Calorimetric observations of the transition of spherical to rodlike

Occurrence of Cloud Points in Sodium Dodecyl Sulfate−Tetra-n-butylammonium Bromide System. Sanjeev Kumar, Damyanti Sharma, Ziya Ahmad Khan, and ...
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7769

J. Phys. Chem. 1993,97, 7769-7773

Calorimetric Observations of the Transition of Spherical to Rodlike Micelles with Solubilized Organic Additives Paul M. Lindemutht and Gary L. Bertrand' Department of Chemistry, University of Missouri-Rolla, Rolla, Missouri 65401 Received: April 12, I993

Incremental calorimetric titration of pentanol into aqueous solutions containing 3.5% cationic (tetradecyltrimethylammonium bromide, TTMAB, in 0.05 M NaBr) or anionic (sodium dodecyl sulfate, SDS,in 0.28M NaCl) surfactant and solubilized additives induced the transformation of spherical to rodlike micelles. The effect of the additive on the transition has been quantified as the change in the concentration of pentanol required to cause the transition divided by the concentration of additive, with a negative value indicating a cooperative effect of promoting the transition and a positive value indicating a retarding effect. Aliphatic hydrocarbon additives retarded the transformation for both surfactants, with the exception of a small cooperative effect for cyclohexane on TTMAB. Alkylbenzenes promoted the transition for TTMAB, as did benzene, toluene, and ethylbenzene for SDS,but the propyl and butyl derivatives showed the opposite effect.

V, = the volume of the amphiphile's hydrocarbon tail

Introduction At low concentrations, an ionic surfactant behaves as a strong electrolyte. Cooperative self-association into spherical micelles becomes apparent over a narrow concentration range and is characterized by a critical micellizationconcentration (cmc). In many systems there is a transition to rodlike micelles at a higher surfactant concentration. This transition is sometimes referred to as a second cmc, perhaps because it is observed experimentally in much the same manner as the primary cmc. Some methods of detection employed to date include light scattering,'q viscosity,e14 conductivity,l5J6 ultrasonic absorption,' 17 small angle neutron scattering (SANS),18.19and, most recently, solution calorimetry.20 Addition of electrolyte or cosurfactant ameliorates the electronic repulsion between surfactant head groups, increasing aggregation number,thus promoting rod formation. Salicylatez233 and thiocyanate23 counterions have been shown to be extremely effective at promoting the transition when coupled with alkyltrimethylammonium and alkylpyridinium cationic surfactants, so effective that rod formation occurs at or near the cmc when the surfactant chain length is Clz or longer.23 At still higher concentrationsof surfactant, SANS measurements have shown24 that the rods of some systems shorten when their rotational volumes begin to overlap and undergo another transition to a different anisometric form, probably to that of a disk. The introductionof certain organicadditivesalso promotes the sphereto-rod transition and the continued growth of the rods to the same limit, at which point some of the rods apparently revert back to spheres or convert to disks, as evidenced by viscosity measurements.25-2' There are many factors, including temperature, and the concentration of surfactant, electrolyte, and cosurfactant, which determine the shape of surfactant association structures. Packing considerations constitute a factor which involves the nature of the head and tail groups of the surfactant. A critical ratio (R,) with associated limits for several of the possible aggregation shapes has been devised by Ninham et al.28929 where

* Author to whom correspondence should be addressed. f

Present address: Nalco Chemical Co., Sugar Land, TX 77487-0087.

0022-3654/93/2097-7769$04.00/0

A, = the optimum cross-sectional area per amphiphile molecule

IC = the length of the fully extended hydrocarbon tail The optimum cross-sectionalarea is determined experimentally by X-ray diffraction of bilayer systems, while the volume and length of the hydrocarbon tail may be calculated following Tanford.30

V, = (27.4

+ 26.9n) %L3;

I, = (1.5

+ 1.2651)A

where n is the number of methylene groups in the hydrocarbon chain. Considering the geometric dimensions, the volume and the surface area of each associationstructure yield critical conditions for the formation of each of the following shapes: spherical structures cylindrical structures bilayer structures inverted structures

R, I 1/3 '/3

I

R, I '/z

l/z I R, I 1 R, 1 1

Amphiphiles with smaller inherent head group areas (high R,) tend to form larger, less curved, or even inverted structures. For ionic surfactants, the same area-shrinking effect may be achieved by addition of a counterion or a suitable cosurfactant. Lengthening or unsaturation of the hydrocarbon chain, particularly cis double bonds, leads to larger structures. Fang31 used this packing ratio to explain a series of phase transitions beginning with normal micelles and ending with inverse micelles in threeand four-component systems. For surfactants to associate in a spherical structure, the surface area occupied by the surfactant's polar head group should be large. If the heads are permitted to pack tightly, on the other hand, the aggregation number will increase, and rod- or disk-shaped micelles will be favored. The essential consideration pertaining to the area occupied by the heads is the work necessary to overcome the electrical repulsion 0 1993 American Chemical Society

Lindemuth and Bertrand

7770 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993

-1

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0.02

0.04

0.06 0.08 m (pentanol)

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Figure 1. Relativepartial molar enthalpy of n-pentanol in 3.54bTTMAB and 0.05 M NaBr, with no additive (M), 0.0034 m n-octane (O), and 0.0035 m n-butylbenzene (A). 7

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- 4 : . . ,

0.00

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TABLE I: Transition Conditions for Nonpolar Additives in 3.5%SDS with 0.28 M NaCl at 25 OC'

Alkanes

none

0.040

cyclohexane cyclohexane n- hexane n-hexane n-heptane n-heptane n-octane isooctane n-nonane n-decane n-dodecane

0.048 0.058 0.060 0.080 0.070 0.105 0.059 0.062 0.058 0.052 0.041

0

Saturated Hydrocarbons

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1 1

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0

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experienced by heads of like charge. A surfactant carrying a large charge on a relatively small charge-bearing atom will inherently be more apt to form spherical micelles due to the high energy needed to overcome the prohibitive charge density of the head group. A surfactant with a high degree of counterion binding may overcome head group repulsion by holding the oppositely charged counterion between head groups of similar charge; head group repulsion is repressed and rod- or disk-shaped micelles become favored. The availability of a cosurfactant also suppresses head group repulsion and promotes a higher aggregation number in much the same manner as a counterion, but with a less marked decrease in the surface area occupied per surfactant monomer. Amines, cosurfactants which may exist in protonated form, have been noted to be more effective with anionic surfactants,3*-33 while the more commonly used alcohols are more effective with cationic surfactants. The depressed chargedensityresulting from the addition of cosurfactant causes a release of bound counterions and forms the basis for conductivity15J6 studies of the sphereto-rod transition. Continued addition of cosurfactant to a normal spherical micellar solution is likely to push the system through the existence of rods and into a liquid crystalline state. A longer hydrocarbon tail produces a micelle with increased aggregation number resulting in a tendency toward rod- or disklike shapes. Longer chains will alsolower theconcentration at which spherical

0.0093 0.0186 0.0078 0.0157 0.0075 0.0140 0.0035 0.0035 0.0031 0.0024 0.0010

0.8 1 .o 2.6 2.5 4.0 4.6 5.4 6.3 5.9 5.0 0.6

108.8 131.6 147.5 163.5 166.4 179.8 196.1 228.6

Aromatic Hydrocarbons benzene 0.022 0.0233 -0.8 89.4 toluene 0.031 0.0115 -0.7 106.9 toluene 0.025 0.0196 -0.8 ethylbenzene 0.037 0.0085 -0.4 123.1 n-propylbenzene 0.050 1.3 0.0078 140.1 n- butylbenzene 0.052 0.0044 2.8 157.0 3.3 isobutylbenzene 0.052 0.0036 158.3 Symbols: m p = concentration of pentanol; mA = concentration of additive. micelles will form (cmc). Branching of the alkyl chain favors rodlike micelles by shortening the length of the surfactant chain relative to thevolume of the hydrocarbon core without significantly altering the amphiphile's head group area. Observation of the sphere-to-rod transition via incremental calorimetric titration was first reported by Nguyen and Bertrand.M Through comparison of their calorimetric data with the light scattering results of Zana,34J5 these authors determined that the end of a break in the plot of partial molar enthalpy of solution vs concentration of cosurfactant (titrant) coincides with thesphere to-rod transition as observed by light scattering. The major advantages of this technique over light scattering, a more common method, are the observance of an enthalpic effect a t rod concentrations which are too small to give a detectable scattering response, the observation of both a beginning and end of the transition, and the avoidance of the painstaking necessity to prepare dust-free samples. In the present work, we have examined the effect of organic additives on the sphere-to-rod transition as observed calorimetrically and gained information pertaining to the orientation of solubilized hydrocarbons. The effects of cosurfactant alkyl chain length and polar head group variation have also been investigated.

The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7771

Transition of Spherical to Rodlike Micelles

TABLE II: Transition Conditions for Nonpolar Additives in 3.5%"TMAB with 0.05 M NaBr at 25 "C. additive none

mp(m)

cyclohexane n- hexane n-heptane n-octane isooctane n-nonane n-decane n-dodecane

0.054 0.068 0.070 0.070 0.069 0.061 0.059 0.057

0.057

m(m)

h / h ~ v(mL/mol)

0

Saturated Hydrocarbons 0.0092 0.0075 0.0063 0.0034 0.0029 0.0020 0.0008 O.OOO8

TABLE Iv: Transition Conditions for Polar Additives in 3.5%Tl'MAB with 0.05 M NaBr at 25

-0.3 1.3 2.1 3.5 4.1 2.0 1.2

0.0

108.8 131.6 147.5 163.5 166.1 179.8 196.1 228.6

Aromatic Hydrocarbons benzene 0.037 0.0110 -1.8 89.4 toluene 0.041 0.0068 -2.4 106.9 0.0059 ethylbenzene 0.042 -2.5 123.1 n-propylbenzene 0.043 0.0054 -2.6 140.1 n-butylbenzene 0.046 0.0035 -3.0 157.0 isobutylbenzene 0.052 0.0048 -1.1 158.3 0 Symbols: mp = concentration of pentanol; m A = concentration of additive.

TABLE IIk Transition Conditions for Polar Additives in 3.5%SDS with 0.28 M NaCl at 25 OC1 additive mP(m) mA(m) h P l h . 4 none 0.040 0 Acids n-hexanoic acid 0.032 0.0041 -2.0 0.015 0.0047 -5.3 n-octanoic acid 0.021 0.0041 -4.5 n-nonanoic acid 0.030 0.0023 -4.3 n-decanoic acid 0.020 0.0057 -3.5 n-dodecanoic acid 0.0037 -2.7 0.030 n-tetradecanoic acid 0.037 0.0035 n-hexadecanoic acid -0.9 0.0021 8.1 0.057 n-octadecanoicacid Alcohols n-pentanol 0.040 1-11 0.032 0.0047 -1.7 n-hexanol -1.4 0.038 0.0014 n-octanol 0.020 0.0047 -4.3 n-octanol 0.0025 -6.0 0.025 n-decanol 0.027 0.0046 -2.8 n-dodecanol 0.034 0.0040 -1.5 n-tetradecanol 0.0034 0.6 0.042 n-hexadecanol Amines 0.016 0.0047 -5.1 n- hexylamine 0.012 0.0048 -5.8 n-octylamine n-decylamine 0.028 0.0026 -4.6 0.036 0.0016 -2.6 n-dodecylamine n-tetradecylamine 0.039 o.OOO9 1.2 n- hexadecylamine 0.047 o.Ooo9 7.9 0 Symbols: mp = concentration of pentanol; mA = concentration of additive. Materials and Methods Materials. Sodium dodecyl sulfate (SDS,Bio-Rad, Electrophoresis Purity Reagent) and tetradecyltrimethylammonium bromide (TTMAB, Aldrich Chemical Co., 99%) were purchased and used as received. All carboxylic acids were from Sigma Chemical Co. and of 99% or better purity. All aliphatic and aromatic hydrocarbons were from Aldrich and of 99% or better purity except n-propylbenzene (98%), isobutylbenzene (98%), and ethylbenzene (Eastman-Kodak Co.). Pentanol and octanol were from Fisher Scientific. Hexanol (99%) was from Sigma. Dodecanol and tetradecanol were from Eastman. Hexyl-, decyl-, and dodecylamine were from Aldrich (99%). Octylamine was from Fluka (99+%). Tetradecylamine (96%) and hexadecylamine (90%) were from Aldrich. Methods. Solutions to be titrated were prepared in 100-mL volumetric flasks. Dry surfactant (3.500 f 0.005 g) was added

additive

mA

Amplhma

0.0042 0.0034 0.0031 0.0019 o.Do10 0.0010 0.0008 0.0009 0.0004

-4.3 -6.1 -6.5 -8.0 -8.6 -7.2

-3.1 -2.8

0.0139 0.0053 0.0073 0.0050 0.0027 0.0024

[-I1 -2.7 -4.9 -4.9 -4.8 -4.4 -1.3

mp

none

0.057

n-hexanoic acid n-octanoic acid n-decanoic acid n-undecanoic acid n-dodecanoic acid n-tridecanoic acid n-tetradecanoic acid n-hexadecanoic acid n-octadecanoicacid

0.040 0.037 0.038 0.042 0.048 0.05 1 0.049 0.051 0.056

n-pentanol n-hexanol n-octanol n-decanol n-dodecanol n-tetradecanol n-hexadecanol

0.057 0.020 0.03 1 0.021 0.033 0.045 0.054

0

Acids

-6.0

Alcohols

Amines n-hexylamine 0.048 0.0110 -0.8 n-decylamine 0.038 0.0040 -4.8 n-dodecylamine 0.042 0.0018 -8.5 n-tetradecylamine 0.048 0.001 1 -7.9 n-hexadecylamine 0.056 0.0010 -1.1 0 Symbols: mp = concentration of pentanol, m A = concentration of additive. to each flask initially. Salt concentrations were attained by pipetting 1 F solutions of NaBr to form 0.05 m solutions with TTMAB or NaCl to form 0.28 m solutionswith SDS. Sonication was employedin many cases to hasten solubilization. Calorimetric measurements were performed on a Tronac Model 450 isoperibol calorimeter with an associated Heath/Schlumberger Model EU205-1 1 strip-chart recorder. The bath temperature was maintained constant to within f0.0002 OC at 25.00 f 0.01 "C. Increments of pentanol (normally 0.1 mL) were titrated into a 100-mL reaction vessel with a 2.5-mL Gilmont microliter buret. Titration was continued for at least four additions beyond an observed transition.

Results and Discussion The enthalpy increment upon addition of pentanol to thevarious micellar solutions was considered to be the partial molar excess enthalpy of pentanol (relative to the pure liquid) and was plotted against the concentration of pentanol. The end of the observed break was taken as the sphere-to-rod transition. Figure 1 shows the nature of the break and an example of a typical shift of the transition caused by the presence of an additive. The solid line corresponds to a solution of 3.5% TTMAB in 0.05 M NaBr requiring a pentanol concentration of approximately 0.057 m to induce the transition. The incorporation of 0.038 g of octane into the 3.5% TTMAB in 0.05 M NaBr solution allows the pentanol concentration to reach 0.07 m (dashed line) before the swollen spherical micelles rearrange into rodlike micelles. These results are used to calculate a ratio (dmp/dmA) as the difference in pentanol concentration at the transition point with and without additive. A negativevalue of this ratio indicates that the additive promotes the sphereto-rod transition and a positive value indicates retardation of the transition. Measurements of some alkanes (Table IV) in the SDS system indicate that this ratio is relatively insensitiveto changes in concentration of additive. The collective results of aromatic and aliphatic hydrocarbon additives to TTMAB and SDS solutions are given in Tables I and I1 and are incorporated into the graphs of Figures 2 and 3. The results of polar additives with the same surfactants are given in Tables I11 and IV and are shown in Figures 4 and 5.

7772 The Journal of Physical Chemistry, Vol. 97,No. 29, 1993

2 0 -2 -4

-6

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12 14 #of Carbons in Alkyl Chain

I

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16

18

Figure 4. Effects of polar organic additives on the transition in SDS.

4

6

8

10

12

14

16

18

# of Carbons in Alkyl C y n

Figure 5. Effects of polar organicadditives on the transition in TTMAB.

I. Nonpolar Additives. (a) Aliphatic Hydrocarbon Additives. For micelles to maintain a spherical form, some of the tails must be able to reach the center of the micelle. Since there can be no vacuum at the center of the micelle, the structure must rearrange into a rodlike shape when the micellar size places an undue conformational stress on the surfactant tails to reach the center. Addition of an aliphatic hydrocarbon, generally thought to reside in the micellar core, relieves this requirement. Now the association structure can maintain spherical form containing the solubilized oil at a radius which was previously prohibitive. It is in this manner, we believe, that the aliphatic hydrocarbon retards the sphere-to-rod transition. Curiously, the curves of Figures 2 and 3 corresponding to solubilized aliphatic hydrocarbons for both SDS and TTMAB show a maximum. Whereas the requirement of the tails to reach the center of the micelle in the case with no additive presents a limit to the maintenance of spherical form, it is, perhaps, the stress of packing associatedwith an additiveof large molar volume that limits the size of the swollen spherical micelle. Smith and AlexanderZ5have determined from sedimentation and viscosity studies that methylcyclohexanewhen added to solutionscontaining cetylpyridinium chloride increases the aggregation number and viscosity only slightly and regularly, while additions of aromatic additives (toluene and trichlorobenzene) showed large increases of viscosity and aggregation number. These findings are interpreted in terms of aromatic hydrocarbons having the ability to

Lindemuth and Bertrand promote rod formationand aliphatic hydrocarbons simplyswelling the preexisting spheres. Similar effects were found for benzene and cyclohexane with cetyltrimethylammonium bromide (CTMAB) as surfactant.26.27 (b) Aromatic Hydrocarbon Additives. Aromatic additives clearly behave differently in the cationic surfactant system than they do in the anionic system. The ability of the aromatics to stimulate rod growth in the cationic surfactant system may stem from interaction of the delocalized a-electron cloud of the benzene ring with the positive charges of the surfactant head groups, a behavior very similar to that of a cosurfactant or counterion. The resulting reduction of head group repulsion favors rods by shrinking the surface area occupied per amphiphile, thereby allowing the aggregation number to increase. The apparent increase of rod promotion with longer side chains on benzene emanates from the increase of aggregation number associated with an increase of radius. The trend toward rod formation of the n-alkyl substituted benzenes in TTMAB is evidence that the aromatic ring prefers the surface for all elements of this class of compounds, toluene through n-butylbenzene. The effect of branching on the alkyl chain is to add surface area to the micelle without appreciably affecting its volume, drastically reducing the ability of the additive to promote rodlike micelles relative to its straight-chain analog. This is evidenced by the dmp/dmA values for n-butyl- and isobutylbenzene (see Figure 3). Benzene, toluene, and ethylbenzene show a slight tendency to destabilize spheres in SDS micelles; then there is a trend toward increasing stabilization of the spherical form with subsequent methylene additions to the side chain. With propyl- and butylbenzenes, the micelles are able to retain spherical form at concentrations of pentanol beyond the spherical limit in the nonadditive case (dmp/dmA is now positive). These results suggest that the *-electrons of the benzene ring do not have as strong an effect when positioned at the anionic SDS micellar surface as in the cationic TTMAB case. With increasing length of the alkyl chain, the aromatic molecule acts more like a saturated hydrocarbon, with apparently a higher preference for the center of the micelle. Residence at the micellar core then promotes the spherical form by relieving the requirement of the surfactant chains to reach the center of the structure. The scant difference in dmp/ dmA upon branching of the n-butyl- to isobutylbenzeneis further evidence for central residence of alkylbenzenesin SDS micelles. In SDS micelles, branching does not appear to affect the area/ volume ratio and shows little or no effect beyond the small change in molar volume incurred; the ability of the additive to promote rods remains approximately the same. II. Polar Additives. (a) Head Group Variation. The hydrophilic ranking of the three different amphiphilic classesof additives used in this study has been treated by Wormuth and Kaler.36 This may be viewed in terms of the partitioning behavior between micellar and aqueous pseudophases. In their study primary amines were determined to be considerably more hydrophilic than either alcohols or carboxylicacids. From this consideration it would be expected to see amines as being less effective at promoting sphereterod transition by virtueofless additive present at the interface, as is indeed the case in TTMAB. These authors also noted that amine hydrophilicitywas lower than they expected when coupled with anionic surfactant in microemulsion systems. In the present study, amiines are found to be much more effective in the SDS system than in the TTMAB system, again in agreement with the phase equilibria studies of Wormuth and Kaler.36 This is indicative of a specific interaction between the amine and the anionic surfactant head group at the micellar interface. The occurrence of the minimum in the amine curve of Figure 4 comes at a lower chain length than that for the other cosurfactants used. This indicates that the amine head group has the ability to sit deeper in the SDS micelle, relieving the requirement of the surfactant tails to reach the center of the micelle at a shorter

Transition of Spherical to Rodlike Micelles alkyl chain length of additive. With respect to Figure 5, it is the carboxylic acid additive that exhibits a similar effect in cationic surfactant. This supports the idea3’ that a cosurfactant with the ability to bear a charge opposite to that of the surfactant head group is more effective at promoting sphere-to-rod transition and has the ability to better penetrate the surfactant-rich film separating the water and oil domains. ( b ) Hydrocarbon Chain Length. Examination of Figures 4 and 5 reveals that the above mentioned minima occur at a considerably shorter additive chain length in SDS than in TTMAB for correspondingchemical classes of polar additives. There are three factors responsible for this phenomenon: (1) The TTMAB alkyl chain is two carbons longer than the SDS chain, and, consequently, a shift of approximately two carbons on the x-axes of Figures 4 and 5 is expected when similar additives associated with the two different surfactants are compared. (2) It is likely that the sulfate head group is more hydrophilic than the trimethylammonium head group. This allows greater water penetration into the SDS micelles than those of TTMAB. Since the cosurfactant head group is expected to be located at the oil/water interface, this allows the polar additivesto sit deeper in the SDS micelles than in the TTMAB micelles. The requirement of the SDS tails to reach the center of the micelle to maintain spherical form is then lifted for a shorter additive chain length. The water penetration effect may account for a one to two carbon shift in the minimum of a particular class of additive. (3) More salt was used in the SDS systems to bring the sphereto-rod transition into an easily titrated range (0.28 M NaCl in SDS and 0.05 M NaBr in TTMAB). The higher salt concentration in the aqueous bulk phase reduces the water solubility of polar additivesinthe SDSsystemsrelativeto theTTMAB systems. This probably has a proportionallygreater impact on the shorter, more soluble derivatives. A reduction in water solubility skews the partitioning of the additive toward residence in the micellar interface where it may act as a charge shieldingagent, promoting the sphere-to-rod transition. One of the more essential considerations pertaining to the solubilization mechanics of polar materials is the distribution of solubilizate between the micellar and aqueous bulk phases. In a employing solubility, density, viscosity,ultrasound,electromotiveforce, and conductivity measurements of a series of alcohols from methanol to decanol in micellar solutions, the more water soluble alcohols (methanol to butanol) were observed to decrease the aggregation number. The moderately soluble alcohols (pentanol to heptanol) affect the aggregation number only slightly, and the sparingly water soluble alcohols (octanol to decanol) increase the aggregation number more dramatically. This effect can readily be seen in the initial trend toward more negative values of dmpldmp, in the curves of Figures 4 and 5 for all of the polar additive series. With alcohols higher than decanol the water solubility becomes negligible and the effect of adding more methylene groups to the chain is simply to increase the volume of the hydrocarbon core. The radial increaseassociatedwith the lengtheningof the additive chain necessitates an increase in the aggregation number. This also contributes to the initial trend toward more negative dmp/ dmp, values. With continued additive chain lengthening the requirement of the surfactant chains to reach the center of the micelle eventuallybecomes relaxed. This yields a minimum and a subsequenttrend toward more positive values of dmpldmp,values in the curves of Figures 4 and 5.

Summary Incremental calorimetric titration proved to be an effective method by which toobservethe sphere-to-rod transition in aqueous solutions of both cationic (tetradecyltrimethylammoniumbromide) and anionic (sodium dodecyl sulfate) surfactant with and

The Journal of Physical Chemistry, Vol. 97,No. 29, 1993 7773 without solubilizedadditives. Aromatic hydrocarbons wereshown to have a greater preference for the micellar interface in cationic surfactant than in anionicsurfactant. This was explained in terms of the delocalized ?r-electronsof the benzene ring having a greater charge shielding effect between the positively charged cationic surfactant head groups. The addition of alkyl side chains to the aromatic additives initiated a trend toward greater partitioning of the additive to the core of the micelle with increased hydrophobicityof additive. Aliphatic hydrocarbons were shown to retard the sphere-to-rod transition by virtue of their residence in the micellar core. Thisrelaxes the requirementof the surfactant tails to reach the center of the micelles to maintain spherical form. Primary amines were shown to have a synergistic effect with the interface of anionic micelles, while the same held true for carboxylic acids in cationicsurfactant. The data also suggested that SDS micelles are more water penetrable than TTMAB micelles.

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