Formation Kinetics of Oil-Rich, Nonionic Microemulsions - Langmuir

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Formation kinetics of oil-rich, nonionic microemulsions Helge Frederic Maria Klemmer, Carola Harbauer, Reinhard Strey, Isabelle Grillo, and Thomas Sottmann Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00738 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 4, 2016

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Formation kinetics of oil-rich, nonionic microemulsions Klemmer, Helge F.M.*a, Harbauer, Carolaa, Strey, Reinharda, Grillo, Isabelleb and Sottmann, Thomasc a

Universität zu Köln, Physikalische Chemie, Department Chemie, Luxemburger Straße 116, 50939 Cologne, Germany. Fax: +49 221 4705104; Tel: +49 221 4704542; E-mail: [email protected]

b

Insitut Laue-Langevin, CS 2015638042 Grenoble Cedex 9, France. Fax: +33 476 207120; Tel: +33 476 207503; E-mail: [email protected]

c

Universität Stuttgart, Institut für Physikalische Chemie, Pfaffenwaldring 55, D-70569 Stuttgart,

Germany. Fax: +49 711 685 64443; Tel: +49 711 685 64494; E-mail: [email protected] *

corresponding author

KEYWORDS formation kinetics, neutron scattering, microemulsions, stopped-flow

ACS Paragon Plus Environment

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ABSTRACT

The formation kinetics of oil-rich non-ionic microemulsions were investigated along different mixing pathways using a fast stopped-flow device in combination with the new high-flux small angle neutron spectrometer D33 (ILL, Grenoble, France). While the kinetics along most pathways were too fast to be resolved, two processes could be detected mixing brine and the binary cyclohexane/C10E5 solution. Here too, the formation of large water-in-oil droplets was found to be faster than 20 ms and therewith faster than the accessible dead time. However, subsequently, both the disintegration of the large water-in-oil droplets (600 Å) and the uptake of water by swollen micelles (50 - 60 Å) could be resolved. Both processes occur on the time-scale of a second. Strikingly, the total internal interface forms faster than 20 ms and does not change over time.

INTRODUCTION

In our everyday life a lot of processes are related to the mixing of polar and non-polar substances, like detergency, medical technology1-3, food processing4, template strategies in nanoparticle synthesis5, 6 and the load-dependent introduction of water into diesel fuel to reduce the NOx- and smoke-emission7-9. In all these processes the usually micrometer- -sized emulsions can be formed by the introduction of high shear forces combined with a low amount of surfactant to mix the two immiscible substances. Although these emulsions might be kinetically stable for weeks or even months, they are inherently unstable from a thermodynamic perspective. Employing a larger amount of surfactant - and thereby covering the complete interface between polar and non-polar substance - thermodynamically stable, nanostructured microemulsions may form10-12. Although their equilibrium properties like phase behaviour13-18, low oil/water


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interfacial tension19, 20 and multifarious nanostructure21-26 have been studied in great detail for the last quarter of a century, far less is known about their formation kinetics and the formation of the internal interface in particular. In older related studies the pressure and temperature-induced micelle formation was investigated in (pseudo-) binary water/surfactant mixtures27-30 as well as the transformation of spherical micelles to elongated ones31,

32

. For these transitions similar

studies have also been conducted using block copolymers33-35. Beyond that most of the other related experiments deal with the kinetics of structural transformations, e.g. the well-studied micelle-to-vesicle transition33,

36-42

induced mainly by changes in the composition (using the

stopped-flow technique)43-45 or the lamellar-to-sponge (L3) transition due to changes in temperature or pressure46. Kinetic experiments on microemulsions concentrate on the exchange kinetics of prestructured inverse w/o-microemulsions47-49 or most-recently on the pressure induced sphere-to-cylinder transitions of CO2-swollen micelles50. Another highly interesting study deals with the formation of dense droplet microemulsions by mixing a binary lamellar phase with oil51. Regarding the application of microemulsions as templates kinetic studies on the growth of nanoparticles have been performed experimentally as well as theoretically52,

53

. As

however none of the investigations carried out so far deals with the formation of microemulsions starting from brine and the binary cyclohexane/C10E5 solution , this study intends to monitor precisely this process.

EXPERIMENTAL DETAILS D2O was obtained from euriso-top, Saint-Aubin Cedex, France at a purity above 99.5% while NaCl was bought from Sigma Aldrich, Munich, Germany at the same purity. Protonated cyclohexane was supplied by VWR prolabo, Darmstadt with 99.5% purity and the nonionic surfactant C10E5 was purchased at Bachem, Budendorf, Switzerland with a purity above 97%.


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The small angle neutron scattering experiments were done at the D33, ILL, Grenoble France at λ = 6 Å with a sample to detector distance of 2 m and 12.8 m for all equilibrium samples (collimation of 7.8 m and 12.8 m), 7 m for all binary samples and 7 m for all kinetic studies (collimation of 7.8 m). A BioLogic SFM 300 stopped-flow equipped with a Berger ball mixer thermostated to the respective temperatures was used with a 1 mm thick planar quartz cell. The stopped-flow head piece was custom made in the mechanical workshop of the Physical Chemistry at the University of Cologne and has a temperature stability of greater than 0.1 K. It is now publically available at the ILL and supervised by I. Grillo. The neutron beam was centered on the cell and had a diameter of 8 mm. As the presented experiment was one of the first experiments ever done at D33 and the first kinetic experiment ever done there, the secondary ring detector was not yet operational. The air gap around the stopped-flow was about 30 cm wide. Mixing was done at a flow rate of 4 ml/s and a total injected volume of 804 µl per repetition. Each experiment was performed at least 40 times to allow for accumulation of at least 106 counts. The phase behavior measurements were done according to the well known method developed by Kahlweit and Strey and have been published numerous times elsewhere (see e.g. 13).

RESULTS AND DISCUSSION

The study at hand investigates the formation kinetics of a pseudo- ternary oil-rich water-in-oil microemulsion of the type D2O/NaCl – cyclohexane - pentaethylene glycol monodecyl ether (C10E5) by means of time-resolved small angle neutron experiments using a fast stopped-flow device. For interpretation of the kinetics the static properties of the water-in-cyclohexane microemulsion have to be known. Therefore, first the phase behavior of the system was


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characterized as a function of temperature T and mass fraction wA of D2O/NaCl (brine) at a constant mass fraction γb = 0.05 of C10E5 in the mixture of C10E5 and cyclohexane. 0.1 wt% of NaCl (ε = 0.001) was added to D2O to screen possible electrostatic interactions due to ionic impurities. The obtained phase diagram is shown in Fig. 1. As can be seen, the phase diagram has the shape expected for non-ionic microemulsion systems. At low temperatures water continuous microemulsion coexists with a cyclohexane excess phase (2), while at high temperatures a cyclohexane continuous microemulsion coexists with a water excess phase (2ത). In between a one phase region (1) can be found at intermediate temperatures which extends up to a maximum brine content of wA,max ≈ 0.16. At larger mass fractions of brine a three phase region (3) appears at the upper critical endpoint temperature Tu, in which a bicontinuous microemulsion phase coexists with a cyclohexane and brine excess phase. Furthermore, a lamellar (Lα) and a sponge (L3) phase including the respective two phase regions were observed at lower water mass fractions, i.e. wA < wA,max and temperatures below Tu. The anisotropic Lα phase can be distinguished from the only streaming-birefringent L3 phase optically using crossed polarizers.


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Fig. 1: Phase diagram of the system D2O/NaCl (brine) - cyclohexane - pentaethylene glycol monodecyl ether (C10E5) with a mass fraction γb = 0.05 of surfactant in the oil/surfactant mixture and a NaCl mass fraction of ε = 0.001 in brine as a function of temperature T and the mass fraction wA of D2O. The blue star and the red cross correspond to the final compositions of the mixing experiments.

The formation kinetics of these water - in - oil microemulsions were studied along different mixing pathways applying a fast BioLogic® SFM 300 stopped-flow device in combination with the new high flux small angle spectrometer D33 (ILL, Grenoble, France, see above for more experimental details). Using D2O and protonated C10E5 as well as cyclohexane the scattering contribution of the film contrast (∆ρfilm = -0.43·10-6 cm-1) is negligible to the contribution of the


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bulk contrast (∆ρcore = -6.68·10-6 cm-1). The temperature was chosen to not cross the lamellar region along the mixing pathway to prevent the highly viscous lamellar phase from blocking or even destroying the stopped-flow device. In all experiments the first scattering curve was recorded 20 ms after mixing due to technical limits based on the stopped-flow itself and the fact that large volumes are required to fill the cuvette. Note, that the kinetic experiment was repeated at least 40 times to obtain scattering curves based on at least 106 counts per detector.

Fig. 2: Left: SANS spectra of the microemulsion D2O/NaCl(brine) -cyclohexane - C10E5 with γb = 0.05, ε = 0.001, T = 24.50°C and wA = 0.05 at equilibrium (black circles), 20 ms (green stars) and 180 s (red squares) after mixing two microemulsions of the same composition. Right: SANS spectra of the microemulsion D2O/NaCl(brine) - cyclohexane - C10E5 with γb = 0.05, ε = 0.001, T = 24.50°C and wA = 0.04 (black circles) and with wA = 0.06 (red squares) at equilibrium, 20 ms (green stars) and 180 s (blue triangles) after mixing the two microemulsion adjusting a mixing ratio of 1:1. The data are described by an appropriate combination of form and structure factors54-56.


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Before the study of the formation kinetics we checked whether the shear forces applied by the stopped-flow device have an influence on the structure of the microemulsion. Therefore two water-in-oil microemulsions with identical composition (at wA = 0.05, γb = 0.05 and ε = 0.001, see Fig. 1, blue star) were mixed at T = 24.50°C adjusting a mixing ratio of 1:1. Figure 2, left shows the recorded small angle neutron scattering (SANS) intensities as a function of the scattering vector q = 4π/λ sin(θ/2) of the equilibrium state microemulsion before mixing (black circles), 20 ms (green stars) and 180 s (red squares) after mixing. Please note, that the measurements in equilibrium and 180 s after mixing were performed at two sample-to-detector distances (2 m and 12.8 m with λ = 6 Å), while the 20 ms measurement was only performed at one sample to detector distance (7 m with λ = 6 Å). All curves resemble the typical scattering pattern of slightly elongated droplets with a q-4-dependence at high and a q0-dependence at low q-values. At q ≈ π/R0 (R0: mean radius of the droplets) an oscillation of the scattering intensity can be observed which is smeared out by the droplet polydispersity σ0/R0. As can be seen, all three scattering curves fall on top of each other which unambiguously proves that the shear forces applied by the stopped-flow device have no influence on the structure of the microemulsion (at least for times larger than 20 ms after mixing).

The scattering data of this and all consecutive experiments were described with a form factor of polydisperse spherical or elongated core-shell droplets (Psphere(q) or Pcylinder(q))54,

56, 57

. The

extracted structural parameters are the mean radius R0, polydispersity σ0/R0, shell thickness d and shape of the radial density distribution function χ (elongated droplets: cross section R0 and length L). To account for interparticle interactions the polydisperse Percus-Yevick structure factor (SPY(q)) was used with the hard sphere interaction radius RHS, its polydispersity σHS/RHS, and the


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volume fraction φHS being the fit parameters55. Applying the decoupling approximation the scattering intensity I(q) is then given by

‫ܫ‬ሺ‫ݍ‬ሻ = ݊ ∙ ܲሺ‫ݍ‬ሻ ∙ ܵሺ‫ݍ‬ሻ

(1)

with n being the particle number density. For more details, e.g. the used form and structure factors see supplementary material.

In the next step we studied the formation of the same water - in -oil microemulsion (wA = 0.05, see Fig. 1, blue star) mixing equal volumes of two microemulsions with different mass fractions of brine, i.e. with wA = 0.04 and wA = 0.06. The respective scattering curves (red squares and black circles) are shown in Figure 2, right. Comparing the scattering intensities at low q a substantially higher intensity is observed for the microemulsion with wA = 0.06. Furthermore, the position of the shoulder shifts to lower values of q. Both trends can be ascribed to the swelling of the microemulsion droplets due to the increasing mass fraction of brine. Mixing the two microemulsions adjusting a mixing ratio of 1:1 already after 20 ms a scattering curve (green stars) is obtained which resembles the curve of the microemulsion with wA = 0.05. Note, that the scattering curves recorded after 20 ms and 180 s (blue triangles) fall on top of each other which shows that the formation kinetics of a microemulsion produced from two microemulsions with different water mass fractions is not resolvable with the accessible dead time of 20 ms.

However, the main goal of this study was to investigate the formation of a water-in-oil microemulsion obtained via the addition of water to an oil/surfactant mixture (the mixing


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pathway within the Gibbs phase triangle is depicted in Scheme 1). Thus, brine and a binary cyclohexane/C10E5 mixture (γb = 0.05) was mixed within the stopped-flow device at T = 24.50°C. Adjusting a mixing ratio of 10:103 a water-in-oil microemulsion with a water mass fraction of wA = 0.12, i.e. a volume fraction of φA = 0.085 is obtained.

Scheme 1: Schematic representation of the mixing pathway for the mixing of an unstructured binary cyclohexane/C10E5 mixture with D2O/NaCl in a Gibbs phase triangle at constant temperature T = 24.50 °C. The red circles represent the single components before mixing, while the arrows point to the composition of the final mixture close to the water emulsification failure boundary (wefb).


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Fig. 3: Left: Small angle neutron scattering spectra of the binary cyclohexane/C10E5 system with γb = 0.05 ( black circles), D2O/NaCl with ε = 0.001 (red squares), and the mixture of the two recorded 20 ms (green stars) and 180 s after mixing (blue triangles). Note that the mixing ratio was adjusted to 103/10 (V(cyclohexane/C10E5)/V(brine)). Right: Small angle neutron scattering spectra recorded at different times t after the mixing. The spectra were recorded at a sample to detector distance of 7 m and λ = 6 Å.

The SANS spectra of the brine solution (red squares), the cyclohexane/C10E5 solution (black circles), the mixture of the two 20 ms (green stars) and 180 s (blue triangles) after mixing microemulsion after mixing are shown in Fig. 3 (left). Thereby, the spectrum recorded 180 s after mixing was measured at three sample to detector distances (2 m and 12.8 m with λ = 6 Å), while the spectrum recorded after 20 ms was measured at one sample to detector distance (7 m with λ = 6 Å) and the scattering of binary cyclohexane/C10E5 and brine were recorded at one sample to detector distances (7 m with λ = 6 Å).

As can be seen, both the brine and the cyclohexane/C10E5 solution show a constant scattering intensity dominated by incoherent scattering contributions. It can therefore be concluded, that the cyclohexane/C10E5 solution is not structured on the length scale under study. However, it is to be noted that Tanaka and Saito and Smith et al. showed that nonionic surfactants form very small aggregates in nonpolar solvents.

58, 59

Mixing brine and the cyclohexane/C10E5 solution already

after 20 ms a scattering pattern can be observed which can be ascribed to the presence of complex droplet-like structures. Comparing this scattering pattern with the one recorded 180s after mixing, it becomes obvious that for q > 0.016 Å-1 both curves fall on top of each other (due to the constant internal interface given by the total amount of surfactant). However, for low q-


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values a clearly different pattern is found. While the scattering curve recorded after 180 s can be described by the form factor of elongated core-shell droplets Pcylinder(q) using the polydisperse Percus-Yevick structure factor (SPY(q)), the one recorded at 20 ms cannot be described using a single form factor. Here the accessible scattering intensity can be quantitatively described assuming a mixture of comparably large unstable water droplets stemming from the mixing process (Rdrop = 600 ± 100 Å (spherical) and elongated microemulsion droplets (RmE = 48 Å) using

‫ܫ‬ሺ‫ݍ‬ሻ = ‫ܫ‬ௗ௥௢௣ ሺ‫ݍ‬ሻ + ‫ܫ‬௠ா ሺ‫ݍ‬ሻ

(2)

with Idrop(q) being the scattering intensity of the larger water droplets and ImE(q) the contribution of the microemulsion droplets. Note, that Rdrop = 600 Å provides the most quantitative description of the scattering data. However, it is obvious that additional data at low q-values would reveal information with regard to the large droplets. Furthermore eq. 2 is an approximation, which neglects the cross term of both scattering amplitudes. This cross term is only of importance in the case where both contributions have a similar intensity, i.e.at q ≈ 0.009 Å-1. An important parameter, which can be deduced from the analysis of the scattering curves is the fraction of the larger unstable water droplets

Φௗ௥௢௣ =

ϕୢ୰୭୮ ϕୢ୰୭୮ + ϕ୫୉

(3)


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with respect to the overall volume fraction of water and surfactant. Thereby φdrop and φmE are the volume fraction of water and surfactant dispersed in the larger unstable water droplets and elongated microemulsion droplets, respectively. Surprisingly, from the fit of the scattering curve recorded 20 ms after the mixing, the fraction dispersed in the large water droplets can be determined to only 14%, while the rest, i.e. 86%, is already dispersed in microemulsion droplets.

To resolve the structural transition from a mixture consisting of large unstable water and microemulsion droplets to a thermodynamically stable microemulsion the temporal evolution of the scattering pattern is shown in Fig. 3 (right). As can be seen, the scattering contribution of the larger water droplets is clearly evident in the first three scattering curves till t = 298 ms. Their contribution to the overall scattering intensity decreases, indicating the fast uptake of the water solubilized in the larger unstable droplets by the stable microemulsion droplets.

Furthermore the shoulder of the scattering curves which is related to the radius of the elongated microemulsion droplets shifts to lower q-values indicating that the cross section radius is increasing. Simultaneously the regime of the scattering curve where the decrease of the intensity is proportional to q-1 shortens indicating a decrease of the length of the cylinders. However, a quantitative determination of the length is not possible due to the limited q-range. Therefore the length of the microemulsion droplets was kept constant at L = 300 Å enabling the fitting of the scattering curves. (Considering the overall composition of the system studied, 300 Å is well within the dimensions commonly encountered and the length describing the scattering data most fittingly.) The most relevant fit parameters are given in table 1. It can be seen, that the cross sectional radius of the microemulsion droplets RmE increases from 48 Å at 20 ms after the mixing to 60 Å in equilibrium due to the uptake of water from the larger unstable droplets. Consistently


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their polydispersity pmE = σmE/RmE decreases strongly from 0.64 to 0.31, which is a typical value for droplet microemulsions56, 57. Last but not least, the fraction Φdrop of larger water droplets with respect to the overall dispersed volume fraction (eq.3) decreases from 0.14 to 0 in below 5 s. For all recorded scattering curves the total internal interface S/V remains constant at about 1.528·103

Å-1 which is reasonable was all surfactant is expected to be at the interface below the

instrumental dead time in this diffusion controlled process.

Table 1 Selected fit parameters used to describe the time-resolved scattering data recorded during the mixing of a binary cyclohexane/C10E5 solution (γb = 0.05) with brine at a mixing ratio of 103/10 (V(cyclohexane/C10E5)/V(brine)). The data are described by the sum of scattering contributions

from

large

unstable

spherical

water

droplets

of

fixed

radius

(∆ρdrop = 6.68·10-6 cm-1, Rdrop = 600 Å) and polydispersity (pdrop = σdrop/Rdrop = 0.18) and from elongated microemulsion droplets using eq. 2. The length of the microemulsion droplets was set to

L = 300 Å,

the

scattering

length

densities

to

∆ρcore = -6.68·10-6 cm-1

and

∆ρfilm = -0.43·10-6 cm-1 using a film thickness of d = 5 Å and a diffusity of the profile of χ = 4 Å. Given are the time after the mixing at which the respective scattering data were recorded, the cross sectional radius RmE of the microemulsion droplets and their polydispersity pmE as well as the fraction Φdrop of larger water droplets with respect to the overall dispersed volume fraction (eq. 3). The times between 4889 ms and 107661 ms have been omitted here, as no significant changes occur in between.

t [ms]

RmE [Å]

pmE

Φdrop

20

48

0.64

0.140

134

52

0.58

0.090


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298

55

0.46

0.070

539

56

0.43

0.045

891

57

0.38

0.020

1408

57

0.35

0.008

2163

57

0.32

0.002

3269

58

0.32

0.001

4889

59

0.32

0.000

107661

60

0.31

0.000

Equilibrium

60

0.31

0.000

Fig. 4 shows the radial growth of the elongated microemulsion droplets (left) and their wateruptake, the fraction Φdrop of larger water droplets with respect to the overall dispersed volume fraction as a function of time in a semi-logarithmic representation (right). As can be seen, the radial growth and the water-uptake can be quantitatively described by a mono-exponential function. The obtained relaxation times τ result to (260 ±30) ms for the radial growth and (450 ±50) ms for the water-uptake. One might have expected that the water-uptake and the growth of the resulting microemulsion droplets to be comparably fast. However, due the limited q-range, changes in the length of the cylindrical droplets could not clearly be resolved.


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Fig. 4: Growth of cross sectional radius of the microemulsion droplets (left) and the fraction Φdrop of larger water droplets with respect to the overall dispersed volume fraction as a function of time in a semi-logarithmic representation (right). The red line corresponds to a monoexponential growth or decay leading to a relaxation time τ of about (260 ± 30) ms for the radial growth and (450 ± 50) ms for the water-uptake of the observed microemulsion droplets. Please note that only the first 5000 ms of the experiment are shown (compare table 1).

CONCLUSION

In the study at hand the formation kinetics of different oil-in-water droplet microemulsions of the type D2O/NaCl - cyclohexane - C10E5 were investigated along different mixing pathways using by combination of a fast stopped-flow device with time-resolved SANS. It turned out that structural changes induced by the mixing of two identical and two different microemulsion were too fast to resolve. Mixing brine with a cyclohexane/C10E5 solution, already the first scattering curve recorded after 20 ms points toward the existence of a mixture of comparable large unstable water droplets stemming from the mixing process (Rdrop = (600 ± 100) Å (spherical)) and elongated microemulsion droplets (RmE = 48 Å). The total internal interface of this first scattering curve and all that follow is identical. From that it can be deduced that the formation of


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self-assembled structures is faster than 20 ms which is good news for many processes in everyday life. However, we were able to resolved two processes: a fast uptake of the water solubilized in the larger unstable droplets by the stable microemulsion droplets which can be described by a mono-exponential decay with a time constant of approx. (450 ± 50) ms and the change of the cross sectional radius of the elongated microemulsion droplets exhibiting a time constant of about (250 ± 30) ms. The discrepancy in time constants is most likely due to the limited q-range which makes it hard to resolve the complete geometrical change of the microemulsion droplets. The observed formation kinetics (Scheme 2) might be either interpreted assuming that the larger emulsion-like droplets are dissolved in an inverse Ostwald ripening-like process or via many coagulation and coalescence processes. Both processes are driven by a lower bending energy60 of the microemulsion droplet, as in this case the amphiphilic film is known to possess almost its preferred curvature61.

In order to monitor and understand the primary structure formation in particular two approaches are plausible: one either has to reduce the dead time of the experiment by improving the stoppedflow apparatus or one has to slow down the formation kinetics. The latter might be tuneable via the variation of the membrane stiffness by varying the surfactant chain length or by introducing amphiphilic diblock co-polymers62. To extend the measurements to even shorter intrinsic experimental dead times a continuous flow technique using microfluidics is promising. Furthermore, the q-range of the experiment has to be increased, which will require a different detector geometry or a larger sample to detector distance (resulting in a lower flux and thus higher number of repetitions) or the use of time-of-flight mode (resulting in a harder to subtract background). In order to gain more comprehensive insights into the formation of droplet


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microemulsions, future studies will deal e.g. with the influence of the number density of droplets and the chain length of the surfactant.

Scheme 2: Proposed microemulsion formation mechanism. At t = 20 ms after the mixing large unstable water droplets coexist with highly polydisperse microemulsion droplets and a few aggregates of surfactant.. Within a few seconds the water solubilized in large unstable droplets is taken up by the stable microemulsion droplets resulting in a lower polydispersity. After that the microemulsion structure equilibrates until it reaches its final state.


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Acknowledgements

We would like to thank H. Metzner and the technical workshop for the development of the custom made highly temperature stable mixing unit for the stopped flow device. Helge Klemmer gratefully acknowledges the support from the IHRS Biosoft. Furthermore, the authors would like to thank the ILL for beam time allocated and travel reimbursement.

References

1. Spernath, A.; Aserin, A., Microemulsions as carriers for drugs and nutraceuticals. Advances in Colloid and Interface Science 2006, 128, 47-64. 2. Paul, B. K.; Moulik, S. P., Microemulsions: An overview. Journal of Dispersion Science and Technology 1997, 18 (4), 301-367. 3. Narang, A. S.; Delmarre, D.; Gao, D., Stable drug encapsulation in micelles and microemulsions. International Journal of Pharmaceutics 2007, 345 (1-2), 9-25. 4. Garti, N.; Yaghmur, A.; Leser, M. E.; Clement, V.; Watzke, H. J., Improved oil solubilization in oil/water food grade microemulsions in the presence of polyols and ethanol. Journal of Agricultural and Food Chemistry 2001, 49 (5), 2552-2562. 5. Towey, T. F.; Khan-Lodhi, A.; Robinson, B. H., Kinetics and mechanism of formation of quantum-sized cadmium sulphide particles in water-aerosol-OT-oil microemulsions. Journal of the Chemical Society, Faraday Transactions 1990, 86 (22), 3757-3762. 6. Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H., Laser and pulse radiolytically induced colloidal gold formation in water and in water-in-oil microemulsions. Journal of the American Chemical Society 1983, 105 (9), 2574-2579. 7. Cornet, I.; Nero, W. E., Emulsified Fuels in Compression Ignition Engines. Industrial & Engineering Chemistry 1955, 47 (10), 2133-2141. 8. Gillberg, G.; Friberg, S., Microemulsions as Diesel Fuels. In Evaporation&Combustion of Fuels, American Chemical Society: 1978; Vol. 166, pp 221-231.


19

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

9. Bemert, L.; Engelskirchen, S.; Simon, C.; Strey, R., Low emissions with microemulsion fuels. Abstr. Pap. Am. Chem. Soc. 2009, 237. 10. Schulman, J. H.; Stoeckenius, W.; Prince, L. M., Mechanism of Formation and Structure of Micro Emulsions by Electron Microscopy. The Journal of Physical Chemistry 1959, 63 (10), 1677-1680. 11. Nakayama, H.; Shinoda, K.; Hutchinson, E., The Effect of Added Alcohols on the Solubility and the Krafft Point of Sodium Dodecyl Sulfate. The Journal of Physical Chemistry 1966, 70 (11), 3502-3504. 12. Friberg, S.; Rydhag, L.; Doi, T., Micellar and Lyotropic Liquid Crystalline Phases Containing Nonionic Active Substances. In Lyotropic Liquid Crystals, American Chemical Society: 1976; Vol. 152, pp 28-42. 13. Kahlweit, M.; Strey, R., Phase-Behavior of Ternary-Systems of the Type H2O-OilNonionic Amphiphile (Microemulsions). Angew. Chem., Int. Ed. 1985, 24 (8), 654-668. 14. Kahlweit, M.; Strey, R., Phase-Behavior of Quinary Mixtures of the Type H2O-OilNonionic Amphiphile-Ionic Amphiphile-Salt. J. Phys. Chem. B 1988, 92 (6), 1557-1563. 15. Lekkerkerker, H. N. W.; Kegel, W. K.; Overbeek, J. T. G., Phase behavior of ionic microemulsions. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 1996, 100 (3), 206-217. 16. Fukuda, K.; Soderman, O.; Lindman, B.; Shinoda, K., Microemulsions formed by alkyl polyglucosides and an alkyl glycerol ether. Langmuir 1993, 9 (11), 2921-2925. 17. Shinoda, K.; Araki, M.; Sadaghiani, A.; Khan, A.; Lindman, B., Lecithin-based microemulsions - phase behavior and microstructure. Journal of Physical Chemistry 1991, 95 (2), 989-993. 18. Shinoda, K.; Kunieda, H.; Arai, T.; Saijo, H., Principles of attaining very large solubilization (microemulsion) - inclusive understanding of the solubilization of oil and water in aqueous and hydrocarbon media. Journal of Physical Chemistry 1984, 88 (21), 5126-5129. 19. Binks, B. P.; Meunier, J.; Abillon, O.; Langevin, D., Measurement of film rigidity and interfacial tensions in several ionic surfactant oil-water microemulsion systems. Langmuir 1989, 5 (2), 415-421. 20. Sottmann, T.; Strey, R., Ultralow interfacial tensions in water-n-alkane-surfactant systems. J. Chem. Phys. 1997, 106 (20), 8606-8615.


20

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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21. Lindman, B.; Olsson, U., Structure of microemulsions studied by NMR. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 1996, 100 (3), 344-363. 22. Kaler, E. W.; Bennett, K. E.; Davis, H. T.; Scriven, L. E., Toward unterstanding microemulsion microstructure - a small-angle x-ray scattering study. J. Chem. Phys. 1983, 79 (11), 5673-5684. 23. Kaler, E. W.; Davis, H. T.; Scriven, L. E., Toward understanding microemulsion microstructure 2. J. Chem. Phys. 1983, 79 (11), 5685-5692. 24. Jahn, W.; Strey, R., Microstructure of Microemulsions by Freeze-Fracture ElectronMicroscopy. Journal of Physical Chemistry 1988, 92 (8), 2294-2301. 25. Gompper, G.; Schick, M., Correlation between Structural and Interfacial Properties of Amphiphilic Systems. Phys Rev Lett 1990, 65 (9), 1116-1119. 26. Chen, S. H., Small-angle neutron scattering studies of the structure and interaction in micellar and microemulsion systems. Annual Review of Physical Chemistry 1986, 37, 351-399. 27. Heitz, M. P.; Carlier, C.; deGrazia, J.; Harrison, K. L.; Johnston, K. P.; Randolph, T. W.; Bright, F. V., Water core within perfluoropolyether-based microemulsions formed in supercritical carbon dioxide. J. Phys. Chem. B 1997, 101 (34), 6707-6714. 28. Hellweg, T.; von Klitzing, R., Evidence for polymer-like structures in the single phase region of a dodecane/C12E5/water microemulsion: a dynamic light scattering study. Physica A 2000, 283 (3-4), 349-358. 29. Baumgardt, K.; Klar, G.; Strey, R., Kinetics of micellization, measured with pressurejump and stopped-flow. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 1979, 83 (12), 1222-1229. 30. Kahlweit, M.; Teubner, M., On the kinetics of micellization in aqueous solutions. Advances in Colloid and Interface Science 1980, 13 (1-2), 1-64. 31. Grell, E.; Lewitzki, E.; Schneider, R.; Ilgenfritz, G.; Grillo, I.; von Raumer, M., Phase transitions in non-ionic detergent micelles. Journal of Thermal Analysis and Calorimetry 2002, 68 (2), 469-478. 32. Ilgenfritz, G.; Schneider, R.; Grell, E.; Lewitzki, E.; Ruf, H., Thermodynamic and kinetic study of the sphere-to-rod transition in nonionic micelles: Aggregation and stress relaxation in C14E8 and C16E8/H2O systems. Langmuir 2004, 20 (5), 1620-1630.


21

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Page 22 of 25

33. Gradzielski, M., Investigations of the dynamics of morphological transitions in amphiphilic systems. Curr Opin Colloid In 2004, 9 (3-4), 256-263. 34. Landazuri, G.; Fernandez, V. V. A.; Soltero, J. F. A.; Rharbi, Y., Kinetics of the Sphereto-Rod like Micelle Transition in a Pluronic Triblock Copolymer. J. Phys. Chem. B 2012, 116 (38), 11720-11727. 35. Lee, J. H.; Jeon, H. S.; Balsara, N. P.; Newstein, M. C., Kinetics of microemulsion formation in polymer mixtures. The Journal of Chemical Physics 1998, 108 (13), 5173-5176. 36. Schmolzer, S.; Grabner, D.; Gradzielski, M.; Narayanan, T., Millisecond-range timeresolved small-angle x-ray scattering studies of micellar transformations. Phys Rev Lett 2002, 88 (25), -. 37. Gradzielski, M., Kinetics of morphological changes in surfactant systems. Curr Opin Colloid In 2003, 8 (4-5), 337-345. 38. Weiss, T. M.; Narayanan, T.; Wolf, C.; Gradzielski, M.; Panine, P.; Finet, S.; Helsby, W. I., Dynamics of the self-assembly of unilamellar vesicles. Phys Rev Lett 2005, 94 (3), -. 39. Gradzielski, M., Dynamics in amphipihilic systems: Morphological transitions studied by small-angle scattering techniques. Abstr. Pap. Am. Chem. Soc. 2005, 229, U642-U642. 40. Barth, A.; Grillo, I.; Gradzielski, M., Dynamics of Formation of Vesicles Studied by Highly Time-resolved Stopped-flow Experiments. Tenside Surfact Det 2010, 47 (5), 300-306. 41. Egelhaaf, S. U., Kinetics of structural transitions in surfactant solutions. Curr Opin Colloid In 1998, 3 (6), 608-613. 42. Egelhaaf, S. U.; Leng, J.; Salonen, A.; Schurtenberger, P.; Cates, M. E., Pathway of vesicle formation. Self-Assembly 2003, 422-431. 43. Egelhaaf, S. U., Kinetics of structural transitions in surfactant solutions. Curr Opin Colloid In 1998, 3, 608-613. 44. Gradzielski, M., Kinetics of morphological changes in surfactant systems. Curr Opin Colloid In 2003, 8, 337-345. 45. Schmölzer, S.; Gräbner, D.; Gradzielski, M.; Narayanan, T., Millisecond-Range TimeResolved Small-Angle X-Ray Scattering Studies of Micellar Transformations. Phys. Rev. Lett. 2002, 88 (25), 258301.


22

Page 23 of 25

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46. Gotter, M.; Strey, R.; Olsson, U.; Wennerstrom, H., Fusion and fission of fluid amphiphilic bilayers. Faraday Discussions 2005, 129, 327-338. 47. Clark, S.; Fletcher, P. D. I.; Ye, X., Interdroplet exchange rates of water-in-oil and oil-inwater microemulsion droplets stabilized by pentaoxyethylene monododecyl ether. Langmuir 1990, 6 (7), 1301-1309. 48. Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J., Kinetics of Swelling of Oil-in-Water Emulsions. Langmuir 1998, 14 (19), 5402-5411. 49. Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J., Kinetics of Swelling of Oil-in-Water Emulsions Stabilized by Different Surfactants. Langmuir 1999, 15 (13), 4495-4501. 50. Müller, A.; Pütz, Y.; Oberhoffer, R.; Becker, N.; Strey, R.; Wiedenmann, A.; Sottmann, T., Kinetics of pressure induced structural changes in super- or near-critical CO2microemulsions. Physical Chemistry Chemical Physics 2014, 16 (34), 18092-18097. 51. Tabor, R. F.; Eastoe, J.; Grillo, I., Time-resolved small-angle neutron scattering as a lamellar phase evolves into a microemulsion. Soft Matter 2009, 5 (10), 2125-2129. 52. Tojo, C.; Blanco, M. C.; Rivadulla, F.; López-Quintela, M. A., Kinetics of the Formation of Particles in Microemulsions. Langmuir 1997, 13 (7), 1970-1977. 53. de Dios, M.; Barroso, F.; Tojo, C.; López-Quintela, M. A., Simulation of the kinetics of nanoparticle formation in microemulsions. J Colloid Interf Sci 2009, 333 (2), 741-748. 54. Foster, T., Universal Analytical Scattering Form Factor for Shell-, Core-Shell, or Homogeneous Particles with Continuously Variable Density Profile Shape. J. Phys. Chem. B 2011, 115 (34), 10207-10217. 55. Percus, J. K.; Yevick, G. J., Analysis of classical statistical mechanics by means of collective coordinates. Physical Review 1958, 110 (1), 1-13. 56. Foster, T.; Sottmann, T.; Schweins, R.; Strey, R., Small-angle neutron scattering from giant water-in-oil microemulsion droplets. I. Ternary system. J. Chem. Phys. 2008, 128 (5). 57. Gradzielski, M.; Langevin, D.; Magid, L.; Strey, R., Small-angle neutron scattering from diffuse interfaces: 2. polydisperse shells in water-n-alkane-C10E4 microemulsions. Journal of Physical Chemistry 1995, 99 (35), 13232-13238. 58. Tanaka, R.; Saito, A., Aggregation of polyoxyethylene glycol monodecyl ether in nonpolar solvents and the effect of water. J Colloid Interf Sci 1990, 134 (1), 82-91.


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59. Smith, G. N.; Brown, P.; Rogers, S. E.; Eastoe, J., Evidence for a Critical Micelle Concentration of Surfactants in Hydrocarbon Solvents. Langmuir 2013, 29 (10), 3252-3258. 60. Helfrich, W., Elastic properties of lipid bilayers - theory and possible experiments. Zeitschrift Für Naturforschung C - A Journal of Biosciences 1973, C 28 (11-1), 693-703. 61. Strey, R., Microemulsion microstructure and interfacial curvature. Colloid and Polymer Science 1994, 272 (8), 1005-1019. 62. Endo, H.; Allgaier, J.; Gompper, G.; Jakobs, B.; Monkenbusch, M.; Richter, D.; Sottmann, T.; Strey, R., Membrane decoration by amphiphilic block copolymers in bicontinuous microemulsions. Phys Rev Lett 2000, 85 (1), 102-105.


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Formation Kinetics of Oil-Rich, Nonionic Microemulsions - Langmuir

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