Langmuir 1990,6, 385-391 assumption of 210 pC/cm2 for the polycrystalline substrate and Pt(lOO)-faceted surface and 230 pC/cm2 for Pt(ll1)-faceted surfaces.22 The turnover numbers of ethylene electroreduction on polycrystalline platinum electrode were calculated as follows: i(6.02 X = 4762i nFA(1.3 X where t n is the turnover number (molecule/surface atom($), i the current (A), A the surface area (cm'), n = 2 the number of electrons involved in the charge transtn =
385
fer, F the Faraday constant, 1.3 X 1015 the number of platinum atoms in polycrystalline (and Pt(100)) platinum, and 6.023 X loz3the Avogadro constant. The number of surface platinum atoms for faceted surfaces was assumed to be 1.3 X 1015 and 1.5 X 1015 for (100) and (111)orientations, respectively. The total number of sites at the real electrode surfaces was calculated in accordance with the hydrogen underpotential deposition charge. Registry No. Pt, 7440-06-4;C,H,, 74-85-1.
Synthesis and Characterization of Zwitterionic Silicone Sulfobetaine Surfactants Steven A. Snow,* William N. Fenton, and Michael J. Owen Fluids, Resins, and Processing Industries Research, Mail #C41Dol,Dow Corning Corp., Midland, Michigan 48686-0994 Received May 15,1989.In Final Form: August 10,1989 A new family of zwitterionic organofunctional siloxanes of the general formula Me SiO(SiMe,O),R(2) = CH,, CH2CH,, CH,Ch,OH; x = 0-3, (SiMeR(l)O),SiMe, (R(1) = (CH2)3NR(2)2+-(CHz)zS03-; y = 1, 2, z = 3,4) have been prepared and characterized both structurally and as aqueous surfactants. They are prepared by the quaternization of the precursor amino functional siloxane with either cyclic 1,3-propanesultoneor cyclic 1,4-butanesultone. They were structurally characterized by elemental analysis, proton and silicon-29 NMR, and IR spectroscopies. Members of this family reduced the surface tension of water to approximately 21 mN/m at concentration levels of 0.005-0.5 w t %. The species where x = 0 and y = 1 displayed very rapid lowering of the surface tension of water. These silicone surfactants demonstrated adequate shelf life stability as the isolated compound or in aqueous solution between pH 5 and 8. Aqueous instability of some of these solutions could be attributed to hydrolysis and subsequent rearrangement of the Si-0-Si bonds in these molecules, yielding hexamethyldisiloxane as a byproduct.
Introduction Historically, silicone-basedsurfactants have been applied in nonaqueous physicochemical systems. Two common applications are their use as mold release agents and as polyurethane foam additives. More recently, there has been interest in the utility of these low surface energy compounds in aqueous systems. There are specialty applications for these compounds, such as the aqueous wetting of hydrocarbon surfaces. Many of these wetting agents are nonionic silicone polyether copolymers of varying molecular weights and compositions. Generally they perform as excellent aqueous surface tension depressants, with surface tension values of approximately 21 mN/m being commonly achieved. This is the same value as the poly(dimethylsi1oxane)(PDMS) homopolymer and indicative of a close-packed array of methyl groups. The flexible siloxane backbone can adopt orientations where it lies flat on the surface of the water, favorably exposing the low surface energy methyl group substituents to the air. When such a parallel orientation of the PDMS backbone with respect to the water is adopted, it results in relatively large areas of molecular surface coverage and low critical micelle concentrations (cmc values). These unique surfactant properties prompted a number of researchers to investigate aqueous silicone surfac0743-7463f 90f 2406-0385$02.50f 0
tants with ionically charged hydrophilic groups including cationic and anionic species. In an early study, Maki et al. reported the synthesis and characterization of (Me,SiO),Si(Me)-(CH,),NMe,+Cl-.'I t is a highly effective aqueous surfactant, reducing the surface tension of water to 21 mN/m. Quaternary ammonium salt type cationic surfactants were also patented by workers from the Union Carbide Corpa2 These surfactants are useful in fire-fighting foam mixtures., Also patented were a number of anionic (sulfate and sulfonate type) surfactants, which are excellent foaming agent^.^ Our interest is in zwitterionic organofunctional silicone aqueous surfactants. Hydrocarbon-based zwitterionic surfactants have extensive commercial utility in part due to their low skin irritation. Patents describing the preparation and use of zwitterionic organofunctional siloxane surfactants were recently issued to the Th. Gold(1) Maki, H.; Horiguchi, Y.; Takaaki,
1970,19, 1029-33.
s.; Komori, s. Yukagaku
(2) Reid, W. G. (a) US. Patent 3,389,160,1968; (b) U.S. Patent 3,658,867,1972; (c) US.Patent 3,836,559,1974; (d) US.Patent 4,059,581, 1977. (3) Rosen, M. R.;Prokai, B. U.S.Patent 3,677,347,1972. (4) Kanner, B.;Pike, R. A.; Morehow, E. L. (a)U.S. Patent 3,507,897, 1970;(b) US.Patent 3,513,183,1970;(c) U.S. Patent 3,531,417,1970; (d) US. Patent 3,531,507,1970;(e) U.S.Patent 3,660,452,1972; (0 U S . Patent 3,707,492,1972;(g) U.S. Patent 3,997,580,1976. 0 1990 American Chemical Society
Snow et al.
386 Langmuir, Vol. 6, No. 2, 1990 Table I. Structures and Abbreviations of Silicone Sulfobetaine Surfactants compd structure abbreviation
(Me,SiO),Si(Me)-(CH,),N+Me,(CH,),SO,(Me,SiO),Si(Me)-(CH,),N+Me,(CH,),SO,(Me,SiO),Si(Me)-(CH,),N+Et&Hz)3S03-
(Me3~O),Si(Me)-(CH,),N+Me((CH2),OH) ((CH,),SO,)Me,SiO(SiMe,O) (SiMeO)SiMe,
L
MD(R)M(I) MD(R)M(II) MD(R)M(III) MD(R)M(IV) MDD(R)M(I)
( H,),N+Me,(CH,),SO,Me,SiO(SiMe,O),( iMeO)SiMe,
r
designation I I1 I11 IV V VI
(CH,),N+Me,(CH,),SO,-
Me,SiO(SiMe,O),(SiMeO)SiMe,
VI1
Me,SiO( iMeO),SiMe,
VI11
Me,SiO(SiMe,O)(SiMeO),SiMe,
IX
I (CH2)3N+Me,(CH,)3S0,-
f; (CH,)3N+Me2(CH2)3S03I
structure MM MWH) MD(H)D(H)M MDD(H)M MDD(H)D(H)M MDDD(H)M MDDDD(H)M
MW 162 148 282 296 356 370 416
Table 11. PreeDaration of Si-H Functional Siloxanes w t % reactants 90 H,wt DC-1107 cyclics MDM MM 0.64 0.45 54.7 0.70 45.3 0.5 77.8 0.34 21.6 1.2 63.1 0.56 35.7 20.5 62.3 0.27 17.2 33.4 51.9 0.23 14.6
Schmidt Co.' and the Beecham Group.' These surfactants were prepared by the quaternization of an organic amine-substituted poly(dimethylsi1oxane) with salts of haloacetic acids. Surface activity data were not reported. In related studies, zwitterionic organofunctional silicones were prepared by the quaternization of organic amine functional silicones with the cyclic compound 1,3propanesultone.' We chose to explore both of these synthetic routes to zwitterionic silicone surfactants. In this paper, we report the preparation of relatively low molecular weight (oligomeric) zwitterionic silicone surfactants via the sulfopropylation and sulfobutylation reactions. Such products are classified as silicone sulfobetaines. A later paper will deal with relatively low molecular weight zwitterionic organofunctional silicone surfactants formed from the quaternization of organic amine functional silicones with haloacetic acids. These we refer to as the silicone betaines.
Materials and Methods I. Preparation of Silicone Sulfobetaine Surfactants. Nine surfactants were prepared and analyzed. Table I lists the complete and abbreviated structures of these compounds. The abbreviations use the common notation for silicone polymers (M = Me,SiO, D = Me,SiO, D(R) = MeRSiO, where R is different than methyl). The first four structures in the table are all trisiloxanes containing different functional groups attached to the inner silicon atoms. All of the other silicone species contain the same R group as MD(R)M(I). a. Overall Synthesis Procedure. The zwitterionic silicones were generally prepared by a three-step procedure. In most cases (except for III), a siloxane containing one or more (5) Kollmeier, H.; Langenhagen, R.; Hoffman, K. (a) DE Patent 3,417,912 C1,1985; (b) DE Patent 3,422,268 C1,1985. (6) Wilkins, A.; Heather, E.; Quinten, R. GB Patent 2,161,172A, 1986. (7) Graiver, D.; Baer, E.; Litt, M.; Baney, R. J . Polym. Sci., Polym. Chem. Ed. 1979,17,3559.
product bp, mmHg/OC
