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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.49 n.3 Concepción set. 2004

http://dx.doi.org/10.4067/S0717-97072004000300004 

 

J. Chil. Chem. Soc., 49, N 3 (2004): 215-217

DEPENDENCE OF THE CRITICAL AMOUNT OF N-HEXANOL REQUIRED TO FORM A WATER-IN-OIL MICROEMULSION WITH THE SURFACTANT CHARACTERISTICS

 

E. Abuin,* E. Lissi and K. Olivares

Facultad de Química y Biología Universidad de Santiago de Chile Santiago-Chile E-mail : eabuin@lauca.usach.cl - Fax: 6812108

(Received: January 28, 2004 - Accepted: April 29, 2004)


ABSTRACT

The amount of n-hexanol required to produce a clear microemulsion in a water-in-oil quaternary system comprising a surfactant, 2,2,4-trimethylpentane, water and the alkanol, was evaluated employing cationic, anionic, zwitterionic and neutral surfactants. The critical analytical n-hexanol concentration, the number of n-hexanol molecules at the interface per surfactant (n), and the distribution constant of the alkanol between the organic pseudophase and the interface are determined by the charge of the surfactant head, being almost independent of the alkyl chain length, the counterion and the surfactant head. The values of n are similar for cationic and anionic surfactants. However, due to a less favorable distribution between the organic solvent and the interface, critical n-hexanol concentrations are significantly higher for the anionic surfactants.


 

INTRODUCTION

With very few exceptions, surfactants form stable water-in-oil micremulsions in aliphatic solvents only in the presence of a co-solvent1 or a co-surfactant such as an alcohol.2-4 The formation of a clear microemulsion takes place at a given alcohol concentration,5 and has been related to the decrease in the oil-water interfacial tension provoked by the incorporation of the alcohol at the interface. In particular, it has been proposed that, at the point of microemulsion formation, the interfacial tension of the microdroplet surface is zero.6 The formation of the microemulsion can be easily detected by a sharp decrease in the turbidity of the sample.5,6 However, there are few studies aimed to evaluate the factors affecting the point of microemulsion formation (surfactant and external solvent characteristics; amount of solubilized water; size and topology of the alcohol, etc.), and to quantify the alcohol distribution between the dispersed microdroplets and the external solvent at the point of microemulsion formation.3,4,7,8

In a previous work, we have evaluated the amount of alcohol required to produce a clear microemulsion in a quaternary water - in - oil system employing a series of alcohols and hydrocarbon solvents of different size or topology.8 It was observed that the amount of n-hexanol and n-decanol required were similar in all the solvents considered. On the other hand, considerably higher concentrations of the branched alcohols (2,4-dimethyl-3-pentanol and 3-ethyl-3-pentanol) were necessary to form the microemulsion, irrespective of the solvent topology (n-hexane or 2,2,4-trimethylpentane). From an analysis of the change in the analytical alcohol concentration with the surfactant concentration the amounts of alcohol present at the microaggregates surface at the point of microemulsion formation were obtained.9,10 It was concluded that the high amounts of branched alcohols needed are due to both, a less efficient incorporation at the interface and to the larger number of alcohol molecules per surfactant required to stabilize the microemulsion.

Besides the alcohol topology, it can be expected that the distribution of the alkanol and the amount of alkanol required to reach the critical microemulsion point, i.e. a nearly zero interfacial tension between the aqueous and non-polar pseudophases,8 be dependent of the surfactant characteristics. In order to address this point, we have evaluated the above mentioned parameters in the system n-hexanol/water/2,2,4-trimethylpentane/surfactant at different values of the water to surfactant mole ratio, W ( from W = 5 to W = 20) employing surfactants of different characteristics. In particular, we have studied surfactants with a common tail and different heads, and surfactants with the same head and different tails. The results obtained are presented in this communication.

EXPERIMENTAL

The microemulsion formation was assessed from the change in the sample turbidity produced by the alcohol addition.8 To a turbid solution comprising a surfactant and water in 2,2,4-trimethylpentane (TMP), at constant temperature, small aliquots of the selected alcohol were added. The point of microemulsion formation was evidenced by a total loss of the sample turbidity quantified by its absorbance measured at 320 nm in a Shimadzu UV-visible spectrophotometer. The sharp decrease in absorbance observed in the sample titration with the alcohol allows a precise determination of the amount of alcohol needed to stabilize the microemulsion. All data were obtained at 25 ± 1 °C.

The surfactants employed were : dodecyltrimethylammonium bromide (DTAB); tetradecyltrimethylammonium bromide (TTAB); cetyltrimethylammonium bromide (CTAB); cetyltrimethylammonium chloride (CTAC); cetylpyridinium chloride (CPC): sodium dodecylsulfate (SDS); trimethylammonium dodecylsulfate (TMADS); N-hexadecyl-N,N-dimethyl-3-ammonium-1-propane sulfonate (HPS); Triton X-100 ; Brij-30. The organic solvent (2,2,4-trimethylpentane, TMP) was a sample from Aldrich (99.8 %, anhydrous).

RESULTS AND DISCUSSION

The minimum amount of n-hexanol required to give a clear microemulsion was determined at two values of the mole ratio W (W = [water]/[surfactant]): W = 5 and W = 20. Determinations were carried out at several surfactant concentrations. For all the surfactants considered but Triton X-100 and Brij-30, the critical amount of n-hexanol increases with the surfactant concentration. This has been ascribed to the removal of alkanol from the organic solvent by the interface, and a simple treatment of the data allows an evaluation of the co-surfactant concentration remaining in the organic pseudophase, and the number of molecules incorporated per surfactant in the microdroplet/external solvent interface.9,10 The values obtained are collected in Tables 1 and 2, where it is shown the amount of alkanol required at 0.1 M surfactant ([n-hex]analyt), the amount of n-alkanol remaining in the organic pseudophase at the critical point of microemulsion formation, ([n-hex]org), the number of n-hexanol molecules per surfactant, n, and the pseudo partition constant of the alkanol at the point of microemulsion formation, defined by,11

K = n/[n-hex]org

The data given in Tables 1 and 2 comprise surfactants with the same tail and different heads (CTAC and CPC; SDS and TMADS), the same tail and head group but differenet counterions (CTAC and CTAB), and surfactants with the same head and different tails (DTAB, TTAB and CTAB). From these data it can be concluded that the alkanol distribution and its critical concentration are very little dependent of the surfactant characteristics. In particular, it is not observed any dependence with the size of the surfactant tail (compare DTAB, TTAB and CTAB), with the counterion (compare CTAB and CTAC) and the characteristics of the head group for surfactants of the same charge (compare SDS with TMADS and CPC with CTAC). On the other hand, there seems to be significant differences between surfactants with different charges at the surfactant head groups. For example, there are significant differences when the head group changes from cationic to zwitterionic or anionic. In particular for HPS, the zwitterionic surfactant, n values tend to be larger than those for the charged surfactants, indicating that more alkanol molecules are required to stabilize the microemulsions. On the other hand, the values of n for the anionic surfactants are similar to those of the cationic. However, a less favorable distribution of the alcohol (lower K) leads to larger values of free n-hexanol and, hence, of the total amount of alcohol required to produce the microemulsion. This effect is observed both at the two W ratios considered. It is interesting to note that the partitioning of n-hexanol in cationic (DTAB) micelles is considerably more favorable that in anionic (SDS) micelles, suggesting a stronger interaction between the alcohol and the positively charged surfactant heads.12 This stronger interaction can also explain the larger values of K obtained in the cationic microemulsions considered in the present work.

The data obtained employing neutral surfactants are more difficult to interpret. For Brij-30, the point of turbidity loss decreases with the surfactant concentration. With Triton X-100, there appear two critical concentrations, indicating a considerably more complex phase diagram in this type of system. The kind of data obtained are exemplified in Figure 1. The data indicate that, with increasing alcohol concentration, the system evolves from

phase separation__→ clear solution__→ turbidity__→ microemulsion

Fig. 1.- Phase diagram for the n-hexanol/Triton X-100 system at W =10.

(A) Phase separation; (B) clear solution; (C) turbid solution (macroemulsion); (D) clear solution (microemulsion).

If this sequence is accepted, the data of Triton X-100 that are comparable to that of the other surfactants correspond to the upper line of Fig. 1, which shows an increase in the critical alkanol concentration with the surfactant concentration. However, the parameters obtained with this assumption, included in Table 2, indicate that the critical concentrations of n-hexanol are considerably larger than those for the ionic and zwitterionic surfactants. This difference is due to the large number of co-surfactant molecules required, per molecule of surfactant, to produce the microemulsion.

(*) Minimum analytical n-hexanol concentration that produces a clear microemulsion in a 0.1 M surfactant solution in TMP.

(*) Minimum analytical n-hexanol concentration that produces a clear microemulsion in a 0.1 M surfactant solution in TMP.


ACKNOWLEDGEMENTS

Thanks are given to DICYT (Universidad de Santiago de Chile) and FONDECYT (Project #1010148) for financial support.

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