Analytical applications of organized molecular assemblies

Analytica Chimica Acta, 169 (1986) l-29 Elsevier Science Publishers B.V., Amsterdam -Printed

in The Netherlands

REVIEW ANALYTICAL ASSEMBLIES

APPLICATIONS

EZIO PELIZZE’ITI*

OF ORGANIZED MOLECULAR

and EDMONDO PRAMAURO

Zstituto di Chimica Analitica,

Universit6

di Torino,

Via P. Giuria 5, 10125

Torino (Italy)

(Received 23rd July 1984)

SUMMARY Organized molecular assemblies have great potential utility in many types of analytical method. This review is concerned with recent studies of micelles, reversed micelles and micro-emulsions in shifting acid-base equilibria, and in electrochemical measurements, ultraviolet-visible spectrophotometry, micellar-enhanced phosphorimetry and fluorimetry, liquid-liquid extraction, flame and plasma atomic spectrometry, and high-performance and thin-layer liquid chromatography.

Amphiphilic molecules (surfactants, detergents) contain distinct hydrophobic and hydrophilic regions. The combination of pronounced hydrophobic and hydrophilic properties within one molecule gives these molecules unique properties on dissolution in water [l-12]. A simple typical surfactant has the structure R-X, where R is a longchain hydrocarbon of 8-18 atoms (usually unbranched) and X is the polar (or ionic) head group. Depending on X, surfactants can be classified as non-ionic, cationic, anionic or zwitterionic. Other surfactants consist of two or more hydrophobic chains and can incorporate functional groups. Examples of these classes of surfactants are listed in Table 1. Depending on their chemical structure and on the nature of the media, amphiphilic molecules can give rise to different organized structures, namely aqueous and reversed micelles, micro-emulsions, monolayers, bilayers and vesicles. These systems, often referred to as organized assemblies, have been shown to exhibit some interesting properties. For example, they can solubilize, concentrate and compartmentalize ions and molecules, they can modify equilibrium, acid-base and redox properties, and they can influence reaction rates, modify chemical pathways and influence stereochemistry. Obviously not all the organized structures are useful in all applications. These aspects may, however, be useful in the field of analytical chemistry, and so the organized structures of main interest in this field (schematically depicted in Fig. 1) and their most important features will be outlined briefly. Micelles. Micelles are formed when the amphiphilic molecule reaches an appropriate concentration (critical micelle concentration, c.m.c.) and aggre0003-2670/85/$10.15

o 1985 Elsevier Science Publishers B.V.

TABLE 1 Structures of the most commonly used classes of surfactants Amphiphile

structural formule

Comments

Sodium dodecyl sulfate (SDS)

CH,(CH,),,SO; Na+ + CH,(CH,),,NWH&Br

anionic cationlc

CH,WH,),,(OCHICH,),OH

non-ionic

Hexadecyltrimethylammonium bromide (HDTB) Polyoxyethylene(6)dodecyl alcohol 4-(Dodecyldimetbylammonio)butyrate

C*H, CH,(CH,),-&CH,-O--8

Sodium bie(2ethylhexyl)sulfosucdnate (Aerosol OT)

8 >

H-SO;Na+

CH,WH,), Dioctadecyldimethylammonium (DODAC) Dlhexadecylphoephate

zwitterionic

CH~(CH,),l~(CH,),(CH,),COO-Ne+

chloride

CH,WH,),~+

,CH,

cl-

CH,(CH,),/ Nb-I

(DHP)

CH,WH,),-O, CH,WH,),,-0

,b /-P<;

-

anionic (reveree micelle forming) cationic (vesicle forming) anionic (vesicle forming)

I-Dodecyl-1’.methyl-4.4’.blpyndine dichloride

redox functional

5-n-Tetradecyl-l,lO~iaza-4,7,13.16tetraoxacyclooctadecane

ligand functional

gates spontaneously. At concentrations close to the c.m.c., micelles are spherical (3-6 nm diameter), consisting of 30-200 monomers. Increasing the surfactant concentration leads to formation of rod-like micelles. The c.m.c. depends on the surfactant structure (the longer the hydrocarbon tail, the lower the c.m.c.) and on experimental conditions (ionic strength, counterions, temperature). The surfactant monomers are in dynamic equilibrium between the micelles and the bulk. Micelles are thermodynamically stable

sphencol micelle F

reversed mlcelle

surfactant

Fig. 1. Schematic representation

w/o mlcroemulsion

F

o/w m~croemulson

cosurfactant

of organized structures of surfactants in different media.

and easily reproducible; obviously they are destroyed if dilution with water brings the surfactant concentration below the c.m.c. Usually micelles have been considered to contain a hydrophobic core and a polar surface. Actually, the water penetration, viscosity, and polarity of the different regions of micellar structure are matters of current investigation and discussion [ 81. An important property of micelles (and of the other organized structures) is the solubilization of organic molecules [2, 3, 7, 91. The site of solubilization varies with the nature of the solubilized species and the surfactant. Information on the sites of solubilization is usually obtained from studies on the solubilizate before and after solubilization (WV., n.m.r., e.s.r. and fluorescence). Based on these investigations, the following models of incorporation have been suggested: (i) adsorption on the surface of the micelle, at the micelle/solvent interface; (ii) in the case of polyoxyethylene groups (nonionic surfactants) between the hydrophilic head groups; (iii) in the palisade layer, between the hydrophilic groups; (iv) more deeply in the palisade layer; (v) in the inner core of the micelle. The most likely solubilization sites are, however, the interface and the Stem layer. The solubilization process is a dynamic one, and the solubilizate is in dynamic equilibrium between the micelle and the aqueous phase (residence time lo”-lo* s). The more hydrophobic a solute molecule, the longer is the residence time. The tendency of a solute to interact with micelles is generally expressed through the binding constant, K, for the reaction S + C $ SC, where S and C are the substrate and the micellar aggregate, respectively. Thus

K = WC1/(PI Cl

(1)

C is generally expressed as the concentration of micellized surfactant, C, (i.e., analytical concentration of surfactant less c.m.c.). Another important effect of micellar systems is that they can modify reaction rates and, to some extent, the nature of the products [ 81. Micelles can inhibit or accelerate reaction rates (by up to several orders of magnitude) and also shift equilibria (acid-base, redox). These changes are due to the hydrophobic and electrostatic interactions of reactants with micellar aggregates. Quantitative kinetic treatments have been proposed in terms of a pseudophase model (i.e., considering the micellar pseudophase as a separate phase), ion-exchange models and electrostatic models. Several reviews and books have been devoted to the effect of micellar and other aggregates on chemical reaction rates and equilibria [ 8,13-161. The proper selection of an ionic surfactant can provide a structure with an interior, an exterior and a charged interface, with an electrical potential at or near the micellar surface that can differ by a few hundred millivolts from that of the bulk; this allows ionic as well as uncharged species to interact with micellar aggregates, thus greatly influencing equilibria and rate of reactions.

4

Reversed micelles. Certain surfactants may also form micelles in hydrophobic solvents, accommodating amounts of water in a hydrophobic bulk phase. Examples of such surfactants are sodium diisooctyl sulfosuccinate (AOT), benzylhexadecyldimethylammonium chloride (BDC) and dodecylammonium propionate (DAP). The dimension of AOT micelles in heptane can vary from 1.5 to 8 nm, as a function of water content. The nature of the water in the pool has been investigated by spectroscopic techniques and is different from the bulk water, with interesting effects on reaction rates. In addition, species insoluble in non-polar media can be solubilized in the water p 001 of reversed micelles . Micro-emulsions. Micro-emulsions are aggregates usually containing water, a hydrocarbon solvent, a surfactant and a co-surfactant (typically an alcohol with an alkyl group of C4 to C,). Detergentless micro-emulsions are formed by water, hydrocarbons and alcohol only [ 8,9,17] . Phase diagrams are usually necessary in order to select a particular microemulsion composition that possesses stability and the desired properties. In spite of their importance and applications, micro-emulsions have not been characterized as carefully as micelles. Micro-emulsions have dimensions ranging from 5 to 100 nm and are believed to have a spherical structure resembling that of micelles. In particular, oil-in-water micro-emulsions (o/w) contain a sizeable hydrocarbon core, in contrast to aqueous micelles, thus providing a highly apolar environment for dissolving relatively high concentrations of hydrophobic molecules in the aggregate and also large organic molecules with dimensions approaching those of many micelles. The leaving rate of monomers and the residence time of solubilizate are of the same order of magnitude of that of micelles. Dilution with water can alter microemulsion droplets. Analogous water-in-oil micro-emulsions also exist. They contain aqueous cores and are similar to reversed micelles. Microemulsions, like micelles, can affect equilibria and catalyze reactions. The peculiar properties of organized assemblies can usefully be considered in the field of analytical chemistry. They can lead to the modification and improvement of existing procedures and to the development of new methods. Organized structures offer means to overcome solubility problems, without appreciable loss in sensitivity, to speed reaction rates of analytical reactions and to reduce side-reactions, to shift acid-base or redox equilibria, and to improve selectivity and efficiency in extraction and chromatographic methods. For these reasons, organized assemblies are receiving increasing interest in analytical chemistry. Few comprehensive reviews on this topic have appeared [18, 191. The present paper is devoted to summarizing recent work and to indicating some of the possibilities of the use of surfactants in analytical chemistry.

5 ACID-BASE

EQUILIBRIA

IN ORGANIZED

ASSEMBLIES

The effect of surfactants on acid-base equilibria was recognized long ago by Hartley who reported on the effect of colloidal aggregates on acid-base indicators [20]. Later, Hartley and Roe [21], using indicators adsorbed to charged micelles, attributed the “apparent” shift of pK,, compared to pure aqueous solutions, to the creation of a concentration gradient of protons between the interface (a’,+) and the bulk solution (a$). This pH shift was related to the surface potential $ &+ = a$ exp(-FJ//RT)

(2)

Because the indicator equilibrium at the micellar surface may be affected not only by the electrostatic potential but also by the local environment (e.g., a low dielectric constant), the “apparent” pK, shift includes also a shift of the “intrinsic” pK, (p&), i.e., pK;m - pK’, = -F$/2.3

RT

(3)

In fact, pK, shifts were observed for phenols and naphthols even in neutral micelles [ 221. The use of amphiphilic pH indicators resulted in a strong fixation of the probe to the aggregate. It was possible to estimate the value of the surface potential for SDS and HDTB micelles; the difference between pK’, and pKr is 1.1 for SDS and Triton X-100 micelles [23]. When the species participating in the equilibrium HA 4 H’ + A- are partitioned between the aqueous and micellar pseudophases, the apparent acidity constant can be expressed as Ky

= (1+ K*-C)/( 1+ Kn*C)

(4)

where KA- and KHA are the binding constants [24]. Equation (4) predicts a monotonic function reaching a plateau value for Kim at high C; it can only account for salt effects by using a salt-dependent value of KA-. Since it was experimentally observed (for phenols in HDTB micelles [25] ) that the value of K,appfirst increases as predicted, but increasing surfactant concentration makes Kim decrease after reaching a maximum, Eqn. (4) was modified to take into account the ion-exchange at the micellar surface between A- and the surfactant counter-ion (i.e., bromide for HDTB) [ 261, giving %”

=K:

[l+

KA/Y(YM/Yw)I/(~

+ KHAC)

where KA,y represents the ion-exchange counter-ion and yM

=(l-_)C-[OHM]

YVI=c&+

CJII.C.

+

-[AM] [OHM] + [AM] + BYT

(5) constant between A- and the

6

where OLis the degree of dissociation of the micelle, BY is an added salt and the subscript M refers to the concentration of the species at the micellar surface. In conclusion, interaction with micellar aggregates induces significant p& shifts which can be rationalized in terms of partitioning of species and electrostatic contribution (including salt effects). A quantitative treatment of equilibrium shift in micellar media based on the ion-exchange model and on the surface potential has been recently developed [ 271. The solubilization property coupled with pK, shifts can be usefully considered in analytical applications [ 281. Potentiometric and visual titrations of sparingly soluble weak acids or bases, such as fatty acids (from decanoic to octadecanoic acid) and long-chain amines (from dodecyl to octadecylamine) [29], benzoic acids [30], sulphonamides [31] and halophenols [ 321, have been reported recently. In addition, investigations of the acid-base equilibria allow the binding constants of a series of compounds to be estimated (e.g., substituted phenols [33-351) and thus the contribution of different substituents to the free energy change in going from aqueous to the micellar pseudophase to be evaluated. It is then possible to predict reasonably the extent of the interaction of a substrate with micelles and also the corresponding pK, shift. The micellar medium offers a convenient and accurate possibility and provides an alternative to the use of mixed solvents for determinations of poorly soluble compounds. ELECTROCHEMICAL

MEASUREMENTS

IN ORGANIZED

ASSEMBLIES

From an electrochemical point of view, a surfactant can be considered as an indifferent electrolyte or substance. When it is adsorbed at the solution/ electrode interface, the interfacial tension and the double-layer structure change. The well-known suppressive action of surfactants on polarographic maxima can be explained by the lowering of the surface tension, thus producing a tendency for the surface to counteract the streaming motion. When an electroactive water-soluble compound that does not interact with aggregates is present in the bulk solution, it will diffuse to, and react at the electrode. The electroinactive adsorbed surfactant will affect the rate of the electrode processes through blocking and electrostatic effects (the latter can be an accelerating effect). Water-insoluble compounds will react on the electrode through a mechanism which will involve entry-exit motion within the aggregate. Water-soluble species interacting with organized structures will also react at the electrode after diffusion from the bulk solution. The concentration and nature of the surfactant can affect not only the shape of electrochemical waves but also parameters such as half-wave potential, electron-transfer rates at the electrode, diffusion and transfer coefficients, and the stability of intermediate species. The effect of surfactants on

7

the electrochemical behavior of organic or inorganic species has been investigated for several systems [36]. The half-wave potentials of electrochemically active species in the presence of organized assemblies were found to be unaffected in the case of some solubilized dyes or for ion couples such as Fe(CN)d-‘3-, Fe(bipy) (CN)i-‘- (bipy is 2,2’-bipyridine) and Fe3+‘2+in the presence of anionic micelles (at 0.5 M sulfuric acid) [37]. In all the other cases tested, the addition of surfactants caused a half-wave potential shift. For example, for a series of ML$+‘2+ couples (M = Ru, OS, Fe and L is a ligand of the 1 JO-phenanthroline type) in the presence of SDS micelles, the potentials were shifted towards more positive values [37]. The shift in the formal potential between aqueous and aqueous/micellar media, is related to the association constant of the species with the aggregates, or to micro-environmental changes. In general, in micro-emulsions, the half-wave potentials are shifted toward more negative values, with respect to water, irrespective of the charge of the microdroplet. Further, not all the polarographic half-waves are reversible 1331. For sparingly water-soluble organic molecules, such as tetrathiofulvalene in HDTC micelles, the observed positive shift in the half-wave potential with increasing surfactant concentration can be related to the partition equilibria [39]. In the case of oxidation of lo-methylphenothiazine to the corresponding radical cation, no relevant effect was reported in the presence of cationic or non-ionic surfactant [40] ; in anionic micelles, however, the formal potential was shifted towards less anodic values with increasing surfactant concentration up to the c.m.c. This behavior was attributed to the preferential stabilization of the cation radical by the surfactant anion monomer, Combining cyclic voltammetry and solubility measurements of sparingly water-soluble compounds, can give the correct value for the formal standard potential in water. This method is particularly useful when either the oxidant or the reductant is so poorly soluble in water that no direct measurements can be made. An example has been reported for the ferrocene/ferrocinium ion system [41]. Electrokinetic studies give additional information on the phase distribution and on the kinetics of phase exchange. It was also shown recently that, in the case of tetrathiofulvalene, the heterogeneous electron-transfer reaction takes place predominantly via the species in the aqueous phase [39]. The kinetic stability of electrogenerated radicals is also influenced by aggregates. Cationic micelles enhance the stability of anion radicals of phthalonitrile and fluorenone, so that the process becomes reversible [42]. In contrast to this example, as well as the above-cited lo-methylphenothiazine/ dodecylsulphate pair formation, is the behavior of nitrobenzene reduction to the corresponding anion radical. The interactions with differently charged micelles have been interpreted in terms of models of various micelle/substrate interactions [ 431 .

8

Electroanaly@cal applications The use of surfactants in electroanalytical techniques can lead to an increase in the sensitivity because of the increase in the yield of oxidized product for less soluble compounds. For example, various compounds that gave only shoulders or no wave in aqueous 2 M sodium hydroxide, gave welldefined anodic waves in both micelles of methyldodecylbenzyltrimethylammonium chloride and emulsions obtained with the same surfactant and an organic solvent (acetonitrile) [44]. Although the heights of these waves were in many cases linearly proportional to the concentration of the compound, the waves for some compounds were lower in height than in the absence of surfactant. An increase in sensitivity was observed in the emulsion system compared to the micelles. In the emulsion system, the potentials tended to group around the oxidation potential of the surfactant, indicating that the mechanism involved oxidation of the compound by electro-oxidized surfactant. In the case of micelles, the broad distribution of the potentials suggests that direct electron transfer to the organic compound is probably also involved; this suggests a possibility of qualitative identification of organic compounds by anodic voltammetry. It is clear nonetheless that micelles and micro-emulsions may be used for the study of electrode reactions of both oil- and water-soluble species; organized assemblies have the potential to provide a great deal of information concerning the electron-transfer processes at oil/water/electrode interfaces. In other cases, a properly selected surfactant may be used as a selective masking agent in polarographic analysis [45]. It is also possible to use a micelle-solubilized species as a mediator/titrant to couple electron transfer between an electrode and other substrates. This is the case for ferrocene solubilized in non-ionic surfactant; the electrochemitally generated ferrocinium ion was used in repetitive reversible equilibrium titrations of the heme proteins of cytochrome c, cytochrome c oxidase and mixtures [46]. Solubilization of mediator/titrant by micelles provides a wide variety of possibilities for designing experiments and analytical applications. It is well known that the presence of surfactants can affect the response of electrodes, such as ion-selective electrodes [47--501 or dropping mercury electrodes in polarography [36]. The investigation and the possible prevention of these effects have been reported extensively [47-511. Electrochemical studies of organized assemblies Electrochemical techniques can be used for the estimation of characteristic parameters of aggregates, such as c.m.c., fractional charge, diffusion coefficient, micellar size. The first estimations of the c.m.c. based on the suppression of polarographic maxima [ 52-541 have been replaced by electrosorption techniques (tensammetry) [55--581 and by methods based on ion-selective electrodes [ 59-631. The latter type of method provides a direct measure of the surfactant monomer activity, information that is difficult to obtain by other tech-

9

niques. When combined with data for electrodes which respond to the counter-ions, they characterize the unaggregated ions in micellar solution. This information can be used to study not only the c.m.c. values but also premicellar aggregation, fractional charge on the aggregate, and micellar reorganization. Data obtained from measurements of the diffusion coefficients of micellar aggregates solubilizing electroactive substances, were found to be in good agreement with those obtained from other techniques, such as light scattering. From these values, the approximate size and hence the aggregation number for the micelles can be derived. The diffusion coefficients of a water-soluble species, such as Cd2+, Tl”, Fe(CN)z-‘4- in non-ionic oil/water micro-emulsions are related to the phase volume and, for water-insoluble compounds, the measured diffusion coefficient is that of the microdroplet [38]. USE OF MICELLAR SYSTEMS SPECTROPHOTOMETRY

IN ULTRAVIOLET-VISIBLE

The use of surfactants to improve u.v.-visible spectrophotometric determinations of metal ions with metallochromic indicators (dyes) has been a very useful innovation. Ternary complexes involving the surfactant molecules usually show an increase in molar absorptivity and bathochromic shift of the absorption maximum relative to that of the binary complexes. Another important effect is the solubilizing capability of micelles towards some metal complexes that are not very soluble in water, so that extractions into organic solvents can be avoided. Although the utilization of surfactant-sensitized spectral methods has received increasing attention and the many published papers in this field have been reviewed [ 181, the mechanism of these reactions has not been elucidated completely. Only recently has attention been devoted to this end. Very sensitive methods for metal-ion determinations have been developed by using chelating organic reagents (e.g., triphenylmethane, xanthene, phenoxazone and other compounds) and cationic or nonionic surfactants [64-68]. The optimization of experimental conditions and studies of the effects of inert electrolytes, interfering ions and buffers have been reported. Ca tionic surfac tan ts Most of the recently published work has been concerned with the use of triphenylmethane dyes (TPM) in the presence of cationic surfactants such as HDTB, hexadecyltrimethylammonium chloride (HDTC), hexadecylpyridinium bromide or chloride (HDPB, HDPC), tetradecyldimethylbenzylammonium chloride (zephiramine) and 1-ethoxycarbonylpentadecyltrimethylammonium bromide (Septonex). The TPM ligands contain oxygen donor atoms which can form five-membered chelate rings with the metal ions. This complex formation implies a delocalization of the n-electrons in the dye molecule, with a corresponding red-shifted absorption maximum. The chelating behavior of

10

some widely used TPM dyes in the presence of cationic micelles was thoroughly investigated; in particular the use of pyrocatechol violet (PCV) in the surfactant-sensitized determination of several lanthanide ions has been examined over a wide range of experimental conditions [69]. The general features of these spectrophotometric methods,with particular attention to the mechanism of formation of ternary complexes with surfactant monomers and micelles, were summarized by Cermakova [ 701. Table 2 reports some selected analytical data concerning the spectrophotometric determinations of metal ions using TPM dyes in the presence of cationic surfactants. Among the other important chelating dyes, the alizarin green series was used in the sensitized determination of uranyl [84] and indium [ 851; p-arsonophenylazochromotropic acid and chromazol KS (azo dyes) were employed for the determination of niobium [86] and aluminum [ 871, respectively; the quinoline derivative, ferron, was used in the determination of iron and aluminum [ 881. TABLE 2 Spectrophotometric factants

determination

of some metal ions with TPM ligands and cationic sur-

Analyte

Liganda

Surfactant

M:Lb

Fe( III) Fe( III)

CAS CAS ECR BPR CAS ECR CAS BPR

HDPB HDTB HDTB HDTB HDTB HDTB HDTB HDPB

1:l

CBG PGR ECR PCV PCV ECR CAS CAS PCV CAS ECR ECR CAS

HDTC HDPC HDTB Zeph. HDPC HDPC Zeph. HDPC Zeph. HDTB HDTB Zeph. HDTB

Fe( III) Ti( IV) Be( II) Ti(IV) Ga( III) UVI) V(IV)

Sc( III)

Y( III) AI( III)

1:3

1:l 1:2 1:4 1:2 1:2 1:3 1:3 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2

%lax

c

PH

Ref.

680 645 635 635 565 560 619 625

6.3 13.5 12.8 5.2 7.3 5.4 9.1

4.0 3.7 + 0.9 4.5 +_0.5 3.7 1.2-1.5 1.5-1.8 5.5 2.5

71 72 72 73 74 74 75 76

662 580 575 660 660 600 610

14.4 3.6, 7.9 7.5 5.0 9.2 15.0 14.2 3.3 3.2 11.5 12.4 11.1

6.5 5.2-6.4 4.9 f 0.3 4.3 f 0.3 3.6 + 0.2 5.5 f 0.1 5.2 * 0.4

77 78 79 79 79 80 80 81 80 82 83 83 83

660 590 600 625

9.0 f 0.1 6.0 ? 0.1 7.3 f 0.2 5.3 i 0.3

*CAS, chrome azurol S; ECR, eriochrome cyanine R; BPR, bromopyrogailol red; PCV, pyrocatechol violet; PGR, pyrogallol red; CBG, chromal blue G; BCG, bromocresol green. bM:L, metal ion to ligand ratio in the ternary complex. =Molar absorptivities, X lo4 1 mol-i cm-i.

11

Comparison of spectral properties of the complexes in the absence and in the presence of the above-mentioned cationic surfactants usually shows oneto ten-fold increases in the absorptivity (e) and bathochromic shifts in the wavelength of maximum absorbance in the range 20-100 nm. The increase in sensitivity depends on surfactant concentration. Usually, when the concentration is lower than the c.m.c., only slightly soluble ion-associates can form between anionic chelates and surfactant monomers and turbidity is observed. At a concentration slightly higher than the c.m.c., solubilization of these aggregates occurs and sensitization is maximal, whereas a further decrease of surfactant concentration usually decreases the absorbance and a hypsochromic shift is observed. The last-mentioned effect can be attributed to a reduction of the chelating ability of the anionic ligand, which interacts strongly with the oppositely charged micelle. If the experimental conditions are chosen properly, the formation of soluble and quite stable ternary complexes is feasible and the spectrophotometric determination can be improved (see Fig. 2). In some cases, cationic surfactants have been used merely as solubilizing agents; for example, the spectrophotometric determination of nitrite through the diazotization reaction of p-nitroaniline and coupling with 2-methyl-& quinolinol was improved in the presence of HDTB because the micelles readily dispersed the azo dye formed and extraction into organic solvents became unnecessary [ 891. A systematic development of new sensitized spectrophotometric methods must include a careful investigation of acid-base equilibria of the ligands, the formation of ternary complexes (including the assessment of their stoichiometry) and the media effects. The variation of pK, values (corresponding to

Warelength

nm

Fig. 2. Absorption spectra of the Al/chromazol KS/CP+ system. A, 5 ml of 0.1% chromazol KS added to 3 ml of buffer solution (pH 6.2) and 10 ml of ethanol, diluted to 50 ml and measured against water in a l-cm cell; B, as A, but 16 pg of Al added and measured against reagent blank; C, as B, but 4 ml of 0.5% HDPB added and measured against reagent blank (from [87]).

12

the phenolic hydroxyl dissociation) for some TPM dyes was measured in the presence of lcarbethoxypentadecyltrimethylammonium bromide [go]. The formation of ion-associates at surfactant concentrations below the c.m.c. was assessed. A quantitative description of the changes in the acidity constants of bromocresol green (BCG) was recently reported, together with the determination of the formation constant of the ion-associates below and above the c.m.c. of the surfactant (Septonex) [ 911. A mathematical interpretation of the absorbance/pH curves of the complexes in the presence of surfactants can be derived; e.g., for concentrations well above the c.m.c., the change in the apparent acidity constant for a ligand HL in the equilibrium HL + S’* (L-, S’) + H+, where S’ is the cationic surfactant monomer, can be described by the expression ApK, = pKipp - pKa = -log PLs - log C

(6)

where Ktpp is the conditional acidity constant of the dye in the presence of surfactant and K, is the acidity constant in absence of surfactant, &s is the formation constant of the ion-pair, and C is as previously defined. If no assumptions are made (concerning the concentration range of surfactant and/or the formation constant of the ion-pair), a more general expression can be written pKtpp - pK, = -log (1 + fir,&)

(7)

or Krpp/K, = 1 + &C

(8)

The formation constants of the ion-pairs can then be evaluated experimentally provided that the K,, value is known. Any study of the mechanism of micellar sensitization needs the selection of some representative dye/metal ion/surfactant systems. The ternary system Be/GAS/S (S is a cationic surfactant) was thoroughly investigated [92]. In particular, an assessment of the salt effects showed that the detection limits for the HDTBcontaining complexes can be lowered and that the observed spectral shifts depend strongly on the anion of the added salt [ 751. The order of the anion sensitization (SO:- < Cl- < Br- < NO;) can be correlated to the ion-exchange equilibrium at the surface of the cationic micelle, on the basis of recently proposed models [ 931. Some of the previously mentioned ternary systems have also been investigated in the presence of a second surfactant (usually nonionic) which acts merely as a solvent. In this case, the solubility of the ternary complexes in the nonionic aggregates depends on the nature of the cationic surfactant used. Some hydrophobic cationic counter-ions (e.g., tetraphenylphosphonium (TPP)) which do not form micelles can be applied [94]. In the sensitized determination of the aluminium with PVC, the distribution of different ion-pairs in Triton X-100 was investigated. The efficiency of solubilization

13

into the micelles was found to be in the order: HDTB > zephiramine > TPP. The bathochromic shift observed when TPP was used can be explained in terms of dielectric constant effects. Nonionic surfactan ts The use of nonionic surfactants together with unionized organic ligands, such as l-(2-pyridylazo)-2-naphthol (PAN), 4-(2-pyridylazo)-resorcinol (PAR), and Squinolinol (Q), has been surveyed [18]. In some recently published work, it was shown that Triton X-100 can provide a suitable medium to disperse metal complexes by solubilization. For example, the sensitivity and selectivity of cadmium determinations based on cadion (4’-p-nitrophenyltriazenobenzene) can be enhanced in the presence of the above-mentioned surfactant [95]. Spectrophotometric determinations of cadmium [96] and silver [97] with the ligand cadion 2B (4’-(4-nitronaphthyltriazeno)azobenzene) was also reported. Indirect determination of traces of cyanide based on suppression of the colour of the silver and copper complexes with cadion 2B were described [ 98,991. Nonionic surfactants of the polyoxyethylenenonylphenol series were recently proposed for the determination of cadmium with PAN [ 1001. Anionic surfactants Although anionic surfactants are not often used as sensitizers, because inhibition or competition effects on the complex formation are often observed, the spectral properties of binary complexes can be improved in a few cases. A sensitized determination of zirconium, which forms a red-violet complex with the azo dye, 2-(6-bromo-2-benzothiazolylazo)-5diethylaminophenol, was achieved in the presence of SDS [ 1011. The measured absorptivity of the ternary complex is higher than those obtained when other methods based on cationic surfactants are used. The role of anionic surfactants is more diffuse in methods based on extraction of positively charged metal complexes into organic solvents. MICELLAR-ENHANCED PHOSPHORESCENCE AND FLUORESCENCE

The heterogeneous structure of micellar aggregates has a strong effect on the properties of partitioned solutes, including the photophysics of probe molecules. Micellar-induced enhanced fluorescence and room-temperature phosphorescence (r.t.p.) have recently been developed as promising analytical techniques, starting from fluorimetric studies on the structure and dynamics of micellar aggregates. Micellar-stabilized room-temperature phosphorescence A detailed review on the physical basis and analytical applications of r.t.p., including more than 130 different examples of determinations of solid-supported analytes has been published [102, 1033. The selective application of

14

this technique in the field of organic analysis, with newer instrumental devices, has also been reported [ 1041. The introduction of micellar-stabilized r.t.p. as a suitable and useful quantitative method was first proposed by Cline Love et al. [105] for the determination of aromatic compounds. Stabilization of the emitting triplet state in mixed anionic micellar solutions containing heavy counter-ions, such as Tl’ or Ag+, was observed; the increase in the quantum yield of phosphorescence obtained by operating in SDS/TlDS (or SDS/AgDS) mixtures can be attributed to the restricted motion of the emitting solutes organized on a molecular basis in the micelle, to the effective shielding from quenchers, and to the higher efficiency of spin-orbital coupling processes with the heavy atom. The last-mentioned interaction depends in turn on the residence time of the emitting molecule in the aggregate. The calibration graphs of phosphorescence intensity against analyte concentration for some compounds investigated (biphenyl, naphthalene, pyrene) showed a maximum, which can be explained in terms of deactivation processes, and by taking into account the distribution of solutes amongst the micelles according to PoissonBoltzmann statistics. Other important factors, such as the inner filter effect, must also be considered. The sensitivity of the above-mentioned determinations was shown to be comparable to that obtained by low-temperature phosphorescence or with r.t.p. of substrates adsorbed on solid supports (usually filter paper). Optimal sensitivity was obtained by using heavy metal/ sodium ratios of about 3:7. Solutions must be carefully degassed in order to prevent oxygen quenching. The micellar-stabilized r.t.p. of various substituted arenes in TlDS/SDS micellar media has also been reported, together with the typical luminescence parameters (phosphorescence lifetime, extent of quenched fluorescence, wavelength of excitation) for the analytes investigated [106]. Limits of detection of 6 X lO+ M, 6 X 10-l’ M and 5 X lo+ M were reported for naphthalene, biphenyl and pyrene, respectively. The enhancement of the triplet state population for substituted compounds was shown to depend on many factors, such as the nature of the substituent, the location of the groups within the aggregates, and the mobility of the solute (related to steric effects). Micellar-stabilized r.t.p. can also be applied to the study of mixtures on the basis of differentiation of triplet-state lifetimes. A detailed investigation of the analyte/heavy atom interaction, based on a kinetic model [107] which accounts for the lumiphor distribution inside and outside the mixed micelles, was reported for some reference compounds (biphenyl, pyrene, naphthalene) [108]. The phosphorescence lifetime (7) is described as a function of several parameters: l/r = k- + kM + k,i,

[&I in - {k-k+ [Ml I(kB + kqex[&I ex + k+ [Ml )I

(9)

where k- is the rate constant for exit from the micelle, kM is the deactivation rate constant within the micelle, kqi, is the quenching rate for an

15

internal quencher, ksex is the quenching rate for an external quencher, kz is the deactivation rate in the bulk, k+ is the reentry rate of the lumiphor, and Q and M represent the quencher and the micelle, respectively. A treatment based on Eqn. 9 led to the consideration of some limiting cases which are useful from the analytical point of view. In particular, it was demonstrated that, if two components are present, their exit rates k_ and micellar phosphorescence deactivation rates k M remain practically unchanged. This allows their individual characterization in mixtures through measurement of the experimental micellar-stabilized r.t.p. lifetimes (7). The recent introduction of the synchronous scanning luminescence technique [109] in studies of micellar-stabilized r.t.p. showed the feasibility of identifying individual compounds in multicomponent mixtures [ 1101. The use of secondderivative synchronous scanning fluorescence can improve the resolution when complex mixtures are present. Cyclodextrins have also been proposed as interesting organized systems for r.t.p. determinations of polynuclear aromatic hydrocarbons [ 1111 and nitrogencontaining heterocycles [ 1121. These structures, which do not form micelles, are very interesting because applications based on size-dependent selectivity for the lumiphor inclusion complexes can be envisaged. The reported high sensitivity is another important factor to be considered in the development of this field. Micellar-enhanced fluorescence Micellar-enhanced fluorescence is another important technique, particularly because the emission intensity of analytes is usually many times greater than in the corresponding homogeneous media. The effect of different micellar aggregates (SDS, HDTC, Triton X-100) on the spectrofluorimetric determination of pyrene was investigated by Singh and Hinze [113] ; a 3-16 fold increase in sensitivity was observed. As for r.t.p., the diminution of deactivation processes for the excited states can be interpreted in terms of the decreased polarity, increased viscosity and shielding against quenching in micellar media. Plots of relative fluorescence intensity vs. analyte concentration usually show a longer range of linearity in micelles compared to that observed in homogeneous (e.g., ethanol) media. The reported reproducibility for pyrene content, even in the pg 1-l range, is satisfactory. Increased fluorescence in micellar systems has been also applied to the detection of other polynuclear aromatic hydrocarbons, separated by pseudophase liquid chromatography (see below). The determination of nonfluorescent analytes through their derivatization with suitable fluorescent compounds is also possible. Micellar-sensitized determinations of amino acids after derivatization with dansyl chloride (l-NJV-dimethylaminonaphthalene5-sulphonyl chloride) and Roth’s reagent (o-phthalaldehyde-2-mercaptoethanol) have been reported [ 1141. The measured detection limits of dansylglycine in micellar media are 12-20 times less than those obtained in water, whereas the increase in sensitivity is about lo-fold for the OPT derivative of lysine.

16

Although micellar spectrofluorimetry seems to be particularly suitable for the determination of organic compounds, its use in inorganic analysis has also been reported. Particularly when inorganic species can form fluorescent compounds, the use of micelles can be effective [ 181. Some of the recently published work concerns, for example, the determination of rare earth metal ions as Pdiketoneltri-n-octylphosphine oxide (TOPO) complexes in the presence of non-ionic surfactants [115, 1161 and the improved determination of aluminium as a complex with morin using ethylene oxide/propylene oxide non-ionic condensates [ 1171. MICELLAR SYSTEMS IN LIQUID-LIQUID EXTRACTION

The liquid-liquid extraction of metal ions as chelate complexes or ion-pairs in organic ‘phases is a widely applied separation technique, which is often coupled with spectrophotometric determination of the extracted species. The formation of suitable ternary complexes in aqueous solutions containing micelle-forming surfactants was discussed in a previous section. In many cases, the neutral ionassociates formed between the metal complexes and the surfactant can be extracted into organic solvents and measured spectrophotometrically in this phase. The choice of surfactant depends on the charge of the metal complex, anionic complexes forming extractable ion-pairs with cationic surfactants and vice versa. In some cases, reverse micelles can be formed in the organic phase. Many examples of these analytical applications were previously reviewed [ 181. Table 3 summarizes some recently reported extraction methods involving anionic complexes and cationic surfactants. The extraction of divalent transition metals with surfactants derivatized to contain a ligand group proved to be effective in nearly neutral solutions. Quaternary ammonium carboxylates, synthesized from trioctylammonium chloride and longchain carboxylic acids, were used for the extraction of Mn2’, Co2+, Cu2+and Ni2+from their aqueous chloride solutions into benzene [ 1231. The synthesis and characterization of new micelle-forming ligands [ 1241, together with detailed analysis of the extraction mechanism and kinetics [ 1251, can offer new perspectives in this field. Studies of the mechanism of transfer (from aqueous solutions to chloroform) of the chloro complexes of zinc (II) and iron(II1) in the presence of methyltrioctylammonium chloride and longchain amines, respectively, suggest that interfacial complexes are formed by ion-exchange reactions (fast processes) and that the ratedetermining step is the replacement of the neutral ternary complexes by the reagent monomers from the organic phase [ 126,127]. A few studies on liquid-liquid extraction with nonionic surfactants have been reported. Extraction of zinc(I1) into different organic solvents (nitrobenzene, 1,2dichloroethane, chlorobenzene) can be achieved, starting from acidic thiocyanate solutions, in the presence of polyethylene glycol and its monoalkylphenyl ether [ 1281. An increase in the extraction efficiency was found when the number of ethylene oxide units was increased.

17 TABLE 3 Extraction of anionic chelate complexes into organic solvents in the presence of cationic surfactants from acidic aqueous solutions Analyte

vo;

All’ Cd=+ AP+ Cd” Sb3+ Bi”’ Ins+ Ga”+

Techniquea

Ligand

Surfactant

Ternary complexb

Organic phase

Ref.

PAR PCV PAR

TOMAC= Zeph. HDBACd TOMAC TOMAC TDEAB’ TDEAB TDEAB TDEAB

1:l:l 1:2 :3 1:2:2 ? ? 1:2:3 1:2:2 1:2:2 1:2:2

CHCI, CHCl, CHCl, CHClJ dichloroethane ccl, Xylene Xylene Xylene

118 119 120 121 121 122 122 122 122

:: PCV PCV PCV PCV

‘S, spectrophotometry ; F, fluorimetry. bStoichiometry of the extracted ternary complexes (metal ion/ligand/surfactant). cTrioctylmethylammonium chloride. dHexadecyldimethylbenzylammonium chloride. e8-Quinolinol-5-sulfonic acid. fTridodecylethylammonium bromide.

The phase separation of nonionic micellar solutions above the cloud point can also be applied to the extraction of metal chelates [129]. At a temperature higher than the cloud point, the micellar pseudophase can be separated by centrifugation, and then the water-insoluble metal complexes (e.g., the Zn-PAN chelate) can be extracted and concentrated into this phase. After dilution with a suitable surfactant solution, the absorbance of the complex in the diluted transparent phase can readily be measured. Recent advances in surfactant-improved extractions include the use of micro-emulsions instead of classical organic solvents. The extraction of gallium from aqueous alkaline solutions into benzene by means of the hydrophobic ligand 7-(lethenyl-3,3,5,5-tetramethylhexyl)-8-quinolinol can be improved if a carboxylate surfactant and a long-chain alcohol are added [130]. The resulting water-in-oil micro-emulsion has a water content which depends on the surfactant concentration and on the co-surfactant chain length. The observed increase in the extraction rate (about 20 times) can be explained in terms of the increased inter-facial area of the micro-emulsion, faster mass transfer and concentration of the reagents in the droplets. The use of macrocyclic crown ethers as carriers for the transport of metal ions through emulsion membrane systems is interesting. The effective and rapid separation of lead ions from a bulk aqueous solution containing alkaline ions was reported [131]. At the end of the transfer process, lead is located inside the emulsion droplet. Another interesting aspect of the surfactant-modified extraction methods concerns their use in separating and quantifying the surfactants themselves which are usually present in waters at only trace levels. These procedures

18

usually exploit the formation of suitable coloured species in the organic phase, which can be determined spectrophotometrically. Many important nonionic surfactants (polyoxyethylene alkyl ethers) can be concentrated in organic solvents, such as 1,2dichloroethane, after the formation of ion-pairs with potassium picrate [132]. The process involves the formation of a cationic coordination complex between the ether groups of the surfactant and the potassium ion, and then the formation of an uncharged extractable species with the picrate anion which absorbs in the visible region. The potassium salt of tetrabromophenolphthalein ethyl ester can provide an ahemative reagent for the extraction/spectrophotometric determination of such surfactants in dichlorobenzene [133]. This method, tested for more than twenty different commercial products is also’suitable for the alkylaryl ethers. Recoveries of 94-100% for nonionic surfactant present in river waters in the concentration range 4-80 ng ml-’ are reported. A method based on the formation of neutral colourless adducts between nonionic surfactants (e.g., Triton X-100) and potassium tetrathiocyanatozincate(I1) involves extraction into 1,2dichlorobenzene and indirect determination of the surfactant by measurement of the metal ion. The zinc(I1) can be backextracted in acidic media and quantified by flame atomic absorption spectrometry (a.a.s.) [ 1341 or converted to a suitably coloured complex and quantified spectrophotometrically [ 1351. Similar procedures have been reported for the determination of anionic surfactants. Among the chromophorecontaining cationic complexes which are capable of forming ion-pairs with anionic surfactants are: the nickel and copper chelates of triethylenetetramine for the selective extraction of longchain carboxylate surfactants in cyclohexane/isobutanol [ 1361; the intensely coloured complex bis[2-(2-pyridylazo)5diethylarninophenol] cobalt(II1) for the determination of sulphonated and sulphated surfactants present in water (benzene as solvent) [ 1371 ; copper( II) complexes of ethylenediamine derivatives for extraction of alkyl sulphates and alkylbenzene sulphonates into chloroform [138]. In this last method, the surfactant content is evaluated indirectly by measuring the copper concentration in the aqueous or organic phase by a.a.s. The increasing interest in extraction methods in the presence of micellar systems has encouraged studies of partition equilibria of solutes between aqueous micellar and organic phases. In a recent paper [ 1391, a distribution model of solutes between octanol and micellar SDS was proposed, which accounts for the partition of the polar compounds investigated and allows the simultaneous determination of the binding constant of solutes to micelles and the partition coefficients between the aqueous and organic phases. Kinetic analysis can also give insight into the distribution processes; for example, the transference of solutes into the amphiphilic aggregates can be monitored by using kinetic techniques [ 1401. Further analytical development of these quite complex multiphase systems will need very accurate studies of the complexation, acid-base and

19

redox processes occurring at the interfaces, and of the kinetics and mechanisms involved in the transfer of analytes. Well-planned syntheses of new suitably functionalized surfactant molecules would also be valuable. SURFACTANT AGGREGATES IN FLAME AND PLASMA ATOMIC SPECTROMETRY

It is well known that the addition of organic solvents can greatly alter the absorption of aqueous solutions of analytes in flame absorption methods. Various mechanisms have been proposed, including the modification of viscosity, lowering of surface tension and formation of very small droplets during the aspiration and nebulizing stages. However, the use of these watermiscible or water-immiscible solvents may present some disadvantages. Although the use of water-soluble surfactants in these techniques has been previously reported [ 181, a complete explanation of the observed effects has still to be proposed. The addition of SDS to an aqueous solution containing chromium(V1) has been shown to improve the analyte determination by flame atomic absorption spectrometry (a.a.s.); the absorption was enhanced and interferences were suppressed [ 1411. A detailed study of surfactant concentration effects on the droplet size distribution of aerosols, conducted on this system under constant flame conditions showed a relevant change in the droplet mean diameter, which is reduced to about half the value in water in the presence of surfactants (50 mmol 1-l SDS or DTAC) [ 1421. An enrichment model which assumes the preferential redistribution of ionic analytes amongst the smaller droplets of the aerosol in the premix burner chamber has been proposed in order to explain the enhanced absorbance of some ions (CL?+, Cr3+ Mn2+, Ni2+, Ca’+, Mg2+) in the presence of SDS [ 1431. Cationic and nonionic surfactants showed only slight effects. More recent studies on the mechanism of surfactant enhancement of sensitivity in flame a.a.s., based on the ionic redistribution in the aerosol and on the effect of many experimental parameters, such as the air flow rate, the viscosity of the aqueous surfactant solutions, the height of the optical path in the burner and the aspiration rate, were reported for copper(I1) in the presence of SDS, HDTB and Triton X-100. The greatest effect was observed when the surfactant had the opposite charge with respect to the analyte, and for concentrations just below the c.m.c. [ 1441. Another important application of surfactants in absorption and emission spectrometry concerns their use as emulsifying agents in order to dilute water-immiscible samples without resort to organic solvents. Metals in liquid organic samples and in creams or ointments by a.a.s. can be quantified after emulsification with appropriate surfactants and direct aspiration of the aqueous emulsion into the nebulizer. Mineralization steps or the use of organic solvents as diluents can thus be avoided. Furthermore, inorganic standards can be used for the calibration curves. The choice of surfactants is usually made on the basis of the hydrophilic/lipophilic balance (h.1.b.) of the oil. For samples containing particulate metal, an acid pretreatment is

necessary, and the stability of the emulsion in acidic media has to be verified. Most of the reported applications in a.a.s. concern oil-in-water emulsions obtained by matching the proper amount of the oil sample with the surfactants and sonication of the mixture until a sufficiently stable dispersion is formed. Table 4 reports some practical examples. One of the more important factors in the choice of emulsions to be directly aspirated into the flame is stability and homogeneity of the emulsion, which are in turn related to the size of the dispersed oil (or water) droplets; the preparation of suitable emulsions containing very small droplets is then an important goal. The use of micro-emulsions, in which the diameter of the dispersed particles is in the range 10-200 nm, can ensure very good properties. Recently, these dispersions were shown to be effective for the direct determination of wear metals in lubricating oils by means of d.c. plasma spectrometry [149]. By treating the sample with a nonionic surfactant (Brij35) and co-surfactant (noctanol), a translucent oil-in-water micro-emulsion with a mean hydrodynamic radius of the droplets of about 25 nm (measured by using the quasi-elastic laser light-scattering technique [150] ) can be obtained easily. The use of these particular microheterogeneous systems in the absorptionemission techniques should be an interesting field for future work. MICELLAR

SYSTEMS IN CHROMATOGRAPHY

A few years ago, aqueous micellar solutions were proposed as new effective mobile phases in liquid chromatography [ 1511. The microheterogeneous micellar environment allows the organization of solutes through hydrophobic and electrostatic interactions so that surfactant solutions can offer an altemative to the use of the organic and hydro-organic traditional eluents in both high-performance liquid chromatography (h.p.1.c.) and thin-layer chromatography (t.1.c.). Micelles in h.p.1.c. The use of SDS as an eluent in h.p.1.c. separations of many phenols and some polynuclear aromatic hydrocarbons on reversed-phase columns [ 1521 showed the dependence of retention times on the concentration of surfactant. TABLE 4 Use of oil-in-water emulsions in some a.a.s. methods Sample

Emulsifiers

Analyte

Ref.

Used lubricating oils Gasoline Undecenoate ointments Used lubricating oils

MS-1 2, Tween-20 Brij-30, Tween80 Brij-30, Tween-20 MIBK, Nemol-KS9

Pb Pb Zn Fe

145 146 147 148

21

A distribution model based on the partition of solutes between three phases: aqueous (bulk), micellar and stationary, can be assumed to describe the chromatographic behavior of eluted compounds [153]. The following equation can be derived v,/(v, - v,)

=

(l/&W)

+

c(pMW

-

~)f%%V

(10)

where V, is the volume of the stationary phase, V, is the elution volume of each solute, V, is the void volume; P MW and Pw are the partition coefficients between the micelle and water and between the stationary phase and water, respectively; v is the partial molar volume of the micellized surfactant and C is the concentration of surfactant exceeding the c.m.c. From the slope and intercept of the plots according to Eqn. 10, the partition coefficients (PM, and Psw) and the binding constant (KMw) of the solutes to the micelles can readily be calculated. The binding constant is related to the partition coefficient through the expression: KMw = (PM~ - l)P [24]. The measurement of KMw from chromatographic data is very important because the quantitation of micelle-solute interaction is at the basis of the practical use of these organized structures. The method can be applied also in cases in which there is spectral overlap between surfactants and solutes (e.g., using a refractive index detector); furthermore, the term Psw can be controlled easily by a proper choice of the stationary phase. When the partition of very hydrophobic solutes with the micelles has to be studied, more polar stationary phases, such as bonded alkylnitrile, must be used in order to reduce the Psw term and thus increase the intercept value in Eqn. 10. As expected from Eqn. 10, selectivity can be controlled by regulating the surfactant concentration. Solutes which bind to the micelle usually show a decreased retention time, thus increasing C, but, because the Psw term is not very sensitive to the surfactant concentration, the rate of variation of capacity factors vs. C is different for solutes exhibiting different hydrophobicity, and inversions in the order of retention can occur [ 1541. This behavior is typical in pseudophase liquid chromatography. If the solutes are present in ionic form, electrostatic interactions with ionic micelles and surfactant monomers may be operative and this introduces another factor which can control the selectivity. These effects are complicated because, for example, an increase in the PMw (or KMw) term, observed when an ionic solute binds to the oppositely charged micelle, can give no decrease in retention (as would be expected on the basis of the mobile phase effect) but, on the contrary, can produce higher retention times. This can be explained in terms of the ion-pair formation between the solute and the surfactant molecules adsorbed on the low-polar stationary phases (the Psw term increases). The elution of acidic phenols, such as nitrophenols, with SDS and DTAB (dodecyltrimethylammonium bromide) showed clearly the abovementioned behavior. The interest in practical applications of micellar chromatography is growing because the sensitivity of the analysis can also be improved by introducing

22

phosphorescence and fluorescence detection. The use of micellar-stabilized r.t.p. for quantifying aromatic compounds (phenanthrene, biphenyl, 2-naphthol) by operating in postcolumn reaction mode was shown to be effective [ 1551. A mixed 0.15 M micellar solution containing SDS and TlDS (7 :3) was used. The basic requirements for the development of new methods based on micelle-mediated luminescence and h.p.1.c. have been recently examined [ 1561. In particular, the effect of temperature, the addition of conventional organic modifiers, the characterization of the signal and the selective enhancement of phosphorescence and/or fluorescence processes, have been considered. The advantages and disadvantages obtained by combining traditional reversed-phase chromatography and micellarenhanced detection have also to be carefully considered. As previously mentioned, micellar chromatography can give insight into the distribution equilibria of solutes. The chromatographic separation of several hydroxy- and dihydroxy-benzene derivatives by using aqueous SDS was recently examined [157] in order to extend the use of the partition model to a series of compounds previously investigated by means of other techniques (variation of acid-base equilibria [34, 351, kinetics of oxidation reaction of diols to the corresponding quinones [158] ). By appropriate operation with different SDS concentrations, the measured chromatographic parameters of each solute, plotted against C according to Eqn. 10 gave the binding constants, which were found to be in agreement with the previously published data. Furthermore, the application of mice&r h.p.1.c. to the separation of some substituted anilines with HDTB as eluent was also shown to be successful; the partition data resulting from h.p.1.c. and from spectral variation measurements compared well [ 1591, Table 5 collects the partition data obtained from h.p.1.c. analysis for several aromatic compounds in SDS micellar media.

Micelles in t.1.c. The first reports on micellar t.l.c. were mainly focussed on the practical use of surfactant mobile phases for separation of hydrophobic and hydrophilic compounds. The separation of a series of chlorinated pesticides and polychlorobiphenyls was achieved by using SDS and HDTB solutions as eluents on polyamide and/or alumina stationary phases [ 1611. Polynuclear aromatic compounds were also separated by operating with SDS on polyamide sheets [162] . The increase in Rf with increasing SDS concentration is higher for more hydrophobic compounds, and this can be connected with the increase in the solute fraction bound to the micelles. Because inverted micelles can provide a microheterogeneous organic phase having properties similar to those of micelles, these systems have proved to be effective in the t.1.c. separation of several hydrophilic solutes, such as nucleosides [ 1611 and amino acids [ 1621. The aqueous core of the inverted micelles of sodium dioctylsulfosuccinate in cyclohexane can partition the

23 TABLE 5 Partition data of aromatic solutes between aqueous and SDS phase, from h.p.1.c. measurements Compound

%w

Benzene Toluene Nitrobenzene Naphthalene Benzene-l-o1 4-MethylO-Ethyl4a-Propy14-t-Butyl 3.5-Dimethyl2.4.5-Trimethyl4-Nitro 4-Fluoro4Chloro4-Bromo4-Iodo3.5~Dichloro-

1-P 215a 908 316c 398

aFrom i321.

95” 24s” 450” 995”

KMW (M-I)

is

40”

230 514 77a 32.5C 28d 62.5e i51e 259e 474e 405e

23 82 10 24 63 117 258 59 133 16 39 61 123 105

Compound Benzene-1.2diol 4-Methyl4CYanO4-i-Propyl4+ButylBenzene-1.3diol Benzene-l,l-diol a-Methyl2Chloro2.3.5-Trimethyl2-Phenyl2-t-Butyl2-Naphthol 4-Nitroaniline

‘MW

wb

79 28b

xx+

43lb

27= 14.5b 2lb 34b s4b 2o2b 244b 3938 848 12C

KMW (M-‘1 7 19 7 58 112 7 3.5 5 9 14 52 63 102 18.5-22

[ 1551. bFrom [ 1581. CFrom [ 1531. dEvaluated from micellar t.1.c. [ 1601. eFrom

solutes to be separated. In this case, the Rf values usually change as a function of water content in the micelle, to a saturation limit. The use of aqueous micellar solutions with nonpolar stationary phases (reversed-phase t.1.c.) was shown to be limited by the instability of these bonded phases. The feasibility of reversed-phase t.1.c. separations was studied for solutions of high ionic strength [ 1631. An optimum range of added salts was defined, with which cationic (HDTC) or anionic (SDS) surfactants can be used. The presence of a double solvent front, the first of which consists mainly of water, whereas the second contains the micellar aggregates, is a unique feature of these chromatograms; the solutes are distributed between the two fronts according to their hydrophobicity. Thus the more hydrophobic solutes are distributed below the lower (micellar) solvent front, whereas the hydrophilic compounds move to the upper aqueous solvent band. Recently, the use of mice&r t.1.c. was also proposed as a tool for the estimation of partition parameters of solutes between water and micellar aggregates. As for pseudophase h.p.l.c., a quantitative description of the chromatographic behaviour of solutes in t.1.c. was proposed by Armstrong et al. [ 1601 Rfl(l

--Rf)

=

(Vm/VsP3W)

-I- (vm/vs)(c(pMW

-

~)T/J’~w)

(11)

where V, and V, are the volumes of the mobile and stationary phases, respectively; the other terms were as previously defined.

24

Recent developments in micellar chromatography The increasing interest in this area has involved assessment of the complex mechanisms of interaction of solutes in organized assemblies. The observation of unusual effects during the elution of a series of compounds (dyes, alkaloids) which do not bind to the micelles has led to a tentative classification of solutes into three categories according to their retention pattern obtained on increasing the surfactant concentration [ 1641. Binding, nonbinding and antibinding character was assigned to the solutes on the basis of the slope of the plots of Eqns. 10 or 11. From the slope values, solute interaction coefficients were assigned; moreover, the dependence of these coefficients on the ionic strength and on the nature of the salts added provides information about the charge of the separated compounds. The observed effects for non-binding or antibinding compounds are still related to the type of stationary phase; for example, alkylnitrile, which does not adsorb the surfactant very much, usually shows plots of negative slope, whereas Cs- or (&bonded phases show plots of zero slope. An explanation of these observations has been proposed [165] ; it was assumed that excluded volume effects operate from electrostatic repulsions between the solute and the micelles. On this basis, salt effects must be important and this behavior was in fact observed. With regard to the use of micellar phases in practical separations, new improvements were reported recently, based on the use of small amounts of organic modifiers (usually alcohols) to the micellar mobile phases, and on separations at higher temperatures [166]. The increases in efficiency observed can be attributed mainly to the faster mass transfer process from the stationary to the mobile phase. Thus, also with highly hydrophobic bonded phases, such as C18, the broadening of the elution peaks is dramatically reduced and chromatographic patterns similar to those obtained with traditional eluents can be obtained. The separation of several aromatic compounds (phenol, acetophenone, benzene, toluene, nitrobenzene, anthracene, ethylbenzene) was reported to be very efficient with 0.05-0.20 M SDS containing 3% n-propanol as eluent on an ultrasphere ODS column (gradient elution) [ 1671. The use of a cationic micellar eluent (HDTB) containing different amounts of methanol was also indicated as a suitable method for the separation of dithiocarbamates on an alkylnitrile column [168] . These examples suggest that micellar mobile phases can be considered as a promising alternative to the use of organic solvents in liquid chromatographic techniques. Conclusions Organized assemblies have been shown to have great potential utility in many aspects of analytical chemistry. Not only can they improve existing procedures but, more interestingly, it is possible in some cases to overcome problems and develop quite new analytical methods. Elucidation of fundamental properties as well as studies on practical systems will certainly increase the range of applications of organized assemblies in analytical chemistry.

Support from the Minister0 della Pubblica Istruzione and the National Research Council of Italy, Progetto Finalizzato “Chimica Fine et Secondaria”, is gratefully acknowledged. NOTE ADDED IN PROOFS

Since the submission of the original manuscript, several publications have appeared on the topic under review, reflecting the intense research activity in this area. For the lack of space, we review them only briefly here. The effect of surfactants in the spectrophotometric analysis of metals have received further attention [169, 1701. In particular, specific methods of analysis of Zn using anionic surfactants [ 1711 and of sulphide in the presence of nonionic surfactants [ 1721 have been reported. The use of long chain quaternary ammonium salts for the extraction of some anions in organic solvents has been investigated [ 1731. Micellar chromatography has been the subject of other recent papers dealing on the study of factors which can control efficiency and capacity [ 174,175]. Application of this technique in protein separation was also recently proposed [ 1761. Papers on miscellaneous analytical applications of micelles reported the use of aggregates as solubilizers for chemiluminiscent analytes [177], the electrokinetic separation of phenols in open-tubular capillaries [178] and the use of aqueous emulsions in a.a.s. analysis of metals [ 1791. REFERENCES 1 C. Tanford, The Hydrophobic Effect, Wiley, New York, 1973. 2 P. H. Elworthy, A. T. Florence and C. B. Macfarlane, Solubilization by Surface Active Agents and its Application to Chemistry and the Biological Sciences, Chapman and Hall, London, 1968. 3 K. Shinoda, T. Nakagawa, B. Tamamushi and T. Isemura, Colloidal Surfactants: Some PhysicoChemical Properties, Academic Press, New York, 1963. 4 M. J. Schick (Ed.), Nonionic Surfactants, M. Dekker, New York, 1967. 5 E. Jungermann (Ed.), Cationic Surfactants, M. Dekker, New York, 1970. 6 W. M. Linfield (Ed.), Anionic Surfactanta, M. Dekker, New York, 1973. 7 M. J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New York, 1978. 8 J. H. Fendler, Membrane Mimetic Chemistry, Wiley, New York, 1982. 9 K. L. Mittal (Ed.), Micelliaation, Solubilization and Microemulsions, Plenum Press, New York, 1977. 10 K. L. Mittal (Ed.), Solution Chemistry of Surfactants, Plenum Press, New York, 1979. 11 K. L. Mittal and E. J. Fendler (Eds.), Solution Behavior of Surfactants: Theoretical and Applied Aspects, Plenum Press, New York, 1982. 12 K. L. Mittal and B. Lindman (Eds.), Surfactants in Solution, Plenum Press, New York, 1984. 13 J. H. Fendler and E. J. Fendler, Catalysis in Micellar and Macromolecular Systems, Academic Press, New York, 1976. 14 E. J. R. Sudholter, G. B. van de Langkruis and J. B. F. N. Engberts, Rec. Trav. Chim. Pays-Bas, 99 (1980) 73. 15 C. A. Bunton, Catal. Rev. Sci.-Eng., 20 (1979) 1. 16 D. Meisel and E. Pelizzetti, Prog. Chem. Kin., in press.

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