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Dielectric analysis for the spherical and rodlike micelle aggregates formed from Gemini surfactant: Driving forces of micellization and stability of micelle Shanshan Wang, and Kongshuang Zhao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01523 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016
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Dielectric analysis for the spherical and rodlike micelle aggregates formed from Gemini surfactant: Driving forces of micellization and stability of micelle Shanshan Wang, Kongshuang Zhao* College of Chemistry, Beijing Normal University, Beijing 100875, China Abstract The self-aggregation behavior of Gemini surfactant 12-2-12 (ethanediyl-1,2-bis (dimethyldodecylammonium bromide) ) in water was investigated by dielectric relaxation spectroscopy over a frequency range from 40Hz to 110MHz. Dielectric determination shows that well defined spherical micelles formed when the concentration of the surfactant was above a critical micelle concentration CMC1 3mM, and rodlike micelles formed above CMC2 16mM. The formation mechanism of the spherical micelles and its transition mechanism to clubbed one was proposed by calculating the degree of counterion binding of the micelles. The interactions between the headgroups and the hydrophobic chains of the surfactant leads to the formation of the micelles, while the transition mainly attributed to the interaction among the hydrophobic chains. By analyzing the dielectric relaxation observed at about 107Hz based on the interface polarization theory, the permittivity and conductivity of micelle aggregates (spherical and clubbed) and volume fraction of micelle were calculated theoretically as well as the electrical properties of solution medium. Furthermore, we also calculated the electrokinetic parameters of the micelle particle surface, surface conductivity, surface charge density and zeta potential, using the relaxation parameters and phase parameters. Based on these results, the balance of forces controlling morphological transitions, interfacial electrokinetic properties and the stability of the micelle aggregates were discussed. 1.
Introduction In 1991, Menger1 assigned the word Gemini to bis-surfactant which is composed of two hydrophobic tails
and two hydrophilic headgroups linked by rigid or flexible spacer group. Because of its unique structue, the Gemini surfactant has lower critical micelle concentration, higher surface activity, and performs varied assembled or aggregate structures in aqueous medium compared with conventional surfactants2. In almost 20 years, both fundamental and applied research on Gemini surfactant has attracted huge attention and special
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interest has recently focused on industrial and biological applications like drug-release3, the synthesis of nano materials4, and the extraction of oil5. Therefore, a lot of studies on the synthese and properties of functional new Gemini surfactants have been reported so far6, 7. Furthermore, as a special surfactant, the aggregation behavior and electrical properties of the Gemini surfactants have received significant attentions in the areas of fundamental research8, synthesis of nano- structured material 9 and so on. Many factors may affect the self-aggregation behavior of Gemini surfactant, of which the most remarkable is the interactions beteween surfactant molecule and solvent because the formation of the aggregate is consequences of subtle balance between the various forces, including mainly the electrostatic interactions between headgroups, the hydrophobic effect10. Besides, changing the types and properties of headgroup, properties of counterions and the length of hydrobhobic tails can also impact the formation of surfactant aggregate2. For instance, sphere-to-rod transitions are known to occur with increasing amphiphile or counterion concentration11, but the concentration associated with the transition depends on the amphiphile headgroup structure and the counterion properties12, 13. As far as the nature of counterion is concerned, the area per polar head group at the surface of surfactant aggregate depends strongly on the nature of the counterion and more specifically on its ability to bind to the surfactant aggregate. Therefore, an easy method to induce morphological transition of surfactant aggregates is to add counterions, because adding counterions will decrease the effective area per head group by screening the repulsive interaction between the head groups14. Indeed, the varied structural motifs of micelles (spheres, rods, lamellar, cubic, etc.) depend on the structure and hydrophobicity of the amphiphilic tail(s), but also on specific ion hydration, ion-pairing and the release of water into the aqueous domain. As far as we know, the situation of interfacial ion-pair formation and release of interfacial water are driven by the hydrophobic effect in morphological transitions, are rarely reported previously and is one of the main subjects of this paper. In this pepar, we look forward to fingding out how the balance of forces in the micellization and transformation is using dielectric spectroscopy. In addition, controlling over stability of micelles is critical to optimize their pharmacokinetics and bio-distribution15. This is because that micelles are generally stabilized by repulsive charges on the micelles surface and by adsorbed surface of micelles which prevent micelles from the close contacting16. The type of surfactant, the size and structure of micelle aggregate all affect the stability of the aggregate, and this has been studied extensively17, 18, However, the effects of shape have received little attention, much of the fundamental research on the stability of the colloid particle dispersion from the perspective of electrical 19, 20
properties, such as the surface conductivity, surface charge density and Zeta potential
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. Especially, Zeta
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potential is an important characteristic of the stability of micelles and a good measure for evaluating its interaction with ionic compounds19, 21. So far, some conventional techniques have been used to characterize and reveal the self-aggregation behaviors and electrical properties of amphiphilic molecules. Such as SANS22 are often used to determine the aggregation number, size and shape of micelles. Transmission Electron Microscopy (TEM)23and Scanning Electron Microscopy (SEM)24 are often used to obtain the size, shape, and state of micelles. Besides, fairly simple and convenient conductivity measurement24 is also widely used to determine the CMC of micelle. In addition, there are lots of studies about the internal information of micelles, despite considerable efforts undertaken over the recent years, our understanding of driving forces resulting the structural change of micelles and electrical dynamical properties of different shaped aggregates are far from complete. One reason is that the number of techniques able to explore the processes of the micellization and transition of micelles and the electrical properties of internal micelles are limited. Alternatively, dielectric relaxation spectroscopy (DRS), owing to its sensibility to all kinds of polarization, and the dynamics of dipoles can be obtained by analyzing the dielectric relaxation25. Until now, there are many researches on the micellar solution using the dielectric spectroscopy. For example, Shikata et al26 studied on the movement of the counterion around the spherical micelles, Buchner et al27 found out the bulk and confined water molecule around spherical micelles, Fan et al28 studied on aggregation behavior and electrical properties of spherical aggregate formed by amphiphilic pyrrole-tailed ionic liquids. However, there is little study about rod micelles by using dielectric spectrum, one of the reasons is that there is lacking of suitable theoretical model and formula. If a quantitative relation between the dielectric measurement and the complex system can be established, it is totally possible to calculate the electrical parameters of the constituent phase by theoretically analyzing dielectric spectra of the complex system. In this work, we use dielectric spectroscopy to study rod micelles based on related model and theoretical formulas (Section 3)for the first time. In the present study, we employed dielectric relaxation spectroscopy to study the aggregation behavior and stability of different shaped micelles of surfactant 12-2-12 (dimeric cationic surfactant ethanediyl-1,2-bis (dimethyldodecylammonium bromide) )aqueous. Its structure of surfactant 12-2-12 as shown in Scheme 1. There is a lot of studies about surfactant 12-2-12, for instance, This special surfactant forms a variety of micellar aggregates with increasing concentration29, 30. Oda et al
31
found the surfactant12-2-12 forms a
network-like superstructure with domain sizes increasing with shear by using electrical conductivity and viscosity measurement. Alargova et al32 reported that the critical micelle concentrations and the micelle
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aggregation numbers in these mixtures of various conventional surfactants and dimeric surfactants by electrical conductivity and time-resolved fluorescence quenching. Bakshi et al33suggested that the effects of the head group and counterion on mixed micelles of surfactant 12-2-12 with conventional surfactants by conductivity, turbidity, and NMR measurements. and so on. However, There is little studies about the internal information of micelles formed by surfactant 12-2-12, driving force in the process of the micellization and transformation of micelles, and the stability of different shaped micelles. In this work, using conductivity data from dielectric measurement, the concentration range of the structure transitions was distinguished. Our aim is to utilize the theoretical analysis based on dielectric spectra of different shapes of micelles. The dynamic parameters of the double layer on the surface and interface of micelles were calculated and the reason for the difference of the stability of the different shaped micelles was also given. Furthermore, the possible microscopic mechanism of the transition, particularly possible microscopic mechanisms for the transition of drive geometry from the microscopic point of view was discussed.
2.
Materials and methods
2.1. Materials ethanediyl-1,2-bis (dimethyldodecylammonium bromide (surfactant 12-2-12) was prepared and purified by using the procedure previously described in the literature
34
.Reaction of alkanediyl-α,ω-bis(dimethy1-amine)
with 1-bromo-n-dodecane for surfactant 12-2-12, which was performed in dry ethanol under reflux (T= 80 OC) for 48 h in the presence of a 5-10% excess of alkyl bromide to ensure as much as possible a complete biquaternization. The surfactant 12-2-12 was recrystallized in various solvent mixtures ethanol-ethyl acetate, and the purity of the surfactant 12-2-12 was checked by elemental analysis. The molecular structure of the surfactant 12-2-12 is depicted in Scheme 1. A series of concentrations of surfactant 12-2-12 aqueous solution were prepared from 0.01 to 29mM. Doubly distilled water was used for all experiments.
Scheme 1 Chemical structures of surfactant 12-2-12 used in this study
2.2. Dielectric measurements The dielectric measurements of the samples were carried out with a HP 4294A Precision
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Impedance Analyzer (Agilent Technologies) over a frequency range from 40Hz to 110MHz. All measurements were maintained at 25.0 ± 0.1 ◦C. A measurement cell with concentrically cylindrical platinum electrodes was employed35. By using several standard substances (air, pure ethanol, and pure water), the determined cell constant Cl, stray capacitance Cr and residual inductance Lr were 0.014 pF, 0.812pF, and 3.19E-8(F/S2) respectively. The experimental data errors arising from the residual inductance and measurement cell were corrected by Schwan method36. Then, the corrected data of capacitance Cs and conductance Gs at each frequency were converted to permittivity and conductivity, using the equations: ε = Cs/Cl and κ = Gsε0/Cl (ε0( = 8.8541 × 10−12 F/m) is the vacuum permittivity). 2.3. Dielectric analysis The complex permittivity ε∗ of a substance or system under an applied electric field with angular frequency ω can be expressed as37
ε ∗ (ω ) = ε (ω ) − j
κ κ (ω ) = ε (ω ) − jε ′′(ω ) − j l ε 0ω ε 0ω
(1)
where ε (ω ) and ε "(ω ) are the frequency-dependent permittivity and dielectric loss of the surfactant solution, respectively, κ (ω ) is conductivity, and j = ( −1)
1
2
, ω ( ω = 2π f , f is measurement
frequency) is the angular frequency. For the aqueous solution systems with higher electrolyte contents like our sample, usually a considerable electrode polarization (EP) occurs in the lower frequency range. The dielectric loss of whole system, κ (ω ) / ε 0ω , contains two contributions, one from the conductivity at low-frequency
κ l (or dc conductivity) which is the equivalent of ε dc′′ ( ε dc′′ = κ l ε 0ω ) , and the
other from dielectric loss
ε ′′(ω )
which can be calculated from the conductivity spectra through the
equation,.
ε ′′(ω ) =
κ (ω ) − κ l ε 0ω
(2)
Generally, the dielectric spectra over whole measuring frequency range can be described by following equation including i (i=1,2,.. indicate the number of dielectric relaxation) Cole-Cole’s terms and one electrode polarization term Aω
−m
(where A and m are adjustable parameters)
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ε ∗ = εh + ∑ i
where
∆ε i + Aω − m βi 1 + ( jωτ i )
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(3)
ε l and ε h are the low- and high-frequency limits of permittivity, respectively. ∆ε = ε l − ε h
indicates relaxation strength (or dielectric increment). Cole parameter (0
ζ potential, and state of motion of dispersed particles on the conductivity of a colloidal suspension. J. Colloid Interface Sci. 2003, 265 (1), 197-201. 62. Shikata, T.; Imai, S. Dielectric relaxation of surfactant micellar solutions. Langmuir 1998, 14 (24), 6804-6810. 63. Hoyer, H. W.; Marmo, A. The Electrophoretic Mobilities and Critical Micelle Concentrations of the Decyl-, Dodecyl-and Tetradecyltrimethylammonium Chloride Micelles and Their Mixtures. J. Phys. Chem. 1961, 65 (10), 1807-1810.
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The formation mechanism of the spherical micelles and its transition mechanism to clubbed one was proposed by calculating the degree of counterion binding of the micelles using dielectric data. The interactions between the headgroups and the hydrophobic chains of the surfactant leads to the formation of the micelles, while the transition mainly attributed to the interaction among the hydrophobic chains.
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