Langmuir 1995,11, 4499-4504
4499
Influence of Multivalent Metal Ions on the Monolayer and Multilayer Properties of Some Unsaturated Fatty Acids Mika Linden* and Jar1 B. Rosenholm Department of Physical Chemistry, Abo Akademi University, Porthansgatan 3-5, SF-20500, Turku, Finland Received June 20, 1995. I n Final Form: August 14, 1995@ The sensitivity of octadecanoicacid (stearic acid, SA) monolayers and of three unsaturated derivatives thereof toward MnC12, CdCl2, and TbCl3 introduced to the subphase has been investigated as a function of electrolyte concentration. The unsaturated fatty acids studied were cis-9-octadecenoicacid (oleic acid, OA), trans-6-octadecenoic acid (petroselaidic acid, PA), and trans-9-octadecenoic acid (elaidic acid, EA). The order of condensation of the monolayers followed the bulk melting points of the acids. The dissociation degree determined for multilayers by attenuated total reflectance Fourier transform IR in excellent agreement with the results obtained for the floating monolayers. The metal ions could be ordered in terms of increasing interaction with the monolayers as Mn2+< Cd2+98%),MnClz (>99%),and CdClz (>99%)were obtained from Fluka. n-Hexane (>99.5%)used as the spreading solvent (1mg/mL) was obtained from Fluka. All the chemicals were used as supplied. The distilled water was purified with a Millipore Milli-Qfiltering system with two carbon and two ion-exchange stages and finally a filtration through a 0.22-pm Zetapore filter. The pH of the pure water was 5.7 and was determined by dissolved COz. Methods. Surface Pressure Measurements. The surface pressure-area isotherms were performed with a KSV 5000 computerized Langmuir trough manufactured by KSV Instruments, Helsinki. The experiments were carried out at 20 i 0.3 "C using a thermostated Teflon trough (750 x 65 mm2). The surface pressure was monitored using a Pt Wilhelmy plate. The initial mean molecular area after spreading was typically 70 Az. The compression was started 8 min after spreading using a constant barrier speed of 3 .&z/(molecule*min),a suitable com(8) Peltonen, J. P. K.; He, P.; Rosenholm, J. B. Thin Solid Films 1992,210l211,372. (9)Peltonen, J. P. K.; He, P.; Rosenholm, J. B. Langmuir 1993,9, 2363. (10) Linden, M.; Gyorvary, E.; Peltonen, J.;Rosenholm, J. B. Colloids Surf In Dress. Cil)Pcltonen, J. P. K.; He, P.; Linden, M.; Rosenholm, J. B. J.Phys. Chem. 1994,98, 12403.
0 1995 American Chemical Society
4500 Langmuir, Vol. 11, No. 11, 1995
Linden and Rosenholm
40 a stearic acid b petroselaidic acid
c
E
2 30 -E
' \
c elaidic acid d oleic acid
\
.u t
20 0
n e,
2
10
2 0
10
20
30 40 mean molecular area [AT
50
mean molecular area [A2]
Figure 1. n/A isotherms for (a) stearic acid, (b) petroselaidic acid, (c)elaidic acid,and (d)oleic acid on a pure water subphase at pH = 2.
Figure 2. The d A isotherms of petroselaidic acid on (a) 5 x 10-6 M, (b) 5 x M TbC13 and (d)pure water M, and (c) (pH = 2).
pression speed for these kind of molecules.12 The precision of the measurements was fO.l mN/m. Surface Potential Measurements. The surface potential, AV, of the monolayer was measured simultaneouslywith the surface pressure by the vibrating plate method. The upper, vibrating electrode (043 mm) was positioned about 2 mm above the subphase surface, and it was perforated to minimize the noise. ATR FTIR Measurements. For the FTIR measurements a Bruker IFS66 spectrometerwas used. Twenty-five layers of the spread monolayer was deposited onto a ZnSe ATR crystal. An average of 1000 scans was used to ensure good signal-to-noise ratios. Spectra were taken at 2.8 cm-l resolution.
On the other hand, the compression isotherm of PA, with the double bond in the 6-position, showed a further transition from the LE state to the LC state, and the monolayer could be compressed to a mean molecular area of 21.5 AZ. However, the PA monolayer had already collapsed a t a surface pressure of 18.6 mN/m. The observed value of A, was about the same as those for EA and OA, 53.5 Az. On a pure water subphase of pH = 5.7, the A, values decreased to about 48 Azfor all three acids. At this pH the monolayers are partly dissociated. The observed condensation may originate from the known attraction between un-ionized and ionized fatty acid molecules"Q7 or simply from dissolution of some of the monolayer material into the subphase.12 However, since the observed final mean molecular area in the LC state for PA was close to 20 A2, dissolution is suggested to be neglible, at least for PA. n, of the partially dissociated OA monolayer was 31.2 mN/m, being slightly higher than the ESP of the pure acid monolayer. Metal Zons Present. An introduction of metal ions to the subphase generally caused a condensation of the monolayer, which was seen in the isotherm as a decrease in the A, value. The LC state characteristic of an SA monolayer gradually disappeared from the isotherms with increasing salt concentrations. The concentrations corresponding to a fully condensed SA monolayer, with an A, value of 22 f 0.5 A2,were 5x and M for Tb/SA, CdISA, and Mn/SA, respectively. PA yielded the most condensed monolayers of the unsaturated fatty acids studied. When the divalent metal ions were compared, slightly lower A, values were observed a t low CdClz concentrations, but a t salt concentrations exceeding however, more condensed monolayers were 5 x obtained on MnClz subphases. The PA monolayers remained in the LC state a t any CdC12 concentration investigated, whereas a transition to the solid state, with n, > 50 mN/m, was observed for [MnClzI > 5 x and [TbCls] > M. The monolayers were occasionally very rigid and could pass over the edges of the trough if the com ression was carried on to mean molecular areas below 20 As a n example, isotherms of PA recorded a t different TbC13 concentrations are shown in Figure 2. In Figure 3 some of the compressionisotherms measured for EA are shown. An LE-LC transition was seen in the M divalent isotherms of EA on subsolutions of
Results Surface Pressure Measurements. Pure Water Subphase. The differences in the chemical and physical properties of the fatty acids studied clearly appeared in the characteristic compression isotherms on a 0.01 M HC1 subphase (pH = 21, as shown in Figure 1. At this pH the monolayers may be regarded as being completely unionized. The isotherm of SA is well-known, and the characteristic transitions to the liquid-condensed (LC) state a t 26 Az and further to the solid state at about 20.5 Azwere clearly seen as abrupt changes in the compressibility of the monolayer. The compression isotherms of the unsaturated acids were much more expanded, and both the position of the double bond and the type of unsaturation (cisltrans)affected the monolayer properties. The importance of the position of the double bond in the hydrocarbon chain was manifested through the fact that both OA and EA with the double bond in the 9-position yielded almost identical isotherms with the EA monolayer having a slightly higher compressibility. A monotonic increase in surface pressure was observed during compression, indicating that the monolayer remained in the liquid-expanded (LE) state. The mean molecular area corresponding to the initial increase in surface pressure, A,, was 55 A2forboth acids, in nice agreement with earlier reports. Since OA is a liquid in the bulk a t room temperature, the monolayer collapse will occur in the liquid state a t a characteristic surface pressure, n,, equal to the equilibrium spreading pressure, ESP, of the mon01ayer.l~The obtained value of the ESP of OA was 29.8 mN/m, in nice agreement with earlier reports in the literature.14J5 n, of EA was significantly lower, 25 mN/m. (12)Tomoaia-Cotisel, M.; Zsak6 J.;Mocanu A,; Lupea, M.; Chifu, E. J. Colloid Interface Sci. 1987, 117, 464. (13)Gaines, G. L., J r . Insoluble Monolayers at the Liquid-Gas Interface; Wiley: New York, 1966. (14)Cary, A,; Rideal, E. K. Proc. Roy. SOC.London Ser. A 1926,109, 318.
wz.
(15)Smith, R. D.;Berg, J. C. J . Colloid Interface Sci. 1980, 74,273. (16) Marsden, J.; Schulman, J. H. Trans. Faraday SOC.1938, 34, 748. (17) Goddard, E.D.; Ackilli, J. A. J. Colloid Interface Sci. 196S,18, 585.
Influence of Metal Ions on Unsaturated Fatty Acids
Langmuir, Vol. 11, No. 11, 1995 4501 I
40 I
a 104M TbC13pH=6.9
a 102MMnC12 b lO'MTbCl3 c 10'4MCdC12 d pH=2
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b 104M TbCl,pH=5.7
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c pH=2
'
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mean molecular area [XI
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30 40 mean molecular area [A2]
50
60
Figure 3. The nlA isotherms ofelaidicacid on (a) M MnC12, (b) M TbC13, (c) M CdC12, and (d) pure water (pH =
Figure 4. The d A isotherms of oleic acid on (a) 10-4 M TbC13 (pH = 6.81,(b) 10-4 M TbC13 (pH = 5.7), and (c) water (pH =
2).
2).
electrolyte or 5 x M TbCl3. If [CdClzI exceeded M, however, the EA monolayer collapsed almost immediately after the transition. Monola ers with no sign of the LE state and an A, value of 22.5 were achieved by introducing TbC13 a t concentrations higher than 5 x M. These monolayers were very unstable, however, and the compression isotherms were strongly affected by changes in the barrier speed. This feature was not observed for MnC12;the condensed state with xc=- 45 mN/m was reached on M MnC12. The mean molecular area in the condensed state was close to 20 A2, corresponding to a close-packed arrangement of the hydrocarbon chains. The LE state was still present in the isotherms, however, and the A, value was 39.5 Az. No sign of the LE state was observed for Mn/EA a t [MnClzI 2 M. For OA, it was still impossible to achieve the condensed state a t any subphase composition a t pH = 5.7. Of the metal ions studied, Tb3+had the most marked influence on the shape of the compression isotherm. At TbC13 concentrations higher than M, xc was clearly lower than the ESP of the pure acid, with a limiting value of 21.1 mN/m a t [TbC131= M. No such change in xcwas observed on divalent salt solutions. Since the different behavior of OA in the presence of Tb3+was thought to be due to hydrolysis of the Tb3+ion, a series of measurements were carried out a t higher pHs a t a salt concentration of M. The increase in pH resulted in a n increase in xc, even to surface pressures well above the ESP of the pure acid. A new solid like phase was apparent in the isotherm as seen in Figure 4. The phase transition can also be seen as a change in the compressibility coefficient, K = -A( a d &4)~.The observed K value of 58.1 mN/m a t x = 15 mN/m for OA was obtained using a pure water subphase, and values 82.1 and 205.7 mN/m were obtained using M TbC13at pH = 5.7 and 6.8,respectively. Some preliminary measurements (results not shown) were also carried out a t pH = 4.4 using A13+as the counterion. A13+is known to hydrolyze readily at low pH. Here again very condensed monolayers were obtained with x, exceeding the ESP of OA. These findings support the suggestion that it is the interaction of the hydrolyzed Tb3+ ion with the OA monolayer which is responsible for the remarkable condensation of the monolayer. Surface Potential Measurements. The surface potential isotherms followed nicely the changes in the surface pressure-area isotherms and were generally very reproducible over the whole area range studied. A typical isotherm is shown in Figure 5. The value of AVincreases very sharply to reach a local maximum atA,, the molecular area corresponding to the initial increase in surface
iz
-
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i 0'
10
20
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Figure 5. The n/A and AVIA isotherms of petroselaidic acid on pure water (pH = 2).
pressure. Upon further compression the value of AVstill increases but more slowly, finally reaching a maximum at a molecular area close to the area of collapse of the monolayer. This maximum value, denoted AV,,,, is usually the most reproducible value for fatty acids and is thus usually taken as the value of comparison between different experimental conditions. The large difference in packing ability of the acids was clearly manifested in the measured values of AV, on a 0.01 M HC1 subphase. The AV, values of OA, 250 mV, and EA, 272 mV, were both obtained in the LE state, while PA yielded a much higher value of 412 mV corresponding to the LC state. These values should be compared with the corresponding value of stearic acid, 390 mV. On a pure water subphase the AV,, was decreased significantly to 146, 157, and 285 mV for OA, EA, and PA, respectively, reflecting the increased dissociation of the monolayers. Again, AV,,,,, for PA was higher than the corresponding value of stearic acid, 259 mV. Metal Ions Present. For all acids the observed value of AVmmwas strongly dependent on the concentration and the counterion present. Under equal experimental conditions AV, increased in the order of OA < EA < SA < PA. The plot of AV, vs salt concentration for SA and PA is shown in Figure 6. The shape of the curves for these two acids is very similar, resembling the shape of a titration curve. The curves have a plateau region a t low salt concentrations, an intermediate region where a large drop in AV, was observed, and finally a second plateau at high salt concentrations. The pronounced decrease in AV,, occurred a t salt concentrations where the conden-
4502 Langmuir, Vol. 11, No. 11, 1995
Linden and Rosenholm
i.
b 350 -
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Figure 6. AVma vs salt concentrationfor (a)petroselaidic acid and (b) stearic acid on ( 0 )TbC12, ( 0 )MnC12, and ( A ) CdC13 solutions at pH = 5.7. I I1W
270
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Figure 9. FTIR ATR spectra of stearic acid (a)pH = 2 and (b) M, (c) 5 x M, and (d) M TbC13.
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Figure 7. AVma vs salt concentration for elaidic acid on (0) CdC12, (0)MnC12, and (A) TbC13 solutions at pH = 5.7.
sation of the monolayer was observed, i.e. between 5 x and M, and M, and 5 x and 5 x M on subphases containingTbC13,CdC12, and MnC12, respectively. The absolute decrease in AV,, was very much a function of the counterion; the largest decrease was observed for Mn2+. The s-shape of the curve may be explained as a conversion of the acid monolayer to the corresponding metal salt (see later). The s-shape was not observed for Tb/EA, since the phase transition from the LE to the LC state with increasing TbC13 concentration was clearly seen as an initial increase in AV,, as seen in Figure 7. Apart from this feature, the shape of the AV,,lc curve is essentially the same for Tb/EA and Tbl PA. The early collapseof the EA monolayer in the presence of CdC12 is also seen in the surface potential results; the obtained AV,,,, value was lower than the corresponding value in the presence of MnC12. A continuous decrease in AV, with increasing CdCl2 concentration without reaching any final plateau region is observed. An even more
featureless AV,,lc curve was observed for OA. At a salt M, the AV, value increased slightly concentration of from 146 mV (no added salt) to about 175 mV, regardless of the counterion present. However, AV, decreased with increasing salt concentration as seen in Figure 8. While Cd2+and Tb3+affected the AV,, only slightly, Mn2+had astronger effect on the surface potential; a t M MnC12, AV, had decreased to 100 mV. If the pH was increased, the AVma increased on subphases containing TbCl3, in agreement with the findings that the OA monolayer was now in a solidlike state. ATR FTIR Results. An ATR FTIR study was carried out in order to be able to correlate the differences observed between the different metal iodfatty acid combinations and the change in dissociation degree of the monolayer. Since both the C=O and COO- groups have strong characteristic absorption bands in the IR region, ATR FTIR has proven to be a very useful technique for this type of study. Since stearic acid was transferable at any subphase condition, was chosen a s the model compound. Twentyfive layers of stearic acidlmetal stearate were deposited on a ZnSe ATR crystal at various subphase conditions. Some measurements were also carried out using petroselaidic acid and elaidic acid. Howeyer, no difference within experimental error in the degree of dissociation for SA, PA, and EA was observed. In Figure 9 the part ofthe IR spectra (1200- 1800 cm-l) which is associated with the polar region of the fatty acid is shown as a function of salt concentration for SA/Tb. Also included is the spectrum obtained a t pH = 2 with no salt added, which corresponds to the free acid (Figure 9).The doublet a t 1695 and 1705 cm-I corresponding to the C=O stretching vibration is clearly seen. The intensity
Influence of Metal Ions on Unsaturated Fatty Acids
-9
-8
.7
.6
-5 Ig c
-4
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Langmuir, Vol. 11, No. 11, 1995 4503
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Figure 10. Dissociation curves for stearic acid as a function of salt concentration: (v,0 )TbC13, (+, A) CdC12, and (0,stacked triangles) MnC12. Filled patterns measured by FTIR ATR, and empty patterns calculated by eq 3.
of these two peaks decreases with increasing salt concentration, and a new peak a t 1540 cm-l, corresponding to the COO- asymmetric streching band characteristic of the dissociated acid, appears in the spectra. At M TbCl the monolayer was totally converted to the soap form, i.e. the C-0 streching a t 1710 cm-' was absent. The degree of dissociation of the monolayer, a, a t different salt concentrations may be obtained by comparing the intensity of the peak a t 1705 cm-' with that of the CH bending vibration a t 1475 cm-' or the CH2 asymmetric strechingvibration a t 2918 cm-l.18 The resulting a c curve for SA on the different salts is shown in Figure 10. It is readily seen that the ions may be ordered with increasing interaction with the SA monolayer as Mn2+ < Cd2+
