868
Langmuir 1991, 7, 868-871
Zwitterionic Organofunctional Siloxanes as Aqueous Surfactants: Synthesis and Characterization of Betaine Functional Siloxanes and Their Comparison to Sulfobetaine Functional Siloxanes Steven A. Snow,' William N. Fenton, and Michael J. Owen Research Department, Dow Corning Corporation, Mail Stop C042A1, Midland, Michigan 48686-0994 Received May 24,1990. In Final Form: August 24, 1990 Novel zwitterionic organofunctional siloxane aqueous surfactants, composed of a short siloxane chain hydrophobic portion and an organobetaine hydrophilic portion, are prepared by the quaternization of precursor tertiary amine functional siloxanes with iodoalkylcarboxylate salts. These surfactants are of the general formula (Me3SiO)(SiMe2O),(SiMeRO)SiMe3(R = (CH2)3+NMe2(CH2)bCOO-;a = 0,l; b = 1, 2). Purified samples reduce the surface tension of water to a minimum of 21 mN/m. The surface areas of coverage of the a = 0 species ( b = 1,70A2; b = 2,85 A2) are similar to the related sulfobetaine functional species. The surface area of coverage of the a = 1species is unexpectedly small, although a similar result was noted for the analogous sulfobetainespecies. Both molecular size and surfactant concentration directly correlate with the dynamic surface tensions of dilute aqueous solutions of these surfactants. These surfactants rapidly decompose in both acidic and basic aqueous solutions, due to the hydrolysis of siloxane bonds. The aqueous solution mixtures of (MegSiO)2Si(Me)-(CH2)3+NMe2CH2COOand ZnClp form addition compounds, possibly due to the complexation of the Zn(I1) metal ion through the carboxylate groups of the surfactant.
Introduction Recently we reported the synthesis and characterization of novel zwitterionic siloxane sulfobetaine surfactants.' These surfactants were of relatively low molecular weight and reduced the surface tension of water to a minimum value of approximately 21 mN/m. The species (MeaSiO)2Si(Me)-R1 (where R = -(CH2)3+N(R2)2(CHz),S03-; R2 = Me, Et, -(CH2)20H; r = 3, 4) also displayed very rapid reduction of aqueous surface tension. These results led us to consider the analogous zwitterionic organobetaine functional siloxanes, of the general molecular formula Me3SiO(SiMe2O),(SiMeRO)SiMe3 (where R = -(CH2)3+NMe2(CH2)bCOO-; a = 0, 1; b = 1, 2). The synthesis of organobetaine functional siloxanes and their use in detergent compositions had been previously claimed by Heckert and Watt of the Proctor and Gamble Company.2 Furthermore, high molecular weight organobetaine functional siloxanes and uses thereof have been patented by workers from Th. Goldschmidt AG3 and The Beecham Groupa4These patents claim the utility of these compounds in cleansing and conditioning formulations. T o investigate the aqueous surface activity of the small organobetaine functional siloxanes, the series described above wm synthesized. We systematically studied the surface activity of these compounds by measuring the equilibrium and dynamic surface tensions of their dilute aqueous solutions. Contrasts and comparisons of these structures and properties were made with the previously reported sulfobetaine functional 5pecies.l Experimental Section I. Synthesis and Characterization of Organobetaine Functional Siloxanes. a. Preparation of Amine Functional Siloxane Intermediates. The zwitterionic siloxanes were generally prepared by a three-step procedure. The first two steps, (1)Snow, S. A,; Fenton, W. N.; Owen, M. J. Langmuir 1990,6 (2), 385-391. (2)Heckert, D.C.; Watt, D. M. U.S.Patent 4,006,176,1977. (3)(a) Kollmeier, H.J.; Langenhagen, R. D.; Hoffman, K. U.S.Patent 4,609,760,1986, (b) U.S.Patent 4,654,161,1987. (4)Wilkins, A.; Heather, E.; Quinten, R. GB Patent 2,161,172A,1986.
0743-7463/91/2407-0868$02.50/0
involvingthe preparation of Si-H functionalsiloxanes and their subsequent conversion to tertiary amine functional siloxanes, are described in detail in ref 1. b. Preparation of Organobetaine Functional Species. The appropriate tertiary amine functional siloxane and either ICHzCOONa or I(CH2)2COONawere mixed in a 1:l molar ratio with an equal weight of dry 2-propanol solvent under nitrogen atmosphere. The mixtures were heated to atmospheric reflux temperature for 24 h. The volatile components were then evaporated from the mixture in vacuo to a maximum condition of 100"C, 2 mmHg pressure. The residues were then redissolved in a 50/50 (v/v) mixture of methanol and 2-propanol (2.5/1.0 weight ratio of solvent to residue),the mixture was filtered, and the solvent was evaporated in vacuo. The residue solid was then washed with toluene and acetone, redissolved in 2-propanol, reprecipitated by addition of acetone, and then dried in vacuo at 100 O C for 3 h (recovered yield typically of 40%). Composition was confirmed by lH nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies (see below). 11. Characterization of the Betaine Functional Siloxanes and Aqueous Surface Activity Measurements. IR spectra for the compounds were acquired on a Beckmann IR 4240 spectrophotometer. The solid compoundswero mulled into Nujol for the spectralacquisition. lH NMR spectrawere acquired on a Varian T-60NMR spectrometer operating in a continuous wave mode. Samples were prepared in 5 mm 0.d. tubes in the presence of deuterated solvents for the spectral acquisition. The 1H NMR spectral assignments were as follows: +0.1 ppm HsCSi, +0.4 ppm H2C-Si, +1.6 ppm H&, +3.1 Hac-N, +3.35 and +3.6 ppm H2C-N. IR assignments were very similar to those recorded for the analogoussulfobetainespeciesand are described in ref 1. The C=O stretching band of the carboxylate group was unique and characteristic (1635 cm-l). Aqueous solution equilibrium surface tension values were obtained by the Wilhelmy plate method using a Rosano surface tensiometer or by a du Nuoy ring attached to an automated Cahn balance. Aqueous solution dynamic surface tension measurements were carried out using an adaptation of the maximum bubble pressure method first suggested by Janule of ChemDyne Research Corp.6 It involves the use of a computer interfaced and controlled differential bubble pressure tensiometer. The (5)Janule, V. Process Analyeis and Control Using Surface Tension Measurement; The Pittsburgh Conference on Analytical Spectroecopy, 1983.
0 1991 American Chemical Society
Zwitterionic Organofunctional Siloxanes
Langmuir, Vol. 7, No. 5, 1991 869
is not hygroscopic. The compounds are slightly to moderately soluble in low molecular weight alcohols such as methanol, ethanol, and 2-propanol. They are slightly soluble in water. They display slight to moderate solubility in low molecular weight halocarbon and ethereal solvents. All are insoluble in saturated, nonfunctional hydrocarbon solvents. These compounds are structurally characterized by their Results and Discussion 'H NMR and IR spectra. The details of these spectral I. Synthesis and Isolation of the Organobetaine characterizations are included in the Experimental Section. Functional Siloxanes. The following organobetaine In the 'H NMR of the compounds, a characteristic functional siloxanes were prepared, also shown is an absorption for the compound is a singlet a t 3.2 ppm. This abbreviated notation of the structures: (Me3SiO)zsinglet corresponds to the presence of the two equivalent Si(Me)-(CH2)3+NMez(CH2)COO-, MD(R)M(l); (Me3methyl groups on the quaternary nitrogen center. The SiO)(Me3SiO)(MeSiRO)SiMe3,MDD(R)M(l) (R = quaternization of the nitrogen atoms of the -(MeSi(CH2)3+NMe&H&OO-; (MesSi0)2SiMe-(CHz)3+NMe2- ((CH&NMe2)0)- groups with the iodoalkylcarboxylate (CH2)2COO-, MD(R)M(2). reagent results in an approximately 0.8 ppm downfield These compounds were prepared by the following series shift of this absorption. Other spectral assignments are of reactions: straightforward. These spectra are appropriately very to that of the related zwitterionic sulfobetaine similar Me3SiO(SiMe,0),(SiMeHO)SiMe3 + functional species.' In the IR spectrum of the betaine H&C4 functional compounds, the absorption band a t 1635 cm-' CH2=CHCH2NMe2 is very characteristic of the C=O stretching mode of a Me3SiO(SiMe,0)a(SiMe((CH,)3NMe2)O)SiMe3 (1) carboxylate groupe8 The other bands in the spectra have straightforward assignments and are compared to those in two key reference^.^*^ The IR spectra for the organoMe,SiO(SiMe,0)a(SiMe((CH2)3NMe,)O)SiMe, + betaine functional siloxanes are, as expected, very similar I(CH,)bCOO-Na+ to those of their sulfobetaine analogues. Me,SiO(SiMe,O),(SiMe((CH,),+NMe,Despite many attempts, highly accurate elemental analysis data could not be obtained for these compounds. (CH2)bCOO-)O)SiMe3+ NaI a = 0,1;b = 1 , 2 (2) This difficulty, could, in part, be due to slight degradation of these compounds, possibly aggravated by the presence The preparation of the tertiary amine functional siloxof trace levels of acidic, basic, and/or ionic impurities. anes, as described in eq 1above,utilized platinum catalyzed The absence of significant impurity absorptions in the lH hydrosilylation chemistry.' Generally, these hydrosilyNMR and IR spectra of these compounds, after isolation lations were run under solventless conditions in the temand purification, was therefore used as our criterion of perature range of 80-120 OC. The product amines were purity and correct structure. These spectra were acquired isolated and purified by fractional distillation. after very careful handling of the compounds. The quaternization of these amines by the iodofunc11. Aqueous Solution Stability and Surface Actional alkylcarboxylate salts is demonstrated in eq 2 above. tivity Studies. The organobetaine functional siloxanes The quaternization reactions proceeded in dry 2-propanol were shelf stable for at least a month under inert in the temperature range 60-110 OC. Attempted quatatmosphere. When exposed to moist air, tacky solids were ernization using the analogous chloro- or bromo-functional formed. Spectroscopic characterization and surface acalkylcarboxylates resulted in complex mixtures of prodtivity measurements indicated that prolonged (>3months) ucts. The isolation and purification of the zwitterionic exposure to atmospheric moisture resulted in slow deorganobetaine functional siloxane products, even when composition of the compounds. Some of the decompothe iodofunctional carboxylate salts were used as starting sition products were soluble in hexane. reagents, was difficult. The compound MD(R)M(l) was tested for its aqueous This difficulty can be attributed to several factors. solution stability. At room temperature, after 1 day of Firstly, there were numerous impurities in the reaction standing in a 0.1 w t % solution, the compound partially mixtures, both organic and inorganic, with similar saltlike decomposed, resulting in a 10 mN/m increase in surface molecular structures to that of the betaines. Secondly, tension. As demonstrated by the data below, heating of exposure of the product mixtures to alcohols or water w t 9% MD(R)M(l)in aqueous solution resulted in more 0.1 resulted, in some cases, in the formation of tacky solids. rapid decomposition: Similar difficulties with the isolation and purification of hydrocarbon-based betaines have been noted by Ernst 100 "C (3 h) 100 OC (21 h) 23 "C 100 "C (1 h) and Miller.' Furthermore, with some samples, these 58.9 mN/m 63.1 m N / m 31.0 mN/m 39.5 mN/m compound mixtures visually degraded during heating above 40 OC. Hypothetically, the quaternary nitrogen As judged by surface tension measurements, MD(R)M(l) centers of the betaine moieties could catalyze siloxane rapidly decomposes within minutes in acidic (pH 9) aqueous solutions. Decomposition of sipresence of trace amounts of water. loxane-based surfactants in acidic or basic aqueous soThe organobetaine functional siloxanes were isolated lutions has been attributed to the acid or base catalyzed as white or yellow powders. The surfactantsMD(R)M(l,B) hydrolysis of Si-0 bonds in the siloxane backbone.' are slightly hygroscopicsolids. The species MDD(R)M(l) Measurement of the equilibrium surface tension of these surfactantsusing the Wilhelmy plate method was, at times, (6) Petroff, L. Dynamic Surface Tension as a Criteria for the Char-
construction of this apparatus and details of ita application have been recently reported by Petroff and co-workers.6 Briefly, the surface tension values of the solution are obtained by measurement of the internal pressure of nitrogen bubbles which are continuously blown into the solution, underneath the solution surface. The rate of the bubbling can be controllably varied from rates less than 1 Hz to rapid bubble evolution (>5 Hz).
-
acterization of Surfactant Performance; 67th Meeting of the Textile Research Institute; Charlotte, NC; 1987. (7) Ernst, R.; Miller, E. J. In Amphoteric Surfactants; Surfactant Science Series;Bluestein, B. R., Hilton, C. L. Me.; Marcel Dekker, Inc.: New York, Vol. 12, Chapter 2, p 112.
(8) Silveratein,R.; Bawler, G.; Morrill, T. SpectrometricIdentif~ation of Organic Compounds, 3rd ed.; Wiley and Sone, New York, 1974. (9) Anderson, D. R. In Analysis of Silicones; Smith, A. L., Ed.;R. E. Kriager Publishing Co.: Malabar, FL, 1983; Chapter 10.
Snow et al.
870 Langmuir, Vol. 7, No. 5, 1991 60
-.
LEGEND MD(R)M(i)
MD(RIM(21
Surface Tension
40
-
30
-
(mN/m)
20. -3
- 2
- 1
-0.3
0
1
Log Surfactant Concentration (Wt. %)
Figure 1. Equilibrium surface tensions of aqueous solutions of the organobetaine functional siloxanes plotted versus concentration. Table I. Equilibrium Surface Tension Values of Organobetaine Functional Silicone Surfactant Solutions concn, wt % 0.001 0.01 0.1 0.5 1.0
(mN/m) MD(R)M(l) MD(R)M(2) 57.5 54.4 44.9 43.6 31.0 28.5 21.5 21.8 21.2 21.0
MDD(R)M(I) 45.4 26.7 22.0 21.4 21.2
Table 11. Comparison of the Cmc and Surface Area/ Molecule of Various Zwitterionic Silicone Surfactants, Consideration of the Silicone Backbone structure X cmc, wt % surface area/molecule,A2 MD(R1)M 1 0.6 70 MD(R1)M 2 0.6 85 75 MD(R2)M 3 0.3 50 MDD(R1)M 1 0.08 MDD(R2)M 3 0.03 70 MDzD(R2)M 3 0.08 >IO0 R1 = -(CH2)3N+Me2(CH2)zC00R2 = -(CH2)sNtMe2(CH2),SOs-
Table 111. Comparison of the Cmc and Surface Area/ Molecule of Various Zwitterionic Silicone Surfactants. Consideration of the Zwitterionic Grouu surface area/ structure cmc, w t % molecule, A2 R-+NMez(CH2)3SO30.3 75 R-+NMez(CH2)&3030.4 75
R-+NEtz(CH2)3SOaR-+NMe((CH2)20H)(CH2)3S03R-+NMeZCHzCOOR-+NMez(CHz)ZCOO-
0.2 1.1 0.6 0.6
65 75 70 85
R = (Me3SiO)gSi(Me)(CH2)3 hampered by irregular wetting of the platinum blade. This irregular wetting suggested the strong adsorption of the surfactant on the blade. Dewetting occurred when the du Nuoy ring apparatus was used to measure the surface tension of dilute aqueous solutions of MD(R)M(l). Possibly the carboxylate group on the betaine functionality bonded to the platinum metal of the blade and ring. Figure 1and Table I display values for the equilibrium surface tensions of dilute aqueous solutions of the organobetaine functional siloxanes. The values for the critical micelle concentrations (cmc) and surface areas of coverage of these Surfactants are shown in Tables I1 and 111. These values were obtained from analysis of the plots of surface tension versus concentration of the surfactants in dilute aqueous solution. They are compared to those reported
for the analogous sulfobetaine species.' The cmc values are obtained from extrapolation of the minimum surfactant concentration necessary to achieve minimum aqueous surface tension (approximately 21 mN/m). Formation of micelles is implied by these measurements; however, further experimentation is necessary to prove their presence. The values for the surface areas of coverage are derived from the slopes of the plots and subsequent application of the Gibbs adsorption equation.1° From Figure 1and Table I it is evident that the organobetaine functionalsiloxanes are highly effective aqueous surfactants, reducing the surface tension of water down to approximately 21 mN/m. This value is equal to that measured for a wide variety of siloxane-based surfactants' and is comparable to the surface tension of high molecular weight homopolymeric poly(dimethylsi1oxane)(20.6 mN/m at 25 "C). The low surface tension of poly(dimethylsiloxane) has been attributed to both the preponderance of highly surface active methyl substituents and a flexible polymer backbone which allows the methyl groups to orient in low-energy configurations." The surface tension values of these surfactants suggest that the siloxane portion lies flat on the water surface, exposing the methyl groups to the air in an *umbrella" conformation.' This molecular model of adsorption a t the air/water interface is consistent with the calculated surface areas of adsorption listed in Tables I1 and I11 for both the betaine and sulfobetaine species. Throughout the MD(R)M species, there are no dramatic differences in surface area, just small differences resulting from altering the hydrophilic groups. The reduction of surface area when the siloxane backbone is increased from MD(R)M to MDD(R)M is difficult to explain, although it is consistent for both the betaine and sulfobetaine species. Unfortunately, we were not able to isolate samples of the MD2D(R)M betaine species, although it is evident that the sulfobetaine analogue has a much larger surface area (>lo0 A2vs 70-80 A2) than the MD(R)M species. There is a complex effect of siloxane structure on the cmc. The data suggest that increasing the length of the hydrophobic siloxane moiety from three to four decreases the cmc. The extraMe2SiO unit in the MDD(R)M species should decrease the solubility of the surfactant and thus increase its efficiency of micelle formation. However, the unexpected increase in the cmc, when the siloxane chain is increased from four to five units (going from MDD(R)M to MD2D(R)M), has no straightforward explanation. It may be that expectations derived from hydrocarbon surfactant systems do not apply to silicone surfactants. Certainly Kanner has reported this same phenomenon with certain polysilicone/polyether copolymer surfactants.I2 Much more data on diverse silicone surfactants are needed, to know what the full structure/property relationship is. We also studied the effects of various metal ion salts on the surface tension of 0.1 w t 5% MD(R)M solutions, for both the betaine and sulfobetaine functional species. As seen in Table IV, the various salts, except for ZnCl2, tended to lower the surface tension of the MD(R)M(betaine) solutions, particularly at higher salt concentrations. The anomalous increase in surface tension of the ZnClz solutions can be explained by postulating that the Zn(I1) metal ion is acidic enough to interact strongly with the carboxylate groups in the betaine moiety. This complex(10)Rosen,MiltonJ. Surfactants and InterfacialPhenomena;WileyInterscience: New York, 1978; pp 56-60. (11)Owen, M. J. Ind. Eng. Chem. R o d . Res. Dev. 1980, 19, 67. (12)Kanner, B.; Reid, W. G.; Petersen, I. H. Ind. Eng. Chem. Prod. Res. Dev. 1967, 6, 88.
Langmuir, Vol. 7, No. 5, 1991 871
Zwitterionic Organofunctional Siloxanes Table IV. Equilibrium Surface Tensions of 0.1 wt % MD(R1)M Solutions Containing Ionic Additives (mN/M) 0.0 wt % 0.25 wt % 2.5 wt % 5.0 wt % surfactant MD(R1)M MD(R1)M MD(R1)M MD(R1)M MD(R1)M
additive additive 30.5 NaCl NaI 30.5 NazSO4 30.5 CaClz 30.5 ZnClz 30.5
MD(R2)M MD(R2)M MD(R2)M MD(R2)M MD(R2)M
NaCl NaI NazSO4 CaClz ZnClz
26.0 26.0 26.0 26.0 26.0
additive 29.4 29.6 29.6 29.4 31.9
additive 28.1 27.7 26.7 28.0 33.5
additive 25.5 25.9 25.9 26.3 35.2
26.9 24.3 25.3 25.4 26.1
25.3 21.5 23.5 24.6 23.7
23.9 21.0 21.1 23.4 23.3
Table V. Comparison of the Dynamic Surface Tension Values for a Series of Zwitterionic Silicone Surfactants at 0.06 wt 9% Concentration (mN/M) structure MD(R1)M MD (R2)M MD(R3)M MDD(R1)M MDD(R2)M MDD(R2)M
1Hz 36.3 36.7 32.7 47.3 64.5 70.5
surface tension at bubble rate 2Hz 3Hz 4Hz 42.3 46.8 48.0 36.8 36.9 37.0 33.4 33.1 33.5 53.4 59.0 62.2 67.6 68.1 68.2 70.6 70.6 70.7
5Hz 49.5 37.1 33.6 65.0 68.3 70.7
R 1 = -(CHz)s+NMezCH2COOR2 = - ( C H Z ) ~ + N M ~ ~ ( C H ~ ) ~ S O ~ R3 = - ( C H Z ) ~ + N E ~ ~ ( C H ~ ) ~ S O ~ -
R 1 = -(CH2)s+NMe&HzCOOdynamic surface tension. Similar results were noted in R2 = -(CH~)~+NM~((CHZ)ZOH)((CH*)~SO~-)
LEGEND MD(RIM(1)Concentratlon 0.05% 0.1% .--_--I MDDIRIM(1) Concentration 0.05% -)cL
-
our previous study with the siloxane sulfobetaine surfactants.' The explanation for these increases in surface tension is that the rate of diffusion of the surfactant through the bulk aqueous solution to the surface strongly affects the concentration of surfactant at the surface and thus the surface tension. Increasing the surfactant size decreases the rate of diffusion, thus, temporarilydecreasing the surfactant concentration at the surface and increasing the dynamic surface tension. Comparisonsof dynamic surface tension values between the betaine and related sulfobetaine species are shown in Table V. Again, there is a direct correlation between the size of the siloxane hydrophobicgroup and dynamic surface tension. More subtle differences between the molecular species could not be explained.
Summary Novel zwitterionic organofunctional siloxane aqueous surfactants, composed of a short siloxane chain hydrophobic portion and an organobetaine hydrophilic portion, are prepared by the quaternization of precursor tertiary amine functional siloxanes with iodoalkylcarboxylate salts. These surfactants are of the general formula (MesSi0)2 3 1 5 (SiM&),(SiMeRO)SiMe (R = (CH2)3+NMe&H2)&00-; Bubble Rate (HI) a = 0, 1; b = 1, 2). The isolation of pure compounds is Figure 2. Surface tensions of aqueous solutions of the orgahampered by their apparent thermal instability above 40 nobetaine functional siloxanes plotted versus bubble rate. "C and the presence of ionic impurities. Equilibrium and ation removes the surfactant from the air/water interface. dynamicsurface tensions were measured on dilute aqueous Formation of a white precipitate was also noted in some solutions of these compounds,and the data were compared of the MD(R)M(1)/ZnCl2 aqueous solutions. This conto the analogous sulfobetaine functional species. The clusion is further supported by an investigation of the MD(R)M(1,2)betaine species have cmc values and surface effects of these salts on aqueous solutions of the sulfoareas similar to the analogous sulfobetaine species. Furbetaine species (Me3Si0)2Si(Me)-(CHz)3+NMe- thermore, the MDD(R)M species, both betaine and sul((CH2)2)OH)((CH2)3503-)(Table IV). Addition of these fobetaine, have unexpectedlysmall surface areas, although salts, including ZnCl2, to the aqueous solutions of this they demonstrated the expected high dynamic surface surfactant resulted in a slight depreasionof surface tension tension. The unexpectedly high cmc value for the a t high salt concentration. Precipitation was not noted MD2D(R)Msulfobetaine compound may be due to similar in these solutions. A strong, specific interaction of the structural features as previously reported for nonionic sisurfactant (in particular the sulfonate group) with the loxane polyether copolymer surfactants. These organoZn(I1) ion is not evident from this measurement. Furbetaine functional siloxane surfactantsdecomposea t slow thermore, since the rise in surface tension is unique to the to moderate rates when heated or exposed to water. They betaine/ZnC12 mixtures, this eliminates the hypothesis rapidly decompose in both acidic and basic aqueous that the rise in surface tension is due to a specific Zn(I1)solutions, due to the hydrolysis of the siloxane bonds in catalyzed degradation (hydrolysis) of the siloxane portion the molecules. The aqueous solution mixtures of the of the surfactant. MD(R)M(l) betaine species and ZnClz appeared to form The effects of surfactant concentration and molecular addition compounds, probably through a complexation of structure on dynamic surface tension for the series of orthe Zn(1I) ion with the carboxylate groups of the betaine ganobetaine functional siloxane surfactants are shown in surfactant. Figure 2. For MD(R)M(l), decreasing the surfactant concentration from 0.1 to 0.05 wt % significantly increases Acknowledgment. The authors thank Ms. Valarie the dynamic surface tension. The effect of the increase Banks and Ms. Jill Revard for assistance in obtaining the in hydrophobic group (siloxane) size on the dynamic surface activity measurements. Also, we thank Dr. Tom surface tension is also shown in Figure 2. Going from Gentle and Mr. Len Petroff for many valuable discusMD(R)M(l) to MDD(R)M(l) significantly increases the sions.
