Article pubs.acs.org/Langmuir
Fusion and Fission Inhibited by the Same Mechanism in Electrostatically Charged Surfactant Micelles Yahya Rharbi,* M. Karrouch, and Paul Richardson Laboratoire de Rhéologie et procédés, UJF/INPG/CNRS, BP 53, Domaine universitaire, 38041 Grenoble, France ABSTRACT: This paper revises the general idea about the role of intermicellar and intramiceller interactions in inhibiting fusion of self-assembled surfactant micelles. Fusion and fission of micelles are usually thought to be limited by different mechanisms. While fission is accepted to be controlled by surface instabilities (intramicellar interactions), fusion is commonly thought to be rate limited by the barrier to the close approach between two micelles due to the steric or Coulombic repulsions (intramicellar interactions). Here we describe the role of electrostatic repulsions in inhibiting fusion and fission kinetics in self-assembled micelles. We use stopped flow-fluorescence technique with hydrophobic pyrene to quantify fusion and fission in ionic/nonionic mixed micelles (Triton X100/SDS). We show that the fusion and fission rates decrease with the same tendency with increasing the fraction of the ionic charges, while their ratio remains constant. Our results are interpreted to mean that, in slightly charged micelles, fusion shares the same limiting step with fission, which most likely involves surface instabilities and intramiceller interactions.
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INTRODUCTION Hydrophilic self-assembled materials such as surfactants and block copolymers can be designed to make micelles with controlled morphologies, such as spheres, rodlike, wormlike, vesicles, and so forth.1−4 The ability of these materials to form micelles from individual molecules and to change their shapes in response to external stimuli is important for both the natural and synthetic worlds.5−15 The dynamical mechanisms of these processes are crucial for controlling their properties and their applications.5−15 The kinetics of these micelles are well accepted to occur via two types of dynamical processes: an individual and a collective.16−41 The individual process involves the exchange of a single molecule between micelles via the aqueous phase by means of expulsion and insertion steps.16−19 Though such a process has been thoroughly investigated during the last three decades, only recent reports began to stress the role of molecular weight on the expulsion barrier in long macromolecules.19−24 The collective processes are dominated by fusion and fission of micelles.25,26 These two mechanisms are particularly important for natural and synthetic matter.9,10 For example fusion and fission of cellular membranes is vital for their function and survival.9,10 They are also crucial for controlling the behavior of amphiphilic aggregates particularly in drug delivery and for the synthesis of controlled nanoobjects.1−4 Fission and fusion in surfactants were suggested to explain experiments involving nonequilibrium kinetics such as a pressure jump, salt jump,26 or temperature jumps.27 The occurrence of these processes at equilibrium was established using a stopped-flow fluorescence technique, which exploits the exchange of hydrophobic pyrene derivatives between empty and full micelles.28,33 Because the exchange of these probes between micelles occurs via three processes, (1) exit−entry, (2), fusion, and (3) fission, the use of low water solubility © XXXX American Chemical Society
probes eliminates the exit−entry and magnifies the fusion and fission. This method was used to quantify fission and fusion in both nonionic and ionic and recently even block copolymer micelles.28−33 The existence of fusion and fission at equilibrium in surfactant micelles and vesicles have also been demonstrated through simulation by several groups.24−42 The dominating mechanisms for fusion and fission are still a subject of debate.28,31,34,40−42 The role of surface charges in inhibiting fission in ionic micelles has been attributed to the surface stabilization by the Coulombic repulsion on the micelle surface.31,32 In this model, fission starts as a surface instability, which involves several steps (Chart 2): (1) bending the micelle surface, (2) pinching, and finally (3) breaking.31,32 The surface bending and pinching require the close approach of the negatively charged head groups in the locus of the instability and the reorganization of the other charges on the micelle surface. Sammalkorpi et al. corroborated this model via numerical simulation and completed such description by suggesting the formation of an interdigitating stalk as a new step before fission.40 The fusion of two micelles is taught to occur via several steps: (i) collision, (ii) adhesion, and (iii) merging. One can imagine several inhibiting barriers for fusion: (1) The diffusion controlled rate, which is several orders of magnitude faster than fusion and can be ruled out as the dominating mechanism.28 2) The barrier to the close approach between micelles, which is dominated by the long-range Coulombic repulsions in ionic micelles and the steric repulsion in nonionic micelles. (3) The barrier to merging the two micelles, which is controlled by the Received: November 23, 2013 Revised: May 26, 2014
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dx.doi.org/10.1021/la501465v | Langmuir XXXX, XXX, XXX−XXX
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interactions within each micelle. Using simulation, Li et al.42 suggested that this step could occur via three stages: (1) molecular contact, (2) formation of a neck, and (3) growth of this neck. They suggest that the limiting steps for merging are the breaking of the water film between two neighboring micelles and the nucleation of a pore between two surfactant films. The rate limiting mechanism for fusion is usually thought to be the barrier to the close approach between two micelles, which is dominated by the long-range Coulombic repulsions in ionic micelles and to the steric repulsion in nonionic micelles.28,32,41,42 For example, the extreme slow fusion rate observed in ionic micelles (SDS or charged copolymer) is usually attributed to the long-range electrostatic repulsion between micelles.12,31,32,42 Yet, how the Coulombic repulsions between micelles (intramicellar interactions) and how the electrostatic repulsions on the micelle surface (intermicellar interaction) affect the various steps of fusion remain open questions. In this paper, we investigate the effect of these interactions on the fusion of surfactant micelles. We investigate the fusion and fission processes in nonionic/ionic mixed micelles, in which the amount of surface charges is varied. We show that, contrary to general belief, both fusion and fission in slightly charged micelles are in fact dominated by the same limiting mechanism, which most likely results from the intermicellar interactions. These results rule out the barrier to close approaches as the dominating step for fusion in slightly charged micelles and suggest that stabilization of the micelle surface with electrostatic charges could be the limiting step for fusion in this case.
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to that of pure TX100 CMC (0.3 mM) for f SDS < 0.4 and increases for f SDS > 0.4 to reach the pure SDS CMC (8 mM). However, electrophoresis experiments have detected a slight heterogeneous distribution of the ionic surfactant among the micelles for f SDS < 0.4.45 When a small amount of probe PyC18 is added to the micellar solution (Py/[micelle] ≪ 1), a fluorescent monomer peak around 375−400 nm appears. As the concentration of probe molecules increases, the fluorescence emission shows an excimer peak at 480 nm that increases with increasing the PyC18 concentration. When a PyC18−micelles solution is mixed with an excess PyC18-free micelle solution, the spectrum evolves and shows a higher monomer emission and no discernible excimer band, which infers the distribution of PyC18 among all the micelles yielding only micelles with one probe (Figure 1). The ratio of excimer to monomer intensities
Figure 1. Emission spectra of PyC18 in aqueous solution of TX100/ SDS micelles. The spectrum labeled “before the exchange” refers to a solution of TX100/SDS containing PyC18. The spectrum “After the exchange” refers to the solution obtained by mixing of the full micelles with an excess of empty TX100/SDS micelles. The excitation wavelength λex = 344 nm. Inset: Molecular structure of the probe PyC18.
EXPERIMENTAL SECTION
Materials. The probe 1-pyrenyl-octadecanone C34H44O (PyC18) was prepared via a Friedel−Crafts acylation of pyrene with stearoyl chloride in dichloroethane and in the presence of aluminum chloride (AlCl3).43 Triton X-100 (TX100, Aldrich) is an octylphenolethoxylate with an average of 9.5 ethylene oxide groups per molecule, and was used as received. N-Octyl pyreneand sodium dodecyl sulfate (SDS), were also used as received. Water used experimentally was demineralized through a Millipore Milli-Q purification system. A trace of PyC18 was added to a solution of Triton X-100 (43.0 g/ L) and heated above the cloud point (75−80 °C) and then agitated with a Vortex Genie 2 apparatus at maximum setting (>10 Hz). This procedure was repeated several times, and then the solution was passed through a 0.2 μm filter to remove any remaining solid traces. SDS solution was added to the PyC18/TX100 solution, at different mole fractions f SDS = [SDS]/([SDS] +[TX100]) to make mixed micelles bearing PyC18. Fluorescence measurements were carried out with a Jobin Yvon Fluorolog III (2-2) spectrometer in the S/R mode. Kinetic experiments were carried out by mixing a micelle solution (1 g/L) containing PyC18 with a probe-free micelle solution (at different concentrations) in a homemade stopped flow. The ratio of micelles containing PyC18 to the probe-free micelles was 1/5. All the measurements were carried out at 23 °C. The excitation wavelength was 344 nm, and the emission was monitored at λem = 480 nm for the excimer and λem = 375.5 nm for the monomer.
(IE/IM) increases linearly with increasing the average number of probe per micelle ⟨n⟩, where ⟨n⟩ = [PyC18]/([micelles] − CMC), which suggests that PyC18 undergoes a random Poisson distribution among the SDS/TX100 mixed micelles up to ⟨n⟩ ≈ 3 (figure not shown here).46 In Figure 2, we show the result of a time-scan experiment in which we monitor the decrease in the excimer intensity (λem =
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RESULTS AND DISCUSSION Nonionic Triton X-100 and ionic SDS surfactants were reported to form randomly mixed micelles in water, with an aggregation number Nagg that decreases with increasing f SDS from 120 in pure TX100 to 60 for f SDS ≈ 0.4 and remains constant for f SDS > 0.4.44 Their hydrodynamic radius (Rh) follows the same trend as the Nagg. On the other hand, their critical micelle concentration (CMC) was found to be constant
Figure 2. Time-scan experiment monitoring the decrease in the excimer emission (λem = 480 nm) and the increase of the monomer emission (λ1 = 375.5 nm) after stopped flow mixing of 0.2 mL of a TX100/SDS solution with a TX100/SDS solution containing PyC18. The excitation wavelength λex = 344 nm. B
dx.doi.org/10.1021/la501465v | Langmuir XXXX, XXX, XXX−XXX
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480 nm, IE) and the increase of the monomer intensity (λem = 375.5 nm, IM) following the mixing of SDS/TX100 micelles (1 g/L) containing PyC18 with PyC18-free micelle (1 g/L). Particularly for the case of small ⟨n⟩ (⟨n⟩
