J. Phys. Chem. 1995, 99, 1120-1123
1120
Infrared Investigation of the Water Structure in PerfluoropolyetherIWater System M. D’Angelo,? G. Martini,*J G. Onori,+S. Ristori,’ and A. Santuccit Dipartimento di Fisica, Universith di Perugia, 06100 Perugia, Italy, and Dipartimento di Chimica, Universith di Firenze, 50121 Firenze, Italy Received: May 9, 1994; In Final Form: September 6, I994@
The structure of water in perfluoropolyether (PFPE) ammonium saldwater systems and in PFPE surfactand PFPE oiVwater ternary systems has been studied as a function of the waterPFPE ratio (W) by using the absorption IR due to 0 - D stretching modes in the 2800-2000 cm-I frequency range. The results show that the IR spectra can be expressed as the sum of contributions from interfacial and bulklike water. The relative amounts were evaluated as a function of W in the 3.8-102 range. The data show that a pool of bulklike water develops in these systems as the water amount increases and suggest the simultaneous presence of two water regions with different structural and dynamical properties.
Introduction It has already been reported by many authors that water in restricted spaces or water-close to ions or biological membranes or at the protein surfaces exhibits a behavior markedly different from that of bulk Several of these studies refer to the water confined in reverse micelle^.^.^^'^-^^ A reverse micelle is an aggregate of surfactant formed in a nonpolar solvent, and it can be considered as a very simple model of bioaggregates (membranes, cells). The ability of reverse micelles to dissolve water in their core provides a unique opportunity to study the properties of water aggregates close to ionic centers (polar head groups and the corresponding counterions of the surfactant) without interference from large amounts of bulk water. In previous papers, the use of infrared spectroscopy as a sensitive tool for the study of water into bis(2-ethylhexy1)sodium sulfosuccinate (AOT) reverse micelles has been de~cribed.~.’ IR spectroscopy is a noninvasive, functional group-selective technique. It has been proved to be sensitive to the chemical environment so that it can be extensively used for the detection and characterization of hydrogen bonding.13 Due to the technical improvement of infrared spectrometers over the last decades, the detection has become more accurate and sensitive. IR experiments involve a measurement on a time scale s) so short that, in principle, any different short-lived species may be recognized and studied separately. It was recently shown that the spectrum in the 0 - H stretching region in the water/AOT/CCl4 system can be expressed as a sum of two distinct contributions associated with interfacial and bulklike water, respectively, thus suggesting the use of the IR measurements to identify interfacial water as vibrationally different from bulk water.6 In this paper we present an IR investigation on the structure of water in perfluoropolyether ammonium salt (PFPE-NH4) water systems. The main advantage of these systems is that their supramolecular structure has been well characterized by different physical technique^'^-^^ and can be easily changed by simply changing the W = [water]/[surfactant] molar ratio. In the PFPE/water systems, the water domains are sandwichlike regions confined between PFPE bilayers. Upon water dilution the only variation which the structure undergoes is an
’ Universiti di Perugia.
* Universiti di Firenze. @
Abstract published in Advance ACS Abstrucrs, November 1, 1994.
increase in the interlamellar water thickness, the interface remaining flat, though highly flexible, with respect to undulations and other distortions. This study has also been carried out on N b - P F P E surfactand PFPE oil/water ternary systems.
Experimental Section The perfluorinated surfactant used in this work has the following chemical structure, CF~(OCFPCF)~OCF~COO-NH~+
I CF3
with equivalent weight EW = 681 and a narrow polydispersity ( ~ 5 %from gas chromatographic analysis). It is produced in the Ausimont S.p.A. research under the form of acyl fluoride and subsequently hydrolyzed and treated with a large excess of aqueous ammonia solution to obtain the ammonium salt. The corresponding PFPE fluorocarbon (henceforth simply indicated with the term “oil”) has the same chain structure of the surfactant, with no polar head, EW = 944, and a higher degree of polydispersity ( ~ 2 0 % ) . D20 (Carlo Erba, 99.8% purity) was used to prepare all the samples. IR spectra were recorded by means of a Shimadzu Model 470 infrared spectrophotometer equipped with a demountable cell and CaF2 windows. The employed optical path lengths ranged between 6 and 25 mm. The molar extinction coefficients were calculated by using the expression E = A/(cd), where A is the absorbance, c the water concentration in molesfliter, and d the cell depth in centimeters.
Results and Discussion PFPE surfactants form lamellar phases in aqueous environment in a wide range of temperature and composition.14,15Table 1 collects the structural data on the investigated systems obtained from scattering techniques (small angle scattering of both X-ray together with and neutrons, SAXS and SANS, re~pectively),’~ the evaluated mean number of water layers between lamellae. For comparison, we also report the composition of two PFPE surfactant/PFPE oiVwater ternary systems in the isotropic domain of the phase diagram.I6 The above ternary systems are microemulsions. Preliminary scattering studies on these mi-
0022-3654/95/2099-1120$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 4, 1995 1121
Water Structure in PerfluoropolyetherrWater System
TABLE 1: Structural Data of the Investigated Systems water layer mean number system composition (w/w) w thickness (A) of water layers D20 = 10% N&-PFPE = 90% 3.8 5.7 2.0 DzO = 17% N&-PFPE = 83% 7 10.5 3.7 D20 = 21% N&-PFPE = 79% 9 12.3 4.4 D2O = 25% N&-PFPE = 75% 11.3 15 5.4 D20 = 38% 20.8 41 15 Nb-PFPE = 62% D2O = 63% N&-PFPE = 37% 58 100 36 Dz0 = 75% 102 135 48 N&-PFPE = 25% Dz0 = 3.3% NH4-PFPE = 22.6% 4.9 PFPE oil = 74.1%
I
______
-
4000
Wr40.1
3500
3000
v
2500
2000
(cm-’1
Figure 1. Infrared spectrum of the PFPED20 system at two selected values of the molar ratio W (---) W = 3.8; (-) W = 40.7.
DzO = 2.8% N&-PFPE = 11.9% PFPE oil = 85.3% croemulsions showed microscopically structured water domains separated from the oil phase by a surfactant monolayer.20 In addition, the waterhrfactant interface in the samples we have chosen for the investigation should not be very different from that of the simpler binary systems. Figure 1 shows the IR spectrum of the PFPE/D;?O lamellar systems at two selected values of the molar ratio W (W = 3.8 and 40.7). Similar spectra for microemulsions were obtained. The spectra were recorded in the 4000-2000 cm-’ range, where the absorptions due to 0 - D stretching of D20 and to vibrational modes of PFPE appear. IR measurements were done in D20 to avoid the spectral complications due to the PFPE contribution at approximately 3000 cm-l, which is superimposed on the 0 - H band of water. Although marked variations of some parameters in the phase diagram of polymer blends have been detected as a consequence of the D/H substitution in the polymer chains,21 the use of D20 instead of H20 in the perfluorosurfactanvwater systems induces changes which are typically very small and tend to vanish at high concentration.22 The same observation has been made in a recent work on ammonium perfluorooctanoate concentrated micellar solutions.23 The PFPE absorption disappeared after substitution of ammonium with K+ or Li’ (work in progress); therefore, it could be assigned to ammonium vibrational modes. Contributions due to vibrational modes of D20 and PFPE were well characterized, because they occurred in distinct spectral ranges. In the 2800-2000 cm-’ range the absorptions due to stretching vibrations of D20 appeared. Pure NH4-PFPE showed absorptions at 3400-2700 cm-’; so, partial overlapping in the region 2800-2700 cm-’ is present. However, the contribution of the PFPE absorption in this region was significant in the spectra of the samples with lowest W (3.8 and 4.9) only. The IR spectrum of water in the PFPED20 system (or PFPW DzO/oil) was significantly different from that of pure DzO, shown in Figure 2. This indicated that the water confined in the PFPE/water lamellar systems and in the PFPE microemulsions lacked the normal hydrogen-bond structure present in bulk water. However, the total peak area of the 0-D stretching band of water was found to increase linearly with water content, as predicted by Beer’s law with E = (21 f 2) x lo3 L mol-’ cm-*.
w.3.a
60
T
E
-? 2w
40
20
0 2800
2600
2400
2200
2000
v (cm-’) Figure 2. Molar extinction coefficient of 0-D stretching absorption bands for pure water (-), bound water (- * -), water in the PFPEDlO system at W = 20.8 (- - -), and W = 7 (..a).
Figure 2 shows the molar extinction coefficient of 0 - D stretching absorption bands for pure water and for water in PFPE/D20 systems at selected values of W. In Figure 2 the absorption attributed to “bound water” is also reported. The meaning of the term “bound water” and the procedure of calculation of its spectrum will be discussed later. The large bandwidth of pure water could be attributed to the range of environments within the liquid, with the more strongly coordinated oscillators absorbing at lower frequencies.6 There was an increase of the intensity of the low-frequency part of the 0-D absorption band as the water content was reduced. This behavior was qualitatively similar to that observed after lowering the temperature of the pure water sample.24 Since the stretching frequency reflects the strength of the interactions between neighboring molecules, the observed behavior (Figure 2 ) could be ascribed to strong interactions of water molecules with the N&+ counterions andor with the ionic head groups of the PFPE molecules. The spectra clearly show the existence of an isosbestic point at V = 2417 f 2 cm-’, and such a circumstance is a strong argument in favor of the existence of a two-site equilibrium. A two-site equilibrium has been recognized by IR6 and by other technique^^^^^^^^ in many cases of water dispersed in restricted spaces, and the distinction has been made between bound and bulklike water. Thus, the spectra can be decomposed as the sum of two concentration independent contributions, corresponding to bound and bulk water, respec-
D’Angelo et a1.
1122 J. Phys. Chem., Vol. 99, No. 4, 1995
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i
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40
-B
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-I
u4
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w 20
20
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0 2600
2400
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zoo0
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2100
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2400
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W = 20.8
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E 9 40
sc
-I
20
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2600
2400
2200
2000
2800
2600
2400
2200
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P (cm“) P (cm”) Figure 3. Molar extinction coefficient of 0 - D stretching absorption bands for water in the PFPE/D2O system at selected values of the molar ratio W (-). Also reported are the two contributions &bulk (- - -) and (1 - X)Ebaund) (. *a).
tively. Following a procedure employed in the case of AOT micro emulsion^,^^^^ we expressed a generic spectrum c(b,W) as the linear combination
where the coefficient X( W)is concentration (but not frequency) dependent. €bulk is the spectrum of pure water and €bound the spectrum of bound water. X(W) therefore represents the relative amount of free water, whereas [ 1 - X(u/?] is the relative amount of bound water. €bound has been determined as it follows: (i) The pure water spectrum was subtracted with an increasing contribution from a given experimental spectrum c(G,W); this subtraction was cut at a value of Xcbulk for which a negative absorption appeared in some region of the spectrum. (ii) Then €bound was calculated as
We checked that the €bound values obtained in this way did not depend on the initially selected spectrum. Obviously, at the highest W values the calculation of the bound water spectrum was affected by large errors because the spectra were more and more similar to that of pure water. On the other hand, at the lowest W values some uncertainty arose from the partial overlap of the 0 - D and PFPE absorption bands. For these reasons we chose to illustrate this calculation procedure for the W = 7 sample, and this allowed us to obtain the €bound values shown in the Figure 2 (dot-dashed line) as the bound water spectrum.
In Figure 2 the bound water spectrum was not significantly different from the one obtained for the sample at the lowest water content (W = 3.8). Note that scattering data relative to this sample indicate a mean number of water layers equal to 2 (see Table 1). In order to quantify the concentration dependence of the two different water structures, any IR spectrum c(V,W) was partitioned into the two contributions previously attributed to the bound and bulk water fractions, according to eq 1. The fitting was then carried out with a computer application of the Marquardt’s algorithm.28 Some examples of the fitting results for selected values of W are shown in Figures 3 and 4. Experimental and calculated spectra were practically not distinguishable and the residuals were in the experimental errors and randomly distributed. We checked that the fittings, even if performed on a restricted frequency range of the spectrum c(V,W), always gave the same value for the X parameter. Figure 5 reports the values of Xbound = 1 - X as derived from the fitting. The number of bound water molecules per polar head was calculated by using the relation Wbound = XboundW. The computed values in the W 0-25 range have been plotted against W, as shown in Figure 6. It is seen from the figure that Wbound remained constant and equal to 3.9 f 0.2. This observation meant that in the W = 3.8 sample all the interlamellar water molecules are involved as bound water molecules and that the structure of the bound water layers did not appreciably change with the increase of the water layers. It is worth noting that the same result was obtained by fitting Xbund values reported in Figure 5 to the relation Xbound = Wbound/W (continuous line in Figure 5). The Xbound values depended on the W ratio only, and the data obtained from microemulsions
J. Phys. Chem., Vol. 99, No. 4, 1995 1123
Water Structure in PeffluoropolyetherNater System
60 I
E
0 3000
2800
2600 P
2400
2200
2000
(cm-'1
Figure 4. Molar extinction coefficient of 0-D stretching absorption bands for water in PFPE microemulsions at W = 4.9 (-). The analysis of this band in terms of two contributions XcbUu;(- - -) and (1 - X)chund has been limited to the 2600-2000 cm-' frequency range (see text).
e**)
X
z 2
Acknowledgment. This work was supported by the Istituto Nazionale di Fisica della Materia (INFM), Consiglio Nazionale delle Ricerche (CNR), and Minister0 della Universitfi e della Ricerca Scientifica e Tecnologica (MURST). Thanks are also due to Ausimont S.p.A., Bollate (Milano), for supplying the peffluoropolyethers.
0.6 0.4 0.2
-
0.0 0
bound region, distinct bands can be assigned to the two populations of water. It was concluded from the results that the two first water layers located near the lamellar interface are composed of bound water molecules. Similar results have been obtained from the IR study of AOT microemulsions.6 The IR absorptions in these systems appear to be sensitive only to the first hydration shell. A comparison between pure water and bound water spectra showed an enhancement of the low-frequency part of the 0-D absorption band for bound water. This fact indicates the presence of strong hydrogen bonds of water molecules with the NH4+ counterions andor with the ionic head group of PFPE molecules. It should be noted that this result is different from that obtained for the AOT/CCk/H20 system recently studied.6 In AOT microemulsions an increase of the intensity of the highfrequency component of the 0 - H band with the decrease of W is found. So, in this case, IR results suggest a minimal amount of hydrogen bonding occuring in the micellar water phase at low water content. In order to establish the origin of this difference, further measurements performed by changing head groups, counterions, and confined geometry are in progress.
,L,References and Notes
20
60
40
80
100
120
W Figure 5. Relative amount of bonded water as a function of W. (0) PFPE/D*O system; (0)PFPE microemulsions.
0
I
I
10
20
W Figure 6. Number of bonded water molecules for the PFF'Emolecule as a function of W. (0)PFF'E/DzO system; (0)PFPE microemulsions;
(- - -) mean value of
!#bound.
(W = 4.9 and 8 in Figures 5 and 6 ) agreed well with the corresponding ones for the lamellar systems.
Conclusions IR measurements in the frequency region of 0-D (or 0-H) stretching modes are shown to be a valid tool for understanding the water behavior in the PFF'E/water systems. The two water regions, namely, bound and bulklike water, existed simultaneously in the water confined between PFPE bilayers. Since IR experiments involve a measurement on a time scale s) shorter than the residence time of water molecules in the
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