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Surface micelles of semifluorinated alkanes in Langmuir–Blodgett monolayers

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

We have studied the molecular patterns formed in Langmuir–Blodgett (LB) films of semifluorinated alkanes C_nF_2n+1C_mH_2m+1 (FnHm diblocks, n = 6, 8, 10, m = 14, 16, 18, 20) by atomic force microscopy.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

All the compounds investigated formed surface micelles whose shape and dimensions varied with the molecular structure of the FnHm diblocks.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

Except for F6H16 and F8H14, which produced exclusively circular micelles, all of the other diblocks also formed elongated micelles that coexisted with circular micelles.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

The mean diameter of the circular micelles increased with the length of the hydrogenated segment of the diblocks.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

By contrast, increasing the length of the fluorinated segments did not have any detectable effect on the diameter.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

The elongated micelles became more numerous and longer for longer FnHm (both the length of the hydrogenated and of the fluorinated segments had a strong effect).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The surface pressure of transfer had an influence on the morphology of the elongated micelles, the latter becoming fewer and shorter for higher surface pressures.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

A detailed X-ray reflectivity study conducted on F8H16 LB films showed that the hydrogenated segments are directed towards the silicon wafer, while the fluorinated segments point outwards toward the air.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

A disc-like model is proposed for the surface micelles.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

Introduction

Colloidal systems involving semifluorinated alkanes C_nF_2n+1C_mH_2m+1 (FnHm diblocks) have recently focussed attention owing to their potential applications in the biological and materials sciences.^1,2

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

These nonpolar, nonetheless amphiphilic molecules were shown to impart specific properties (increased stability, reduced permeability, reduced fusion rate, etc.) to various colloidal systems of biological interest including vesicles (liposomes),^3,4 fluorocarbon-in-water emulsions^5,6 and hydrocarbon-in-fluorocarbon emulsions.⁷

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

One of our objectives is to exploit the mutual antipathy between fluorinated and hydrogenated chains for the elaboration of nanopatterned surfaces.⁸

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Compression of Langmuir monolayers and Langmuir–Blodgett (LB) films of combinations of FnHm diblocks and phospholipids provided evidence for both lateral and vertical phase separation.^9,10

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Precise control of the shape, size and molecular organization of the molecular clusters decorating the solid substrates is necessary to achieve mastered construction of 2D supramolecular architectures.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

Despite several studies conducted on Langmuir monolayers of semifluorinated alkanes,^11–13 the molecular orientation of the diblock molecules at the air/water interface has remained controversial.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot3

We present here an investigation by atomic force microscopy (AFM) of the morphology of Langmuir monolayers of a series of FnHm diblocks (n = 6, 8, 10 and m = 14, 16, 18, 20) after transfer onto silicon wafers.

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj1

The AFM images were analyzed using an image analysis software (Visilog®).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

Experimental section

Materials

FnHm diblocks were synthesized according to ^{ref. 14} and were thoroughly purified by repeated crystallization from acetone.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

Their purity (>99%) was determined by TLC, NMR and elemental analysis.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

Spreading solutions of FnHm (1 mmol L⁻¹) were prepared in chloroform (analytical grade).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

Water was purified using a Millipore system (surface tension: 72.1 mN m⁻¹ at 20 °C, resistivity: 18.2 MΩ cm).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Monolayer isotherms–monolayer transfer

Surface pressure versus molecular area (π–A) isotherms were recorded on a Langmuir minitrough (Riegler & Kirstein, Germany) equipped with two movable barriers (compression speed: 0.1 nm² min⁻¹).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The surface pressure π was measured using the Wilhelmy plate method.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

Temperature was regulated at 20.0 ± 0.5 °C.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

25 μL of FnHm solution were spread on the water surface and 5 min were allowed for solvent evaporation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

The monolayers were compressed up to the desired surface pressure and transferred onto a silicon wafer treated with piranha solution (conc. H₂SO₄ + 30% H₂O₂ 3∶1), using the LB technique (one monolayer transferred, lift speed of 5 mm min⁻¹).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

AFM images–X-ray reflectivity

The transferred films were analyzed with an atomic force microscope (NanoScope III) in tapping mode.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The cantilever (Olympus) was fitted with a very sharp tip (5 nm).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The resonance frequency was 300 kHz and the spring constant 42 mN m⁻¹.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The digitized AFM images were analyzed with Visilog® Image Analysis Software (Noesis, France) using filtering techniques, a morphological gradient method, mathematical morphology functions, and adaptive thresholding.¹⁵

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The grazing incidence X-ray studies of the transferred films of F8H16 were performed with an EXPERT-MPD device from Philips (divergence slit 1/32°, parallel plate collimator, flat Ge monochromator and Xe detector).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

A Cu Kα beam at wavelength 0.1542 nm was used.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

The data were analyzed using Parratt32 software (version 1.6.0)¹⁶.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Results and discussion

Surface pressure–molecular area (π–A) compression isotherms of FnHm diblocks

The isotherms of the FnHm diblocks investigated indicate that, except for F6H16, the shortest diblock, which is also the less fluorinated one, all the other diblocks form stable Langmuir monolayers (Fig. 1), as shown in .^{ref. 11}

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

The longer the FnHm molecule, the higher the surface pressure of collapse, π^c.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

Moreover, by comparing two diblocks of same length, the most fluorinated one gave the most stable monolayer.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

For example, F10H16 had a higher π^c (∼22 mN m⁻¹) than F8H18 (∼15 mN m⁻¹).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

All the monolayers formed by the FnHm diblocks (except F6H16) had a similar extrapolated molecular area of about 0.31 nm², corresponding to the cross-section of a perfluorinated chain, which is larger than the cross-section of a typical hydrogenated chain (ca. 0.20 nm²).¹⁷

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Because the van der Waals radius of fluorine is larger than half the C–C bond distance, steric hindrance causes the chain to adopt a (15/7) helical conformation, and renders the carbon backbone rigid.¹⁸

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

The isotherms were reversible, without hysteresis, upon compression–expansion cycles.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

Influence of the FnHm molecular structure on the shape of the micelles: circular vs. elongated

The monolayers of FnHm diblocks were investigated by AFM after transfer onto silicon wafers.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

All the FnHm diblocks formed surface micelles.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

Two-dimensional surface micelles have been visualized by AFM in monolayers of partially fluorinated carboxylic acids spread on a cadmium acetate subphase and transferred on cover glass.¹⁹

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

The shape and dimensions of the FnHm surface micelles varied with the molecular structure of the diblock.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

As illustrated in Fig. 2, some diblocks, e.g. F8H16, led principally to circular micelles, while in the AFM images of other diblocks (e.g. F8H20), elongated micelles that coexisted with circular micelles were clearly visible.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

The variation of the sum area fraction of the long micelles, as determined by the image analysis software Visilog®, is plotted on Fig. 3a–c as a function of the lengths of the Fn and Hm segments and of the total length of the diblock molecule.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

The monolayers were transferred at 5 mN m⁻¹.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

The results showed that the area fraction of the elongated micelles increased from 0 to 10% with the length of the diblock molecule.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

Both the lengths of Fn and Hm segments had a strong effect.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

Further studies are presently underway to determine whether the shape of the micelles is changing from circular to elongated, or whether circular domains coalesce to form elongated domains.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

X-ray reflectivity study of F8H16 LB films

Although F6H16 and F8H14 exclusively produced circular micelles (Fig. 2), they did not lead to LB films stable enough to carry out an X-ray analysis.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

F8H16 was therefore selected for this study because of the stability of its LB films, and because it led to a monodisperse population of circular micelles with only a very limited number of long micelles (fraction area of ∼3%).

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met11

The F8H16 monolayer, compressed at 7 mN m⁻¹, was analyzed by X-ray specular reflectivity after transfer onto a silicon wafer.¹⁹

Type: Method | Advantage: None | Novelty: New | ConceptID: Met11

The experimental curve was fitted with a two-layer model that provided an excellent agreement with the data.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

The electron density distribution of the two-layer model (Fig. 4) consisted of an upper layer with a thickness of 1.0 nm and an electron density of 487 e nm⁻³ (a fluorinated layer) and of a lower layer with a thickness of 1.93 nm and an electron density of 290 e nm⁻³ (a hydrogenated layer).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

The total thickness of the monolayer was 2.93 nm, close to the length of a fully extended F8H16 molecule (3.32 nm).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

These results definitely demonstrate that the hydrogenated segments of the F8H16 molecules are directed toward the substrate, while the fluorinated segments are pointing outwards toward air.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

Altogether, the results of the AFM and X-ray reflectivity showed that the size of the surface micelles is controlled by the density mismatch between the fluorinated and hydrogenated segments.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

A disc-like shape model was proposed for the circular micelles formed by F8H16 (Scheme 1)²⁰.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

Influence of the FmHn molecular structure on the dimensions of the surface micelles

Diameter of the circular micelles

Fig. 5a–c displays the variations of the mean diameter of the circular micelles as a function of the lengths of Fn, Hm, and of the overall molecule.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

The monolayers of FnHm diblocks were transferred at 5 mN m⁻¹.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met12

It is noteworthy that, for a given length of the Hn segment, increasing the length of the Fn segment did not significantly influence the diameter of the micelles (Fig. 5a).²¹

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

On the other hand, the diameter of the circular micelles increased strongly with the Hm segment length for a given Fn length (Fig. 5b).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

As a consequence, for a given carbon number, increasing the weight of the fluorinated part of the FnHm diblocks resulted in a decrease of the size of the micelles (Fig. 5c).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

Length of the elongated micelles

Fig. 6a–c displays the variations of the length of the elongated micelles, as determined by Visilog®, as a function of the length of the Fn segment, the length of the Hm segment, and the overall length of the FnHm molecule.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

It can be seen that the longer the Hm segment and/or Fn segment, the longer the elongated micelles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

It is noteworthy that both the hydrogenated and fluorinated segments have an impact on the length of the elongated micelles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

Influence of the surface pressure of transfer on the elongated micelles

Although the surface pressure of transfer, π_t, of the monolayer onto the silicon wafer had no significant influence on the diameter of the circular micelles,²¹ it had a substantial impact on the morphology of the long micelles.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

The higher π_t, the smaller the area fraction of the elongated micelles.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

This was observed for both the sum area fraction (Fig. 7) and for the average area fraction of one single domain (data not shown).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

It was also observed that the higher π_t, the shorter the length of the elongated micelles (Fig. 8).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

This suggests that the elongated micelles are transformed into circular micelles at high π_t, providing a more regular arrangement.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

The long micelles may either break or curl to rearrange into circular micelles.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

Conclusions

We have demonstrated that monolayers of semifluorinated alkanes C_nF_2n+1C_mH_2m+1 (FnHm, n = 6, 8, 10, m = 14, 16, 18, 20) transferred onto silicon wafers consist of surface micelles.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

The shape of the micelles, circular or elongated, depends on the FnHm molecular structure; the longer the Hm segment, the larger the diameter of the circular micelles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

By contrast, the length of the Fm segment does not influence the diameter significantly.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

The elongated micelles are more numerous and longer for the longer FnHm, both the Fn and Hm segments having a strong effect.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

The surface pressure of transfer influences the shape and the length of the elongated micelles; the higher the surface pressure, the smaller their area fraction and the shorter their length.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

This work implicates that semifluorinated alkanes may be used to decorate surfaces with molecular clusters of controlled shape and predetermined size in the nanometer range.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7