ESR Probes for the Study of the Aggregational Behavior of

J. Phys. Chem. 1995,99, 17886-17890

17886

ESR Probes for the Study of the Aggregational Behavior of Perfluoropolyether Surfactants and of the Physical Status of Interlamellar Water. 2. Mn(I1) Probe Sandra Ristori, Elena Ottomani, Maurizio Romanelli, and Giacomo Martini* Dipartimento di Chimica, Universita' di Firenze, 50121 Florence, Italy Received: April 5, 1995; In Final Form: September 20, 1995@

The physical status of water in self-assembling aggregates (lamellar phases mainly) of the perfluoropolyether (PFPE) surfactant was investigated by the use of the Mn2+ (S = 5/2, I = 9 2 ) ion as a paramagnetic probe. From the ESR line shape as a function of temperature and of surfactant content, the dynamic parameters were obtained. The results are compared with those previously obtained by using nitroxides and Cu2+ ions and discussed in terms of the structural features derived from previous IR and scattering data. The rheological properties of water confined between lamellae of PFPE are differentiated, depending on the distance from the charged surface, as (i) water directly bound to the charged surface of the lamellae; (ii) water layers not interacting with the lamellar surface, with a motional behavior resembling that of free water, (iii) water layers with a motional behavior intermediate between (i) and (ii). This picture agrees with the generally observed properties of water confined in restricted spaces of inorganic, organic, and biological systems.

Introduction In the past few years we have studied the behavior in solution of typical fluorosurfactants with similar properties such as the branched-chain perfluoropolyether anion, 1 (henceforth simply indicated as PFPE), and the straight-chain perfluoroalkanoate anions, 2 (PFA), particularly the perfluorooctanoate CF,( OCF,CF), -0 -CF2COO-

I

CF3

1, PFPE

Cu(II), namely Mn(I1) ion. The solution ESR spectra of this 6S ion exhibit a marked dependence on the environmental conditions that may integrate the results obtained for the 2D Cu(I1) ion. The main electron spin relaxation mechanism for Mn(I1) ion is the time modulation of the zero-field splitting (ZFS) term, S*D.S, where D is a traceless tensor. Both molecular reorientation and distortion caused by collisions with solvent or ligand molecules cause spin relaxation. Analytical equations for the expected transitions cannot be obtained;** an averaged longitudinal relaxation rate is given by the relation23

TI&'

= (2A2/50)[4S(S

+ 1) - 31 x

CF3(CF,),COO-

2, PFA (n 2 6) Perfluorinated surfactants show several interesting properties, such as chemical and thermal resistance, biological inertness, low surface tension, and high oxygen permeability.*-I0 Moreover, if compared with hydrogenated surfactants, they exhibit a remarkable ability in forming aggregates where low surfaces curvatures are required, mostly vesicles and lamellar (lyotropic smectic) phases. Depending on concentration, temperature, and structure, perfluorinated compounds such as 1 and 2 also form micellar phases. The structure of these aggregates has been studied mainly by direct (small-angle neutron and X-ray scattering, S A N S and SAXS, respectively, and nuclear magnetic resonance techniques) and indirect (electron spin resonance in continuous and in pulsed wave method^).'-^,^-^,^ Small and large nitroxide radicals have been extensively used as the probes for obtaining dynamic and structural features by the indirect methods. ,4-6 Very recently, we made use of the Cu(I1) probe for a continuous-wave ESR investigation of both aggregation properties and counterion effects in PFPE, 1, solutions at different concentrations. l 7 We report here the natural extension of that work. We use a spin probe with different magnetic properties with respect to

* Author to whom correspondence @

should be addressed. Abstract published in Advance ACS Abstracts, November 15, 1995.

0022-3654/95/2099- 17886$09.00/0

where A2 is the inner product of the ZFS tensor, wo is the electron spin resonance frequency, and tcis the correlation time for the electron-lattice interaction, which can be either the rotational correlation time or the correlation time for any other fluctuation of the ZFS tensor.22 Since the ESR sextet of Mn2+ ion is extremely sensitive to symmetry changes around the paramagnetic center, most of the ESR work with Mn(I1) in fluid solution has been carried out to study the ionic interactions in pure inorganic systems and in Mn-containing soluble material^.^^-*^ It is accepted that, in the fast exchange domain, changes in the outer solvation sphere simply broaden the line width of the hyperfine components and the evaluation of the broadening has been used to extract dynamic information on the systems under study. This is well documented in papers and reviews.'9-21 Replacement of inner solvation molecules induces ZFS changes largely enough to cause the ESR spectra to become unobservable around room temperature, so that a fraction of Mn2+ ions may escape detection.27 This particular behavior is therefore suitable to follow the dynamic features of aggregated systems such as micellar and lamellar phases of perfluorinated surfactants, whose characteristics strongly depend on the water content and on the thickness of the interlayer water. The physical status of water in these self-assembling aggregates is a task of major interest when these systems are studied. Because of its peculiar ESR activity, Mn(I1) can be helpful in solving this problem, as it is largely documented in the literature, when the study of water finely dispersed in 0 1995 American Chemical Society

J. Phys. Chem., Vol. 99,No. 51, 1995 17887

Perfluoropolyether Surfactants restricted space is approached by magnetic resonance techn i q u e ~ ? ~In - ~this ~ work we show that the rheological properties of water confined between lamellae of PFPE are differentiated depending on the distance from the charged lamellar surface and three motional domains are recognized: (i) water bound directly to the polar heads of the surfactant molecules forming the self-assembled aggregate; (ii) water layers not interacting with the surface, with a motional behavior very similar to that of free water, (iii) water layers with a motion intermediate to those of (i) and (ii). The above findings agree wth, and integrate, all of the results obtained in the studies carried out on this system with other paramagnetic probes and with other techniques, including SAXS, SANS,596and infrared spectros~opies.~

Mn'+ IO'M in H,O

t

104

Experimental Section The PFPE surfactant 1 with equivalent weight 664 (n = 3) was obtained in the acid form as almost monodisperse material (dispersivity I5%, from chromatographic analysis; that is, the 664 EW material represented a fraction higher than 95% of the total) as intermediate in the synthesis of PFPE polymer^.^^,^^ Its ammonium salt was obtained after exchange with a large excess (molar ratio PFPE/NH3 = 1/10) of aqueous NH3. The solid salt was obtained after removing the ammonia excess and drying under vacuum at 60 "C overnight. The Mn(II) ions were introduced under the form of Mn(C104)2.6H20 (Fluka), because no salt precipitation could lead to the Mn(PFPE)2 formation, contrary to the case of the Cu(I1) ion. Lamellar and micellar systems were prepared by mixing Mn(C104)2 and NI-&-PFPE solutions in the appropriate amounts in order to obtain NH4/Mn atomic ratio >50/1, except when stated (see, for instance, Figure 1 and its discussion). In no case was the total Mn(II) concentration over 3.6 x lo-' mol/ L, so that spin-spin effects did not alter the ESR line shape. A two-day equilibration was necessary to reach homogeneous and stable systems, as is shown for PFPE lamellar phases by SANS and SAXS measurement^.^.^ This is, moreover, quite typical for several surfactants. The samples were sealed in 1 mm inner diameter quartz capillaries to be used in the ESR experiments. These were carried out with a Bruker 200D spectrometer operating at X-band (9.5 GHz), equipped with the Stelar data handling system and with the Bruker ST100-700 variabletemperature assembly. Results and Discussion Aqueous solution of Mn2+ at concentrations ranging from 5 to 3.6 x m o l 5 to which the stoichiometric amount of NI&-PFPE was added gave the typical six-line ESR spectrum of Mn(C104)~~olutions?~ with the same line shape and the same line width within the experimental uncertainty ( f 0 . 1 mT). Because of the invariance of the line width in this concentration range, the simple peak heights were checked to determine the free Mn(I1) contribution. This meant that Mn(H20)62+was the ionic form prevailing in the presence of the above concentrations of NI&-PFPE. The ESR spectrum of hexaaquomanganese(I1) was also observed at PFPE concentrations higher than the stoichiometric value. Figure 1 shows the peak-to-peak distance, AH, of the fourth hypefine components (mI = 1/2) of the ESR spectrum of a lod3m o l 5 solution Mn2+ as a function of [PFPE] at T = 297 K. The choice of the fourth hpf component was dictated by the fact that this line is the less affected by second-order effect^.'^-^^ Although the critical micellar concentration, cmc, of PFPE in pure water is 3 x the line width of the ESR spectrum was the same as that of Mn(H20)62+in pure x

1

io4

10-1

io*

io"

io0

[NH,PkPEl, movLive

Figure 1. Peak-to-peak distance of the fourth hyperfine line of the ESR spectrum of a lo-' m o m aqueous solution of MnZf as a function of PFPE concentration.

water at the same temperature up to [PFPE] = 2 x mom (the magnetic parameters, i.e., the g factor and the hyperfine coupling constant, were also the same). On the other hand, a sudden line broadening was observed at [PFPE] = 3 x man, and the width slightly decreased with increasing the surfactant concentration. Although only dynamic features could be derived from these results, we might suggest that at [PFPE] < 3 x mom, where the system consists of micellar aggregates, Mn(II) ions were confined in a water phase with properties typical of free water, that is, same mobility, structure, and viscosity. The Mn2+-micelle interactions, if any, would occur at the level of the inner solvation sphere whose result should be an intensity decrease of the absorption. However, no signal intensity decrease was observed in the limits of the experimental error so that interaction in both inner and outer solvation spheres could be ruled out. The sudden increase of the line width at [PFPE] re 3 x m o m can be due either to the increase of the microviscosity around the paramagnetic ion and/or to the increase of ZFS as resulting from interaction in the outer sphere with the polar heads of the aggregates. From previous ~ t u d i e s ~changes * ~ * ~ in the aggregate shape have been evidenced as a function of the surfactant concentration. Surface tension2 and nitroxide ESR measurements2s3suggested these changes to occur at [PFPE] > 2 x mol5. At this concentration no changes in the spectral ESR features of the Mn(I1) sextet were observed. This can be attributed to the different location of the ESR detected Mn(II) ions that resided in the water layers between aggregates, in contrast with the TMT+(4-trimethylammonium-2,2,6,6-tetramethylpiperidine- 1-0xyl) nitroxide directly bound to the carboxylate heads of the aggregate surface. A further evolution of the system toward a lamellar topology has been suggested on the basis of small-angle scattering5 and ESR TMT+ results3 at higher PFPE concentrations ( 1 2 x mom). The observed increased line width of Mn(I1) at [PFPE] = 3 x m o l 5 could be related to such a change. A more quantitative analysis requires the estimation of the parameters determining the h4n(II) line width i.e., the correlation time rc and the inner product Az of the ZFS tensor. Usually this is accomplished by comparing simulated and experimental spectra. It is known that AH of the ESR spectra of 6S ions exhibits a maximum value as a function of temperature and this maximum corresponds to the condition wo2r> = 1.36,37This maximum is quite often observed for Mn(II) solutions adsorbed onto porous supports, where the solvent, typically water, does not crystallize, and it is used to obtain the value of the correlation

17888 J. Phys. Chem., Vol. 99, No. 51, 1995

Ristori et al. PFPE 25% W / W

I

56

-

52

~n~+2.10-~~

PFPE 83% wiw in 40

Md' 3 3 \ 1O'M

UI

40

PFPE 50% W/W Mn2'2.1O-'M

40

1

364 240

PFPE 83% W/W Mn2' 3.3.10"M

,

,

I

260

280

I

'

300

320

340

T(K)

PFPE 63% W/W Mn2' 3.3.lom3 M

Figure 2. Dependence on the temperature of the peak-to-peak distance of the fourth hyperfine line of the ESR spectrum of Mn2+ in a 83% (w/w) aqueous solution of PFPE.

T9 267 K A,

time otherwise ~ n o b t a i n a b l e . ~Figure ~ , ~ ~2 shows the temperature dependence of the peak-to-peak distance of the fourth line V of Mn2+in a very concentrated waterPFPE binary system (83% 1 Z T w/w of PFPE). Although the large uncertainty of this kind of Figure 3. Experimental (full lines) and simulated (dashed lines) ESR measurement, the maximum AH was observed at 267(f3) K. spectra at 297 K (lines a, b, and c) and at 267 K (line 6 ) of Mn2+ in At this temperature z, = w0-I = 1.7 x lo-" s and this value pure water and in PFPWwater solutions. was therefore assumed as the basis for the calculation of the TABLE 1: Best-Fit Parameters for the Simulation of the temperature dependence of the correlation time. ESR Spectra of Mu2+in Pure Water and in PFPE Lamellar The temperature of maximum line width depended on the Phases surfactant content in the system, being higher for higher [PFPE]. system temp, K rc, ps ZFS, cm-' For instance, at [PFPE] = 90% w/w the maximum AH = 5.3 f 0.5 mT occurred at 285 & 8 K. This is in agreement with Mn(H20)62f/H20 297 3 0.0 18 Mn2+/PFPE25% 297 4 0.024 an increasing immobilization of Mn2+ due to the decrease of Mn2+/PFPE50% 297 6 0.024 water content. However, the quality of the ESR spectra, due Mn2+/PFPE 83% 267 17 0.024 to the low water content and to the consequent low Mn(I1) Mn2+/PFPE83% 297 10 0.024 concentration, prevented any accurate simulations. When feasible, a spectral simulation procedure was followed same activation energy as free water. This fact perfectly agreed to obtain the correlation times at different temperatures and the with the results of a recent IR study on the same systems that procedure was based on the program written by Luckhurst and indicate that about 50% of the water behaves as free water even P e d ~ l l i and ~ ~ modified ,~~ by Romanelli and B u r l a m a c ~ h i . ~ ~ * in ~ ~the highly concentrated PFPEIwater binary systems,' this is This consists in UP to [PFPE] % 80-85% W/W. (i) calculation of the position of the 30 transitions arising The calculated ZFS value in the 83% w/w system markedly from S = 9 2 and I = 5/2 on the basis of the perturbation theory, increased as a consequence of the structural variations from Mnwith the hyperfine interaction assumed as second order pertur(H20),j2+in free water to M n ( H ~ 0 ) 6 ~in+lamellar systems. This bation with respect to the electronic Zeeman interaction; finding was not surprising if the results obtained with Cu(ii) writing of the Redfield relaxation matrix, through the (H20)62+are considered," for which a distortion toward a square spectral densities at the frequencies 0, wo, and 2w0, for each of planar symmetry is assumed in interlamellar water. In Mnthe five electron transitions coupled to the other transitions (H20k2+the lamellar surface effects could induce perturbation which contribute to alter the population of the spin level on the outer solvation sphere with the ZFS values increased interested; from 0.018 to 0.024 cm-'. (iii) diagonalization of the relaxation matrix for each transition A careful comparison between simulated and experimental in a chosen magnetic field range; eigenvalues give line widths spectra revealed that the agreement was not as good as usually and eigenvectors give the related intensities. obtained in regular solution of Mn(I1). These discrepancies Figure 3 gives some examples of the fit between experimental could be attributed to weak background signals overlapped to and simulated spectra. Table 1 reports zc and ZFS values used the main sextet absorption. This effect is also observed in a for the fit. in more evident way in several examples of heterogeneous systems, where water is finely dispersed in For [PFPE] = 83% w/w, the calculated correlation times porous supports. It has been attributed t o the occurrence of agreed with those evaluated from the temperature dependence on the basis of zc = 1.7 x lo-" s at 267 K and by using the a distribution of correlation times because of a mobility activation energy for the viscous process of bulk water (E, = gradien~*~-~O This feature of the ESR spectrum of the Mn(I1) probe has also been more formally treated in terms of a 4.5 k J / m ~ l ) the ; ~ ~same result is obtained by using the Cu2+ probe;" this meant that the interlamellar water had almost the distribution function of the relaxing site^.^^^^^ The presence of ~~

Perfluoropolyether Surfactants PFPE/H.,O 83%

J. Phys. Chem., Vol. 99, No. 51, 1995 11889 W/W

Mn(CI0,)?=3.3xlU3M

Y T-257 K

W

(ii) Water layers not interacting with the surface, where the Mn2+ behavior is very similar to that in free water; this is the predominant water at low surfactant concentration, where the bound water fraction is negligible with respect to the free water; (iii) Intermediate water layers, where the surface interactions are effective in modifying the water structure, which prevents the usual liquid-solid phase transition to be observed42 and induce the symmetry distortion in the M I I ( H ~ O ) ~outer ~ + sphere which causes larger ZFS values. The resultant line broadening depends on the overall thickness of this water layers, that is on the ratio between free and bound water. The main scope of this work was therefore to study the behavior of type (ii) and (iii) water.

Conclusions The representation of the interlamellar water in PFPE lamellar phases as parted among three different motional domains had been derived from the ESR data of small and large nitroxides and of Cu(I1) probe and from IR results as coupled with structural data given by S A N S and SAXS patterns. Here we I further prove by the use of the Mn(I1) probe that this picture 03 0 32 0 34 0 34 perfectly agrees with the general behavior of water in restricted Magnetic field (mT) spaces of inorganic, organic and biological systems. This Figure 4. ESR spectra of Mn2+ in the PFPJYwater (83% w/w) lamellar happens, for instance, for water dispersed in silica gels, zeolites, phase at different temperatures. aluminas, et^.;^^,^^-^^ in some of these cases the propagation of the surface effects is ill-defined and variable thicknesses up such a distribution could be the reason of the ESR spectra 100 nm have been suggested. Many cases have been to “anomalies” in the present system, that is, spectral wings proposed in which this propagation extends in the range 4-8 flanking the first and the sixth lines, which are not correctly nm, which is in line with the thickness of the interlamellar water reproduced by simulation with single correlation time and ZFS layer studied in this work. A better agreement exists on the values. number of water layers involved in the region (i), which results The spectral simulations of the 25 and 50% w/w systems to be 2-3, independent of the nature of the surface. (Figure 3a,b) and the best-fit data shown in Table 1 require The main difference between the systems studied in this work some comments. In these cases the water content was so high and those reported in the above references resides in the high that the observation of maximum AH was prevented because fluctuational mobility of the lamellar structures in the fluid of the freezing of water that was not surface-bonded. Thus a environment. This induces a higher uncertainty of both direcreference value of the ZFS was unobtainable. In order to obtain tionality of the surface interactions and number of the layers semiquantitative values of rc we used ZFS = 0.024 cm-’ involved in these interactions. From this point of view, the calculated for the 83% w/w system, which certainly is a rough systems more similar to the PFPE lamellar phase are some approximation. Anyway the obtained values of z, were in line biological system^.^'-^^ It has indeed been proposed that almost with those expected from the behavior of TMT+ and C U ( I I ) . ~ ~ ’ ~ the same law governs the freezing effects of water either in mol/ Figure 4 shows the ESR spectra of Mn2+ (3.3 x inorganic materials or in biological organisms.54 For example, L) as a function of decreasing temperature in the waterPFPE two different phases have been suggested for water in the binary system (83% w/w of PFPE). Liquidlike spectra were oriented system of brain phospholipid^^^.^^ with an oriented observed down to 250 K, with a progressive variation toward a water thickness of 0.22 nm. This observation, older than 30 solidlike spectrum from 250 K downward. Forbidden transitions years, has received further proofs. The different structure of (Am1 f 0) appeared as is typical of M I I ( H ~ O ) ~in~ +a glassy water has therefore to be considered as an essential feature in matrix.41 This was a further proof of lack of crystallization of interlamellar water, which is also observed with C U ( H ~ O ) ~ ~ + . ’the ~ formation and stability of aggregated phases of proteins, lipids, and nucleic acids.54 Very similar results were given by mixtures at lower [PFPE], the main differences being in the tempertures where the lineAcknowledgment. Thanks are due to Italian Minister0 dell’ shape effects were observed. Universita’ e della Ricerca Scientifica e Tecnologica (MURST) On the basis of the above results and taking into account and to Consiglio Nazionale delle Ricerche for financial support. those obtained with the Cu(I1) probei7 and with IR spectrosThe authors are also indebted to Ausimont S.p.a. for kindly copy,’ we may summarize that three different domains of water providing samples of PFPE. are recognized in the lamellar phases of PFPE surfactant: (i) Water bound directly to the Stem layer near the lamellar References and Notes surface; from IR data7 and from other spin probes such as T M F radical3 and Cu2+ ionI7 it results that the water layers in this (1) Ristori, S.; Martini, G. Langmuir 1992,8, 1937. immobilized region are 2-3. Their structure is strongly affected (2) Ristori, S.; Ottaviani, M. F.; Lenti, D.; Martini, G. Langmuir 1991, by the hydrogen bond with the surface, which increases the 7, 1958. ( 3 ) Martini, G.;Ottaviani, M. F.; Ristori, S . ; Lenti, D.; Sanguineti, A. vibrational freedom degrees as a consequence of the decrease Colloids SUI$ 1990,45, 177. in the coordination number. Mn2+ ions in this region are (4)Romanelli, M.; Ristori, S.; Martini, G.; Kang, Y .-S.; Kevan, L. J . strongly perturbed in the inner sphere and they are ESR silent Phys. Chem. 1994,98, 2125. because of the resultant very large ZFS, as it is reported in other (5) Gebel, G.; Ristori, S.; Loppinet, B.; Martini, G. J . Phys. Chem. sy~tems.30,~~,3~ 1993,97, 8664.

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