4476
Langmuir 1996, 12, 4476-4478
Polymer/Surfactant Interactions. The Controlled Desorption of Sodium Dodecyl Sulfate (SDS) from a Polymer/SDS Complex in Aqueous Solution Yan Li, Derek M. Bloor,* and E. Wyn-Jones Division of Chemical Sciences, University of Salford, Salford M5 4WT, U.K. Received February 14, 1996. In Final Form: June 13, 1996
Introduction The interactions between ionic surfactants and nonionic polymers have received much attention.1,2 As a result of recent developments in the use of techniques such as neutron reflection,3 isothermal titration calorimetry,4-9 and also the versatility of established methods such as surfactant selective electrodes6-8 and fluorescence techniques,10,11 it is now possible to characterize the various critical concentrations associated with the binding of surfactants to polymers.6 An approach which we have found to be successful in a recent study involves measuring the wavelength shift in a polymer labeled with a covalently bonded solvatochromic probe following the addition of surfactant.8 The macromolecule in question (PAPR*) is a copolymer of N-(vinylacryloyl)pyrrolidine (98.6%) containing covalently bonded 4-vinylpyridine dicyanomethylide (1.4%) chromophore RC5H4N+C-(CN)2. The red wavelength shift of the chromophore at ∼390 nm was successfully used as a probe to monitor the binding of sodium dodecyl sulfate (SDS) to this polymer.8 In this communication we have used the spectroscopic method to study the properties of various mixed PAPR*/SDS/cosurfactant solutions.
PAPR* This work has been carried out for two reasons. Firstly, commercial preparations of most detergent products are mixtures of ionic and nonionic surfactants often mixed in combination with other additives, in particular polymers. (1) Goddard, E. D. Colloids Surf. 1986, 19, 255. (2) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadnanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (3) Purcell, I. P.; Thomas, R. K.; Penfold, J.; Howe, A. M. Colloids Surf. 1995, 94, 125. (4) Olofsson, G.; Wang, G. Pure Appl. Chem. 1994, 66, 527. (5) Persson, K.; Wang, G.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1994, 90, 3555. (6) Bloor, D. M.; Holzwarth, J.; Wyn-Jones, E. Langmuir 1995, 11, 2312. (7) Wan-Yunus, W. M. Z.; Wan-Bahdi, W. A.; Li, Y.; Holzwarth, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (8) Bloor, D. M.; Li, Y.; Wyn-Jones, E. Langmuir 1995, 11, 3778. (9) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (10) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (11) Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Colloid Polym. Sci. 1995, 273, 83.
S0743-7463(96)00138-2 CCC: $12.00
Figure 1. Plot of ∆λmax as a function of SDS concentration for the SDS/PAPR* (0.5% (w/v)) system.
The preparations of such mixtures are essential to maximize the effectiveness of the desired process taking place. Secondly, as far as we are aware there is a paucity of data available on the properties of mixed ionic/nonionic surfactant and neutral polymers. It is the purpose of this communication to address this problem. The main surfactant used was sodium dodecyl sulfate (SDS) and the cosurfactants were hexaethylene glycol monododecyl ether (C12EO6), 3-(N-tetradecyl-N,N-dimethylammonio)propanesulfonate (C14SB), N,N-dimethyldodecylamine N-oxide (C12DMAO), dodecyltrimethylammonium bromide (C12TAB), octyltrimethylammonium bromide (C8TAB), and sodium tetradecyl sulfate (STS). Experimental Section (i) Materials. Sodium dodecyl sulfate (cmc ) 8.3 mM; method of Davidson12) and the cosurfactant C12DMAO (cmc ) 2.5 mM; method of Kaimoto13) were prepared in the laboratory. The remaining cosurfactants and their suppliers were as follows: C12EO6 (cmc ) 0.087 mM; Nikkol), C14SB (cmc ) 2.88 mM; Sigma), C12TAB (cmc ) 14 mM; Fluka), C8TAB (cmc ) 130 mM; Lancaster), STS (cmc ) 1.4 mM; Lancaster). The probe labeled polymer PAPR* was synthesized according to the method described by Velasques and Galin.14 The molecular weight of the polymer was determined to be 10 000 by gel permeation chromatography. (ii) UV-Visible Spectrometer. The spectrophotometer used in this work was a Hewlett-Packard 8452A spectrophotometer. All measurements were made on solutions of the mixed surfactants and 0.5% (w/v) PAPR* using a 1 mm cell at 298 K.
Results and Discussion The background related to the methodology used for the interpretation of the present work has been reproduced in the plot of wavelength shift of the chromophore (∆λmax) as a function of added SDS concentration to a 0.5% (w/v) solution of PAPR*. This plot, which was reported earlier,8 is shown in Figure 1. By combining these spectral data with complementary emf and isothermal titration calorimetry measurements, the following noteworthy features relating to this system have been established.8 1. Following the onset of binding (denoted T1) at 4 × 10-3 mol dm-3 added SDS, there is a very sharp rise in ∆λmax. The bound SDS exists in the form of aggregates, and this pronounced red wavelength shift indicates that (12) Davidson, C. J. Ph.D. Thesis, University of Aberdeen, 1983. (13) Kaimoto, H.; Shoho, K.; Sasaki, S.; Maeda, H. J. Phys. Chem. 1994, 98, 10243. (14) Valasques, D. L.; Galin, J. C. Macromolecules 1986, 19, 1096.
© 1996 American Chemical Society
Notes
Langmuir, Vol. 12, No. 18, 1996 4477
Figure 2. Plot of ∆λmax as a function of C12EO6 concentration for the SDS/C12EO6/PAPR* (0.5% (w/v)) system. Concentrations of SDS were ([) 80 × 10-3 mol dm-3, (2) 16 × 10-3 mol dm-3, (9) 6 × 10-3 mol dm-3, and (b) no SDS. Refer to Table 1 for key A-C.
Figure 3. Plot of ∆λmax as a function of cosurfactant concentration for the SDS/cosurfactant/PAPR* (0.5% (w/v)) system. Cosurfactants were ([, ]) C14SB and (9, 0) C12DMAO. Concentrations of SDS were ([, 9) 16 × 10-3 mol dm-3, (], 0) 6 × 10-3 mol dm-3, and (b) no SDS. Refer to Table 1 for key A-B.
Table 1
solution
PAPR* (% (w/v))
SDS (mol dm-3)
initial SDS monomer (mol dm-3)
initial λmax (nm)
A B C
0.5 0.5 0.5
6 × 10-3 16 × 10-3 80 × 10-3
3.8 × 10-3 4.8 × 10-3 4.5 × 10-3
392.4 394.3 393.4
the solvatochromic probe experiences a more hydrophobic environment as binding proceeds. 2. At 16 × 10-3 mol dm-3 added SDS, ∆λmax reaches a maximum value of ∼5.5 nm, decreases slightly, and eventually levels off. This small blue shift is caused by the redistribution of a small amount of SDS aggregates on the polymer from the chromophore to the more abundant N-(vinylacryloyl)pyrrolidine moiety of the PAPR*. 3. The data also show that in the SDS/PAPR* system “free” micelles occur in solution at ∼ 29 × 10-3 mol dm-3 SDS (denoted Cm) and the polymer becomes fully saturated with SDS aggregates at 45 × 10-3 mol dm-3 SDS (denoted T2). We now discuss the situation in the presence of the cosurfactants. (i) PAPR*/SDS/C12EO6. The data presented in Figure 2 show the variation in the value of ∆λmax on the addition of C12EO6 to PAPR* alone and also to the PAPR*/SDS system for three different concentrations of SDS as detailed in Table 1. In solutions A and B, the SDS/PAPR* complex, in the form of SDS aggregates bound to the polymer chain, is in equilibrium with free SDS monomers in bulk solution in the absence of any free micelles. Solution C contains sufficient SDS such that the PAPR* is fully saturated with surfactant and also “free” SDS micelles occur in the bulk solution, both of which are in equilibrium with monomer SDS. In the absence of SDS, it is clear from the ∆λmax data for PAPR*/C12EO6 that the wavelength of the chromophore remains constant and equal to the value of PAPR* in water showing that no interaction between C12EO6 and the polymer takes place. However, on addition of C12EO6 to solutions A and B a solvatochromic effect is immediately observed in the form of a pronounced blue shift. This shows that the microenvironment of the chromophore becomes less hydrophobic until it eventually almost reaches the value found for PAPR* in water. This means
Figure 4. Plot of ∆λmax as a function of STS concentration for the SDS/STS/PAPR* (0.5% (w/v)) system. Concentrations of SDS were ([) 16 × 10-3 mol dm-3 and (9) 6 × 10-3 mol dm-3.
that on addition of C12EO6 to the SDS/PAPR* system the bound SDS is stripped or desorbed from the polymer to form mixed SDS/C12EO6 “free” micelles in solution. As a result of the above observations the third solution C was investigated. The variation of ∆λmax with added C12EO6 shows a small red shift followed by a more pronounced blue shift (Figure 2). The maximum in ∆λmax (5 nm) corresponds to the maximum in Figure 1. We believe that the following sequence of events occur. Firstly the added C12EO6 forms mixed micelles with the “free” SDS micelles in bulk solution. This results in a decrease in the monomer SDS concentration15 and in response to this decrease the equilibrium between monomer SDS and the polymer bound aggregated SDS is readjusted in such a way that some of the bound SDS is desorbed from the polymer to form more “free” mixed micelles in bulk solution. As a result of the reduction in the concentration of bound SDS aggregates, the reverse of the effect described for PAPR*/SDS alone occurs; i.e., there is a redistribution of bound SDS aggregates from the more abundant N-(vinylacryloyl)pyrrolidine moiety of PAPR* to the chromophore, thus rendering the chromophore in a more hydrophobic environment and producing a red shift. As (15) Hall, D. G.; Meares, P.; Davidson, C.; Taylor, J.; Wyn-Jones, E. ACS Symp. Ser. 1992, No. 501, 128.
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further C12EO6 is added to the system, this behavior continues until the maximum in ∆λmax occurs, after which the desorption mechanism described above for PAPR*/ SDS solutions A and B occurs. (ii) PAPR*/SDS/(C14SB, C12DMAO, C8TAB, C12TAB, STS). The data for the SDS/PAPR* solutions A and B in the presence of the added cosurfactants C14SB and C12DMAO are shown in Figure 3 in the form of a plot of ∆λmax against added cosurfactant. They behave in a similar way to C12EO6 in the sense that they desorb the bound aggregated SDS from the polymer chain via the same mechanism as C12EO6 albeit in a less efficient manner. The two cationic surfactants C8TAB and C12TAB were also found to show some evidence of promoting SDS desorption, but unfortunately, as expected, precipitation occurred at an early stage. Finally, addition of the cosurfactant STS (C14 chain) to the PAPR*/SDS systems initially shows a red shift (Figure 4) indicating that the chromophore exists in a more hydrophobic environment. A maximum ∆λmax of 6.25 nm is observed which is higher than the value (5.5 nm) found
Notes
in Figure 1 for the SDS/PAPR* system. Following the maximum a redistribution of bound aggregates described above occurs. In this case the C12 and C14 alkyl sulfates form mixed aggregates on the polymer chain resulting in a more hydrophobic environment than pure bound SDS aggregates. Conclusion It has been shown that addition of nonionic/zwitterionic cosurfactants to a solution containing PAPR*/SDS complexes causes desorption of SDS aggregates from the polymer chain to form “free” mixed micelles in solution. The amount of SDS that is desorbed can be controlled by the concentration and type of added cosurfactant. Acknowledgment. Y.L. wishes to thank the Division of Chemical Sciences and the O.R.S. for financial contribution toward his postgraduate studies. LA960138O
