C6 C12 Br2 micelle Fig. 3 shows the steady-state fluorescence spectra of Cur3− in micellar solutions of 3 mM C12 C6 C12 Br2 with different concentrations of sodium bromide (CNaBr ) at pH 13. It is seen

D. Ke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 80–86

CNaBr=5 (mM)

a 1.62 1.60

CNaBr=10 (mM)

4

CNaBr=50 (mM) CNaBr=120 (mM) CNaBr=150 (mM)

1.58

I1/I3

CNaBr=100 (mM)

12

1.56 1.54

8

1.52 4

0 500

0

50

100

150

0

50

100

150

b 0.14 550

600 650 700 Wavelength (nm)

750

800

Fig. 3. Steady-state fluorescence spectra of Cur3− in micellar solution of 3 mM C12 C6 C12 Br2 with different concentrations of NaBr at pH 13.

0.12 0.10

r

Fluorescence Intensity ( x 10 )

CNaBr=0 (mM)

16

83

0.08 that the fluorescence intensities of Cur3− first increase when CNaBr = 0–100 mM but then drop down markedly at CNaBr = 120–150 mM. The common salt screening of electrostatic interaction should bring about the decreasing tendency of fluorescence intensity of Cur3− above CNaBr = 120 mM [27–29], which also provides the evidence for the discussion about the electrostatic attractions of negatively charged groups of Cur3− with quaternary ammonium headgroups of gemini surfactants. However, at CNaBr = 0–100 mM, we have to consider that the added salt may alter the structural properties of C12 C6 C12 Br2 micelle because it is not possible that the addition of salt directly promotes the electrostatic attraction between Cur3− and C12 C6 C12 Br2 micelle. The fluorescence probes of pyrene and DPH have been employed to investigate the salt effect on the micropolarity and microviscosity of C12 C6 C12 Br2 micelle, respectively [30]. As shown in Fig. 4, the plots of I1 /I3 of pyrene and r of DPH in C12 C6 C12 Br2 micelle without Cur3− are provided as a function of CNaBr . It is noted that as CNaBr increases, I1 /I3 values decrease gradually but r values increases gradually. These results reveal that added salt can promote the formation of gemini surfactant micelle with more compact structure, in consistent with the growth of ionic surfactant micelles to large aggregates with big microviscosity upon the addition of salts [31,32]. Similar to the salt-enhanced interaction in the system of oppositely charged polyelectrolyte with surfactants [33,34], the added salt at CNaBr smaller than 100 mM may lead larger C12 C6 C12 Br2 micelle with higher surface charge density to have a stronger electrostatic association with Cur3− . This may give the explanation for the gradual enhancement in the fluorescence intensity of Cur3− when CNaBr increases from 0 to 100 mM.

0.06 0.04

CNaBr Fig. 4. Changes of polarity ratio I1 /I3 of pyrene (a) and anisotropy r of DPH (b) in micellar solution of 3 mM C12 C6 C12 Br2 with different concentrations of NaBr at pH 13.

micelles and proteins [25,35]. There are somewhat variations of packing of alkyl chains inside C12 Cs C12 Br2 micelles, especially for the micelle of C12 C12 C12 Br2 with the looped conformation of long spacer within the hydrophobic core. But Cur3− is mainly restricted in the palisade layer of C12 Cs C12 Br2 micelles due to the electrostatic attraction of negatively charged groups of Cur3− with quaternary ammonium headgroups of C12 Cs C12 Br2 . This layer is essentially composed of quaternary ammonium headgroups, Br− counterions, water molecules, and ␣-methylene groups of surfactant alkyl chains. The similar environments of the palisade layers may be responsible for the nearly constancy of r values of Cur3− in different C12 Cs C12 Br2 micelles, irrespective of different length of the spacer unit. Furthermore, the previous results of close position of the absorption and fluorescence maximum of Cur3− in water and in C12 Cs C12 Br2 micelles indicate that the local environment of Cur3− in C12 Cs C12 Br2 micelles has some water content; that is, Cur3− is located in the palisade layer of micelles but not deeply inside the micelle core.

3.3. Anisotropy of Cur3− in C12 Cs C12 Br2 micelles

3.4. pKa1 of curcumin in C12 Cs C12 Br2 micelles

Fluorescence polarization measurement can provide the information on the anisotropy (r) of the environments of Cur3− . The anisotropy of Cur3− indicates the extent of the rotation restriction of Cur3− ; that is, higher r value reveals greater restriction of the rotation of Cur3− . In comparison with r = 0.32 obtained in water, r of Cur3− was determined to be the close values at 0.35–0.36 in five C12 Cs C12 Br2 micelles at pH 13. The result of higher r values of Cur3− in C12 Cs C12 Br2 micelles than in water is consistent with the increase of r values in the case of curcumin bound to

The spectrometric titrations of curcumin have been conducted to determine the lowest pKa (pKa1 ) of curcumin in C12 Cs C12 Br2 ¯ micelles. Fig. 5a provides the absorption spectral mean () curves of curcumin as a function of pH in micellar solutions of ¯ values exhibit clear sig3 mM C12 Cs C12 Br2 . As the pH increases,  moidal changes in five micellar systems. Here, the half point of ¯ curves let us determine the pKa1 of cursigmoidal transition in  cumin [16]. The variation of pKa1 values of curcumin against the number of carbons in the spacer (s) of gemini surfactants is shown

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λ (nm)

a

C12C2C12Br2

480

C12C3C12Br2 C12C4C12Br2

460

C12C6C12Br2 C12C12C12Br2

440 420 2

b

4

6 pH

8

10

7.4

pKa1

7.2 7.0 6.8 6.6

2

4

6

s

8

10

12

Fig. 5. (a) pH titration curves of curcumin in micellar solutions of 3 mM C12 Cs C12 Br2 ; (b) variation of pKa1 values of curcumin against the number of carbons in the spacer of gemini surfactants.

in Fig. 5b. While these pKa1 values of curcumin in C12 Cs C12 Br2 micelles are found to be much lower than the value of 8.38 in water [11], the pKa1 values decrease gradually as s changes from 2 to 6, but turn into an increased value at s = 12. Because pKa1 corresponds to deprotonation of the enol group of curcumin with the equilibrium Cur = Cur− + H+ , our result indicates that the electrostatic attractions of anionic Cur− with positively charged C12 Cs C12 Br2 micelles may effectively shift the acid-base equilibrium to the right side. The smallest pKa1 value of curcumin in C12 C6 C12 Br2 micelle suggests that there is the strongest electrostatic attraction between Cur− and C12 C6 C12 Br2 micelle, which makes the right-shift of the equilibrium Cur = Cur− + H+ to a largest degree. 3.5. Interaction mechanism of C12 Cs C12 Br2 micelles with Cur3− While Cur3− carries three negative charges in the enol group and the phenolic parts at pH 13, C12 Cs C12 Br2 micelles are

made of cationic gemini surfactant molecules with two charged headgroups. The dominant electrostatic attractions of negatively charged groups of Cur3− with quaternary ammonium headgroups of surfactants and the additional hydrophobic interactions between the aryl groups of Cur3− and the alkyl chains of surfactants lead Cur3− to be trapped in the palisade layer of C12 Cs C12 Br2 micelles. The results of absorption and fluorescence spectra suggest that the electrostatic attraction of C12 Cs C12 Br2 micelles with Cur3− is significantly influenced by the spacer length of gemini surfactants. Fig. 6 shows an illustration for the interaction pattern of Cur3− with C12 Cs C12 Br2 micelles at pH 13. The spacer group controls the separation between the headgroups of one gemini surfactant molecule that may be greater or less than the thermodynamic distance (dT ) between the headgroups of neighboring aggregated molecules within a gemini surfactant micelle [36]. As indicated by Zana et al., C12 Cs C12 Br2 micelles with s = 2–4 have the bimodal distribution of distance of headgroups [37]. It exhibits a narrow maximum at the distance, ds , corresponding to the fully extended spacer length, which is smaller than the thermodynamic equilibrium distance dT (about 0.7–0.9 nm). On the other hand, it is expected that the negative charges in Cur3− are equally distributed by the length corresponding to about six methylene groups, because a phenyl group is equivalent to an average of 3.5–4 methylene groups [38,39]. In the case of electrostatic interactions of Cur3− with C12 Cs C12 Br2 micelles with s = 2–4, owing to the high electron density in the central negative charge in the enol group of Cur3− [24], the central negative charge of Cur3− may be first anchored to one headgroup of gemini surfactant. One side negative charge in the phenolic group of Cur3− can electrostatically interact with the cationic headgroup from neighbor surfactant molecule by matching the charge separation in Cur3− to the value of dT inside gemini surfactant micelles. But another side negative charge of Cur3− is free to associate with any surfactant headgroup due to longer charge distance in Cur3− than ds value. As a consequence, it is reasonable that the total electrostatic interactions between Cur3− and C12 Cs C12 Br2 micelles will become stronger as s increases, owing to better charge matching between Cur3− and C12 Cs C12 Br2 micelle. So, as s increases from 2 to 4, we observe the enhanced maximum intensities of absorption and fluorescence of Cur3− in C12 Cs C12 Br2 micelles. Zana et al. reported the value of ds for C12 C6 C12 Br2 is comparable to dT value in C12 C6 C12 Br2 micelle [37]. It is thus expected that the charges are nearly equally distributed in C12 C6 C12 Br2 micelle with the distance of about 6 methylene units. This charge distance is appropriate to the length of half curcumin molecule. So we could deduce that all three negative charges in the enol group and the phenolic parts of Cur3− electrostatically associate with the cationic headgroups of C12 C6 C12 Br2 . Because of the optimum matching of the positive charge distribution in C12 C6 C12 Br2 micelle to the negative charge separation in Cur3− , there is the strongest electrostatic attraction between Cur3− and C12 C6 C12 Br2 micelle, which is supported by the changes of the absorption and fluorescence spectra of Cur3− .

Fig. 6. The simplified mechanism for interaction of C12 Cs C12 Br2 micelles with Cur3− at pH 13.

D. Ke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 80–86

residual amount and kobs of Cur3− in C12 Cs C12 Br2 micellar systems reveal that the cationic micelle composed of C12 C6 C12 Br2 is most highly effective than other micelles in improving the stability of Cur3− . Although curcumin can lose its hydrogens easily into Cur3− at pH 13, the highest electrostatic attractions of Cur3− with positively charged C12 C6 C12 Br2 micelle may lead to the strongest protection of Cur3− in C12 C6 C12 Br2 micelle. On the contrary, the observed fastest degradation of Cur3− in C12 C12 C12 Br2 micelle may be probably resulted from the combination of the weak electrostatic interaction of C12 C12 C12 Br2 micelle with Cur3− and the much loose micellar structure of C12 C12 C12 Br2 .

100

3-

Residual Cur (%)

85

85 C12C2C12Br2 C12C3C12Br2 C12C4C12Br2

4. Conclusions

C12C6C12Br2 C12C12C12Br2

0

2

4

6 8 Time (h)

10

12

Fig. 7. The pseudo-first-order plots for the degradation of Cur3− in C12 Cs C12 Br2 micelles at pH 13. The data are normalized to a value of 100 at zero time. Points represent the experimental data and the solid lines were drawn using pseudo-first-order kinetic equation.

When s reaches 12, the value of ds is longer than dT value, although the long spacer partly bends toward the hydrophobic core of C12 C12 C12 Br2 micelle. Like in the micelles of C12 Cs C12 Br2 (s = 2–4), only two negative charges of Cur3− bind to the headgroups from two neighboring aggregated C12 C12 C12 Br2 molecules by electrostatic attraction, except one side negative charge in the phenolic part of Cur3− . Meanwhile, compared to the surfactants of C12 Cs C12 Br2 (s = 2–6), the much long spacer in C12 C12 C12 Br2 makes the two ionic headgroups more separated and reduces the charge density of surfactant headgroups to a greater extent, as suggested by the reported data of micelle ionization degree and zeta potential of C12 Cs C12 Br2 micelles [40,41]. Therefore, the electrostatic attraction of Cur3− with C12 C12 C12 Br2 micelle is weaker than C12 C6 C12 Br2 micelle. Additionally, the maximum intensity of fluorescence of Cur3− in C12 C12 C12 Br2 micelle is observed to be higher than those in micelles of C12 C2 C12 Br2 and C12 C3 C12 Br2 , which may be due to the enhanced hydrophobicity of the palisade layer of C12 C12 C12 Br2 micelle induced by the bending of the spacer group. 3.6. Degradation of Cur3− in C12 Cs C12 Br2 micelles In order to further understand the interactions between C12 Cs C12 Br2 micelles and Cur3− , we have compared the alkaline degradation of Cur3− in five C12 Cs C12 Br2 micelles. Fig. 7 shows the changes of the residual amounts of Cur3− in C12 Cs C12 Br2 micelles as a function of time, signifying the degradation of Cur3− at pH 13. After 12 h incubation, the residual amounts of Cur3− in C12 C2 C12 Br2 , C12 C3 C12 Br2 , C12 C4 C12 Br2 , C12 C6 C12 Br2 and C12 C12 C12 Br2 micelles decay to about 84.5%, 89.5%, 93.6%, 96.9% and 76.3%, respectively. These results reveal that increase of s from 2 to 6 can improve the stability of Cur3− , whereas the residual amounts of Cur3− show a smallest value at s = 12. Similar to other people’s work on the degradation of free curcumin and micelle-encapsulated curcumin [10,13,42], the residual amounts of Cur3− in C12 Cs C12 Br2 micelles were fitted to the pseudo-first-order kinetic equation. From the linear regression lines of the residual amount of Cur3− against time, which are shown in the fitting lines in Figure 7, the observed pseudo-first-order rate constants of Cur3− degradation (kobs ) are determined to be 0.013, 0.0087, 0.0048, 0.0024 and 0.022 h−1 as s changes from 2 to 12, respectively. The changes of values of

This work has revealed that the electrostatic interactions of C12 Cs C12 Br2 (s = 2, 3, 4, 6, and 12) micelles with Cur3− are significantly regulated by the spacer length of gemini surfactants. Among five gemini micelles of C12 Cs C12 Br2 , the maximum intensities of absorption and fluorescence spectra of Cur3− exhibit the highest values in C12 C6 C12 Br2 micelle with the smallest alkaline degradation at pH 13. These findings are explained as the strongest electrostatic attractive interaction between Cur3− and C12 C6 C12 Br2 micelle due to the optimum matching of the positive charge distribution in C12 C6 C12 Br2 micelle to the negative charge separation in Cur3− . Significantly, the novel gemini surfactants with diverse structures may give great possibilities to design new effective systems as well as to control the physicochemical properties and the bioactivities of the natural drugs like curcumin.

Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant 21173081), the Natural Science Foundation of Shanghai (Grant 11ZR1408600), the Fundamental Research Funds for the Central Universities (Grant WK1213003), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry of China.

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