Langmuir 1992,8, 169-175
169
Ion Permeability of Dilauroylphosphatidylethanolamine Monolayer at the Polarized Nitrobenzene/Water Interface Takashi Kakiuchi,* Tetsuya Kondo, Mitsuyoshi Kotani, and Mitsugi Senda Department of Agricultural Chemistry, Faculty of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606, Japan Received May 17,1991. I n Final Form: August 6, 1991 The phase behavior and ion permeability of dilauroylphosphatidylethanolamine(DLPE) monolayers formed at the polarized nitrobenzene (NBYwater(W) interface have been studied by measuring ac impedance of the interface under the precise control of the potential drop across the interface. With increasing concentration of DLPE in NB, the monolayer formed by adsorption of DLPE from NB undergoes a phase transition from a gaseous to a liquid-expanded state at an occupied area of a DLPE molecule ( A ) being 0.83 nm2 and from the liquid-expanded to a liquid-condensed state at A = 0.50 nm2. The permeability of the monolayer with respect to ions depends on packing density of the monolayer, surface charge density, and charge and size of transferring ions. When the condensed DLPE monolayer is introduced to the interface, the decrease in the rate of ion transfer is detectable for tetraethylammonium ion (TEA+)transfer, but not C104- ion having a smaller ionic radius. The negatively charged DLPE monolayer appreciably reduces the rate of ion transfer for C104- ion and accelerates the transfer of TEA+ ion. The effect is reversed in the case of the positively charged DLPE monolayer. These facts clearly indicate the importance of hydrodynamic friction between the condensed monolayer and transferring ions and also of the electrical double layer effect on the kinetics of ion transfer across the DLPE monolayer.
Introduction Phospholipid monolayers formed a t the polarized oil/ water interface have recently been studied as a promising model to elucidate the electrical properties of lipid membrane/solution interfaces, e.g., structure of the electrical double layer and the rate of ion transfer across membrane/solution interfaces.l-ll An advantage of this system over monolayers a t an airlwater interface and a t a nonpolarizable oil/water interface is that the potential drop across the monolayer is precisely controlled externally and hence various electrochemical methods can be applied to characterize the m o n ~ l a y e r . ' ~ JSince ~ the free energy of ion transfer across the polarized oillwater interface is determined by the compositions of the oil and water phases and should not be influenced by the presence of a monolayer, the monolayer at the polarized oil/water interface is suitable for studying the kinetics of ion transfer across the monolayer. In particular, the effect of electrical double layer on the transfer of ions across the membrane/solution (1)Koryta, J.; Hung, L. Q.;Hofmanova, A. Stud. Biophys. 1982,90, 25. (2)Girault, H.H. J.; Schiffrin, D. J. In Charge and Field Effect in Biosystems; Allen, M. J., Usherwood, P. N. R., Eds.; Abacus Press: England, 1984;p 171. (3)Cunnane, V. J.; Schiffrin, D. J.; Fleischman, M.; Geblewicz, G.; Williams. D. J . Electroanal. Chem. Interfacial Electrochem. 1988.243. , . 455. (4)Girault, H. H. J.; Schiffrin, D. J. J . Electroanal. Chem. Interfacial Electrochem. 1984,179, 277. (5)Kakiuchi,T.; Nakanishi, M.; Senda, M. Bull. Chem.SOC.Jpn. 1988, 61, 1845. Nakanishi, M.; Senda, M.Bull. Chem. SOC.Jpn. 1989, (6)Kakiuchi, T.; 62,403. (7)Kakiuchi, T.; Yamane, M.; Osakai, T.; Senda, M. Bull. Chem. SOC. Jpn. 1987,60,4223. (8) Wandlowski, T.; MareEek, V.; Samec, Z. J. Electroanal. Chem. Interfacial Electrochem. 1988,242,277. (9)Kakiuchi, T.;Kondo, T.; Senda, M. Bull. Chem. SOC.Jpn. 1990, 63,3270. (10)Kakiuchi, T.;Kotani, M.; Noguchi, J.; Nakanishi, M.; Senda, M. J . Colloid Interface Sci., in press. (11)Girault, H. H. J.; Schiffrin, D. J. Biochim. Biophys. Acta 1986, 857. .. 251.
(12)Gavach, G.; Mlodnicka, T.; Guastalla, J. C. R. Seances Acad. Sci., Ser. C. 1968,266, 1196. (13)Koryta, J.; Vanfrsek, P.; Bfezina, M. J . Electroanal. Chem. Interfacial Electrochem. 1977,75, 211.
0743-7463/92/2408-0l69$03.00/0
interface14may be studied with the same exactness as that in the electron transfer across an electrode/solution interface.l5 In previous studies, we reported the monolayer prope r t i e ~double ,~ layer structure: and ion penetrabilitygJ0 of the phosphatidylcholine (PC) and phosphatidylserine (PS)monolayers adsorbed a t the nitrobenzene (NB)/water (W) interface by using electrochemical techniques, e.g., capacitance measurement, cyclic voltammetry, electrocapillarity, and ac impedance measurements. One of the interesting facta in the previous studies is that PC and PS monolayers in the liquid-expanded state do not reduce, or even increase, the rate of the transfer of tetramethyl- and tetraethylammonium ions,1° although the liquid-condensed PC monolayers do give rise to a 5-fold decrease in the rate of ion transfer.1° This unexpected observation suggests the importance of the double layer effectsl4J6 in addition to the packing density of the monolayer in ion permeation. Phosphatidylethanolamine (PE) is known to form a t an aidwater interface a monolayer denser than that of PC having hydrocarbon chains of the same length.I6 This characteristic of P E is of decisive importance in designing liposomes which possess desired permeability against certain substances and in fact, has been utilized to seal leaky liposome membranes.17J8 An amino group of P E in the hydrophilic head group makes it possible to change the surface charge density by changing pH in W. Hence, the P E monolayer is an interesting system also in elucidating the double layer effect on ion transfer across a lipid monolayer. The purpose of the present study is to estimate the permeability of P E monolayers in different charged states by determining the kinetic parameters of (14)Hainsworth, A. H.; Hladky, S. B. Biophys. J. 1987,51,27. (15)Delahay, P. Double Layer and Electrode Kinetics; Interscience: New York, 1965; Chapter 7. (16)Mingins, J.; Llerenas, E.; Pethica, B. A. In Ions in Macromolecular and Biological Systems; Everett, D. H., Vincent, B., Eds.; Colston Papers No. 29;Scientechnica: Bristol, 1978; p 41. (17)Noordam, P. C.; Killian, A.; Oude Elferink, R. F. M.; De Gier, J. Chem. Phys. Lipids 1982,31,191. (18)Van Hoogevest, P.; Du Maine, A. P. M.; De Kruijff, B.; De Gier, J. Biochim. Biophys. Acta 1984,771, 119.
0 1992 American Chemical Society
Kakiuchi et al.
170 Langmuir, Vol. 8, No. 1, 1992 ion transfer across the monolayer and to compare the results with those of PC and PS. Since the properties of PE monolayers at the polarized O/W have not fully been characterized yet,4 we first deal with the monolayer properties of L-a-dilauroylphosphatidylethanolamine (DLPE) at the NB/W interface, and then describe the permeability of the monolayer on the basis of the kinetic measurements.
Experimental Section DLPE was obtained from Sigma Ltd. and was used without further purification. Tetrapentylammonium tetraphenylborate (TPnATPB) was prepared from tetrapentylammonium iodide and sodium tetraphenylborate and was twice recrystallized from acetone-ethanol mixture. An aqueous solution of reagent-grade tetrapentylammonium chloride ((TPnA)Cl) was treated with silverchlorideto removetrace iodideion. Lithium chloride monohydrate (Merck, Spurapur grade) was dissolved in water to prepare the stock solution. The concentrations of (TPnA)Cl and LiCl solutions were determined with potentiometric titration. Nitrobenzene was distilled under reduced pressure. The middle 60% of the distillate was shaken with active alumina and then equilibratedwith water after filtrating out the precipitates. Triply distilled water was used throughout the measurements. Allother chemicals used were of reagent grade. The electrochemical cell is represented by I
I1
Ag
1 I
I11
50 mM
I
IV 0.1 M TPnATPB
AgCl (TPnA)Cl apMDLPE (W (NB)
v
1 Lgl 1
VI
VI1
0.1 M
AgCll Ag
The interface between phases IV and V is the polarized NB/W interface. The potential of the right-hand side of the cell with respect to the left is denoted as E. By using the potential of zero charge for the present system at a = 0, E = 0.34 V,5 E can be converted to the inner potential difference between W and NB in the rational potential scale, &"(p, defined by LWi=pE - E,.'$ The potential range negative (positive)to E = 0.34V will hereafter be called the negative (positive) branch. We adopted a two-electrode system for electrochemical meas~rementa.~ The cell design was similar to a type reported previously.9 A flat polarized NB/W interface was formed at the upper part of the narrowed cylindrical portion of the cell. The area of the polarized interface was 0.196 cm2. Other details of the cell have been described el~ewhere.~ The impedance characteristic of a two-electrode system in determining kinetic parameters of ion transfer has been discussed elsewhere.20s1DLPE monolayers were formed by two methods: adsorption method and spread m e t h ~ d .In ~ the former, DLPE molecules were allowed to adsorbat the interface from the NB containing DLPE. In the latter, DLPE was dissolved in a 3:2 toluene-chloroform mixture and 1 X lo4 dm3 of the DLPE solution containing a certain amount of DLPE was applied to the interface. In the spread method, DLPE molecules applied to the interface gradually desorbed into the NB phase, as is the case of DLPC monolayers.' The time-dependent capacitance values were extrapolated to t = 0 to obtain a capacitance value corresponding to the monolayer composed of an added amount of DLPE. The r was changed by changing the concentration of DLPE in the spreading solution, while the volume of the solution was kept constant at 1X 104 dm3. The details of this method have been described el~ewhere.~ Cyclic voltammograms for the transfer of tetraethylammonium ion, TEA+, and Clod- ion were recorded at a scan rate between 20 and 200 mV 5-1. The real and imaginary parts of the impedance were measured by means of a phase-selective ac polarography,as described elsewhere.1° Apositive feedback method (19) Grahame, D. C. Chem. Rev. 1947,41, 441. (20) Kakiuchi, T.; Senda, M. Collect. Czech. Chem. Commun. 1991, 56, 112.
(21)Kakiuchi, T.; Noguchi, J.; Kotani,M.; Senda,M. J . Electroanal. Chem. Interfacial Electrochem. 1990, 296, 517.
I 0.2
i
l
0.4
0.3 E/V
Figure 1. Double layer capacitance vs applied potential curves at adsorption equilibria for the interface between a 0.1 M LiCl aqueous solution and a nitrobenzene solution containing 0.1 M TPnATPB and 0 (- - -1, 1 (O),2 (A),5 ( O ) , 10 (0),20(A),or 50 (M) pM dilauroylphosphatidylethanolamine (2' = 25.0 OC).
was employed for the compensation of an ohmic potential drop. An ac voltage of 5 mVPP with a frequency between 10 and 100 Hz was superimposed on the dc voltage. In the adsorption method, ac polarograms were usually recorded with a scan rate of 0.5 mV s-* from E = 0.15 to 0.48 V for a cation transfer and from E = 0.48 to 0.15 V for an anion transfer. In the spread method, the impedance was measured at a fixed dc potential and at 50 Hz as a function of time. The double layer capacitance of the interface, Cd, was calculated from the imaginary component of the admittance at 50 Hz.7 All measurements were made at 25.0 i 0.5 O C .
Results and Discussion Monolayer Formation by Adsorption Method. After the formation of a polarized NB/ W interface, the c d l value at a given value of E decreased with time and reached a time-invariant value, which ensured the establishment of adsorption equilibrium. Figure 1shows the equilibrium c d l vs E curves at six different concentrations of DLPE in NB, The decreased with and up to cEFPE= 20 pM;a saturated DLPE monolayer was formed above this concentration. The Cd was lowered in the entire range of E studied; the adsorption of DLPE is stable in both positive and negative branches, in distinction from monolayers of phosphatidylcholines with aliphatic chains shorter than palmitoyl r e s i d ~ e . ~In~the J ~latter case, the c d l value in the positive branch exceeds the value for the base solution probably due to the partial desorption and reorientation of adsorbed PC molecules.6 When the concentration of DLPE was lower than 5 pM, the Cdl vs E curves had a common intersection point at E = 0.25 V. A thermodynamic relation for the adsorption of organic compounds at an electrified interface22
cE!~~.
(ac,,/wE= r,(d2e/aE2)
(1)
predicts that this point corresponds to a point of inflection on the adsorbed amount of DLPE vs E curves. In eq 1, p is the chemical potential of an adsorbate, rm is the maximum adsorption,and 8 = r/r, is the surfacecoverage, where r is the adsorbed amount of an adsorbate. The presence of this point implies that the rearrangement of adsorbed molecules with the concentration of the adsorbate in a solution is negligible.22 When c;fpE was higher than ~~
(22) Kakiuchi, T.; Senda,M. J.Electroanal. Chem. Interfacial Electrochem. 1978, 88, 219.
Langmuir, Vol. 8, No. 1, 1992 171
DilauroylphosphatidylethanolamineMonolayer
rxdO/ mo~cm2 0
1 I
1
2
3
1
I
!
4 I
I
- 0
0
10
20
30
40
1
I
50
/ pmol d m 3 Apparent surface coverage eappvs the concentration %fPE
Figure 2. and 0.455 of DLPE in the nitrobenzene phase at E = 0.380 (0) ( 0 )V. See eq 4 in the text.
10 p M , the curves deviated from the point downward (Figure 1). This change in the mode of concentration dependence of the c d l vs E curves suggests the change in the adsorbed state of DLPE molecules.22 The change in the adsorbed state of DLPE molecules is more clearly seen in the plots of eappvs c;fpE a t E = 0.380 and 0.455 V (Figure 21, where 8app = (OCdl- c d l ) / ( o c d l - '&I), and OCdl and 'Cdl are the c d l values in the absence and presence of a saturated DLPE monolayer. The two curves in Figure 2 show kinks around c:FpE = 5 p M . When an adsorbate takes only one adsorbed state, the Frumkin's two parallel plate condenser model of the form23 (1 - 8)OCdl + 8'Cdi (2) may be applied to describe the change in c d l with adsorption. OaPp in Figure 2 then represents the actual surface coverage 8. If the DLPE monolayer undergoes a phase transition, there is a range of r in which the DLPE molecules in two states (states 1 and 2) coexist in the monolayer. Then, eq 2 may be extended to cdl
+
(3) (1 - 81 - 82)OCdI 8l1CdI + 8:cdl where ea and aCdl are the surface coverage defined by I?,/ J?m,a and the saturated capacitance for state a (a = 1 or 2). rm,a is the maximum adsorption of the adsorbate in state a. If we let 'Cdl be the observed c d l value a t the observed saturated monolayer, eappis given by cd1
eapp = 82 + [(OCd] - l c d I ) / ( o c d l - 2Cd1)le, (4) That is, the ordinate in Figure 2 represents the sum of the two surface coverages for the two states, the coverage of state 1 being scaled by [(o~dl-1~dl)/(ocdl-2~dl)181. Figure 2 yet evidences the phase transition of the monolayer. Monolayer Formed by the Spread Method. The phase transition of DLPE monolayer was confirmed by using the spread method. In Figure 3, the c d l values at E = 0.250 Vextrapolated to t = 0, Cdl,t=O, are plottedagainst r. The r values were calculated from the volume and the concentration of the solution applied to the interface, assuming that a t t = 0 all DLPE molecules applied stayed a t the interface as a monolayer. The Cdl,t=O decreased linearly with l? (Figure 3), as predicted by eq 2. A further increase in r brought no change in C ~ I , ~up= O to l? = 2.75 X mol cm-2. The increase in r beyond this value
(23)Frumkin, A. N.;Damaskin, B. B. In Modern A s p e c t s of Electrochemistry; Bockris, J. O'M., Conway, B. E., Eds.; Butterworths: London, 1964;Vol. 3, p 149.
O
0
L
I
50
1
1
100
Figure 3. Double layer capacitance extrapolated to t = 0, Cd,t.~, against the spread amount of DLPE at the interface at E = 0.25 V. The volume of the spread solution was kept constant at 1rL. CDLPE,~denotes the concentration of the DLPE solution, and r represents the amount of DLPE applied to the interface. diminished the Cdlvalue again, and eventually gave a stable value a t r = 3.20 X 10-lO mol cmW2.The point of inflection in Figure 3 a t r = 1.35 X 10-lomol cm-2 indicates a phase transition of the DLPE monolayer. The value of r = 1.85 X mol cm-2 corresponds to 0.90 nm2 for the area occupied by a DLPE molecule a t the interface and suggests that the monolayer is in a liquid-expanded state. The c d l value was constant a t 10.5 pF cm-2 between r = 1.85 X and 2.75 X mol cm-2. This c d l value is comparable with a value of 11 pF cm-2for the DLPC monolayer in the liquid-expanded state.7 The transition a t r = 1.85 X 10-lo mol dm+ is therefore attributable to the transition between a gaseous state and a liquid-expanded state of the monolayer. At the point of the second transition in Figure 3 a t = 3.2 X 10-lo mol cm-2, A is calculated to be 0.52 nm2, which is larger than the value of A obtained for the DLPE monolayer in the solid phase a t the aidwater interface,24 and also than A = 0.386 nm2for DLPE in a solid crystal.25 The c d l value of 7 pF cm-2 a t this stage, which well agrees with 7 pF cm-2 for the saturated monolayer formed by the adsorption method (Figure 11, is significantly lower than the C d l value for a saturated monolayer of DLPC in a liquidexpanded state, 11 pF cm-2, a t the NB/W interface and larger than the c d l value of 1-2 pF cm-2 obtained for the densely packed PC and PS m o n ~ l a y e r s . ~ItJ ~is thus likely that the P E monolayer having an A value of 0.52 nm2 is in a liquid-condensed state, similar to the liquid-crystal state or gel state of P E monolayers and bilayers having an A value of about 0.50 nm2.24925The results of the spread method thus confirm the phase transition from a gaseous to a liquid-expanded state and the further transition to a liquid-condensed state. Since the adsorbed monolayer of DLPC at the NB/W interface is in a liquid-expanded state even a t a saturation coverage6*'a t 5 OC,l0 the observed denser DLPE monolayer at the NB/ W interface indicates a stronger interaction (24)Standish, M. M.;Pethica, B. A. Trans. Faraday SOC.1968,64, 1113. (25)Hauser, H.; Pascher, I.; Pearson, R. H.; Sundell, S. Biochim. Biophys. Acta 1981,650, 21.
Kakiuchi et al.
172 Langmuir, Vol. 8, No. 1, 1992
I
01
0.2
I
I
0.3
I
1
0.4
1
1 1
I
I
I
E / V
Figure 4. Double layer capacitance vs applied potential curves in the presence of DLPE monolayers formed by the adsorption of DLPE from the NB phase containing 20 pM DLPE. The composition of the aqueous solution is 0.09 M LiCl + 0.01 M LiOH (pH 11.8, curve l),0.1 M LiCl (pH 6.2, curve 2), and 0.1 M HCl (pH 1.4, curve 3).
I
1
0.2
03
I 1 0.4
I
0.2
I
03
I
0.4
E I V
between the head groups of adsorbed P E molecules, as is observed in other P E a ~ s e m b l i e s . ~ This ~ - ~ ~stronger interaction between the P E molecules and larger hydrophilicity of the PE head group probably endows the monolayer with stronger stability against the applied voltage in the positive branch in comparison with PC monolayer~.~~8JO Effect of pH on the State of DLPE Monolayer. Figure 4 shows the c d l vs E curves for c:&~ = 20 pM recorded a t three compositions of the W phase: 0.01 M LiOH + 0.09 M LiCl (pH 11.8, curve 11,O.l M LiCl (pH 6.2, curve 2), and 0.1 M HC1 (pH 1.4, curve 3). A t pH 11.8, the position of the minimum in curve 1 is located in the positive branch; i.e., DLPE is surface-active in the positive branch. This potential dependence of the capacitance is similar to that found in the adsorption of an anionic surfactant a t the oil/water interface,28and indicates that a t pH 11.8the P E monolayer bears net negative charge due to the deprotonation of the amino group (pK, = 9.02993010.924). The c d l value a t the minimum of the c d l vs E curve increased from 7 to 9.5 pF cm-2 on going from pH 6.2 to 11.8. This change in c d l can be ascribed to t.he expansion of the monolayer due to the electrostatic repulsion between the adsorbed DLPE molecules, as is observed in the P E monolayers a t the air/water interface in contact with an aqueous alkaline e 0 1 u t i o n . l ~ ~ ~ ~ In contrast, the minimum in curve 3 in Figure 4 is in the negative branch. A backward scan of E to the negative direction after the forward scan from 0.18 to 0.43 V (broken line in Figure 4) gave much higher c d l values, suggesting partial desorption of DLPE molecules in the positive branch. Thus, a PE monolayer in contact with an aqueous solution a t pH 1.4 behaves as a cationic surfactant32and demonstrates that the monolayer bears net positive charge. A range of pKa values has been reported for the phosphate group in P E in various systems.33 The present result suggests that the pKa value is above 1.4, since a considerable portion of the P E molecules are presumably (26)Seddon, J . M.;Harlos, K.; Marsh, D. J . Biol. Chem. 1983,258, 3850. (27)Chang, H.; Epand, R. M. Biochim. Biophys. Acta 1983,728,319. (28)Wiles,M. C.; VanderNoot,T.; Schiffrin,D. J . J.Electroanal. Chem. Interfacial Electrochem. 1990,281,231. (29)Cadenhead, D. A.;Demchak, R. J.; Phillips, M. C. Kolloid 2.2. Polym. 1967,220,59. (30)Minones, J.; Sandez Macho, M. I.; Iribarnegaray, E.; Sanz Pedreno, p.; Colloid Polym. Sci. 1981,259,382. (31)Gorwyn, D.; Barnes, G. T. Langmuir 1990,6 , 222. (32)Kakiuchi, T.; Kobayashi, M.; Senda, M. Bull. Chem. SOC.Jpn. 1987,60,3109. (33)Tocanne, J . F.;Teissie, J.Biochim.Biophys. Acta 1990,1031,111.
Figure 5. Change in the imaginary part of the admittance, Y”, when, after the formation of a saturated DLPE monolayer at the interface between an aqueous solution of 0.1 M LiCl and a nitrobenzene solution 0.1 M TPnATPB (a) and 10 pM DLPE, the aqueous phase was changed successively with 0.1 M LiCl + 1mM TEACl (b),0.1 M LiCl + 1mM LiC104- (c), and 0.1 M LiCl (d). The broken line in (a) illustrates the Y” in the absence of the monolayer. positively charged. The c d l value a t the minimum, 6 pF cm-2, is 1pF cm-2 lower than the minimum value of c d l in curve 2. Apparently, the positive charge on the monolayer does not induce an expansion of the monolayer. No sign of the expansion of positively charged P E monolayers has been reported also in the P E monolayers a t the aidwater interfaces3’ We hereafter call the P E monolayers in contact with 0.01 M LiOH + 0.09 M LiC1, 0.1 M LiCl, and 0.1 M HCl, negatively charged, neutral, and positively charged monolayers, respectively. Kinetic Parameters of Ion Transfer across the DLPE Monolayer. The rate of the transfer of TEA+ and ClC4- ions was measured in the absence and presence of the DLPE monolayer formed by the adsorption of DLPE from a NB solution containing 20 or 50 pM. After the formation of a stable monolayer, the aqueous solution (phase V) was replaced with an aqueous solution containing 0.5 or 1mM (TEA)Cl or LiC104. The stability of a DLPE monolayer against this procedure is exemplified in Figure 5, in which the imaginary parts of the admittance vs E curves are shown in the presence of a neutral DLPE monolayer formed a t c:EPE = 20 pM when the aqueous phase is successively changed from 0.1 M LiCl (Figure 5a) to 0.1 M LiCl + 1mM (TEA)Cl (Figure 5b), 0.1 M LiCl + 1mM LiC104 (Figure 5c), and 0.1 M LiCl (Figure 5d). A comparison of Figure 5a with Figure 5d shows that the monolayer is still intact after the ac impedance measurements. Prior to each admittance measurement, we recorded a cyclic voltammogram. Within the range of scan rate between 20 and 200 mV s-1, the voltammograms for both TEA+ and Clod- ion transfers were dc reversible even in the presence of a P E monolayer. The values of midpoint potential, E,, obtained from the voltammograms are shown in Table I for TEA+ and C104-. The real and imaginary parts of the admittance for an ion transfer, YIT’ and YIT”,were obtained by subtracting the real and imaginary parts of the admittance in the
Langmuir, Vol. 8, No. 1, 1992 173
Dilauroylphosphatidylethanolamine Monolayer
Table I. Kinetic Parameters of the Transfer of Tetraethylammonium Ion and Perchlorate Ion across the Nitrobenzene/ Water Interface in the Presence of Dilauroylphosuhatidylethanolamine Monolayer TEA+ c104~% ,c' uM E,, mV k., cm 5-l a E,, mV k., cm s-1 a ~~
~
~
~~
~~
0.05 (0.02)" 0.04 (0.01)" 0.03 (0.01)" 0.07 (0.02)"[+Id 0.04 (0.01)" [-Id
275 279 282 298 294
0 20 50 206 2oE
0.5 0.6 0.5 0.7 [+Id
237 237 238 247 245
0.05 (0.01)" 0.06 (0.01)" 0.05 (0.01)" 0.03 (0.01)"[-Id 0.07 (0.02)"[+Id
0.4 0.5 0.5 0.1 [+Id 0.5 [ + I d
0.8 [+Id Standard deviation. The aqueous phase contains 0.09 M LiCl + 0.01 M LiCH (pH 11.8). The aqueous phase contains 0.1 M HC1 (pH 1.4). d The direction of the change in k, or a due to the double layer effect predicted by eqs 8 and 9.
61
I
I
0.10
0.15
I
l a
C
\
FI
I
a 1 bq
I
N
1
N
-I
0
0.05
I
-1/2/s1/2
i
0.2
0.4
0.3 E / V
0.5
C
4
1
'
1
'
\
'
a 3 bq
hi
ru
I
1
0.15
0.10
0.05
0
w-I/2/s1/2
0.4 0.3 E / Y Figure 6. The real parts (0,A) and imaginary parts (0,A) of ion transfer admittancesat 50 Hz for TEA+(a)and c104-(b)ions in the absence ( 0 , O ) and presence (A,A) of a saturated DLPE monolayer at the interface between an aqueous solution of 0.1 M LiCl and a nitrobenzene solution of 0.1 M TPnATPB and 50 pM DLPE. 0.2
absence of a transferring ion from those in the presence of a transferring ion. Typical results are shown in Figure 6a for TEA+ ion transfer and in Figure 6b for Clod- ion transfer. The peak potentials (E,) in YIT' and YIT" agree within experimental error (*2 mV) with the E, values obtained from cyclic voltammetry, except one in the YIT" vs E curve for the clod- ion transfer in the presence of a negatively charged monolayer. The positive deviation of the peak potential from the E, in the latter case implies that the transfer coefficient is significantly smaller than 0.5.21 The admittance was converted to the real and imaginary parts of the ion-transfer impedance, ZIT'and ZIT".^^ Figure 7 shows for TEA+ and clod- ion transfer the ZIT' and ZIT" as a function of d I 2 , where w = 27rf and f is the ac frequency. The plots in Figure 7a represent the ZIT' and ZIT" in the presence of a neutral DLPE monolayer formed = 50 p M and, in Figure 7b, the ZIT' and ZIT'' in at the presence of a negatively charged DLPE monolayer formed a t c:EpE = 20 p M . In each case, the plots gave two
Figure 7. Real (0) and imaginary ( 0 )parts of the ion transfer impedance vs w-l/* for TEA+ ion and C104- ion transfers. The monolayer was formed at the interface between an aqueous solution of 0.1 M LiCl and a nitrobenzene solution of 0.1 M TPnATPB and 50 pM DLPE (a) and the interface between an aqueous solution of 0.09 M LiCl + 0.01 M LiOH and an nitrobenzene solution of 0.1 M TPnATPB + 20 pM DLPE (b). The broken lines indicate the real part of the impedance in the absence of a DLPE monolayer. straight lines in parallel. The line for ZIT' in the absence of the monolayer is shown as a broken line in Figure 7a for TEA+ and clod- in Figure 7b. The corresponding plot for the ZIT"was perfectly overlapped on the plot for ZIT" in the presence of the monolayer in both cases. The diffusion coefficientsof TEA+ and clod- ions in W obtained from the slopes of these plots were 7.8 X lo+ and 14.8 X lo4 cm2 s-*, respectively, and are in excellent agreement with the literature value^.^^^^^ Thus, the transfer of TEA+ and clod- ions across the monolayer can be considered to be a simple ion transfer
(5) ion (W) i- ion (NB) without complication due to the adsorption of reactant ions a t the interface35or due to the reduction of effective area for ion transfer36 which is expected if solidifed P E assemblies in the monolayer partially block the ion ~
~
~
~
~
~~
~
(34) Wandlowski, T.;MareEek,V.; Holub, K.; Samec,2.J.Phys. Chem. 1989, 93,8204.
(35) Senda, M.; Delahay, P. J . Phys. Chem. 1961, 65, 1580. (36) Tokuda, K.; Gueshi, T.; Matsuda, H. J. Electroanal. Chem. Interfacial Electrochem. 1979, 102, 41.
Kakiuchi et al.
174 Langmuir, Vol. 8, No. 1, 1992 51
0 ' -3
I
I
-2
-1
0
1
2
3
5 Figure 8. In [&/(Dw)1/2] vs ( in the absence (A)and in the presence of DLPE monolayer formed at the interface between an aqueous solution of 0.1 M LiCl and an nitrobenzene solution and at the interface of 0.1 M TPnATPB + 50 fiM DLPE (0) between an aqueous solution of 0.09 M LiCl + 0.01 M LiOH and a nitrobenzene solution of 0.1 M TPnATPB + 20 fiM DLPE (0). transfer. The interaction of CQ- ions with P E assemblies reported a t higher concentrations of LiC10437is apparently negligible in the present system. We may then write for the current, I , due to the transfer of i ionz1 I/ziFA = AociW- LociNB
(6)
where zi is the electric charge on i ion, F is the Faraday constant, A is the cross-sectional area of the interface, k and h are the rate constants for forward and backward reactions in eq 5, and Ociw and OciNB are the surface concentrations of i ion in W and in NB. Standard proceduresz1 were applied to real and imaginary components of the ion transfer impedances to obtain kinetic parameters of ion transfer for the simple firstorder reaction represented by eq 6. The values of diffusion coefficients of TEA+ and clod- ions in YB were assumed to be half those in W.zl Some of the In [k/(DW)1/23 values are plotted against E = (ziF/RT)(E - ' E l p ) in Figure 8, where Dw is the diffusion coefficient of TEA+ or Clod- ion in W and 'E1p is a reversible half-wave potential. Curves 1, 2, and 3 correspond to the TEA+ ion transfers in the absence of a P E monolayer, in the presence of a neutral DLPE monolayer a t c;tpE = 50 p M , and in the presence of a negatively charged DLPE monolayer a t c;fpE = 20 pM,respectively. The apparent standard rate const@ of ion transfer, k,, was calculated from the value of k at *E1pand the diffusion coefficients of the ion in W and NB. The results are listed in Table I together with the values of apparent transfer coefficients, a,defined by a = (d In i / d E ) / ( z i F )
(7)
estimated a t 'Elp It is discernible in Table I that the introduction of a neutral DLPE monolayer in the liquidcondensed state a t c:EpE = 20 or 50 pM reduces the k , value of TEA+ ion transfer. A positively (negatively) charged P E monolayer decreases (increases) the k, value for TEA+ ion transfer, and both monolayers give rise to an appreciable increase in a. In the case of clod- ion transfer, the neutral DLPE monolayer induces no appreciable change in k, and a (Table I). In contrast, the negatively charged DLPE monolayer reduces both k, and a €or Clod- ion transfer.
Hydrodynamic Friction of Neutral PE Monolayer on Transferring Ions. The differences in the influence (37) McLaughlin, S.; Bruder, A.; Chen, S.; Moser, C. Biochim. Biophys. Acta 1975, 394, 304.
of the neutral DLPE monolayer on the TEA+ and clodion transfers is attributable to the difference in hydrodynamic friction between the monolayer and the transferring ions. The Cdl value and the occupied area for the neutral DLPE monolayer suggest that the interface is not completely covered by DLPE molecules, and free surface filled with solvent molecules still exists even in the liquidcondensed state. Under this condition, a smaller ion like Clod-can pass through the monolayer without significantly interacting with the DLPE molecules in the monolayer. The observed transparency of phospholipid monolayers to ion transfer is also the case of the transfer of tetramethylammonium ion and TEA+ ion across the DLPC monolayer in the liquid-expanded state.'O The ion transfer in this case resembles the permeation of uncharged substances across the monolayer a t the air/water interface, in that the amount of free area in the monolayer is a decisive factor in determining p e r m e a b i l i t ~ . ~ ~ However, the DLPE monolayer in the liquid-condensed state is yet more densely packed than the DLPC monolayer in the liquid-expanded state. A hydrodynamic friction on TEA+ ions whose hydrodynamic radius is 0.28 nm and is 2.1 times larger than that of clod- 39 should then be greater than that on clod- ion. In other words, the transferring TEA+ ions have higher probability to collide with DLPE molecules a t the interface than c104ions. This explains the observed decrease in k, only for TEA+ ion. If this kinetic barrier of DLPE monolayers against the ion transfer is simply attributable to the increase in microviscosity a t the interface, the degree of retardation of ion transfer would be independent of E, provided that the structure of the monolayer is not appreciably dependent on E. Indeed, the intzoduction of the neutral DLPEmonolayer shifted the In [k/(DW)l/zlvs [ plot for the TEA+ ion transfer downward in parallel with the plot in the absence of the monolayer (Figure 8, curves 1and 2). In charge-pulse experiments of ion transfer across bilayer lipid membranes, Benz found that the rate constants of the translocation of a series of lipophilic anions are greater in dioleoylphosphatidylethanolaminemembrane than in a dioleoylphosphatidylcholinemembrane.40 This is a t variance with the present results together with our results for PC monolayers.lO It is not clear a t this moment whether the observed difference in the effect of the head group of phospholipids on the rate of translocation of ions across the monolayer and bilayer reflects the difference in mechanism between the ion transfers across the monolayer and the bilayer. Double Layer Effect on Ion Transfer across the PE Monolayer. Since the negatively charged DLPE monolayer is probably in the liquid-expanded state, the observed decrease ink,, not to say the acceleration, cannot be attributed to the friction exerted by the DLPE molecules. The k, is related to the true standard rate constant of ion transfer, i.e., the standard rate constant of ion transfer in the absence of the diffuse double layers in both NB and W, throughd1
k, = k,,t exp[-atzif(vz 0 - cp011 exp[-(l- at)zif(vzW- vW)1 (8) where at is the transfer coefficient in the absence of the (38)Blank, M. In Progress in Surface and Membrane Science; Cadenhead, D. A., Danielli, J. F., Eds.; Academic Press: New Yolk, 1979; Vol. 13, p 107. (39) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Butterworths: London, 1959, p 124. (40) Benz, R. Biophys. J. 1988,54, 25. (41) Samec, Z. J.Electroanal. Chem. Interfacial Electrochem. 1979, 99, 197.
Dilauroylphosphatidylethanolamine Monolayer
Langmuir, Vol.8,No.1, 1992 175
double layer effect, f is FIRT, eoand (ozw are the outer Helmholtz potentials in NB and W phases, and qo and qw are the inner potentials in NB and W phases, respectively. The a is related to at
The predicted change is less obvious in the case of C104ion transfer across the positively charged monolayer. The negative scan of E required in the measurements Of C104ion transfer might have destroyed a certain part of the positively charged DLPE monolayer (see curve 3 in Figure a = at - atd((p: - (pO)/dA,"(p- (1- at)d(qkW- (pw)/dA,Wq 4). In C104- ion transfer across the negatively charged DLPE monolayer, the decrease in k, is in the same trend (9) predicted by the double layer effect. However, the decrease in a is opposite to the trend expected from the In the absence of specific adsorption of ions, eo- (Po and double layer effect. Moreover, the E, value in the Y" vs ew- pw are in the same order of magnitude but with E plot significantly deviated from E , as described above, opposite signs.43 The double layer effects in NB and W which does not conform to the case of a simple ion transfer on k,,t are thus counteractive. Since at is about 0.5 for the transfer of monovalent ions across the NBIW interface,2l~~~ reaction.21 Specific interaction between clod- and the DLPE monolayer3' which cannot be detected by the 2' k , is not very much different from k,,t. In this case, a also and 2" vs u1/2plots (Figure 7b) might be involved in the does not deviate from at, since the second and third terms ion transfer process. on the right-hand side of eq 9 are canceling each other. However, in the case of the positively charged and Conclusions negatively charged DLPE monolayers, the double layer structure on both sides of the interface is likely to be similar The DLPE monolayer a t the polarized nitrobenzene/ to those in the presence of the adsorption from NB of water interface can be made negatively charged, neutral, cationic and anionic surfactant, respectively. Then, eqs and positively charged, depending on the pH in the aqueous 8 and 9 predict the change in k, and whose direction solution. Using these DLPE monolayers whose phase is indicated by k, and a values in Table I. The observed properties are well-characterized, we have shown on the tendency in the change of k , and a upon charging of the basis of kinetic measurement of ion transfer that the DLPE monolayer for TEA+ ion transfer is in qualitative permeability of the monolayer is primarily determined by agreement with the prediction, although more accurate the two factors, Le., the hydrodynamic friction and the data of the potential distribution is required to make a double layer effect against transferring ions. Detailed quantitative double layer correction. The increase in both pictures of the double layer structure in the vicinity of the k , and a for the negatively charged DLPE monolayer is monolayer and more precise measurements of kinetic also clearly seen in curve 3 in Figure 8. parameters of ion transfer will allow us to understand the mechanism of the ion transfer on a more quantitative basis. (42) Kakiuchi, T.; Noguchi, J.; Senda, M. Bull. Chem. SOC.Jpn. To be submitted for publication. (43) Kakiuchi, T.; Senda, M. Bull. Chem. SOC. Jpn. 1983, 56, 1753.
Registry No. DLPE, 42436-56-6; NB,98-95-3;TEA+,6640-0;Clod-,14797-73-0.
