Interaction of DPPC monolayer at air–water interface with hydrophobic ions

V.L. Shapovalov

Institute of Chemical Physics RAS, Kosygina 4, Moscow 117977, Russia

Abstract

Interaction of a Langmuir monolayer of dipalmitoylphosphatidylcholine (DPPC) with different hydrophobic ions added to the aqueoussubphase was studied using surface pressure-area and surface potential-area isotherm techniques. Strongly hydrophobic tetraphenylborate(TPB) anions cause marked expansion of the monolayer and drastic decrease of surface potential. Both effects grow gradually with the TPBconcentration, the first shows also an increase with the ionic strength. The influence of the less hydrophobic picrate anions is qualitativelysimilar, but essentially weaker than that of TPB. The results obtained can be described in terms of the competition of two effects: (i) boththe hydrophobic interaction and local electrostatic attraction between anions and ammonium moiety of DPPC promote incorporation ofhydrophobic ions into the monolayer, that results in its charging; (ii) long-range electrostatic repulsion between the charged monolayer andions prevents further incorporation of the later. In contrast to TPB anions, the strongly hydrophobic tetraphenylphosphonium (TPP) cationshave only a minor influence on the DPPC monolayer. This marked difference can be explained by taking into account dissimilarities in thedegree of hydration, charge density and migration ability between the negatively-charged phosphate and the positively-charged ammoniummoieties of DPPC head group. 1998 Elsevier Science S.A. All rights reserved

Keywords:Dipalmitoylphosphatidylcholine; Hydrophobic ions; Ion–monolayer interaction; Surface potential

1. Introduction

It is well known that the behaviour of charged Langmuirmonolayers is strongly affected by the ionic composition ofthe subphase solution. By now the influence of various inor-ganic (hydrophilic) ions [1,2] as well as ionic organic dyes[3,4] and polyelectrolytes [5–7] on the properties of someLangmuir monolayers has been widely investigated. Aswell as listed groups of ions, there is one more particulargroup – hydrophobic ions. It includes strongly hydrophobicand typically highly symmetric ions, which possess only aminor surface activity. Illustrative examples of hydrophobicions are the tetraphenylborate (TPB) anion and the tetraphe-nylphosphonium (TPP) cation. Recently, we have observeda strong interaction of TPB anions with positively-chargedmonolayers of stearylamine and dioctadecyldimethylam-monium (DODA) which results in the formation of drasti-cally-expanded mixed monolayers [8,9].

A strong effect of hydrophobic ions on oppositely-charged monolayers is evidently a results of both electro-static and non-electrostatic (hydrophobic) interactions. It isclearly manifested in the following two facts:

1. We could not detect any effect of hydrophobic ions onmonolayers with like charge and uncharged one (fattyalcohol) [8].

2. The effect of ions on oppositely-charged monolayersbecomes stronger with an increase of their hydrophobicproperties [9].Nevertheless, we expected that opposite charge of the

amphiphile head group is not a necessary condition fornoticeable interaction between the monolayer and hydro-phobic ions. Provided that an uncharged head group con-tains two spatially-separated oppositely-charged moieties(e.g. it is a zwitterion), there is a possibility of strong elec-trostatic attraction between a hydrophobic ion and appro-priate local charge. Head groups of common extensively-studied amphiphile – dipalmitoylphosphatidylcholine(DPPC, for structure see Fig. 1a) matches this requirementsin a wide range of subphase pH [10,11].

To test the proposed hypothesis experimentally, we havestudied the interaction of the DPPC monolayer at the air–water interface with various hydrophobic ions added to thesubphase by means of surface pressure- and surface poten-tial-area isotherm techniques.

Thin Solid Films 327–329 (1998) 599–602

0040-6090/98/$ - see front matter 1998 Elsevier Science S.A. All rights reservedPII S0040-6090(98)00721-4

2. Experimental details

DPPC (Sigma, approx. 99% purity) was used withoutadditional purification. Monolayers were spread from5 × 10−4 M solution in freshly-distilled chloroform. Sodiumtetraphenylborate (STPB), tetraphenylphosphonium chlor-ide (TPPC) and sodium picrate (SP) (practical grade) werepurified by multiple recrystallisation from water. Tetraethy-lammonium chloride (TEAC) (Sigma, St. Louis, MO) wasused as purchased.

Subphase solutions were prepared from double-distilledwater passed through a column with active carbon. Addi-tions of NaCl (analytical grade, heated at approx. 900 K)were used for ionic-strength variation.

Surface pressure-area (p-A) and surface potential-area(J-A) isotherms were recorded at a 25× 10 × 0.8 cm rec-tangular trough. A Wilhelmy balance with a 1-cm widthfilter paper plate was used for surface pressure registration(0.05 mN/m reproducibility). The surface potential wasmeasured by means of the vibrating capacitor method (5mV accuracy).

3. Results and discussion

Both (p-A) and (J-A) isotherms of DPPC monolayer atthe surface of dilute NaCl solutions (Fig. 1) are very close tothose previously reported [10,12], except for minor distinc-tions caused by dissimilarities in subphase composition andtemperature. Detectable values ofp appear nearA = 0.8nm2 reflecting the formation of continuos LE phase. Theplateau atp ≈ 5 mN/m represents the liquid expanded–liquid condensed (LE–LC) coexistence region, and furthersharp increase ofp corresponds to the formation of a com-pact LC phase. Evolution of the (J-A) isotherm reflects that

of (p-A) isotherm; atA . 0.8 the values ofJ are notstrongly reproducible because the monolayer likely consistsof large domains of LE and rarefied gas phases.

It is to be mentioned especially, that large variations ofionic strength cause only a minor changes of surface poten-tial (Fig. 1b). These data give the possibility of estimation ofan average charge per molecule,z – an indicator of theionisation state of DPPC and/or the amount of chargedimpurities in monolayer. Assuming that bothz and thedipole term of monolayer surface potential [13,14] are notaffected by ionic strength, the Gouy–Chapman model gives|z| , 0.01 (of electron charge). This result suggests thatDPPC monolayer at the surface of 0.1÷ 100 mM NaClsolution (with no buffer added) is practically uncharged.

When STPB is added to subphase solution, both the (p-A)and the (J-A) isotherms of the DPPC monolayer changedramatically (Fig. 2). The surface pressure rises withSTPB concentration, the increase is largest in the LE–LCcoexistence region of the parent monolayer. Starting at[STPB] = 10−6 M, the onset ofp gradually shifts towardslargerA with STPB concentration, i.e. the condensed mono-layer expands. The surface potential drastically decreaseswith STPB concentration, it even becomes slightly negativefor [STPB] = 10−4 M and A near 1 nm2 (Fig. 2b). Thedescribed changes in the (p-A) and (J-A) isotherms forthe DPPC monolayer are comparable with those observedfor positively-charged stearylamine and DODA monolayers[8,9] in the presence of TPB anions. They suggest that con-siderable amount of TPB anions incorporates into the headgroup region of the DPPC monolayer.

We have observed also a pronounced effect of ionicstrength on TPB–DPPC interaction. Variations of (p-A)and (J-A) isotherms with ionic strength at the constantSTPB concentration are presented on Fig. 3. An increase

Fig. 1. Surface pressure-area (a) and surface potential-area (b) isotherms ofthe DPPC monolayer at the surface of water containing different concen-trations of NaCl.T = 294 K.

Fig. 2. Surface pressure-area (a) and surface potential-area (b) isotherms ofthe DPPC monolayer at the surface of water containing different concen-trations of STPB.T = 294 K, [NaCl] = 10−2 M.

600 V.L. Shapovalov / Thin Solid Films 327–329 (1998) 599–602

in the ionic strength results in a transformation of the (p-A)isotherm which resembles that caused by increase of STPBconcentration (cf. Figs. 2a and 3a), however the behaviourof (J-A) isotherms is strongly different (cf. Figs. 2b and3b).

The evolution of (p-A) and (J-A) isotherms with concen-tration of TPB and ionic strength can be described in termsof a competition of two effects

1. Both hydrophobic interaction and local electrostaticattraction promote incorporation of TPB anions intothe monolayer, accompanied by negative charging ofthe last.

2. Long-range electrostatic repulsion between ions and thecharged monolayer prevents further incorporation of thefirst.

The observed values of surface potential shift (DJ) arevery large:DJ reaches 380 mV for [STPB]= 10−4 M andA = 0.4 nm2. Therefore, they cannot be attributed only to theGouy–Chapman potential of a charged monolayer (for theexpression, see e.g. [2,3]). Estimates of surface charge den-sities to be necessary to produce such shifts appear to beincredibly large (more than ten charges per 1 nm2). Wepropose that incorporation of TPB ions results in a rearran-gement of the monolayer head group region in such amanner that the dipole component of surface potential[13,14] (Jd) decreases. The observed shift of surfacepotential is consequently a sum of two negative terms:DJ =JGC +DJd. It should be taken into account, that onlythe first term affects directly the incorporation of TPB ionsinto DPPC monolayer. An increase in the ionic strengthcauses a decrease ofJGC which results in further incorpora-tion of TPB anions, manifested in monolayer expansion(Fig. 3a). The behaviour of the surface potential (Fig. 3b)

is more complex sinceJGC and DJd change in oppositedirections.

The effect of less hydrophobic picrate anions on theDPPC monolayer (Fig. 4) is similar, but clearly less pro-nounced than that of TPB anions (Table 1). A resemblingeffect of negative charging of DPPC liposomes in the pre-sence of picrate anions is reported in [11]. In this study thepicrate anions are most hydrophobic among all anions testedand their effect is the strongest. This result, along with thedifference in effects of picrate and TPB anions on DPPCmonolayer, suggests that hydrophobic properties of ions areof critical importance for their interaction with DPPC.

The TPP cation is evidently very similar to the TPB anionin terms of hydrophobic properties. Nevertheless, its effecton both (p-A) and (J-A) isotherms of DPPC monolayer isincomparably smaller than that of TPB (cf. Figs. 2 and 5). Itis remarkable that a surface potential shiftDJ caused byaddition of TPPC, changes its sign in the course of themonolayer compression (Fig. 5, Table 1). This effect,along with the extremely small absolute value ofDJ resultsprobably from the opposite signs ofJGC andDJd. However,it does not mean that two large quantities compensate eachother – the surface pressure isotherms unambiguously evi-

Fig. 3. Surface pressure-area (a) and surface potential-area (b) isotherms ofthe DPPC monolayer at the surface of water containing 10−5 M STPB anddifferent concentrations of NaCl.T = 294 K.

Fig. 4. Surface pressure-area (a) and surface potential-area (b) isotherms ofthe DPPC monolayer at the surface of water containing different concen-trations of sodium picrate (SP).T = 293 K, [NaCl] = 10−2 M.

Table 1

Effect of hydrophobic ions on DPPC monolayer

Ion Concentration(mM)

DJ at0.8 nm2

(mV)

DJ at0.4 nm2

(mV)

DA at1 mN/m(nm2)

TPB− 0.1 −320 −380 0.22picrate− 1 −120 −60 0.04TPP+ 0.1 25 −30 0.04TEA+ 1 ,5 ,5 ,0.01

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dence that amount of TPP cations in the monolayer is verysmall. With a less hydrophobic cation – tetraethylammo-nium (TEA), there is no detectable effect of 10−3 M ofTEAC added to the subphase on DPPC monolayer (Fig. 5,Table 1).

The marked difference between hydrophobic cations andanions in their interaction with DPPC monolayer (Table 1)can be easily understood taking into account followingassumptions, concerning the structure and properties ofDPPC head group (for the structure see Fig. 1a)

1. Both the phosphate group and neighbouring glycerolmoiety are capable of strong hydrogen bonding withwater molecules. Therefore, the negatively-chargedgroup is surrounded with water, which is unfavourablefor either an approach of hydrophobic cations or astrong electrostatic interaction. The positively-chargedammonium group is incapable of hydrogen bonding,hence it is accessible for interaction with hydrophobicanions.

2. The negative charge of the phosphate group is distrib-uted among four oxygen atoms, therefore its spatialdensity is low, while the positive charge of ammoniumgroup concentrates on a single nitrogen atom which isfavourable for electrostatic interaction with anions.

3. The phosphate moiety of the DPPC molecule is linkedto two others (glycerol and choline ones), which pre-vents its free motion, while the ammonium group isterminal, therefore it is able to change its normal posi-tion in the head group region to another one, favourablefor interaction with hydrophobic anions.

All of these assumptions look reasonable, however thesecond one seems to be the most probable.

4. Conclusions

Hydrophobic ions are able to interact strongly not onlywith oppositely-charged monolayers but with someuncharged monolayers as well. The possibility appearsthen that an amphiphile head group with zero total chargecontains two moieties, that are spatially separated and oppo-sitely charged, such as in the zwitterionic head group ofDPPC. Both hydrophobic interaction and local electrostaticattraction between ions and an appropriate charged moietyof the head group are of importance for the incorporation ofhydrophobic ions into the monolayer.

A strong interaction depending on both the concentrationof hydrophobic ions and the ionic strength of subphase isdemonstrated by the example of DPPC monolayer and TPBanions. A simple qualitative model of ion-monolayer inter-action is discussed, a quantitative model based on the ther-modynamic approach is now in the development stage.

Acknowledgements

Many thanks are due to Dr. Yu.V. Il’ichev for reading themanuscript. The work is partially supported by the RussianFoundation for Basic Research (Project No. 96-03-34122a).

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Fig. 5. Surface pressure-area (a) and surface potential-area (b) isotherms ofthe DPPC monolayer at the surface of water containing 10−2 M NaCl anddifferent electrolytes containing hydrophobic cations.T = 294 K.

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