A new simple method for determining the critical micelle concentration of surfactants using surface plasmon resonance of silver nanoparticles
© Karimi et al. 2015
Received: 11 August 2015
Accepted: 2 December 2015
Published: 10 December 2015
A new and simple determination method for critical micelle concentration (CMC) has been developed based on surface plasmon resonance (SPR) technique. In the current work, the CMCs for sodium dodecyl sulfate (SDS), cetyl trimethyl ammonium bromide (CTAB), and Triton X-100 (TX-100) were determined in aqueous media by SPR of silver nanoparticles (AgNPs).
The variation of the absorbance of AgNPs in respect of bulk water is a consequence of the micelles formation from the surfactants monomer, here, SDS, CTAB, and TX-100. Under the optimal experimental conditions, CMCs for SDS, CTAB, and TX-100 using SPR technique were 8.6 × 10−3, 9.1 × 10−4, and 2.6 × 10-4 mol L−1, respectively.
The potential application of SPR technique for the CMC determination has been demonstrated in this work. Comparing to the other CMC determination methods, this SPR method is universal, sensitive, and nonadditive; it can be used for the direct measurements of CMC without adding any dyes or fluorescent probes. CMC values of various surfactants including SDS, CTAB, and TX-100 determined by this method are in agreement with those determined by the other methods. The results obtained by our simple method have comparable accuracy with those obtained from more elaborate experiments.
KeywordsCritical micellar concentration (CMC) Surfactants Silver nanoparticles (AgNPs) Surface plasmon resonance (SPR)
Surfactants are compounds with molecular structure consisting of hydrophilic and hydrophobic parts. These important classes of industrial chemicals are widely used in modern industries. The hydrophilic part consists of an ionic or polar group whereas the hydrophobic part is generally a long hydrocarbon chain. In diluted solutions, the surfactants are found as individual monomers. At a concentration above a critical value, i.e., critical micelle concentration (CMC), surfactant molecules tend to aggregate forming micelles. At CMC point, some properties of surfactant solutions like conductivity, surface tension, osmotic pressure, absorption, and fluorescence as a function of concentration suffer abrupt changes due to the micelle formation (Tran & Yu 2005; Van Os et al. 1993).
CMC value of surfactants must be known in many applications like solubilization, stabilization, isolation, crystallization, etc. Therefore, CMC determination as an important characteristic of a surfactant is of great importance. CMC point can be determined by examining the changes in the chemical and physical characteristics of surface activator solutions after increasing their concentrations. Several methods have been reported for the determination of CMC such as UV-visible spectrophotometry (Khamis et al. 2005; Mondal & Ghosh 2012), fluorimetry (Mondal & Ghosh 2012; Topel et al. 2013; Zhu et al. 2014), infrared spectroscopy (Tran & Yu 2005), light scattering (Topel et al. 2013), nuclear magnetic resonance (NMR) (Yan & Palmer 1969), chromatography (Lin & Lin 2000), sound velocity (Zielinski et al. 1987), calorimetry (Simonović & Momirović 1997), and electrochemical techniques (Racaud et al. 2010; Nesmerak & Nemcova 2006).
In the past years, the theory and applications of surface plasmon resonance (SPR) have been discussed by several researchers (Yong et al. 2009; Abdulhalim et al. 2008; Amendola et al. 2010). Several of these applications take advantage of the engineering of silver nanoparticle (AgNP) plasmonic response that depends on their shape, size, dielectric environment, and on mutual electromagnetic interactions among particles in close proximity (Ren & Tilley 2007). SPR is a powerful technique to retrieve information on optical properties of nanomaterials. Essentially, SPR depends on the optical properties of metal layer and environmental changes, so it is related to charge density oscillation at the interface between them (Homola 2006). Hence, the main potential of SPR is the characterization of medium after the metal layer. Resize of the SPR signal is proportional to the mass of material that has immobilized SPR data, mass and thickness of absorbed layers can be inferred (Pattnaik 2005).
To the best of our knowledge, CMC determination by SPR method using AgNPs has not yet been reported. The aim of this work was to evaluate the possibility of using this method for CMC determination in aqueous solutions.
This paper describes a method for the CMC determination of sodium dodecyl sulfate (SDS), cetyl trimethyl ammonium bromide (CTAB), and Triton X-100 (TX-100) in aqueous media by SPR of AgNPs. The method is based on the variations observed in the absorbance of AgNPs.
Materials and methods
Instruments and reagents
A GBC UV-Visible Cintra 6 Spectrophotometer model, attached to a Pentium (IV) computer, with 1-cm quartz cell was used for the evaluation of optical characteristics of the SPR of AgNPs and for recording the spectra data. Metrohm 781 pH-meter was used to adjust pH of the buffered solutions.
All chemicals were of analytical grade and used as received without further purification. Silver nitrate (AgNO3), sodium borohydride (NaBH4), SDS, CTAB, and TX-100 which are used for the synthesis of AgNPs and CMC determination of surfactants were purchased from Merck. SDS, CTAB, and TX-100 solutions (1.0 × 10−2 mol L−1) were prepared by dissolving 0.288, 0.364, and 0.647 g in water and diluting to 100 mL with water, respectively. The minimum number of possible dilution steps was used for the preparation of more dilute solutions. All other common laboratory chemicals were of the best available grade.
All the measurements were performed at 25.0 ± 0.2 °C. The change of absorbance was recorded spectrophotometrically by measuring the absorbance of the mixture solutions at 410 nm that is λmax of SPR peak of AgNPs at these conditions against a reagent blank. In a typical procedure, 2.0 mL of AgNO3 solution (2.0 × 10−3 mol L−1) and 2.0 mL of KH2PO4 buffer solution (pH 7.0) were transferred into 10-mL volumetric flasks. Then, appropriate volumes of SDS, CTAB, and TX-100 solutions, in the range of 0.0 to 12.0, 0.0 to 1.6, and 0.0 to 0.5 mmol L−1, were added respectively to this solution and were put on magnetic stirring. After they were being mixed completely, the solutions were titrating by 1.0 mL of quite cold NaBH4 (1.0 × 10−2 mol L−1). Yellow colored solution confirmed the presence of AgNPs, and subsequently the solution diluted up to the volume mark with deionized water and measured its absorbance at 410 nm.
Results and discussion
Preliminary investigations of the system
The AgNPs were prepared according to the previous reported method with minor modification (Fan et al. 2009). Chemical synthesis of AgNPs and dimensions, shape, and distribution of NPs were influenced by several parameters, such as the AgNO3 and NaBH4 concentrations, time, temperature, pH, and surfactant concentration. All experimental parameters affecting the AgNP synthesis and CMC determination were optimized by “one at a time” method.
The effect of AgNO3 concentration
The effect of NaBH4 concentration
In order to establish the effect of NaBH4 concentration on the amount of absorption of AgNPs, different concentration values of NaBH4 in the range of 5.0 × 10−4 to 1.0 × 10−2 mol L−1 were studied. For this purpose, the solutions containing 2.0 × 10−3 mol L−1 AgNO3 were studied. It was founded that 1.0 × 10−2 mol L−1 NaBH4 was sufficient for further studies.
The effect of pH
The effect of temperature
The effect of time
CMC determination of SDS, CTAB, and TX-100
In order to determine the CMC points by SPR technique, anionic, cationic, and nonionic surfactants with different concentrations were utilized, and the curves were prepared from the related SPR peaks. All of the tested surfactants showed a sharp inflection point in the curves that enabled sensitive determination of their CMC values. In addition, all displayed similar magnitude of absorbance changes and shared similar curves around their CMC independent of the charge, class of surfactant, and order of magnitude of the CMC, indicating the universality of the method. Based on the mechanism of micelle formation, there is an inflection point in the absorbance of SPR of AgNPs versus surfactant concentration for all of the tested surfactants (Figs. 5b, 6b, and 7b). As can be seen, in all of the curves, a fracture in the slope of the surfactant concentration versus absorption at λmax of SPR peak of AgNPs (410 nm) is observed, that is the CMC point. Under the optimal experimental conditions, CMCs for SDS, CTAB, and TX-100 using this method were 8.0 × 10−3, 9.0 × 10−4, and 2.5 × 10−4 mol L−1, respectively.
Comparison of the CMC of SDS, CTAB, and TX-100 between typical published methods and the proposed method in water solution
CMC, × 10−3 mol L−1
(Khamis et al. 2005)
(Khamis et al. 2005)
(Mondal and Ghosh 2012)
(Aguiar et al. 2003)
(Prazeres et al. 2012)
(Mondal and Ghosh 2012)
Resonance Rayleigh scattering
(Shi et al. 2011)
Fiber-optic refractive index
(Tan et al. 2010)
(Brooks et al. 1988)
(Fuguet et al. 2005)
(Ma et al. 1998)
(Liu et al. 1999)
SPR of AgNPs
In summary, we have successfully developed a new and facile surfactant CMC determination by using SPR of AgNPs. Our goal was principally determining the CMCs of ionic and nonionic surfactants with a method that is comparable in scope and resolution to those currently obtainable from other techniques such as conductometric, voltammetric, spectrophotometric, fluorescence, and surface tension measurements. It could be seen that, the obtained CMC values of SDS, CTAB, and TX-100 by this method agree very well with those determined by previously reported methods. Furthermore, this method is universal, sensitive, and nonadditive; accordingly, it can be used for the direct measurements of CMC without the addition of any dye or fluorescent probe. Values obtained by our simple method have accuracy which is comparable with the data obtained from more elaborate experiments.
This work was supported by the Nanoscience and Nanotechnology Research Laboratory (NNRL) of Payame Noor University of Sirjan.
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