The association of some novel cationic surfactants in benzene

The Association of Some Novel Cationic Surfactants in Benzene DO(~AN E M i N G U V E L i 1

Institute of Physical Chemistry, University of Basel, Klingelbergstrasse 80, Basel CH-4056, Switzerland Received December 16, 1983; accepted March 14, 1984 The mean aggregationnumber, (n), of di-(ethyl, 2-ethylhexanoate)ammonium salts, methyl, dimethyl di-(2-ethylhexylpropionate) ammonium salts and dielectric increment, h~, and dipole moment, ~t, of dimethyl di-(2-ethylhexylpropionate)ammonium nitrate and perchlorate progressivelyincreased with increasing surfactant concentration in benzene, while g/(n) (/~ = dipole moment of an aggregate)for latter surfactantsdecreasedas (n) increased, indicating nonmicellar associationpatterns. The substituents constituting the polar head and counterions affectedthe association. The increase in (n) with different counterions was in the order HSO~ > NO~-> C104 > CI-. The application of pseudo-phase model to the experimental data failed to detect micellar association and the critical miceUeconcentration for the cationic surfaetants and no agreement was observed between multiple-equilibrium model and real systems. The experimental results were correlated with those of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in benzene. The other factors affectingthe aggregation process were discussed in terms of the solute-solute and solute-solvent interactions. INTRODUCTION The association of different types of surfactants in apolar media has been studied intensively (1, 2). However, there have been relatively few detailed studies on the association of cationic surfactants in apolar solvents. The question of micellar aggregates by cationic surfactants remains open for discussion and argument (3). In general, cationic surfactants display progressive association patterns while anionic surfactants demonstrate the process of micellization similar to that in aqueous solutions. The difference in association reveals the difference in structural effects rather than the difference in charge types. It is known that surfactant polar head is the main driving force with respect to hydrocarbon chain for the aggregation process in apolar media and association increases with decrease in total carbon n u m b e r of the alkyl groups (1, 2). Therefore, it was considered that surfactants having different polar heads and hydrocarbon chains I Present address: 88 rue du Bac, 75007 Paris, France.

similar to that of sodium bis(2-ethylhexyl)sulfosuccinate, which shows micellar association patterns, would be ideal to investigate the nature of the effects involved for the different behavior of cationic surfactans in apolar media. With this objective, a series of cationic surfactants have been synthesized and their association behaviors were studied in detail by using different techniques. In this work benzene was used as a solvent because o f its intensive use in studies for the aggregation of cationic surfactants. This paper reports data on the association of some novel cationic surfactants in benzene as obtained from vapor pressure and dipole m o m e n t measurements and interprets these data in terms of pseudophase and multiple-equilibrium models. EXPERIMENTAL

Materials 2-Ethylhexyl acrylate (purum), diethanolamine (puriss), 2-ethylhexanoic acid (purum) (Fluka, Buchs SG), and thionylchloride 344

0021-9797/84 $3.00 Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984

ASSOCIATION

OF

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345

(Merck AL grade) were distilled under reduced pressure. Phosphorous trichloride (BDH AL grade), methylamine (purum, 33% in methanol), silver nitrate (puriss p.a.), silver perchlorate monohydrate (puriss cryst.), conc. sulfuric acid (puriss p.a.), methyl iodide (puriss p.a.), and potassium fluoride (purum p.a.) (Fluka Buch SG) were used as received. 2-Ethylhexanoyl chloride: Distilled 2-ethyl hexanoic acid, 60 g (0.416 M) and thionyl chloride, 160 g (1.345 M) were refluxed for 6 hr (4). The mixture was distilled under nitrogen atmosphere at 13 Torr and 2-ethylhexanoyl chloride was collected at a boiling point of 64-65°C. Diethanolamine HCI: Distilled diethanolamine, 12.67 g (0.12 M) was dissolved in 100 ml dried ether and HCI gas was passed through until HCI gas evolved, solvent evaporated, and dried under vacuum. Solvent: Benzene (Merck AL grade) was dried over a sodium alloy (Dri-Na, Baker AR) in a distilling apparatus with circulation of benzene. Its specific conductivity and water quantity measured by Karl-Fisher titration were 7.10 -15 ohm -1 cm -1 and 0.01%, respectively, at room temperature.

Hitachi 101 spectrophotometer fitted with 1 mm thickness cell and W + W 1000 recorder. The eluents of 17 g material were collected at 240 nm in small portions at a flow rate of 0.8 ml/min. Separated tertiary amine was convetted to salt with an equivalent methanolic HCI, solvent evaporated, dried under high vacuum for 5 hr, and desiccated under vacuum and over phosphorous pentoxide. Elemental analysis showed C, 60.9%; H, 10.2%; N, 3.6%; O, 16%; C1, 8.4%. Di-(ethyl, 2-ethylhexanoate) ammonium bisulfate: An equivalent conc. sulfuric acid was added to the neat di-(ethyl, 2-ethylhexanoate) amine and the mixture was kept 2 hr at 60-70°C, then dried and stored as described above. Di-(ethyl, 2-ethylhexanoate) ammonium nitrate and perchlorate: These surfactants were prepared by adding an equivalent ethanolic silver nitrate and silver perchlorate monohydrate to the ethanolic solution of di-(ethyl, 2-ethylhexanoate) ammonium chloride. The obtained salts were dissolved in dried benzene, treated with charcoal for 24 hr, then filtered, solvent evaporated, dried, and stored as described above. Methyl di-(2-ethylhexylpropionate) ammonium chloride: Methylamine, 8.1 ml (0.071 The Preparation of Cationic Surfactants M) was reacted by 2-ethylhexylacrylate, 26.28 Di-(ethyl, 2-ethylhexanoate) ammonium g (0.142 M) in 40 ml absolute ethanol for 2 chloride: 2-Ethylhexanoyl chloride, 32.53 g hr at 60°C. Through the column separation, (0.2 3/) and diethanolamine HC1, 14.16 g (0.1 obtained tertiary amine was converted to salt M) was reacted (5) using 0.15 ml phosphorous by an equivalent methanolic HCI. Following trichloride as a catalyst (6) first at 70°C for 3 the treatment described above elemental hr, then at 140°C until the liberation of HCI analysis showed C, 63.4%; H, 10.9%; N, 3.2%; gas stopped. The neutralized reaction mixture O, 15%; C1, 8.2%. Methyl di-(2-ethylhexyl propionate) amwas treated by dried benzene, solvent evaporated, and dried under vacuum. Free chloride monium nitrate, perchlorate, and bisulfate: and carboxylic acid was checked by 1 M They were prepared from methyl di-(2-ethylAgNO3 and 1.1 X 10-2M ethanolic NaOH. hexyl propionate) ammonium chloride with For the column separation of reaction mix- an equivalent silver salt according to the proture, a column (56 X 75 cm i.d.) with glass cedure described above. Dimethyl, di-(2-ethylhexyl propionate) wool at the bottom prepared by Kieselgel 60 (0.04-0.063 nm, 230-400 mesh, ASTM ammonium iodide: A slight excess of methyl Merck) using light petroleum bp 60-80°C and iodide and neat methyl di-(2-ethylhexyl proabsolute ethanol (2:1), was connected to the pionate) amine in 1:1 ratio was kept 48 hr in Journal of Colloid and Interface Science, Vol. 10 I, No. 2, October 1984

346

DOriAN EM[N GUVELI

a dark place, treated with charcoal, filtered, dried, and stored as described above. Dimethyl, di-(2-ethylhexyl propionate) ammonium fluoride: It was prepared from the mixture 0.01 M dimethyl, di-(2-ethylhexyl propionate) ammonium iodide in benzene and 0.1 M aqueous potassium fluoride solution by benzene extraction. The collected benzene solutions was treated with charcoal for 24 hr, filtered, solvent evaporated, dried, and stored as described above. Dimethyl, di-(2-ethylhexyl propionate) ammonium nitrate and perchlorate: They were prepared from dimethyl, di-(2-ethylhexyl propionate) ammonium iodide or fluoride by an equivalent silver salt according to the method described above. Measurements Vapor pressure. Vapor pressure osmometry is a superior technique for establishing the aggregation behavior of surfactants in apolar solvents and it has been used intensively (1, 2, 7-10) to determine the mean aggregation numbers, (n), as a function ofsurfactant concentration in apolar media. A vapor pressure osmometry (Hitachi-Perkin-Elmer, type 115) was used for the determination of (n) per aggregate of the surfactant concentrations at 25 + 0.1°C. The instrument was calibrated by using a recrystallized benzil according to the instrument manual and a calibration curve relating the variation of AR to concentration of benzil was used to evaluate (n) for each surfactant solution that was measured in replicate at least six times. AR is the resistance difference between pure solvent and solution. In order to check the accuracy and the precision of the vapor pressure osmometric measurements for (n) of various surfactant solutions, (n) of AOT at 8.80 × 10-3 mole dm-3(CMC = 2-2.7 × 10-3 mole dm -3 at 2028°C (2)) in benzene was determined at 25 +_ 0.1 °C. The obtained (n) = 15.4 compared favorably with that found ((n) = 15) by Eicke and Arnold (8) using the same technique. Journal of Colloid and Interface Science,

Vol. 101,No. 2. October1984

Dipole moment measurements. Dielectric and dipole moment studies (1, 2) provide important information on the nature of solutesolute and solute-solvent interactions in solutions. As the system is not disturbed by the measurements, these methods are suitable for the investigation of surfactants association characteristics in apolar solvents. Thus, a number of notable studies were reported (8, 1l, 12) for some anionic surfactants in apolar media. Dipole moment measurements were made at 25 _+ 0.05°C using a WTW dipole meter, type DM 01 (Weilheim, Germany) with a measuring frequency of 2 MHz. The dielectric constants of solvent (es) and solutions (~ol) were determined at the same temperature. Dipole meter was calibrated with the solvents oflow dielectricconstant usingasample holding cell, DFL 2, according to the instrument manual. The measuring sensitivity of the instrument and the accuracy of the dielectric constant determination were AE/~s = 5 × 10-5 and ___2. l 0 -4, respectively. AEis the dielectric constant variation and E~is the dielectric constant of the nonpolar solvent. RESULTS AND DISCUSSION

The mean association numbers, (n), of various cationic surfactants at different concentration ranges are given in Table I. R21~H2C1- showed no tendency to aggregation in benzene. This can be explained in terms of the benzene solubility parameter and its high polarizability which led to the stabilization of the surfactant more effectively in monomeric form thereby shielding the dipole moment of the surfactant molecule. The replacement of C1- by C104 influenced the aggregation in benzene, and (n) increased with increasing surfactant concentration (Fig. 1). This is in accord with the observation of Kertes and Markovits (13) for tri-dodecyl ammonium salts in nonpolar solvents. Surfactant association improved by NO~ and (n) was more pronounced with HSO4 (Table I). The increased in (n~ of R2~4H2X- with different

ASSOCIATION OF CATIONICS

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Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984

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REI~H2X-. There are a number of factors to consider for the aggregation of cationic surfactants in nonpolar solvents and to some extent one can isolate the head group effect on association as demonstrated by Geer et al. (14) and by Anacker and Geer (15) for ionic surfactants in aqueous solution. The effect of polar head on association can be examined in term of its composition and structure. At conA 3 C stant substituents, an increase in effective V counterion radius increases its mean distance 2 of the closest approach to coion because of 3 the geometrical and steric restrictions. Thus, ( n ) increases with decreasing counterion size 2~ supporting the previous observations (13, 15) for the counterion effect on surfactant association in nonpolar and aqueous solutions. The hydration interaction between counterion and water is also one of the factors affecting the association in apolar media (16). In order to avoid different hydration interaction effects produced by different quantity of the water Loq. Con. mot, dm3 FIG. 1. Dependence of mean aggregation numbers of in solutions, extreme care was taken to keep cationicsurfactantson concentrationin benzeneat 25°C. the water content constant in solutions (see ×, R2~IH2C102; /x, R2~H2NO~; F-l, R2~H2HSO4; A, experimental section). However, since the waR2I~HCHaC10~; O, R2~HCH3NO~; m, R2/~IHCH3- ter content of the surfactant and solvent canHSO2; ,, R2~I(CH3)2F-; ~, R2~I(CH3)2C104; [~, not be removed completely (16), the hydration R2I~(CH3)2NO~. interaction is considered to affect the association of cationic surfactants in benzene which in turn is reflected in the concentration-decounterions was in the order HSO4 > NO~ pendent aggregation process, as noted by Eicke > C104 > CI-. A similar increase was reported and Christen (16) for AOT with alkylated amby Kertes and Markovits (13) for the coun- monium ions and that the hydration interterion effect on the association of tri-dodecyl action between counterion and water is to ammonium salts in apolar media. be expected in the order HSO4 > NO3 Substitution of a metyl group also affected > C104 > C1-. Other factors, such as counthe surfactant aggregation in benzene (Table teflon polarizability and hydrophobicity (1, 2) I). R2~HCH3C1- showed no association also influence to the aggregation in apolar solas noted for R2~H2C1- and ( n ) for vents. Nevertheless, as seen from Table I it is R21~IHCH3X- was dependent on the surfac- difficult to generalize the changes in ( n ) in tant concentration. Second methyl group sub- terms of polarizability and hydrophobicity of stitution was accompanied by an additional the counterions. increase in ( n ) and R21~I(CH3)2F- indicated a slight rise in ( n ) in contrast to R2~(CH3)2I-, DIPOLE MOMENT while ( n ) showed further increase by NO3 Information on the aggregates smaller than and C10;, and that the dependence of ( n ) on counterion was the same as observed for micelles can be obtained from dielectric studJournal of Colloid and Interface Science, VoL 10l, No. 2, October 1984

ASSOCIATION OF CATIONICS

349

R2~(CH3)2C104 which form larger aggregates were evaluated according to Guggenheim o (17) and Smith (18). The U values for R2]C4(CH3)2CIO4 were higher than that of R21~I(CH3)2NO~. As polar heads of surfactants are usually undissociated in apolar solvents, they behave as dipoles (19). Hence, it is considered that dipole-dipole interactions between polar head groups of present surfactants make a significant contribution to aggregation in benzene. Thus, as seen in Fig. 3, tl/ t~ increases with increasing (n). In the range 2 ~< (n) ~< 4.5 an average increase in/~ per surfactant monomer was 1.8 D and this value 1C reduced to the average value of 1.2 D at high (n) due to the formation of larger aggregates and it was not possible to compare the experimentally observed changes in # as (n) increases by the variation of # with respect to (n) in terms of the pseudo-phase and multipleequilibrium association models (will be dis1 5 1'0 cussed later) as the present program does not -2 -3 Con. 10. tool. dm calculate (n) for either model. Figure 3 also indicates the relationship FIG. 2. Plot of dielectric incrementsof cationic surfactants versus concentration in benzene at 25°C. O, between (n) of R2]C4(CH3)2C104 a n d R2I~ (CHa)2NO3 and one monomer contribution R2]~(CH3)2NO3; m, R 2 ] ~ ( C H 3 ) 2 C I O 4 . to the dipole moment of the aggregates. It is apparent that there is a sharp fall in ~/(n) at ies. Thus, the dielectric increments, Ae = (esoZ low (n) followed by a steady decrease in ~t/ -- e~) of the well-behaved surfactants were de- (n) as (n) increases. The fall of #/(n) at low termined as a function of concentration in (n) is similar to that of the micellized surbenzene. Figure 2 illustrates the dependence factants in nonpolar solvents. However, at high of Ae on the concentration that exhibits no (n), the dependence of # / ( n ) on (n) was conparticular surfactant concentration at which trary to the finding of Eicke and Christen (l 1) the cooperative interaction takes place. who observed constancy of ~t/(n) for AOT Whereas, an apparent discontinuity in Ae ver- above (n) with the value of 3 or 4. sus concentration plots was reported (16) for It is evident that #/(n) indicates no consurfactants showing micellar association and stancy rather than gradual changes at high the noted break was quite comparable to those (n). This supports the observations for the obtained by other techniques (16, 2). It is ev- dependence of (n), Ae, and/z on surfactant ident that cationic surfactants display non- concentration (Figs. 1-3). Although, the presmicellar association patterns which are re- ent surfactants have different polar heads and flected in Ae as concentration increases. This counterions, their hydrophobic chains to some is consistent with the idea of progressive as- extent resemble AOT (see Table I), compared sociation of surfactants in benzene. with cationic surfactants reported in the litFor additional information, the average di- erature (1, 2). In view of this fact, a comparison pole moments, #, of RE]C~(CH3)2NO3 and can be made between the cationic surfactants 30 ¸

Journal of Colloid and lnte(face Science, Vol. 10t, No. 2, October 1984

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DO(~AN EMiN GOVELi

14-

.4

A

1~-

10"



FIG. 3. Mean dipole moment of the monomer as a function of the mean number of monomers per aggregate of cationic surfactants, A, and mean dipole moments of aggregates versus mean number of ~4monomers of aggregates, B, in benzene at 25°C. O, R2N(CH3)2NO~; II, R2N(CH3)2C10~.

and AOT to understand the competing effects involved that lead to the different behavior of surfactants in benzene. As seen in Table I cationic surfactants have longer chains and larger polar heads and counterions with respect to AOT and the mean distance of closest approach of counterions to colons is different with comparing Na + to coion. For cationic surfactants, two -C2Hsgroups are close to the hydration sphere of the head group. Whereas -CH2- and the polar groups of the alkyl chains of AOT being in close proximity to the polar head lead to the location of the side chains near to the head group. This may increase the attraction between carboxyl groups of the chains and water molecules located near the head group through hydrogen bonding. However, this is not the case for cationic surfactants as carboxyl groups of the chains are further away from the polar Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984

head. In addition, their chains are more bulky and there is an excess crowding near the center compared to AOT. The positive charge of cationic surfactants is less exposed and the hydrocarbon chains appear to be inflexible due to the more compact head structure which fails to shield sufficiently the polar head with respect to AOT. Thus, in view of these differences, it can be proposed that the bulk of material close to the neighborhood of the polar head and the size of the head group and counteflon are the key steric parameters for the formation of larger aggregates in apolar media.

Association of Surfactants A survey of reported data (1, 2) suggests that most surfactant-solvent systems which show strikingly different aggregations can be classified into two association classes. In type I, ( n) increases progressively as concentration

351

ASSOCIATION OF CATION1CS

rises without reaching a limiting constant value as noted for cationic surfactants and there is no well-defined change in a given concentration range. This type of association is well defined by a multiple-equilibrium model. In contrast, for type II, aggregation numbers are large and ( n ) reaches a constant value at high surfactant concentrations and a moderately defined change occurs in a plot of physical property versus surfactant concentration. This behavior is found for the anionic surfactants in nonpolar solvents and is well fitted by the m o n o m e r ~- n-mer model. Although, cationic surfactants generally display type I, there is still considerable disagreement (3) as to whether type II exists for cationic surfactants in apolar solvents. One of the problems associated with the study ofmicellar aggregation and the CMC phenomenon in apolar solvents is the lack of reliable experimental methods and none of the commonly employed physical techniques sufficiently provide a quantitative support for the CMC when ( n ) remains small. For such systems the existence of a CMC is questionable. In fact, the concept of a CMC cannot be used satisfactorily for the systems in which the aggregates consist of a few molecules. The uncertainty still exists for the definition of CMC in relation to the micellar and nonmicellar association patterns. The curves in Figs. 1-3 indicate gradual changes in (n), A~, and # as the surfactant concentration rises and they may show that CMC does not exist and systems do not obey m o n o m e r ~ n-mer association. However, those results are inappropriate to reach a conclusion for the existence of CMC and micellar association. The aggregation of surfactants could be analyzed in relation to the micellar and nonmicellar associations by examining ( n ) and its independence on the surfactant concentration. This approach has been introduced by Eicke and Denss (20) who discussed micellar association, the existence of CMC, and nonmicellar association for some cationic surfactants in apolar solvents by fitting the experimental data

points to the plots of the pseudo-phase (PM) and multiple-equilibrium (MEM) models. The PM treats the micelle formation as a phenomenon similar to phase separation where the CMC is a saturation concentration and it is defined with the assumption that monomers, S, are in equilibrium with miceUar species, S., having a fixed aggregation number, ?/, a s

Kn

nS

s.,

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tS.VtS]"

where K, is the equilibrium constant for the formation of the micelles. This model considers the micelle formation at a distinct surfactant concentration and constant m o n o m e r concentration above the CMC and does not take into account the intermediate sizes smaller than micelles. In contrast, MEM is generally characterized by the aggregates of several different sizes which are in a dynamic equilibrium with the m o n o m e r and with each other. Although a number of MEM systems have been presented (10, 21, 22) the application of this model has been restricted due to the fact that more parameters are required to define the equilibria than can be evaluated experimentally. Therefore, to improve the applicability of the model, it is necessary to use simplifying assumptions (10, 21). In this model aggregates are formed by a stepwise process involving the addition of monomers to already existing aggregates as /(2

$1 + Sl

= $2

S2 + S 1

K3 = S3

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where Ki is the equilibrium constant for each aggregation step (i = 2, 3, 4, • • • ) with the assumption that Ki = Ki-i - K. This model does not predict a CMC but permits the discussion of association numbers and distribution of aggregates at given surfactant concentrations. Both models represent limiting Journal of ColloM and Interface Science, Vol. 101, No. 2, October 1984

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DO(~AN EMiN GOVELi

cases which are more or less approximated by the investigated systems. The theoretical plots of these models derived according to the quantitative expressions described in the literature (23) were fitted to the experimental data in terms of the Marquardt algorithm (24). For PM the examination of experimental resuits was started by n = 2 which corresponds to the monomer-dimer, then continued by increasing, n, value such as monomer-trimer (n = 3 ) . . . m o n o m e r - n - m e r until the better fitting with small variance was achieved. In the case of MEM experimental data were fitted by the program considering Ki --- Ki-1 - K. For the accurate analysis of surfactant aggregates it is necessary to check carefully the degree of coincidence between the model plots and the real systems. This can be achieved by calculating the variance or by examining the residuals of the experimental points, (n~, as a function of concentration of the surfactants. The evaluated equilibrium constants (Kn and

numbers. Although n values are close to the experimental values, there is no experimental indication for the existence of m o n o m e r - n mer association for the cationic surfactants in benzene and the uncertainty of finding a position for a CMC is clearly demonstrated in Figs. 4-6 by the variance of the fitted experimental data. Figures 4-6 also show that theoretical curves of both models deviate considerably from the plots of real systems and it is apparent that experimental results disagree with either model. Therefore, any definition of CMC in terms of PM will become arbitrary or ambiguous. When the experimental data are well fitted by PM a search for the existence of CMC is meaningful. But failing to observe a CMC does not reject the existence of the micelles, if the association process obeys the PM.

K for PM (nS ~KSn) and MEM ($1 + S1 K2 = $2. • • S H + S1 = Si), respectively, the accuracy was within 66% confidence limit (20)), variances and n values are given in Table II. The variances of the fitted experimental results to the models indicate the degree of compatibility of the real systems with the models and the n values correspond to the aggregation

Mean aggregation numbers, (n), of di(ethyl, 2-ethylhexanoate) ammonium salts and methyl, dimethyl di-(2-ethylhexyl propionate)ammonium salts increase with increasing surfactant concentration indicating nonmicellar association patterns in benzene. Dielectric increments and dipole moments of dimethyl di-(2-ethylhexyl propionate)-

CONCLUSIONS

/G

TABLE I1 Pseudo-Phase and Multiple-Equilibrium Models Data of Cationic Surfactants in Benzene at 25°C a Pseudo-phase model

(n)

n

Surfactant

(a)

(b)

Variance

R2NH2HSO2 R2NHCH3HSO~ R2N(CH3)2FR2N(CH3)2C104 R2N(CH3)2NO~

1.7-3.2 3.0-5,2 2.2-3.4 1.9-4.9 1.7-7.1

3.2 4.0 3.8 4.7 6.3

0.01 0.01 0.01 0.17 0.09

Multiple-equilibrium model

K.

X

[mole din-3] ~-"

Variance

[mole dm-~]-~

0.05 0.07 0.02 0.12 1.25

130 140 379 381 633

+

2.5 4.8 2.6 1.5 2.8

X × X × ×

107 10 II I09 1012 1019

a Note: a, experimental values for the concentration ranges used (Table I); b, aggregation numbers; variance indicates the degree of compatibility of the experimental results with the model; Kn and K correspond to the pseudo-phase and multiple-equilibrium models within the 66% confidence limits.

Journal of Colloid and Interface Science. Vol. 101, No. 2, October 1984

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354

DOt3AN EMIN GOVELI

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ammonium nitrate and perchlorate show similar trends. Polar head and counterion affect the association. The increase in ( n ) with different counterions is in the order HSO4 > NO~ > C104 > C1- and the observed association behaviors are not compatible with the pseudo-phase and multiple-equilibrium models. ACKNOWLEDGMENTS The author acknowledges the assistance provided by the Swiss Research Council by way of a research grant Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984

), pseudo-

and thanks Professor H. F. Eicke for reading the manuscript and comments, Dr. R. Hopmann for discussions, and to the referees for their thoughtful comments. REFERENCES 1. Kertes, A. S., and Gutmann, H., "Surface and Colloid Science" (E. Matijevic, Ed.), Vol. 8, p. 194. Wiley, New York, 1976. 2. Eicke, H. F., Topics Current Chem. 87, 85 (1980). 3. Mittal, K. L. (Ed.), "Micellization, Solubilization and Microemultion," Vol. I. Plenum, New York, 1977. 4. Hilzbadur, M., and Bergmann, E., J. Org. Chem. 13, 307 (1948).

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Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984