THE INTERFACE SYIifPOqIUM 1
-2
‘
Surface Active Agents -Their Behavior E. G . SCHWARZ ~~
~
W. G.REID
~
For producing foams or preventing foams - theoretical and empirical approaches to industrial use of surfactants are collected and explained. physical properties exhibited by surface active Thematerials . .m . aqueous solution, viz., surface tension, interfacial tension, critical micelle concentration, micelle aggregation number, contact angle, spreading coefficient, etc., depend on the composition and structure of the surfactant molecule as a whole and also in part-i.e., on the composition and structure of the surfactant hydrophobe and hydrophile. Hartley considered the possession of hydrophobic and hydrophilic tendencies and their asymmetrical distribution so fundamental a property of surface active agents that he coined the word “amphipathy” for it (70). This article considers how molecular composition affects the fundamental properties that define surface activity, and tries to relate the importance of both the theoretical approach and the empirical approach to the progress of surfactant technology.
in the surface tensions and solubility parameters (both measures of the cohesive forces) of these materials. Comparing liquids containing about the same number of atoms, the order of decreasing cohesive force is hydrocarbon > dimethylsilicone> fluorocarbon (Table 11). In the light of our own experience, the surface tension attainable with a particular type of surfactant in aqueous solution is never less than the surface tension of the parent hydrophobe. Any surface tension greater than this value would be ascribed to the absence of a complete monolayer of surfactant at the water-air interface. Thus the order of decreasing aqueous surface tension for surfactants having about the same water solubility would be hydrocarbon > dimethylsilicone > fluorocarbon. This is shown by aqueous surface tension data (Table 111), in which, as far as possible, comparisons were made between agents having the same or similar hydrophiles and approximately the same water solubilities for both nonionic and ionic surfactants. Nonionic surfactants are materials that do not ionize in aqueous solution; water solubility is imparted by hydrogen-bonding groups such as ether, oxygen, and hydroxyl. Such surfactants usually have a “cloud point,” or temperature at which they become insoluble in aqueous solution. As the temperature of an aqueous solution of surfactant is increased, hydrogen bonds between surfactant hydrophile and water are disrupted, thereby causing insolubility. Ionic surfactants are those materials that form ions in aqueous solution. The names anionic and cationic refer to the charge on the surfactant hydrophobe.
Surface Tension
Surfactant molecules are adsorbed at an aqueous surface because the attraction between water-water dipoles is much greater than that resulting from hydrophobe-water plus hydrophile-water attractions. The surfactant hydrophobe is “squeezed” out of solution. If we keep the same hydrophile, such as a polyalkylene oxide, and compare fluorocarbon, silicone and hydrocarbon hydrophobes, the ultimate aqueous surface tensions obtained will depend on the surface energy (van der Waals’ attractions) of the hydrophobes themselves. Fluorocarbons, silicones, and hydrocarbons possess different cohesive forces (van der Waals’ attractions) because of the relative deformability (polarizability) of the valence electron clouds around C-C-F, Si-C-H and C-G-H bonds (5). This difference is reflected 26
INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
Interfacial Tension
Surfactants that appreciably lower the surface tension of water will usually lower the interfacial tension between water and organic liquids. The extent of the lowering of interfacial tension will depend in a complex way upon the solubility of the surfactant in both the aqueous and organic phases. This will be a function not only of the surfactant hydrophile-lipophile balance (HLB) (7, 8), but also of the nature of the hydrophobe and hydrophile. In general, a low solubility of the surfactant in both phases will promote interfacial activity. The effects of several types of surfactants on the mineral oil/water interfacial tensim (52.0 dynedcm.) and a dimethvlsilicone/water interfacial tension (32.8 dynes/ cm.) are listed in Table IV.
and Industrial Use The oil/aqueous solution interfacial tensions attained with silicone and hydrocarbon surfactants were generally lower than the compondmg interfacial tensions resulting from the fluorocarbon surfactants. This might be due to too low a solubility of the fluorocarbon portion of the surfactant in the oil phase, so that the surfactant cannot ideally orient at the water/oil interface and thus gives low interfacial tensions. &her (2) believes that his data indicate that high interfacial tensions occur when surfactants are not ideally oriented because of perturbing effects of either the surfactant hydrophobe or hydrophile. Applying Becher's theory, Figure 1 shows the possible orientations of a surfactant at the oil/water interface. Figure 1, A and C corresponds to high interfacial tension. In l A , this arises from the perturbing effect of the hydrophobe on the water structure. This would describe fluorocarbon surfactants at the mineral oil/ water interface, since the fluorocarbon hydrophobe would not be very soluble in mineral oil. As for Figure 1C, the high interfacial tension arises from the perturbing effect due to presence of the hydrophile in the oil phasc. This occurs when a surfactant is quite soluble in the oil. Figure 1B represents the ideal position (perturbations a t a m i n i u m ) and corresponds to the case of silicone and hydrocarbon surfactants at the mineral oil/water and silicone/water interface discussed above.
TABLE 1.
SOURCE OF MATERIALS
Componr
SUrfncIanI
FC-170 FC-128 FC-134 L-77 L-79 Y-4723
3M company
Not dinclmed
"
'L
Union Carbide Gorp., Dimethylsilicone-polySilicones Division alkylcnc oxide 'I " Dimethylsiliwnc cationic " Dimcthylsiliwnc-polyakylcncoxide
.'
"
Y-4724 Y-4725 "
Y-4726
"
Dimethylsiliconc-anionic Dimethylsiliwne-polyalkylene oxidc CIHn(CI")O-
Triton X-100
Rohm and Haan Co.
TMN-6
Union Carhide Corp.. C,,Hs,O(CsH,O)IH Chcmicak Division " CirHmO(C*HdO)IH American Cyanamid CsHnOOCCH&Ha. (C0OCsHn)SOINa E. I. duPont de ClnHmOSOlNa Nemours & Co. General Mills
(C!HD),.IH TMNJ Acrasol OT
Na lauryl sulfate Aliquat 204
Mieellirmion and Aggregation Number
A surfactant molecule can escape from the bulk aqueous solution by being adsorbed at the water/& interface and also by forming clusters of molecules, called micelles, in the bulk solution. (A surfactant molecule can also be adsorbed on the container walls.) In micellization the surfactant hldrophobe is again "squeezed" out of the water and the cluster of hydrophobes is shielded from the water by the outer layer of hydrophiles. These processes are represented by: KI
surfactant e in micelles
surfactant in bulk solution
K.
surfactant at the surface
The equilibrium constants K , and K I are dependent on surfactant composition, structure, and activity. The number of molecules per micelle, called the aggregation number, will be largely dependent on the structure of the surfactant molecule and nature of the surfactant hydrophobe and hydrophile. Organic sur-
TABLE Ii. Swfnce No. of Tmwn, 25'C., Dynrs/Cm. AIom
Hydrophoba
CHdCHdsCHs (CHdrCCHzCH(CHI). (CH,),SiOSi(CH& CFdCHdaCFa CHI(CH~CHI (CH&SiOSi( CHdr OSi(CH& CF~CFAOCFI
26
21.8
26 27 26 38
18.3 15.7 13.6 25.0
6.9 6.0 5.7 7.8
37 38
16.9 m.p. 74.5' C.
5.5 AHv not available
VOL. 5 6
NO. 9 S E P T E M B E R 1 9 6 4
27
factants usually have aggregation numbers ranging from about 20 to 150. Our measurements, which utilized a vapor phase osmometer, have shown that dimethylsilicone surfactants have aggregation numbers less than five. These small numbers could be attributed to the presence of hydrophilic oxygen groups along the dimethylsilicone chain (small hydrophiles within the hydrophobe). In order to allow contact of these oxygen groups with water, only small micelles could form, with the methyl groups forming the micelle core. Zisman has shown that polydimethyl siloxane molecules are adsorbed at an aqueous surface with the oxygen
I
i Figure 7 . PossibIe orimdions of
a surfom atiw agent d an oil/
water irdtrfme
74
66
d 2%
E
=-
E
50
Y
z 5 42 w 34 0
0.8 1.2 20 2.0 AGING TIME, SECONDS X 10’
3.6
Figure 2. Thc +naniic .ufocc Unsion of silzcone-oxyaikylene mpoIymns in 7% aqutous soluhon. (Ross and Chcn, 7960)
atoms attracted to the water because of the semi-ionic nature of the Si-0 bond (7). Silicone surfactant micelles solubilize liquids insoluble in water, but to a lesser extent than do hydrocarbon surfactant micelles. Fluorocarbon surfactants probably form micelles also, since the same driving force that causes surfactant molecules to be adsorbed at the surface of an aqueous solution is involved in the phenomenon of micellization. Our literature survey revealed no studies of micellization by fluorocarbon surfactants. Wetting
The ability of a liquid to spread on a solid is measured by the spreading coefficient:
28
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
free energy of the solid surface
y,
=
y,
= free energy of the liquid surface =
free energy of the solid/liquid interface.
If the spreading coefficient is positive, a liquid should wet-out a solid-i.e., spread to a thin film. Materials like polyethylene and polytetrafluoroethylene have low surface energies (y,) and are thus harder to wet than more polar materials such as glass or steel. There are no direct means of measuring the surface energy of a solid, but an estimate can be obtained by measuring the contact angles of a homologous series of liquid-.g., alkanes-n the solid. Plotting the surface tension of the spreading liquid, y l , us. contact angle on the solid gives a straight line. Extrapolating the line to zero contact angle, representing the wettingout of the solid by a liquid, gives the surface tension of liquid needed to just wet-out the solid. Examination of the forces involved in this spreading shows that: y , cos e = y, - yell,where 8 is the contact angle (2) Thus when 8 = 0, y z = y. - yIi,. At zero contact angle (wetting-out), if it is assumed that yair is small, then y 1 S y,. Surface tension, y o of a liquid that just wets-out the solid has been termed the critical surface tension of the solid, ye, by Zisman (77). Although Zisman’s critical surface tension theory generally applies to the spreading of pure, nonhydrogen bonding liquids on low surface energy solids, he has extended the theory to aqueous surfactant solutions on low energy solids (3). Fluorocarbor., silicone, and hydrocarbon surfactants lower the surface tension of water, thereby increasing the ability of water to spread. A comparison of the aforementioned surfactants was made by measuring the spreading of 0.02 ml. of 1.0 wt. %, and 0.10 wt. % aqueous solutions of these surfactants on clean, smooth polyethylene. The per cent increase in diameter of a spread droplet over that of water alone was measured. Table V shows that even though all aqueous solutions of the surfactants tested have surface tensions lower than the critical surface tension of wetting (7.) of polyethylene (31d ynes/cm.) (75)only one material, L-77, allowed water to wet-out the polyethylene. The inability of some of these agents to wet-oqt when the theory above predicts that they should wet-out could be accounted for by higher interfacial energy, y,ib than the small value that was assumed, or another phenomenon might occur. Zisman (g, 78) describes a situation where the wetting agent alters the surface to be wet by being adsorbed with the hydrophobe outermost so that the critical surface tension of wetting of the new surface is less than that of the spreading liquid itself. Such liquids were described as “autophobic.” For example, we have shown how a 1 wt. % solution of L77, a silicone-oxyalkylene copolymer, wets-out clean, smooth polyethylene. If an identical polyethylene specimen is deliberately coated with a film of dimethylsilicone oil, the L-77 solution will not wet-out. Foam Slabilizalion
Pure liquids do not foam. To form a foam and maintain it a liquid must contain a surface active agent.
OJW7A NONIONIC: .Hydmph~hc Fluorocarbon
FC-170
60
18.5
is.5
Dimcthykilimne
Y-4723
54
20.8
21.2
Hydrocarbon
Triton
30.4
30.0
Dimethybilkone
Y-4724
21.3
21.2
x-loo
-
,.65 40
Hydrocarbon
TMN-6
Dimethylsilicone
677
