Transformation from Globular to Cylindrical Mixed ... - ACS Publications

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Transformation from Globular to Cylindrical Mixed Micelles through Molecular Exchange that Induces Micelle Fusion Grethe V. Jensen,† Reidar Lund,‡ Theyencheri Narayanan,§ and Jan Skov Pedersen*,† †

Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, DK-8000 Aarhus, Denmark ‡ Department of Chemistry, University of Oslo, Postbox 1033 Blindern, NO-0315 Oslo, Norway § ESRFThe European Synchrotron, 71 Avenue des Martyrs, F-38043 Grenoble, France S Supporting Information *

ABSTRACT: Transformations between different micellar morphologies in solution induced by changes in composition, salt, or temperature are well-known phenomena; however, the understanding of the associated kinetic pathways is still limited. Especially for mixed surfactant systems, the micelles can take a very wide range of structures, depending on the surfactant packing parameter and other thermodynamic conditions. Synchrotron-based small-angle X-ray scattering (SAXS) in combination with fast mixing using a stopped-flow apparatus can give direct access to the structural kinetics on a millisecond time scale. Here, this approach is used to study the formation of cylindrical micelles after mixing two solutions with globular micelles of the nonionic surfactant dodecyl maltoside (DDM) and the anionic surfactant sodium dodecyl sulfate (SDS), respectively. Two separate processes were identified: (i) a transition in micellar shell structure, interpreted as exchange of surfactant molecules resulting in mixed globular micelles, and subsequently, (ii) fusion into larger, cylindrical structures.

M

Both SDS and DDM have a hydrophobic C12 hydrocarbon tail. SDS has a small anionic headgroup, whereas DDM has a bulkier, nonionic maltoside headgroup. Both form globular micelles in aqueous solution at moderate salt concentration (here, 0.3 M NaCl). However, it is observed (see SAXS data in Supporting Information) that cylindrical micelles are formed when SDS and DDM are mixed at ratios from 1:9 to 3:7 by weight. This is ascribed to a decrease in the effective headgroup area of the SDS molecules when they are mixed with DDM in the micelles, leading to a decrease in the repulsion between them (Figure 1a). The decreased headgroup area results in a lower preferred surface curvature and a transition from globular to cylindrical micelles. It is essentially the same effect that is observed in the well-known transition from globular micelles of SDS to cylindrical at higher salt concentrations, where the electrostatic repulsion between the head groups in the micelle shell is screened.31,32 Furthermore, the mixing of different head groups in the micelle shell might lead to a more efficient steric headgroup packing, and hence an effective reduction of headgroup area. Three different kinetic experiments were performed, following the structural change after mixing of SDS and DDM solutions in the stopped-flow apparatus with synchrotron SAXS (Figure 1b). Solutions were mixed to give a final surfactant concentrations of 2 wt % with a weight fraction of SDS, f w,SDS = 0.2, and of 1 and 2 wt % for f w,SDS = 0.3. The

icellar structures in surfactant solutions are dynamic entities1,2 which readily form, dissolve, or change their shape in response to changes in their environment.3−5 The wide range of structures obtained makes surfactant solutions attractive for a variety of applications from drug delivery and foods to materials templating,6−10 in many of which a mixture of different surfactants is used to obtain the desired properties. As the surfactant molecules in the micelles mix, new structures are formed that do not necessarily have intermediate properties of the unmixed micelles,11−14 leading to synergistic effects. The kinetics of micelles typically falls in the millisecond time scale,15−17 which not only is of fundamental interest but also determines their stability, that again control their performance in, for example, foaming, wetting, and emulsifying.18,19 Here, we use stopped-flow mixing in combination with synchrotron SAXS to study the formation of cylindrical micelles after mixing solutions of sodium dodecyl sulfate (SDS) with dodecyl maltoside (DDM) with millisecond time resolution up to 1.8 s. This approach has previously been used to study formation of vesicles20−22 and micelles23,24 and allows investigation of the structural details of the kinetic process as opposed to techniques that have typically been applied, such as fluorescence,25 light absorption,26 light scattering at a fixed angle,27,28 or conductivity.17,29 Theoretical investigations of the structural transitions can in principle be done by molecular dynamics simulation methods; however, the characteristic times of micelle reorganization and transfer of monomers exceed currently accessible time scales for atomistic surfactant models in explicit solvent.30 © XXXX American Chemical Society

Received: April 11, 2016 Accepted: May 16, 2016

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DOI: 10.1021/acs.jpclett.6b00767 J. Phys. Chem. Lett. 2016, 7, 2039−2043

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pattern, as shown on a log-log scale, appears flat up to quite large values of the scattering vector modulus, q, indicating globular micelles. Data for a corresponding premixed surfactant solution, corresponding to the expected equilibrated state at t → ∞, are also plotted. Data from the initial and the final kinetic SAXS frames are plotted as lines. The initial kinetic frame at t = 2.5 ms shows no sign of rod-like micelles, and the entire kinetic transition from globular to cylindrical micelles, thus, is captured. However, it clearly does not fully correspond to the average scattering of SDS and DDM, indicating that a slight degree of surfactant mixing or structural rearrangement has occurred already at t = 2.5 ms. The final kinetic frame at 1.8 s corresponds well with the premixed sample, but with a small discrepancy, which might be related to minor differences in the mixing ratio or to slight difference in temperature of the sample during the kinetic measurements. Alternatively, it could reflect that the structural transition has not reached completion at the last kinetic data frames, where the average aggregation numbers are determined to 81−93% of the values for the premixed samples. Both data sets show a q−1 behavior at intermediate to low q, characteristic of rod-like micelles. The kinetic data are shown in panel b together with fits of a structural model, given by a combination of scattering from ellipsoidal (Pcs‑ell(q)) and cylindrical (Pcs‑cyl(q)) core−shell particles

Figure 1. (a) Schematic drawing of reduced electrostatic repulsion when nonionic DDM head groups (orange) are mixed with the smaller, anionic SDS head groups (green). (b) Schematic drawing of the stopped-flow experiment, where solutions of SDS micelles (top left) and DDM micelles (bottom left) are mixed to give SDS weight fractions, f w,SDS, of 0.1−0.3. With time, the surfactants mix and cylindrical micelles are formed.

resulting SAXS data for 2 wt % surfactant and f w,SDS = 0.3 are plotted in Figure 2 (data for the other experiments are shown in Supporting Information). Data for the unmixed solutions, 0.8 wt % SDS and 3.2 wt % DDM, are shown in Figure 2a together with the calculated average of the two scattering curves, corresponding to the expected scattering at t = 0. The intensity

⎡f ⎤ 1 − fn ,cyl n ,cyl Pcs ‐ cyl(q) + Pcs ‐ ell(q)⎥ I(q) = Nsurf ⎢ ⎢⎣ p ⎥⎦ pell cyl

Nsurf is the surfactant number concentration, f n,cyl is the number fraction of surfactants in cylindrical micelles, and pi is the aggregation number of the micelles (see full description in Supporting Information). The radius of the ellipsoid core, Rcore,ell, was fixed at 16.7 Å, corresponding to the length of an all-trans C12 tail4 (attempts to optimize this parameter resulted in slightly larger values). The aspect ratio, ε, of the ellipsoid cores was fixed at a value of 1.8, as obtained for the initial frame, corresponding to a prolate shape. The radius of the cylinder core, Rcore,cyl, was fitted for the last data frame where the cylinders contribute most, resulting in a value of 13.7 Å, which was used for all other frames. The shell thickness D was fixed at a value of 10 Å for both the ellipsoidal and cylindrical micelles, which is comparable to the shell thickness in DDM micelles33 and is a reasonable estimate because DDM has the largest headgroup of the two surfactants and therefore can be expected to define the thickness of the shell. To obtain a good fit to the data, the core−shell and shell−solvent interfaces were graded in the model, rather than sharp: Gaussian smearing was introduced through the parameters σcore and σshell, giving the respective interfacial widths, applying the same values for ellipsoidal and cylindrical micelles. σcore was fixed at 1 Å, as the very hydrophobic core is expected to have a sharp surface. σshell was used as a free fit parameter. The modeling was done on absolute scale and the scattering contrasts for core and shell were calculated from molecular volumes of the surfactant tail and head groups, respectively, assuming a homogeneous micelle shell consisting of perfectly mixed SDS and DDM for both ellipsoidal and cylindrical micelles at all times t. This assumption can clearly not hold, and a correction factor, SΔρ,head, was applied for the headgroup contrast and optimized for each data set. The transition from ellipsoidal to cylindrical micelles is only partial, which indeed suggests nonidentical compositions of the two types of micelles. However, adding

Figure 2. (a) SAXS data for 0.8 wt % SDS (blue), 3.2 wt % DDM (red), the average of the two (magenta), a premixed 1:1 sample (black), and the first and final kinetic frames obtained after mixing 1:1 (magenta and black lines). (b) Data from the kinetic experiment, following the structural evolution after mixing (magenta to black) with model fits (black lines) and data from a premixed 1:1 sample (black points). 2040

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associated with formation and growth of cylindrical micelles, is observed in the time interval 0.1−1 s, and seen as an increase of the fraction of surfactant in cylindrical micelles, f n,cyl (panel b), with lengths L of a few hundred angstroms (insert in panel b). The clear separation of the two processes in time suggests that the molecular mixing is a prerequisite for the micelle growth. In a catanionic mixture, Zhang and Liu also found two sequential processes, upon mixing of oppositely charged surfactants (SDS and dodecyltriethylammonium bromide, DEAB), leading to formation of cylindrical micelles or vesicles depending on temperature.27 They ascribed the first process to formation on nonequilibrium mixed aggregates, followed by rearrangement into the final micelle structures. However, as the processes were detected through changes in the light scattering intensity at a fixed angle, structural details were not probed, and their assignments are of a more tentative, qualitative nature. Micelle growth of SDS, induced by addition of NaCl, has been the subject of a previous study, using an approach similar to the present.24 Here, the relaxation does not involve surfactant mixing, and for similar surfactant concentrations, the process was finished an order of magnitude faster than in the present case. The salt concentration of at minimum 0.5 M was higher than in the present case; however, doubling it to 1 M only resulted in a slightly faster relaxation. Therefore it is not expected that the relaxation time would be significantly increased at 0.3 M NaCl, corresponding to the salt concentration in the present study. A computer simulation study of cetyltriethylammonium chloride (CTAC) mixed with aromatic salts also showed rapid transition from spherical to cylindrical micelles although the growth mechanism was not analyzed in detail.34 The much slower equilibration in the present study indicates that the mixing of the two types of surfactants by molecular exchange is in fact delaying the micelle fusion that follows subsequently. These two processes, based on interpretation of the structural evolution (Figure 3a−d), are shown schematically in Figure 3e. The final value of f n,cyl obtained is less than unity; thus, the transition to cylindrical micelles is only partial. The cylinders are relatively short (panel b, insert). Both f n,cyl and L are larger for the higher concentration of 2 wt %. They are also larger for the smaller weight fraction of SDS, f w,SDS, indicating that the cylindrical micelles might contain an excess of DDM, and the ellipsoidal micelles, correspondingly, an excess of SDS. The fit parameters obtained for premixed, equilibrated samples (Table 1) are comparable to the values for the final kinetic frames, but with larger values for f n,cyl and L, indicating that the full equilibration might not have obtained in the kinetic experiment within 1.8 s. The suggested molecular mechanism behind the micelle growth can be validated by comparing the results to theoretical predictions. The classical theory for kinetics and dynamics in micelle solutions, given by Aniansson and Wall,1,16 describes the kinetics as a series of insertion/expulsion events of single molecules from the micelles, each characterized by the relaxation time τ1, and formation/dissolution of entire micelles through a series of insertion/expulsion steps, characterized by the relaxation time τ2. This theory is only valid for small perturbations where the kinetic equations can be linearized. For larger perturbations, leading to a substantial change of micelle size or shape, the process of micelle fusion/fission must be included to account for experimental observations35 and simulation results.2,36 In the present case, the initial mixing of SDS and DDM is likely to happen through rapid exchange of

further details to the model would require very specific assumptions on the distribution, and as this simpler model can describe the data, it was decided not to introduce an unequal composition. Nevertheless, it should be noted that the fit parameters σshell and SΔρ,head are expected to be sensitive to compositional and structural changes in the micelle shell. The structural parameters obtained from the model fits are plotted in Figure 3. Two separate processes can be identified.

Figure 3. Parameters from model fits to SAXS data for the kinetic experiments: (a) scale factor for the headgroup contrast, (b) number fraction of surfactant molecules in cylindrical aggregates with inset showing cylindrical micelle length, (c) width of smearing of the micelle shell surface, (d) reciprocal of the number-average micelle aggregation number (derived from the structural model parameters) with exponential fits (see text), (e) schematic representation of the processes identified for the formation of cylindrical micelles, as interpreted from the evolution of the structural parameters in panels a−d; a mixture of SDS micelles (green head groups) and DDM micelles (orange head groups) (left) exchange surfactant molecules, resulting in mixed micelles (center), a fraction of which fuses to form cylindrical micelles (right).

The first process occurs in the time interval 0.01−0.1 s, and is associated with an increase of the scale factor for the headgroup contrast, SΔρ,head (panel a) and of the smearing width of the outer micelle surface, σshell (panel c). As noted, the variation of these parameters might not describe the actual physical process, but indicates that a structural rearrangement is occurring within the micelle shell. This is compatible with a change of composition in the micellar shell and therefore is interpreted as associated with the fast mixing of SDS and DDM by molecular exchange between micelles. The second process, 2041

DOI: 10.1021/acs.jpclett.6b00767 J. Phys. Chem. Lett. 2016, 7, 2039−2043

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The Journal of Physical Chemistry Letters Table 1. Fit Parameters for the Model Described in the Text sample

premixed samples

micelle growth kinetics

c (wt %)

f w,SDS

f n,cyla

L (Å)b

σshell (Å)c

SΔρ,headd

1/pavee

Δ(1/pave)f

1/pave,eqg

τf (s)h

2 2 1

0.2 0.3 0.3

0.708(8) 0.484(9) 0.452(1)

381(7) 286(7) 221(6)

4.9(1) 4.16(7) 4.7(2)

1.056(5) 1.087(2) 1.111(5)

0.0041 0.0062 0.0068

−0.00518(9) 0.00369(7) 0.0026(1)

0.00505(7) 0.00669(6) 0.0076(1)

0.32(1) 0.36(2) 0.53(6)

a

Fraction of surfactants in cylindrical micelles. bLength of core of the cylindrical micelles. cSmearing width of the outer interface of the micelle shell. Scaling factor for the headgroup contrast. eThe reciprocal number-average aggregation number of the micelles, derived from fcyl and the core volumes of cylindrical and ellipsoidal micelles. fChange in reciprocal aggregation number. gFinal value of reciprocal aggregation number. hRelaxation time for the micelle growth. d

>99%) resulting in surfactant concentrations of 1 and 2 wt %, respectively. The kinetic stopped-flow SAXS data were collected at the beamline ID02 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France.22,24 After kinetic times of 1.8 s, the evolution of the SAXS intensity was not perfectly reproducible, possibly due to a change in sample composition via cross diffusion, and we restricted our analysis to t < 1.8 s. The SAXS data for solutions of pure SDS or DDM micelles could be described by the scattering from a model of core−shell ellipsoidal particles. For the mixed SDS-DDM samples (with f w,SDS of 0.1 to 0.3), scattering from cylindrical core−shell micelles was included in the model. See Supporting Information for further details.

single surfactant molecules, however, fusion might play an important role for the formation and growth of cylindrical micelles as also observed in other mixed surfactant systems.22 A growth mechanism dominated by micelle formation/dissolution through expulsion/insertion of single surfactant molecules would result in relaxation times increasing with surfactant concentration.16 We obtained relaxation times for the micelle growth by following the approach by Waton,37 who suggests that the number of micelles, which is inversely proportional to the average aggregation number, pave, will equilibrate following an exponential decay. The variation of pave−1(t) is plotted in Figure 3d together with fits of exponential functions, pave−1(t) = pave,eq−1 + Δ(pave−1) exp(−t/τf). The fits provide the relaxation times, τf, given in Table 1 together with the values of 1/pave,eq and Δ(1/pave). The relaxation time decreases with surfactant concentration, which strongly supports a growth mechanism dominated by fusion/fission of micelles.2,37,38 Observations by Michels and Waton for kinetics in solutions of worm-like SDS micelles in NaCl solutions, showed that the fusion/fission mechanism is also dominant in this case and furthermore suggested that the fusion was not limited by diffusion but by an activation barrier for the fusion process, which might be related to an unfavorable curvature (“neck formation”)39 at the fusion point or to residual electrostatic repulsion. The same aspects might also hold in the present case, indicated by the slight decrease in relaxation time for the lower fraction of SDS, f w,SDS, corresponding to a lower surface charge of the mixed micelles and thereby to a lower energy barrier for the fusion process. We note that an alternative equilibration mechanism, suggested by Poole and Bolhuis,40,41 consisting of growth through the uptake of single surfactant molecules followed by micellar fission and rearrangement of the unstable micelles is not supported by our data. In conclusion, the formation of cylindrical micelles after mixing of globular micelles of SDS and DDM was followed on a millisecond time scale by SAXS, revealing a two-step mechanism: First, there is a change in the micellar shell structure, interpreted as the formation of mixed, unstable micelles by molecular exchange, which appears to be a prerequisite for the second process, associated with formation and growth of cylindrical micelles, showing behavior in accordance with micelle fusion. To our knowledge, this mechanism has not been clearly identified before and contributes to the basic understanding of mixed surfactant systems and of the dynamic behavior of surfactant micelles useful for tailoring their physical properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00767. (i) plots of SAXS data for 2 wt % solutions of SDS and DDM with varying weight fraction of SDS, and the corresponding pair−distance distribution functions, p(r), (ii) plots of SAXS data for kinetic SAXS measurements after mixing of SDS and DDM solutions with concentrations of the SDS and DDM to obtain final total surfactant concentrations of 2 wt % with a weight fraction of SDS, f w,SDS = 0.2, and 1 wt % for f w,SDS = 0.3, (iii) details and mathematical expressions for the structural model of ellipsoidal and cylindrical micelles, applied to fit the SAXS data (PDF). (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. +45 87155921. Present Address

(G.V.J.) Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark. Author Contributions

(G.V.J. and R.L.) These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The ESRF is acknowledged for provision of synchrotron beam time. R.L. acknowledges grants from the Norwegian Research Council, under the SYNKNOYT program (218411 and 228573).

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EXPERIMENTAL METHODS SDS (Aldrich, >99%) and DDM (Glycon Biochemicals GmbH, >99.5%) were dissolved in a solution of 0.3 M NaCl (Fluka, 2042

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