1330
Langmuir 1990,6, 1330
Reduction of Surface Tension by Novel Polymer Surfactants Keizo Ogino, Yasushi Onoe, and Masahiko Abe' Faculty of Science and Technology and Institute of Colloid and Interface Science, Science University of Tokyo, Noda, Chiba 278, Japan
Hisao Ono and Keiichi Bessho Development Center, Japan Synthetic Rubber Co. Ltd., Yokkaichi, Mie 510, Japan Received November 7,1989. In Final Form: May 21, 1990 We measured the reduction of surface tension by novel polymer surfactants having different molecular weight and/or acid content. The sodium polyolefin sulfonate with molecular weight 10 OOO and an acid content of 2.5 mequiv/g was able to reduce surface tension below 40 dyn/cm. Moreover, the effectiveness in surface tension reduction increased with decreasing molecular weight and with decreasing acid content. Polymer surfactants are useful mainly as demulsifiers, dispersing agents, and emulsion stabilizers. Their wetting, foam formation, and detergency are much inferior to those of low molecular weight surfactants due to low effectiveness in surface tension reduction. In this paper, we report the effectiveness of novel polymer surfactants in reducing the surface tension of aqueous solutions. The polymer surfactants are sodium polyolefin sulfonates (SPS) having different molecular weight and/or acid content, which are prepared from polybutadiene or polyisoprene by sulfonation with sulfuric anhydride. Acid content is defined as mequiv of sulfonate/g of polymer. The molecular weights and acid contents of the SPS samples are shown in Table I. Molecular weights (as polystyrensulfonate) were determined by using gel permeation chromatography, and acid contents were obtained by titration with NaOH. Water used in this experiment was twice distilled, followed by deionization; its resistivity was 18.0 M k m , and its pH was 6.7. The surface tension of aqueous solutions of polymer surfactants were measured at 30 f 0.1 "Cby using a Wilhelmy-type surface tensiometer (A-3, Kyowa Kaimenkagaku Co., Ltd.) with a glass plate. The time to reach a constant value of surface tension increased with increasing molecular weight of polymer surfactant or with decreasing surfactant concentration, the maximum time required being approximately 48 h. The effect of SPS concentration on surface tension is shown in Figure 1. T h e surface tensions decreased monotonically with increasing SPS concentration. The effectiveness in surface tension reduction increased with decreasing molecular weight and with decreasing acid content. The behavior of SPSl is of particular interest since it shows a discontinuity in the surface tension at a specific SPS concentration, above which the surface tension is nearly constant. This is similar to the behavior of a low molecular weight surfactant which also shows this break in the surface tension at the cmc or concentration where micelles first form.' For SPS1, the concentration at the break in surface tension is 0.108 mequiv/L, which is low compared to typical anionic surfactants (e.g., sodium dodecy1 sulfate (or SDS) cmc = 8 mequiv/L).2 When converted to a weight basis, this concentration is still much lower (1)Abe, M.; Tsubaki, N.; Ogino, K. Colloid Polym. Sci. 1984,57,831.
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I
0.001 0.01
0.1
1
10
" 100 1000
Concentration (g/L) Figure 1. Variation of surface tension with concentration for SPS at 30 O C : (0) SPS1; ( 0 )SPSB; ( 0 )SPS3; ( 0 )SPS4.
Table I. Specification of SPS sample
molecular weight
acid content, mequivlg
SPSl SPS2 SPS3 SPS4
10 OOO 10 OOO 26 OOO 66 OOO
2.5 3.5 5.9 6.1
for SPSl (0.043 g/L) than for SDS (2.3 g/L). Polymeric surfactants with high molecular weights generally are unable to reduce surface tension below approximately 50 d y n / ~ m . As ~,~ shown in Figure 1,SPS polymers can attain surface tensions as low as 38.0 dyn/ cm at very low solution concentrations. At this time, we do not have sufficient experimental results to explain this observed behavior in detail. Further studies are in progress and will be published in the near future. (2) Ogino, K.; Kubota, T.; Uchiyama, H.; Abe, M. J. Oil Chem. SOC.
1988. - - - -, 37. - . , 588. - - -.
(3) Bistline, R. C., Jr.; Stirton, A. J.; Weil, J. K.; Port, W. S. J. Am. Oil Chem. SOC.1956,33, 44. (4) Tanizaki, I. J. Oil Chem. SOC.1985,34, 973.
0 1990 American Chemical Society
