Alpha-tocopherol in CTAB/NaCl systems — The light scattering studies

https://doi.org/10.1016/j.molliq.2017.02.116Get rights and content

Highlights

  • Vitamin E increases the dissociation degree of micelles in the CTAB/NaCl systems.

  • Increase of the vit.E/CTAB molar ratio causes the rise in electrophoretic mobility.

  • Mixed aggregates can serve as vitamin E carriers due to their small size.

  • The α-tocopherol/CTAB molar ratio modifies polydispersity of the studied systems.

  • Incorporation of vit.E into the CTAB micelles slightly changes the aggregation number.

Abstract

The light scattering techniques - laser Doppler electrophoresis (LDE) and dynamic light scattering method (DLS) - were applied to characterize properties of the cetyltrimethylammonium bromide (CTAB) micellar solutions in the presence of α-tocopherol (vitamin E) and NaCl at 25 °C. The surfactant concentration changed from 2 mM to 25 mM and the molar ratio of vitamin E to CTAB was in the range 0.01–0.18. It was found that the incorporation of α-tocopherol molecules into the palisade layer of CTAB micelles increases the ionization degree of mixed aggregates even at 0.5 M NaCl. At the same time it does not cause a significant increase of the mixed micelle sizes (the number of CTAB molecules in the aggregates). The observed increase of surface electrical charge of micelles in the presence of vitamin E leads to an increase of their electrophoretic mobility. A small size of the mixed micelles (about 5.0 and 6.0 nm at 0.15 and 0.50 M NaCl, respectively) points at full solubilisation of the active substance. This effect can be useful for elaboration of bioactive substance carriers in the micellar systems in the presence of electrolyte.

Introduction

Alpha-tocopherol (vitamin E) is a fat-soluble substance [1], [2], [3] of significant importance for pharmaceutical, cosmetic and food industry. The area of vitamin E applicability is related to its antioxidative and antitumour properties [4]. Alpha-tocopherol molecules contain the phenolic hydroxyl group which readily undergoes oxidation [5]. Solubilisation of vitamin E in the micellar solutions and emulsions inhibits its decomposition and enhances the stability of products [6]. Additionally, it is a way to create highly bioavailable nanomedicines [7], [8]. Micellar solutions containing vitamin E are investigated as potential drug delivery systems in, e.g. cervical cancer, glioma and breast cancer treatment [9].

Takahashi and Underwood [10] showed that vitamin E is well solubilised in the micelles created by bile salts and phospholipids. To dissolve α-tocopherol in the systems composed in major portion of water, the addition of amphiphilic molecules is necessary [5]. The molecules of hydrophobic substance are incorporated into the micellar core or are located in the palisade layer of mixed aggregate [5], [11].

Cetyltrimethylammonium bromide is a cationic surfactant used e.g. as a model ionic amphiphilic molecule in the investigations of solubilisation process of different substances in the aqueous phase [5], [12]. In the pharmaceutical research it is used e.g. as a stabilizer, emulsifier or drug carrier [13], [14], [15] with the additional antimicrobial activity [16], [17], [18].

Numerous investigations aimed at elaboration of the micellar systems that can be used as potential nano-carriers to transport active substances by different bio-membranes were carried out [4], [19], [20]. They focused on the determination of shape and size of aggregates, using e.g. dynamic light scattering (DLS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) methods [21]. Composition and stability of micellar systems were evaluated e.g. by investigations of electrical properties like conductivity and electrophoretic mobility [4], [15], [22], [23], [24], [25]. For quantitative determination of the amount of the hydrophobic substance solubilised in the surfactant micelles the UV–Vis techniques are applied [26].

In our previous studies it was found that solubilisation of vitamin E strongly influences properties of the CTAB micellar solution. Its incorporation into the surfactant aggregates led to lowering of CMC, decrease of electrolytic conductivity as well as increase of the aggregates size, electrophoretic mobility and zeta potential [11]. It would be of some interest to check if such influence of vitamin E on the CTAB solutions is maintained in the electrolyte presence.

The aim of the paper was to determine physicochemical properties of the CTAB micellar solutions in the presence of α-tocopherol at low and high concentrations of NaCl. It was done by the use of the light scattering techniques: laser Doppler electrophoresis and dynamic light scattering.

Section snippets

Materials

Cetyltrimethylammonium bromide (CTAB), sodium chloride (NaCl) and α-tocopherol (vitamin E) of > 99.8 wt% purity were purchased from Sigma Aldrich (Germany).

All solutions were prepared using the redistilled filtrated (Whatman Anodisc Membrane Filter with 0.02 μm pores size) water (electrolytic conductivity of 0.13 · 10 2 mS cm 1 at 25 ± 0.1 °C).

Preparation of samples

CTAB solutions (0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 2.00, 4.00, 6.00, 12.00 and 25.00 mM) with the NaCl concentration equal to 0.15 and 0.50 M each were prepared by

Characterization of CTAB/NaCl systems

The electrolyte presence in the ionic surfactant solution facilitates the aggregation process due to the increase of ionic strength and reduction of intermolecular repulsion. As a result, the critical micelle concentration (CMC) is lowered [37], [38]. The CMC values for CTAB determined by Roy et al. [39] in 0.01 M NaCl at 30 °C from the surface tension and conductance measurements were 0.55 mM and 0.60 mM, respectively, whereas the mean value of CMC obtained by Naskar et al. [40] using the

Conclusions

Alpha-tocopherol affects physicochemical properties of the CTAB/NaCl micellar systems. Location of vitamin E molecules among the CTAB monomers in the palisade layer of the micelle leads to the increase of the dissociation degree of mixed aggregates even at 0.50 M NaCl. This causes increase of electrophoretic mobility of the micelles in solution.

Incorporation of α-tocopherol molecules into the surfactant aggregates does not lead to a significant change in the micelles size and aggregation number.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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