J. Phys. Chem. B 2003, 107, 4573-4578
4573
Mechanism of Lecithin Adsorption at a Liquid|Liquid Interface Vladimir Marecˇ ek,* Alexandr Lhotsky´ , and Hana Ja1 nchenova´ J. HeyroVsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsˇkoVa 3, 182 23 Prague 8, Czech Republic ReceiVed: October 11, 2002; In Final Form: March 6, 2003
Adsorption of L-R-lecithin (dipalmitoyl) at the water|1,2-dichloroethane interface has been studied by cyclic voltammetry. It has been found that the voltammetric peak in acidic medium corresponds to the desorption of the protonated form of lecithin. Its surface concentration is controlled by a surface chemical reaction of the zwitterionic form of adsorbed lecithin with hydroxonium ion. The whole process is complemented by the potential dependent adsorption/desorption equilibrium of the protonated form of lecithin. Its concentration in the organic phase is affected by the acid-base equilibrium.
1. Introduction
2. Experimental Section
Ion transfer across a liquid|liquid interface modified by adsorption of phospholipids has been of increasing interest in recent decades not only because it can mimic one side of a biological membrane but also from the point of view of how it influences the structure of the interphase and the role it plays in the charge transfer kinetics. The relevant studies have focused primarily on the formation and description of the adsorbed monolayer. Girault and Schiffrin1 have shown that surface tension in the presence of lecithin at the water|1,2-dichloroethane (1,2-DCE) interface in an unbuffered or a slightly acidic medium is strongly potential dependent. This has been explained by the potential dependent protonation of the phosphate moiety of the adsorbed zwitterion. The decrease in the interfacial tension at negative potentials has been related to the adsorption of phospholipid as a zwitterion, while the increase at positive potentials has been accounted for by the adsorption of the cationic form of the phospholipid.1 Later it has been pointed out that the surface tension increase at positive potentials can be ascribed to desorption of the lipid,2-5 and an attempt has been made to maintain the surface coverage constant during a voltammetric cycle by controlling the surface pressure using the electrochemical Langmuir trough.4,5 An explanation of the phospholipid desorption has been offered on the basis of polarographic and drop time experiments in concentrated solutions of a phospholipid where micellization cannot be ruled out. The authors6 have suggested formation of a complex between hydrophilic cations in the aqueous phase and a phospholipid layer followed by the desorption of the complex from the interface. However, analysis of the lipid desorption process is still lacking.
2.1. Chemicals. Reagent grade chemicals from Fluka AG (LiCl, HCl, tetrabutylammonium chloride (TBACl), tetrabutylammonium tetraphenylborate (TBATPB), potassium tetrakis(4-chlorophenyl)borate (KTPBCl), potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTPBCF3), and Serva (L-Rlecithin, dipalmitoyl) were used as received. TBATPBCl was prepared by mixing acetone solutions of TBACl and KTPBCl. KCl was filtered out and TBATPBCl was crystallized from the filtrate. Highly purified and deionized water ( 0.25 V (curve d, Figure 3). The ratio of the rate constant k1/k2 calculated from eq 6 is shown in Figure 5, points 9. Application of eq 9 yields E0 ) 0.402 V (Ep ) 0.41 V) and (R + β)x ) 0.46 (Figure 5, curve a). A small deviation of k1/k2 from the straight line in the log scale observed at potentials below 0.27 V indicates that close to the potential of zero charge Epzc the surface charge density qw does not correspond to ΓH+(E). This is understandable, because ΓH+(E) can hardly be zero or negative. The values of ΓHL+(t,E) and ΓL((t,E) (Figure 4) at potentials close to Epzc were calculated using eqs 6, 4, and 3; the potential dependence of the ratio log k1/k2 was extrapolated to E ) 0.22 V. The dissociation constant
4576 J. Phys. Chem. B, Vol. 107, No. 19, 2003
Figure 5. Potential dependence of the rate constant ratio k1/k2 calculated according to mechanisms 1 [9, a] and 10 [b, b]. Points (9, b) were calculated by eq 6 for the experimental data in Figure 2, curves a and c. The fit (a, b) was by eq 9.
Marecˇek et al.
Figure 7. Surface concentration of L((ads) and HL+(ads) calculated according to mechanism 1: (9) ΓHL+(o)(t,E) ) q/F; (b) ΓH+(E) ) qw/ F. Experimental data are given in Figure 2, curves b and d.
Figure 6. Surface concentration of L((ads), HL+(ads), L((o), and HL+(o) calculated according to mechanism 10 for pKw ) 1.7. Experimental data are given in Figure 2, curves a and c.
Figure 8. Potential dependence of the rate constant ratio k1/k2 calculated according to mechanisms 1 [9, a] and 10 [b, b]. Points (9, b) were calculated by eq 6 for the experimental data in Figure 2, curves b and d. The fit (a, b) was by eq 9.
Kw (eq 3) is invariant in the whole potential range, and its value Kw ) 3.3 × 10-8 mol m-2 corresponds to pKw ) 7.5. Analysis of the voltammogram according to mechanism 10 does not provide the dissociation constants Kw and Ko but only their ratio Ko/Kw ) 33 m-1. The potential dependence of the ratio log k1/k2 (points b, Figure 5) does not change in the range of Kw between 1 × 10-5 and 1 mol m-2. Fit by eq 9 (curve b, Figure 5) yields E0 ) 0.41 V and (R + β)x ) 1.2. These data and pKw ) 1.7 found for the phosphate group of dimyristoyl phosphatidylethanolamine12 were used to calculate the surface concentrations of L( and HL+ shown in Figure 6. The positions of the voltammetric peaks of lecithin desorption/ adsorption depend on the pH of the aqueous phase, because production of the protonated form of lecithin (reaction mechanisms 1 and 10, step 2) depends on the H+ concentration. The forward peak potential of the voltammogram shifts by 0.06 V to less positive potentials (Ep ) 0.35 V) if 0.01 M HCl in water is replaced by 0.1 M HCl. Analysis of the voltammogram using eqs 1-6 and 9 (mechanism 1) yields the same value of Kw ) 3.3 × 10-8 mol m-2 as with 0.01 M HCl. Fit of eq 9 to the potential dependence of the ratio of the rate constants k1/k2 calculated from eq 6 yields E0 ) 0.34 V and (R + β)x ) 0.47. Analysis of the same voltammogram according to mechanism 10 yields E0 ) 0.347 V, (R + β)x ) 1.08, and the dissociation constants’ ratio Ko/Kw ) 34 m-1. The surface concentration of
ΓH+(E) ) qw/F was derived in both analyses from the surface charge density qw in 0.1 M HCl (Figure 3, curve e). 3.4. Base Electrolyte TBATPBCl. The shift of the voltammetric peaks of 0.06 V to positive potentials is observed when TBATPB is replaced by TBATPBCl without changing the concentration of HCl in the aqueous phase (Figure 2, curves a and b). This can be explained in terms of the proposed mechanisms by a decrease of the surface concentration of H+. Indeed, the surface charge density qw derived from the ac impedance measurement at potentials positive to Epzc is lower in TBATPBCl than in TBATPB (curves f and d in Figure 3). Analysis of the voltammogram shown in Figure 2, curve b, using eqs 1-6 (mechanism 1) supposing that surface concentration ΓH+(E) equals qw/F (points b in Figure 7), gives Kw) 3.2 × 10-8 mol m-2 and the k1/k2 potential dependence (points 9 in Figure 8). Fit of the log k1/k2 potential dependence (curve a in Figure 8) by eq 9 yields E0 ) 0.457 V and (R + β)x ) 0.455. The value of E0 is close to the peak potential Ep ) 0.47 V. The value of (R + β)x is close to that obtained in TBATPB, but the deviation of log k1/k2 from the straight line is larger than that in TBATPB. The surface concentration of the desorbed lecithin ΓHL+(o)(t,E) (points 9, Figure 7) was calculated from the passed charge q (curve d, Figure 2). Concentrations of ΓHL+(t,E) and ΓL((t,E) (Figure 7) were calculated using a linear fit to the log k1/k2 potential dependence (curve a, Figure 8).
Lecithin Adsorption at a Liquid/Liquid Interface
Figure 9. Surface concentration of L((ads), HL+(ads), L((o), and HL+(o) calculated according to mechanism 10 for pKw ) 1.7. Experimental data are given in Figure 2, curves b and d.
Analysis of the voltammogram according to mechanism 10 yields E0 ) 0.465 V, (R + β)x ) 1.28, and the ratio of dissociation constants Ko/Kw ) 225 m-1. These data and the value of pKw ) 1.7 (see above) were used to calculate the surface concentrations of L( and HL+ shown in Figure 9. The value of the dissociation constant pKw ) 7.5 obtained in the analysis of the voltammograms both in TBATPB and TBATPBCl according to mechanism 1 is much higher than the expected value,
