Progress in Colloid and Polymer Science Æ Volume 122 Æ 2003

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Progress in Colloid and Polymer Science

Editors: F. Kremer, Leipzig and G. Lagaly, Kiel

Volume 122 Æ 2003

Aqueous Polymer –Cosolute SystemsSpecial Issue in Honor of Dr. Shuji Saito

Volume Editor:

Dan F. Anghel

1 23

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IV

This issue is dedicated to Doctor Shuji Saito on theoccasion of the 50th anniversary of his first paper publishedin the Journal of German Colloid Society. The paperentitled ‘‘Die Solubilisation von Polyvinylazetat’’ by N.Sata and S. Saito appeared in the Kolloid Zeitschrift (1952)128: 154. According to the title, the paper dealt with thesolubilization of poly(vinyl acetate) (PVAc) in aqueoussodium dodecyl sulfate (SDS) solutions. The authorsobserved that PVAc, a water-insoluble polymer, is com-pletely dissolved by micellar SDS solutions, a result thatmostly intrigued them. One has to recall that at that time,the micellar solubilization was the only available theory toexplain the dissolution of hydrophobic compounds inmicellar systems. The theory worked well in the case oflow molecular weight compounds like hydrocarbons,oleophilic dyes, etc., which are solubilized in the inner coreof the micelle, but how can a small micelle accommodate agiant polymer molecule? To solve this problem Sata andSaito originally proposed a model consisting of surfactantaggregates formed along the polymer backbone. The modelbased on simple viscometric measurements was later in theeighties confirmed by the advent of the more sophisticatedneutron scattering technique and is nowadays called the‘‘necklace model’’.

During his career, Dr. Saito published about 70 originaland review papers, which is a very good score for a personwho mainly worked in a cosmetic company and not in theacademic field. The majority of his papers belong to theinteraction between ionic or nonionic polymers andcharged and uncharged surfactants. Dr. Saito also paidattention to the interactions of ions with polymers, ofnonionic polymers with polymeric acids, as well as to thesurfactant micelles and to aqueous solutions of tetraalkyl-

ammonium salts. His review chapter on ‘‘Interactions of Polymers and Surfactants’’ published in Nonionic Surfactants.Physical Chemistry, M. J. Schick (Ed.), Dekker, New York, 1987, became a classic for all those who act in this field. I wouldlike to mention in this short introduction about the work of Dr. Saito the lines written by Dr. E. D. Goddard: ‘‘As regardsthe subject matter of this chapter, it is appropriate first to mention the name of S. Saito, who can properly be termed thefather of this field of research’’ (E. D. Goddard and K. P. Ananthapadmanabhan, Interactions of Surfactants with Polymersand Proteins, CRC Press, Boca Raton, 1993).

My first encounter with Dr. Saito’s work was more than 30 years ago, when I started a research project on polymer-surfactant interaction. The search of literature revealed as the most frequent author the name of S. Saito. His papers were asource of inspiration for our research group, and after we published the first paper in Kolloid Zeitschrift und Zeitschrift furPolymere in 1972, we were deeply impressed to receive a congratulation letter fromDr. Saito. In 1989 we had the privilege tohave Dr. Saito as invited lecturer at the 3rd Romanian Symposium on Colloid and Surface Chemistry held in Timisoara.During the visit, he truly enjoyed the people and the country, and decided to come back in the future. This happened afterhis retirement and in 1992 he spent almost two months in our institute. On this occasion we had the opportunity not only tolearn useful things from a master, but also to get a friend.

The most distinctive feature of Dr. Saito that stroked me from the very beginning was his eagerness. I remember the visitspaid together to the Village Museum, the National History Museum, and the National Gallery or in the outskirts ofBucharest. His vivid eyes scrutinize everything and he asked a lot of questions about our history and customs, culture, artand traditional architecture aiming to know as much as possible about us. A proof in this respect is that he started to learnRomanian, to be able of reading in original the poems of Eminescu, the Romanian National poet. Dr. Saito is also a talentedpainter. His gift was revealed to us in many landscapes and portraits he did during the stay in Bucharest, and everyone whoopens this volume will have the proof of his skill.

I can not conclude these lines without mentioning the unanimous enthusiasm with which both the Editor-in-Chief ofColloid and Polymer Science, Professor Gerhard Lagaly, the Springer Publishing House and the authors invited tocontribute a paper responded to the initiative to publish this volume. I wish to express my gratitude to all persons whohelped me in this endeavor of bringing an idea to life.

Dan Florin Anghel

Dr. Shuji Saito

Progr Colloid Polym Sci (2003) 122 : V� Springer-Verlag 2003 PREFACE

Anghel DF: Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

Wang C, Tong Z, Zeng F, Ren B,Liu X, Wu S:

Surfactant structure effects on bindingwith oppositely charged polyelectrolytes observed by fluorescenceof a pyrene probe and label . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Dragan S, Schwarz S: Dependence of the aggregation mode of two bidentate azo dyesin polycation/dye multilayers on the dye structureand the polycation conformation . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Anghel DF, Saito S, Iovescu A,Baran A, Stınga G, Neamtu C:

Counterion effect of cationic surfactants on the interactionwith poly(acrylic acid) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Koetz J, Gunther C, Kosmella S,Kleinpeter E, Wolf G:

Polyelectrolyte-induced structural changes in the isotropic phaseof the sulfobetaine/pentanol/toluene/water system . . . . . . . . . . . . . . 27

Bakshi MS, Kaur I: Surfactant–polymer aggregates of mixed cationic micellesand anionic polyelectrolytes: a surfactant head group contribution . 37

Burke SE, Palepu RM, Hait SK,Moulik SP:

Physicochemical investigations on the interactionof cationic cellulose ether derivatives with cationic amphiphilesin an aqueous environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Berglund KD, Timko AE,Przybycien TM, Tilton RD:

Use of nonionic ethylene oxide surfactants as phase-transfer catalystsfor poly(acrylic acid) adsorption to silica againstan electrostatic repulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Vıjan LE, Volanschi E,Hillebrand M:

Molecular modeling of anthracycline–DNA interaction . . . . . . . . . . 67

Muller AJ, Garces Y, Torres M,Scharifker B, Saez Eduardo A:

Interactions between high-molecular-weight poly(ethylene oxide)and sodium dodecyl sulfate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

ShirahamaK, KogaH, TakisawaN: Diverse actions of added alkanols on the binding of dibucaine cationto an anionic polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Olteanu M, Mandru I, Dudau M,Peretz S, Cinteza O:

The aqueous liquid/liquid interphases formedby chitosan–anionic surfactant complexes . . . . . . . . . . . . . . . . . . . . . 87

Barreiro-Iglesias R,Alvarez-Lorenzo C, Concheiro A:

Microcalorimetric evidence and rheological consequencesof the salt effect on carbopol–surfactant interactions . . . . . . . . . . . . 95

Piculell L, Sjostrom J, Lynch I: Swelling isotherms of surfactant-responsive polymer gels . . . . . . . . . 103

Roscigno P, D’Errico G, Ortona O,Sartorio R, Paduano L:

A comparison study between sodium decyl sulfonateand sodium decyl sulfate with respect to the interactionwith poly(vinylpyrrolidone) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

von Klitzing R, Kolaric B: Influence of the polycation architecture on the oscillatoric forcesof aqueous free-standing polyelectrolyte/surfactant films . . . . . . . . . 122

Tiefenbach K-J, Durchschlag H,Schneider G, Jaenicke R:

Thermodynamic analysis of serum albumin denaturationby sodium dodecyl sulfate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

Progr Colloid Polym Sci (2003) 122 : VI–VII� Springer-Verlag 2003 CONTENTS

Lopez-Fontan JL, Martınez-Land-eira P, Santamarina C, Ruso JM,

Prieto G, Sarmiento F:

The surfactant characteristics of short-chain lecithins analyzedthrough lecithin–lecithin and lecithin–biopolymer interactions . . . . . 141

Miyazawa K, Winnik FM: Isothermal titration calorimetry and fluorescence spectroscopy studiesof the interactions between surfactantsand a phosphorylcholine-based polybetaine . . . . . . . . . . . . . . . . . . . 149

VII

Introduction

Mixed surfactants producing mixed micelles are ofgreat interest for practical use and for scientificresearch. In recent years many studies on mixedmicelles have been reported by a number of workers[1, 2, 3] and important theoretical models have beenproposed and used [4, 5, 6, 7, 8, 9, 10, 11]. In general,mixtures of nonionic surfactants show ideal behaviour,while other combinations exhibit nonideality throughattractive or antagonistic interactions between thecomponents. Among the various types of surfactants,those of the zwitterionic type are of particular interestbecause they often exhibit significant synergism (i.e.,

favorable interactions) when mixed with ionic surfac-tants, while comparably little synergism is observedwhen mixed with nonionic surfactants [12, 13, 14]. Thenature and factors determining these interactions havenot been sufficiently assessed, so investigations of thesematters are of research interest.

Zwitterionic surfactants exhibit unique properties inaqueous solution, such as pH dependence of the criticalmicelle concentration (cmc), foam stability, lowering ofthe Krafft point by addition of salts, good solubility inwater and reduced sensitivity to salts and temperature[15, 16, 17, 18]. Short-chain lecithins are very goodmodel of zwitterionic molecules. From a structuralpoint of view, these lecithins are composed by two acyl

Progr Colloid Polym Sci (2003) 122 : 141–148DOI 10.1007/b10535� Springer-Verlag 2003

Jose L. Lopez-Fontan

Pablo Martınez-Landeira

Cibran Santamarina

Juan M. Ruso

Gerardo Prieto

Felix Sarmiento

The surfactant characteristics of short-chainlecithins analyzed through lecithin–lecithinand lecithin–biopolymer interactions

J.L. Lopez-Fontan Æ P. Martınez-LandeiraC. Santamarina Æ J.M. Ruso Æ G. PrietoF. Sarmiento (&)Group of Biophysics and Interfaces,Department of Applied Physics,Faculty of Physics, University of Santiagode Compostela, 15782 Santiagode Compostela, Spaine-mail: fsarmi@uscmail.usc.esTel.: +34-981-563100Fax: +34-981-520676

Abstract The micellar behaviour ofthe binary mixed systems di-hexanoylphosphatidylcholine(diC6PC)/diheptanoylphospha-tidylcholine (diC7PC), diC7PC/dioc-tanoylphosphatidylcholine (diC8PC)and diC6PC/ diC8PC has beenstudied. Critical micelle concentra-tions of the mixtures were quantita-tively estimated from surface tensionmeasurements versus solution com-position plots. The micellar compo-sition in the micelles was determinedby the Motomura thermodynamicmodel. The nature and the strengthof the interaction between two surf-actants in the mixture were estima-ted by calculating the values of theirbM12 parameter from Holland andRubingh’s treatment. DiC6PC anddiC7PC mix ideally with an interac-tion parameter near zero. DiC7PC/

diC8PC and diC6PC/diC8PC shownonideality for about 75 and 40%diC8PC in the mixture, respectively.The nonideality has been attributedto differences in the acyl chainlength, the size of the micellesformed by pure components and thecomposition of mixed micelles. Zetapotential and microcalorimetricmeasurements were used to investi-gate the effect of diC8PC on humanserum albumin. The results revealedthat the interaction is hydropho-bically driven and depends onlecithin concentration.

Keywords Mixed micelles Æ Leci-thin–lecithin interaction Æ Lecithin–protein interaction Æ Surfacetension Æ Thermodynamic models

chains, each with 4–8 carbon atoms, one glycerolmolecule, a phosphate group and a choline residue(Scheme 1).

As observed, the molecules are zwitterionics owing tothe negative charge attached on the phosphate group andthe positive charge on the nitrogen atom of choline.Then, the properties of solutions of these amphiphilesdepend, on the one hand, on structural parameters of theamphiphile itself, i.e., on its chemical structure and, onthe other hand, on the pH of the medium which canchange the charge of the head group towards positive ornegative values.

The structures and thermodynamic parameters ofpure one-component short-chain dialkylphosphatidylch-oline (diCnPC, with n¼ 4)8) micelles have been wellstudied and characterized by different physical tech-niques [19, 20, 21, 22, 23, 24]. Hauser [ 25] has recentlyreviewed the physicochemical properties of short-chainphosphatidylcholines, emphasizing their important de-tergent-like behaviour. An important application of thisproperty is the use of short-chain lecithins in thesolubilization of biological membranes and the reconsti-tution of membrane proteins into simple, well-definedmembrane systems.

Our most recent studies [26, 27, 28] deal with theinfluence of pH and acyl chain length on the micellizationof diC5PC, diC6PC, diC7PC and diC8PC and theinteraction of diC8PC with human serum albumin(HSA) [29]. Since the properties of diC6PC, diC7PCand diC8PC are well determined, the mixed diC6PC/diC7PC, diC6PC/diC8PC and diC7PC/diC8PC are idealmodel systems for studying mixed micelles. Somethermodynamic modelling on the diC6PC/ diC7PCsystem has been performed [30, 31] to predict the micellesize and the composition of mixed micelles at differentmixing ratios.

In this work, we extended the previously mentionedstudies to the mixtures diC6PC/diC7PC, diC7PC/diC8PCand diC6PC/diC8PC, and determined their cmcs by thesurface tension technique. The results are analysed inlight of the treatments of Holland and Rubingh [6] andMotomura et al. [7]. With the model of Motomuraet al., the composition of each component in themixture can be obtained. The model of Holland andRubingh permits us to obtain the bM

12 parameter as ameasurement of the interactions and nonideality of themixtures. The lecithin–lecithin interaction in mixedmicelles was compared with that of the lecithin–proteininteraction using the data obtained in a previous study

on diC8PC and HSA [29]. The study of the HSA–diC8PC interaction is of interest in relation to thewidespread occurrence of phosphatidylcholines in bio-logical organisms, whereas their short-chain precursorsand breakdown products are present in anabolic andcatabolic processes, respectively.

Experimental

Materials

The lecithins diC6PC, diC7PC and diC8PC were purchased fromAvanti Polar Lipids (Birmingham, AL) in powder form with puritygreater than 99%, stored at )20 �C and used without any furtherpurification. The buffer system used was phosphate, pH 7.4. Waterwas from a Milli-Q reverse-osmosis purification system.

HSA (fraction V A 1653) was obtained from Sigma ChemicalCompany and was used as supplied. The phosphate buffer systems,pH 7.4, used in the HSA–diC8PC interaction were at an ionicstrength of 0.188 and 0.0188, respectively.

Surface tension measurements

Surface tension wasmeasured by theWilhelmy plate technique usingaKruss K12 surface tension apparatus, equipped with a processor toacquire the data automatically. The equipment was connected to acirculating water bath to keep the temperature constant to298.15 � 0.01 K. Phospholipid solutions were prepared by dilutingwith buffer and determining the concentration by mass with aprecision of �0.00001 g. The instrument was calibrated against theMilli-Qwater andmeasurements were taken until the surface tensionwas constant at least for two periods of 20 min each. Thereproducibility of the surface tension was �0.01 mN m)1. The usualprecautions were taken to ensure cleanliness.

Electrophoretic mobility measurements

The electrophoretic mobilities, u, of HSA plus diC8PC weremeasured using a Malvern Instruments, Zetasizer 3000. f potentialswere calculated from the Henry equation [32, 33]:

f ¼ 3gu2e0erf ðjaÞ ; ð1Þ

where the permittivity of a vacuum, e0, the relative permittivity, er,and the viscosity ofwater, g, were taken as 8.854 · 10)12 J)1 C2 m)1,78.5 and 8.904 · 10)4 N m)2 s, respectively. To calculate the Henryfactors, f(ja), theDebye length, 1/j, and themeasured complex radii,a, from dynamic light scattering were used [29].

Microcalorimetry

Enthalpy measurements were made at 298.15 K using an LKB-Produkter 10700 twin-cell batch microcalorimeter system [34]. Theinstrument was used on the 30-lV range, where the mean sensitivityof the detectors in the heat sinks of the two vessels was14.66 � 0.32 W V)1. The sample cell was charged with 2 g bufferedHSA, concentration 0.25% w/v, and 2 g diC8PC of the requiredconcentration in a range up to approximately 0.3 mM.The referencecell was charged with 2 g diC8PC solution of identical concentrationto that in the sample cell and 2 g buffer solution. On mixing, theenthalpies of dilution of the phospholipid solution cancel and theenthalpy of dilution of the HSA was measured in a separateexperiment and was used to correct the data.

142

Results and discussion

Thermodynamic model

A plot of surface tension, c, against total molarconcentration, c012, for the diC7PC/diC8PC binary systemis shown in Fig. 1. Similar plots (not shown) wereobtained for the other mixtures.

The cmcs, cM12, were calculated from the intersection ofthe best straight lines through the two branches of thec/c012 curves and the values are given in Table 1. The cM12values are lower than the cmc of component 1 and arenear the cmc of pure component 2 for a mixturecomposition higher than 1:1. This suggests the stronginfluence of the second component in the micellizationprocess.

The composition of the monomeric and micellarphases of these systems was evaluated by the treatment ofMotomura et al. [7], which is based on excess thermo-dynamic quantities. This method of estimating theequilibrium distribution of components in a mixedsystem has been shown to give values in good agreementwith those derived from direct measurements by gelpermeation chromatography [35]. The surface tensionfrom an aqueous solution of a two-component surfactantmixture as a function of temperature, T, and pressure, p,is written as

dc ¼ �sEdT þ vEdp � CE1 dl1 � CE

2 dl2; ð2Þwhere sE and vE are the excess entropy and volume perunit interfacial area defined with respect to the twodividing planes, making the excess number of moles ofwater and air zero. li is the chemical potential of thecomponents and GE

i represents their surface densities.By assuming the aqueous solutions to be ideal, and

substituting the chemical potentials expressed as afunction of the total composition, c012 ¼ c01 þ c02, of themole fraction of surfactant 2 in the aqueous solution, x2,Eq. (2) can be written

dc ¼� DsdT þ Dvdp � RT CE12

c012

� �dc012

� RT CE12

x1x2

� �xE2 � x2� �

dx2 :

ð3Þ

Dy is the thermodynamic quantity of adsorption ofsurfactant per unit surface area defined by [31]

Dy ¼ yE � CE1 y1 þ CE

2 y2� �

; ð4Þ

CE12 ¼ CE

1 þ CE2 ð5Þ

is the total surface density of surfactants and

xE2 ¼CE2

CE1 þ CE

2

ð6Þ

is the mole fraction of surfactant 2 in the surfaceseparation.

Fig. 1 Surface tensions, c, as a function of molar concentration, c012,for the binary system diheptanoylphosphatidylcholine (diC7PC)–dioctanoylphosphatidylcholine (diC8PC) at 298.15 K and pH 7.4:10% diC8PC (diamonds); 20% diC8PC (down triangles); 40% diC8PC(up triangles); 60% diC8PC (circles); 80% diC8PC (squares)

Table 1 Critical micelle concentrations, cM12, of the mixed systems dihexanoylphosphatidylcholine (diC6PC)–diheptanoylpho-sphatidylcholine (diC7PC), diC7PC–dioctanoylphosphatidylcholine (diC8PC) and diC6PC– diC8PC in aqueous solution (pH 7.4) at298.15 K

Molar fractiondiC7PC

cM12 diC6PC/diC7PC(mM)

Molar fractiondiC8PC

cM12 diC7PC/diC8

PC (mM)Molar fractiondiC8PC

cM12 diC6PC/diC8

PC (mM)

0.00 11.0 0.00 1.3 0.00 11.00.14 5.2 0.10 0.72 0.01 6.00.22 4.6 0.20 0.51 0.05 2.20.41 2.8 0.40 0.33 0.10 1.20.45 2.4 0.60 0.24 0.40 0.330.65 1.9 0.80 0.22 0.78 0.221.00 1.3 1.00 0.21 1.00 0.21

143

The micelle formation is described by the analogue ofEq. (2) by using the excess thermodynamic quantity yM

of mixed micelles, which is defined with respect to thedividing surface, making the number of moles of waterzero:

�sMdT þ vMdT � NM1 dl1 � NM

2 dl2 ¼ 0 ; ð7Þwhere NM

i is the number of molecules of surfactant i inthe mixed micelle. By introducing the total excessnumber of molecules, NM

12 and the mole fraction, xM2, ofsurfactant 2, respectively, by

NM12 ¼ NM

1 þ NM2 ð8Þ

and

xM2 ¼NM2

NM1 þ NM

2

; ð9Þ

xM2 is evaluated by

xM2 ¼ x2 �x1x2cM12

� �@cM12@x2

� �T ;p

; ð10Þ

where cM12 is the cmc.Diagrams of cM12 versus composition for the diC6PC/

diC7PC, diC7PC/diC8PC and diC6PC/diC8PC systemsare shown in Figs. 2, 3 and 4, respectively. In these curvesline a represents the relationship between cM12 and themole fraction of surfactant 2 (diC7PC, diC8PC anddiC8PC, respectively) in the solution, x2, line b thevariation of cM12 with the mole fraction of surfactant 2 inmicelles, xM2, calculated by Eq. (10) and line c thevariation of cM12 with xM2 of ideal mixing. As a criterion

for ideal mixing we used the equation proposed byAratono et al. [11]:

cM12 ¼ cM1 þ cM2 � cM1� �

xM2 : ð11ÞThe analysis of Fig. 2 suggests that diC6PC and

diC7PC mix practically as the ideal form in all rangeof compositions. Slight differences exists when thepercentage of diC7PC in the mixture is bigger than

Fig. 2 Phase diagram of micelle formation of the dihexanoyl-phosphatidylcholine (diC6PC)—diC7PC system. Line a representsthe relationship between cM12 and the mole fraction of diC7PC in thesolution, x2, line b represents the variation of cM12 with themole fraction of diC7PC in micelles, xM2, calculated by Eq. (9) andline c the variation of cM12 with xM2 of ideal mixing

Fig. 3 Phase diagram of micelle formation of the diC7PC– diC8PCsystem. Line a represents the relationship between cM12 and the molefraction of diC8PC in the solution, x2, line b represents the variationof cM12 with the mole fraction of diC8PC in micelles, xM2, calculatedby Eq. (9) and line c the variation of cM12 with xM2 of ideal mixing

Fig. 4 Phase diagram of micelle formation of the diC6PC– diC8PCsystem. Line a represents the relationship between cM12 and the molefraction of diC8PC in the solution, x2, line b represents the variationof cM12 with the mole fraction of diC8PC in micelles, xM2, calculated byEq. (9) and line c the variation of cM12 with xM2 of ideal mixing

144

approximately 90%. This behaviour could be explainedby the influence that diC7PC has on the size of the mixedmicelle. As generally known, diC7PC forms rodlikemicelles in solution, while diC6PC forms sphericalmicelles [22, 23, 24]. An increase in the percentage ofdiC7PC in the mixture must cause the micelles to becomelarger. Lin and coworkers [30, 31] have elaborated athermodynamic theory to predict the microscopic changein the formation and growth of the diC6PC/diC7PCmicelles. Their predictions, corroborated by dynamiclight scattering experiments, show that the mixeddiC6PC/diC7PC micelles are still rodlike micelles as theratio of diC7PC to diC6PC increases. Our observationsconfirm this fact, although the thermodynamic modelapplied by us is different.

DiC7PC/diC8PC and diC6PC/diC8PC systems havemore complexity. The number of carbon atoms in the acylchain determines the variety of the shapes and sizes of theaggregates formed in aqueous solution: from micelles(spherical and nonspherical) to vesicles and lyotropicliquid-crystalline phases [36, 37, 38, 39]. DiC8PC repre-sents the transitions between the straightforward micellarstructures that in aqueous solution are formed byphosphatidylcholines with acyl chains of fewer than eightcarbon atoms and vesicles formed when this number isbigger than eight [22, 23, 24]. This suggests that theinfluence of diC8PC becomes more dramatic in thenonideality of the mixtures that contain this componentthan, for example, diC7PC mixed with diC6PC. This canbe seen in Figs. 3 and 4. When comparing these figures,note that the difference in the hydrophobicity betweendiC6PC and diC8PC, which is bigger than betweendiC7PC and diC8PC, influences the observed behaviour.

Lecithin–lecithin interaction

The nature and strength of the interaction between twosurfactants in the mixture can be determined by calcu-lating the values of their bM

12 parameter [6, 40]. Followingthe simplified approach proposed by Holland andRubingh [6], the monomer concentration of each com-ponent in the micelle mixture, cMi, is given by

cMi ¼ aicM12 ; ð12Þ

where ai is the mole fraction of the ith component in thetotal mixed solute.

For nonideal binary mixtures of surfactants, theactivity coefficients are given by

fi ¼aicM12xMi cMi

: ð13Þ

Using a simple regular solution approximation, theactivity coefficients can be expressed as functions of themole fractions of each of the components in the mixed

micelle, xMi, and an appropriate interaction parameter,bM12, that is related to the net interaction between lecithin

molecules in the mixed micelles. The regular solutionapproximation for the activity coefficients in the mixedmicelles gives

f1 ¼ exp bM12 1� xM1� �2 ð14Þ

and

f2 ¼ exp bM12 XM

1

� �2: ð15Þ

bM12 can be readily determined when the mixed cmc for

the binary system is known, by interactively solving forxMi at the cmc using a relationship such as

XM1

� �2ln

a1cM12xM1 cM1

� �¼ 1� xM1� �2

lna2cM12

1� xM1� �

cM2

!ð16Þ

obtained from Eqs. (13), (14) and (15). bM12 can then be

directly obtained by combining Eq. (13) with Eq. (14) togive

bM12 ¼

ln a1cM12=xM1 cM1� �1� xM1� �2 : ð17Þ

Negative bM12 values indicates attractive interaction

and positive, repulsive interactions.

Values of bM12 were obtained for the mixtures diC6PC/diC7PC, diC6PC/diC8PC and diC7PC/diC8PC understudy using the computational method proposed byHolland and Rubingh [6] which is based on Eq. (16) andusing cmc data of pure components.

The values obtained are shown in Fig. 5 as a functionof molar fractions of component 2. The mixture of

Fig. 5 Variation of molecular interaction parameter, bM12, against the

mole fraction of component 2 in the solution, x2, for the binarysystems diC6PC–diC7PC (circles), diC7PC–diC8PC (triangles) anddiC6PC–diC8PC (squares)

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diC6PC with diC7PC does not show any significantinteraction, as already discussed. The bM

12 are near zero(ideal mixing) over practically all the concentration rangeand confirm the results shown in Fig. 2. Values of bM12 ofabout zero have been reported by Hines et al. [12] formixtures of the zwitterionic C12 betaine and the nonionicC12 maltoside. Rosen and Zhu [41] obtained small valuesof bM

12 for the system C12BMG/C12E8, which is a mixtureof a zwitterionic similar to C12 betaine and the nonionicoctaethylene glycol monodecylether. At pH 7.4 thelecithins studied are electrically neutral (electrophoreticmobility measurements [42] show an isoelectric point forsynthetic lecithins of about 6) so there are no electrostaticinteractions and hence the nonideality must be due to theinfluence of the acyl chain length of pure diC6PC,diC7PC and diC8PC. DiC8PC has more influence on thenonideality than diC7PC as can be see by comparing thecorresponding lines in Fig. 5. The interactions betweenmolecules in the diC6PC/diC8PC system change dramat-ically at a ratio of approximately 1.5:1, from attractive(bM12 » )3) to strongly repulsive (bM

12 » 12). Repulsiveinteraction in mixed micelles in aqueous media is unusualand for hydrocarbon-chain surfactants has been reportedonly in mixtures of long-chain carboxylates and long-chain alkylbenzensulfonates [43]. It is more commonlyfound in mixtures of anionic fluorocarbon-chain andanionic hydrocarbon-chain surfactants [1].

Lecithin–protein interaction

The HSA–diC8PC interaction can be followed frommeasurements of the f potentials and by microcalorime-try.

The f potential of HSA+diC8PC at pH 7.4 and twoionic strengths is shown in Fig. 6. As can be seen these fpotentials are negative, which is consistent with previ-ously reported isoelectric points of 4.2 [44], 4.9 [45], and4.7–4.9 [46] for HSA and become less negative withincreasing phospholipid concentration, smoothly at lowionic strength and more sharply at high ionic strength.The decrease in the negative f potential of HSA onaddition of diC8PC must arise from hydrophobic inter-actions of the acyl chains of the phospholipid and achange in the structure of the double layer.

The enthalpy of interaction between HSA and diC8PCat pH 7.4 and ionic strength 0.188 M is shown in Fig. 7as a function of the final diC8PC concentration aftermixing. Note that these data predominantly relate todiC8PC concentrations below the cmc. The data werecorrected for the enthalpy of dilution of the HSA, whichwas found to be exothermic ()1.04 � 0.02 J g)1), andfor a final HSA concentration of 0.125% w/v. Theenthalpy of dilution of diC8PC was cancelled by mixingof diC8PC and buffer in the reference cell. The exother-mic effect of the HSA+diC8PC interaction increases

with phospholipid concentration. A possible explanationof this fact is the dissociation of dimers of HSA tomonomers that become incorporated into HSA–diC8PCcomplexes [29]. If we write the process occurring at lowdiC8PC concentrations as an interaction between HSAdimers and xdiC8PC molecules as

HSA2 þ xdiC8PC$ HSA2diC8PCx ð18Þand at high diC8PC concentration an interaction betweenHSAmonomers and the dimer–diC8PC complexes whichrequires the partial dissociation of n dimers HSA2, thus

nHSA2 $ ðn� 1ÞHSA2 þ 2HSA ð19Þ

Fig. 6 f-potentials of human serum albumin (HSA) in aqueoussolution at pH 7.4 in presence of diC8PC. Ionic strength: 0.0188 M(circles), 0.188 M (squares)

Fig. 7 Enthalpy of interaction of HSA in aqueous solution (pH 7.4,ionic strength 0.188 M) with diC8PC at 25 �C

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ðn�1ÞHSA2diC8PCxþ2HSA$ðn�1ÞHSAð2n=n�1ÞdiC8PCx

ð20Þthen at high diC8PC concentrations the overall processmay be written

nHSA2þ ðn� 1ÞdiC8PCx$ ðn� 1ÞHSAð2n=n�1ÞdiC8PCx:

ð21ÞIf the dissociation of dimeric HSA to monomers is

exothermic as the dilution data shows (see previously),then the increasing exothermicity observed at higherdiC8PC concentrations arises from dissociation of dimersto monomers that are incorporated into complexes.

Conclusions

Mixtures of short-chain lecithins are strongly influencedby the higher acyl chain component and the size and

shape that the lecithin molecules form. DiC6PC anddiC7PC mix ideally, with an interaction parameter nearzero. DiC7PC/diC8PC and diC6PC/diC8PC show no-nideality for approximately 75 and 40% diC8PC, respec-tively. This fact suggests the prevalence of diC8PC in themicellar behaviour of the mixture related with the type ofaggregate that forms in solution.

The interaction between diC8PC and HSA, followedfrom measurements of the f potential of HSA plusdiC8PC, shows a decrease in the negative f potential ofHSA on addition of diC8PC which must arise fromhydrophobic interactions of the acyl chains of thephospholipid and a change in the structure of the doublelayer. The study of the interaction between diC8PC andHSA, followed by microcalorimetry, shows an exother-mic interaction at the pH studied (7.4). A possibleexplanation for the increasing exothermicity with in-creasing diC8PC concentrations is the exothermic disso-ciation of HSA dimers to monomers that areincorporated into the HSA–diC8PC complexes.

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