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Effect of N-Glycinylmaleamic Acid on Microstructural Characteristics and Solubilization Properties of Sodium Dodecyl Sulfate Micellar Assemblies L. J. Milton Gaspar, Geetha Baskar,* and B. S. R. Reddy Industrial Chemistry Laboratory, Central Leather Research Institute, Adyar, Chennai 600020, India
Radha Ranganathan* and Miroslav Peric Department of Physics, California State University, Northridge, Northridge, California 91330 Received June 9, 2004. In Final Form: August 5, 2004 The cosurfactant activity of N-glycinylmaleamic acid (NGMA) in sodium dodecyl sulfate (SDS) micelles has been demonstrated. The complementary techniques of electron spin resonance (ESR) and fluorescence spectroscopy have been used to draw information on hydration index (H), microviscosity (η), and aggregation number (N) of micellar assemblies. The estimate of the critical micelle concentration of SDS in the presence of NGMA suggests a synergistic effect of NGMA. The enhanced solubilization of butyl propionate in the presence of NGMA in SDS micelles is explained on the basis of availability of larger interfacial area calculated from a simple spherical geometric model, combined with a low hydrophilicity index as estimated from ESR. Thus, addition of NGMA contributes to an increase of about 50% in ratio of area of polar shell (AP)/volume of hydration (Vh) ratio. The decrease in H accompanied by a decrease in η with the incorporation of butyl propionate probably arises from solubilization of a butyl component inside the core with the adsorption of propionate ester on the interface.
* Corresponding author. Fax: 91-44-24911589. Tel.: 91-4424911386/24911108. E-mail:
[email protected].
micelles.8,9 The changes in degree of dissociation and hence micellar charge characteristics enabled through addition of medium chain alcohols, e.g., butanol to SDS micelles, have been demonstrated in increased quenching rates of naphthalene due to Br- quenchers.3 The incorporation of butanol through dehydration in cationic micellar surfaces has been demonstrated by Romsted et al. through a chemitropic method developed by them.10 The modifications in micellar characteristics contributing to enhanced solubilization characteristics have been exploited in enhanced oil recovery,11 environmental treatment methodologies,12 and generation of nanolatex through microemulsion polymerization reaction.13 In the recent work of Sabatini et al.,4 the idea of combination of hydrophilic and lipophilic linkers in enhancement of solubilization of chloroalkanes highly significant in environment related processes has been well documented. The difference in segregation characteristics of these two types of linkers between core and interface of micellar assemblies by virtue of their typical hydrophilic and lipophilic structures has been attributed to the observed effect of increased solubilization properties. The choice of systems to modify micellar assemblies is basically decided by the factors, viz., nature of surfactants, and the ultimate functional properties expected from modified micellar assemblies. The academic importance and industrial potential of aqueous nanolatex of polymers
(1) Graciaa, A.; Lachaise, J.; Cucuphat, C.; Bourrel, M.; Salager, J. L. Langmuir 1993, 9, 3371. (2) Burnside, B. A.; Knier, B. L.; Raymond A.; Mackay, H.; Durst, D.; Longo, F R. J. Phys. Chem. 1988, 92, 4505. (3) Rubio, D. A. R.; Zanette, D.; Nome, F.; Bunton, C. A. Langmuir 1994, 10, 1151. (4) Sabatini, D. A.; Acosta, E.; Harwell, J. H. Curr. Opin. Colloid Interface Sci. 2003, 8, 316. (5) Rhabri, Y.; Winnik, M. A. J. Phys. Chem. B 2003, 107, 1491. (6) Bales, B. L.; Zana, R. J. Phys. Chem. B 2002, 106, 1926. (7) Hassan, P. A.; Raghavan, S. R.; Kaler, E. W. Langmuir 2002, 18, 2543.
(8) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (9) Ali, A. A.; Makhloufi, R. Phys. Rev. E. 1997, 56, 4474. (10) Chaudhuri, A.; Romsted, L. S. J. Am. Chem. Soc. 1991, 113, 5052. (11) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Marcel Dekker: New York, 1988. (12) Uchiyama, H.; Acosta, E.; Sabatini, D. A.; Harwell, J. H. Ind. Eng. Chem. Res. 2000, 39, 2704. (13) Pokhriyal, N. K.; Sanghvi, P. G.; Shah, D. O.; Devi, S. Langmuir 2000, 16, 5864.
Introduction The modifications effected in size and shape of micellar structures of surfactants using additives are important in view of scientific significance and immediate relevance in ultimate functional characteristics, e.g., solubilization,1 catalytic activity,2 and charge-specific reactions.3 From literature, it could be recognized that additives are termed as cosolvents, cosurfactants, lipophilic or hydrophilic linkers, and hydrotropes, depending on their chemical features and site of location in micellar solutions.4 The additives drawn from various classes as briefed above basically bring about modifications in aggregation number, degree of dissociation in the case of ionic surfactants, and degree of hydration, which are surfactant specific and also additive specific. To cite a few examples, addition of a high concentration of (>0.6 M) NaCl or NaBr effects micellar growth of sodium dodecyl sulfate (SDS)5 or dodecyltrimethylammonium bromide (DTAB)6 from spherical to rod morphology due to a large increase in N > 160. Recently, Hassan et al. have shown that a very small amount of p-toludine hydrochloride7 could promote rodlike micelles of SDS contributing to a considerable increase in viscosity by virtue of its typical hydrotropic effect, similar to that of salicylic acid derived salts in cationic
10.1021/la0485834 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/15/2004
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are well documented.14-16 The design of such nanolatex of the classic systems such as poly(styrene) or poly(butyl acrylate) using a microemulsion polymerization method involves use of micellar assemblies of surfactants in the presence of cosurfactants, wherein, the spherical or quasispherical geometry of the micelles is retained and still enhanced solubilization of oil-soluble monomer components is favored. Cosurfactants drawn from medium-chain alcohols are especially popular in view of their typical adsorption characteristics at the micellar surfaces. The alcohols modify dissociation characteristics and favor reduction of electrostatic repulsion among headgroups by suitably altering the size of micellar assemblies.17 By this process, the alcohols inhibit formation of liquid crystals or high viscous solution18-20 and, thus, are suitable in polymerization reactions requiring good diffusion characteristics of reactive species. The cosurfactants capable of providing functionalization and getting chemically linked to latex particles are desirable in various processes such as drug delivery, surface modifiers, and emulsion stabilizers.21,22 The generation of functionalized latexes through mini- or microemulsion polymeriztaion has been demonstrated. Antonietti et al.23 have established the scope for functionalization of polystyrene lattices using short-chain polar acrylate monomers and also the lecithinderived surfactants in place of conventional ones, in combination with bovine serum proteins as cosurfactants in generation of nanolatex. In our efforts to develop novel polymerizable cosurfactants, we have chosen to synthesize N-glycinylmaleamic acid (NGMA), which has a previously unexplored structural motive, i.e., is potentially biodegradable, in the first phase. The incorporation of amino acid is expected to provide benefits of interesting charge characteristics and stability over a wide range of pH. We have chosen a maleyl polymerizable group in view of the unique tendency of a maleyl group to undergo copolymerization rather than homopolymerization and also simple synthetic methodologies involving cheaper and less hazardous maleic anhydride. To the best of our knowledge, this is the first report on investigations on the effect of NGMA on microstructures of SDS micellar assemblies and scope for enhancement in solubilization of n-butyl propionate, a model saturated analogue of butyl acrylate. From our preliminary investigations on the microemulsion polymerization reaction involving polymerizable cosurfactants consisting of reactive sites such as carboxyl groups, quite interestingly we observed that NGMA is capable of generating functionalized polybutyl acrylate latex with particle size distribution of 30 nm < R < 150 nm as controlled by the surfactant/cosurfactant:monomer ratio. It is noteworthy to mention here that NGMA is able to effect considerable reduction in surfactant/monomer ratio 0.5m is observed. These results are indicative of promotion of adsorption of NGMA especially at concentrations >0.5m on the micellar interface. The plot of Rm vs concentration of butyl propionate in the presence of NGMA is presented in Figure 6. The Rm undergoes small variations with respect to SDS, e.g., in
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the presence of 0.5m NGMA, Rm changes in the range of 0.68-1.83 Å over a concentration range of 0.33-0.67m BP. Similar shifts are observed for other concentrations of NGMA (Figure 6a). Fitting the volume contribution of BP, the Rm changes in range of 0.83-3.03 Å (Figure 6b). Interestingly, the variations in Rm with butyl propionate almost remain same about 4% ( 1% in a simple geometric model. Thus, for example, in the presence of 0.67m NGMA, A0 changes by 3% over the entire concentration range of BP from 0.33 to 0.67m. A similar trend of increase in Rm with BP has been observed also in the model taking BP volume into account. The incorporation of volume contribution from BP, in effect, contributes to about 0.19% increase in Rm/SDS chain. These results support retention of average spherical geometry of SDS with adsorption and solubilization of NGMA and BP and this is in fact desirable especially in microemulsion systems, as spherical geometry provides maximum area for the available volume. Conclusion The present study demonstrates the scope of NGMA as a potential cosurfactant for SDS micelles. NGMA provides synergistic effects on surface activity of SDS promoting a micellization process as evidenced from decrease in cmc by almost an order. The small changes in N with dehydration and increase in microviscosity of SDS micelles in the presence of NGMA, as low as 0.67m suggest interfacial adsorption of NGMA. It is understood that the decrease in hydrophilicity index equivalent to DEC of 85% aqueous methanol provided by NGMA might serve as the prime factor underlying the enhancement in solubilization of BP, four times that of SDS micelles. The availability of a larger interfacial area with lower hydrophilicity index as defined by the ratio Ap/VH in the presence of NGMA promotes solubility of BP in SDS micelles. The dehydration of SDS micelles with a decrease in microviscosity suggests the probable distribution of BP between interface and core. The retention of spherical shape identity with small changes in size of the assemblies coupled with the low viscosity in the presence of BP strongly support cosurfactant activity of NGMA especially suitable in microemulsion polymerization reaction wherein, fluidity of solutions underlying the diffusion process of reactants and proximity of reactants are very significant. Acknowledgment. The authors thank Dr. T. Ramasami, Director, CLRI, Chennai, for his support and permission to publish this paper. G.B. is thankful to DG CSIR and Director, CLRI for the approval of a collaborative project, 5/258/2003-TNBP, between CLRI, India, and CSUN, USA, and grant of sabbatical leave to carry out a part of work at CSUN, USA. G.B. acknowledges the support of Dr. A.B. Mandal, and R.R. and M.P. acknowledge the financial assistance for the part of this work from NIH, through Grant NIH/GM 48680. L.J.M.G. thanks CSIR for a senior research fellowship. LA0485834
