Polycarbinol-siloxane surfactants: synthesis, characterization, fire suppression evaluation and foaming-defoaming properties

2.1 Materials

Reagent chemicals were used as received, unless indicated otherwise, and obtained from the following sources: Sigma-Aldrich: ethanolamine, diethanolamine, N-methyl-d-glucamine, diethylene glycol monobutyl ether (DGBE); Fisher Chemical: tris(hydroxymethyl)aminomethane, chloroplatinic acid hexahydrate; Alfa Aesar: tris(trimethylsiloxy)silane (NMTS), glucono-1,5-lactone; TCI America: allyl glycidyl ether, 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS); Gelest: 3-aminopropylmethyl bis(trimethylsiloxy) silane; platinum-divinyl tetramethyl disiloxane complex; BASF: Glucopon^®225DK.

Solvents were used as received, unless indicated otherwise, and obtained from the following sources: Sigma-Aldrich: methanol (MeOH), 2-propanol (2-PrOH), diethyl ether (Et₂O), tetrahydrofuran (THF), chloroform.

NMR solvents and reference compounds obtained from the following sources: Acros Organics: CD₃OD, CDCl₃, CD₃COCD₃; Cambridge Isotope Laboratories: D₂O, Si(CH₃)₄.

2.2 Instruments

Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Advance 300 NMR system operating at 300 MHz using 5 mm-diameter cells. Chemical shifts are referenced to Si(CH₃)₄. Ultraviolet–visible (UV–Vis) spectra were obtained using a Thermo Spectronic UNICAM UV 500 spectrometer. Surface tension measurements of surfactants at the air-water interface were conducted with a platinum Wilhelmy plate (dimensions: 0.038 cm thickness × 0.988 cm width) connected to a Nima PS4 surface pressure sensor calibrated with a 100.0 mg weight. Ross-Miles foaming ability was measured according to ASTM D1173-53 with a Wilmad LabGlass apparatus (LG-3941-100) with an attached ThermoElectron NESLAB RTE7 water circulator bath having Digital One temperature control.

2.3 Synthesis

Procedures for the preparation of polycarbinol-siloxane surfactants described below are adaptations from those described in references cited for specific syntheses. In our previous experience, the hydrosilylation catalyst selected is of particular importance and has involved a choice between the Speier’s catalyst (H₂PtCl₆·6H₂O 1.0 wt% in 2-propanol) and the Karstedt’s catalyst (platinum-divinyl tetramethyl disiloxane complex 5.0 wt% in xylenes). ¹² The former has a low activity relative to that of the latter. ^{24 , 25} Their selection is correlated with the structure/reactivity of the reagents with the Speier’s catalyst used with the trisiloxane and allyl ether reagents and the Karstedt’s catalyst used with the tetrasiloxane and allyl amine reagents. ¹H and ²⁹Si NMR spectra are presented Supplementary Material Figures S-1–S-4.

1,1,1,3,5,5,5-heptamethyl-3-[3-(2-oxiranylmethoxy)propyl]-trisiloxane (HMTS-Pr-Glycidyl). This synthesis is adapted from Wagner et al. ^{13 , 26} The reagents allyl glycidyl ether 11.414 g (100 mM) and HMTS 22.251 g (117 mM) were weighed into a 3-neck 100 mL round-bottom flask then fitted with thermometer, magnetic stirring bar, condenser and N₂ inlet. After mixing, a ¹H NMR spectrum was recorded and 0.064 g Speier’s catalyst were added. This reaction was stirred at room temperature for 1 h and then gradually heated to 65 °C over a period of 3.5 h. Periodic NMR monitoring of (15–20) mg reaction mixture samples in CDCl₃ showed the decline and disappearance of the reagent resonances (Si–H, 4.65 ppm and =C–H, (5.2, 5.3 and 5.9) ppm), the growth of the three trimethylene product resonances ((0.50, 1.6 and 3.5) ppm). The product was isolated by vacuum distillation (120 °C/<1 mm). Yield 30.189 g (89.7 %). ¹H NMR: (CDCl₃, 298 K, 300 MHz) δ(ppm): 0.018 (s, 3H, –OSi(CH₃)O); 0.091 (s, 18H, ((CH₃)₃SiO)₂); 0.46 (m, 2H, –Si–CH₂–CH₂); 1.62 (m, 2H, SiCH₂–CH₂–CH₂–); 2.61 (m, 1H, oxirane–O–CH₂–CH); 2.80 (m, 1H, oxirane–O–CH₂–CH); 3.16 (m, 1H, oxirane–O–CH–CH₂–); 3.42 (m, 2H, –O–CH₂–CH₂–); 3.46 (d, 1H, glycidyl–O–CH₂–CH–); 3.70 (d, 1H, glycidyl–O–CH₂–CH–). Spectrum with resonance assignments is presented in Supplementary Material Figure S-1(b).

1,1,1,3,3,3,5,5,5-nonamethyl-3-[3-(2-oxiranylmethoxy)propyl]-tetrasiloxane (NMTS-Pr-Glycidyl). This synthesis is also adapted from Wagner et al. ^{13 , 26} using the more active Karstedt’s catalyst. ²⁴ The reagents allyl glycidyl ether 5.707 g (50 mM) and NMTS 14.833 g (50 mM) were weighed into a 3-neck 50 mL round-bottom flask then fitted with thermometer, magnetic stirring bar, condenser and N₂ inlet. After mixing, a ¹H NMR spectrum was recorded, and 0.036 g Karstedt’s catalyst were added. This reaction was stirred at 38 °C for 1 h then gradually heated to 50 °C over a period of 1 h. The reaction was monitored by periodic NMR spectra (allyl glycidyl ether allyl (5.2, 5.3 and 5.9) ppm and NMTS Si–H, 4.23 ppm resonances) over the next 25 h. It was observed after 12 h that the Si–H resonance intensity did not decrease proportionately to the allyl resonances and that new weak allyl resonances were slightly increasing in intensity after 2 more hours. The reaction mixture also changed from colorless/clear to dark amber/clear. This is consistent with an isomerization of the C=C double bond from the terminal H₂C=CHCH₂O– position to the internal CH₃CH=CHO– position in the allyl glycidyl ether structure. Additions of 1.05 g and 0.50 g NMTS were made at the respective 14 h and 20 h reaction times, and the intensities of the Si–H and allyl glycidyl ether reagent allyl 5.2, 5.3 and 5.9 monitored for disappearance at the 25 h reaction time. The reaction mixture was distilled at various levels of vacuum, yielding three colorless clear fractions (Fraction #1 (40–63) °C/2 mm, 0.97 g; Fraction #2 (65–110) °C/<1 mm, 0.91 g; Fraction #3 (110–120) °C/<1 mm, 18.42 g) and a strongly colored distilland (0.10 g). The NMR spectra of Fractions #1 and #2 are consistent with isomerization of the double bond to the 2-position in allyl glycidyl ether, and the NMR spectrum of Fraction #3 is consistent with a purified NMTS-Pr-Glycidyl product with a yield of 18.42 g (89 %). ¹H NMR: (CDCl₃, 298 K, 300 MHz) δ(ppm): 0.10 (s, 27H, ((CH₃)₃SiO)₃Si–; 0.50 (m, 2H, –Si–CH₂–CH₂); 1.63 (m, 2H, SiCH₂–CH₂–CH₂–); 2.63 (m, 1H, oxirane–O–CH₂–CH); 2.80 (m, 1H, oxirane–O–CH₂–CH); 3.17 (m, 1H, oxirane–O–CH–CH₂–); 3.42 (m, 2H, –O–CH₂–CH₂–); 3.45 (d, 1H, glycidyl–O–CH₂–CH–); 3.70 (d, 1H, glycidyl–O–CH₂–CH–). Spectrum with resonance assignments is presented in Supplementary Material Figure S-1(c).

HMTS–(CH₂)₃–OCH₂CH(OH)CH₂–NH(CH₂)₂OH (Si3(OH)2). To a 3-neck 50 mL round bottom flask fitted with thermometer, magnetic stirring bar, condenser and N₂ inlet were added 6.00 g 2-PrOH and 0.908 g (14.85 mM) ethanolamine. The HMTS-Pr-Glycidyl 5.004 g (14.85 mM) was added dropwise over a 20 min during which time a mild exotherm (38 °C) was observed and reaction mixture became clear. The reaction mixture was stirred 16 h at 32 °C then 1 h at 70 °C. The product was isolated by removal of 2-PrOH via rotary evaporation followed by vacuum desiccator drying (<1 mm/12 h) to yield 5.744 g (97 %) of a colorless clear liquid. ¹H and ²⁹Si NMR spectra with resonance assignments are presented in Supplementary Material Figures S-2(a) and S-4.

HMTS–(CH₂)₃–OCH₂CH(OH)CH₂–N(CH₂CH₂OH)₂ (Si3(OH)3). To a 3-neck 50 mL round bottom flask fitted with thermometer, magnetic stirring bar, condenser and N₂ inlet were added 10.00 g 2-PrOH and 1.509 g (14.35 mM) diethanolamine. The HMTS-Pr-Glycidyl 5.001 g (14.35 mM) was added dropwise over a period of 30 min during which time a mild exotherm (35 °C) was observed and the reaction mixture became clear. The reaction mixture was stirred at 32 °C for 2 h, then the reaction temperature was gradually increased to 75 °C over a period of 4 h. The product was isolated by removal of 2-PrOH via rotary evaporation followed by vacuum desiccator drying (<1 mm/12 h) to yield 6.43 g (98 %) of a colorless clear liquid. ¹H NMR and ²⁹Si NMR spectra with resonance assignments are presented in Supplementary Material Figures S-2(b) and S-4.

HMTS–(CH₂)₃–OCH₂CH(OH)CH₂–NC(CH₂OH)₃ (Si3(OH)4). This procedure is adapted from Racles. ²⁶ To a 3-neck 100 mL round bottom flask fitted with thermometer, magnetic stirring bar, condenser and N₂ inlet were added 20.00 g 2-PrOH, 20.00 g MeOH and 1.739 g (14.35 mM) tris(hydroxymethyl)aminomethane. The HMTS-Pr-Glycidyl 5.000 g (14.35 mM) was added dropwise over 20 min, during which time no exotherm was observed and reaction mixture became clear. The reaction temperature was gradually increased to 75 °C over a period of 1 h and remained at that temperature for 24 h during which time a faint clear yellow coloration developed. The MeOH and 2-PrOH were removed at 75 °C by slowly increasing the vacuum to < 1 mm and monitoring for constant mass which required 48 h. The isolated product was a clear faintly yellow colored gum with a 6.529 g (97 %). ¹H NMR and ²⁹Si NMR spectra with resonance assignments are presented in Supplementary Material Figures S-2(c) and S-4.

HMTS–(CH₂)₃–OCH₂CH(OH)CH₂–N(CH₃)CH₂(CHOH)₄CH₂OH (Si3(OH)6). This surfactant synthesis has been recently reported, but little purification detail is provided. ²¹ After several attempts, we found that the reaction of the two reagents in this synthesis, when conducted above room temperature, resulted in significant byproducts with altered Si–CH₃ NMR resonance patterns and that purification of the desired product based on solubility was needed. These issues are detailed in Supplementary Material, and our best procedure is described below. To a 50 mL Erlenmeyer flask containing 40 g MeOH were added 2.812 g (14.40 mM) N-methyl-d-glucamine with stirring followed by dropwise addition of HMTS-Pr-Glycidyl 5.022 g (14.41 mM) producing a fine opaque suspension. The reaction was stirred for 4 days at room temperature with visual and NMR observations. After 1 day the reaction mixture became a clear solution. After 4 days NMR of the reaction mixture indicated the glucamine reagent N–CH₃ resonance intensity was 15 % of its initial intensity with minor degradation observed in the Si–CH₃ trisiloxane resonance pattern (Supplementary Material). The reaction was stopped, MeOH solvent and other volatiles were removed by rotary evaporation followed by drying in an evacuated desiccator for 2 days to give a colorless gum crude product (7.56 g). This product was extracted by addition of 40 mL THF and gently stirred for 2 days. The THF solution was decanted, rotary evaporated and vacuum dried to yield 3.40 g (45 %). Comparison of NMR spectra of the THF-soluble with the THF-insoluble product indicated a significant purity improvement for the THF-soluble product as presented in Supplementary Material Figure S-3(a).

NMTS–(CH₂)₃–OCH₂CH(OH)CH₂–N(CH₃)CH₂(CHOH)₄CH₂OH (Si4(OH)6). Analogous to the Si3(OH)6 procedure, to a 25 mL Erlenmeyer flask containing 16 g MeOH were added 2.139 g (10.95 mM) N-methyl-d-glucamine with stirring followed by dropwise addition of NMTS-Pr-Glycidyl 4.501 g (10.95 mM). The fine opaque suspension was stirred for 1 day at 25 °C, then the temperature was elevated to 35 °C resulting in a clear colorless solution which was stirred for 3 more days at 35 °C. The ¹H NMR spectrum of the glucamine N–CH₃ resonance intensity in the reaction mixture indicated 80 % conversion with no degradation to the Si–CH₃ tetrasiloxane resonance pattern). The crude product was collected by rotary evaporation and vacuum drying to give 5.67 (81 %) of a sticky gum crude product. No degradation was observed in the Si–CH₃ tetrasiloxane resonance pattern. A small scale THF extraction purification was attempted, but the solubility is lower than that of Si3(OH)6, and no apparent NMR-based purity improvement was observed. ¹H NMR and ²⁹Si NMR spectra with resonance assignments are presented in Supplementary Material Figures S-3(b) and S-4.

HMTS–(CH₂)₃–NHC(O)(CHOH)₄CH₂OH (Si3(OH)5). This procedure is adapted from Guoyong et al. ²⁷ The as-received 3-aminopropylmethylbis (trimethylsiloxy)silane reagent was fractionally distilled (80 °C/<1 mm) immediately before use because NMR spectral analysis indicated that an impurity and/or a partial but significant degradation of the trisiloxane structure had occurred. 3.597 g (20.19 mM) glucono-1,5-lactone and 50 mL MeOH were added to a 100 mL round bottom 3-necked flask equipped with a thermometer, magnetic stirring bar, condenser and N₂ inlet. After dissolution, 5.000 g (20.19 mM) 3-aminopropylmethyl-bis(trimethylsiloxy)silane was added dropwise over 10 min. The reaction solution was then run at 65 °C for 12 h. The reaction was worked up by rotary evaporation of the CH₃OH, vacuum drying of the crude product, washing of the white solid with 10 mL Et₂O, filteration and vacuum drying to yield 8.419 g (94.5 %) of purified product. ¹H NMR and ²⁹Si NMR spectra with resonance assignments are presented in Supplementary Material Figures S-3(c) and S-4.

2.4 Surfactant characterization

Surfactant solution preparations for solubility, surface tension, fire extinguishing, and foaming experiments were made by first dissolving the desired amount of polycarbinol-siloxane surfactant in an equal weight of MeOH in a small vial and then adding this concentrate dropwise to the amount of water or formulation solution intended for the experiment. This procedure ensures rapid and uniform distribution in suspensions and complete dissolution in solutions for the intended measurements. For surfactants in the physical form of a gum or solid, this procedure appears to be necessary. Control experiments showed that the small amount of MeOH had no effect on the measurements.

Surfactant solubility measurements were made by visible spectroscopic absorption measurements on successive water dilutions to a hazy over-saturated solution of the surfactant. Typically, a surfactant suspension was prepared by adding analytical quantities of 0.1 g surfactant dissolved in 0.1 g MeOH to 10 g deionized water, ultrasonicating for 30 min, and stirring for 30 min. The light scattering of this suspension is measured from a 1 cm optical cell as absorbance in a UV–Vis spectrometer at 400 nm. This suspension is quantitatively diluted until the absorbance at 400 nm is less than 0.2. At that point, absorbance measurements are recorded at (400, 500, 600, 700 and 800) nm, and successive dilutions of this suspension accompanied by absorbance measurements are made such that four or five data points are obtained above and below the point where optical clarity is obtained. The solubility is determined by the intersection of a plot of these data from above and below this point.

Surface tension measurements of surfactants at the air-water interface were conducted with a platinum Wilhelmy plate (dimensions: 0.038 cm thickness x 0.988 cm width) connected to a Nima PS4 surface pressure sensor. Calibration was conducted with a series of platinum wire weights ranging from 19 mg to 147 mg and measurement accuracy verified with surface tension measurements on a series of 12 purified solvents over the range (12–73) mN m⁻¹ as previously described. ¹² Water (3x quartz distilled) with verified 72 mN m⁻¹ surface tension was used for surfactant solution preparation. Glassware used for solution preparations was acid-washed, rinsed with 3x quartz distilled water, and confirmed to have no surface released contamination via passing a 72 mN m⁻¹ measurement of distilled water in the container. Surfactant solutions for surface tension measurement were prepared by initial dissolution of weighed quantities of 30 mg–40 mg surfactant in 30 mg–40 mg MeOH followed by 40 g distilled water in the freshly tested container and 5 min stirring. Surface tension measurements were made by an initial 60 s wetting of the Wilhelmy plate followed by four successive measurements at 30 s and 60 s equilibration times with the average of the 60 s measurements being used in the data workup. Dilutions of the initial and successive solutions to approximately half the previous concentration for surface tension measurement were prepared by analytically weighing 15 g of the current measurement solution into a freshly cleaned and tested container followed by 15 g distilled water and 2 min stirring.

The alkyl polyglycoside surfactant Glucopon^® 225DK is a BASF commercial concentrate. Vendor information indicates this surfactant’s structure consists of a polyglycoside chain average of 1.7 units with an alkyl chain length range of 8–10 units, and its concentration in the concentrate is (68–72) wt% in water. ²⁸ Experimental measurement of surfactant concentration in this Glucopon^® 225DK sample is 65 wt% which was the value used for this surfactant’s CMC determination (Supplementary Material Section II).

Fuel fire suppression testing was conducted using a bench-scale apparatus and procedure described in earlier work ⁹ and is briefly described in Supplementary Material Section III. Surfactant foam formulations were prepared by combining the polycarbinol surfactant with Glucopon 225DK (0.30 wt%) and DGBE (0.50 wt%) in water. The quantities of the Si3(OH)2, Si3(OH)3, Si3(OH)4, Si3(OH)6, Si3(OH)5 and Si4(OH)6 surfactants in these foam formulations were wt% amounts corresponding to 6x of their respective CMC. Fire suppression testing was conducted with alcohol-free gasoline and heptane fuels.

Foaming ability was evaluated by the Ross-Miles method according to ASTM D1173-53 (Reapproved 2001) with a Wilmad LabGlass apparatus conforming to this standard’s specifications. The temperature of the surfactant solution and water-jacketed foam receiver column was maintained at 49 °C via the water circulator bath. Surfactant solutions (300 g quantity) were prepared at (1.00, 0.500 (or 0.300) and 0.100) g kg⁻¹ concentrations, stirred for 30 min then equilibrated at the 49 °C temperature of the column for 30 min. A 50.0 mL quantity of the surfactant solution is retained at the bottom of the receiver column and 200.0 mL is transferred to the pipet positioned at the top of the column. The pipet stopcock is opened, and a stream of the 200 mL solution drains and impacts at the center of the standing 50 mL solution at the base of the column. When the draining is complete, the height of the foam above the 50 mL mark is measured (foaming ability metric), and 5.0 min later the foam height above the 50 mL mark is again measured (foam stability metric).

3.1 Surfactant synthesis

The polycarbinol-siloxane surfactants depicted in Figure 1 have the attractive practical property that they can be prepared in two steps from readily available reagents and catalysts. The use of an allyl glycidyl ether in the preparation siloxane surfactants dates back to the 1970s and is very versatile. ^{8 , 13 , 21 , 25 , 26 , 27 , 29 , 30} Of particular importance is the avoidance of any degradation of the tri- and tetra-siloxane tail structures. This can occur by use of impure allyl-co-reagents particularly if such impurities have acidity or basicity and by use of elevated reaction temperatures. A particularly useful diagnostic is the ¹H NMR resonance pattern of the Si–CH₃ protons. In the Supplementary Material, Section I are recorded spectra for each surfactant. A monitoring of the reaction progress by proton resonances of the reagent oxirane and the amine proton resonances is also helpful. A challenging feature is the purification of the surfactants after reaction workup when needed as they are non-volatile and non-crystalline. A selective solvent extraction method involving Et₂O or THF is utilized. When the surfactant’s isolated physical form is a gum, its purification and handling can be challenging.

3.2 Surfactant characterization

The six surfactants depicted in Figure 1 were characterized by physical form, water solubility, and surface tension measurements with the results depicted in Table 1. In physical form, only Si3(OH)5 is a solid, while the others are liquids or gums. This may correlate with it having a shorter linking structure between the head and tail and with having a rigid amide structure instead of the more flexible amine in the head structure.

As mentioned above the physical form of the isolated surfactant is an important issue with regard to handling the surfactant and solution preparations. Gums tend to stick to glassware and spatula surfaces and are very slow to disperse/dissolve in water. The liquid and solid surfactants transfer and disperse much more readily but water dissolution is quite slow.

Surfactant solubility was determined by measured dropwise addition of the 1:1 surfactant:methanol concentrate to a rapidly stirred volume of water producing a hazy suspension, followed by analytical water additions until optically clear solutions were obtained. During this process scattered light is measured by absorbance/transmission measurements at (400, 500, 600, 700 and 800) nm. An absorbance versus concentration plot consists of a concentration-dependent light scattering region and a concentration-independent horizontal line at zero absorbance, with the intersection of the two regions determining surfactant solubility. This is illustrated in Figure 2a for the Si3(OH)2 surfactant. Siloxane surfactant micelles/aggregates are typically very large, with non-spherical disk-like shapes or large bilayers or vesicles. ³¹ As such, they are very effective in scattering visible light, with the shorter wavelengths of light being more highly scattered. In Figure 2b are comparative plots for the solubility of the surfactants showing a significant increase in solubility with increasing number of hydroxyl groups. It is also interesting to note that the transition from a trisiloxane to a tetrasiloxane tail group (i.e. Si3(OH)6 versus Si4(OH)6) causes an order of magnitude decrease of solubility.

Figure 2:

Surfactant solubility measurements: (a) solubility measurement of Si3(OH)2 surfactant via optical absorbance measurements at (400–800) nm as a function of concentration; (b) comparative solubility measurements for the six surfactants in Figure 1 using the 400 nm absorbance measurements.

The critical aggregation concentration (CAC) or critical micelle concentration (CMC) of a surfactant has very important implications for practical applications as well as being a fundamental surfactant property. In the firefighting foam application, the role of the surfactant is to maintain the foam structure by stabilizing the air-water interface of the vesicles within the foam, and to do this the surfactant reservoir must be present at a concentration significantly above the CMC. ³² Typically, surfactants are used in premix firefighting foam formulations at concentrations 5 to 10 times their CMC. For effectiveness and practicality, the CMC of a candidate surfactant should be in the range of 0.1 g kg⁻¹ to 5 g kg⁻¹. Plots of these data are presented in Figure 3 and the results are reported in Table 1.

Figure 3:

Equilibrium surface tension of aqueous solutions of the polycarbinol-siloxane surfactants versus log molar concentrations.

The curves in these plots correlate with the surfactant head and tail structures and are analyzed using the Gibbs equation. The CMC (or CAC) and the surface tension at the CMC, γ_CMC, are derived from the break points in the plots defined by linear extrapolation of the curves above and below the CMC. The surfactant parameters corresponding to the excess surface concentration at the air-water interface, Γ ₁ , and the area per surfactant molecule at the interface, A ₁ , are obtained from the slopes of the plots below the CMC via the Gibbs equation. ³³

where R is the gas constant, T is absolute temperature and C is the molar concentration. The results of this characterization for these surfactants is summarized in Table 1.

The correlations between surfactant structure with property are very interesting. The first four surfactants in Table 1 have the same tail and linking structure, a systematic increase in γ_CMC and a significant increase in CMC. Their packing at the air-water interface (A ₁ values) correlates with the branching and size of the head structures in this series. Comparison of the properties of the Si3(OH)6 and Si4(OH)6 surfactants shows the effects of substituting the tetrasiloxane for the trisiloxane particularly on solubility and packing density. The structure of the Si3(OH)5 surfactant differs from the others in that it has a 3-atom chain linking structure as opposed to seven atoms in the other linking structures and an amide instead of an amine functionality. This would intuitively make the structure’s confirmation more rigid and, interestingly, slightly lower its surface tension.

3.3 Fire suppression

In addition to water, the composition of firefighting foams includes several surfactants, concentrate solvents, and other additives. The roles for the surfactants beyond foam generation are: a low surface tension to promote spreading on burning fuel surfaces; a foam stabilization at the air-water and fuel-water interfaces; a CMC or CAC range that provides for an effective aqueous phase reservoir of surfactant to accommodate the large increase in air-water surface area when the foam is generated; a foam expansion ratio in the range of 6–10; and a blocking of fuel-vapor diffusion from the liquid fuel surface to the fire above the foam. ³² Traditionally, fluorocarbon and hydrocarbon surfactant mixtures have addressed these roles. As described in the introduction these formulation compositions can be complex, and a simplified formulation, Ref AFFF, composed on one fluorocarbon surfactant, one hydrocarbon co-surfactant, an organic concentrate solvent, and water was developed for the purpose of screening candidate silicone surfactants as a substitute for the fluorocarbon surfactant. ⁹

The hydrocarbon co-surfactant is a commercial alkyl polyglycoside, Glucopon 225DK, which has a structure represented by 1.7 glucoside groups and an alkyl chain of 8–10 carbon units, and a high foaming ability by itself and synergistically with siloxane surfactants. ^{10 , 28} A benchtop apparatus for screening foam formulation fire suppression performance on a small scale has been developed. ^{9 , 34} The fire suppression performance is assessed by a profile plot of foam flow rate (typically (150–2,300) mL min⁻¹) versus fire extinction time (typically 30 s to 5 min). See Supplementary Material Section III and Reference 9 for additional pool fire suppression testing detail and data.

Each of the surfactants in Figure 1 was substituted for the fluorocarbon surfactant in the Ref AFFF formulation at an amount of 6x their CMC then tested for benchtop fire extinction. Two key fire-extinguishing foam performance parameters are initially measured at a foam deposition rate of 1,000 mL min⁻¹ onto the 19 cm pool fire. These parameters are the foam coverage time (time from the start of foam deposition onto the burning pool fire surface until it is 100 % covered) and the extinction time (coverage time plus additional time for complete extinguishment). These results for alcohol-free gasoline and heptane pool fires are presented in Table 2. Somewhat surprisingly, the formulations of Si3(OH)2, Si3(OH)3 and Si3(OH)4 either produced no foam or rapidly decomposing foam, none of which was able to cover the 19 cm burning pool fire. A control experiment using only the Glucopon 225DK surfactant alone with DGBE solvent in the formulation resulted in < 100 % coverage with no extinguishment of the gasoline fire and 17 s coverage with a 66 s extinction for the heptane fire. This clearly indicates that these three surfactants in addition to being ineffective have a significant deteriorating effect on the ability of the Glucopon 225DK co-surfactant to generate foam. Fire suppression testing for the Si3(OH)6 and Si4(OH)6 surfactant formulations resulted in fire extinction data for both gasoline and heptane pool fires. The 1,000 mL min⁻¹ foam flow rate data are presented in Table 2, and the profile plots of foam flow rate versus extinction time and cover time are presented in Supplementary Material Figures S-8 and S-9, respectively.

The Si3(OH)5 surfactant has been reported to be effective in extinguishing fires via its spreading behavior, although fire suppression results were not provided. ²² Results for this surfactant’s formulation are reported in Table 2 and in the Supplementary Material, Section III. These results show that this Si3(OH)5 surfactant is the most effective in rapidly covering and extinction of the gasoline and heptane burning fuel pool fires.

For this set of polycarbinol-siloxane surfactant formulations, the fire suppression behavior ranges from no foaming, to foaming without coverage, to foaming with coverage and extinction. The control experiment clearly indicates that the presence of the Si3(OH)2, Si3(OH)3 or Si3(OH)4 surfactant has a degrading effect on the foaming ability, foam stability and fire suppression performance of the Glucopon 225DK co-surfactant. This is consistent with an observed synergism between the siloxane and alkyl polyglycoside surfactants in foam formulations for the suppression of fuel fires where the alkyl polyglycoside co-surfactant stabilizes the spreading of the foam and the siloxane surfactant promotes extinction of the fire. ¹⁰ The results in Table 2 indicate a strong negative influence on foaming ability and foam stabilization for the trisiloxane surfactants with 2, 3 and 4 hydroxyl groups in the head structure.

3.4 Foaming ability and stability

To obtain an understanding of the fire suppression results for this series of polycarbinol-siloxane surfactants, direct measurements of foaming ability and foam stability were made for these surfactants alone and in formulation with the Glucopon 225DK. Of particular interest is the level of antifoaming (prevention of foam formation) versus defoaming (rapid degradation after foam formation) that may accompany the variation in the number of hydroxyl groups in the siloxane surfactant head structure. The reference surfactant used is Glucopon 225DK. As indicated above, this is a commercial alkyl polyglycoside with a carbon chain length of 8–10 units and 1.7 glucoside groups. As a member of the BASF Glucopon product series, it is a nonionic high foaming surfactant. ³⁵ It has a higher CMC (0.41 g kg⁻¹; 0.98 mM; see Supplementary Material, Section II) than the polycarbinol-siloxane surfactants and was therefore used at a higher concentration in these foaming experiments.

Traditionally, the Ross-Miles method for evaluating foaming ability and foam stability has been in practice since the 1950’s. ³⁶ The foaming ability is measured by the height of the foam generated in a column above the liquid immediately after the pipette is emptied, and the foam stability is measured as the height of the foam 5 min after the initial foam height measurement. These Ross-Miles measurements were first made on the individual surfactant alone at a concentration of 0.1 g kg⁻¹ to 1.0 g kg⁻¹, which brackets their solubility limits. The results are presented in Figure 4.

Figure 4:

Ross-Miles measurements of foaming ability (immediate column height) and foam stability (5 min column height) for aqueous solutions the individual surfactants at concentrations ranging from 0.1 g kg⁻¹ to 1.0 g kg⁻¹.

It is immediately apparent that the Si3(OH)5 surfactant has the highest foaming ability, which decreases significantly as its concentration falls below its CMC value of 0.195 g kg⁻¹. In a parallel trend, the Si3(OH)6 surfactant displays a similar profile with less foaming ability but does not drop as rapidly as the concentration falls below its CMC, which would correlate with its slightly higher CMC (0.224 g kg⁻¹) and solubility. The next surfactant, Si3(OH)4, displays a significantly reduced foaming ability, which correlates with its lower CMC and solubility (0.138 g kg⁻¹ and 0.31 g kg⁻¹, respectively). The next two surfactants, Si3(OH)3 and Si3(OH)2, have quite low solubilites (0.20 g kg⁻¹ and 0.15 g kg⁻¹, respectively) and CMCs (0.0575 g kg⁻¹ and 0.0339 g kg⁻¹), and attempts to work with more concentrated solutions produced hazy suspensions. What little foam they produced at 0.10 g kg⁻¹ disappeared very quickly and almost completely. The tetrasiloxane surfactant, Si4(OH)6, is much less soluble (0.071 g kg⁻¹) with an equivalent and low CMC (0.078 g kg⁻¹), which is about a factor of 10 less than its trisiloxane analog, Si3(OH)6. The three Si4(OH)6 foaming ability data points correlate with saturated solution effects, and the corresponding level of foaming stability is significantly better than that of the Si3(OH)4, Si3(OH)3 and Si3(OH)2 surfactants.

The second half of the antifoaming and defoaming question above is directed at how the presence of members of this polycarbinol-trisiloxane surfactant series affects the foaming of the Glucopon 225DK co-surfactant. A concentration range of 0.10 g kg⁻¹ to 0.50 g kg⁻¹ was selected to bracket the water solubility of most surfactants (see Table 1). The presence of the Glucopon 225DK has some enhancing effect on their solubility, and observations of haze/precipitation formation were made at the higher concentrations (see Supplementary Material, Section IV). The effect of these polycarbinol-trisiloxane surfactants on the Glucopon 225Dk foaming are presented in Figure 5.

Figure 5:

Ross-Miles testing of polycarbinol-trisiloxane surfactant successive additions (0.1, 0.3 and 0.5 g kg⁻¹) to a Glucopon 225DK solution (10 g kg⁻¹) to diagnose defoaming and enhanced foaming effects.

In this bar graph the black bars at the left of a series of measurements represent the initial foam heights of the Glucopon 225DK solution, and the successive bars to the right within a colored group represent the foam heights resulting from small successive increments of the polycarbinol-trisiloxane surfactants to this solution. There is a systematic defoaming trend that decreases as the number of hydroxyl groups bonded to the trisiloxane surfactant structure increases from 2 to 4. The foaming ability is moderately reduced while the foam stability is strongly reduced. This trend is reversed for the trisiloxane surfactants with 5 and 6 hydroxyl groups. Foaming ability becomes enhanced and foam degradation is small. The addition of the Si3(OH)6 surfactant at and above 0.3 g kg⁻¹ increases the foaming ability with much enhanced foam stability. This effect is even more pronounced with the addition of Si3(OH)5. Its measurements are positioned to the far right of Figure 5 due to a difference in the head-to-tail linking structure compared to the other polycarbinol-trisiloxane surfactants (see Figure 1). The amide structure and shorter linking structure may play a role in its foaming enhancement.

Finally, a direct comparison between the Si3(OH)6 and Si4(OH)6 surfactants on their additions to the foaming of Glucopon 225DK is made in Figure 6. It shows that Si3(OH)6 has a positive effect, while the latter is negative and seems to be independent of its added concentration. However, the water solubility of this surfactant (0.071 g kg⁻¹) is the lowest of the group (Table 1) and lower than the 0.1 g kg⁻¹ in the initial test. The initial 0.1 g kg⁻¹ addition solution was clear, while the subsequent 0.3 g kg⁻¹ and 0.5 g kg⁻¹ additions were hazy (Supplementary Material,Section IV).

Figure 6:

Ross-Miles testing of Si3(OH)6 versus Si4(OH)6 surfactant additions (0.1, 0.3, and 0.5 g kg⁻¹) to the Glucopon 225DK solution (10 g kg⁻¹) and methanol control experiment with Glucopon 225DK.

The Si4(OH)6 additions are apparently above the solubility saturation concentration, and, after an initial decrease in foam height at the 0.1 g kg⁻¹ concentration, the Glucopon 225DK surfactant foam is unaffected by the second and third additions, as is the 5 min foam stability height measurement. The methanol control experiment was conducted to determine if the methanol contained in the polycarbinol-siloxane additions had any effect on the Glucopon 225DK foam. The methanol alone additions to the 10 g kg⁻¹ Glucopon 225 solution were conducted to produce MeOH concentrations of (0.2, 0.6 and 1.0) g kg⁻¹ concentrations, and no significant effect on the Glucopon 225DK foam was observed.

The potency of a surfactant as a foaming agent depends on its effectiveness in reducing the surface tension of the foaming solution and on the intermolecular forces of surfactants at the air-water interface. ³³ The stability of a bubble wall/lamella in a foam depends on viscosity/drainage rate, elasticity, evaporation and other properties related to the surfactant structure. ^{37 , 38} The results in Figures 4 and 5 indicate that the foaming ability of the polycarbinol-trisiloxane surfactants alone and with Glucopon 225DK correlates with chemical structure increases in the following order:

Within this series, clearly the trisiloxane tail structure is a constant and the number of hydroxyl groups increases. Within the solubility limits of the surfactants, it is the low surface tension (19.5 mN m⁻¹ to 21 mN m⁻¹) that drives the surfactant to the air-water interface of the foam lamellae. The hydroxyl groups engage in intramolecular hydrogen bonding and intermolecular hydrogen bonds with water and other surfactants. In the isolated surfactants, this number of hydroxyls, hence the degree of hydrogen bonding, correlates with their physical character as they progress from liquid to gel to solid (Table 1). In a bubble lamella the close positioning of these surfactants at the air-water interface will have much to do with their foaming ability and stability, and an increasing number of hydroxyl groups per surfactant molecule result in a more extensive hydrogen bonding to both adjacent surfactants and water molecules. As such, the surface viscosity and lamellar drainage associated with increased hydrogen bonding will promote the higher foaming ability and stability which is observed in Figure 4. With respect to the surfactant packing density at the air-water interface, an interesting metric can be obtained by dividing surface area per molecule derived from the Gibbs equation by the number of surfactant-bonded hydroxyl groups (Table 1). The values of 16 Ų per hydroxyl to 12 Ų per hydroxyl would correlate with poor foaming, and the values of 10 Ų to 8 Ų per hydroxyl would correlate with high foaming. This ignores the role of the amine and amide functionalities in these structures, which also would also be involved in the hydrogen bonding. Another correlation-based surfactant parameter is the hydrophile-lipophile balance (HLB), which has been used as a guide to formulate stable emulsions, and for nonionic surfactants is calculated by the general formula (Eq. (3)): ³³

where M _H is the formula weight of the hydrophilic portion of the surfactant molecule and M _L is the formula weight of the lipophilic portion of the surfactant. The HLB values calculated for the six surfactants in Figure 1 are entered in the bottom row of Table 1. Five of the six surfactants have HLB values in the 8–10 range, with the two most effective surfactants for fire suppression (Si3(OH)5 and Si3(OH)6) having values of 8.5 and 10.1. Within this 8–10 range, there does not appear to be a definitive correlation, as two relatively ineffective surfactants (Si3(OH)4 and Si4(OH)6) have respective values of 8.5 and 8.8, respectively. The HLB concept has been criticized as inappropriately assigning a number to a single surfactant for a balance between water and a multiple number of oils ³⁹ and as failing when applied to silicone surfactants. ⁴⁰

With respect to the effect of these polycarbinol-trisiloxane surfactants on the foaming of the alkyl polyglycoside co-surfactant presented in Figure 5, significant degrading effects are observed by the addition of small quantities of the siloxane surfactants with four or fewer bonded hydroxyl groups, and enhancement is observed for those with five and six hydroxyl groups. The Glucopon 225DK surfactant in aqueous solution above its CMC has a surface tension of 29.0 mN m⁻¹ (see Supplementary Material, Section II) and can be readily displaced at the lamellar interface by siloxane surfactants with lower surface tension. In fact, low molecular weight polydimethylsiloxane without hydroxyl groups is an excellent foam control agent. ³⁷ The glucoside head of Glucopon 225DK has six to seven hydroxyl groups. Apparently, the addition of the Si3(OH)5 and Si3(OH)6 surfactants have sufficient hydrogen bonding capability with the Glucopon 225DK surfactant to reinforce the lamellae’s surface viscosity and mechanical properties creating a synergism for improved foaming.

Finally, the comparison of Si4(OH)6 with Si3(OH)6 on the Glucopon 225DK foaming in Figure 6 illustrates the problem of a surfactant with both a low solubility and low CMC. The solubility Si4(OH)6 (0.071 g kg⁻¹) is a factor of 10 lower than the solubility of Si3(OH)6, while the CMC of Si4(OH)6 (0.0708 g kg⁻¹) is a factor of three lower than that of Si3(OH)6. When solubility and CMC have the same value, it also corresponds to a Krafft temperature, which means that increases in micellar concentration at room temperature are not supported. Also, the data in Figure 4 indicate that this Si4(OH)6 surfactant has very little foaming capability on its own.