Indian J ournal of Chemistry Vol. 39B, July 2000, pp. 509 - 516

Acidic hydrolysis of hydroxamic acids in mixed' cationic - cationic, cationic -nonionic and anionic - nonionic micelles

Kallol K Ghosh* & Alka Pandey School of Studies in Chemistry,

PI. Ravishankar Shukla University, Raipur 492 010, India

Received 6 November 1998; accepted (revised) 6 March 2000

Acidic hydrolyses of some carbon and nitrogen substituted hydroxamic acids (R-C(O) N(OH)R', R = CHJ, R'=H; R=C6Hs, R' = H; R = C6Hs, R' = C6Hs, R = C6Hs, R' = 4-CHJ-C6H4) have been studied at 55 °C in mixed micellar solution of surfactants (cationic - cationic, cationic-nonionic and anionic - nonionic). T he results indicate that addition of non ionic surfactant to an acid solution of anionic surfactant (SDS) strongly decreases the observed rate constant. In other mixed micellar systems inhibition has been observed. Binding constants and rate constants for the reaction in the micellar pseudophase, obtained from the-kinetic analysis, have been reported. The inhibition effects depend on the hydrophobic chain length of the surfactants and the hydrophobicity of the substrate.

The study of the mixed surfactant systems is a field of intense activity of investigation mainly due to the known synergistic (e.g. enchanced surface activity, lower CMC's) and incompatibility effects that present as consequence of the strong interactions between the molecules of the surfactants in solutionI-4. Mixed micelles of two or more components are also important in biology. Several theories and analyses of the thermodynamics of micellization, structure and shapes for mixed micelles have appeared5-1 I. In contrast to the numerous studies of mixed - micelle formation, there have been few reports analysing the influence of mixed surfactant systems on reaction processesI2-16. When surfactants of two different types are mixed together, micropolarity of the Stern layer and microviscidity are different from that of the single micelles of the surfactantsI2. Polarity and viscidity of the reaction medium have a great effect on chemical reactions. In order to reveal the principle of the effect of mixed micelle on reactions, we have studied the acidic hydrolysis of some hydroxamic acids I in mixed micelles of cationic-non ionic, cationic -cationic and anionic - non ionic surfactants,

Hydroxamic acids have been successfully used for a large variety of applications in analytical, biological and medicinal fields, such as durg delivery system�, siderophores - iron transportI8 and DNA c\eavageI9, etc. It is our intention to investigate how the incorporation of these chemicals In molecular

R = H, A' =CH3

R = H, R ' = C.H5

R = C,H5, R' = C.H.

R = 4 - CH3 C.H., R' = C.H5

R-N-OH I R'-C=O

Acetohydroxamic acid (AHA)

Benzohydroxamic acid (BHA)

N - Phenylbenzohydroxamic acid (PBHA)

N-p-Tolylbenzohydroxamic acid (p-TBHA)

assemblies can improve interest in these fields. The micellar effects on hydrolysis of hydroxamic acids in acidic20-23 and alkaline solutions24-26 have frequently been a matter of study in the past few years; nevertheless, further efforts have not been made to investigate mixed micellar effects on hydrolysis reactions' or to determine the mechanism. The present study aims at addressing this oversight in part. The following mixed systems were chosen for studying acidic hydrolysis of hydroxamic acids.

Materials and Methods Reagents. All the hydroxamic acids were prepared

by standard methods. The cationic surfactants tetradecyl (trimethyl) ammonium bromide (lTAB) (Sigma), cetyl (trimethyl) ammonium bromide (CT AB) (Sigma), cetyl (pyridinium) chloride, (CPC) (Sigma), anionic surfactant sodium dodecylsulfate (SDS) (BDH) and nontontc surfactants

510 INDIAN J CHEM, SEC B, JULY 2000

[Polyoxyethylene (23) lauryl ether (Brij-35)] (Sigma), Triton-X- IOO (Loba Chemie) were used without further purification. Their solutions were prepared in double distilled water. All other chemicals were of analytical reagent grade.

Kinetics. Kinetics were followed spectro­photometrically by measuring the concentration of the hydroxamic acid by the colour reaction with Fe3+ ions. An aliquot of the reaction mixture was periodically removed, diluted to 1 OmL and its absorbance measured on a "UNICAM UV-2-300" spectro­photmeter. Beer's law was obeyed by the system, Pseudo first order rate constants have been obtained with a standard deviation below 0.5%. Least squares analysis was carried out on a WI PRO - Pentium computer under MS-DOS.

CMC. The CMC values of mixed micellar systems were determined by conductometric studies using a Systronics conductivity meter (type 303) . In prepar­ing the mixed surfactant soultions (molar ratio, I: 1), desired amounts of both surfactants were weighted and dissolved in convenient volume of water.

Results and Discussion The influence of pure and mixed cationic, anionic

and non-ionic surfactants on the acidic hydrolysis of different hydroxamic acids was investigated at fixed concentrations of W (HCI = 0.35 M) and hydroxamic acid in 10% (v/v) 1,4-dioxane at 55°C. Addition of cationic and non-ionic surfactants decreased the

observed first order rate constant (data not given), on the contrary, a catalytic effect with anionic surfactant was observed (Tables I-IV). Figures 1-7 show typical plots.

In most cases, simple electrostatic considerations suffice for prediction of whether a reaction will be catalysed or inhibited by cationic, anionic, or non­ionic micelles27• Ionic micelles typically accelerate the bimolecular reactions of counterions and inhibit those of co-ions. The influence of micellar systems on chemical reactivity is usually analysed in terms of the micellar pseudo-phase model28. When a non-ionic surfactant'is mi?,ed with an ionic surfactant, the uncharged non-ionic heads reduce the electrostatic repulsions between the charged ionic heads, thus facilitating mixed micelle formation.

CMC. Critical micelle concentration plays a very important role in describing the behaviour of binary surfactant solutions5. The CMC's of mixed surfactants have been determined using conductivity measurments. The experimentally determined values of specific conductance were plotted (figures not shown) against the logarithm of the total surfactant concentration and the break points of the curves are taken as the CMC of the respective systems. The results are shown in Table V. For ionic surfactants, micelle formation is governed by the balance between the tendency of alkyl chains to avoid contact with water and the electrostatic repulsion between the head groups, whereas for non-ionic surfactants the

Table I-Effect of mixed micelles on acidic hydrolysis of N-phenylbenzohydroxamic acid (PBHA) at 55°C

kljl. 1 06 s- I

Mx \03 SOS CTAB CTAB TTAB TTAB CPC SOS CTAB

+ + + + + +

Brij-35 TX- 1 00 Brij-35 TX- I OO Brij-35 Brij-35 CPC

\,4 I \.0 9.5 9.2 6.8 6.2 9.7 9.0 7.8 2.7 I \.6 9.3 8.9 6.2 5.6 9.4 8.3 7.5 3.0 4. 1 1 2.0 8.4 8. 1 5.8 5. 1 8.8 7.6 6.6 6.9 1 3.6 7.4 7.3 5. 1 4.5 8.0 7.0 5.7 7.3 4.9 8.0 1 4.0 7.0 6.4 4.5 4.3 7.8 6.7 5.5 1 0.3 1 4.3 6.4 5.9 4.0 3.7 7.4 6.2 5.2 I \,7 1 5.6 6.0 5.3 3.7 3.4 6.4 5.7 4.5 12.5 1 3.7 1 6.4 5.3 5. 1 3.5 3. 1 5.9 5.4 3.9

\5.0 17.0 5. 1 4.7 3.3 2 .8 5.7 4.9 3.7 \8.4 3.0 20.6 17 .8 4.4 3.7 2.7 2 .5 4.6 4. 1 2.4 25.0 18 . 1 Cmixed=C)+C2:C)=C2 ko= I O.3x I 06

S') HCI=0.35M, 1,4-0ioxane=JO% (v/v)

GHOSH el al.: ACIDIC HYDROLYSIS OF CARBON & NITROGEN SUBSTITUTED HYDROXAMIC ACIDS 511

Table II--Effect of mixed micelles on acidic hydrolysis ofN-p-tolylbenzohydroxamic acid (p- T BHA) at 55°C

/nv. 106 s- I

Mx 103 SDS CT AB CTAB TTAB TTAB CPC SDS CT AB + + + + + + +

Brij-35 TX-100 Bri j-35 TX-100 Brij-35 Brij-35 CPC

1.4 10.0 7.7 9.2 7.0 6.5 7.9 8.2 6.0 2.7 10.6 7.0 7.6 6.0 5.4 7.3 6.3 5.6 4.1 11.4 5.9 6.6 5.2 4.7 6.4 5.1 5.0 6.9 11.8 4.9 5.4 3.9 3.6 5.7 4.2 4.5 8.0 12.2 4.7 4.8 3.5 3.1 5.3 4.0 4.2 10.3 12.5 4.6 4.4 3.2 2.7 4.9 3 .7 4.1 11.7 12.8 4.3 4.2 3.0 2.6 4.6 3.5 3.8 12.5 13.5 2.4 13.7 13.8 4.0 3.9 2.9 2 .3 4.4 3.4 3.7 15.0 14.2 3.8 3.7 2.8 2.2 4.2 3.3 3.5 18.4 14.6 4.0 � : ; . 20.6 15.5 3.7 3.6 2.7 2.1 3.9 3.2 3.3 25.0 16.8 Cmixed =C1+C2:C1=C2 ko= I O.3x I 06

S-I

HCI=0.35 M, 1,4-Dioxane=lO% (v/v) Table III-Effect of mixed miicelles on acid hydrolysis of acetohydroxamic acid (AHA) at 55°C

kyo 106 s-1

M x 103 SDS CT AB CT AB TTAB T T AB CPC SOS CT AB + + + + + + +

Bri j-35 TX- I OO Brij-35 TX-100 Bri j-35 Bri j-35 CPC

1 .4 21.8 17.2 17.5 16.5 17.7 17.4 18.4 17.8 2.7 26.0 17.0 16.2 16.4 15.8 16.8 17.5 3.0 35.9 15.6 16.2 15.2 4.1 40.0 16.6 14.8 15.9 14.2 15.4 18.2 17.1 6.9 42.3 16.0 14.0 14.8 12.6 15.2 16.4 7.3 15.6 13.6 12.0 16.2 8.0 44.5 13.5 14.6 11.5 15.1 18.0 15.9 10.3 54.1 15.2 13.4 13.8 11.0 14.4 17.6 15.6 11.7 14.8 13.6 10.4 13.8 17.0 12.5 14.6 13.0 13.6 16.8 15.4 13.7 14.4 13.2 10.2 16.6 15.0 15.0 55.1 14.2 13.0 12.9 13.4 16.4 14.8 18.4 55.5 12.7 10.0 13.0 16.0 20.6 55.9 14.1 12.9 12.5 9.8 12.7 15.9 14.7

Cmixed=CI+C2:C ,=C2 �= I 0.3x 106S-1 HCI=0.35 M, I ,4-0ioxane=1 0% (V/V).

tendency of alkyl chain to avoid contact with water is balanced by the hydration and space requirement of oxyethylene chains29. The nonionic surfactants with bulky head groups will favour compact structure and form spherical aggregates whereas ionic surfactants with small head groups will favour rod like micelles.

The mixing CMC is smaller than that of either individual components. This is because of chain length and molecular weight of non-ionic surfactant (Brij-35), which is higher than that of ionic surfactant. Since longer chain length decreases the CMC and the fact non ionic surfactants have low· CMC's, it is expected the effect of Brij-35 on CT AB would be

substantial compared to the effect of CT AB on Brij-35. The radius of the mixed micelle with electrical double layer increases with an increasing concentration of non-ionic surfactants. The surface charge density of counterions would decrease with an increasing concentration of non-ionic surfactant. Consequently the degree of ionic dissociation, a of the mixed micelle, increases with the proportion of non-ionic surfactant in the micelle. During the formation of mixed micelles, a solubilization process between both the surfactants may take place. Theoretical equations based on the regular solution theory have been proposed30 in order to calculate the

512 INDIAN J CHEM, SEC B, JULY 2000

Table IV-Effect of mixed micelles on acidic hydrolysis of benzohydroxamic acid ( BHA) at 55°C kyo 106 s-I

Mx 103 SDS CT AB CT AB TT AB TT AB CPC SDS +

Brij-35 +

TX-IOO +

Brij-35

2.1 5.6 3.6

+ TX-IOO

5.5 4.4 3.5 5.2 6.9 3.8 3.4 4.9 8.0 4.8 3.1 4.6 10.3 6.5 3.5 2.8 4.3 11.7 7.3 3.2 4.5 12.5 2.9 2.4 4.2 13.7 8.0 2.6 4.1 3.8 15.0 8.3 2.2 3.8 2.0 3.5 20.0 8.7 3.5 1.8 3.1 25.0 9.6 2.0 3.1 1.7 2.7 35.0 10.4 2.9 2.3 40.0 11.4 2.6 50.0 12.1 1.9 2.3 1.3 2.1 60.0 13.3 1.8 80.0 14.7 1.3 89.2 15.9 1.8 100.8 16.5 1.1 0.9 1.9 120.0 18.5 Cmixe4=CI+C2 : C1=Cz ko= I O.3x I 06 s-I HCI=0.35 M, I ,4-Dioxane=10% (v/v)

30 .PBHA

.p·T8HA

25

ao 70 60

'i 'i SO CI) CI) 20 '" 0 .... 0 40 ....

T'" � � 30 x.

� 15 20 10

10 0 0

5

+ + Brij-35 Brij-35

5.6 5.9 5.0 5.7 4.8 4.5 5.2 4.2

5.1 4.1

5.0 4.0 3.8 4.8 3.6 4.6

3.4 4.5 3.1 3.6 2.8 3.3

2.5 2.1

2.6

5 10 15 1 03 [Surfactant] mol dm-J

CTAB

+ CPC

4.5 4.2

3.9 3.7 3.5 3.3 2.8 2.5 2.3 2.0

1.8

1.6 1.5

. .."

20 25

0 5 10 15 20 103 [Surfactant] mol dm-3

25 30 Figure 2-Effect of anionic surfactant (SDS) on the acidic hydrolysis of AHA at 55°C.

Figure I-Effect of anionic surfactant (SDS) on the acidic hydrolysis of P BHA andp-T BHA at 55°C.

extension of miscibility between the surfactants. In the case of SDS and Brij-35 the mixed micelles may become stabilized primarily through the contact with hydrocarbon chains of both surfactants; the SDS polar head groups are located in the ethylene oxide region of the non-ionic surfactant in a hydrophilic region (second, third ethylene oxide of the non ionic surfactant). Usually, the CMC's of mixed surfactants fall in between the CMC's of the individual pure components.

Inhibition effect of mixed micelles of surfactants The data in Table V show kM / kw are less than

for all the mixed surfactant systems investigated. This indicates inhibition of the mixed micelles on the acidic hydrolysis of hydroxamic acid. The addition of surfactant decreases the observed rate constants (Figures 4-7). The addition of anionic surfactant (SDS) results in an increase of the first order rate constant (Figures 1-3). But, addtion of nonionic surfactant to acid solution of SDS inhibits the reaction significantly. When an ionic surfactant and a non ionic surfactant are mixed together to form mixed micelle,

Table V -Ks' kM and kM / kw values for t he hydrolysis ofhydroxamic acids. (DT Range = 1 .4 x 10 3 M to 120.0 xlO -3 M)

Micellar / 3 CMCx 10

Mixed micellar -3 Ks mol. dm

System SDS 1.00 3.3 CTA B- Brij-35 0 . 10 98.7 CT A B-T X- IOO 0.48 46.7 TT A B-Brij-35 0.24 34.3 TT A B-T X-IOO 0.51 52.9 CPC-Brij-35 0. 19 1 1 7. 1 SDS-Brij-35 0.032 30.8 CTA B-CPC 0.71 125.8

::r.., '< _. c..1JQ (3 !; -<" k",. 1 a's·' o a '" ... '" "" -'" [!

'1' � 01 I� -io tI:l-:to >;: � '" � c 0 - ... ... v.� _ VI 0 U'J 0 - c 0� :l. _ til III 0 3" !l -. III � :J c.. .=; til C ... � n

� o ::l

3 � Q. a. 3 " �

�t?tftt o I/> � � � 0 0 >fiI » » 0> C. � If If If If I(,�' '''' ' ����I :- ", (l!�tI!8t:8bi

o '" c.: ;:;.

BHA P BHA kM k / k K k k / k K

s M w s M M W -6 -\ -6 -\ ( 1 0 S ) ( 1 0 s )

50.0 2.7000 25.8 33.6 1 .8564 1 75.5 0.4 0.2222 69.8 1 .5 0.3409 296.9 0.06 0.0545 226.2 4.5 2.4 0.6667 453.5 2.6 0.3 0. 1 579 604.5 2.4 2.5 0.961 5 64.6 2.5 2.0 0.9524 1 70.3 3.6 1 .02 0.6800 269.7 2.3

::r.., '< _. c..1JQ ... c:: o .. Cii'

k",.10's·'

� "! o _ _ � 0 N � � m 0 N • t8� 01 I ,=4) \ :tn >0 � ...., - til � VI c o '" v.� w o � � n n U'J § c

@. :l. _ 3 � 0 x' iii G :J c.. _ � 3' � Q. o a. 3

�

� til " � o ::l

o I/> a -i -i 0 0 � fiI <'S > > > > CD W w If If If 0> � .... ' "I" � g> � � � 1(l!h\�8bi8hil � t)l

c.: n'

AHA p-T BHA k kM/ kw K k M S M' -5 -\ -6 -\

( 1 0 s ) ( 1 0 s ) 66.7 1 . 1 932 3 1 .7 25.8 1 4.0 0.9929 3 1 6.5 2.8

::r.., '< _. c..1JQ ... c:: o .. k",.10es-1

-<"

[I �� >n I>l 0 -...., VI 0 VI I>l ... o ::l � n o' U'J . ::l C n' =l. III til 0 C _ ... III � :J o _

§ 3 - 0 '""' -[/) a. o 3 [/) , � ... o ::l :;. G

� c.: n'

o ... o 1 I �

N o

... o

.. o

-o

i .... III

t -� o

.... .... o ...

k / k M W

1 .5357 0.7568

N o N ...

o :t o [/) :I: � I:> �

> n 8 () :I: -<: o � r -<: [/) Vi o 'Tl

� � � Ro Z � 8 tTl Z [/) 53 [/) -i =i � tTl o :t -<: o

� ::: () > n 8 [/)

Vl Vol

\ \ \

514 INDIAN J CHEM, SEC B, JULY 2000

__ CTAI .. ,,·3S

__ CTAI.TX·CO

24 --'- TTA8+1,,·3S

-6-TTAI.TX·CO

� cl'C+I,,·n

__ IOS+I,,·35

19 -o- CTAI+CPC

i en ... 0 or- 14 � .l<:

9

4 � __ � __ � ____ � __ -+ ____ +-__ � o 5 10 15 20 25 30

1 03 [Surfactant) mol dm-3 Figure &-Effect of surfactantslmixed surfactants on the acidic hydrolysis of AHA at 55 0c.

7 __ CTAB<811-35 __ CTAB_TX·1)() 6 --.-TTAB <8 11-35 -6-TTAI-TX·1)()

5 �CPC<811-35 __ 505<811-35

4 -o-CTAB+CPC i en • 0 3 or-� .l<: 2

o 10 20 30 40 50 60 70 60 90 100 1 10

", 1 03 [Surfactant) mol dm-3 Figure 7--Effect of surfaclantslmixed surfactants on the acidic hydrolysis of BHA at 55 0c.

molecules of the non ionic surfactant penetrate between molecules of the ionic surfactant I 2 . This makes the surface �harge density of the mixed micelle lower than that of the single ionic micelle, and leads to the decrease of exclusion between ionic head groups of the ionic ·surfactane I and the increase of the ion-dipole interaction between the ionic and the nonionic surfactants32• Thus, molecules and ions of surfactants of different types in the mixed micelle are closer, and the Stern layer becomes more compact. The polarity effect of Stern layer of mixed micelles becomes larger. When the Stern layer becomes more compact in the mixed micelle, its water content decreases, causing the polarity of the Stern layer to decrease and the acidic hydrolysis of hydroxamic acid becomes slower in a lower polar situation. Moreover,

with the rising compactness of the Stern layer in the mixed micelle, its viscosity becomes higher than that of a single one, and it is more difficult for the hydroxamic acid to make translation and rotation in the mixed micelle33, so the increased microviscosity will result in the decrease of reaction rate.

As the head group of Brij-35 [C'2H25 (OCH2CH2 )23 OH] is larger than that of SOS (C'2H2S0S03" Na+), the radius of the mixed micelle with the electrical double layer increases with an increasing concentration of Brij-35. Thus, the surface charge density of counterion would decrease with an increasing concentration of Brij-35. Consequently, the degree of ionic dissociation (a.) of the mixed micelle increases with the proportion of non-ionic surfactant in the mixed micelle. Several studies on the influence of cations in the binding of non-ionic surfactants to SOS show that the cation plays a role in the phenomenon of binding34-36• The results of these studies suggest that the electrostatic affinity of the cations for the micelle and the co-ordination with the polyoxyethylene surfactant energetically contribute to the binding force between Brij-35 and SOS micelles. Therefore, during the formation of mixed micelles a solubilization process between both the surfactants may take place. The mixed micelles may become stabilized primarily through the contact with hydrocarbon chains of both surfactants with the result that SOS polar head groups are located in the ethylene oxide region of the non-ionic surfactant in a hydrophilic region. Both surfactants have dodecyl head hydrocarbon tails, and therefore, synergism arises from interactions between the surfactant heads.

The results implied that a more open and water­penetrated structure for the mixed micelles, the properties of which being highly ionized, more hydrophilic and with a more voluminous interface seem to be completely different from those corresponding to pure surfactants. The above characteristics ensure that the important catalytic effect observed in the acidic hydrolysis of hydroxamic acids in the presence of SOS micelles may be completely suppressed by the addition of a certain quantity of a non-ionic surfactant, probably because of the combination of three effects operating in the same direction : (i) the [H+] is reduced in the micellar region as a. increases, (ii) the increase of the molar reaction volume, which produces a dilution of both reactants in the miceller region and (iii) a probable change in the apparent dielectric constant of the reaction site in the micelle. Similar results have been

GHOSH et af. : ACIDIC HYDROL YSIS OF CARBON & NITROGEN SUBSTITUTED HYDROXAMIC ACIDS 5 1 5

observed i n the acidic hydrolysis of alkyl nitritesl5

and alkaline hydrolysis of estersl2. Cationic-cationic mixed micelles effects are not very significant.

Effect of hydrophobic chain length of surfactants in mixed micelles

From Table V, we can see that kM / kw increases as the hydrophobic chain length of quarternary cations increases from IT AB to CT AB, resulting in the inhibition of mixed micelles on the acidic hydrolysis of hydroxamic acid. This is because the polyoxyethylene chain of nonionic surfactant Brij-35 partially goes into the micellar core, whole of the carbon chain length of cationic surfactants increases. It is equal to a shortening in the "efficient chain length" of the polyoxyethylene chain, and this leads to the increase of water content and polarity of Stern layer, therefore inhibition effect on hydrolysis of hydroxamic acid decreases.

When the chain lengths of hydrophobic groups of surfactants have relatively large difference as in CTAB-Brij-35 and CPC - Brij-35, mixed micellar systems have the smallest inhibition; when the chain lengths of hydrophobic groups of surfactants have relatively small difference as in TTAB - Brij-35, mixed micellar system has the stronger inhibition. The order obtained in experiments is as follows.

CPC - Brij -35 < CTAB - Brij-35 < TTAB - Brij-35 inhibition effect increases.

Quantitative Treatment The pseudophase model treats the overall reaction

rate as the sum of rates in the aqueous and micellar pseudophases

kw Products kM Scheme I

In this scheme Ks is the binding constant of the substrate to micellized surfactant and kw are kM are first order rate constants for hydrolysis reaction in the aqueous and micellar pseudophases. [Dn] is the total concentration of micellized surfactant. Mixed C MC's at the mixing ratios used in this study are lower due to non ideal interactions in the mixed micelles. The first order rate constant is given by Eq. ( I ) .

k = k w + kM ·K, [ D n ] '/I I + K, [ Dn ]

. . . ( I )

This equation in Eq. (2).

leads to the relationship given

kw �k. � kw - kM + kw �kJ K, (D,; - CMC 1 . . . (2)

A plot of the left-hand side of equaiton (2) vs l /(Dn-CMC) allows the calculation of kM and Ks. Therefore, dealing with the data in Tables I-IV according to equation (2) we obtain kM and Ks for different reaction systems. The results are presented in Table V. These results indicate that rearly all the surfactant are in micellized form and substrates are essentially fully bound under all our conditions.

In single surfactant systems, the concentration of micellized surfactant is generally taken to be the total concentration minus CMe. In mixed surfactant systems, this provides a lIseful "rule of thumb", but does not strictly hold due to a gradual increase in total monomer concentration above CMC37 • A complete explanation of all the available data would require a much more detailed knowledge of the local aggregate structure and improved quantitative models. Thus, the field still needs high quality data for systematically vaired hydroxamic acid and surfactant structures. Still the source of interaction forces between both surfactants, the role played by the counterions in complex formation and kinetic process, and even the structure of the mixed micelle itself have not been • completely explored.

Acknowledgement The authors thank the MP Council of Science and

Technology, Bhopal and Pt. Ravishankar Shukla University, Raipur for financial support. They are also thankful to the Head SOS in Chemistry for providing laboratory faci I ities.

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516 INDIAN J CHEM, SEC B, JULY 2000

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