КОЛЛОИДНЫЙ ЖУРНАЛ, 2010, том 72, № 6, с. 780-790
УДК 541.18
PHYSICOCHEMISTRY OF MIXED MICELLIZATION: BINARY AND TERNARY MIXTURES OF CATIONIC SURFACTANTS IN AQUEOUS MEDIUM
© 2010 Chanchal Das*, Tanushree Chakraborty**, Soumen Ghosh**, 1, Bijan Das*
* Department of Chemistry, North Bengal University Darjeeling-734013, W.B, India **Centre for Surface Science, Department of Chemistry, Jadavpur University Kolkata-700 032, W.B., India Поступила в редакцию 09.12.2009 г.
The importance of studying mixed micellization lies in tuning the performance of an amphiphile to bend through variation of stoichiometry of the blend. In this study, the binary and ternary mixed systems of cetylpy-ridinium chloride (CPC), tetradecyltrimethylammonium bromide, and dodecylpyridinium chloride (DPC) have been studied at 30°C using tensiometry and conductometry. In most cases, the cmc observed from either method is in close proximity whereas in CPC/DPC mixtures, tensiometric cmc precedes conductometric cmc which may arise from a lowering in degree of counterion binding on micellar interface in the mixed system with lower stoichiometric mole fraction of CPC. Various existing theories have been used and the results were compared with the experimental observations.
1. INTRODUCTION
Mixed surfactant systems exhibit better performance compared to the individuals [1, 2] in terms of lowering surface tension of aqueous solution and hence, exhibit enhanced detergency, dispersing water insoluble substances in aqueous solution, and are widely used in the area of suspension, wetting, emulsification, and different technological, biochemical, pharmaceutical directions [3]. The molecular structure of amphiphiles, their concentration, and composition along with the environmental conditions, such as temperature, pH, pressure, and presence of additives [3] significantly govern the activity of the surfactant mixtures.
Because of the amphiphilic chemical structure, surfactant has a preference towards interfacial adsorption at low concentration region; whereas above a critical concentration, it self-aggregates to form assembled structure whose size, shape and average number of amphiphile per aggregated structure depend on the amphiphile concentration and other physicochemical parameters like temperature, presence of salt, etc. The critical amphiphile concentration required for the onset of formation of more or less spherical aggregated structure, called micelle, in aqueous medium is called critical micellar concentration (cmc). Micelles can be treated as a separate phase within a surfactant solution and various physico-chemical properties change dramatically depending on surfactant concentration. Measurement of any of such properties with variation of surfactant concentration leads to a discontinuity in the profile leading to determination of cmc of the surfactant. Frequent measurements of surface tension [4—9], conductance [4—9], fluores-
1 Corresponding author. E-mail: gsoumen70@hotmail.com
cence intensity [10], heat capacity [11, 12], light scattering [13], etc., are used in determination of cmc. The change in these properties occurs over a narrow region of total surfactant concentration. Moreover, concentration dependent changes ofproperties ofpremicellar and post-micellar regions can be joined by two straight lines and the point of intersection of these two lines is taken as the cmc of the surfactant mixture.
The self-aggregation and associated thermodynamics of different mixed surfactant combinations, such as, nonionic—nonionic [7, 8, 14, 15], cationic—nonionic [4, 7, 9, 16, 17], anionic—nonionic [8, 18, 19], cationic-an-ionic [20, 21], anionic-biosurfactant [22], cationic-cat-ionic [4, 11, 23], anionic—anionic [24, 25], etc. have been studied for a long period of time. In this report, we have physicochemically studied three cationic surfactants namely, dodecyl pyridinium chloride (DPC), cetylpyridinium chloride (CPC), and tetradecyltrimeth-ylammonium bromide (TTAB) in pure and mixed states. The first two (DPC and CPC) have the same pyridinium head group and different tail lengths (12 and 16 C) whereas TTAB has a methyl substituted quaternary ammonium head group linked with a 14 C tail. Three permutations of binary combinations, CPC/TTAB, DPC/TTAB, and CPC/DPC and ternary mixtures of CPC/DPC/TTAB have been studied. Tensiometry and conductometry were used for determining the cmc of all the mixtures. Various parameters such as cmc, Gibbs surface excess (rmax), minimum area of exclusion per surfactant monomer at the air/solution interface (^min), pC20 (where C20 is the surfactant concentration required to decrease the surface tension of solvent by 20 unit), degree ofcounterion binding (g) on micellar interface along with various thermodynamic parameters like Gibbs ad-
y/mN m 1 70
65
60
55
50
45
40
35
V - 1
□ - 2
V a - 3
- V. - 4
-4.0
3.6
-3.2
-2.8
-2.4
2.0
3\
log([surfactant]/mol dm 3)
k/^S cm
800
700 600 500 400 300 200 100
0 0.002 0.004 0.006 0.008 0.0010 0.012 0.014
_3
[Surfactant]/mol dm 3
Fig. 1. Tensiometric plot of binary mixtures (1) CPC/TTAB, aCPC = 0.25; (2) DPC/TTAB, aDPC = 0.4; (3) CPC/DPC, aCPC = 0.9; and ternary mixtures (4) DPC/CPC/TTAB (0.333/0.333/0.333) at 303 K.
Fig. 2. Conductometric plot of binary mixtures (1) CPC/TTAB, aCPC = 0.1; (2) DPC/TTAB, aDPC = 0.4; (3) CPC/DPC, aCPC = 0.75; and ternary mixtures (4) DPC/CPC/TTAB (0.125/0.625/0.250) at 303 K.
sorption energy (AG^), Gibbs micellization energy
(AGm), can be found out by using these two methods. Existing theories of Clint [26, 27], Rosen [27_29], Rubingh [27, 30], Motomura [27, 31_33], Blankschtein [27, 34_ 36], and Rubingh and Holland [37, 38] are applied in these systems to evaluate theoretical cmc, micellar and interfacial mole fractions and interaction parameters among the surfactants in micellar and interfacial mono-layer, surface free energy, activity coefficient, etc.
2. EXPERIMENTAL
2.1. Materials
The cationic surfactants DPC, CPC, and TTAB were purchased from Sigma (USA). All the products were used without further purification. All solutions were prepared in doubly distilled water and the experiments were performed at 303 ± 0.1 K.
2.2. Methods 2.2.1. Tensiometry
The tensiometric experiments were performed using a platinum ring by the ring detachment method in a calibrated K9 Tensiometer (Kruss, Germany). Detailed procedure has been reported earlier [4_10, 19]. Each experiment was repeated several times to achieve good repro-ducibility. The measured surface tension (y) values were corrected according to the procedure ofHarkins and Jor-don. The y values were accurate within ±0.1 mN m_.
2.2.2. Conductometry
The conductance measurements were taken with a Pye-Unicam PW-9509 conductivity meter at a frequency of2000 Hz using a conductivity cell of cell constant 1.0 cm_'. The same procedure of addition of surfactant as in tensiometry was followed. The accuracy of the measurements was within ±1%. The measurement details can be found elsewhere [4_10, 19].
3. RESULTS AND DISCUSSION 3.1. Determination of critical micellar concentration (cmc)
Tensiometrically, cmc corresponds to the surfactant concentration at which there is a discrete break in the air/solution interfacial tension vs. log [surfactant] isotherms (Fig. 1). Phenomenologically, this cmc corresponds to the saturation ofinterfacial adsorption. Beyond this concentration, the added surfactants can hardly affect the interfacial topology of the interfacially adsorbed surfactant monolayer but prefer to self-associate in the bulk solution to form micelles whereby the hydrophobic tails of individual surfactant monomers are buried within a hydrophobic encapsulation provided by the polar head group of the surfactant structures [5, 19]. On initiation of the self-association, the counterions of ionic surfactants with significantly large ionic mobility start to adsorb onto the micelle_solution interface leading to a dramatic change in the electrical transport property of the solution as a result of decrease in number of effective charge carriers in solution and is reflected in a sharp break in the specific conductance of the solution (k) vs. surfactant concentration isotherm (Fig. 2). The conductometric cmc, therefore, corresponds to onset ofself-association ofion-ic surfactant in bulk solution. If the interfacial saturation
Table 1. Critical micellar concentrationa (cmc) of pure, bi-
nary and ternary mixtures of surfactants at 303 K
aCPC (I) or Tensiometry Conducto- Average C cmcC
»dpc (II) metry cmc
CPC/TTAB (103 cmc, mol dm-3) (I)
0.00 3.715 3.524 3.620 -
0.10 2.564 2.716 2.640 2.846
0.25 1.556 2.255 1.906 2.154
0.50 1.247 1.562 1.404 1.534
0.75 0.998 1.250 1.124 1.191
0.90 0.920 1.313 1.026 1.050
1.00 0.910 1.036 0.973 -
DPC/TTAB (103 cmc, mol dm-3) (II)
0.00 3.715 3.524 3.620 -
0.10 3.908 3.992 3.950 3.931
0.25 4.645 4.731 4.688 4.513
0.40 5.297 5.781 5.539 5.297
0.50 5.929 6.435 6.182 5.991
0.60 7.211 7.131 7.171 6.895
0.75 9.453 9.387 9.420 8.910
0.90 11.695 12.414 12.054 12.588
1.00 17.132 17.608 17.370 -
C PC/DPC (1 03 cmc, mo l dm-3) (I)
0.00 17.132 17.608 17.370 -
0.10 4.291 8.631 6.461 6.468
0.25 3.254 7.764 5.509 3.332
0.40 2.218 5.190 3.704 2.244
0.50 1.527 3.950 2.739 1.843
0.60 1.465 2.775 2.120 1.563
0.75 1.445 2.469 1.957 1.273
0.90 1.096 2.296 1.696 1.074
1.00 0.910 1.036 0.973 -
a The average error in cmc is ±2%.
a concentration delay (lag) between interfacial saturation and self-aggregation process. In all the mixtures, the cmc values are close to the component having lower cmc (higher tail length) and increases with increasing stoichi-ometric mole fraction of the shorter tail component in the mixture. Later, a thorough discussion in this context will be addressed in the theoretical section. The cmc values of ternary mixture, DPC/CPC/TTAB have been presented in Table 2 showing that the cmc determined by
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