Effect of anionic surfactants on the surface plasmon

Journal of Molecular Liquids 223 (2016) 771–774

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Effect of anionic surfactants on the surface plasmon resonance band of silver nanoparticles: Determination of critical micelle concentration Jamil K. Salem ⁎, Issa M. El-Nahhal, Bassam A. Najri, Talaat M. Hammad, Fawzi Kodeh Department of Chemistry, Al-Azhar University, PO Box 1277, Gaza, Palestine

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Article history: Received 24 March 2016 Received in revised form 8 August 2016 Accepted 4 September 2016 Available online 06 September 2016 Keywords: Silver nanoparticles Anionic surfactants Critical micelle concentration

a b s t r a c t We have utilized surface plasmon resonance (SPR) band sensitivity to surfactant concentration to investigate the critical micelle concentration (cmc) of AOT and SDS. The process is based on an in situ formation of silver nanoparticles (AgNPs) through the reduction of silver ions (Ag+) by diethylene triamine (DETA) at room temperature. The shift of the SPR band position (λmax) at cmc can be observed with the naked eye in the form of a color change from brown to red for SDS and from brown to yellow for AOT. © 2016 Published by Elsevier B.V.

1. Introduction Surfactant is a unique group of compounds containing both a hydrophobic long-chain and a hydrophilic polar head group in their structures [1,2]. In complex systems, surfactants were widely used in membrane protein separation, crystallization, purification and stabilization. Since the physicochemical properties of solutions below and above cmc demonstrate drastic changes, such as surface tension, electrical conductivity, turbidity, osmotic pressure, density, viscosity, and light scattering [3]. Many methods have been proposed to determine the cmc of surfactants, e.g. spectrophotometry [4], colorimetric [5], potentiometry, refractometry, conductometry, capillary electrophoresis, light scattering [6], pyrene 1:3 ratio method [7], and so forth. Metal nanomaterials such as silver nanoparticles (AgNPs) and Au nanoparticles (AuNPs) have been found wide applications as ideal probes for colorimetric detection owing to their unique optical and electric properties [7–10]. When gold/silver ions are reduced to AuNPs/AgNPs, solutions show a distinctive color attributed to differences in their size and concentration. Besides, surface plasmon resonance bands of noble metal nanoparticles are typically located in the visible region and the band is strongly dependent on a nanoparticle's size, shape, composition, crystallinity and interparticle spacing [11]. Therefore, colorimetric sensors can be established relying on the color and UV–vis spectrum response of metal nanoparticles suspension. In the present work, we have utilized surface plasmon resonance band sensitivity to surfactant concentration to investigate the critical micelle concentration (cmc) of SDS and AOT. The process is based on an in situ formation of silver nanoparticles (AgNPs) through the reduction of silver ions (Ag+) by diethylene ⁎ Corresponding author. E-mail address: [email protected] (J.K. Salem).

http://dx.doi.org/10.1016/j.molliq.2016.09.014 0167-7322/© 2016 Published by Elsevier B.V.

triamine (DETA) at room temperature. Anionic surfactants are the most frequently used surfactant class, they have a negatively charged head-group balanced by a positively charged counter ion mostly sodium. Among them, AOT (sodium bis (2-ethylhexyl) sulfosuccinate Aerosol-OT) and SDS (sodium dodecyl sulfate) have many applications in emulsion chemistry, in catalysis in cosmetics industry, biochemistry and other. The cmc of those surfactants has been widely investigated in aqueous solutions or in the presence of non-polar organic solvents [12–16]. 2. Experimental 2.1. Materials All reagents used in the present work were analytical grade and directly used without further treatment. Dodecyl Sulfate Sodium Salt (SDS) (Merck, 99% purity); Sodium Bis (2-ethylhexyl) Sulfosuccinate Aerosol-OT (AOT) (Merck, 99% purity) , diethylene triamine (DETA) (Loba Chemie, 98% purity), silver nitrate (AgNO3) (HiMedia Laboratories Pvt. Limited, India, 99.8% purity) and deionized water were used in the preparation of solutions. 2.2. cmc determination by AgNPs probe Initially, a 0.2 ml of DETA was added to a certain amount of surfactant. Ten samples of the above solution of various amounts of surfactants were used in this work: ([AOT] = 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.5 and 5.0 mM) and ([SDS] = 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12 mM). The samples of each surfactant are numbered as 1 to 10 from the lower to higher concentration, respectively. The surfactant concentrations are covering the range below and above cmc. The solutions

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J.K. Salem et al. / Journal of Molecular Liquids 223 (2016) 771–774

were mixed completely. A 10-ml solution of AgNO3 (1.0 mM) was slowly added into each sample of the above solutions. pH of all solutions were the same (pH = 11–12). These solutions were shaken and kept in dark at 25 °C. UV–vis absorption spectra were collected after 3 h from the addition of silver nitrate solution using a UV–vis spectrophotometer (Shimadzu, UV-1601) in the wavelength range from 200 to 800 nm. 2.3. Conductivity measurement Conductivity measurements were performed on a conductivity meter (AC-13, Japan) equipped with a conductivity cell having cell constant of 0.943 cm−1. The changes in specific conductivity were measured during titration of surfactant into aqueous solution at 25 °C, and the cmc values of the surfactants have been estimated at the break point of nearly two straight line portions of the specific conductivity vs. concentration plots. 3. Results and discussion Diethylene triamine (DETA) can reduce silver ions to AgNPs in the presence of anionic surfactants and a stable suspension was obtained. Due to electron-rich DETA containing nitrogen on both sides, Ag+ is reduced to AgNPs by solvated electron released by DETA [17,18]. Then surfactant micelles are adsorbed onto AgNPs and modify their surface either by reducing or enhancing the degree of particle agglomeration depending on the molecular structure and concentration of the surfactant. Molecular structures of SDS, AOT and DETA are shown in Fig. 1. Fig. 2 shows the absorption spectra of AgNPs at different concentrations of SDS and AOT. In all cases, single surface plasmon absorption peaks were observed in the range of 425–475 nm and 420–433 nm in the presence of SDS and AOT, respectively. The obtained absorption spectra of AgNPs colloidal solution are similar with earlier report for the Ag nanospheres [19]. According to Mie's theory [20], spherical

O

O

+ O S O Na

Dodecyl Sulfate Sodium Salt (SDS)

+

Na O O O

S O O

O O

Sodium Bis (2-ethylhexyl) Sulfosuccinate - Aerosol-OT (AOT)

H2N

H N

NH2

Diethylene Triamine (DETA) Fig. 1. Molecular structures of SDS, AOT and DETA.

Fig. 2. UV–vis absorption spectra of AgNPs prepared in the presence of (A) SDS and (B) AOT. Insets show the absorption band intensity against (A) [SDS] and (B) [AOT].

AgNPs will give a single absorption peak, but anisotropic AgNPs will exhibit two or more bands. The wavelength corresponding to the absorption maximum of the SPR band is highly sensitive to the size and dielectric properties of the AgNPs, and is known to shift to longer wavelengths upon aggregation [21–24]. The insets of Fig. 2 display the impact of varying concentrations of SDS and AOT on the absorption spectra properties of AgNPs. The SPR band intensity of AgNPs shows constancy at [surfactant] below 6.0 mM SDS and 2.0 mM AOT. A further increase in SDS and AOT concentrations shows a sudden increase in the intensity of SPR band. Fig. 3A shows sigmoidal fit of the SPR band position (λmax) against [SDS]. The SPR band is red shifted from 425 nm to 475 nm by increasing [SDS] from 3 to 12 mM. The break point in the sigmoidal fit is obtained from the Gaussians of the second derivative (dash blue curve), occurs at 7.3 mM SDS which can be observed with the naked eye in the form of a color change from brown to red as seen from the optical images of silver suspensions (Fig. 3B). Fig. 4A shows the sigmoidal fit of the SPR band position (λmax) against [AOT]. The Gaussians of the second derivative (dash blue curve) gives an inflection point at 2.1 mM. The sigmoidal fit of AOT shows blue shift contrary to the sigmoidal fit of SDS (red shift). This blue shifting can be observed with the naked eye in the form of a color change from brown to yellow (Fig. 4B). The utilization of the SPR band shift for the evaluation of cmc is based on the shift of the SPR band position (λmax) at different surfactant concentrations covering the range below and above cmc. The Gaussians of the second derivative fit of the λmax versus [surfactant] is regarded as the cmc [25]. The colorimetric sensing of the AgNPs solution at cmc is in good relation with the cmc value obtained by Gaussian fit of the experimental points. For further investigation, electrical


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Fig. 3. (A) Maximum wavelength of the absorption band (λmax) as a function of [SDS] and in (B) are given the photographic images of AgNPs solutions containing different [SDS]. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig. 4. (A) Maximum wavelength of the absorption band (λmax) as a function of [AOT] and in (B) are given the photographic images of AgNPs solutions containing different [AOT]. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

conductivity method has been used to study the aggregation behavior of surfactant in deionized water and in the presence of AgNO3 (1.0 mM). The conductivity can be linearly correlated to the [surfactant] in both the pre-micellar and post-micellar regions, having a slope in the former region greater than that in the latter. The intersection point of the two straight lines gives the cmc [26]. The obtained cmc values from Fig. 5A are 7.9 and 7.1 mM which is in the range of the cmc of pure SDS. The cmc value (7.1 mM) obtained in the presence of 1.0 mM AgNO3 by conductivity method confirms the cmc value (7.3 mM) obtained by AgNPs probe. Similar effect was reported [27] when 0.6 mM AgNO3 is added during the synthesis of AgNPs; the cmc of SDS is decreased from 8.1 to 7.4 mM. The cmc value (2.0 mM) of AOT obtained by conductivity method (Fig. 5B) is in good agreement with the cmc value (2.1 mM) determined by AgNPs probe. The cmc of AOT is decreased from 2.4 to 2.1 mM by the addition of 1.0 mM AgNO3. The literature CMCs are in a range of ~2.0–2.70 mM for AOT [12–14] which are in good agreement with our results. It is seen from Fig. 5 that the cmc values of SDS and AOT in H2O in the presence of AgNO3 were found to be lower than that of pure water. This is a result of increased screening by the Ag+ ions of the electrostatic repulsion between the negatively charged head groups of the surfactant molecules, making the formation of aggregate structures more energetically favorable, thus resulting in the formation of micelles at a lower concentration. The interaction of growing small nanoparticles with a negatively head group of surfactant aggregates also lowers the cmc of ionic surfactants. Possibly the above two effects work together on the cmc values of the anionic surfactants. The results clearly show that the sizes of the Ag seed clusters were dependent on the surfactant concentration. When SDS concentration

is below cmc (b7.3 mM) Ag seed clusters grew because the SDS concentration is not high enough to prevent aggregation of Ag nanoparticles in aqueous solution. When the concentration of SDS was above cmc (N7.3 mM) the growth of the Ag nanoparticles was arrested and the Ag nanoparticles were well-dispersed in the aqueous solutions. This was due to the protection of the Ag nanoparticles by SDS micelles. Therefore the SPR band is initially red shifted from 425 nm to 475 nm and then it remains constant (Fig. 3A). On the contrary our results suggest that AOT at low concentrations below cmc (2.1 mM) acts as a dispersing agent to effectively prevent Ag clusters aggregating in the aqueous solution. As the AOT concentration was increased above cmc (2.1 mM), the Ag seed clusters dramatically reduced in size and the SPR band is blue shifted from 433 nm to 420 nm as shown in Fig. 4A. Therefore the obtained cmc values of SDS and AOT in the presence of AgNO3 are the effective micelle concentrations necessary for the synthesis of AgNPs. 4. Conclusion To the best of our knowledge, these results constitute the first report for determining the critical micelle concentration of anionic surfactants based on the SPR band shift and color change of Ag nanoparticles. Thus the cmc of an anionic surfactant can be monitored by the naked eye or UV–vis spectrophotometer. We conclude that the surfactant concentration and structure decided the SPR band position (λmax) of AgNPs. The cmc detection is carried out during the formation of AgNPs, which provides not only a detection method for cmc but also a synthesis method of AgNPs.

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[23] [24]

[25] [26] [27]

Fig. 5. Plot of specific conductivity vs. [surfactant] in water and in the presence of 1.0 mM AgNO3.

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