J. Phys. Chem. 1994, 98, 7591-7598
7591
Perfluoropolyether Carboxylic Salts in Water: Liquid Crystals by Optical Microscopy and NMR Maura Monduzzi,' Alba Chittofrati,? and Viviana Bosellit Dipartimento Scienze Chimiche, Universita' Cagliari, Via Ospedale 72, 091 24 Cagliari, Italy, and AUSIMONT, R & D Centre, Via S. Pietro 50, 20021 Bollate, MI, Italy Received: January 3, 1994; In Final Form: May 3, 1994'
The ammonium salts of three perfluoropolyether (PFPE) carboxylic acids differing in average molecular weight were examined for their phase behavior in water. Micellization was detected by surface tension measurments, whereas lyotropic liquid crystalline phases were identified by optical miscroscopy and NMR techniques. Lamellar phases have been found with shorter chain surfactants (average M W of 450 and 740) while a hexagonal phase appears to form with the surfactant with the highest M W (940). Although the composition of the three PFPE surfactant mixtures certainly contributed to determine the local microstructure, the packing features in terms of V/al wererather clearly outlined. The increase of the surfactant chain length and the addition of a cosurfactant or a cosolvent produced significant variations of the self-association structures in agreement with the expected variations of the packing parameter.
Introduction Perfluorinated surfactants, e.g. perfluoroalkanoates, have previously been proved to be highly hydrophobic with low values of cmc and minimum surface tension.1.2 Their lyotropic phases in water have been investigated by several authors, first by Tiddy and co-workers, and Fontel and Lindman found lamellar liquid crystals (L,) with the salts of the pentadecafluorooctanoic acid (HPFO) in water394 and with the ammonium or alkylammonium heptadecafluorononanoate (HFN).S Recently Boden et al. demonstrated also the occurrence of discotic micelles ordered in the nematic phase of the cesium (CsPFO) and ammonium (APFO) pentadecafluorooctanoate in D Z O . ~ ? ~ Here, the investigation focuses on the ammonium salts of perfluoropolyether (PFPE) carboxylic acids, having the following general structure:
-
with n >> m,p, m - p 0, and Rf = Cl-C3 perfluoroalkyl group. Optical microscopy and 2H and I4N NMR techniques have been used to identify the lyotropic phases formed in water by three surfactants having short (Sl, MW = 434), intermediate (S2, MW = 722), and long (S3, MW = 923) PFPE chains, which, in turn, corresponds to an average number of perfluoropropylene oxide units "nn of 1.4, 3.2, and 4.4, respectively. In previous studies, earlier micellization data can be found in refs 8 and 9, while some L, regions were identified by ESR, SANS, SAXS, and SAXRD techniques.l"12 In addition, a few water-in-PFPE oil microemulsions13revealed interestinganalogies between a S2-type PFPE surfactant and double-chain surfactants, i.e. didodecyldimethylammonium bromide (DDAB) and AOT, which are known to form L, phases in water.l4J5 Theaimofthisstudy is toreport the first detailedphasediagram of these interesting PFPE compounds and to establish a relation between the average chain length and the type of the lyotropic phases. Some preliminary results of these data were recently presented.16 Albeit the presence of isomers and/or slightly different MW homologues in each fraction of surfactant affects the self-
* Author to whom correspondenceshould be addressed at the Universita' Cagliari. Fax: (39)70-669272. E-mail: MONDUZZI@VAXCAl. UNICAJT. t AUSIMONT. 0 Abstract published in Advance ACS Abstracts, June IS, 1994. 0022-365419412098-7591$04.50/0
association behavior, the results presented here depict the formation of lamellar, cubic, and hexagonal mesophases upon varying theaveragechain length and theconcentration. Examples of the packing modification induced by a change of solvent (hydroalcoholic solution) or by addition of a PFPE alcohol as cosurfactant hint at the observed lamellar-hexagonal transitions. Experimental Section Materials. The general features of the PFPE carboxylic surfactants have been described el~ewhere.~J3J~ The PFPE carboxylic acids are prepared by photooxidation of hexafluoropropene17over a wide range of molecular weight (MW), and fractionsof narrow MW distribution (gas chromatographic purity of 95-99%) can be obtained by distillation. The ammonium salts S1, S2, and S3 are prepared by simple neutralizationwith aqueous ammonia followed by thorough drying. The absence of -COOH in the salts was confirmed by IR spectroscopy. The acids may contain residual amounts of PFPE oil of similar MW that can be removed from the ammonium salts by recrystallizationfrom ethanol. The oil impurity in the surfactants was 1 wt % for S2 and 4 wt % for S3 while S1 did not contain oil impurities. S 1 and S2 were used without further purification, while S3 was recrystallized. Traces of ethanol and ethyl ester were detected in S3 by NMR spectroscopy. Milli Q water (surface tension 71 mN/m at 25 "C) was used for the preparation of the samples. Ethanol (Carlo Erba) and a PFPE alcohol (average MW 680) obtained by reduction of the corresponding PFPE acid were used as cosolvent and cosurfactant, respectively, in the three-component systems. The samples for NMR studies were prepared with deuterium oxide (Carlo Erba, 99.8% enriched). Methods. Surface tension of surfactant solutions was measured at 25 0.1 "C by the De Nouy ring method with a Lauda tensiometer TElC. The values are the average of 5-6 measurements (reproducibilitywithin 1%)with equilibration time within 30 min, depending on the concentration of the solution,corrected by the Harkins-Jordan factor to account for ring geometry and density. The approximate two-component phase diagrams were determined by observing a series of samples made of weighed amounts of water and surfactant, mixed by a Vortex mixer and equilibrated at constant temperature for 24 h (no detectable changes were found with longer equilibration). At 25 "C, the observation of the samples (after 1 h of centrifugation at 3000 rpm) was first visual, through crossed polaroids, to detect birefringence and roughly evaluate number, volume, and type of
*
0 1994 American Chemical Society
7592
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994
Monduzzi et al.
le:
[nl
IC
I -
I
Figure 1. D-NMR spectra of S1/2H20 system at various compositions, at 25 OC: [A] 69.7 wt 5% (L.), [B] 79.7 wt % (La),[C] 94.6 wt 9% (Lz),[D] the same as [C] at 40 OC where ammonium and water NMR signals display fast exchange.
phases. The apparently homogeneous liquid crystalline phases were then observed by optical microscopy (Zeiss Universal 11) in polarized light, in comparison with the typical textures of other surfactants.l*J9 The samples for NMR studies were prepared by weighing appropriate amounts of surfactants and deuterium oxide into glass tubes (0.d. 7 mm) which were flame sealed prior to cycles of back and forth centrifugation until homogeneity was achieved. Samples, with compositions differing by 4-5 wt %, were stored at 30 O C for several months while ZH NMR spectra were periodically recorded to verify the achievement of equilibrium. Each sample, before NMR measurement, was heated at 35 OC for 24 h and then kept in the probe at 25 OC for 30 min to reach thermal equilibrium and to allow alignment of the magnetic field. The determination of the phase boundaries by this procedure leads to slightly larger L. ranges than those elsewhere reported for similar surfactants.12 NMR measurements were performed by Varian FT-80A (1.88 T) and Varian VXR-300 (7.05 T) spectrometers, at the operating frequencies for 2H of 12.28 and 46.05 MHz, respectively; 14N NMR spectra were run on the Varian VXR-300 at the operating frequency 21.67 MHz. In the case of the FT-80A spectrometer an external 2H lock was used while in the case of the VXR-300 deuterium NMR spectra were recorded without lock. TheVarian FT-80A spectrometer, equipped with a pulsed magnetic field unity (by Stelar, Italy) with a maximum gradient power of 250 mT/m, was used for water self-diffusion measurements by the usual PGSE technique.20
Results and Discussion Interpretation of N M R Data. Extensive literature is available on the investigation of water binding at the polar-apolar interface
in surfactant-water systems by NMR of quadrupolar nuclei.6JJI.z The interaction of the electric quadrupole moment (of nuclei with a spin quantum number I = 1, such as 2H and 14N) with non-zero electric field gradients (due to anisotropic orientation) produces 21 resonance peaks whose separation A is given byz3
where m = 4 for the lamellar phase and m = 8 for the hexagonal phase, pb is the fraction of the observed nucleus in the bound state, x is the quadrupolar coupling constant, and s b = 1/2(3 cos2 OD - 1) is the order parameter relating the time averaged orientation (OD) of the nucleus with respect to the surfactant chain axis. Generally, for water molecules, Pb is linearly dependent on the surfactant/water (S/W molar ratio, and thus eq 1 can be written as
where nb is the number of bound water molecules per polar head. The straight line of eq 2 would pass through the origin at low surfactant concentration while, with increasing surfactant concentration, the line should reach a maximum A splitting at the concentration occurring at nb = (W/S+ l).24 Here, the interpretation of deuterium NMR spectra of PFPE surfactant/water systems is complicated by the Occurrence of slow exchange between ammonium and water deuterons which can be observed especially at high surfactant concentration as shown in Figures 1-3, which report selected D-NMR spectra of S1, S2, and S3, respectively, at various concentrations. The spectra display two deuterium quadrupolar splittings originating
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7593
PFPE Carboxylic Acids in Water
lOLW
IC:
IW
11000
!Won
MW
0000
10100
SOW0
9MO
W O
9700
-0
9500
WOO
9300
ID]
1OW
Figure 2. D-NMRspectra of S2/2H20 system at various compositions, at 25 'C: [A] 14.5 wt 96 (LI+L.), [B]58.5 wt 4% (La),[C]82.5 wt 96 (La), [D] 93.4 wt 96 (V2).
from different magnetic sites for birefringent samples and two D-NMR signals for isotropic samples. The same frequency difference (-2.6 ppm), observed between the isotropic signals of Figures 1Cand 2D, is retained for the twoquadrupolar splittings as clearly shown in Figure 1A,B, Figure 2B,C,and Figure 3. This shift is independent of either the type and concentration of the surfactant or even of the nature of the liquid crystalline phase. The possibility that the twosplittingspertain to water molecules oriented in different magnetic environments (Le. a 2-phase system), which are in slow exchange (thus giving different splittings), must be ruled out since, in such a case, no significant difference of chemical shifts would be e ~ p C c t e d . ~Obvious ~~~* considerations allowed the assignment of the downfield NMR signal and thus of the smaller doublet to the fully deuterated ammonium groupND4+. This interpretationof deuteriumspectra is further supported by observingthe coalescenceof the two peaks at 40 OC in the isotropic solution of S1 (S= 94.6 wt %I),as shown in Figure 1C and 1D. Conversely, coaltscence was not observed in the anisotropic phases even at 50 OC: we did not deepen this topic; however, the different time scales of the chemical shifts and of the quadrupolar splittings are certainly responsible for the observed behavior. I4N NMR spectra of some samples of the three PFPE surfactants, prepared in water to avoid N-D quadrupolar broadening of the signals, displayed only one doublet, thus confirming the Occurrence of real lamellar or hexagonal liquid crystalline 1-phase regions (see below). A similar slow exchange is rather unusual; only Tiddy and co-workers, in a NMR study on the dynamics of the APFO/ water system, suspected a slow exchange affecting the dipolar relaxation times of water, but they did not observe slow exchange on the NMR chemical shift time scale.3
Notwithstanding all this, the rationalization of deuterium quadrupolar splittings may be pursued in terms of the above equations. Only the nuclei at a close distance from the oriented interface experience the anisotropic motion, and the observed quadrupolar sdittings are reduced by the exchange with the unbound species. If the interface region is consideredas a general binding site while the real sites where the water molecules can be localized are neglected, we can also assume that a rapid exchange,on the NMR time scale, between bound and free species occurs; thus, as a first approximation, eqs 1 and 2 still hold (see below). The ammonium counterion probably does not represent the best choice for this NMR study, but it is of interest for promoting surfactant solubility and decreasing Krafft temperatures, as also noted in other fluorinated surfactants.3.s Two-Component Systems: Phase Boundaries. In the low concentration region, the micellization of S1 and S2 is detected by surface tension measurements (Figure 4). At high surfactant concentration the three PFPE surfactants form liquid crystalline phases. Figure 5 shows the substantially identical textures of the L, phases formed by S1 and S2 in water at 25 O C . Figure 6 displays the hexagonal (H2) texture observed for S3. The phase diagram, with the identification of the monophasic regions, was attained by deuterium and nitrogen NMR. Table 1 summarizes the phases identified by optical microscopy and NMR, at 25 OC. The achievement of the equilibrium was a crucial point for several samples which displayed constant quadrupolar splittingsin the D-NMRspectra after 10-12 months of storage at 30 O C , Such a long equilibration was not necessary with the three-component samples.
7594 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994
Monduzzi et al.
[AI
IB]
[c:
16000
1lCOO
1WCO
14000
I :COG
17033
16000
15000
14300
11000
Figure 3. D-NMRspectra of S3/2H20 system at various compositions, at 25 "C: [A] 74.5 wt 96 (L,+H2), [B] 80.3 wt 96 (Hz), [C] 84.2 wt 96 (H2), [D] 94.3 wt 96 (Hz).
60
20
0
10.'
10"
IO'*
10"
I
M (mol/l) Figure 4. Surface tension vs concentration for S1 and S 2 in water at 25 OC.
Micellar Solutions. The solubility limits, at 25 OC, in water are 5 5 wt % surfactant for S1 and 0.5 wt % for S2,while thevalue for S3 is below the limit of visual detection (0.01 wt %). In the case of S1 and S2, a Krafft point lower than room temperature could be evaluated by the plot of the specific conductivity vs temperature in the range 5-90 OC,at constant concentration: approximate values of 10-13 O C for S1 and 1618 "Cfor S2 were found at "c/cmc" ranging from 2 to 8. In the case of S3, a Krafft point higher than room temperature might be expected, but no evidence was found by conductivity up to 90 "C. The measurements of surface tension have been limited to solutions of S1 and S2, as shown in Figure 4, and gave evidence of micellization at surfactant concentrations exceeding 7 X 10-3
I' Figure 5. Optical micrographs, at 25 OC. Typical textures in polarized light of lamellar La liquid crystals in water for [A] S1 70 wt 96 (X256) and [B] S 2 85 wt 96 (X160).
mol/L for S1 (5% error) and 2.7 X 10-4mol/L for S2 (1% error). The values of the ultimate surface tension are 14.8 and 15.3 mN / m, respectively. It can be noticed that PFPE surfactants give either higher Krafft temperatures or lower cmc than APFO, whereas the cross sectional areas at the water-air interface, obtained by the Gibbs
PFPE Carboxylic Acids in Water
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7595 Water Deuterium Quadrupolar Splittings
Ammonium Deuterium Quadrupolar Splittings
Figure 6. Optical micrograph, at 25 OC (X256). Typical textures in polarized light of hexagonal H2 liquid crystals. S3 90 wt 96, in water at 25 "C.
300 0
TABLE 1: Phase Boundaries at 25 OC of PFPE Surfactant/
0.1
0.2
SIW
0.3
0.4
C
Water Svstemsa range S wt %
optical aspect
identified phases
Ammonium Nitrogen Quadrupolar Splittings 4
s1 0-55 55-70 70-89 91-95
I solution I + B viscous B viscous B + I viscous I solution
0-0.5 0.5-10 10-32 32-90 90-9 1 91-97
I solution I + B solution B + I visc. sol. B homog. B + I solid I solid
0.05-75 75-95
I+Bviscous B viscous
89-90
m
.-
LI LI + La La La + L2 L2
s2
L1
L1 dominant + (La) LI (long rod micelles) + La La La + v2
r
2
m
0
. . ' . I . . . . ~ . . . . , . . ' . , ' . . . 0.1 0.2 0.3 0.4 0.5
SIW
v2
s3
LI+H~ H2
Lcgend: I = Isotropic, B = Birefringent, LI = Micellar Solution, L2 = Isotropic Solution, La = Lamellar Liquid Crystal, H2 = Inverse Hexagonal Liquid Crystal, V2 = Bicontinuous Cubic Liquid Crystal. Notation from ref 22.
equation and given in Figure 4, are in agreement with earlier findings of perflu~roalkanoates.~~ The deuterium NMR spectra of the systems containing less than 5 wt 5% of the surfactant merely displayed the single narrow signal due to deuterated water. As to the micelles structure, it should be mentioned that whereas evidencesof cylindrical APFO micelles25 and discotic CsPFO micelles7have been reported, here the formation of nonspherical aggregates for S2, previously suggested in ref 10, is supported by the 13CNMR spectra which display very broad signals, typical of a short-range order such as in long rod micelles. Lyotropic Phases. The attribution of the lamellar (Latype) and the hexagonal (H2, inverse hexagonal type) liquid crystalline phases was easily attained by optical microscopy (Figures 5 and 6). Besides the D-NMR spectra typical of oriented mesophases (cf. Figures 1-3). the water deuterium quadrupolar splittings, reported as a function of the S / W molar ratio for the three surfactants in Figure 7A, gave the unambiguous attribution of a monophasic lamellar mesophase for S1 (range 70-90 wt 96) and S2'(range 33-90 wt %jand of a monophasic hexagonal
Figure 7. NMR quadrupolar splitting AVQ (Hz) vs ~urfactant/~HzO S/Wmolar ratio at 25 OC: (0)S1; (0)S2; ( 0 )S3; (A) S2 + RfCos(S = S2 RfCos). [A] 2H splittingsof water, [B] 2Hsplittingsofammonium, [C] 14N splittings of ammonium.
+
mesophase for S3 (range 75-97 wt %). Figure 7B,C reports the 2 H and 14Nquadrupolar splittingsof the ammonium counterions; these data do not follow eqs 1 and 2 since many equilibria other than bound-free species occur (particularly dissociation and partially deuterated species equilibria). As to the phase boundaries summarized in Table 1, we can notice that the region of isotropic micellar Ll solution is rather wide for S1 and much smaller for S2, whereas S3 gives a large two-phase system of a Ll solution in equilibrium with a H2 phase. The phase transition of S1 and S2,at high surfactant concentration, from Lato an isotropic phase produces a very narrow twophase region. Generally, an appropriate equilibrium led to an easy identification of the various one- and two-phase regions, but to establish the types of the microstructures of the S2 samples in the range 5-30 wt % S appeared more difficult. These samples, upon aging, became more viscous and did not flow as easily as the Lasamples. They did not present appreciable phase separation even upon centrifugationwhile some heterogeneousbirefringence (multihued streaks) appeared when observed between crossed polarizers, as reported for nematic liquid crystals.26 Their D-NMR spectra displayed small quadrupolar splittings overlapped with isotropic signalsonly afterlongaging (cf. Figure 2A), while nematic phases
7596 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994
are known to undergo magnetic field orientation easily, thus giving well-separated quadrupolar splittings for deuterium.' Indeed 2H and I3C NMR spectra seem to indicate the Occurrence of a twophase region of small lamellae dispersed in a solution of long rodlike micelles, but the present data are rather limited, and the identification of the microstructure remains an open question. Turning attention to the NMR results of Figure 7A, it should be noted that the observed trends are not affected by the deuterium exchange problem mentioned above, at least to a significant extent. As predicted by eq 2, at low surfactant concentration, the lamellar A splittings of both Si and S2, reported as a function of S / W , give straight lines crossing through the origin with approximately the same slope. That is indicative of similar order parameters and quadrupolar coupling constants, thus implying similar arrangements of the water binding at the interface. The quadrupolar splittings due to the H2 phase of S3 give more scattered data which do not cross through the origin and have a lower linear correlation. It has been often observed that hexagonal splittings do not increase with increasing surfactant concentration as much as the theory predicts,21 but here the splittings are approximately halved (cf. eq 1) with respect to those pertaining to the lamellar phase of S2. The maxima predicted by eq 2, at high surfactant concentration, cannot be clearly identified in Figure 7A. The degree of water binding per polar head, nb, should be roughly estimated between 4 and 9 for the lamellar packing of S1 and S2, whereas n b < 4 should hold for the hexagonal packing of S3. It is worth noting that a fittingof the experimental splittings (at low Sconcentration) to eq 2, for the three surfactants, introducing n b = 8 and x = 222 KHz,z7 gave the order parameters S b 0.01 1 and S b 0.007 (errors around 1O%), for the lamellar and the hexagonal packings, respectively, which are in excellent agreement with several literature data on hydrocarbon surfactants.22 As to the phases formed at high surfactant concentration (S wt % > go), the different association structures areclearly related to the surfactant chain length. S1 forms an isotropic L2 solution (Figure 1D,E). In this region the W/Smolar ratio is lower than 2 and a water self-diffusion coefficient D, = 2 X 10-11 m2/s is measured. This value might either originate from the water core of "inverse microstructures" or pertain to tightly bound hydration water only. The PFO and HFN ammonium salts do not display a similar behavior, while L2 phases have been reported, at higher temperatures, for HFN acid and its dimethyl- and diethylammonium salts,5 without any structural identification. S2 (at W/S< 4) forms a highly viscous isotropic crystalline phase with a typical cubic appearance. In consideration of the observed D, = 5-7 X 1&lo m2/s, a bicontinuous cubic structure (V2 type) may be proposed in analogy with Fontell's suggestions for DDAB.28 This finding agrees also with the similarly high 0, values found earlier for PFPE microemulsions, where the high water self-diffusion coefficients along with the high conductivity data observed at W/S= 4-6 were associated with the formation of a water continuous network.13 As mentioned above, S3 needed a long aging time before the well-resolved D-NMR spectra reported in Figure 3 weredisplayed. This surfactant forms a hexagonal mesophase almost in the whole concentration range, easily identified by microscopy for S3 > 10 wt %, whereas a one-phase H2 microstructure occurs for S3 > 75 wt % only. The optical micrograph of Figure 6 refers to a high surfactant concentration, but a similar texture was evident in moredilute samples also. The Occurrenceof a hexagonal packing at any composition and the wide two-phase region are somehow unusual features, but the formation of a H2 mesophase is in agreement with the increase of the chain length and the volume of the PFPE tail. Tbree-Component Systems. The modification of S2 and S3 two-component systems upon addition of a suitable alcohol was
-
Monduzzi et al. F
v1
llrl
Ih
II
II
I
I
I
IC
-
Figure 8. Three-componentS2 system, at 25 O C : S2(723)/RfCos/W. Optical micrograph with typical HLC textures of composition (wt%) [A] 69/20/11 (X400) and [B] 70/27/3 (X256). [C] D-NMR spectrum of a sample in 2H20of composition 69/20/11.
examined for two cases: (a) the addition of a cosurfactant to S2 which produces a transition from a lamellar to an inverse hexagonal mesophase, and (b) the use of a hydroalcoholic solvent for S3 (addition of cosolvent) which produces a transition from an inverse hexagonal to a lamellar mesophase. Addition of Cosurfactant to S2. The PFPE alcohol used as cosurfactant (Cos) was chosen according to previous findings on W/PFPE micro emulsion^.^^ A partial S2/Cos/ W phase diagram by optical microscopy made evident the formation of a wide hexagonal liquid crystal phase, in the surfactant-rich corner. A S2/ W sample at about 90110 weight ratio of cubic structure can incorporate up to 5 wt 5% of Cos without macroscopic modification. The onset of the formation of the H2 phase is found between 5 and lo%, and it is apparently pure up to 30 wt % Cos (see Figure 8A,B). A higher Cos concentrationleads to an isotropic solution in equilibrium with the H2 phase. The D-NMR spectra of a few samples of this system displayed a rather complicated pattern of not well resolved splittings (see Figure 8C). No attempt was made to evaluate to what extent the exchange of deuterons between the water and the alcohol -OH group affects the water splitting. Actually, theconcentration of the fluorinated alcohol never exceeded 20 wt %, and for a few samples it was possible to assign the two ammonium and water doublets. The observed quadrupolar splittings A of deuterated water, reported in Figure 7A vs the (S + Cos)/W molar ratio, agree with the occurrence of a H2 microstructure, as found for s3. Similarobservations on the modification induced by fluorinated or hydrogenated alcohols on the liquid crystalline phases of the lithium pentadecafluorooctanesulfonate (LiFOS)/water system have been recently r e ~ r t e d . 2 ~
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7597
PFPE Carboxylic Acids in Water
Besides all that, however, it is worth noting that the drastic change in the mesophase structures, at least in the high concentration range, must reflect a significant change in the packing features upon changing solvent polarity. The presence of ethanol causes the complete swelling of the hexagonal phase occurring in pure water, favoring the lamellar packing. Analogies can be roughly drawn between the phase behavior of S2 in water and S3in hydroalcoholicsolution. Theaggregation of surfactants in polar solventsother than water has been addressed by several group~.3~J* In particular, lyotropic phases with water soluble alkanols appear to be limited to LiFOS.29 Concluding Remarks on Packing Features
IB.
E l4 N-NMR
Figure 9. Three-component S 3 system, at 25 OC: S3/(W+EtOH). [A] Optical micrograph (X256) of S3 80 wt 9G in W/EtOH ( l / l by wt). [B] 2H NMR and [C] 14N NMR spectra of S3 64.3 wt % in 2H20/EtOH (1/1 by wt).
Addition of a Cosolvent to S3. S3 gives a H2 structure in water at almost all compositions, while a lamellar structure appears to form in hydroalcoholic solution (50 wt % ethanol in water). In the hydroalcoholicmedium, the solubility of S3 is widely increased, but no data on Krafft point and micellization are available yet. The hydroalcoholic solvent simply increases the solubility of S1 and S2, and the lamellar phases are found at higher surfactant concentration. For concentrationsof S3 above 70 wt %, monophasic samples with the typical La texture (Figure 9A) form, whereas below 70 wt % the La phase is in equilibrium with an isotropic solution. In this case, the D-NMR spectra of samplesprepared with EtOH/ D20 displayed an isotropic NMR signal superimposed on a very broad (about 500 Hz or more) signal which never revealed a resolved Pake doublet pattern typical of an oriented liquid crystalline phase. An example is shown in Figure 9B. A clear quadrupolar splitting of 1 KHz is instead observed in the 14N NMR spectrum (Figure 9C), although it cannot be fully understood why the ammonium splittings decrease more than 1 order of magnitude with respect to those observed in water (La or H2 phases; cf. Figure 7C). It can be hypothesized that the exchange of deuterons among water, ammonium, and ethanol along with the equilibria between bound and free species produced NMR spectra of no easy interpretation.
-
In summary, it has been shown (Table 1) that the La phases form with the shorter chain surfactants (S1 and S2) while the H2 phase forms with the surfactant with the highest MW (S3). In terms of the packing parameter "V/al", as defined by Ninham and Mitchell,32the large volume of the PFPE tails is expected to increase the packing ratio drastically. The packing of these surfactants is strongly influenced by the addition of a PFPE cosurfactant (La to H2 phase transition) or by the substitution of water with a hydroalcoholicsolution(H2to La phase transition). The trend toward the increase of V/al upon MW increase is quite obvious, but the microstructure features, observed at high surfactant concentration,need a further comment. If theevolution from a bicontinuous cubic V2 arrangement proposed for S2 (V/ a1 = 1) to a H2 structure found for S3 (V/al > 1) is consistent with an increase of the PFPE chain volume, the Occurrence of water droplets of spherical shape for S1 seems to be unlikely, unless other factors might occur. Indeed, a water self-diffusion around lo-" m2/sdues not necessarily imply spherical shape, but it might bedue to any nonsphericalsurfactant aggregate (disklike or vesicles bilayers) with V/al ranging between 0.5 and 1. Unambiguous interpretation of the experimental data could not always be attained, and further work is in progress to clarify the structural details of the various phases. Although the composition of the three PFPE surfactant mixtures certainly contributed to determine the local microstructure, it is worth stressing that the packing features in terms of V/al are now clearly outlined. Acknowledgment. M.M. thanks Ulf Olsson and Hakan Wennerstrom (Lund) for useful comments. A.C. thanks the EEC, Brite Euram Program for financial support of a part of this work. Italian CNR and MURST arealso thanked for financial support. Carlo De Rubeis, Cesare Morriconi, Luciano Mura, Giuseppe Paolucci, and Nicoletta Zinnarosu are gratefully acknowledged for their technical assistance. References and Notes (1) Kunieda, H.; Shinoda, K. J . Phys. Chem. lW6.80, 2468. (2) Shinoda, K.; Hato, M.; Hayashi, T. J . Phys. Chem. 1972, 76, 909. (3) Tiddy, G. J. T. J . Chem. Soc. Faraday Trans. 1 1W2,68,608. (4) Kekicheff, P.; Tiddy, G. J. T. J . Phys. Chem. 1989, 93, 2520. (5) Fontell, K.; Lindman, B. J. Phys. Chem. 1983.87, 3289. (6) Boden, N.; Clements, J.; Jolley, K. W.; Parker, D.; Smith, M. H. J. Chem. Phys. 1990, 93,9096. (7) Boden, N.; Jollcy, K. W.; Smith, M. H. J. Phys. Chem. 1993, 97, 7678. (8) Chittofrati, A.; Lenti, D.; Sanguineti, A.; V i m , M.; Gambi, C.; Senatra, D.; Zhou, Z. Colloids Surf.1989, 41, 45. (9) Martini, G.; Ottaviani, M. F.; Ristori, S.; Lenti, D.; Sanguineti, A. Colloids Sur/. 1990, 45, 177. (10) Gebel, G.; Ristori,S.;Loppinet, B.; Martini,G. J . Phys. Chem. 1993, 97, 8664. (1 1) Martini, G.; Ristori, S.; Gebel,G.; Chittofrati, A.; Vim, M. Appl.
Magn. Reson., in press. (12) Chittofrati, A.; Boselli, V.; V i m , M.; Friberg, S. E. J. Dispersion Sci. Technol., submitted for publication. (1 3) Monduzzi, M.; Chittofrati, A.; V i m , M. Langmuir 1992,8, 1278. (14) Fontell, K. J . Colloid Interface Sci. 1973, 44, 318. (1 5) Fontell, K.; Ceglie, A.; Lindman, B.; Ninham, B. W. Acta Chem. Scand. Ser. A 1986.40, 247.
7598 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 (16) Chittofrati, A.; Bmlli, V.; Monduzzi, M. In 205th ACS Narl. Meetins Denver. 1993: . ~~ , . . rI) . 239.(17) Sianesi, D.; Pasetti, A.; Fontanelli, R.; Bernardi, G. C.; Capriccio, G., Chim. I d . 1973,55,208. (18) Rosevear. F. B. J. Am. Oil Chem. Soc. 1954. 31. 628. (19) Mandel1,'L.; Fontell, K.; Ekwall, P. In Ordered Fluids and Liquid Crystals; Porter, R. S., Johnson, J. F., Eds.; American Chemical Society: Washington, DC, 1967; p 891. (20) Stilbs, P. Prog. Nucl. Magn. Reson. Specrrosc. 1987,19, 1. (21) Blackmore, E. S.;Tiddy, G. J. T. Liq. Crysr. 1990, 8, 131. (22) Bleasdale, T. A.; Tiddy, G. J. T. In Organized Solurions; Friberg, S.E., Lindman, B., Eds.; M. Dekker, Inc.: New York,1992; Vol. 44, p 125. (23) Wennerstrom, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974,6, 97. (24) Carvell, M.; Hall, D. G.; Lyle, I. G.; Tiddy, G. J. T. Faraday Discuss. Chem. SOC.1986,81,223.
.
Monduzzi et al. (25) Burkitt, S.J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987,265,619. (26) Harrison, W.J.; McDonald, M.P.; Tiddy, G. J. T. J. Phys. Chem. 1991,95, 4136. (27) Halle, B.; Wennerstrom, J. J . Chem. Phys. 1981,75, 1928. (28) Fontell, K.Colloid Polym. Sci. 1990,268, 264. (29) Ishikawa, A.; Tamori, K.;Esumi, K.;Meguro, K.J. Colloidlntcrjace Sci. 1992,151, 310. (30) Auvray, X.;Perche, T.; Anthore, R.; Anthore, C.; Petipas, C.; Rim, I.; Lattes, A. Longmuir 1991,7 , 2385. (31) Bergenstal, B.; Jonsson, A.; Sjoblom, J.; Stenius, P.; Warnheim, T. Colloid Polym. Sci. 1987,74, 108. (32) Mitchell, J. D.; Ninham, B. W. J. Chem. Soc. Faraday Trans. 2 1h1,77,601.
