Chemiluminescence as a Novel Indicator for Interactions of Surfactant

Jan 13, 2014 - Chemiluminescence (CL) has been employed as a novel technique to monitor the interactions between poly(ethylene glycol) (PEG) and ...
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Chemiluminescence as a Novel Indicator for Interactions of Surfactant−Polymer Mixtures at the Surface of Layered Double Hydroxides Weijiang Guan, Wenjuan Zhou, Qianwen Huang, and Chao Lu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Chemiluminescence (CL) has been employed as a novel technique to monitor the interactions between poly(ethylene glycol) (PEG) and sodium dodecyl benzenesulfonate (SDBS) at the surface of CO3-layered double hydroxides (LDHs). The CL data demonstrated that the interactions of PEG and SDBS at the LDH surfaces were dependent on the SDBS concentration, the PEG molecular weight, and the PEG concentration. Furthermore, powder X-ray diffraction (XRD), zeta-potential measurements, thermogravimetric analysis (TGA), CL spectrum, and radical scavenging methods clarified the relationship between the CL intensity and the interactions of PEG with SDBS at the LDH surfaces. At low concentrations of SDBS, few interactions between PEG and SDBS took place. The aggregation of the LDH colloidal solution occurred as a result of SDBS hydrophobic tails pointed to the aqueous environment. As the concentration of SDBS increased, the PEG chains were bound onto the SDBS bilayers to reduce the electrostatic repulsion between anionic head groups of SDBS due to the structural transformation of SDBS at the surface of LDHs from monolayers to bilayers. This work would provide an attractive route to manipulate the adsorption and composition of polymer−surfactant mixtures at the particle surface by tuning the CL signals.

1. INTRODUCTION Surfactant−polymer mixtures have attracted considerable attention in a variety of industrial applications including detergency, mineral separation, and pharmaceuticals for their ability to self-assemble in bulk solution and at interfaces.1−3 It is demonstrated that the behavior of surfactant−polymer mixtures can be quite different from individual polymer or surfactant solution due to an attractive interaction between surfactant micelles and polymers in bulk solution or at the surface of solid materials. The interaction between anionic surfactant sodium dodecyl sulfate (SDS) and neutral poly(ethylene glycol) (PEG) has become an attractive model system for studying polymer− surfactant interactions in bulk solution to take the form of the so-called necklace-like structure.4−6 However, there is currently no good robust evidence showing the adsorption process of surfactant−polymer mixtures onto the particle surfaces.7,8 In surfactant−polymer mixtures at the particle surfaces, surfactant and polymer components might adsorb at the particle surfaces independently, competitively, or cooperatively.9−11 In recent years, a variety of techniques have been performed to measure the process of polymer−surfactant interactions in bulk solution or at the particle surfaces,12−16 including surface tensiometry, isothermal titration calorimetry, viscosity measurements, small-angle X-ray scattering, photon correlation spectroscopy, nuclear magnetic resonance, and fluorescence spectroscopy; However, most of these methods are subject to © 2014 American Chemical Society

certain limitations. Chemiluminescence (CL) is defined as the production of electromagnetic radiation observed when a chemical reaction yields an electronically excited intermediate, which either luminesces or transfers its energy to another molecule.17 As unique CL reaction media, various micellar microenvironments (e.g., normal micelles, microemulsions, and surfactant−polymer aggregates) possess lots of unique and advantageous properties as follows: solubilize, concentrate, and organize reactants; alter electronic microenvironments; alter chemical and photophysical pathways and rates; and facilitate energy transfer.18 Accordingly, they have been widely applied in CL signal amplification.19−21 It is reasonably believed that micellar microenvironment-tuned CL intensity may offer new opportunities in potentially studying polymer−surfactant interactions. Layered double hydroxides (LDHs), known as inorganic layered compounds, have positively charged brucite-like layers and interlayer balancing anions with well-defined two-dimensional (2D) nanostructures.22 LDHs can be found in nature as minerals and readily synthesized in the laboratory. These layered compounds present high porosity, high surface area, and interlayer anion mobility, and thus they can be widely Received: October 9, 2013 Revised: January 7, 2014 Published: January 13, 2014 2792

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applied as adsorbents.23−25 However, their adsorption capacity is directly related to the nature of the surface and intercalated species of LDHs. The intrinsic hydrophilic surface property of LDHs can be modified into hydrophobicity by incorporating anionic surfactants (e.g., SDS) into the interlayer anions (socalled organo-modified LDHs),26 facilitating their usages as adsorbent material for hydrophobic organic contaminants in water treatment.27−29 More recently, the structural changes in LDH-based polymer nanocomposites have been the subject of considerable interest for their novel or enhanced properties in colloid stability and dispersity as a result of the polymer steric hindrance.30,31 However, there is no report on the polymer− surfactant interactions at the surface of LDHs. Sodium dodecyl benzenesulfonate (SDBS) can strongly interact with PEG molecules to form surfactant−polymer mixtures.32 In addition, SDBS and SDBS-modified LDHs can enhance the CL from periodate (IO4−)−hydrogen peroxide (H2O2) reaction.33 Therefore, the SDBS−IO4−−H2O2 system was chosen as a model one to investigate the polymer− surfactant interactions at the surface of LDHs. In this study, we presented a detailed investigation of the binding interaction of PEG−SDBS (denoted as PEG@SDBS) at the surface of Mg− AlCO3-LDHs using the IO4−−H2O2 CL system. It was found that the PEG concentration, the PEG molecular weight, and the SDBS concentration could strongly affect the configuration of PEG@SDBS at the surface of Mg−AlCO3-LDHs, accompanying with the varying CL intensity. To clarify the mechanisms governing these results, we carried out powder X-ray diffraction (XRD), zeta-potential measurements, elemental analyses, thermogravimetric analysis (TGA), CL spectrum, and radical scavenging methods. Our investigations provided new clues to understand the nature of the PEG@SDBS interactions at the particle surfaces. In principle, this technique is applicable to a variety of the interactions of polymer−surfactant (e.g., ionic polymer−cationic surfactant interactions) at the surface of particles by tuning different CL systems. To the best of our knowledge, this work is unique in its examination of polymer− surfactant interactions at the solid surfaces using the CL technique, which will have a significant impact on the study of polymer−surfactant interactions.

obtained by the coprecipitation method. Solution A containing Mg(NO3)2·6H2O and Al(NO3)3·9H2O was prepared in 60 mL of deionized water (the total metal concentration was 1.0 M). Solution B containing NaOH and Na2CO3 was also prepared in 60 mL of deionized water (the concentrations of NaOH and Na2CO3 were as follows: [NaOH] = 1.6[Mg2+ + Al3+] and [NaCO3] = 2[Al3+]). The pH value of the synthesis process was maintained at 10 by adding dropwise these two solutions into a 250 mL four-necked flask under vigorous stirring. Afterward, the resulting white precipitate was continually stirred for 24 h at 65 °C. Finally, the aged precipitate was washed with deionized water for three times and diluted to its original concentration for further use. 2.3. Adsorption of PEG and SDBS Mixture at the Surface of CO3-LDHs. 2.0 mL of 0.1 M SDBS solution was mixed with 200 μL of 20% w/v PEG aqueous solution. After the mixed solution was completely transparent after gentle stirring for 10 min, it was added to 1.0 mL of the as-prepared CO3-LDHs suspension and left to stir for 15 min to make adequate self-assembly of PEG@SDBS at the surface of CO3LDHs (Figure S1). Also, the mixture of a series of SDBS concentrations and different PEG molecular weights were selfassembled onto the surface of CO3-LDHs according to the same procedure. 2.4. Apparatus. Powder X-ray diffraction (XRD) measurements were performed on a Bruker (Germany) D8 ADVANCE X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.5406 Å). The 2θ angle of the diffractometer was stepped from 2° to 75° at a scan rate of 10°/ min. Zeta-potential measurements and particle size analysis were made using a Zetasizer 3000HS nano-granularity analyzer from Malvern Instruments Ltd. (Malvern, UK) with the water RI of 1.33, the water dielectric constant of 78.55, and the water viscosity of 0.8872 cP. The Brunauer−Emmett−Teller (BET) specific surface areas were determined by N2 adsorption at 77 K with a Quadrasorb SI apparatus (Quantachrome) on sample and outgassed at 573 K. A Vario EL cube elemental analysis (Elementar Analysensy steme GmbH, Germany) was used to determine the adsorbed quantities of SDBS and PEG. A TGA/ DSC 1/1100 SF (Mettler, Toledo) was used to carry out the TGA in N2 at a heating rate of 5 °C/min. Note that all the samples were dried in vacuo at 60 °C for 24 h to remove free water before each TGA experiment. The CL detection was conducted on a Biophysics chemiluminescence (BPCL) luminescence analyzer (Institute of Biophysics, Chinese Academy of Sciences, Beijing, China). The CL spectrum of this system was measured in the same instrument with highenergy cutoff filters from 400 to 640 nm between the flow cell and the photomultiplier tube (PMT). 2.5. CL Measurements. The CL flow injection analysis (CL-FIA) system (Figure S2) was designed to monitor the adsorption of PEG and SDBS mixture at the surface of CO3LDHs using the IO4−−H2O2 CL system. The solutions of 0.01 M NaIO4 and 0.01 M H2O2 were pumped into the flow cell by peristaltic pumps at 2.0 mL/min. The suspension of PEG@ SDBS−CO3-LDHs was injected through a six-port sample injection valve with a 50 μL sample loop. The signals generated from the CL reaction were recorded by a BPCL luminescence analyzer with PMT (−1000 V). The profile of CL intensity was acquired by BPCL software under Windows 7 at an interval of 0.1 s.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Solutions. All experimental chemicals were analytical grade and used without further purification. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Na2CO3, NaOH, and NaIO4 were purchased from Beijing Chemical Reagent Company (Beijing, China). PEG polymers with 200, 400, 600, 1500, 2000, and 4000 average molecular weights were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO). SDBS was purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan). Thiourea and NaN3 were purchased from Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Ascorbic acid was purchased from Beijing Aoboxing Biotech Co. Ltd. (Beijing, China). Nitro blue tetrazolium chloride (NBT) was purchased from Nacalai Tesque Inc. (Tokyo, Japan). All working solutions were prepared by deionized water (Milli-Q, Millipore). 0.01 M aqueous H2O2 solution was prepared freshly by diluting 30% (v/v) H2O2 (Beijing Chemical Reagent Company, China) with deionized water. 0.01 M aqueous NaIO4 solution was prepared by dissolving solid NaIO4 in deionized water. 2.2. Preparation of Mg−AlCO3-LDHs. The carbonate intercalated LDHs with Mg/Al molar ratios of 3.0 were 2793

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3. RESULTS AND DISCUSSION 3.1. CL Performances of PEG, SDBS, and PEG@SDBS Self-Assembly at the Surface of CO3-LDHs. In a flowinjection CL setup, the effects of PEG, SDBS, and PEG@SDBS at the surface of CO3-LDHs (with the average size of 235 nm, 93 m2/g BET surface area, 0.03 g/mL) on the IO4−−H2O2 CL system were investigated in detail. As shown in Figure 1, the

the attractive CL amplification of PEG@SDBS assembled at the surface of CO3-LDHs, we investigated the geometrical configuration of SDBS at the surface of CO3-LDHs by measuring the change of zeta-potential of the CO3-LDH particles with an increasing in the loading amounts of SDBS from 0 to 0.12 M. Figure 2 shows that the zeta-potential of

Figure 1. CL intensity of the IO4−−H2O2 system in the presence of (a) CO3-LDHs, (b) PEG, (c) PEG at the surface of CO3-LDHs, (d) PEG@SDBS, (e) SDBS, (f) SDBS at the surface of CO3-LDHs, and (g) PEG@SDBS at the surface of CO3-LDHs. Note that the concentrations of SDBS and PEG were 0.1 M and 1% w/v, respectively.

Figure 2. Plots of the zeta-potential of CO3-LDHs versus the concentration of SDBS (red), and the CL intensity of the IO4−−H2O2 system varying the concentration of SDBS at constant PEG (1% w/v, blue).

CO3-LDHs was positively charged (29.3 mV) in the absence of SDBS. When SDBS was added up to 0.015 M, the zetapotential was decreased sharply to 0.3 mV, meaning that selfassembled monolayers of SDBS anions at the LDH surfaces occurred by electrostatic attraction.35 However, the loading amounts of SDBS continued to increase, the zeta-potential was inverted from positive to negative values, indicating that the self-assembled monolayer of SDBS anions at the surface of CO3-LDHs was converted into the SDBS bilayer through the interactions between hydrophobic chains of SDBS, resulting in the hydrophilic head groups exposed to the continuous aqueous phase.36 In addition, when the mixture of 1% w/v PEG and SDBS displaced SDBS to be attached to the surface of CO3-LDHs, the values of zeta-potential became a little higher, demonstrating the wrapping of PEG molecules around the anionic head groups of SDBS via ion-dipole interactions.37 3.3. Sorption Quantity of SDBS and PEG at the CO3LDH Surface. The sorption quantities for SDBS and PEG at the surface of CO3-LDHs were established by elemental analysis technique. Figure 3 shows that the adsorption capacities of PEG were close to zero when the concentration of SDBS in solutions was below 0.02 M, meaning the formaiton of SDBS monolayer at the LDH surface. In this case, the limited interaction occurred between PEG molecules and hydrophobic chains of SDBS adsorbed at the LDH surface.4,37 However, the PEG adsorption capacity increased at a fast rate until it gradually approached a plateau due to the complete bilayer formation of SDBS at the surface of LDHs. The findings provided compelling evidence for the binding interactions of PEG@SDBS at the surface of CO3-LDHs. 3.4. Thermogravimetric Analysis. To further verify the structure of SDBS at the surface of CO3-LDHs, TGA plots and their derivatives (Δmass/ΔT) were measured in the study. The TGA data of CO3-LDHs, 0.01 M SDBS attached to the surface of CO3-LDHs, and 0.1 M SDBS attached to the surface of CO3-

self-assembly of PEG@SDBS at the surface of CO3-LDHs could generate the strongest CL signal. Note that XRD patterns of CO3-LDHs were investigated before and after they were mixed with PEG@SDBS (Figure S3), revealing that neither SDBS nor PEG@SDBS molecules were intercalated into the interlayers of CO3-LDHs. Furthermore, the self-assembly of 0.1 M SDBS at the surface of CO3-LDHs had an obvious enhancement on the CL intensity, which is presumably ascribed to the organo-modified LDH microenvironment.33 However, the adsorption of 1% w/v PEG at the surface of CO3LDHs took no effect on the CL intenstiy. It can be explained that there was no interaction between neutral PEG molecules and CO3-LDHs, which was in conformity with the previous literature.34 Under the same experimental conditions, blank experiments were also carried out including PEG, CO3-LDH, SDBS, and PEG@SDBS. The results indicated that no CL enhancement or quenching effects were observed in the presence of PEG and CO3-LDH; however, SDBS showed a slight enhancement on the CL intensity as a result of SDBS micellar microenvironment.18 Interestingly, in comparison to SDBS alone, a slight decrease of the CL intensity was observed in the presence of PEG@SDBS. This might be due to the fact that PEG molecules around SDBS micelles might prevent the contact between SDBS micelles and CL reactants.32 These findings indicated that the IO4−−H2O2 CL intensity could be highly sensitive to the binding interaction of PEG@SDBS at the surface of CO3LDHs. In the following sections, more experiments would be carried out to explore the potential relationship between the CL intensity and the binding interactions of PEG@SDBS at the surface of CO3-LDHs. 3.2. Zeta-Potential. The self-assembly of surfactant− polymer mixtures at the particle surface is usually related to their zeta-potential.14 Therefore, to further clarify the original of 2794

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bilayer at the surface of CO3-LDHs, which was in good agreement with those of zeta-potential data. 3.5. Influence of SDBS and PEG Concentrations at the LDH Surface on the CL Intensity. First, in the presence of 1% w/v PEG, the effect of the SDBS concentration on the CL intensity at the surface of CO3-LDHs was investigated (Figure 2). The results showed that the CL intensity was almostly constant with increasing the concentration of SDBS (lower than 0.02 M), accompanied by the formation of flocculation. These results indicated that PEG cannot interact effectively with SDBS at the surface of CO3-LDHs when the loading amounts of SDBS were relatively low.6 On the contrary, the CL intensity was increased with increasing the SDBS concentrations in the range from 0.02 to 0.12 M, meaning that the interactions between PEG and SDBS at the surface of CO3LDHs were gradually strengthened.7 On the other hand, different molecular weights and concentrations of PEG at constant SDBS concentration were mixed with the CO3-LDH colloidal solution to examine their effects on the IO4−−H2O2 CL intensity. Figure 5A displays that

Figure 3. Sorption quantities of SDBS (circles) and PEG (squares) at the surface of CO3-LDHs (0.03 g).

LDHs (curves a, b, and c, respectively) are shown in Figure 4. For all the samples, the first weight loss occurred in the

Figure 5. Effect of (A) different molecular weights of PEG at 1% w/v PEG and 0.1 M SDBS and (B) different concentrations of PEG 200 (% w/v) at 0.1 M SDBS on the IO4−−H2O2 CL intensity.

the CL intensity was decreased with increasing PEG molecular weights from 200 to 4000. This phenomenon can be explained by the same theory proposed by Nagarajan,40 which was extensively used to investigate interfacial tension of polymer− surfactant interactions. In this case, the larger PEG molecules own lower hydrophilic character, decreasing the critical micelle concentration (CMC) of SDBS.4 Accordingly, the content of SDBS absorbed at the surface of CO3-LDHs was decreased. In addition, Figure 5B shows that the CL intensity was increased with increasing PEG200 concentrations from 0.1 to 1% w/v and then remained constant up to 5% w/v, indicating the formation of the PEG@SDBS at the surface of CO3-LDHs. 3.6. CL as a Novel Indicator for PEG-SDBS Interactions at the LDH Surfaces. Currently, the exact process of PEG− SDBS interactions at the solid surfaces is incomplete.6 In this study, we tried to employ the IO4−−H2O2 CL system to study the process of PEG−SDBS interactions at the surface of CO3LDHs. Based on the above investigations, the interactions between SDBS and PEG at the surface of CO3-LDHs are summarized as follows (Figure 6): Stage I: at low concentrations of SDBS, few interactions between PEG and SDBS took place;4 SDBS molecules were preferentially adsorbed at the surface of CO3-LDHs via electronic attraction to form the monolayer configuration.35

Figure 4. TGA plots and their derivatives of (a) CO3-LDHs, (b) 0.01 M SDBS attached to the surface of CO3-LDHs, and (c) 0.1 M SDBS attached to the surface of CO3-LDHs (inset: possible configurations of SDBS monolayer and SDBS bilayer at the surface of CO3-LDHs).

temperature range from 80 to 240 °C due to the water molecules dehydrated from interlayer of LDHs, followed by the decomposition of interlamellar carbonate from 280 to 410 °C.38 Interestingly, the desorption temperature of the low concentration SDBS (0.01 M) adsorbed at the surface of CO3LDHs was higher than that of the high concentration SDBS (0.1 M) (see curves b and c in Figure 4). These results might be due to the presence of the weak hydrophobic interactions from hydrocarbon chains of SDBS bilayer molecules and the strong electrostatic interactions between anionic groups of SDBS monolayer molecules and positive-charge surfaces of the LDHs.39 Therefore, we could draw a conclusion that 0.01 M SDBS can form the monolayer while 0.1 M SDBS can form the 2795

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Figure 6. Schematic diagram of the relationship between the CL intensity and the interactions of PEG@SDBS at the surface of CO3-LDHs.

The aggregation of the LDH colloidal solution occurred as a result of SDBS hydrophobic tails pointed to the aqueous environment. Stage II: as the concentration of SDBS increased, further adsorption of SDBS can occur, resulting in the structural transformation of SDBS from monolayers to bilayers with the hydrophilic head groups exposed to the continuous aqueous phase.36 This configuration can facilitate the interactions between SDBS bilayer molecules and PEG molecules on the solid phase.37 The PEG chains were bound onto the SDBS bilayers to reduce the electrostatic repulsion between anionic head groups of SDBS, resulting in the compact arrangement of PEG@SDBS at the surface of CO3-LDHs.9 As a result, the CL intensity was increased due to the partial disappearance of electrostatic repulsion between SDBS anionic head groups and anionic CL reactants/intermediates (e.g., •O2−) in organomodified LDH microenvironment.33 3.7. Mechanism Discussion. In this work, the CL spectrum of this present system was measured using a BPCL instrument with high-energy cutoff filters from 400 to 640 nm, which were placed between the flow cell and the PMT. As shown in Figure S4A, there was a maximum emission peak at about 460 nm, corresponding to an emission band of excited singlet oxygen molecules (1O2)2*.41,42 Furthermore, the scavengers of various reaction oxygen species were used to further confirm the emitting species. As shown in Figure S4B, the CL intensity was totally quenched by the addition of 5 mM ascorbic acid or thiourea (scavengers of •OH radical), implying that abundant •OH radicals were released from the reaction;43 a remarkable decrease of CL intensity was observed by adding 1.0 mM NBT (reductant of superoxide radical anion) and 5.0 mM NaN3 (scavenger of singlet oxygen).21 These results demonstrated that the excited singlet oxygen molecules acted as the luminophore for the present CL system. In comparison, the luminophore of the IO4−−H2O2 system in the presence of SDBS was still the excited singlet oxygen molecules. When SDBS molecules were mixed with CO3-LDH colloidal solution, SDBS molecules can self-assemble into unique bilayers at the surface of CO3-LDHs. On the other hand, the wrapping of PEG chains onto the SDBS bilayers could partially screen the electrostatic repulsion between anionic head groups of SDBS.9

As a result, CL intermediates could easily be concentrated on the formed microheterogeneous system.33 Therefore, the stronger CL signals were generated in such a case.

4. CONCLUSIONS In summary, the CL technique provides a new method to study the interactions between PEG and SDBS at the LDH surfaces. The relationship between PEG@SDBS interactions and the CL intensity was verified by some common techniques, such as XRD, zeta-potential measurements, TGA, CL spectrum, and radical scavenging methods. The results revealed that the PEG chains bound onto the SDBS bilayers could make PEG@SDBS at the surface of CO3-LDHs arrange compact as a result of the reduced electrostatic repulsion between anionic head groups of SDBS. The configuration of the formed PEG@SDBS can induce the partial disappearance of electrostatic repulsion between SDBS anionic head groups and anionic CL reactants/ intermediates in organo-modified LDH microenvironment, making an obvious increase in the CL intensity. This study has opened new ways to investigate the surfactant−polymer interactions by the CL technique, and it can facilitate both fundamental research in colloidal chemistry and potentially analytical applications in heterogeneous microenvironmentamplified CL emissions. Current experiments are underway with other polymer−surfactant systems to check the universality of the CL technique.

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ASSOCIATED CONTENT

S Supporting Information *

Effect of stirring time for the IO4−−H2O2 CL intensity; schematic diagram of the flow injection CL analysis system; powder XRD patterns of CO3-LDHs before and after mixing with PEG@SDBS; CL spectrum of the PEG@SDBS at the surface of CO3-LDHs; and effects of radical scavengers on the CL intensity. This material is available free of charge via the Internet at http://pubs.acs.org. 2796

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86 10 64411957 (C.L.). Author Contributions

W.G. and W.Z. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program, 2014CB932103), the National Natural Foundation of China (21375006), the Program for New Century Excellent Talents in University (NCET-11-0561), and the Fundamental Research Funds for the Central Universities (JD1311). We also thank Prof. Xue Duan, Beijing University of Chemical Technology, for his valuable discussions.

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