Complexation between Sodium Dodecyl Sulfate and Amphoteric

The complexation between negatively charged sodium dodecyl sulfate (SDS) and positively charged amphoteric polyurethane (APU) self-assembled ...
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J. Phys. Chem. B 2007, 111, 11134-11139

Complexation between Sodium Dodecyl Sulfate and Amphoteric Polyurethane Nanoparticles Yong Qiao, Shifeng Zhang, Ouya Lin, Liandong Deng, and Anjie Dong* School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin, 300072, China ReceiVed: April 12, 2007; In Final Form: July 12, 2007

The complexation between negatively charged sodium dodecyl sulfate (SDS) and positively charged amphoteric polyurethane (APU) self-assembled nanoparticles (NPs) containing nonionic hydrophobic segments is studied by dynamic light scattering, pyrene fluorescent probing, ζ-potential, and transmission electron microscopy (TEM) in the present paper. With increasing the mol ratio of SDS to the positive charges on the surface of APU NPs, the aqueous solution of APU NPs presents precipitation at pH 2, around stoichiometric SDS concentration, and then the precipitate dissociates with excess SDS to form more stable nanoparticles of ionomer complexes. Three stages of the complexation process are clearly shown by the pyrene I1/I3 variation of the complex systems, which only depends on the ratio of SDS/APU, and demonstrate that the process is dominated by electrostatic attraction and hydrophobic aggregation.

1. Introduction

SCHEME 1: Structure of APU

The interaction of polymers and surfactants has been extensively studied in the last decades.1,2 As one of the most important branches in this field, complexation of polyelectrolyte (PE) and oppositely charged surfactant has received considerable attention both theoretically3-5 and experimently6-12 for its potential application in cosmetics, pharmaceutical formulations, rheological control, oil recovery, etc. The strong interactions such as electrostatic attraction and hydrophobic assembly between PE and oppositely charged surfactant in aqueous medium normally leads to phase separation.13-15 Most studies observed that insoluble complex formed when a small amount of surfactant was added into oppositely charged PE solution, which was induced by electrostatic interaction and charge neutralization. Further adding surfactant, the insoluble complex can be redissolved at excessive surfactant capacity because of micellar solubilization.16,17 For widely reported linear polyelectrolyte/surfactant systems, a contraction of oppositely charged polymer/surfactant complex before precipitation and an expansion of the polymer coil in the case of excessive surfactant have been observed.18,19 Various techniques such as turbidimetry, pyrene fluorescence probing, ζ-potential measurements, dynamic light scattering, tensiometry, potentiometry, etc. were used to study the interaction mechanism of PE with oppositely charged surfactants. Growing certifications of theoretical simulations and experiments supported the cooperative interaction and surface charge neutralization of surfactant micelles with the oppositely charged flexible polymers.4 In some researches, noncooperative surfactant binding20 and specific binding mechanism including cooperative and noncooperative steps are also proved.21 It has been shown that the structure of PE/surfactant depends considerably on the PE chemistry, including the hydrophilic or hydrophobic character of the polyion, flexibility, charge density, chemical nature of its ionizable groups, and molecular architecture of the polymers.22,23 Different models for complexation of linear PE and oppositely charged surfactant have been proposed by several groups.24-28 * Corresponding author. E-mail: [email protected].

The block ionomer complexes (BIC) formed in aqueous solutions by “dual hydrophilic” block ionomers containing ionic and nonionic water-soluble segments and oppositely charged surfactants were studied and were used to obtain small vesicles or nanoparticles.29-33 Little is known about the interaction of block ionomer self-assembled micelles or nanoparticles containing hydrophobic nonionic blocks with oppositely charged surfactant. Lysenko and co-workers had studied the interaction of cationic block copolymers of polystyrene-block-poly(N-ethyl4-vinylpyridinium bromide) (PS-b-PE4VP) micellar solutions with oppositely charged surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and polystyrene-block-poly(sodium acrylate) micelles with N-cetylpyridinium bromide ions.34,35 The results showed that BICs with unique microheterogeneous structure varied with changing the polyion and surfactant ratio as well as the lengths of the copolymer blocks. Significantly, they discovered nonstoichiometric and stoichiometric PS-bPE4VP/AOT complexes whose formation strongly depended on the length of ionic PE4VP block.35 Recently, we synthesized a kind of amphoteric polyurethane (APU).36 As shown in Scheme 1, hydrophobic polyether soft segments and hard segments with pendent -COOH and -CH2N(CH3)2 groups arrange in APU chains randomly. APU can be dissolved in both acid (pH 1.5 to ∼4) and basic (pH 9 to ∼13) aqueous solution and, respectively, self-assemble into positively charged and negative charged nanoparticles depending on the charged -CH2N(CH3)2 and -COOH groups.36-38 In the present study, we focus on the interaction between positively charged APU nanoparticles (pH 2) and anionic surfactant. The specific three interaction stages in the complexation process were observed and analyzed. 2. Experimental Section 2.1. Materials and Synthesis of APU. Poly(propylene oxide) glycol (N210, Mn ) 1000) (Tianjin University Chemical) was

10.1021/jp072874e CCC: $37.00 © 2007 American Chemical Society Published on Web 09/06/2007

Complexation between SDS and Polyurethane NPs

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Figure 2. Plots of SDS concentration at special turning points vs the concentration of -CH2N(CH3)2 groups. CmSDS presents the concentration of SDS while the complex size reaches the maximum; CzSDS means the concentration of SDS at the zero ζ-potential point.

Figure 1. Particle sizes (A) and ζ-potential (B) of APU/SDS complex systems. The concentration is 2.5 g/L for APU and 1.7 mmol/L for -CH2N(CH3)2.

demoisted by a distilling method before use. Dimethylol propionic acid (DMPA) (TRIMENT Technical Products, Inc.) and dimethylamino-1,2-propanediol (DAP) (Acros Organics, Sweden) were used as low molecule weight diols in order to provide pendent carboxylic acid and dimethylamino groups. DMPA was dried at 100 °C in a vacuum for 3 h, and DAP was distilled before use. 1,4-Butanediol (BD) (Beijing Chem. Agent Inc.) as the chain extender and N-methyl-2-pyrrolidone (NMP) (Beijing Chem. Agent Inc.) and ethyl acetate (EAC) (Tianjin Chem. Agent Inc.) as solvents were distilled before use. Sodium dodecyl sulfate (SDS) (Nankai University Chemical) was used after recrystallization in ethanol for several times. Pyrene was delivered from Sigma-Aldrich. Distilled water was purified by filtering with a 0.22 µm Millpore filter paper. Isophorone diisocyanate (IPDI) (Taisen Chemical) was used as received. APU was synthesized according to ref 36. The average molecular weight of APU is 4.06 × 104 g/mol measured by SLS. The molar content of -CH2N(CH3)2 and -COOH groups in APU macromolecules are both 0.683 mmol/g. The charge density, defined as the molar ratio of the sum of -CH2N(CH3)2 and -COOH groups to N210, is 4. 2.2. Preparation of APU/SDS Complexes. APU NPs waterborne dispersions were prepared by dispersing a given amount of APU powder in HCl aqueous solution (pH ≈ 1.8) at room temperature, and then the pH of the solution was adjusted to 2. After that, the prepared NPs dispersions were kept for 12 h at room temperature, and then SDS (pH 2) solutions were added isometrically. All complex systems were kept at room temperature for 24 h before characterization. 2.3. Measurements. 2.3.1. Hydrodynamic Diameter and ζ-Potential. The measurements of particle size and ζ-potential of APU/SDS complexes were performed on a BI 90 Plus/ Zetaplus instrument of Brookhaven Corporation, employing a laser source (618 nm) at an angle of 90° and with the temperature of 25 ( 0.5 °C. 2.3.2. Fluorescence Measurement. Saturated pyrene aqueous solution with a concentration of 2.0 × 10-6 M and pH 2 was prepared first, and then an appropriate amount of SDS or APU

Figure 3. Particle size distribution f(Dh) of APU/SDS complexes with different SDS concentrations. The concentrations are all 2.5 g/L for APU and 1.7 mmol/L for -CH2N(CH3)2 groups.

was dissolved to obtain solutions containing pyrene and kept for 24 h. Second, APU and SDS were mixed isometrically. It should be noticed that pyrene was contained only in one solution system, SDS or APU particles solution before mixing. Therefore, the concentration of pyrene is the same in all the complexes, i.e., 1.0 × 10-6 M. The fluorescence spectra were recorded on a Hitachi F-3010 spectrofluorimeter at an excitation wavelength of 334 nm, and the scanned range was from 350 to 400 nm. The intensities, I1 and I3 of the first and the third vibrational peaks were taken in the emission spectrum of pyrene at approximately 373 and 384 nm, respectively. All samples were examined at controlled temperature of 25 ( 0.5 °C. 2.3.3. TEM Measurement. The transmission electron microscopy (TEM) specimens for the complexes of APU/SDS were observed under a Philips EM400 ST transmission electron microscope. A droplet of mixture was placed onto an acclivitous Formvar-coated copper TEM grid, and then the water was absorbed using a piece of filter paper after absorption of about 30 s. 3. Results and Discussion When APU and oppositely charged SDS were mixed, with increasing of SDS, the complex systems turned to milky from clear and then precipitated; subsequently, the precipitate

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Qiao et al.

Figure 4. TEM images of APU/SDS complexes in pH 2 HCl aqueous solution with SDS concentration equal to (a) 0, (b) 0.3, (c) 0.75, (d) 2.5, (e) 15, and (f) 30 mmol/L. The concentration is 2.5 g/L for APU.

dissociated and the system was clear again. We characterized the complexation process by different methods. 3.1. Particle Size and ζ-Potential Measurement. When SDS is added into APU NPs dispersion, the particle size presents a complicated variation which is shown in Figure 1A. The particle size does not change when the concentration of SDS is very low. Upon further addition of SDS, the average diameter increases gently at first and sharply increases to a maximal value. Once beyond the maximal value, the particle size of the complex decreases and reaches a plateau gradually. The shift in particle size is in accordance with the optical phenomena from clear to milky, precipitate, milky, and to clear again. The maximal particle size emerges around 1.7 mM SDS which just equals the concentration of -CH2N(CH3)2 groups in the complex system. These results mean that the complex of APU/SDS reaches the maximal size at electroneutral point, i.e., the stoichiometric SDS concentration. When SDS is added into APU NPs dispersion, the potential of the complex, as shown in Figure 1B, varies from positive to negative and passes through zero around the electroneutral point, which verifies the electrostatic interaction between APU nanoparticles and SDS. The concentrations of SDS at the maximal particle size (CmSDS) and zero-potential point (CzSDS) both increase with APU concentration as shown in Figure 2. The slopes of the two fitted line are both near 1, which means, when the complex is neutralized, aggregation reaches the maximum at the same time. Therefore, it can be concluded that the neutralization manipulates the precipitation. This is similar with the general features of oppositely charged linear PE/surfactant systems.16,17 The particle size distribution in Figure 3 clearly shows the aggregation and redispersion processes of the APU/SDS complex. The complex size shifts to high values with SDS concentration increasing until the electroneutral point. It identifies the interaction of APU and SDS induced by charge neutralization. Further addition of SDS to the complex system leads to particle size shift to small area, which shows redispersion of the aggregate. It can be seen that almost all the

complexes present a single-peak distribution of particle size, which means the interaction of APU nanoparticles with SDS is a homogeneous process. 3.2. TEM. The TEM photos in Figure 4 further confirm the interaction between SDS and APU nanoparticles. APU takes spherical morphology in HCl aqueous solution (pH 2) as shown in Figure 4a. When a little amount of SDS is added, as shown in Figure 4b, the size slightly increases and core-shell structure can be observed to demonstrate that SDS associates on APU particles. Figure 4c clearly shows aggregation of complex particles when the concentration of SDS increases to 0.75 mM. Upon further addition of SDS, large aggregates occur as shown in Figure 4d. Once the concentration of SDS is far from the electroneutral point, smaller sized particles appear again as shown in Figure 4e. When the SDS concentration reaches 30 mM, smaller and more compact particles are observed as shown in Figure 4f. 3.3. Fluorescence Measurement. As a kind of fluorescence probe, pyrene has often been used to study the interaction of polymers and surfactants. Five vibronic peaks can be observed in pyrene fluorescence emission spectroscopy. The ratio of I1/ I3 is proportional to the polarity of the environment where pyrene located. So I1/I3 is commonly used to probe the formation of hydrophobic microdomains.39,40 In the present study, we used pyrene to track the complexation process of APU nanoparticles and SDS. As can be seen in Figure 5a, the critical micelle concentration (cmc) of SDS in HCl aqueous solution (pH 2) is about 2.0 mM which is much lower than that in pure water (about 8 mM) because of the effect of ionic strength of the solution.41,42 Figure 5b shows that I1/I3 tends to decrease when SDS is added into pyrene contained APU NPs dispersion. Four critical points (P1, P2, P3, and P4) are observed. The value of I1/I3 decreases slightly in the range of low SDS concentration. This is in accordance with the above results of the particle size measurement. The value of I1/I3 decreases dramatically after P1 because of hydrophobic aggregation of SDS on APU particles. The aggregation of SDS strengthens the hydrophobic

Complexation between SDS and Polyurethane NPs

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Figure 5. I1/I3 value of pyrene as a function of the SDS concentration: (a) SDS solution at pH 2; (b) complex made by adding SDS to pyrene contained APU dispersion at pH 2. The concentration is 2.5 g/L for APU and 1.7 mM for -CH2N(CH3)2 groups.

character of APU nanoparticles. So we assign P1 as the critical aggregation concentration (cac) of SDS, which is much less than the cmc of SDS in pH 2 aqueous solution. It was widely reported that aggregate formation of surfactant on polyelectrolyte often started at the cac which is far below the cmc. When the concentration is lower than the cac, interaction primarily takes place between polymer and surfactant monomers. However, polymer-micelle aggregates dominate the interaction above the cac.43,44 With increasing of SDS concentration, I1/I3 declines and fixes in a short range of SDS concentration (from P2 to P3), then decreases again until leveling off after point P4. Considering the results of Figure 1 and Figure 5b, we assign P2 as the initial point of precipitation, P3 as the dissociated point, and P4 as the equilibrious point from which stable complex NPs are formed. Electrostatic attraction dominates the interaction of SDS and APU nanoparticles before P1, but hydrophobic gathering gradually appears after P1. The hydrophobic interaction between surfactant tails and the electrostatic attraction between oppositely charged groups jointly lead to precipitation. The constant value of I1/I3 from P2 to P3 indicates that polarity of the complex in the solution phase does not change during the precipitation process. It is obvious that the electroneutral point (CSDS ) 1.7 mmol/L) is between P2 and P3. After point P3, more and more SDS molecules associate to the precipitate through hydrophobic binding, so the net charge of the system changes from positive to negative. The increase of negative charge induces dissociation of the precipitate because of electrostatic repulsion. The decrease in I1/I3 in this stage means that the complex particles become more and more hydrophobic. Finally, a stable complex structure forms after P4. The value of I1/I3 is constant at 0.89, which is lower than both of I1/I3 values in APU NPs and SDS micelles. In order to know the interaction process more clearly, the change of I1/I3 with the mixing time was tracked. SDS was added into APU solution, and the fluorescence measurement was taken immediately without any agitation. The concentration of APU was kept at 2.5 g/L, and three SDS concentrations, 0.3, 1.7, and 10 mM, respectively, around the cac, electroneutral point, and final equilibrium P4 according to Figure 5b were chosen. The results are shown in Figure 6.

Figure 6. I1/I3 ratio as a function of the mixing time when SDS was added into APU dispersions: (a) pyrene contained in APU dispersion initially and (b) pyrene contained in SDS solution initially. The concentrations of APU in the complexes are all 2.5 g/L.

Pyrene was originally contained in APU NPs dispersions in Figure 6a and in SDS solutions in Figure 6b. We can see that all the complex systems reach their equilibriums after a short time disturbance led by addition of SDS. The complex system containing 0.3 mM SDS reaches its equilibrious state “Plat I” in about 10 min. For the complex system containing 1.7 mM SDS, a short stay is observed at “Plat I” before it reaches its equilibrious state “Plat II”. Similarly two short stays, respectively, at “Plat I” and “Plat II” are observed for the complex system containing 10 mM SDS before it reaches its equilibrious state “Plat III”. The values of I1/I3 at the three equilibrious states are just in accordance with those at points P1, P3, and P4 in Figure 5b. As can be seen in Figure 6, parts a and b, even though the pyrene exists in different environments before mixing, three similar plateaus are observed. Therefore, it is concluded that the value of I1/I3 at equilibrium depends on the ratio of SDS/ APU and is independent of the environment where pyrene initially exists. These results further indicate that there are definitely three interaction stages in the associative process of SDS on APU nanoparticles. During the initial 10 min, the change of I1/I3 relates to the original environment of pyrene. The value of I1/I3 increases at the beginning and then declines in Figure 6a, but it decreases all the time in Figure 6b. This is probably due to a redistribution of pyrene between SDS congeries and APU particles. A step-by-step mixing process was studied in order to further understand the complexation between SDS and APU nanoparticles. According to Figures 5 and 6, the mol ratios of -CH2N(CH3)2/SDS (M) are 5.7 at the cac, 1.0 at the electroneutral point, and 0.17 at equilibrium for the complex system containing

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Figure 7. I1/I3 value as a function of mixing time in the step-by-step mixed process. The process was as follows: 20 mM SDS solution containing pyrene (SP-SDS, 10 mL) and 5 g/L APU dispersion (SAPU, 10 mL) were prepared, respectively, and stored for 24 h. Then, solution a (mol ratio of -CH2N(CH3)2/SDS, M ≈ 5.7) was prepared by mixing 0.3 mL of SP-SDS with 10 mL of SAPU, and I1/I3 was measured after 24 h. Then, 1.4 mL of SP-SDS was added into solution a to form solution b (M ≈ 1.0), and the I1/I3 was measured immediately. Similarly, 0.7 mL of SP-SDS was blended with solution b after 24 h to produce solution c (M ≈ 0.71), and finally 7.6 mL of SP-SDS was added into solution c 24 h later to prepare solution d (M ≈ 0.17).

10 mM SDS and 2.5 g/L APU. Therefore, the three ratios were chosen in the step-by-step mixing process, as shown in Figure 7. It can be seen that the I1/I3 of solution a (mol ratio of -CH2N(CH3)2/SDS (M) 5.7) is 1.16. When the M decreases from 5.7 to 1.0, I1/I3 decreases from 1.16 to 1.03, which is in accordance with Figure 6. Whereas the decrease of -CH2N(CH3)2/SDS from 1.0 to 0.71 (curve c) does not induce the change of the I1/I3 value, curve c presents the precipitation process, in which the structure of the complex in solution does not change. Further addition of SDS makes I1/I3 decrease initially and then level off at 0.9 as shown in Figure 7, curve d. The step-by-step experiment proves that there exist definitely three interaction stages for a certainty. According to the above results, electrostatic attraction and hydrophobic assembly are thought to dominate the complexation between APU nanoparticles and SDS. There are obviously three distinct stages in the interaction process. The first stage is electrostatic association of SDS monomers on APU nanoparticles before the cac. The positive charge of APU is partially neutralized, but polarity is still unchanged. The second stage is formation of SDS clusters on the surface of APU NPs and appearance of precipitation. After the cac, SDS molecules begin to self-aggregate into micelles. These negative micelles can be cross-linkers of positively charged APU particles. Therefore, the average particle size increases upon further addition of SDS and becomes a maximum around the electroneutral point. The formation of SDS micelles strengthens the hydrophobicity of complex system, so I1/I3 of pyrene decreases. The third stage is a hydrophobic association process of the added SDS molecules with SDS clusters in the precipitation. This leads to a charge reversal and dissociation of precipitation. Finally, stable complex NPs of APU/SDS are formed. The abundant SDS makes the complex particles smaller and more compact, and they redisperse into water stably, so the system turns clear again, and I1/I3 of pyrene reaches a minimum. Our previous study36 has shown that the APU NPs aqueous dispersion was stable in pH 1.5 to ∼4 and 9 to ∼13. Phase separation will happen apart from the above-mentioned pH range. APU nanoparticles also precipitate when the concentration of NaCl

Qiao et al. is higher than 0.03 M. However, the APU/SDS complex at high SDS concentration can keep stable in a broad pH range from 1 to 14, and phase separation will not occur until NaCl is higher than 0.5 M. Balomenou and Bokias25 have reported a complex micelle that consisted of anionic comb-type copolymers and cationic surfactant dodecyl trimethylammonium bromide (DTAB). It has a water-insoluble core (polyelectrolyte/surfactant complex), which is protected by a hydrophilic nonionic corona. We proposed that a similar structure maybe exists in the APU/SDS complex. This structure may consist of a water-insoluble core (APU/SDS complex) and cationic corona (SDS). Therefore, it is reasonable to think that the high stability of the APU/SDS complex is attributed to electrostatic repulsion between complex coronas. Furthermore, the complexation between SDS and APU NPs at a different mixed sequence, i.e., APU NPs was added into SDS solution, was also studied in our lab. Similar results as above in the I1/I3 of pyrene were obtained and have been listed in the Supporting Information. These results certify that the APU/SDS interaction only depends on the ratio of APU/SDS but is independent of the mixing manner. 4. Conclusions In this paper, the complexation of SDS and positively charged APU nanoparticles was studied in pH 2 HCl aqueous media. The study results reveal that the complexation of SDS and APU nanoparticles is dominated by electrostatic interaction and hydrophobic aggregation of SDS molecules. The complexation is a reversible process, which depends on the mol ratio of the -CH2N(CH3)2 groups of APU to SDS but is independent of the mixing manner. Three definite interaction stages with the cac of SDS and the electroneutral point as boundaries are confirmed. The three stages are (i) the electrostatic association of SDS monomers on APU micelles before the SDS concentration reaches the cac, (ii) the cluster forming of SDS between APU nanoparticles, which leads to precipitation of the complex because of electroneutralization, and (iii) the dissociation of precipitate until stable complex nanoparticles with excess negative charges are formed. The formed stable complex nanoparticles in exceeding SDS existence present high stability in a broad pH range and high salt concentration, which maybe provide potential application in a broad pH range and higher salt environment. Further work needs to be done to know the structure and properties of the stable complex nanoparticles. Acknowledgment. This project was supported by the Program for New Century Excellent Talents in University (NCET) and the Programme of Introducing Talents of Discipline to Universities (No. B06006). We also thank Dr. Wangqing Zhang for fruitful discussions. Supporting Information Available: Details of particle sizes and ζpotentials measurements, stability tests of the complex, and the results at different mixing manners. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zana, R. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1998; Chapter 10, pp 59 and 108. (2) Goddard, E. D. Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; Chapter 4. (3) Thu¨nemann A. F. Prog. Polym. Sci. 2002, 27, 1473-1572. (4) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604-613.

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