Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (2004) 195–201

Microstructures in aqueous solutions of hybrid fluorocarbon/hydrocarbon catanionic surfactants

Andreea Pasc-Banua, Raluca Stana, Muriel Blanzata, Emile Pereza, Isabelle Rico-Lattesa,∗,Armand Lattesa, Thomas Labrotb, Reiko Odab

a Laboratoire des Interactions Moléculaires et Réactivité Chimique et Photochimique, Université Paul Sabatier,UMR 5623, 118 route de Narbonne, 31062 Toulouse Cedex 4, France

b Institut Européen de Chimie et Biologie, Molécules, Biomolécules et Objets Supramoléculaires (MOBIOS), UMR CNRS 5144,2 rue Robert Escarpit, 33607 Pessac Cedex, France

Received 8 January 2004; accepted 3 May 2004

Abstract

Mixtures of new hybrid fluorocarbon/hydrocarbon catanionic sugar-based surfactants are prepared by an acid–base reaction betweenN-alkylamino-1-lactitols and fluorinated carboxylic or phosphinic acids, respectively. The presence of the sugar moiety allows the synthesisof water soluble catanionic mixtures in an equimolar ratio over a wide range of concentrations (0.1–50 mM). These catanionic salts are ableto form spontaneously supramolecular assemblies in water. Dynamic light scattering is used to identify the aggregate size in water, whereastransmission electron microscopy is used to identify the morphology of the catanionic mixtures. Slight variation of the molecular structure(perfluorooctyl moiety or perfluorodecyl-moiety) can lead to marked differences in the shape of the molecular aggregates: vesicles or coiledmembranes. Moreover, perfluorodecyl-derived catanionic mixtures show an original behaviour forming lamellar dispersions in water even atconcentrations as low as 1 mM.© 2004 Elsevier B.V. All rights reserved.

Keywords:Fluorocarbon/hydrocarbon catanionic surfactant; Vesicle; Lamellar phase; Sugar; Phosphinic acid

1. Introduction

Catanionic surfactants have gathered the attention of re-searchers because of their various aggregate microstructures[1] (micelles, spontaneous vesicles, and lamellar phases).The type of equilibrium structure in water depends on dif-ferent factors: those which favour aggregation (e.g. the hy-drophobic effect) and those which oppose aggregation (e.g.the electrostatic interaction). Mixing a hydrocarbon and flu-orocarbon surfactant provides a new approach for influen-cing the aggregation process due to the repellence betweenthe two hydrophobic chains. Therefore, these systems mayexhibit novel features which are important for both theore-tical [2] and applied interest[3].

Previous work on hydrocarbon/fluorocarbon systems hasfocused on surfactants mixtures of the same charge. Ex-

∗ Corresponding author. Fax:+33 561 558 155.E-mail address:rico@chimie.ups-tlse.fr (I. Rico-Lattes).

tremely heterogeneous micelles have been reported and thecoexistence of two types of micelles, fluorocarbon-rich orhydrocarbon-rich was explained by the mutual phobicity ofthe two surfactant chains[4].

Concerning mixtures of oppositely charged hydrocarbonand fluorocarbon surfactants, only few examples have beenreported in the literature[5]. The attractive interaction be-tween the electrical charges seems to be sufficient to over-come the mutual antipathy of the hydrocarbon and fluoro-carbon chains[6]. In general, phase behaviour indicates thatthese systems tend to form bilayer structures, with vesiclesforming in a few cases. However, when using long hydrocar-bon chain length (>8C), catanionic surfactants are practicallyinsoluble in water in equimolar conditions and therefore theuse of one of surfactants in excess is often necessary[7].

Here, we investigate the self-assembling properties andsolution structure of mixed oppositely charged fluoro-carbon/hydrocarbon surfactants bearing a hydrocarbonlactose-derived polar head and various fluorinated hy-drophobic tails (Fig. 1). The presence of water-soluble sugar

0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfa.2004.05.003

196 A. Pasc-Banu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (2004) 195–201

O

OH

OHOH

OH

OHO

OHOH

NH2

OH

RH

X CH2 CH2 RF

RH = CnH2n+1, n = 12, 16RF = CmF2m+1, m = 8, 10

X- = COO-, PHO2-

Fig. 1. Structure of hybrid fluorocarbon/hydrocarbon sugar-based catan-ionic surfactants.

moiety is suitable for their hydrophilic properties, in orderto avoid the precipitation phenomenon when working withlong hydrophobic chains[8]. Furthermore, the presence ofthe sugar moiety can be a chirality source and can influencethe biological and the molecular recognition properties[9].

The main purpose of this study is to determine theamphiphilic behaviour of catanionic mixtures of fluorocar-bon/hydrocarbon surfactants. Their synthesis is achievedquantitatively by acido-basic reaction between carboxylicor phosphinic fluorinated acids andN-alkylamino-lactitols,according to a procedure already reported by our group[10,11].

2. Experimental section

2.1. Materials

1-Perfluorooctylethene and 1-perfluorodecylethene wereElf-Atochem products.N-alkylamino-1-deoxylactitols wereprepared as previously described[10,12] from reaction ofcorresponding alkylamine and lactose. Water was a Mili-pore Q-grade with a specific resistance of 18 M� cm. Thecatanionic mixtures were prepared[4] by mixing in waterequimolecular amounts ofN-alkylamino-1-lactitols and flu-orinated carboxylic or phosphinic acids, respectively.

2.2. General methods

NMR spectra were recorded on a Brucker AC 400 spec-trometer at nominal frequencies of 400.1 MHz for1H,100.1 MHz for 13C, 376 MHz for 19F and 81.01 MHz for31P, respectively. Chemical shifts are reported in ppm. IRspectra were recorded on a Perkin-Elmer FT 1760-X spec-trometer (0.5% in KBr) and the frequencies are expressedin cm−1.

The formation of aggregates was observed through trans-mission electron microscopy using a JEOL JEM 200 CXelectron microscope, operating at 120 kV. Aliquots of solu-tion (5 mM) were applied on carbon-coated Formvar grids,negatively stained with a 2% (w/v) solution of phospho-tungstate (pH 7.5).

Freeze-fracture experiments were performed with aBalzer vacuum chamber BAF (Balzer, Liechtenstein). Asmall droplet of mixture was sandwiched between twocopper specimen holders and was kept at the desiredtemperature to reach equilibrium. The environment wassaturated with water to avoid evaporation. The sandwichwas than frozen with liquid propane cooled with liquidnitrogen. The frozen sandwich was additionally fixed toa transport unit under liquid nitrogen and transferred tothe fracture replication stage in a chamber that was thenpumped down to 10−6 mbar at−120◦C. Immediately afterfracturing, replication took place by first shadowing withplatinum/carbon at 45◦C and then with carbon deposition at90◦. The sample was allowed to warm to room temperature.Replica were retrieved and cleaned in water and mountedon 200 mesh copper grids. Observations were made witha Cryo-Electron Microscope FEI EM120 (120 kV), andthe images were recorded with SSCCD 2k× 2k Gatancamera.

The size of aggregates (at 25◦C and for a concentration of5 × 10−2 M) was also evaluated by dynamic light-scatteringusing a Malvern Instruments Zetasizer 3000, which can beused for samples containing particles from 1 nm to 10�m.

2.3. Synthesis of perfluoroalkylethylphosphinic acids,1

To a solution of NaH2PO2·H2O (4.62 mmoles, 2.5 eq.) in10 mL EtOH and 1.5 mL H2O were added the 1-perfluoroal-kylethene (1.85 mmoles, 1 eq.), 0.45 g (1.85 mmoles, 1 eq.)benzoyl peroxide and 0.272 g (2.77 mmoles, 1.5 eq.) H2SO4.The mixture was heated under reflux, with stirring, for 24 h.After removal of the solvent, the white residue was succes-sively washed with 20 mL acidified water (HCl, pH 1) and20 mL toluene. The fluorinated phosphonic acid was puri-fied by recrystallisation (AcOEt for1a and AcOH/toluenefor 1b).

2.4. (1-Perfluorooctyl)ethylphosphinic acid,1a

Yield 84%; Mp. 104◦C.1H NMR (AcOH-d4): 7.31, d, 1H(1JHP = 575 Hz, P–H); 2.52, m, 2H (CH2–CF2); 2.19, m, 2H(CH2–P). 13C NMR (AcOH-d4): 121.3–109.8 (CF2, CF3);25.2, t (2JCF = 23 Hz, CH2–CF2); 22.2, d (1JCP = 95 Hz,CH2–P).31P NMR (AcOH-d4): 36.6.19F NMR (AcOH-d4):−4.0, t (3JFF = 9.8 Hz, CF3); −37.6, m (CF2–CH2); −44.5to −46.1, m (CF2 internal);−49.0, m (CF2–CF3). IR (KBr):2366.2 (νP–H); 1202.8 (νC–F); 1148.7 (νP=O). Analysis cal-culated (%) for C10H6F17O2P (511.98), C: 23.45; H: 1.18;found, C: 23.68; H: 1.02. MS (m/z, NH3) 530 (MNH4

+).

2.5. (1-Perfluorodecyl)ethylphosphinic acid,1b

Yield 68%; Mp. 144–145◦C. 1H NMR (AcOH-d4): 7.14,d, 1H (1JHP = 575 Hz, P–H); 2.45, m, 2H (CH2–CF2);1.94, m, 2H (CH2–P). 13C NMR (AcOH-d4): 120.8–110.5(CF2, CF3); 23.1, t (2JCF = 23 Hz, CH2–CF2); 22.6, d

A. Pasc-Banu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (2004) 195–201 197

(1JCP = 94 Hz, CH2–P). 31P NMR (AcOH-d4): 36.8.19FNMR (AcOH-d4): −4.6, t (3JFF=10.1Hz, CF3); −38.2,m (CF2–CH2); −45.1 to−46.6, m (CF2 internal);−49.6,m (CF2–CF3). IR (KBr): 2366.2 (νP–H); 1211.6 (νC–F);1151.0 (νP=O). Analysis calculated (%) for C12H6F21O2P(611.98), C: 23.55; H: 0.99; found, C: 23.31; H: 0.66. MS(m/z, NH3) 630 (MNH4

+).

3. Results and discussion

3.1. Synthesis of fluorinated phosphinic acids

Fluorinated phosphinic acids CmF2m+1–CH2–CH2–PO2H(m = 8, 10) were prepared by reaction of sodium hy-pophosphite with the corresponding fluorinated alkenes(CmF2m+1–CH=CH2) under radical conditions (Scheme 1).

Sodium hypophosphite is known to react with alkenes inthe presence of peroxides[13], AIBN [14] or triethylborane[15] leading to alkylphosphinic acids in good yields. How-ever, the use of the same type of reactions on fluorinatedalkenes is limited by the electron-deficiency of the substrate.Preliminary attempts of synthesis using equimolar ratio flu-orinated alkene:sodium hypophosphite and catalytic amountof initiator (CmF2m+1–CH=CH2:NaH2PO2:(C6H5COO)2= 1:1:0.05) led to desired products in modest yields (entries1, 3 of Table 1).

Low conversions could be explained both by the decreasedreactivity of the substrate[16] and by inefficient chain pro-pagation promoted by P-centered radicals[17]. Therefore,by working with 1:2.5:1 olefin/hypophosphite/initiator ra-tio, fluorinated phosphinic acids were obtained in improvedyields (entries 2,4 ofTable 1).

3.2. Catanionic surfactants

The synthesis of fluorocarbon/hydrocarbon sugar-basedcatanionic surfactants was achieved using the method pre-

CmF2m+1 CH CH2 + NaH2PO2

(C6H5COO)2 , H2SO4

EtOH/H2O reflux , 24 hCmF2m+1 CH2 CH2 P

O

OH

H

1a m = 8 1b m = 10

Scheme 1. Synthesis of fluorinated phosphinic acids.

Table 1Synthesis of fluorinated phosphinic acids CmF2m+1–CH2–CH2–PO2H

Entry Compound m NaH2PO2 (eq.) (C6H5COO)2 (eq.) Yielda (%)

1 1a 8 1 0.05 342 1a 8 2.5 1 843 1b 10 1 0.05 254 1b 10 2.5 1 58

a Isolated, recrystalised product.

viously described[10], by mixing in water equimolecu-lar amounts ofN-alkylamino-1-lactitols and fluorinatedcarboxylic or phosphinic acids, respectively: the generalmethod is shown inScheme 2.

For the catanionic mixtures2a and2b, the complete con-version of the carboxylic acid to the carboxylate form wasdetermined by IR: the frequency of the carboxylic group(νC=O) moves from 1712 cm−1 to 1589 cm−1 (asymmetric)and 1410 cm−1 (symmetric) for2a and 1585 cm−1 (asym-metric) and 1407 cm−1 (symmetric) for 2b, respectively.In the case of phosphinic acids, the completion of the re-action was proved by31P NMR spectrometry: the signalcorresponding to phosphorus atom shifted from∼36.5 to∼24.5 ppm.

The ammoniumlactitol head and the carboxylate or thephosphinate group are hydrophilic, and they form the polarhead of the catanionic surfactants, while the hydrocarbonchain CnH2n+1 (n = 12, 16) and the fluorocarbon chainCmF2m+1 (m = 8, 10) constitute the hydrophobic part.

3.3. Dynamic light scattering and transmission electronmicroscopy

Photon correlation spectroscopy measurements were per-formed in order to determine both the presence and the sizeof objects in aqueous solution. A broad distribution of par-ticles, centered between 300 and 500 nm was observed forperfluorooctyl-derived catanionic compounds2a,b and3a,bwhile two populations near 500 and 1300 nm were observedfor perfluorodecyl-derived catanionic compounds3c,d. DLSexperiments also showed that the aggregates were stable forat least 6 days at room temperature.

The aggregate morphology of all surfactants was deter-mined by TEM. In the case of perfluorooctyl derivatives,2a,b and 3a,b spherical vesicles were observed in a widediameter range from 50 to 400 nm. (Fig. 2). These resultsare in good agreement with those obtained by DLS mea-surements.

198 A. Pasc-Banu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (2004) 195–201

O

OH

HOOH

OH

OHO

HO NH

OH

RH

OHO

HOOH

NH2

OH

RH

O

OH

HOOH

OH

OHO

HOOH

NH2

OH

RH

PH CH2 CH2 RF

O

O

C CH2 CH2 RF

O

O

OO

HO

C CH2 CH2 RF

HOOH

PH CH2 CH2 RF

1a RF = C8F17

1b R C F

O

OH

HOOH

OH

2a RH = C12H25, RF = C8F17 3a RH = C12H25, RF = C8F17

3b RH = C16H33, RF = C8F17

3c R = C H R = C F

Scheme 2. Synthesis of hybrid fluorocarbon/hydrocarbon catanionic surfactants2 and 3.

Fig. 2. Negatively stained electron micrographs of vesicles formed by2a (a) and3a (b).

Fig. 3. Negatively stained electron micrographs of rolled membranes and vesicles formed by3c (a) and3d (b).

A. Pasc-Banu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (2004) 195–201 199

Fig. 4. Schematic representation of the formation of coiled membranesfrom perfluorodecyl-derived hybrid catanionic mixtures3c,d.

In the case of perfluorodecyl derivatives3c,d, coiledmembranes like open tubules of widths ranging in the mi-crometer size were observed (Fig. 3). The changing in mor-phology of the aggregates from vesicles to tubules seemsto be related to the increase both in hydrophobicity andlipophobicity introduced by the additional two CF2 units.Due to their reduced solubility in water, these compoundscould form lamellar dispersions even at low concentra-tions.

The shape of the tubules obtained is very different fromthose we had previously described with non-fluorinatedsugar surfactants[18,19] (closed tubules, filaments and he-lices). In this case, we have flat membranes sheets whichrolled up simultaneously from opposite edges with a more

Fig. 5. Freeze-fracture electron micrographs of3b (a) multilamellar vesicles (b) unilamellar vesicles.

important rolling at the vertexes, leading to open tubuleswith a tapered shape (Fig. 4).

Moreover, as observed by other authors for fluorinatedsurfactants[20], we have a mixture of tubules, small vesi-cles and some V-shaped coiled membranes. The originalshapes of these aggregates result probably from the chira-lity of the sugar and an important segregation of the mixedchains (fluorinated and hydrogenated) along the edges of themembranes.

3.4. Freeze-fracture electron-microscopy

The negatively stained TEM results of two samples,3band 3d were also confirmed from FF–TEM as following.In Fig. 5a and bwe show the FF–TEM micrographs ofthe3b catanionic salt. The large multilamellar vesicles witha diameter of about 600 nm (Fig. 5a) coexist with muchsmaller unilamellar vesicles whose population is centeredon 90 nm (Fig. 5b).

Fig. 6 shows two different micrographs of the L�-phaseformed by3d catanionic mixture. One can observe the typi-cal pattern for stacked bilayer of the normal L�-phase withina multilayer arrangement whose thickness is about 80 nmwith a bilayer “step” of about 6 nm. This permits us to havemore details on the precise morphology of the coiled mem-branes observed in TEM.Fig. 6bshows the coexistence ofmultilayers and small unilamellar vesicles with a diameterof about 20 nm, which is in good agreement with the TEMexperiments. The coexistence of two different types of ag-gregates could be a result of a partial segregation betweenthe hydrophobic tails (fluorocarbon and hydrocarbon). Theexcess of the fluorinated chains enhance the rigidity of themembrane giving rise to straight multilayers.

200 A. Pasc-Banu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (2004) 195–201

Fig. 6. Freeze-fracture electron micrographs of3d (a) multilayers (b) mixture of small vesicles and fractural multilayers.

4. Conclusion

The first hybrid fluorocarbon/hydrocarbon catanionic sur-factants derived from sugars were reported. They were syn-thesised by acido-basic reaction ofN-alkylamino-1-lactitolswith fluorinated carboxylic or phosphinic acids, respectively.The method used here was adapted for a modular synthesisof a large family of mixed fluorinated/hydrogenated sugarsurfactant. Furthermore, the presence of the sugar moietyenabled us to prepare stable and water soluble catanionicsurfactants.

All the catanionic salts prepared are able to form sponta-neously L�-phase in water. The variety of the morphologiesobtained is very large as for the classical catanionic sur-factants: vesicles, rolled membranes like open tubules andV-shaped coiled membranes. The differences of shape of

the L�-phase seem to depend only on the fluorocarbon chainlength. For instance, when using perfluorooctyl derivativesonly vesicles, unilamellar and multilamellar, were formedwhereas perfluorodecyl derivatives gave rise to mixtures ofsmall unilamellar vesicles and multilayers. Thus, increasingthe fluorofilicity of catanionic mixtures leads to more rigidand more hydrophobic aggregates like dispersed multilayersin water even at low concentration.

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