Core Freezing and Size Segregation in Surfactant CoreâShell

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Core Freezing and Size Segregation in Surfactants Core-Shell Micelles Beatrice Plazzotta, Jing Dai, Manja Annette Behrens, Istvan Furo, and Jan Skov Pedersen J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b06041 • Publication Date (Web): 30 Jul 2015 Downloaded from http://pubs.acs.org on August 3, 2015

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The Journal of Physical Chemistry

Core Freezing and Size Segregation in Surfactants Core-Shell Micelles Beatrice Plazzotta1, Jing Dai2, Manja A. Behrens1, István Furó2, Jan Skov Pedersen*1 1

Aarhus University, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark

2

Division of Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 36, S - 100 44 Stockholm, Sweden * [email protected]

1 § Manja A. Beherens is currently employed as a consultant at Danish Technological Institute Kongsvang Allé 29, 8000 Aarhus C, Denmark

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ABSTRACT: Nonionic surfactants containing Poly(Ethylene Oxide) are chemically simple and bio-compatible and form core-shell micelles at a wide range of conditions. For those reasons, they and their aggregates have been widely investigated. Recently, irregularities that were observed in the low-temperature behavior of surfactants of the kind [CH3(CH2)nO(CH2CH2O)mH], (abbreviated CnEm) were assigned to a freezing-melting phase transition in the micellar core. In this work we expand the focus from the case of single component systems to binary surfactant systems at temperature between 1 and 15 °C. By applying Small-Angle X-ray Scattering (SAXS), Differential Scanning Calorimetry (DSC), Nuclear Magnetic Resonance (NMR) and density measurements in pure C18E20 and C18E100 solutions and their mixtures, we show that core freezing/melting is also present in mixtures. Additionally, comparing SAXS data obtained from the mixture with those from the single components, it was possible to demonstrate that the phase transition leads to a reversible segregation of the surfactants from mixed micelles to distinct kinds of micelles of the two components.

Keywords: Surfactants, Segregation, Phase Transition, Micelles, Binary Systems, SAXS INTRODUCTION Amphiphilic block copolymers with a hydrophobic and a hydrophilic block may show a surfactant-like behavior and, when dissolved in water, they may self-assemble into micelles. In those assemblies, the hydrophobic chains make up the compact core of the micelles, while the hydrophilic blocks form a swollen shell around it.1 Among others, block copolymers containing Poly(Ethylene Oxide) (PEO) have been widely studied2-7 both due to their relatively simple chemical structure and their bio-compatibility that permits to use them as drug carriers.8-10

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Recently, studies on spherical micelles formed by polymers consisting of a short hydrocarbon block and a PEO block11 showed that the hydrocarbon chains forming the micellar core can undergo a phase transition akin to the liquid-solid transition of corresponding alkanes.12 Even though micelles with a crystalline core were known to exist13,14, cases reported were dominantly for copolymers with much longer hydrophobic chains and therefore with a better ability to fold in an compact structure. Hence, the new results11 are indeed very interesting, but so far limited to systems containing only one surfactant. There are numerous investigations in the literature on binary systems of surfactants15-18 showing how the interactions between surfactants of compatible sizes or chemical nature lead to the formation of mixed micelles. In those studies the influence of concentration, composition and chemical structure on the formation and characteristic of mixed micelles has been extensively investigated, but there are almost no studies that explore the effect of temperature on the systems. In this article we extended the scope and performed a study of mixed micelles as a function of temperature, first identifying surfactants that display phase transition in the micellar core and then use them to form a binary system, which is investigated as a function of temperature to see how the phase transition in influenced. More specifically we deal with polymers/surfactant with similarly short hydrophobic blocks, the commercially available Brij S20 and Brij S100, both composed of an 18 unit hydrocarbon chain and of a PEO chain of, respectively, 20 and 100 repeat units. Those polymers are often abbreviated as C18E20 (Brij S20) and C18E100 (Brij S100), and as such they will be referred to in the manuscript. Preliminary studies (forming the background to a study of surfactants behavior in the presence of salt4) showed irregularities in their low-temperature behavior similar to that previous-



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ly observed.11 In particular, C18E100 (purchased under the earlier designation Brij 700), showed a discontinuity in the partial molar volume as a function of temperature and a corresponding variation in Small-Angle X-Ray scattering (SAXS) data. A similar behaviour was found for C18E20. As previously stated, as a first step we analyze systems containing either C18E20 or C18E100 in aqueous solution, to verify if freezing of the micellar core occurs in any of the samples and, if so, where in relation to the phase transition temperature of pure octadecane (28°C)19 it takes place. Secondly, in samples containing the two surfactants in different proportions we study if and how the presence of two components affects the solidification process. To investigate the systems we used SAXS to determine the structure and dimension of the micelles at different temperatures, while density measurements were used to identify possible discontinuities in the partial molar volumes of pure systems. Nuclear Magnetic Resonance (NMR) provided information on the segmental mobility of the hydrocarbon and PEO chains at different temperatures and about the hydrodynamic radii while Differential Scanning Calorimetry (DSC) identified phase transitions by their enthalpy of freezing/melting. MATERIALS AND METHODS Materials. Brij S20 (C18E20) and Brij S100 (C18E100) from Sigma Aldrich were used as received. Both polymers were dissolved in Milli-Q water, at concentrations of 0.5%, 1%, 2% and 5% in weight. Note that the results shown are solely from 1 wt% samples. The samples of the other concentrations were analyzed only by SAXS and show the same trends and results as data obtained for 1%wt samples. Samples containing both polymers were obtained by mixing the 1 wt% stock solutions in set ratios 1:1, 3:1 and 9:1 between C18E100 and C18E20, corresponding to 1:4, 3:4 and 9:4 corresponding molar ratios.


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The mixed samples were either prepared in a cold room (4 °C), working in ice, or at room temperature. The samples mixed in the cold room were transferred into the SAXS capillary in cold environment, and stored on ice until the measurement. SAXS. SAXS data were recorded at a wavelength of 1.54 Å, on a flux- and background optimized camera from Bruker AXS20. The instrument uses a rotating Cu anode as source and it is equipped with Montel multilayer optics and a home-build compact ‘scatterless’ slit21 is placed before the samples, which were kept in quartz capillaries of 2 mm diameter. The acquisition time for the data was 1800 s. For pure surfactants, data were recorded at temperatures between 1 and 15 °C, with 1 degree steps. For mixtures, data were recorded upon a cooling-heating cycle. First the samples were heated from 1 to 15 °C and then cooled again to 1 °C, with collection of the scattering curves at every 1 degree steps. The mixture with 1:1 weight ratio of the polymers underwent two subsequent cooling-heating cycles. Temperature was regulated using a Peltier element (Anton Paar), in which the homebuilt sample (quartz capillary) holder was inserted. Particle scattering was obtained by subtracting from the scattering of the sample the background obtained for pure Milli-Q water at the same temperature. The initial data treatment was performed using the SUPERSAXS program package (C.L.P. Oliveira and J.S. Pedersen, unpublished) using Milli-Q water as a standard.22 The intensity of scattered radiation is presented as a function of the scattering vector, ‫= ݍ‬ tion and 2ߠ is the scattering angle.

ସగ ఒ

sinߠ, where λ is the wavelength of the incident radia-

The total measured intensity depends on the number concentration in the illuminated sample N, and can be expressed as ‫ܫ‬ሺ‫ݍ‬ሻ = ܰܲሺ‫ݍ‬ሻܵሺ‫ݍ‬ሻ

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where P(q) is the normalized form factor, determined by the shape of the particles, and S(q) is the structure factor that originates from interference between X-rays scattered by different particles. The latter mainly influences the region at low q and it is negligible for dilute solutions, as the 1% wt solutions shown here. For more concentrated samples, as the 2%wt and 5%wt the structure factor is not negligible and has to be included in the modelling. SAXS modeling. As stated in the introduction, C18E20 and C18E100 have been shown to form spherical core-shell structures at the temperatures of interest for this study. In the model used here, the internal structure of the shell is also taken into account, in terms of the polymer chains that form it. The contribution from those chains is in fact dominating the high-q scattering region. Also, the grading of the interfaces between core and shell and between shell and solvent are taken into account, giving the form factor23 ܲሺ‫ݍ‬ሻ = ቈ∆ߩ௦௛௘௟௟ ܸ௧௢௧ ߔሺ‫ܴݍ‬௧௢௧ ሻ݁

మ

మ

഑ ೜ ି ೚ೠ೟ మ

− ሺ∆ߩ௦௛௘௟௟ − ∆ߩ௖௢௥௘ ሻܸ௖௢௥௘ ߔሺ‫ܴݍ‬௖௢௥௘ ሻ݁

ܰ௔௚௚ ሺ∆ߩ௖௛௔௜௡ ܸ௖௛௔௜௡ ሻଶ ܲ௖௛௔௜௡ ሺ‫ݍ‬ሻ

మ

మ

഑ ೜ ି ೔೙ మ

ଶ

቉ + (2)

where ∆ߩ௦௛௘௟௟ , ∆ߩ௖௢௥௘ and ∆ߩ௖௛௔௜௡ are the excess scattering length densities, Rtot and Vtot the radius and the volume of the particle and Rcore and Vcore are the radius and volume of the inner core, Nagg and Vchain the number and volume of the polymer chains in the shell, while exp ቀ− and exp ቀ−

మ మ ఙ೔೙ ௤

ଶ

మ ௤మ ఙ೚ೠ೟

ଶ

ቁ

ቁ describe the grading of the interfaces. Pchain(q) is the form factor for Gaussian

chains, described by the Debye function,24 while ߔሺ‫ܴݍ‬ሻ describes the form factor amplitude of a sphere25


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ߔሺ‫ܴݍ‬ሻ =

ଷሾୱ୧୬ሺ௤ோሻି௤ோୡ୭ୱሺ௤ோሻሿ ሺ௤ோሻయ

(3)

The modeling was done on arbitrary scale. Fitting parameters were the core radius and the width of the shell (Rtot-Rcore), the ratio between ∆ߩ௖௢௥௘ and ∆ߩ௦௛௘௟௟ , the width of the interfaces, the contribution from the polymer chains in the shell, and an overall scale factor. The structure factor of hard spheres26,

27

was used for describing the concentration effectsfor

samples at 2%wt and 5%wt. The parameters entering S(q) are the effective hard-sphere volume fraction and interaction radius. Data obtained from the mixtures were not modeled directly, but were compared to the appropriate (set by composition) linear combination of the C18E20 and C18E100 data. Agreement between the experimental data and the linear combination indicates that different kind of surfactants form different micelles while disagreement shows mixed micellar structures are formed containing both kinds of polymers. The SAXS spectra were also used to obtain pair distance distribution functions, using an indirect Fourier transform procedure.28,29 This function is the distribution of distances between volume elements inside the particle weighted by the excess density distribution in these elements. Density and Apparent Specific Volume. The density of C18E20 and C18E100 solutions was measured using an oscillating U-Tube densitometer (DMA 5000, Anton Paar. Accuracy of density: 5×10-5 g/cm3. Accuracy of temperature: 0.01°C). The density was obtained from 1 to 90°C for all samples (with Milli-Q water for SAXS and calorimetry and with deuterated water for NMR) with 1 degree steps between 1 and 15°C and 5 degrees steps between 15 and 90°C. The measured densities were converted to apparent specific volume of the solute vsolute by 7 ACS Paragon Plus Environment


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‫ݒ‬௦௢௟௨௧௘ = ቀ

ଵ

௖ೞ೚೗ೠ೟೐

ቁቀ

ଵ

ఘೞ೚೗ೠ೟೐

ቁ−ቀ

ଵି௖ೞ೚೗ೠ೟೐ ௖ೞ೚೗ೠ೟೐

ቁቀ

ଵ

ఘೞ೚೗ೡ೐೙೟

ቁ

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(4)

where csolute is the solute concentration in weight fraction, ρsolute and ρsolvent are the densities of, respectively, the solute and the pure solvent30. The vsolute values have an accuracy of about 0.2% due to the low concentration of the solutions analyzed. Calorimetry. Differential Scanning Microcalorimetry was performed on solutions of pure C18E20 and C18E100 and on their 1:1 mixture, using a VP-DSC (MicroCal, Northampton, MA). Milli-Q water was used as a reference. Measurements were run from 5 to 40°C with a rate of 2 degrees per minute. NMR. The 1H NMR experiments were performed on a Bruker Advance III 500 MHz spectrometer using a Bruker GREAT 60 gradient amplifier and a Bruker DIFF 30 probe with z-gradient. The samples for NMR experiments were prepared with D2O as solvent. Measurements were made upon decreasing the temperature from 20 to 0°C and, on some occasions (see below), also upon increasing the temperature in the same range. The step size was 2 degrees from 20 to 10°C, and 1 degree from 10 to 0°C, with a stabilization time of 5 minutes prior to experiments for each step. In the 1H NMR spectrum, the oxyethylene (from the shell) and main methylene (from the core) peaks are well separated by about 2 ppm. Hence, their respective line widths ∆1/2 can be simply obtained as peak widths at half amplitude. In addition, the longitudinal relaxation time T1 for each moiety was measured using inversion recovery. In NMR diffusion measurements, the double-stimulated-echo pulse sequence31 was used to obtain the decay of the signal intensity with increasing gradient strength. From the decay, the diffu-


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sion coefficient D was extracted by least-square fitting of the conventional Stejskal-Tanner expression32 ఋ

ܵ/ܵ଴ = expሾ−‫ ߛܦ‬ଶ ݃ଶ ߜ ଶ ሺ߂ − ሻሿ

(5)

ଷ

to the data where S and S0 are the signal intensity with and without gradient, respectively, γ the gyromagnetic ratio, g the gradient strength, δ the duration of gradient pulses (set to 1ms), and ∆ the diffusion time (set to 200 ms). Since the oxyethylene and methylene moieties belong to the same molecular entity, the diffusion coefficients obtained from data extracted from those respective signals coincided, as expected, within experimental error. Since the methylene peak is subject to a large broadening at low temperatures, it is the data extracted from the oxyethylene peaks that are shown below. From the diffusion coefficients, the hydrodynamic radius Rh of micelle were obtained via the Stokes-Einstein equation ௞ ்

ಳ ‫଺ = ܦ‬గఎோ

(6)

೓

where kB is the Boltzmann factor, T the absolute temperature, and η the solvent viscosity. RESULTS AND DISCUSSION SAXS. Scattering data obtained from single-component C18E20 and C18E100 solutions at different temperatures are presented in Figure 1A-B. For C18E100 (1A) only small changes in scattering with temperature are observed, while for C18E20 (1B) there are large differences between the data recorded at different temperatures. It can be seen that the changes are gradual and that the interesting temperature range is between 6 and 12 °C. At temperatures above and below that range the scattering curves coincide.



I(q)

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q(Å )

0.1

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I(q)

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-3

10

-4

10

0.01

0.1 -1

q (Å )

Figure 1 Scattering curves (A) for a solution of C18E100 at temperatures between 1 and 15 °C. The different curves coincide. (B) for a solution of C18E20 at temperatures between 1 to 15°C. Data at 1°C (blue line) and 15°C (red line) are highlighted The lines merely represent the experimental data and are not fitting curves. A comparison of data collected at 1°C and 15°C (Figure 1B) shows that the intensity of the peak around q=0.1 Å-1 increases with increasing temperature, while the corresponding minimum is shifted towards lower q values. It can also be seen that the intensity of radiation scattered at low q decreases as the temperature increases. Figure S1 of the Supporting Information show data obtained for C18E20 at different concentrations, which display the same general behavior as the 1


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wt% data. The data were fitted using the spherical core-shell model summarized by Eq. (2), using a least-squares method. Example of fits are shown in figure S2 of the Supporting Information. As shown in Figure 2, the core radius increased upon increasing temperature in C18E20 from 18 to 20 Å, while the shell thickness remained constant around 30 Å. This can be rationalized as freezing of the hydrocarbon chains, with a shrinking of the core size due to the better packing of the chains, in accordance with what seen by Zinn et al.12 The pair distance distribution function presented in Figure 2B also changes significantly with temperature. Rather than being connected to a change in particle shape, this is instead caused by a variation in the relative contrast between the core and the shell, a consequence of the decrease in the core volume and the resulting increase of core electron density. C18E100, core radius and shell thickness remained both constant at, respectively, approximately 20 Å and 70 Å, showing no evidence of a phase transition for this system.

A 30 25 20 15

20

r core (Å)

r core, d shell(Å)

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B 0.0002

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p(r)

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0.0000

-0.0001 0

20

40

60

80

100

120

r(Å)

Figure 2 (A) The core radius (filled symbols) and shell thickness (open symbols) for C18E20 micelles as a function of temperature as obtained from evaluating the data in terms of Eq.(2). The inset present with higher resolution the core radii. (B) Pair distance distribution functions, obtained from C18E20 SAXS data via an indirect Fourier transform procedure,28,29 at temperatures between 1 and 15 °C. The profiles at 1 (blue) and 15 °C (red) are highlighted. The data obtained in mixtures of C18E20 and C18E100 are reported in Figure S3 in the supporting information and exhibit a temperature dependence in qualitative agreement with that observed for pure C18E20, hinting that the presence of a second component (C18E100) does not hinder the phase transition of C18E20. Comparing the three mixtures, the temperature effect is more pronounced when the content of C18E20 in the sample is higher. The data shown are from the samples mixed in the cold room, however practically identical data were obtained for samples mixed at room temperature.


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A 70

cooling cycle 1 heating cycle 1 cooling cycle 2 heating cycle 2

60 50

χ

2

40 30 20 10 0 0

2

4

6

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T(°C)

B

cooling heating

70 60 50

χ

2

40 30 20 10 0 0

2

4

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T(°C)

C

cooling heating

40 35 30

2

25

χ

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20 15 10 5 0

2

4

6

8

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T(°C)

Figure 3 Discrepancy in terms of reduced chi-square between scattering data and linear combination of the data for the two pure constituents as a function of temperature upon heating (red sym13 ACS Paragon Plus Environment


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bols) and cooling (blue symbols) for mixture with 1:1 (A), 3:1 (B) and 9:1 (C) proportions of C18E100:C18E20. For the 1:1 mixture, two heating-cooling cycles were performed and filled symbols refer to the first cycle, while empty symbols refer to the second one. The vertical lines represent the approximate phase transition temperatures. Below those temperatures, the system can be described as composed by segregated micelles, while above it cannot. This latter feature is taken as being the result of the presence of mixed micelles. The data obtained from mixtures were fitted with a linear combination of the scattering from pure surfactant solutions, as explained in the Material and Methods section. Figure 3 shows the discrepancy (߯ ଶ ሻ between the scattering data of the mixture and the linear combination as a function of temperature. At low temperatures, there is a good agreement that indicates that the system can be described as a mixture of pure C18E20 micelles and pure C18E100 micelles. At around 6 °C, there is a large increase in the discrepancy, which is a signature of having the two surfactants mixed in a single kind of aggregate. It can be noticed that the temperature of transition is slightly lower when the content of C18E100 is higher. The jump in discrepancy is present both upon heating and cooling, indicating reversibly mixing and separation. Also, the discrepancy is in all cases higher for data collected under cooling than for the corresponding temperatures during heating. This is particularly evident for the 1:1 mixture. The same behavior has been observed during the second thermal cycle. This can be explained considering that the mixing and de-mixing of surfactants in micelles are two different processes and therefore the intermediate structures may be different; also, the electron density contrast variations between different components may be different for the mixing and de-mixing. In the 1-1 mixture, the discrepancy reaches a maximum around 7-8 °C, after which it decreases to a constant value. This can be explained considering that there is a difference in transition temperature between the mixture and the pure C18E20. In fact, while at 6.5 °C the mixtures is already undergoing a phase transition, the pure C18E20 that it is compared to is still core-frozen. Increasing the temperature above 7-8 °C, C18E20 undergoes a phase transition as well, and its 14 ACS Paragon Plus Environment

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structure is therefore less incompatible with that of the mixed micelles. For mixtures of the two other compositions, such a peak is not observed and the discrepancy reaches immediately a sort of plateau; this is probably due to the lower amount of C18S20 present. Analogous trends have been observed for mixtures at different overall concentrations. Apparent Partial Specific Volume. The density was converted to apparent partial specific volume vsolute as described in the Material and Methods section, and the results are shown in Figure 4. For both copolymers, vsolute increases with increasing temperature, reflecting increasing thermal motion. For C18E20, there is an abrupt increase of vsolute between 6 and 11 °C. The initial step of this effect is also present in C18E100, but at lower temperatures and therefore is demonstrated less clearly (measurement below 0°C were not possible because of freezing of the solvent). The data above 20 °C are not shown, but follow the same trend as data from 10°C onward. Again, the abrupt increase of vsolute can be due to the shrinking of the core subsequent to its freezing. The results suggest that such a freezing may occur for C18E100 as well, even if at lower temperatures compared to C18E20.

3 -1

)

A Apparent Specific Volume (cm g

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0.84

y=0.0012 x+0.817

0.83

0.82

0.81 0

5

10

15

20

T(°C)



B 3 -1

)

0.93 Apparent Specific Volume (cm g

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0.92 y=0.0014 x+ 0.896

0.91 0.90 0.89 y=0.0015 x+ 0.882

0.88 0

5

10

15

20

T(°C)

Figure 4 Apparent specific volume of (A) C18E100 and (B) C18E20 as a function of temperature. Linear fits to the data in two regions are shown. Calorimetry. Heating scans for the C18E20 systems show an endothermic peak centered around 7.4 °C, evidence of a phase transition taking place. For the 1:1 weight mixture of the two surfactants, an endothermic peak is still present, but shifted to 6.5 °C (Figure 5) and its intensity scales with the C18E20 content of the systems. A similar peak was not observed for C18E100, indicating that there is no phase transition in that system in the explored temperature range. This again confirms the presence of a freezing for pure C18E20 and the presence of a related phase transition for the mixture as well. The lower transition temperature indicates that C18E100 is affecting the transition and the behavior of the hydrocarbon chains of C18E20, suggesting the presence of mixed structures at high temperatures, where the two kinds of surfactants are in close contact with one another.


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2

-1

Cp (J K )

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1

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25

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35

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T (°C)

Figure 5 Heat capacity as a function of temperature for C18E20 (continuous line) and the 1:1 mixture of C18E20:C18E100 (dash-dotted line), showing the presence of endothermic transitions. The vertical lines emphasize the peak positions and illuminate the peak shift. The plot corresponding to C18E100 (dotted line) shows no peaks in the interval investigated.

Figure 6. 1H NMR spectra obtained for a C18E20 micellar solution at different temperatures between 5 and 20 °C. The peak at 3.7 ppm is the signal from the ethylene oxide chain (shell), while the peaks at 3.45 ppm, 1.55 ppm, and 1.3 ppm are from the different CH2 moieties in the hydrocarbon chain and the peak at 0.88 ppm arises from the terminal CH3 (core). The inset presents the CH3 peak with higher resolution. See assignment and an expanded view of the ethylene oxide peak in Figures S7 and S8 of the Supporting Information.



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NMR. As the other techniques, NMR parameters also show characteristic changes34 upon varying temperature, especially around 7 °C, the temperature at which we suspect core freezing takes place. 1H NMR spectra for C18E20 at the different investigated temperature are shown in Figure 6. NMR spectra and peaks assignation for C18E100 are shown in Figures S9 and S10 in the Supporting Information. Clearly, the peaks from core moieties become broader when the temperature is decreased below 8 °C, indicating that the segmental mobility of the hydrocarbon chains is significantly reduced. Additionally, the core hydrocarbon peaks are slightly shifted towards higher chemical shift as is particularly evident for the CH3 group in the inset of Figure 6. Note that in the spectra recorded at 8 and 9 °C two CH3 peaks are present, one at 0.88 ppm and one at ca 0.94 ppm. This peak splitting may indicate that in the transition region we have micelles both with frozen and molten cores, probably due to the unavoidable polydispersity of the PEO-blocks and thereby that of the micelles created. Figure 7 shows the temperature dependence of the 1H line width for core and shell signals as extracted from the spectra while Figure 8 shows the variation of the 1H longitudinal relaxation time T1 as provided by the inversion recovery experiments. Clearly, there is a large difference between the micellar core and shell behaviors. In general, the line width for the core increases upon decreasing temperature, starting from temperature points particular to the samples and roughly in the phase transition temperature ranges identified by the calorimetric and density data. At roughly the same respective temperatures, the longitudinal relaxation time T1 for the core also increases. On the other hand, the NMR parameters recorded for the shell exhibit smooth monotonic trends with no sudden departures from them.


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Figure 7. The temperature dependence of the 1H line width for the micellar core and shell signals for (A) pure C18E20 and (B) pure C18E100 solutions and (C) for their 1:1 mixture. In C, data recorded upon warming and cooling are reported jointly.



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The most straightforward explanation for the observed line width and relaxation time33,34 behavior is core freezing which results in strongly reduced segmental (chain isomerization) and overall (motion of chains around the micellar core) chain mobility. Within the general frame, the samples exhibit subtle differences. For C18E20, the core mobility is suppressed abruptly at slightly below the phase transition temperature observed by calorimetry. On the other hand, the core mobility for the C18E100 starts to slow down far above its calorimetrically indicated phase transition region. Interestingly, it is the 1:1 mixture that exhibits the sharpest behavior as concerning the line width. Yet, for the same sample the transition marked out by the T1 points is broad which indicates that overall chain dynamics and segmental motion become arrested in different manners.


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Figure 8. The temperature dependence of the 1H longitudinal relaxation time T1 for the micellar core and shell signals for (A) pure C18E20 and (B) pure C18E100 solutions and (C) for their 1:1 mixture. There are several additional minor details to discuss. First, one may observe that the line width for the shell groups is roughly constant. This is in part an artifact – the measured line width in the high-temperature region has large contributions from field inhomogeneity and spin-spin coupling. As the molecular dynamics slows, transverse relaxation becomes dominant and the line width correctly reflects the slowing down of molecular motions. That the dynamics shows a conventional (that is, thermally activated) temperature dependence even above the respective phase transition temperatures is more accurately reflected by the smooth evolution of the shell T1 times in Figure 8. Second, above the respective phase transition temperatures, the overall chain mobility in the core is slower than that in the shell as is shown by the core line widths being higher then shell line widths. Third, above core freezing temperatures the line width for the shell moieties in C18E20 is larger than that in the other two samples. This is the consequence of having the shell consisting of shorter PEO chains in the region closest to the core and thereby being affected by the slower core dynamics. Here it is interesting to note that the signal from the ethylene oxide moiety closest to the hydrocarbon chain suffers a large broadening (and perhaps some shift) upon



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core freezing as is shown by Fig. S8 in Supporting Information. This indicates that the decoupling of core and shell molecular dynamics is not entirely abrupt but happens over an extended (larger than an –O–CH2– group) region. Finally, as is shown by Figure 7C, there is no hysteresis in the molecular dynamics observed upon increasing and decreasing temperatures.

Figure 9. The temperature dependence of the hydrodynamic radius of micelles in pure and mixed solutions, as obtained via Eq.(6) from the self-diffusion coefficients measured by 1H NMR stimulated-echo-type experiments. While the molecular dynamics exhibits interesting changes with temperature, the hydrodynamic radius as seen by self-diffusion experiments (Figure 9) remains roughly constant in all investigated samples. First, one should notice that there is a good general agreement between the SAXSderived micellar radius (in pure C18E20 solution, 48 – 50 Å) and the corresponding hydrodynamic radius obtained by NMR (55 Å). The slight discrepancy is probably arising from hydrating water molecules within the micellar shell. The radii of the C18E100 micelles and the average radii in the mixed system are rather close to each other; interestingly, the mixed system seems to contain somewhat larger aggregates.


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In contrast to the X-ray data which indicate a slight (ca 4%) decrease of the micellar radius upon core freezing, the NMR-derived hydrodynamic radius for the pure C18E20 micelles remains constant. Since the error in the diffusion data is approximately 1-2 %, this discrepancy must have molecular reasons. Increasing hydration of the shell, perhaps permitted by a change in local conformation of the PEO blocks adjacent to the core, may be the explanation. CONCLUSIONS All the different techniques show that a change is happening in the C18E20 system, at temperatures between 7 and 10 °C. The peak in the DSC measurement suggests that the changes correspond to a phase transition, with a corresponding contraction of the micellar structure as evidenced by the reduced apparent specific volume. In particular, analysis on the SAXS data show that only the core is contracting and thus the phase transition concerns the core part. We can then affirm that the C18 chains confined in the core are undergoing solidification, with a more compact rearrangement of the chains similar to what observed for analogous systems by Zinn et al.12. This is additionally confirmed by the reduced segmental mobility of C18 chains at low temperature as observed from NMR data. The fact that there is no variation in the segmental mobility of the PEO block with temperature, except in the groups bordering the core, is again confirming that the transition is in the core region. NMR and density measurement show also indicates of a phase transition for the C18 in C18E100, however, with the presence of a longer PEO block causing a significant reduction in the transition temperature, which can be identified as below 3 °C. The NMR data hint in addition to that global and local (segmental) chain dynamics might be decoupled when the slowing down appears upon freezing.SAXS measurement did not bring evidences of such a transition, but that may be due to the dominance of the PEO scattering to the total scattering signal. In fact the PEO block are more than five times longer than the C18 chain and this 23 ACS Paragon Plus Environment


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means that the scattering is mostly originated by the PEO structure, with a smaller contribution from the C18 chains. As a consequence small variations in the core size may not influence the overall SAXS pattern in a significant way. Unfortunately, due to instrumental limitations it was not possible to obtain heat capacity measurements at such temperatures, thus preventing us to confirm or refute the hypothesis of a phase transition for C18E100. It is interesting that the phase transition is not hindered by the addition of another surfactant, as can be seen from SAXS, DSC and NMR data on the mixtures. The plots of the discrepancy between experimental SAXS data and linear combination for the mixed systems shows that, while above the transition temperature mixed micelles containing both surfactants are present, below the samples contain surprisingly separate micelles of the individual surfactants. This means that when cooling the transition leads to a separation of the different components in different micellar structures. Such a separation has to our knowledge not been observed before in a mixed surfactant system. . The separation probably happens with a mechanism analogous to that of purification through crystallization and it is possible due to the difference in transition temperature between the two surfactants. The perfect reversibility of the phase transition is in accordance with the surfactants in the micelles being dynamically active also at low temperatures35 and is an evidence of a high energetic advantage connected to the C18E20 solidification. Finally, it can be seen how increasing the concentration in C18E100 does not prevent the transition but simply shifts the critical temperature to lower values, as commonly happens for phase diagrams of mixtures.


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Acknowledgements This work was supported by the Hungarian Scientific Research Fund (OTKA K 108646), the Swedish Research Council VR as well as by the NanoS3 – 290251 ITN and the COST Action CM1101 project of the European Commission, which is gratefully acknowledged. Supporting Information Available: Additional experimental information and graphs as discussed in the text. This information is available free of charge via the Internet at http://pubs.acs.org.

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"table of contents" Core Freezing and Size Segregation in Surfactants Core-Shell Micelles Beatrice Plazzotta, Jing Dai, Manja A. Behrens, István Furó, Jan Skov Pedersen*

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