Langmuir 1989,5, 1037-1043 Table I. Comparison of Spectrophotometric"and Kinetic Parametersbof Iodine-Egg Lecithin Complex in Water and Cyclohexane solvent: cycloparameters solvenkwater hexane ACT, nm 293 295 ce, m2 mol-' 7250 1174 K,, mol-' dm3 1.60 x 103 7.35 x 103 kinetic k l , min-' (at 28 "C) 6.22 X 1.64 X lo-' kl, min-' (at 35 " C ) 8.01 X 2.93 X E,,kJ 24.10 55.44 6.20 x 10-4 K,, mol dm-3 V,, min-' 1.92 References 1 and 2. Data are average of 5-6 different experimental values with a mean deviation of 5 % .
formed in water as well as in cyclohexane, and the values of V , and K , for the CT complex in water only. Our results show that the strength of the lecithin-iodine interaction is more in the case of cyclohexane than in water, reasons for which have been given in detail in ref 2. The activation energy for complex formation is also more in cyclohexane than in water. This may be explained from the fact that in the case of liposomes more surface
1037
area is provided, and the polar head group of the lecithin molecule is readily available for interaction with the adsorbed iodine. This provides a path for reaction with a lower energy of activation. However, when lecithin is forming reverse micelles in cyclohexane, iodine molecules have to approach from outside the aggregate structure in order to interact with the polar head group, which is located inside, and this path for reaction requires the extra activation energy. The structural similarity between biomembranes and pure lipid bilayers makes the study of lipid dispersion in aqueous medium quite u ~ e f u l .As ~ there is no other experimental evidence of liposome-catalyzed phospholipidiodine interaction reported in the literature, this study is quite relevant, and the results may be extrapolated to the case of similar interaction in actual membrane-based life-sustaining process.
Acknowledgment. Financial support from the Department of Science and Technology, Government of India, is gratefully acknowldged. Registry No. Iodine, 7553-56-2. (9) Tanford, C. The Hydrophobic Effect, 2nd Ed.; Wiley Interscience: New York, 1980.
Monolayers and Langmuir-Blodgett Films of Doxylstearic Acids. Spreading Isotherms and ESR Study Francesca Bonosi, Gabriella Gabrielli, Giacomo Martini,* and M. Francesca Ottaviani Department of Chemistry, University of Florence, Via G. Capponi 9, 50121 Firenze, Italy Received December 7, 1988. I n Final Form: March 20, 1989 The spreading isotherms of monomolecular films of stearic acid nitroxides with a five-membered paramagnetic unit in different positions of the hydrocarbon chain were studied at 293 K. The presence of the substituent rendered more expanded monolayers, whose properties depend on the distance between the NO and COOH groups. Langmuir-Blodgett films of these nitroxides were deposited on quartz plates. The ESR spectra of both collapsed monolayers and oriented multilayers indicated a more opened structure with higher chain flexibility with respect to pure, bulk compounds. The dipole-dipole effects and the spin-exchange interactions were determined to mainly depend on the ring position along the hydrocarbon chain.
Introduction Langmuir-Blodgett (LB) films are usually built up with amphiphilic molecules that often contain saturated and/or unsaturated hydrocarbon chains.' Increasing efforts are being directed toward the preparation of LB films bearing uncommon optical, electrical, and magnetic properties in light of their versatile applications.2 Several papers have recently appeared on the use of fatty acids and their derivatives for the preparation of ultrathin and homogeneous molecular edifices such as the LB Moreover, LB films constituted of fatty acid (1) Roberts, G. G. Adu. Phys. 1985, 34, 475 and references therein.
(2) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelashvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (3) Laxhuber, L. A.; Moehwald, H. Langmuir 1987, 3, 837. (4) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1034, 1045. ( 5 ) Kamata, T.; Kato, A.; Umemura, J.; Takenaka, T. Langmuir 1983, 3, 1150. (6) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (7) Dote, J. L.; Mowery, R. L. J. Phys. Chem. 1988, 92, 1571.
0743-7463/89/2405-l037$01.50/0
bilayers are often considered the first step for a simple model of natural membra ne^.^^^ Stearic acid nitroxides (the so-called doxy1 nitroxides), with the oxazolidinyl paramagnetic moiety inserted in different positions of the aliphatic chain, are well-known to biology-concerned scientists because of their very large use as paramagnetic probes in the study of several features of natural and synthetic membranes.lOJ' The perturbative effects that these materials may introduce into biological systems under study are, at the same time, well-known. These compounds are potentially surfactant substances. They are therefore suitable for monolayer formation on (8) Quinn, P. J. The Molecular Biology of Cell Membranes; McMillan: London, 1976. (9) Kinnunen, P. K. J.; Virtanen, J. A.; Tulkki, A. P.; Ahuja, R. C.; Mobius, D. Thin Solid Films 1985, 132, 193. (10) Smith. I. C. P.: Butler. K. W. In S ~ i n Labeling. Theory and Applications; Berliner; L. J., Ed.; Academic: London, 7976; Vo1.-1, pp 411-451. Griffith, 0. H.; Jost, P. C. Ibid. pp 453-523. (11) Marsh, D. In Membrane Spectroscopy; Grell, E., Ed.; Springer: Berlin, 1981; pp 51-142.
0 1989 American Chemical Society
1038 Langmuir, Vol. 5, No. 4 , 1989 a polar s u b p h a s e and for LB film deposition on solid substrates. The ESR s t u d i e s on LB films of stearic nitroxides initially have at least two m a i n objectives: (i) the analysis of the p r o p a g a t i o n and of the a n i s o t r o p y of the magnetic interactions, as a part of the s t u d y of the cooperative effects a m o n g constituents in membrane-like films, and (ii) the analysis of the m o t i o n a l a n i s o t r o p y of t h e hydrocarbon c h a i n s and of their order degree. For point i we used LB films of pure nitroxides, and this paper describes their spreading monolayer properties, their LB film formation, and their ESR spectra, particularly f r o m the p o i n t of view of spin-spin interactions, mobility, and stability. Point ii required the use of LB films built up w i t h nitroxides diluted in a stearic acid as a diamagnetic host. The results will be described in a forthcoming paper.
Experimental Section The nitroxides used in this work were obtained from Molecular Probe, Eugene, OR, and from Sigma, Munchen (West Germany), and used without purification. All nitroxides were derivatives of stearic acid: CH3 -(CHz)"
Bonosi et al.
30.
5 7 10 14
n = 12 10
SA
10.
0.
60 80 100 Area (A2/Molecuie)
8 5 1
From ESR measurements, the purity of 9-doxylstearic acid (9DXSA) was too low, probably because of reduction to the corresponding diamagnetic hydroxylamine. Crystallization from alcohol or CHC13 did not largely improve purity. The results obtained with this spin probe were used only to check the calculation procedures we employed in the analysis of the pure systems. Chloroform (Merck, purity >99%) solutions of stearic acid (4.2 X mol/L) and of the above nitroxides (concentrations ranging from 2.4 X mol/L to mol/L depending on the nitroxide) were used for both spreading isotherm determination and LB film deposition. Monomolecular films of the nitroxides were obtained by spreading the pure substance solutions on a water subphase a t pH 5.6. Before isotherm determination, we waited at least 10 min to allow solvent evaporation. The water was twice distilled and further purified with the Milli-Q water system supplied by Millipore. Only water with a resistence higher than 18 Mibcm was used. The spreading isotherms were measured with the LaudaFilmwaage according to the Langmuir method. The balance was modified in order to be computer controlled.12 Surface pressures, H , were determined with an accuracy of k0.07 dyn/cm and the areas A with an accuracy of 4~0.01m2/mg. The .n/A isotherms were determined with discontinuous compressions, and time intervals between two successive compressions were in the 90-120-s range, depending on the monolayer type. Three pressure readings were taken a t each area. All the isotherms discussed in this work were obtained at 293 K. Temperature was controlled with a Haake thermostat Model PG-40C (accuracy f O . l K). The L B films were prepared a t room temperature with a Langmuir Trough 4, Joyce-Loebl, according to the standard Langmuir-Blodgett method of vertical dipping of quartz plates previously coated with a monomolecular film of stearic acid. This precoating was found to improve the subsequent deposition of monolayers. The dimensions of the quartz plates were 42 X 8 X 1 mm or 40 X 4 X 0.5 mm in order to be inserted into the ESR resonating cavity. The surface pressures were varied continuously at the lowest compression speed, and the monolayer deposition (12) Baglioni, P.; Carl&,M.; Dei, L.; Martini, E. J. Phys. Chem. 1987, 91, 1460.
120
Figure 1. Spreading isotherms at 293 K of pure doxylstearic acids and of stearic acid on water a t p H 5.6. Table I. Collapse Pressures and Limiting Areas of Monolayers of Doxy1 NitroxidesO system H,,u, dyn/cm Ab, A* molecule-' stearic acid 28.4 24 5-doxylstearic acid 33.9 76 7-doxylstearic acid 30.8 63 9-doxylstearic acid 26.8 36 12-doxylstearicacid 24.5 33 16-doxylstearicacid 22.1 23
(CHz),,,-COOH
5 - D X S A (solid) 7-DXSA (oil) 9-DXSA (oil) 1 2 - D X S A (oil) 1 6 - D X S A (solid)
0
SDXSA 7DXSA SDXSA 12DXSA 16DXSA
20.
X "+
m-3
h\
0
a
Values at 273 K; subphase is water at pH 5.6.
on the quartz plate was carried out at the highest surface pressure before collapse took place. The rates of transfer were 3 mm/min for the stearic acid coating and 15 mm/min for film deposition. The ESR spectra were taken with the aid of the Bruker Model 200D ESR spectrometer operating in the X-band (-9.7 GHz), interfaced with the Bruker Aspect 2000 data-handling system for spectral accumulation and magnetic parameter calculation. In order to obtain spectra with different plate orientations, the quartz plates containing either collapsed monolayers or L B films were mounted on a quartz rod connected to a goniometer. Temperature variations were achieved by using the Bruker ST 100/700 variable-temperature assembly. Modulation amplitude and microwave power were such to avoid signal overmodulation and saturation. Hyperfine coupling constants and g values were obtained through field calibration with the D P P H radical (g = 2.0036).
Results and Discussion Spreading Isotherm. Figure 1 shows the spreading isotherms at 293 K of the pure nitroxides on water subphase at pH 5.6. For comparison, the stearic acid (SA) isotherm at t h e same temperature is also reported. T a b l e I r e p o r t s the collapse pressures and the limiting a r e a values, A,,) that is, the area occupied b y e a c h molecule in the condition of t h e highest film packing as valued at zero surface pressure. Table I1 summarizes the significant p h a s e s that were identified i n the i s o t h e r m of each nitroxide f r o m t h e calculation of the compressibility coefficient, C;I:
CL1 = -A(dr/dA)T
(1)
In the past, Cadenhead and c o - w o r k e r ~ ~have ~ - ~used ~ monomolecular films of 5-DXSA, 12-DXSA, 16-DXSA, its (13) Cadenhead, D. A.; Muller-Landau,F. Adu. Chem. Ser. 1975,144, 294. (14) Cadenhead, D. A.; Muller-Landau, F. J. Colloid Interface Sci. 1974, 49, 131. (15) Cadenhead, D. A.; Muller-Landau, F. Biochim. Biophys. Acta 1976, 443, 10.
(16) Cadenhead, D. A.; Muller-Landau, F. Biochim. Biophys. Acta
1973, 307, 279.
Monolayers and LB Films of Doxylstearic Acids
~~~
Table 11. Compressibility Coefficients and Phases in the 273 K SDreadina Isotherms of Doxy1 Nitroxides system phasesa C[l, dyn/cm SA G ( X < 1) 123-100 L2 (1 < x < 23.5) 290-285 L2-S (23.5 < T < 28.4) 5-DXSA G (T < 12.1) 42-23 L1 (12.1 < x < 32.8) 7-DXSA G ( X < 22.3) 29-21 L1 (22.3 < x < 29.8) 9-DXSA G (T < 20.3) 35-16 L1 (20.3 < T < 26.1) 12-DXSA G ( X < 16.4) 29-17 L1 (16.4 < T < 24.6) 16-DXSA G (T < 10.6) 18-13 L1(10.6 < x < 15) Lz (T > 15) 66-60 ~
G,gaseous; L1,expanded liquid; L2,condensed liquid; S,pseudosolid; ?r, surface pressure (in dyn/cm).
ester, and 8-DXPA (8-doxylpalmitic acid) to study the implications of using these substances as spin labels in the study of lipid bilayers and of cell membrane structure and dynamics. These experiments were carried out under continuous compression. We repeated the isotherm determination for five doxylstearic acids (from 5- to 16DXSA) under a discontinuous compression procedure which gave results that better approximate equilibrium conditions without attaining it, because the 90-120-s delay might be shorter than required for a proper equilibrium. However, we choose these delay times as a compromise between isotherm determination under discontinuous compression and LB film deposition under continuous compression. This also allowed us to minimize the possible, slow chemical deterioration to which such molecules underwent. Mainly for these reasons, both collapse pressures and limiting area values in Table I did not completely agree with those values from the isotherms reported in ref 13-16. The different experimental procedure and the differences in the subphase pH also explained the poor agreement of SA surface parameters with the values reported in 1iterat~re.I~ These discrepancies, however, did not mean alterations in the surface phase and orientations of the SA monolayer. This monolayer is known to be oriented with the hydrocarbon chains held together through hydrophobic forces and protruding into the gaseous phase, whereas the COOH group hydrophilically interacts with the water polar ~ubphase.'~ The isotherms of 5-DXSA and 16-DXSA have also been analyzed by Baglioni et a1.12in a study of the interaction between such nitroxides and a-poly(?-methyl L-glutamate). These authors employ a more acidic subphase (water at pH 2 after addition of mol/L HCl), and the limit area values are significantly lower because of the lower dissociation of the carboxyl group. We did not carry out runs as a function of temperature since the valuation of the thermodynamical parameters of these systems was beyond the scope of this work, and the temperature of 293 K, at which the isotherms were obtained, was the same used for the LB film deposition. From the spreading isotherms and from the data reported in Tables I and 11, it is clear that the presence of a five-membered ring as a substituent in the hydrocarbon chain strongly modified the isotherm shape, the limiting areas, and the collapse pressure, with respect to unsub(17) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interface;
Interscience: New York. 1966. (18) Kivelson, D. J. Chem. Phys. 1957,27, 1087. D.J Chem. Phys. 1967,47, 3312.
Plachy, W.; Kivelson,
Langmuir, Vol. 5, No. 4, 1989 1039
stituted stearic acid. Despite the partial decomposition of 9-DXSA,the features of the spreading isotherm for this nitroxide were well in line with those from the other doxylstearic acids since the change from N-0 to N-OH did not appreciably affect either the stearic hindrance or the net hydrophilic properties. The films obtained from pure nitroxides were significantly more expanded. The presence of the oxazolidine ring appreciably decreased the strength of the hydrophobic interactions among aliphatic chains. This perturbation depended on the ring position. In the case of 12-DXSA and 16-DXSA,the COOH and doxyl groups are sufficiently apart so that these nitroxides behave as typical bipolar molecules at low surface pressure.13 They assume a bent structure in which both polar heads interact with the polar subphase. The stability of this structure was higher in 16-DXSA than in 12-DXSA, since the spreading isotherm from the first nitroxide showed a marked plateau at a = 15-16 dyn/cm. This clearly indicated a transition from a closed-packed bent to an erect conformation with a strong decrease of the limiting area. This conformation prevailed at high surface pressure, as suggested by Cadenhead.l3 The erect conformation was favored by the lower hydrophilicity of the N-0 group with respect to the COOH group. Although 5-DXSA, 7-DXSA, and 9-DXSA also were formally bipolar molecules, they behaved as monopolar substances, since they assumed almost erect configurations at every surface pressure. The presence of the doxyl substituent led to A, values that increased with the decrease of the distance between N-0 and COOH groups. This was particularly marked for 5-DXSA and 7-DXSA, whereas 16-DXSA had almost the same area value as stearic acid (SA). This meant that the limiting area values were determined both by the size of the entire polar group at the air/liquid interface and by the steric hindrance induced by the oxazolidine ring presence. LB Films and ESR Spectra. All of the LB films whose ESR spectra are discussed in this section were prepared on quartz plates that were rendered hydrophobic through the preliminary coating of a stearic acid monolayer. Before discussing the ESR spectra we obtained, we feel it necessary to present (i) a short digression on the spinspin interactions which occur in highly concentrated paramagnetic systems and (ii) a definition of the nitroxide molecular axis orientation with respect to both the normal to the quartz plate and the magnetic field direction. The ESR spectra of interacting radicals are described by the Hamiltonian 7f = PB-gS + S.A.I + 7feX, %dip
+
where the last two terms are related to the spin-spin Heisenberg exchange and to the dipole-dipole interactions. In nitroxide ESR spectra, the exchange term is usually isotropic and leads to homogeneous line broadening depending on the exchange frequency We=. When conditions of strong exchange are verified (as frequently occurs in the spectra described in this paper), the three lines of the nitroxide spectrum collapse into a single, exchange-narrowed line,lgwhose width, AB,,, is related to the exchange frequency, Wexc: s e x ,
= aN2/Wexc
where a N is the isotropic coupling constant. The dipole-dipole interaction is anisotropic in nature and leads to homogeneous line broadening of each line. It (19) Unpublished results.
Bonosi et al.
1040 Langmuir, Vol. 5, No. 4, 1989
a)
b)
axis and the pw molecular orbital axis: we define B as the angle between B and Z n. The rotation of the plate inside the cavity was along the X axis in the ZY plane. Figure 3 shows the ESR spectra of the freshly prepared LB films of the doxy1 nitroxides at 298 K with B = 90" (Figure 3a) and 0 = Oo (Figure 3b). In every case, the films were prepared with 20 dippings of the quartz plate in the water subphase. Of course, this does not ensure that exactly 40 layers (20 for each plate surface) were experiencing magnetic field, since it is known that repeated dippings do not always lead to a continuous and regular overlapping of monomolecular layers. All of the experimental spectra (except those from the 9-DXSA LB film) were broad, and there was a very poor resolution of the 14N hyperfine (hpf) components. Both line width and hpf resolution depended on the position of the oxazolidine ring in the hydrocarbon chain and on the angle. Outside the field range corresponding to g 2, we did not observe transitions attributable to spin states with S I1. The hpf resolution in the ESR spectrum of the 9-DXSA LB film was higher for both orientations. This finding agreed with the lower purity of this sample and resulted in lower exchange frequency and dipole-dipole interactions due to the increased mean interelectron distances. The observed spectra therefore seemed more similar to the spectra given by LB film of diamagnetically diluted nitro~ides'~ than to those from pure nitroxides described in this paper. The spectra of the pure (solid or oily) nitroxides and of the collapsed monolayers were also registered, to be compared with the ESR spectra of the LB films. The spectra of the pure compounds consisted of a single, exchanged narrowed Lorentzian line centered a t g 2 and with a peak-to-peak distance in the 16-19-G range, without resolution of the hpf structure due to 14N. On the contrary,
-
Figure 2. Orientation of the quartz plate in the external magnetic field (a) and of the p~ orbital of the doxylstearic acid molecule in a LB film with respect to both magnetic field and quartz plate (b).
is the predominant effect in solids or in very highly viscous liquids. Figure 2 represents sketches of the orientation of the quartz plate in the magnetic field (Figure 2a) and of the pyy orbital of the nitroxide molecule containing the unpaired electron with respect to both magnetic field and quartz plate. Assuming a Y-type deposition (as for SA), the normal to the plate surface, n , almost coincided with the z molecular axis, directed along the hydrocarbon chain
-
\
7-DXSA
12-DXSA
16-DXSA
F i g u r e 3. Experimental spectra of LB films of doxylstearic acids (40 layers on quartz plate coated with a stearic acid monolayer). Temperature 298 K. (a) Field direction perpendicular to the plate normal (0 = goo). (b) Field direction parallel to the plate normal (e = 00).
Langmuir, Vol. 5, No. 4, 1989 1041
Monolayers and L B Films of Doxylstearic Acids
A \
/---
GAIN l x
/
7 DXSA
20 G
\\r
20 G
H
Figure 4. ESR spectra at 298 K of collapsed monolayers of doxylstearic acids.
a partial resolution of the hpf structure was observed from 9-DXSA oily samples even though they arose from different lots. Their spectra were typical of highly viscous systems containing very high concentrations of randomly oriented radicals. The ESR spectra of the collapsed monolayers (with the usual exception of 9-DXSA) also consisted of a single, exchanged narrow line centered at g 2 with line widths of 24-26 G (Figure 4). These were higher than those of the corresponding bulk nitroxides. A higher line width (-40 G ) was observed from collapsed monolayers of 12DXSA. Both the line shape and the line width were independent of the orientation in the magnetic field. The collapsed systems may be considered to be tridimensional phases which arose from bidimensional phases according to the collapse mechanisms.20s21 The above results suggested that the collapsed nitroxides had different structural features with respect to those of the bulk phases. The larger widths indicated a weaker spin-exchange interaction due to a higher mean distance between unpaired electrons. The collapsed monolayer structures should therefore be more open (and this was particularly true for 12-DXSA) than in pure compounds. Since the collapse process occurred through nucleation and growth, exactly as in a true crystallization,20*21the higher mean distances between unpaired electrons in collapsed monolayers with respect to pure compounds could arise from water incorporation in the material and/or from different packing of the first nuclei in the two cases. No experimental evidence allowed us to distinguish between the above two possibilities. Let us now consider in more detail the ESR spectra of the LB film of 16-DXSA,as a representative example. The B = 90° spectrum consisted of a broad unstructed absorption. As generally observed for the other nitroxides, this was a very complex signal whose shape was determined by several effects including dipole-dipole interactions and Heisenberg spin exchange (strong exchange conditions)
-
(20) Gabrielli, G.; Guarini, G. G. T.; Bastianini, F. J.Colloid Interface Sci. 1979, 69, 352. (21) Gabrielli, G.; Guarini, G. G. T.; Ferroni, E. J.Colloid Interface Sci. 1976, 54, 424.
Figure 5. Comparison between ESR spectra a t 298 K of 12DXSA LB films registered immediately after preparation (a, t9 = 90"; b, t9 = Oo) and 2 days after preparation (c, 8 = 90°; d, 0 = 00).
besides line effects due to molecular motion. The peakto-peak distances in the spectra of the LB film (40-41 G), of the collapsed monolayer (25 G), and of the pure solid compound (16 G) were clearly indicative of different molecular packings. In the B = Oo spectrum of the same sample, a very weak resolution of the A,, components appeared. The main reason for the line shape differences between the B = Oo and the B = 90° spectra should reside in the relative contributions of the dipole-dipole effects and in the molecular reorientation frequencies. After the film deposition, the resolution of the hpf components increased with time, and this was associated with an intensity decrease. Figure 5 compares the ESR spectra of the 12-DXSA LB films registered immediately after the deposition (Figure 5a,b) and 2 days after (Figure 5c,d). A progressive decomposition of the radicals in the LB film occurred with a resultant increase of the mean distance between unpaired electrons and a decrease of exchange frequencies and dipole-dipole effects. This finding was generally observed in the LB films of all nitroxides investigated, and it was clear evidence of poor stability with time of the magnetic properties of these molecular assemblies. A detailed analysis of the time dependence of the magnetic properties of these films was, however, beyond the aim of this paper. As outlined above, the theory of the magnetic interactions among separate spins is straightforward in liquid and solid systems, that is, in two of the limit phases of matter, and the calculation procedures for the separate contributions from those effects are quite well established. Unfortunately, in intermediate situations such as those encountered in biological or colloidal systems, approximations and simplifications must be made, depending on the
1042 Langmuir, Vol. 5, No. 4, 1989
Bonosi et al.
characteristics of the system under study. In order to obtain details on both molecular mobility and spin-spin interactions, we tried to use the spectral computation procedure for nitroxides in fluid systems developed by Freed and co-workers.22 This procedure is based on the modulation of the magnetic anisotropies g and A, through a correlation time for the motion, T. It is generally used for diluted spin systems.23 The following values of the g and A components were used in the calculation procedure: g,, = 2.0088 g,, = 2.0061 g,, = 2.0027
A,, = 6.3 G A,, = 5.8 G A,, = 33.6 G
The line width of each mI manifold is given byz4
B = CY’
+ CY” + pmI + ym:
where a”, p, and y depend on the motional properties. Their analytic expressions22 include spectral densities dependent on a correlation time for the diffusional reorientation, T . In fluid systems, this parameter is usually identified with the Debye-Stokes-Einstein reorientational time, and its value is obtained from the spectral computation outlined above. Depending on the T value with respect to the time-dependent part of the spin hamiltonian, three time domains are identified:22 (i) fast motion ( T < s). However, we attributed the motional effects represented by the 7 values to a bidimensional wobbling motion (segmental motion) of the NO group along the doxylstearic acid molecule more than to a true tridimensional reorientation. The term a’ includes terms other than those dependent on motion (spin rotation, unresolved hyperfine structure, etc.). In our computation, we included dipole-dipole broadening in the term a’,that is, the residual, motion-independent line width. Furthermore, we took into account the Heisenberg spin-exchange effects by simply summing to the calculated line shape a Lorentzian single line whose width was related to the spin-exchange frequency predicted by the modified Bloch equations.25 The summing coefficients were those that gave the best fit. Our procedure differed from those used by Sackman and Trauble= for androstane spin labels incorporated in dipalmitoyllecithin membranes and by Mann and McGregor2’ for cholestane nitroxide monolayers, because it allowed the determination of the correlation time T characterizing the NO motion rate. The procedures quoted above are based on the sum of three Lorentzian lines with different widths and include both exchange and dipole-dipole effects and do not include the evaluation of the correlation times. For instance, Sackman and TraubleZ6estimate T by comparison of their experimental spectra with theoretical spectra calculated by ItzkowitzZ8 and Alexandrov et al.29 Figure 6 shows the comparison between the experimental 9 = 90’ spectrum of 16-DXSA LB film and the spectrum computed in slow motion conditions, with T = (22)Freed, J. H.In Spin Labeling. Theory and Applications; Berliner, L. J., Ed.; Academic: New York, 1976;Vol. 1, pp 53-132. (23)Romanelli, M.; Ottaviani, M. F.; Martini, G . J. Colloid Interface Sci. 1983,96,373. (24)Kivelson, D.J . Chem. Phys. 1960,33, 1094. (25)McConnell, H.M. J . Chem. Phys. 1958,28,430. (26)Sackman, E.; Trauble, H. J . Am. Chem. SOC.1972,94, 4492. (27)Mann, J. A.; McCregor, T. R. Adu. Chem. Ser. 1975,144, 321. (28)Itzkowitz, M. J . Chem. Phys. 1967,46, 3048. (29)Alexandrov, I. V.;Ivanova, A. N.; Korst, N. N.; Lazarev, A. V.; Prikhozhenko, A. I.; Stryvkov, V. B. Mol. Phys. 1970,18, 681.
\
Figure 6. Experimental (-, T = 298 K) and computed (---) spectra of 16-DXSALB film. For the computation,the following parameters were used: T = 3 X s, residual line width T2,0= 20 G,ABexc= 35 G (spectrum a, 0 = 90’); T = 3 X lo4, T2,0= 18 G,AB,,,= 35 G (spectrum b, B = OO). 3X s and a’ = 20 G, summed with a Lorentzian line with a peak-to-peak width of 35 G. If the very low intensity of the spectrum and the resultant base line shift are kept in mind, the fit for both line shape and line width was quite good. A similar fit was obtained for the 9 = 0’ spectrum of 16-DXSA,with T = 3 X lo4 s, a’ = 18 G, and AB,,,= 35 G. The decrease of the CY’ value should arise from a decreased dipole-dipole contribution. This agreed with the increased molecular flexibility (shorter T) that led to a partial modulation of both the magnetic anisotropies and the anisotropic dipolar interactions. Both 9 = 0’ and 9 = 90° ESR spectra were simulated with correlation times in slow motion domains as in a highly viscous systems. This might seem to disagree with the almost rigid model proposed in Figure 2b, which predicted a coincidence of the principal axis of the nitrogen hyperfine tensor with the plate normal. In this case, for 9 = 90°, the p~ orbitals of all the spin labels were perpendicular to B, and because of the small value of A,, no hpf splitting should be observed. For % = O’, all pw orbitals were parallel to B, and Allcomponents should be observed. This was, in fact, the general trend of the experimental results for the two orientations (see Figure 3a,b). However, in Figure 2b the hydrocarbon chains were assumed as really perpendicular to the plate, whereas it is known that long-chain acids show an appreciable tilting with respect to the film normal.30 The presence of the doxy1 group induces additional perturbation in the chain packing. This perturbation was expected to be at maximum a t the middle of the chain. The calculated correlation times reflected therefore a significant motional interexchange of A,, and A , because of the wobbling of the NO group by itself or of the oscillations of the chains with respect to a equilibrium position, which is tilted with respect to the film normal. This effect (30) Chollet, P. A,; Messier, J. Thin Solid Films 1983,99, 197.
Monolayers and L B Films of Doxylstearic Acids
Langmuir, Vol. 5, No. 4, 1989 1043
Table 111. Best-Fit Parameters for the Simulation of the ESR Spectra at 298 K of Nitroxide LB Films 7,
LB film
0 = 90'
5-DXSA 7-DXSA 9-DXSA 12-DXSA 16-DXSA
3X 3 X lo-' 3X
3X
/Y
s
0 = 0'
lo-' 5X 3 x 10-8 3X
Ad, G 15 8 3 3 2
AB..,, G 50 60
\
35 35
was apparently the highest in 7- and 9-DXSA films (see Figure 3). As a further source of uncertainty, it is important to note that the calculated 7 values were highly approximated when a' was high (a'> 10-15 G). Therefore, the lower mobility in the 0 = 90° spectrum should be considered more qualitatively than strictly quantitatively. More precise valuations of 7 have been obtained in LB films of doxylstearic acids diluted in stearic acid,lQand they confirm the above trend. All of the three motional models (Brownian, free, and jump d i f f ~ s i o n ) ~used ~ b l in the calculation gave almost the same line shape. Moreover, we could have also taken into account the following: (i) anisotropic motion (711# T ~ that ) certainly better represented the actual physical situation, (ii) a distribution of the correlation times due to mobility differences in the various layers of the films, and (iii) a distribution of the values of the components of g and A because of nonuniformity of the LB film. It was, however, very difficult to valuate the above effects, which should not largely affect the calculated line shape. This, in fact, resulted in a line shape largely dominated by a' and AB,,,. This calculation procedure was used for all the spectra, and Table I11 reports the best-fit correlation time at 298 K. The same table also reports the spin-exchange contributions and the anisotropies of the dipole-dipole interactions expressed as the differences of a' values in the 0 = 90' and 0 = Oo spectra. In each system, the exchange frequency was isotropic as expected, and its value was lower than in the corresponding collapsed monolayer (for which strong exchange conditions hold) that, in turn, was lower than in pure compounds. This ensured a more opened and more flexible structure by passing from the three-dimensional phases to the two-dimensionalsystems. The exchange interaction depended on the oxazolidine ring position on the hydrocarbon chain because it increased with the increase of the distance of the NO group from the hydrophylic COOH group. This reflected both the decreased interlayer distances among paramagnetic moieties and an increased mobility of the hydrocarbon chains outside the anchoring, polar carboxyl group. The anisotropy of the dipole-dipole effects, as evaluated from the difference between a' in parallel and perpendicular orientations, was more pronounced when the nitroxyl group was nearer to the polar head (5- and 7-DXSA). This (31) Goldman, S. A.; Bruno, G. V.; Freed, J. H. J. Phys. Chem. 1972, 76, 1858.
A
b)
Figure 7. Experimental (-, T = 298 K)and computed (---) spectra of 9-DXSA LB film (40 layers on quartz plate coated with a monolayer of SA). For the computation, the following parameters were used 7 = 3 X lO-'s, T2,0= 16 G (spectrum a, 0 = 90°); T = 5 X lo-* s, Tz,o= 13 G (spectrum b, 0 = O O ) .
proved a lower mean mobility of these nitroxides in their LB films. In addition, the dipole-dipole contribution to a' was in every case higher with the B direction perpendicular to the z molecular axis. Even with the uncertainty in their values, the calculated 7 values also proved the dependence of the NO mobility on both the orientation and the position of the paramagnetic unit in the hydrocarbon chain. According to the dipole-dipole spin-spin effects, the rotational mobility was higher with B aligned along the z axis than with B perpendicular to this a i s , and on the average, it increased with the increase of the distance between NO and COOH. No exchange-narrowing contribution was necessary in the simulation of the ESR spectra from 9-DXSA (Figure 7), which were appreciably more resolved than those of the other spin probes. The line-broadening dependence on the orientation clearly indicated the predominance of dipole dipole spin effects. Since the ESR spectra from 9-DXSA films were fairly well reproduced by the Freed procedure, the calculated correlation times reported in Table I11 could be accepted as realistic values and confirmed the trend of higher mobility in the direction perpendicular along the z axis, that is, for 0 = Oo.
Acknowledgment. Thanks are due to the Minister0 della Pubblica Istruzione (MPI) and to the Consiglio Nazionale delle Ricerche (CNR) for financial support. We are indebted to Prof. E. Ferroni for a useful discussion during the elaboration of this work.
