Review -Tong Quan

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7alma, Vol. 33, No. 3, pp. 255-264, 19RbPrinted in Great Britain. All rights reserved0039-9140/86 $3.00 + 0.00Copyright @>1986 Pergamon Press Ltd

DYE-SURFACTANT INTERACTIONS: A REVIEWM. E. DIAZ GARCIA and A. SANZ-MEDEL*

Analytical Chemistry Department, Chemistry Faculty, University of Oviedo, Oviedo. Spain

Summary-The present state of knowledge of the mechanisms of dye-surfactant interactions for normalaqueous micelles is surveyed. The nature of the forces which lead to the binding of dye molecules inmicelles, the influence of the cationic, anionic or non-ionic character of a surfactant on the absorptionand/or fluorescence behaviour (below and above the critical micelle concentration), ion-associationprocesses and the influence of additives on these processes are discussed. Some discussion along these lineson related systems (reverse micelles, vesicles, polyelectrolytes) is included.

In analytical chemistry the main use of surfactants isin spectrophotometry and fluorimetry, particularly inthe development of new methods of metal-ion deter-mination. The addition of cationic surfactants to anegatively-charged coloured binary complex may re-sult in the formation of new analytical systems (sen-sitized reactions). In such sensitized systems, theaddition of a surfactant may lead to lowering of thepH at which the complex is formed, red-shifts inabsorption bands, and increases in molar absorptivityor fluorescence intensity. Usually, the metal-chelatecomplexes formed in the micellar systems are morestable than those formed in the absence of micelles.6Micelles are responsible for many of the practicalapplications of detergents such as: (i) enhancement ofthe soIubility of organic compounds in water,.owing to their incorporation in the micelle, wherethey experience an altered micro-environment; (ii)catalysis of many reactions,12 usually explained interms of a concentration effect in the micellarpseudophase; (iii) alteration of reaction pathways,rates and equilibria. I3 I6 Additionally, micelle systemsare convenient to use because they are opticallytransparent, stable and relatively non-toxic..

These beneficial effects show the advantage of suchsurfactant systems in the development of newspectrophotometric and Buorimetric methods fordetermining micro amounts of metal ions, anions,biological compounds, drugs and pesticides.Typical chromophoric reagents which have beenused to determine metal ions by use of surfactants asa third component include derivatives of the tri-phenylmethane series, azo~ompounds,8~zo anthra-quinone dyes, phenoxazone,*23 and oxine deriva-tives.24.5 Although considerable attention has beenpaid to the analytical applications, the nature andmechanism of these types of reaction are still notclearly understood. The electrostatic interactions be-tween oppositely-charged surfactants and dyes arewell understood (Hartley rules),2h but do not in*Author for correspondence.

themselves explain the spectral changes observed. ftseems probable 25.27hat once the electrostatic forceshave brought together the oppositely-charged mole-cules, hydrophobic interactions take place, dra-matically changing the micro-environment experi-enced by the chromophoreZS or lumophore.7Knowledge of dye-surfactant interaction should beof great value in understanding the chemical equi-libria, mechanisms and kinetics of surfactant-sensitized colour and/or fluorescence reactions. Inthis review, data scattered throughout the literatureon dye-surfactant interactions are discussed in thelight of our own experience.

NORMAL MICELLE FORMATIONA surfactant (surface active agent) is a molecule or

ion that possesses both polar (or ionic) and non-polarmoieties, i.e., it is amphiphilic. Large variations instructure are possible; the polar group can havevaried charge and nature (e.g., alkylsulphate, alkyl-phosphate or alkylammonium) and be attached toalkyl groups of varying lengths (8-18 carbon atoms)or to other hydrophobic moieties (Table I).In very dilute solutions, surfactants dissolve andexist as monomers, but when their concentrationexceeds a certain minimum, the so-called criticalmicelle concentration (c.m.c.), they associate sponta-neously to form aggregates. The term micelle isused for an entity of colloidal dimensions, in dynamicequilibrium with the monomer from which it isformed. As the surfactant concentration increasesabove the c.m.c., the addition of fresh monomerresults in the formation of new micelles, so themonomer concentration remains essentially constantand approximately equal to the c.m.c. Micelle for-mation is a result of the dual nature of the surfactantmolecule, the hydrophobic part trying to escape fromthe bulk water, and the hydrophilic part interactingstrongly with the water. Water has an open structurebecause of three-dimensional hydrogen-bonding,which permits the existence of clusters of water

255


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2% M. E. DIAZ GARCIA and A. SANZ-MEVEITable I. Micelle-forming amphiphilic molecules

Cationic swfactantsCH,-(CH,),,-fi-(CH,), X0

X0 = F-, Cl-, Br-,I-, NO,

CH,-(CH,),-OPO;- Mm Me = Li+, Na+, K+,Ca*+, MgZ+

i--J-+0CH,-W;),,-N N-RIL/OyOcI

Zwitterionic swfactantsCK

CH,-(CH2),,-@i!LCH,-SO~c!H,

molecules containing cavities of specific sizes, whichcan accommodate non-polar chains.* The flickeringcluster model of water structure29 postulates that theformation of hydrogen bonds is predominantly aco-operative phenomenon; formation of a hydrogenbond between any two given atoms results in thebinding of each atom by hydrogen bonds toneighbouring molecules. In this way, cavities ofdifferent volumes (Fig. l), surrounded by hydrogen-bonded water molecules, are formed.29

For a given surfactant, at a given temperature, onlya certain amount of monomer can be accommodatedin the cavities and any further addition of surfactantwill result in the formation of micelles. In otherwords, the further addition of surfactant provides adriving force to minimize contact of the monomerhydrocarbon chains with water. Therefore, accordingto Langmuirs principle of differential solubihty, thehydrocarbon chains cluster to form a core (micellarcore), while the polar groups interact with the water.j

Each micelle consists of a certain number of mono-mer molecules (aggregation number, N), which deter-mines its general size and shape. The exact size andshape of micelles is still uncertain but it is assumedthat an ionic micelle in dilute aqueous solution isroughly spherical (Fig. 2). The charged (or polar)

ClustersFig. 1.Schematic representation of hydrogen-bonded watermolecules in liquid water.hydrophilic groups are directed towards the aqueousphase (Stern layer), while the hydrocarbon chains aredirected away from the water (forming the hydro-phobic central core). The region adjacent to the Sternlayer contains a high density of counter-ions of thepolar heads (~ouy~hapman double layer) and separates the hydrophobic interior from the bulk aque-ous phase.16 This model visualizes a micelle as an oildrop with an ionic or polar coat.31 On the macro-scopic scale, a micellar medium could be described asa mixed aqueous-organic solvent3*It is interesting to note that although it is usuallyassumed that there is a fairly well-defined water layeraround the micellar surface, there is no agreement onthe composition of the micellar core, i.e., whether itconsists of pure hydrocarbon or of hydrocarbonchains mixed with water. Water penetration of themicellar core is still a matter of controversy. Experi-mental evidence has been produced supporting theview that water cannot rigorously be excluded from

Gout-ChaRman

XX >X

X

I-Sternlayer

X

buik ohase

Fig, 2. Two dimensional representation of a model sphericalionic micelle. x : counter-ions, 0: ionic head-groups,-: hydrocarbon tails.


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Dyeesurfactant interactions 257the micellar core. This conclusion is supported notonly by early NMR studies, but also from themore recent C-NMR investigations.4 Manyspectroscopic studies indicate that there is signi-ficant penetration of water into micelIes. Recentfluorescence studies on the hydrogen-bonding inmicellar aggregates? indicate that the distinctionbetween polar and non-polar sites in the micelles isinaccurate; in fact, it has been proposed that micellesare loose and porous structures in which water andhydrophobic regions are constantly in contact.8,9

Current thought on this controversial water ex-posure of micelles is founded mainly on low-angleneutron-scattering experiments which allow the studyof unperturbed micelles. 4o This modern concept dis-cusses the main characteristics of the molecular con-formation in micelles in terms of the predictions ofthe interphase model.4 Interphase theory predic-tions are in agreement with experimental data and areparticularly consistent with some principal features ofmicellar structure:40

surfactant salts. The actual species formed dependsmainly on the nature of the dye: Bromophenol Blueand Bromocresol Green solutions at lowconcentrations of l-carbethoxypentadecyltrimethyl-ammonium bromide, in acid and alkaline solution,show turbidity;54 Bromopyrogallol Red in solution atcetylpyridinium bromide concentrations below thec.m.c. and at pH 2-3 precipitates as a dyeesurfactantsalt.56 Formation of an insoluble salt between ionicdyes and oppositely-charged detergents is most com-mon, but is not a completely general phenomenon.In fact, some dyes, such as Phenol Red4 or8-hydroxyquinoline-5-sulphonic acid produce neitherturbidity nor precipitation2 along with the spectralchange induced by addition of the cationicsurfactant.

(1) the micellar core is virtually devoid of water,according to Langmuirs original principle ofdifferential solubility;

(2) micellar chains are randomly distributed andsteric forces determine the final structure;

(3) contact of the hydrophobic sections of themicelle with water results from a disordered structurein which the terminal groups or chain ends are nearthe micellar surface and thus exposed to bulk water.4

The nature of the dyes and their own tendency toaggregate5 59 have to be considered to explain suchphenomena. Dyes are also amphiphiles, in the sensethat bulky non-ionic moieties are attached to theionic or analytical groups, but as they lack long-chainalkyl groups they have weak surface activity and donot form micelles in water. Depending on the balancebetween the hydrophobic and hydrophilic tendenciesof any particular dye, increases in dye concentrationcan lead to stepwise aggregation, i.e., the formationof dimers, trimers, polymers and finally colloids:

dye (monomer)+dimer+...,+n-mer

Although the water penetration concept of thehydrophobic sections of micelles is now less accept-able than the water exposure concept, this contro-versial topic is still under debate.42.43

Swfactant -induced spectral changesIn aqueous solution, micelle formation is usually

detected by some change in the physical properties ofthe solution, such as surface tension, conductivity,viscosity, and emf,4 46 or some optical or spec-troscopic property of the solution (e.g., light-scattering behaviour or spectral changes accom-panying solubilization of dyes in surfactantmicelles).47~50 Since this latter spectral method isbased on dye-surfactant interactions, it deserves fur-ther elaboration.

HartleyZh first noticed that the colour of sul-phonaphthalein indicators changed on the additionof detergents, and this effect occurred only when thecharge on the detergent aggregate was opposite insign to that on the dissociated indicator molecule.This behaviour proved to be quite general, as azo,5triphenylmethane 4 and merocyanine dye9 all ex-hibited the same effect.

If a surfactant is added to such a dye solution atsubmicellar concentrations, both the surfactantmonomer and the dye aggregates can interact to forma special kind of micelle (mixed micelIe) at concen-trations far below the normal c.m.c. characteristic ofthe surfactant. This dye-surfactant interaction ac-counts for the often observed fact that the so-calledspectral change dye method47 does not provide atrue c.m.c. value. In fact, in such cases, the change inabsorbance or fluorescence intensity of a dye solutionin the presence of increasing surfactant concen-trations may not reflect the formation of micelles ofthe surfactant (homomicelles) but that of mixedmicelles or dye-surfactant salts. A comparison ofsome c.m.c. values of cetylpyridinium bromide, asdetermined in our laboratory by different methodsunder different conditions, is presented in Table 2 andclearly illustrates this point. As can be seen, not onlythe nature and concentration of the dye. but also thereaction medium can markedly influence the surfac-tant c.m.c. values.

Once the surfactant concentration has reached avalue close to or above the c.m.c. neither turbiditynor precipitation is observed. Solubilization of thedyeesurfactant salt-like ion-pairs in the micellarphase and/or the final incorportion of the dye into themicelles (homomicclles) is taking place.

At concentrations below the c.m.c., addition of a Many of the features observed in the spectralsurfactant to a dye solution may bring about the behaviour of dye-surfactant systems carrying op-formation of colloidal dye-surfactant submicellar posite charge can often be extended to general sensi-aggregates (mixed micelles) or insoluble dye-- tized reactions in micellar media.


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258 M. E. DIAZ GARCIA and A. SANZ-MEDELTable 2. Comparison of critical micelle concentrations of cetylpyridinium bromide, obtained by differentmethodsMethod Additive c.m.c.. 10e4M DH ReferenceConductivity

Surface tension

Spectral dye-change

none 6.39 5 25none 6.86 5 50KC], O.OlM 3.16 5 56HCI, IM 4.96 56Buffer HAc/NaAc 0.2M 2.58 4.5 508-Hydroxyquinoline-5-sulphonic acid 3.76 25Dimethylformamide 2.39 0 (HC!, IM) 57Eosin 0.04 6 58Pyrogallol Red 2.80 2.8 59Bromopyrogallol Red 0.99 0 (HCI, IM) 56Bromopyrogallol Red ppte. 2 50Thorin ppte. 9 56Eriochrome Black T depends on 9 60

dye concn.

Mukerjee and Mysels4* using spectrophotometricand electrical conductivity measurements of thepynacyanol-sodium dodecylsulphate system identi-fied the presence of two types of dye-surfactantaggregates: (i) below the c.m.c. a dye-surfactant saltwhich formed a coarse (visible suspension) stableslurry in the presence of more than a stoichiometricamount of surfactant, and (ii) dye-rich micelles, atbelow and around the c.m.c., which solubilized thewater-insoluble dye-detergent salt. Malik et uI.~-~~reported that spectra1 changes for several dyes aredue to electrostatic forces involving interactions be-tween the anionic (or cationic) surfactant and thebasic (or acidic) dye. They claimed, however, thatchemical interaction giving a stoichiometricdye-surfactant complex was very improbable. Guhaet aI. attributed the changes in the absorptionspectra and the decrease in fluorescence intensity ofthionine to the formation of a dye-surfactant com-plex at sodium dodecylsulphate concentrations belowthe c.m.c.; at concentrations above the c.m.c. theappearance of the dye absorption spectrum, with asmall red-shift and increased extinction coefficient,was interpreted as due to the incorporation of the dyeinto the micelles.Nature of the dye-surfactant interaction

The existence of true ion-association complexesformed at below the c.m.c. between ionic surfactantsand dyes with opposite charge is supported by mostof the published data. 54,7 4 These complexes areelectrically neutral, and often poorly soluble in waterbut readily extractable by low-polarity solvents. Theyhave stoichiometric surfactant/dye ratios. At surfac-tant concentrations at the c.m.c. value and above, thesolubilizing effect of the micelles begins to be im-portant and the ion-association complexes are incor-porated into the micelles.

Electrostatic interaction of anionic dyes with thesurface of cationic surfactant micelles takes placethrough the negatively charged groups of the dye

(--SO;, -COO-). However, this kind of electro-static interaction could not explain by itself thespectra1 changes observed during the interaction. Infact, bulky non-micelle forming species such as thediphenylguanidinium or tetraethylammonium ion,have no effect per se. Moreover, simple ion-pairingbetween a negative group such as -SO, or -COOof the dye and a quaternary ammonium ion does notperturb the chromophore.75 In the presence of cat-ionic surfactants, aromatic compounds withsulphonic76 or carboxylic acid groups do not actsimply as counter-ions, but are incorporated into thewater-rich Stern layer of the micelle in a sandwicharrangement. This permits not only the hydration ofthe hydrophilic --SO; (or --COO-) group, but alsothe solvation of the aromatic ring of the dye by the-&(CH,), group and the participation of van derWaals interactions between adjacent surfactantchains and the dye organic moiety (hydrophobicforces). In this situation, the micro-environment ofthe chromophore has clearly changed, from thatexisting in the bulk aqueous phase, and this changeis the cause of the spectra1 shifts observed. Since dyesbased on aromatic rings are widely used in spec-trophotometry and fluorimetry, this picture can beconsidered genera1 (and is probably operative in mostanalytical dye-surfactant systems at concentrationsabove the c.m.c.). In this context, it is worth men-tioning the importance of the presence of an --SO,group in the dye. The electronic parameters foraromatic substituents78 indicate that an -SO; groupgives less resonance interaction with the aromaticsystem than does a -COO- group. In the latter case,the negative charge is delocalized and distributedover the terminal -COO- group and the aromaticring; thus the cationic end of the surfactant will tendto interact electrostatically less with the -COO-group than with an -SO; group, on which thecharge is localized. The -COO- group will thereforebe buried deeper in the micelle Stern layer than willthe --SO, group, leading to diminished electrostatic


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Dye-surfactant interactions 259interaction between the -400~ groups and thecharged head-groups of the surfactant.

It has been reported79 that the ionic association ofcharged micelles with an aromatic dye through the--SO< group promotes electron-withdrawal from thearomatic rings along the conjugated n-system, soleading to the ionization of easily dissociated groupsin the dye (--OH groups). The resulting ionizedgroup may further associate with another surfactantmolecule. The spectra1 shifts then observed may bedue to the deprotonation of an -OH group on thedye, with the incorporation of the chromophore intoa single conjugation plane.79 In this case, maximumdelocalization of the n-electronic system of the dyecan occur.Russian authors claim5~80~8hat the sulphonic acidgroup in the triphenylmethane dye series is isolatedand that its electrons are not able to interact with thearomatic n-electron system. However, it has beendemonstrateds2 that the electrons on the sulphonicacid group may participate in the rr-system throughthe empty d-orbitals of the sulphur atom. Associationof the ----SO, group with the surfactant reduces thefraction of total charge on it, so promoting electron-withdrawal from the entire n-electron system of thedye and causing energetic dissymmetry, with con-sequent dissociation of ionizable hydroxyl groups. Inthis way, micelles affect not only the electronic struc-ture of the dyes but also their basicity and hence thepK, of indicators. I6 Micelles can either stabilize ordestabilize charged dye species, depending on the signof their surface charge, as shown in Table 3.The proton-release occurring during the reactionbetween an anionic dye and a cationic surfactantproduces a change in the spectrum w hich s simi la r t ot hat observ ed on ncreasi ng t he pH of the dye solution.Such pK, shifts for solubilized indicators have beenattributed to the influence of the surface potential ofmice11es.85 The pK, changes also appear to berelated to the reduction of the difference in freeenergy between the acidic form of the dye and itsanion in the micelle.77~88oExtensive incorporation ofan anionic dye into a cationic micelle implies that the

free energy of the anionic form decreases more thanthat of the unionized form, as the anion is morepolarizable and firmly attached to the positive end-groups of neighbouring surfactant molecules.There is multiple binding in these associated mi-cellar species: evidence has been produced indicatingthat hydrophobic interaction, not charge compen-sation, plays the main role in binding between dyesand surfactants. The exact nature of this interaction,however, has not yet been satisfactorily explained.Chiang and Luktong2 report that their results onthe interaction between 2-p-toluidinylnaphthalene-6-sulphonate and sodium dodecylsulphate (NaDDS)micelles suggest that the binding force is hydro-phobic. Analogously, Birdi et al. claim that theinteraction of NaDDS micelles with l-anilinonaph-thalene&sulphonate is hydrophobic in nature. Theinteraction between some mono-azo dyes with aseries of non-ionic surfactants has been shown9 to behydrophobic in nature and occurs between dyes andthe ethylene oxide chains of the non-ionic surfactant.Minch showed, from spectral changes of mero-cyanine dyes in cationic and anionic micelles, that inall cases the spectra were red-shifted when the dyewas incorporated into micelles and that the mag-nitude of the shift increased with more hydrophobicdyes. Biedermann and Datyner94 also suggested thatthe interactions of some azo dyestuffs with NaDDSmicelles increased with increasing hpophihcity of thedyes.According to current thought,40 the inclusion of adye molecule within a micelle is not strictly akin toplacing it in a hydrophobic region in the micellarcore, but is more like placing it in a hydrophobicenvironment where it is exposed to water. A consid-eration of hydrocarbon chains in micelles as disor-dered structures could explain why the nature of thedye may determine its binding site within the micelleassembly. 94 n other instances, the factors responsiblefor the spectral changes have been ascribed to thedeaggregat io n of t he dye molecules by associationwith micelles,95 to the joint effect of deaggregationand the change in t he mol ecular envi ronment,h~v7 r to

Table 3. pK,, shifts of various species, as a function of surfactant charee-tvoeCationic Anionic Non-ionicSpecies Alone surfactant surfactant surfactant Reference

Phenol Red, pK,, 7.68 7.66* 54Bromophenol Blue, pK,,: 3.89 3.02* 54Bromocresol Green, pK,, 4.58 4.08* 548-Quinolinol, pK$,, 5.02 4.261 5.726 5.021 83, 84

PK.,: 9.67 9.35t 10.294 9.76: 83, 84Quinine, PK.,, 4.13 4.20t 5.355 83PK.,? 8.52 7.57t 9.796Umbelliferone, pK,, 7.75 6.7511 8.25# 85Methyl Red, pK,, 4.95 3.6711 6.638 5.20$ 854-Nitrophenol, pK,, 7.15 7.11: 84*Carbethoxypentadecyltrimethylammonium bromide.tDodecyltrimethylammonium chloride.$Triton X-100@odium dodecylsulphate.1/Dodecyltrimethylammonium bromide.


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260 M. E. DIAZ GARCIA and A. SANZ-MEDELthe localization of the chromophore within the hydro -phobic micellar interior.

INFLUENCE OF ADDITIVES

Strong electrolytesMicelles are sensitive to small changes in the ionic

strength of the aqueous solution. The change in thec.m.c. of cetylpyridinium bromide in aqueous solu-tion with electrolyte concentration62 reveals twotrends, one occurring at low and the other at highconcentrations of the added salt (see Table 4). Addi-tion of salts to ionic micelle solutions reduces themutual electrostatic repulsions of charged head-groups:

/-J-J+@:: X-Gv- - d-_&-Lfy_y---n&&x-x-k x-y-X-Thus, addition of a salt anion, X-, to cationic micellesolutions results in an increase of counter-ion dis-sociation (~1). The degree of displacement of the Y-counter-ion will depend on the nature of the X-anion, and usually follows the order in the lyotropicserieslOOOn the other hand, electrolyte addition leads to anincreased aggregation number and micellardiameter99.0 because if the electrical surface potentialis reduced, more polar heads, and hence more mono-mers, can constitute a given micelle (increase in N andsize).

Regarding the usual decrease observed in the c.m.c.values after salt addition (Table 4) some authorsspeculate that it is related to the ability of the salt tomelt Frank--Evans icebergs; micelle formation isan entropy-directed process and is influenced bychanges in the water structure surrounding surfactantions.02,03 If structure-breaking ions are present insolution, the water icebergs2x will thaw to a morerandom state. Destruction of icebergs around themonomeric surfactant ions would result in easier

Table 4. Effect of added electrolytes onthe c.m.c. of cetvlpvridinium bromide62Added electrolyte c.m.c., 10-4M*Aqueous solution 6.86KCI. O.OlM 3.76KCI, O.IM 4.20KCI. IA4 7.14HCI. 0.2M 3.50HCI, IM 4.96NaCI, 0. I M 3.40NaCI. IA4 4.96N&I, 1.5M 4.56*Surface tension measurements al 20 C.

micelle formation at a lower surfactant level.04 How-ever, Schick claims from the iceberg picture that astructure-breaking ion should reduce micelle for-mation. Whatever the theoretical approach to thiseffect, it has to be borne in mind that c.m.c. decreaseswith electrolyte addition and that the c.m.c. is ameasure of the ease of micelle formation: the lowerthe value of the c.m.c. the higher the tendency tomicelle formation.

If the strong electrolyte concentration becomessufficiently high, not only the size of the micellechanges but also its shape, e.g., spherical sodiumdodecylsulphate micelles become rod-like at high salt(NaCl) concentrations05 and cetylpyridinium bro-mide micelles grow steadily to form elongated semi-flexible rods, with increasing sodium bromide con-centration. In the case of non-ionic surfactants, thec.m.c. is only slightly dependent on salt concen-tration.

As the magnitude of the ionic strength has anegligible effect on the spectral behaviour of dyesalone,55~7408 he main role in the spectral changesobserved when the salt concentration is increased ina dye-surfactant solution should be played by thesurfactant-electrolyte interactions. The absorptionspectrum of Bromopyrogallol Red at pH 4 and ionicstrength 0.2M (fixed by the acetate buffer used) showsa maximum at 580 nm when cetylpyridinium bromidemicelles are present. 62 The addition of sodium chlo-ride releases the dye from the micelles and theabsorption spectrum changes to resemble that ofBromopyrogallol Red in the absence of cetyl-pyridinium micelles. The band at 580 nm decreasesin intensity with sodium chloride concentrationsranging from 0.2 to 0.6M. At the higher salt concen-trations the absorbance decrease is much less pro-nounced and a break-point is observed (at[NaCI] N 0.48M), which may correspond to thechange in shape and size of the micelles.

The fact that the absorption spectrum of the freeBromopyrogallol Red reappears on addition of so-dium chloride suggests that electrostatic forces play afundamental role in dyeemicelle binding, maintainingthe dye in or near to the micelle.

The saturation of the micelle surface (Stern layer)by strong electrolytes should dissociate thedye-micelle associate, restoring the properties of theaqueous dye. This would also account for the ob-served increases in pK, of the dye-surfactant associ-ation complex when the electrolyte concentration isincreased.54,y It has to be stressed, however, thatother authors have indicated that addition of strongelectrolytes may cause, conversely, an acceleration ofthe rate of dye penetration into micelles or even amore effective inclusion of the dye into the micelIe.This is another example of how the studies and basicknowledge on micellar interactions available so farmay be contradictory and insufficient to permit aclear choice between different possible interpretationsof micellar reactions.


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Dye-surfactant interactions 261Hydrophobic addit ives

Ionic micelles contain binding sites for both hydro-phobic and hydrophilic solutes. For instance, addi-tion of alcohols to aqueous solutions of surfactantaggregates is known to influence the properties ofmicelles. Alcohols penetrate the interior of the micellewithout appreciably changing its volume, formingmixed micelIes. The alcohol hydroxyl group ishydrogen-bonded to the surfactant head-groups, in-creasing the distance and hence decreasing the re-pulsion between them. I The effect of added alcoholson the c.m.c. of surfactant solutions is dependent onthe nature of the alcohol, e.g., I-propanol (or2-propanol) decreases the c.m.c. of n-dodecyl-trimethylammonium bromide and sodium dodecyl-sulphate more effectively than does ethanol2 owingto some stabilization of the mixed micelles throughdirect hydrophobic interactions. I-Propanol is moresoluble than ethanol in the micellar phase, therebypromoting micelle formation.

In general, organic molecules (or ions) tend toreduce the c.m.c. of surfactants, the reduction in-creasing with the size of the alkyl group. The c.m.c.for n-dodecyltrimethylammonium bromide micellesis decreased by a factor of 40 as n changes from 0 to5 in the series CH3+CH2);;-COO-.3 The alkylchain length of the additive also has an appreciableeffect on the total number of monomers which forma micelle; it has been shown that if a longer hydro-carbon chain is used, in order to enhance the hydro-phobic interaction with micelles, the aggregationnumber of the micelles is reduced.

The effect of some hydrophobic solutes has beenconsidered in a number of studies2 I4 but very littleattention has been directed5.6 towards the mea-surement of the effect of added hydrophobic mole-cules on the properties of solutes already incorpo-rated in micelles.

THE NATURE OF THE SURFACTANT

The spectra1 changes observed for different anionicdyes in the presence of surfactant micelles show thatcationic surfactants affect the spectral characteristicsof such dyes more strongly and over wider acidityranges than other types of surfactant do. This meansthat charge-type effects are operative.Changes in the nature of the cationic head-groupof the surfactant, however, apparently play only aminor role, and it is the length of the hydrocarbon tailwhich is the predominant factor in determining theappearance of new peaks and/or band-shifts in thespectra.75 Solubilization of a dye in micellar solutionscan be attributed to hydrophobic interactions and itseems clear that the same kind of interactions areresponsible for the dye spectral changes observed inmicelles. In the light of solubilizdtion experimentsLianos et al. concluded that there is a limiting chainlength (more than IO carbon atoms) for the solu-

bilization of arenes and that such solubilization doesnot seem to be sensitive to probe size, as pyrene andnaphthalene showed similar behaviour. In a similarway, the spectra1 changes are first observed when thehydrocarbon chain length of the surfactant rises to I Ior I2 carbon atoms, which coincides with the appear-ance of surface-active properties in the molecule.75 Inother words, the length of the hydrocarbon chain inthe surfactant is primarily responsible for the hydro-phobic properties and could represent its degree ofhydrophobicity. If the chains of the surfactants arevery short, the corresponding micelles are extremelylabile, with very short lifetimes. This would explainwhy such small micelles are unable to solubilize thearenes.8

Owing to electrostatic repulsion, the interactionbetween anionic dye ions and the head-groups ofanionic surfactants should produce neither new spec-tral bands nor changes in absorbance or fluorescenceintensity. However, as mentioned earlier, lipophilicitymay often be the driving force for interaction, ratherthan the electrostatic interaction55,90,9294 and somespectral changes can be explained in this way. Asimilar explanation can also be given for non-ionicsurfactant effects on the spectral behaviour of dyes:Coomassie Brilliant Blue G-250 does not show anyspectra1 shift with anionic detergents such as sodiumdodecylsulphate or sodium deoxycholate. but doeswith non-ionic surfactants, probably owing to trans-fer of the dye from a hydrophilic to a hydrophobicmicellar environment.

If a charge-type effect can combine with the classi-cal hydrophobic interactions then both kinds ofinteractions, electrostatic and hydrophobic, seem toact concurrently, bringing about the largest spectralchanges, as shown for anionic dyeecationic surfac-tant complexes by Savvin et a/.75 or for metalchelatecationic surfactant species by Sanz-Medel eta[.25.27

In any case, it seems clear that the surfactantcharacter has the decisive role in determining theobserved spectral changes, since bulky ions, whichare non-micelle-forming (e.g., tetraethylammonium)do not give rise to effects similar to those observed inthe presence of micelle-forming agents.5h.75

SOME RELATED SYSTEMS

The implications of a model for the interactions inmicelles are significant not only for micelles in waterbut also for related assemblies, since the principles oforganization are thought to be quite general. Forthis reason the following related assemblies are re-viewed.Reverse mi cel l es

The surfactant interactions in non-aqueous mediahave been investigated less than those in aqueoussurfactant systems. The surfactant aggregates in or-ganic solvents are described as having a reverse


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262 M. E. DIAZ GARCIA and A. SANZ-MEDELmicellar structure, in which the hydrocarbon tailsare in contact with the solvent and the polar head-groups form the micellar core.

The aggregation number in such reverse micelles isrelatively small, e.g., less than 10 for alkylammoniumcarboxylates, compared with up to 100 for aqueousmicelles, but it is supposed that these systems wouldexhibit an experimentally determinable c.m.c. Al-though many of the common methods for c.m.c.determination in aqueous solution are not applicableto reverse micellar systems, because of the low degreeof aggregation and because ionic surfactants do notionize in organic media, the spectral changemethod has been proposed for determinationof the c.m.c. of Aerosol-OT [sodium di-(2-ethylhexyl)sulphosuccinate]* with the dye 7,7,8,8-tetracyanoquinodimethane. Breaks in the plots ofabsorbance against surfactant concentration wereinterpreted as corresponding to the surfactant c.m.c.However, the concept of c.m.c. as explained fornormal micelles is no longer applicable in thesesystems and is still subject to controversy. Reversemicelles alter the micro-environment of solubilizedreactants and thus affect their stereochemistry, dis-sociation constants, redox potentials and reac-tivities.22

In analytical chemistry scant use has been made ofreverse micelles. Many organic reactions have beenstudied in reverse micelle systems but few studieshave been made on inorganic reactions.23-25 In viewof this situation, the study of analytical systems inreverse micelles is an unexploited research field.Synthetic bilayer membranes (vesicles and lamellae)

Vesicles are the simplest membrane-mimetic col-loidal systems and their use as membrane modelshas been recently reviewed.26,27Although they areusually made of biomaterials such as lecithin,vesicles have recently been made from syntheticsurfactants.28 The main difference between vesiclesand micelles is geometric: single-chain surfactants,e.g., cetyltrimethylammonium bromide, form mi-celles, while double-chain surfactants, such as diocta-decyltrimethylammonium bromide, form vesicles. A

typical diagram of a vesicle and a bilayer membraneis shown in Fig. 3. The hydrophobic sections are incontact and separated from the inner and outer waterphases by the polar head-groups.Single-compartment vesicles (and bilayer mem-branes) are able to encapsulate and retain a numberof substrates: 8-azaguanine is successfully incorpo-rated (34% entrapment) in positively charged di-octadecyldimethylammonium chloride vesicles, whilein cationic single-compartment liposomes (phos-pholipid membranes) the uptake of this molecule isonly 1.8% .29Some cyanine and merocyanine dyes show unusualspectral behaviour when bound to synthetic mem-branes;13 the spectral variation is highly specific tothe chemical structure of the membrane-formingamphiphile. The fluorescence of a probe molecule isdrastically increased when the probe is added to asuspension of bilayer aggregates. This enhancement iscaused by the entrance of probe molecules into thebilayer. The lower polarity of the environment andthe restriction of the twisting of the excited probemolecule result in a pronounced increase in thequantum yield.3.32These phenomena provide a wayto achieve control of the spectra of dye molecules inthe bilayer membranes, useful not only in modelstudies of biological chromophores (membrane-bound chlorophyll) 33~34ut also from an analyticalpoint of view.As with micelles, it is difficult to define the natureof the spectral changes after the addition of a probeto a bilayer membrane solution and even to deter-mine clearly whether it is hydrophobic or hydrophilicin character.

PolyelectrolytesThe phenomenon called metachromasia resultsfrom the interaction between a cationic dye and apolyelectrolyte in aqueous solution. Metachromatic

changes of colour have been studied for a number ofdyes such as Crystal Violet,3s,36Triplafavine13 andMethylene Blue38,39 ith simple polyelectrolytes (so-dium polyphosphate, polymethacrylic acid).

(4 IDlCaflO Single-cqmpartmentvesicleDouble-tailed

surfactant

(b)

Sphericalmicelle

Rod-shapedmicelle

Bilayer

Fig. 3. Schematic representation of: (a) single-compartment vesicle, (b) conversion of detergents intospherical micelles. rod-like micelles and bilayers.


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Dye-surfactant interactions 263The exact nature of these changes is again not

known, but it has been shown40 that hydrophobicattractive forces between a dye and a poly-ion may insome instances predominate over electrostatic forces.The binding of Eosin-Y to poly+lysine has beenfound to be purely electrostatic in nature, in contrastto its binding to poly(p-xylylviologen), which hasboth an electrostatic and a hydrophobic com-ponent.14 Changes in apparent acidity constants ofindicators in polyelectrolyte solutions have been at-tributed mainly to the large charge density of poly-ions and also to non-electrostatic interactions.42

The optical behaviour of a metachromatic dyebound to polyelectrolytes depends on the chemicalstructure of the dye, on the nature of the poly-electrolyte and on the binding equilibrium.36.43 It canbe related, in some aspects, to the behaviour ofmicellar systems.Acknowledgement-Thanks are due to Professor W. L.Hinze (Wake Forest University, North Carolina, U.S.A.)for his thoughtful and critical reading of this manuscript.

I2.3.4.5.6.

7.8.9.

10.11.12.13.14.15.16.

171819.20.21.22.

23.24.

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