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A study of the solution, interfacial and wetting properties of silicone surfactants
G&ids and Surfaces, 44 (1990) 281-297 Elsevier Science Publishers B.V., Amsterdam -
281
Printed in The Netherlands
A Study of the Solution, Interfacial and Wetting Properties of Silicone Surfactants K.P. ANANTHAPADMANABHAN,
E.D. GODDARD and P. CHANDAR
Union Carbide Corporation, Specialty Tarrytown, NY 10591 (U.S.A.)
Chemicals Division,
Tarrytown
Technical
Center,
(Received 20 April 1989; accepted 19 July 1989)
ABSTRACT This work reports an investigation of the solution, interfacial and special wetting properties of a group of three silicone surfactants using surface tension, wetting, adsorption, fluorescence and fluorescence decay techniques. Aqueous solutions of two of these surfactants exhibit a c.m.c. type break in the surface tension versus concentration plot. Fluorescence and turbidity measurements show that for one (SSl ) the break corresponds to the formation of well dispersed droplets rather than micelles. A second surfactant, SS2, forms micelles in solution of aggregation number around 30 which is significantly lower than that for conventional surfactant micelles. SSl, with a compact hydrophobic group and a long polyether tail, reduces the surface tension of water to 20.5 mN m-’ and exhibits a unique ability to wet low energy surfaces such as polyethylene. Interestingly, fluorocarbon surfactants which can lower the surface tension of water even further do not wet polyethylene as efficiently. The special wetting properties of SSl are attributed to a variety of factors, i.e., its unique structure, ability to lower the liquid-air surface tension to extremely low values, fast kinetics of adsorption at the liquid-air and solid-liquid interfaces, high affinity of the surfactant for low energy surfaces, and favorable orientation and structure of its adsorbed molecules.
INTRODUCTION
Conventional ionic and nonionic surfactants, which are extensively used in many industrial applications, have hydrocarbon groups as their hydrophobic moiety. A class of silicone surfactants receiving increasing attention has polydimethylsiloxane groups as the hydrophobic group. Members of this class when dissolved in water are able to achieve extremely low surface tension values (20 mN m-’ ), an ability exceeded only by fluorocarbon surfactants [ 1,2]. Another feature of silicone surfactants, recognized for some time, is their unique ability to wet such low energy surfaces as polyethylene. Interestingly, even fluorocarbon surfactants which lower the surface tension of water to 16 mN m-’ or less, do not wet polyethylene surface as efficiently. Some of the recent work by
282
Zabkiewicz et al. [ 31 shows that silicone surfactants wet the hydrophobic surfaces of plant leaves very efficiently and therefore find application in herbicide formulations. The molecular processes and the structure performance relationships which govern the behavior of silicone surfactants in some of the above processes have not been fully explained. In this paper, solution, interfacial and wetting properties of selected silicone surfactants are presented and the results are compared with those for some conventional surfactants. A molecular mechanism which can account for the unique wetting ability of silicone surfactants is proposed. EXPERIMENTAL
Materials The three nonionic silicone surfactants used in this study are designated as SSl, SS2 and SS3. The structure and some of the relevant properties of these surfactants are shown in Table 1. They were synthesized using the procedure TABLE
1
Relevant properties
of surfactants
Property
Surfactant
Structurea
SSl
ss2
ss3
n=O,y=l, 2=1.5
x=o,y=1.5,
x=ZO,y=3.2, 2z7.5
MW
620
2=7.5 855
9%EO Cloud point ( ‘C)
53 O-20 24.1
58 43-53 25.2
34 O-20 21.6
20.7
22.4
22.1
Surface tension bulk (mN m-l) Surface tension, 1% solution “Nominal
formula and structure.
3120
described by Bailey and Snyder [ 21. Note that the structures indicated in Table 1 represent the average or nominal structures present in products and do not imply that they are discrete, pure individual components. The polyethylene powder (Polymist B-6, average particle size 6 pm) was obtained from Allied Signal Corporation. Pyrene used in the fluorescence probe study was purchased from Aldrich Chemicals Company. The viscosity probe, dipyrenyl propane, was obtained from Molecular Probes Company. Methods Surface tension was measured using the Wilhelmy plate technique with a sand blasted platinum blade as the sensor. Dynamic surface tension was determined by quickly removing, by suction, a thin surface layer from the surface of the surfactant solution contained in a dish and then monitoring the surface tension with time. The procedure involved is described in Refs [ 41 and [ 51. Fluorescence measurements were conducted using a Perkin Elmer LS-5 spectrophotometer. Surfactant solutions for measurements were prepared using water presaturated with a commonly used fluorescence probe, pyrene [ 61. (Concentration of pyrene in a saturated solution is < lop6 moll-l. ) The measurements involved exciting the sample at 332 nm and monitoring the emission spectrum in the 360-400 nm region. The dynamic fluorescence studies on aqueous surfactant solutions were done using a single photon counter at Columbia University (see Ref. [ 71 for a description of the equipment) and the results were interpreted to obtain information on the size of micellar aggregates. A brief description of the method is as follows. Pyrene is solubilized in a micellar solution to yield a pyrene concentration well below the molar concentration of micelles. This solution, when excited at the appropriate wavelength, exhibits a single exponential decay curve. A second experiment is then conducted in which the molar concentration of pyrene ( cp) is almost the same as that of micelles. The decay curve of this system exhibits a double exponential curve, a fast decay associated with the micelles containing two or more pyrene molecules and a slow decay associated with the micelles containing one pyrene. The results are then analyzed using a theory developed by Atik et al. [ 81 which allows the determination of the number of pyrene molecules, p, per micelle. With a knowledge of p, c,, C( concentration of the surfactant ) and the c.m.c., the aggregation number, N, can be calculated using the following equation: N= (C-c.m.c.)
- (p/c,)
(1)
In addition to using the above technique, an attempt was made to determine the micellar size using the photon correlation spectrophotometer (PCS ) manufactured by the Brookhaven Instruments Company. In this equipment, laser light scattering by the particles suspended in solution is analyzed to yield the particle size.
284
The relative spreading ability of various surfactant solutions on a low energy surface such as Parafilm (paraffin wax film) was determined in the following manner. A 50 ~1 drop of the solution was placed using a microsyringe on a clean sheet of the Parafilm stretched across an optically flat glass plate resting upon a horizontal optical bench. After 5 min, the largest diametric dimension of the spread drop was measured using a calibrated grid. The spreading factor is calculated as the ratio of the diameter of the surfactant solution drop to that of a doubly distilled water drop. Wetting kinetics experiments, under quiescent conditions, were done in the following manner. During the surface tension measurements of surfactant solutions, a fixed amount (0.1 g) of polyethylene powder was placed on the surface of the solution. An instantaneous increase in surface tension occurred because of extraction of surfactant molecules by the powder. This change of surface tension will, of course, depend upon the rate of extraction and the rate of replenishment of the molecules from the subsolution/interface. Experimentally, the surface tension was observed to change slowly after the initial increase and to finally attain a new equilibrium value. The time required for complete wetting corresponded to that at which surface tension attained its new equilibrium value, thus affording a means to monitor wetting time. Adsorption experiments were done under agitation conditions and the amount adsorbed was determined by the solution depletion technique. Essentially this involved contacting a fixed weight ( 1 g) of polyethylene powder with a given amount (20 ml) of solution in appropriate vials and shaking the vials (1 h) using a wrist action shaker. The solution was then separated from the solid by centrifugation. The supernatant solutions were analyzed by total carbon analysis. In select cases supernatant analysis was also done by gel permeation chromatography (GPC ). A Beckman Tocamaster was employed for carbon analysis. Turbidity of solutions was measured using a HACH turbidimeter (model 2100 A). All measurements were at ambient temperature (23 + 1 “C ) . RESULTS
AND DISCUSSION
Surface activity and micellization/dispersion Surface tension versus log concentration plots for the three silicone surfactants are shown in Fig. 1. The effectiveness of these surfactants in lowering the surface tension of water follows the order: SSl > SS2 > SS3. The ultimate value of surface tension attained using SSl is close to 20 mN m-‘, a value close to the surface tension of liquid polydimethyl siloxane (PDMS) itself. It is reasonable that the ultimate value of surface tension attainable using a silicone surfactant would correspond to that of PDMS. Evidently, the silicone surfac-
285
Fig. 1. Surface tension versus log concentration SSl, SS2 and SS3.
plots for aqueous solutions of silicone surfactants
tants can decrease the surface tension of water to much lower values than are attained by conventional ionic and nonionic surfactants. The lowest surface tension value in the case of SSl suggests the presence of silicone groups exposed to “air”. The long polyether chain, presumably, will be entirely in the water. Paradoxically, the structure of SSl (Fig. 2A) with its compact hydrophobic group and extended hydrophilic chain seems like an “inversion” of the structure of a regular ionic surfactant (possessing a compact polar head group and a long hydrophobic alkyl chain). Note also the high density of hydrophobic methyl groups evident in the “plan” view of the SSl molecule (Fig. 2B ). It would not be surprising if this “inversion” endowed the SSl molecule with special, if not unique, properties. (See below.) In this regard the effectiveness in a surfactant molecule of a compact hydrophobic group and a long hydrophilic group was recognized by Hartley [9] as early as 1941. Adam and Elliott [lo] demonstrated, furthermore, that a “CH3” surface is more hydrophobic than a “CH2” surface. In the case of SSl, the silicone backbone provides a means to expose a dense layer of CH, groups on the surface. The packing area of the molecule which can be obtained from the limiting slope of the surface tension versus log concentration plot, via the Gibbs equation, turned out to be 70 A’ for the SSl molecule, probably representing close packing of the hydrophobic groups and allowing fairly flexible packing of the hydrophilic chains. The area per molecule for the SS2 molecule is somewhat higher (around 90 A” ) . The breakpoint in the surface tension versus concentration plot occurs at a concentration of 7*10e3% for SSl and of 3.8*10-‘% for SS2. SS3, on the other hand, does not exhibit any sharp breakpoint in the tested range. For conventional surfactants, the breakpoint indicates the onset of micellization. In the case of silicone surfactants, on the other hand, formation of micelles has not
287
Fig. 2. (A) A space filling model of silicone surfactant SSl. (B) “Plan view” of SSl molecule. Both photographs illustrate the compact nature of the hydrophobic head group.
been clearly established for the reason that some solutions develop slight turbidity even at concentrations as low as the “critical” concentration. SSl is visibly cloudy at and above about 0.05% which is in the plateau region of the surface tension versus concentration plot. SS2 is, however, soluble at concentrations as high as 5%. SS3, on the other hand, is turbid, but well dispersed at all tested concentrations. Possible formation of micelle type aggregates in silicone surfactants was tested by using a fluorescence probe technique with pyrene as the probe. The characteristic property of pyrene, which indicates the polarity of the environment in which it is solubilized, is the ratio of the intensity of its fluorescence peaks at 373 (II) and 383 (13) nm. The ratio 1,/I, has a value around 1.7 in water, 1.0 in alcohol and 0.6 in oil. In sodium dodecyl sulfate (SDS) type micelles, 1J& is around 1.0-1.1. The change in the 1,/I, ratio of pyrene in SSl and SS2 solutions can be seen in Fig. 3. The steep reduction in 1J& in SSl solutions occurs at a concentration of 0.007%. The 1J& change in SS2 solutions is not as steep and it occurs at a slightly higher concentration of the surfactant. Both these changes indicate that above these transition concentrations, pyrene is solubilized in a much more hydrophobic environment than in water. The final value of II/I.1 is higher
288 1.8
Pyrene O-o-o_
1.6 -
----!
ss2
1.4 r _
SSl
\\
0
l
\\
1.2 .
1.0 -5
-4
-3
-2
Fig. 3. Fluorescence
.
0
%
characteristics
in Z1/13 indicates
-0-o
-1
Log Concentration,
decrease
.--.\ 0
of pyrene
the solubilization
in aqueous
solutions
of SSl
of pyrene in a hydrophobic
and SS2.
The sharp
environment.
5 I 4 --
3 -$ g
2
E 2
1 --
i 0
Om -6
/,
D---o-0-0-~ -5
-4
-3
Log CONCENTRATION,
Fig. 4. Turbidity
of aqueous solutions
-2
-1
%
of SSl.
NTU represents
nephelometric
turbidity
units.
than in conventional ionic surfactant micelles, but is similar to that previously observed for regular nonionic surfactant micelles. This indicates that the pyrene is possibly solubilized near the hydrophobic/hydrophilic boundary of the surfactant molecules in the aggregates. The sharp change in the case of SSl indicates that the shape and size are conducive for solubilization to occur over a narrow concentration range. In contrast to this, the smooth change in the case of SS2 is a clear indication of the lower capacity of SS2 aggregates to solubilize the pyrene. A comparison of the fluorescence curve of SSl with its surface tension plot in Fig. 1 shows that the concentration at which II/I3 exhibits a marked decrease coincides with the concentration at which the surface tension attains a constant value, but the development of turbidity above the transition concentration complicates the picture. Turbidity measurements on S’S1 solutions, plotted in Fig. 4, show that the onset of turbidity in fact coincides with the apparent c.m.c. indicating that SSl does not form regular micelles. The tur-
289
bidity is evidently due to microdroplets of SSl. The density of SSl is very close to that of water (1.007 g ml-‘). This fact plus likely peripheral hydration of microdroplets can account for the observed stability of these droplets. In the case of SS2, the concentration at which 1J& exhibits a sharp decrease and the absence of turbidity development appear to be consistent with a conventional critical micelle concentration. The question of micelles versus liquid droplets was examined using another fluorescence probe which is sensitive to the fluidity of the medium in which it is solubilized. The probe molecule, dipyrenyl propane, can form an internal excimer involving the 2 pyrene groups and the extent of excimer formation is an indication of the mobility of the probe in its particular environment. The ratio of the intensity of the excimer peak to that of the monomer peak represents the relative amounts of excimer and monomer in the system. The values of 1,/I, obtained in selected solutions is shown in Table 2. It is evident that 1,/Im in SS2 solutions approaches the value observed in SDS micellar solutions supporting the view that SS2 forms micelles in solution. On the other hand, the value of I,/&, for SSl solutions at a level of 0.1% (at which the solutions exhibit turbidity) is markedly different and is closer to that observed in SSl and SS2 neat liquids. This indicates that the “supramicellar solutions” of SSl are in fact dispersions of SSl in water. An attempt was made to determine the size and aggregation number of SS2 micelles using dynamic fluorescence measurements. PCS measurements TABLE 2 Fluorescence characteristics of dipyrenyl propane in silicone surfactant solutions System
UInI
Remarks
ss2 1% Solution
3.19
SS2- 1% solution is similar to an SDS micellar system
SSl
0.1% Solution
0.26
SSl Neat fluid
0.485
ss2 Neat Fluid
0.42
SDS 1% Solution
3.85
SSl-0.1% solution is similar to neat SSl and SS2 fluids. SSl solution has microdroplets of neat SSl
showed that the size of these micelles was below the range of the equipment. This would imply that the size is less than 100 A. The dynamic fluorescence measurements of SS2 solutions were analyzed using Eqn (1) from which the value of the aggregation number of SS2 micelles is estimated to be 31. Thus, the size of these micelles is small compared to that of conventional surfactant micelles, but the technique does not provide any information on their shape. As always, that structure allowing maximum contact of hydrophobic groups and maximum polyether group immersion in the solution would be favored. Neutron scattering or small angle X-ray scattering experiments may provide more information about the shape of these micelles. Wetting of polyethylene Certain silicone surfactants are known to wet polyethylene extremely efficiently. The ability of different surfactants to wet a low energy surface is compared in the data on the wetting of Parafilm (paraffin wax film) given in Table 3. Interestingly, fluorocarbon surfactants which lower the surface tension of water to as low as 16 mN m-l do not wet the surface as efficiently as SSl and SS2. Evidently surface tension is not the only criterion governing the wetting of this low energy surface. In fact, similar conclusions that the spreading of a liquid on a solid is not a simple function of surface tension were reached some time ago [ 11-141. TABLE 3 ability of
surfactants” relative
System
factor
water Surface (mN m-‘)
1.0 SSl ss2 surfactant, FSAb Fluorocarbon surfactant, Nonionic Tergitol NP-10” Fluorocarbon surfactant, Sodium sulfate
2.3
20.5 23.5
1.8
16.8 31.1
1.4 1.2
concentration 0.1%. from DuPont “Obtained from Carbide Corporation.
44.3
291
The availability of polyethylene powder made it possible to test wetting in a different way. The fate of a small amount of polyethylene powder placed on the surface of various surfactant solutions was studied. In the case of SSl, wetting was seen to begin at the periphery of the powder aggregate. In this process the wetted particles get dislodged from the mass in a very vigorous way. The concentration at which this process occurs is at about, and above, 0.005% SSl. In contrast to this, SS3 did not exhibit rapid wetting behavior. In this case intense shaking was needed to wet the powder. SS2 which is structurally similar to SSl did not exhibit these properties at the 0.005% level. At higher concentrations ( > 0.03% ), however, SS2 did show mildly vigorous wetting of the powder. In contrast to SSl and SS2, conventional nonionic surfactants did not wet the polyethylene powder efficiently. A typical fluorocarbon surfactant, Fluorosurfactant FSB (DuPont product) at 0.1 and 1% levels (J+~=16.5 mN m-l) also did not wet the polyethylene powder placed on the surface of the solution. The reasons for the unique wetting behavior of SSl and the differences in the behavior of various silicone surfactants are not clear at present. It appears from all the available results that the structure itself of the SSl molecule plays a governing role in determining its superior properties. Some mechanistic possibilities involved in this process are examined below. Spreading of a liquid on a solid surface involves the replacement of solidgas interface by solid-liquid and liquid-gas interfaces. This will occur spontaneously if the spreading coefficient, S, defined by S=YSg-&‘sl-Y,p
(2)
is positive. The criterion for immersion of the solid powder in a liquid is different from that of spreading. The process of immersion involves the replacement of solidgas interface by solid-liquid interface. Therefore, thermodynamically, immersion of solid powder will occur if Y*l< Ysg
(3)
Wetting of a solid surface by a surfactant solution is complicated by the fact that the interfacial tension values in Eqns (2) and (3) are influenced by surfactant adsorption and changes in them with time. In the case of powders, spreading of the solution on the solid may be necessary for adsorption of the surfactant to occur at the solid-liquid interface (or vice versa). Note that the equations imply that if the conditions for spreading [ Eqn (2) ] are favorable (i.e., JJ,- yBl- yipis positive), then the conditions for wetting immersion [ Eqn (3 ) ] must be favorable (7, - yslis positive). The process of wetting/spreading in surfactant-solid systems, as mentioned above, is a dynamic phenomenon. The spreading process involves a number of kinetic processes which themselves involve molecular events. There is clearly
292
an interplay between advancement of the leading edge of the liquid and adsorption occurring at the newly wet solid surface. This in turn can result in a momentary reduction in surfactant concentration near the leading edge which can be corrected by diffusion of surfactant molecules from the liquid-air interface and the subsurface region of the trailing film. The rate of spreading, therefore, can be expected to be governed by any one of the subprocesses involved. Interest in the details of the spreading process has recently increased and there is now evidence that, at least in some instances, the leading edge of the spreading fluid is actually a thin satellite film. Hardy [15], in his classical study of spreading of polar liquids on glass and steel, postulated that the leading edge is an almost invisible film of liquid ( - 1 ,um in thickness) and he referred to this film as the primary spreading entity. He also postulated that the surface tension of the primary film is always higher than that of the spreading fluid. According to Hardy, this film, in order to attain equilibrium, pulls the drop, leading to the formation of a secondary film. This type of mechanism may exist in the present system. It is reasonable to expect that, because of surfactant adsorption at the solid-liquid interface, the primary film will have a higher surface tension than the rest of the drop. A study of the spreading process by various techniques such as ellipsometry, interference microscopy, high speed microscopy, etc. should provide better insight into the presence of such films and, in turn, into the unique spreading characteristics of SSl. A number of indirect tests including kinetic and equilibrium measurements of adsorption, kinetics of wetting of powder, and dynamic surface tension were conducted to examine the role of various factors on wetting. The results obtained showed that SSl at the 0.005% level (ylp=21.5 mN m-l) wets the surface in - 8 min. In contrast to this, SS2 did not wet the powder until its concentration was raised to about 0.03% ( ylln= 24 mN m- ’
) wets the powder in - 2.7 min. Note that the surface tension of the latter solution is only slightly lower than that of the 0.03% solution which wetted polyethylene much more slowly. The difference between the two evidently involves the more rapid diffusion and adsorption which can occur at the higher concentration. The same reasoning can also account for other differences in wetting behavior which evidently involves molecular kinetics. For example, when SSl was used at the 0.003% level where its surface tension (23.5 mN m- ’ ) is identical to that of a fast wetting, 0.1% SS2 solution, wetting by SSl occurred only after - 1 h. This again shows that when the fluid
293
surface tension is below a critical value required for wetting, the availability of surfactant molecules and rapid diffusion control the wetting. Another experimental illustration is that at the 0.01% level, SSl gives complete wetting in less than 1.2 min, whereas SS2, at an order of magnitude higher concentration, takes over 2 min. The higher mobility of SSl molecules over that of SS2 is evident from the kinetic results for their adsorption at the liquid/air and solid/liquid interfaces (See Fig. 5 and Table 4). The dynamic surface tension results show that any depletion of SSl molecules from the liquid/air interface will be replenished by an almost instantaneous diffusion of molecules from the bulk solution. Note that in the adsorption studies (Table 4)) unlike the wetting studies, the powder samples were well mixed with the solution by intense shaking using a wrist action shaker. Interestingly, kinetic measurements of adsorption at the solid/ liquid interface clearly showed that the attainment of equilibrium corresponded to complete wetting of the powder.
20.0
0.0
40.0 TIME,
60.0
min.
Fig. 5. Dynamic surface tension of aqueous solutions of silicone surfactants SSl, SS2 and SS3. TABLE 4 Kinetics of adsorption of SSl and SS2 on polyethylene powder (solution agitated) Adsorption (mol g-l)” Time (min) 0
1 3 10 15 20
SSl
ss2
0
0
1.3*1o-6 2.9*10-6 2.9*1o-6
0 1.7*1o-6
2.9*10-6
2.0. 1o-6 2.0-10-s
“Initial surfactant concentration 0.02%.
294
As mentioned earlier, the leading edge of the spreading film has a lower surfactant concentration than the bulk of the spreading drop. The fast kinetics of adsorption of SSl at interfaces can be expected to help replenish the leading edge quickly with SSl molecules. Results of equilibrium adsorption at the solid/liquid interface are shown in Tables 5 and 6. As expected, adsorption is found to increase with an increase in concentration and to attain a constant or almost constant value for both SSl and SSZ. The presence of a plateau is consistent with monolayer adsorption rather than multilayer condensation. Interestingly, the equilibrium adsorption value for the two surfactants is similar in the plateau region. In the low concentration range, however, more SSl than SS2 is adsorbed. These results are consistent with the surface wetting results in the sense that complete wetting with SS2 occurs at a much higher concentration than that for SSl. An estimate of the area occupied by SSl and SS2 molecules at the interface can be made from the plateau adsorption values. On assuming a surface area TABLE
5
Adsorption of SSl on polyethylene /fin size), 1 g solid, 20 g solution Initial concentration
powder (solution
agitated).
System: Polyethylene
powder (6
agitated).
System: Polyethylene
powder (6
Adsorption (mol g-‘)
(%) 0.003 0.01 0.02
6.65*10F7 2.14.10-s 3.07.10-s
0.03 0.1
3.44*1o-6 3.44.1o-6
0.3
3.68.10V6
TABLE
6
Adsorption
of SS2 on polyethylene
powder (solution
/cm size), 1 g solid, 20 g solution Initial concentration
Adsorption (mol g-‘)
(70) 0.005 0.01 0.03 0.1
5.4. 1o-7 1.3-10-6 2.5-1o-6 2.9.10-s
0.2 1.0
3.8.10V6 3.8-1O-6
295
Polyether
Conventional Non-Ionic Sulfactant
-
7
WATER
J
TIME
$i<;Cg;
/,////// POLYETHYLENE
/,//
/
Fig. 6. A schematic depiction of transfer of surfactant molecules from the liquid-air interface to the polyethylene surface. The progressive advance of SSl solutions can be likened to “molecular zippering” of the polyethylene-water interface. Conventional surfactants surface exposing hydrophobic patches which impede spreading.
tend to lie flat on the
of 1.1 m2 g-l for polyethylene powder (average particle size 6 pm, density 0.93 g cmp3), the area occupied by an SSl molecule is estimated to be about 50 A”. This would be in line with the projected area of the hydrophobic head group of the SSl molecule. Interestingly, the adsorbed layer, on the basis of this calculation, appears to be more compact at the solid-liquid interface than at the liquid-air interface (area per molecule 68 A”). In the case of SS2, the packing area at the solid-liquid interface is almost the same as that of SSl but the reasons for this are not clear at present. It is interesting that the rapid wetting of a powder by SSl is not restricted to polyethylene. Preliminary experiments showed that similar rapid wetting of hydrophobic silica also occurred in SSl solutions, but in this case at still
296
higher concentrations of the surfactant (i.e., 0.01% SSl, yla=20.5 mN m-l). Note that the critical surface tension (23 mN m-l ) for the wetting of an alkane type surface containing methyl groups is appreciably lower than that for a polyethylene surface (31 mN m-’ ). As mentioned earlier, the critical surface tension criterion is not the only one governing the wetting/spreading characteristics of surfactant solutions. The unique structure of SSl must play a central role in determining its rapid wetting/spreading behavior. We believe that the unique ability of SSl solutions to spread on hydrophobic surfaces is related to its compact structure which in turn facilitates a transfer of surfactant molecules from the liquid-air interface to the solid surface. A depiction of possible events at the liquid interface during spreading is presented in Fig. 6. Note that the dynamic advancing angle of the satellite film is shown, on a molecular scale, as being obtuse - a real possibility in our view. Because of their compact nature, efficient adsorption and packing SSl molecules are readily transferred to the polyethylene interface facilitating progressive advance of the liquid film in a process which can be likened to “molecular zippering” of the polyethylene/water interface. This behavior can be contrasted to that observed with conventional surfactants (and other silicone surfactants) in which the more cumbersome hydrophobic groups impede the molecular transfer and wetting process (Fig. 6). On this basis, it is understandable why “contaminant surfactant molecules” (advantitious or deliberately added) can interfere with the molecular processes involved in the wetting action of SSl. Direct determination of the thickness of the adsorbed surfactant layer and that of the leading edge of the spreading film would provide considerable insight into the spreading process. Determination of the advancing dynamic contact angle by high speed video-microscopy could also provide valuable information on the mechanism of spreading. These aspects are currently being examined.
REFERENCES
2 3 5 6 I 8 9 10 11 12
E.G. Schwarz and W.G. Reid, Ind. Eng. Chem., 56 (1964) 26. D.L. Bailey and N.Y Snyder, U.S. Pat., 3.299.112,1967. J.A. Zabkiewicz, R.E. Gaskin and J.M. Balneaves, BCPC Monogr., 28 (1985). R.D. Kulkarni and P. Somasundaran, AIChE Symp. Ser. 150,71 (1975) 124. M.J. Schick, J. Colloid Interface Sci., 18 (1963) 378. J.K. Thomas, ACS Monogr., 181 (1984). N.J. Turro and Y. Tanimoto, Photochem. Photobiol., 34 (1981) 173. S.S. Atik, M. Nam and L.A. Singer, Chem. Phys. Lett., 67 (1979) 75. G.S. Hartley, Trans. Faraday Sot., 37 (1941) 130. N.K. Adam and G.E.P. Elliott, J. Chem. Sot., (1962) 2206. J.L. Moilliet, and B. Collie, Surface Activity, Van Nostrand, New York, 1951. M.K. Bernett and W.A. Zisman, J. Phys. Chem., 63 (1959)
1241.
297
13 14 15 16
A. El-Shimi and E.D. Goddard, J. Colloid Interface Sci., 48 (1974) 242. P.G. de Gennes, C.R. Acad. Sci., Paris, Ser. II, 298 (1984) 111. W.B. Hardy, Collected Works, Cambridge Univ. Press, Cambridge, 1936, p. 38,667. W.A. Zisman, Adv. Chem. Ser., 43 (1964) 1.
281
Printed in The Netherlands
A Study of the Solution, Interfacial and Wetting Properties of Silicone Surfactants K.P. ANANTHAPADMANABHAN,
E.D. GODDARD and P. CHANDAR
Union Carbide Corporation, Specialty Tarrytown, NY 10591 (U.S.A.)
Chemicals Division,
Tarrytown
Technical
Center,
(Received 20 April 1989; accepted 19 July 1989)
ABSTRACT This work reports an investigation of the solution, interfacial and special wetting properties of a group of three silicone surfactants using surface tension, wetting, adsorption, fluorescence and fluorescence decay techniques. Aqueous solutions of two of these surfactants exhibit a c.m.c. type break in the surface tension versus concentration plot. Fluorescence and turbidity measurements show that for one (SSl ) the break corresponds to the formation of well dispersed droplets rather than micelles. A second surfactant, SS2, forms micelles in solution of aggregation number around 30 which is significantly lower than that for conventional surfactant micelles. SSl, with a compact hydrophobic group and a long polyether tail, reduces the surface tension of water to 20.5 mN m-’ and exhibits a unique ability to wet low energy surfaces such as polyethylene. Interestingly, fluorocarbon surfactants which can lower the surface tension of water even further do not wet polyethylene as efficiently. The special wetting properties of SSl are attributed to a variety of factors, i.e., its unique structure, ability to lower the liquid-air surface tension to extremely low values, fast kinetics of adsorption at the liquid-air and solid-liquid interfaces, high affinity of the surfactant for low energy surfaces, and favorable orientation and structure of its adsorbed molecules.
INTRODUCTION
Conventional ionic and nonionic surfactants, which are extensively used in many industrial applications, have hydrocarbon groups as their hydrophobic moiety. A class of silicone surfactants receiving increasing attention has polydimethylsiloxane groups as the hydrophobic group. Members of this class when dissolved in water are able to achieve extremely low surface tension values (20 mN m-’ ), an ability exceeded only by fluorocarbon surfactants [ 1,2]. Another feature of silicone surfactants, recognized for some time, is their unique ability to wet such low energy surfaces as polyethylene. Interestingly, even fluorocarbon surfactants which lower the surface tension of water to 16 mN m-’ or less, do not wet polyethylene surface as efficiently. Some of the recent work by
282
Zabkiewicz et al. [ 31 shows that silicone surfactants wet the hydrophobic surfaces of plant leaves very efficiently and therefore find application in herbicide formulations. The molecular processes and the structure performance relationships which govern the behavior of silicone surfactants in some of the above processes have not been fully explained. In this paper, solution, interfacial and wetting properties of selected silicone surfactants are presented and the results are compared with those for some conventional surfactants. A molecular mechanism which can account for the unique wetting ability of silicone surfactants is proposed. EXPERIMENTAL
Materials The three nonionic silicone surfactants used in this study are designated as SSl, SS2 and SS3. The structure and some of the relevant properties of these surfactants are shown in Table 1. They were synthesized using the procedure TABLE
1
Relevant properties
of surfactants
Property
Surfactant
Structurea
SSl
ss2
ss3
n=O,y=l, 2=1.5
x=o,y=1.5,
x=ZO,y=3.2, 2z7.5
MW
620
2=7.5 855
9%EO Cloud point ( ‘C)
53 O-20 24.1
58 43-53 25.2
34 O-20 21.6
20.7
22.4
22.1
Surface tension bulk (mN m-l) Surface tension, 1% solution “Nominal
formula and structure.
3120
described by Bailey and Snyder [ 21. Note that the structures indicated in Table 1 represent the average or nominal structures present in products and do not imply that they are discrete, pure individual components. The polyethylene powder (Polymist B-6, average particle size 6 pm) was obtained from Allied Signal Corporation. Pyrene used in the fluorescence probe study was purchased from Aldrich Chemicals Company. The viscosity probe, dipyrenyl propane, was obtained from Molecular Probes Company. Methods Surface tension was measured using the Wilhelmy plate technique with a sand blasted platinum blade as the sensor. Dynamic surface tension was determined by quickly removing, by suction, a thin surface layer from the surface of the surfactant solution contained in a dish and then monitoring the surface tension with time. The procedure involved is described in Refs [ 41 and [ 51. Fluorescence measurements were conducted using a Perkin Elmer LS-5 spectrophotometer. Surfactant solutions for measurements were prepared using water presaturated with a commonly used fluorescence probe, pyrene [ 61. (Concentration of pyrene in a saturated solution is < lop6 moll-l. ) The measurements involved exciting the sample at 332 nm and monitoring the emission spectrum in the 360-400 nm region. The dynamic fluorescence studies on aqueous surfactant solutions were done using a single photon counter at Columbia University (see Ref. [ 71 for a description of the equipment) and the results were interpreted to obtain information on the size of micellar aggregates. A brief description of the method is as follows. Pyrene is solubilized in a micellar solution to yield a pyrene concentration well below the molar concentration of micelles. This solution, when excited at the appropriate wavelength, exhibits a single exponential decay curve. A second experiment is then conducted in which the molar concentration of pyrene ( cp) is almost the same as that of micelles. The decay curve of this system exhibits a double exponential curve, a fast decay associated with the micelles containing two or more pyrene molecules and a slow decay associated with the micelles containing one pyrene. The results are then analyzed using a theory developed by Atik et al. [ 81 which allows the determination of the number of pyrene molecules, p, per micelle. With a knowledge of p, c,, C( concentration of the surfactant ) and the c.m.c., the aggregation number, N, can be calculated using the following equation: N= (C-c.m.c.)
- (p/c,)
(1)
In addition to using the above technique, an attempt was made to determine the micellar size using the photon correlation spectrophotometer (PCS ) manufactured by the Brookhaven Instruments Company. In this equipment, laser light scattering by the particles suspended in solution is analyzed to yield the particle size.
284
The relative spreading ability of various surfactant solutions on a low energy surface such as Parafilm (paraffin wax film) was determined in the following manner. A 50 ~1 drop of the solution was placed using a microsyringe on a clean sheet of the Parafilm stretched across an optically flat glass plate resting upon a horizontal optical bench. After 5 min, the largest diametric dimension of the spread drop was measured using a calibrated grid. The spreading factor is calculated as the ratio of the diameter of the surfactant solution drop to that of a doubly distilled water drop. Wetting kinetics experiments, under quiescent conditions, were done in the following manner. During the surface tension measurements of surfactant solutions, a fixed amount (0.1 g) of polyethylene powder was placed on the surface of the solution. An instantaneous increase in surface tension occurred because of extraction of surfactant molecules by the powder. This change of surface tension will, of course, depend upon the rate of extraction and the rate of replenishment of the molecules from the subsolution/interface. Experimentally, the surface tension was observed to change slowly after the initial increase and to finally attain a new equilibrium value. The time required for complete wetting corresponded to that at which surface tension attained its new equilibrium value, thus affording a means to monitor wetting time. Adsorption experiments were done under agitation conditions and the amount adsorbed was determined by the solution depletion technique. Essentially this involved contacting a fixed weight ( 1 g) of polyethylene powder with a given amount (20 ml) of solution in appropriate vials and shaking the vials (1 h) using a wrist action shaker. The solution was then separated from the solid by centrifugation. The supernatant solutions were analyzed by total carbon analysis. In select cases supernatant analysis was also done by gel permeation chromatography (GPC ). A Beckman Tocamaster was employed for carbon analysis. Turbidity of solutions was measured using a HACH turbidimeter (model 2100 A). All measurements were at ambient temperature (23 + 1 “C ) . RESULTS
AND DISCUSSION
Surface activity and micellization/dispersion Surface tension versus log concentration plots for the three silicone surfactants are shown in Fig. 1. The effectiveness of these surfactants in lowering the surface tension of water follows the order: SSl > SS2 > SS3. The ultimate value of surface tension attained using SSl is close to 20 mN m-‘, a value close to the surface tension of liquid polydimethyl siloxane (PDMS) itself. It is reasonable that the ultimate value of surface tension attainable using a silicone surfactant would correspond to that of PDMS. Evidently, the silicone surfac-
285
Fig. 1. Surface tension versus log concentration SSl, SS2 and SS3.
plots for aqueous solutions of silicone surfactants
tants can decrease the surface tension of water to much lower values than are attained by conventional ionic and nonionic surfactants. The lowest surface tension value in the case of SSl suggests the presence of silicone groups exposed to “air”. The long polyether chain, presumably, will be entirely in the water. Paradoxically, the structure of SSl (Fig. 2A) with its compact hydrophobic group and extended hydrophilic chain seems like an “inversion” of the structure of a regular ionic surfactant (possessing a compact polar head group and a long hydrophobic alkyl chain). Note also the high density of hydrophobic methyl groups evident in the “plan” view of the SSl molecule (Fig. 2B ). It would not be surprising if this “inversion” endowed the SSl molecule with special, if not unique, properties. (See below.) In this regard the effectiveness in a surfactant molecule of a compact hydrophobic group and a long hydrophilic group was recognized by Hartley [9] as early as 1941. Adam and Elliott [lo] demonstrated, furthermore, that a “CH3” surface is more hydrophobic than a “CH2” surface. In the case of SSl, the silicone backbone provides a means to expose a dense layer of CH, groups on the surface. The packing area of the molecule which can be obtained from the limiting slope of the surface tension versus log concentration plot, via the Gibbs equation, turned out to be 70 A’ for the SSl molecule, probably representing close packing of the hydrophobic groups and allowing fairly flexible packing of the hydrophilic chains. The area per molecule for the SS2 molecule is somewhat higher (around 90 A” ) . The breakpoint in the surface tension versus concentration plot occurs at a concentration of 7*10e3% for SSl and of 3.8*10-‘% for SS2. SS3, on the other hand, does not exhibit any sharp breakpoint in the tested range. For conventional surfactants, the breakpoint indicates the onset of micellization. In the case of silicone surfactants, on the other hand, formation of micelles has not
287
Fig. 2. (A) A space filling model of silicone surfactant SSl. (B) “Plan view” of SSl molecule. Both photographs illustrate the compact nature of the hydrophobic head group.
been clearly established for the reason that some solutions develop slight turbidity even at concentrations as low as the “critical” concentration. SSl is visibly cloudy at and above about 0.05% which is in the plateau region of the surface tension versus concentration plot. SS2 is, however, soluble at concentrations as high as 5%. SS3, on the other hand, is turbid, but well dispersed at all tested concentrations. Possible formation of micelle type aggregates in silicone surfactants was tested by using a fluorescence probe technique with pyrene as the probe. The characteristic property of pyrene, which indicates the polarity of the environment in which it is solubilized, is the ratio of the intensity of its fluorescence peaks at 373 (II) and 383 (13) nm. The ratio 1,/I, has a value around 1.7 in water, 1.0 in alcohol and 0.6 in oil. In sodium dodecyl sulfate (SDS) type micelles, 1J& is around 1.0-1.1. The change in the 1,/I, ratio of pyrene in SSl and SS2 solutions can be seen in Fig. 3. The steep reduction in 1J& in SSl solutions occurs at a concentration of 0.007%. The 1J& change in SS2 solutions is not as steep and it occurs at a slightly higher concentration of the surfactant. Both these changes indicate that above these transition concentrations, pyrene is solubilized in a much more hydrophobic environment than in water. The final value of II/I.1 is higher
288 1.8
Pyrene O-o-o_
1.6 -
----!
ss2
1.4 r _
SSl
\\
0
l
\\
1.2 .
1.0 -5
-4
-3
-2
Fig. 3. Fluorescence
.
0
%
characteristics
in Z1/13 indicates
-0-o
-1
Log Concentration,
decrease
.--.\ 0
of pyrene
the solubilization
in aqueous
solutions
of SSl
of pyrene in a hydrophobic
and SS2.
The sharp
environment.
5 I 4 --
3 -$ g
2
E 2
1 --
i 0
Om -6
/,
D---o-0-0-~ -5
-4
-3
Log CONCENTRATION,
Fig. 4. Turbidity
of aqueous solutions
-2
-1
%
of SSl.
NTU represents
nephelometric
turbidity
units.
than in conventional ionic surfactant micelles, but is similar to that previously observed for regular nonionic surfactant micelles. This indicates that the pyrene is possibly solubilized near the hydrophobic/hydrophilic boundary of the surfactant molecules in the aggregates. The sharp change in the case of SSl indicates that the shape and size are conducive for solubilization to occur over a narrow concentration range. In contrast to this, the smooth change in the case of SS2 is a clear indication of the lower capacity of SS2 aggregates to solubilize the pyrene. A comparison of the fluorescence curve of SSl with its surface tension plot in Fig. 1 shows that the concentration at which II/I3 exhibits a marked decrease coincides with the concentration at which the surface tension attains a constant value, but the development of turbidity above the transition concentration complicates the picture. Turbidity measurements on S’S1 solutions, plotted in Fig. 4, show that the onset of turbidity in fact coincides with the apparent c.m.c. indicating that SSl does not form regular micelles. The tur-
289
bidity is evidently due to microdroplets of SSl. The density of SSl is very close to that of water (1.007 g ml-‘). This fact plus likely peripheral hydration of microdroplets can account for the observed stability of these droplets. In the case of SS2, the concentration at which 1J& exhibits a sharp decrease and the absence of turbidity development appear to be consistent with a conventional critical micelle concentration. The question of micelles versus liquid droplets was examined using another fluorescence probe which is sensitive to the fluidity of the medium in which it is solubilized. The probe molecule, dipyrenyl propane, can form an internal excimer involving the 2 pyrene groups and the extent of excimer formation is an indication of the mobility of the probe in its particular environment. The ratio of the intensity of the excimer peak to that of the monomer peak represents the relative amounts of excimer and monomer in the system. The values of 1,/I, obtained in selected solutions is shown in Table 2. It is evident that 1,/Im in SS2 solutions approaches the value observed in SDS micellar solutions supporting the view that SS2 forms micelles in solution. On the other hand, the value of I,/&, for SSl solutions at a level of 0.1% (at which the solutions exhibit turbidity) is markedly different and is closer to that observed in SSl and SS2 neat liquids. This indicates that the “supramicellar solutions” of SSl are in fact dispersions of SSl in water. An attempt was made to determine the size and aggregation number of SS2 micelles using dynamic fluorescence measurements. PCS measurements TABLE 2 Fluorescence characteristics of dipyrenyl propane in silicone surfactant solutions System
UInI
Remarks
ss2 1% Solution
3.19
SS2- 1% solution is similar to an SDS micellar system
SSl
0.1% Solution
0.26
SSl Neat fluid
0.485
ss2 Neat Fluid
0.42
SDS 1% Solution
3.85
SSl-0.1% solution is similar to neat SSl and SS2 fluids. SSl solution has microdroplets of neat SSl
showed that the size of these micelles was below the range of the equipment. This would imply that the size is less than 100 A. The dynamic fluorescence measurements of SS2 solutions were analyzed using Eqn (1) from which the value of the aggregation number of SS2 micelles is estimated to be 31. Thus, the size of these micelles is small compared to that of conventional surfactant micelles, but the technique does not provide any information on their shape. As always, that structure allowing maximum contact of hydrophobic groups and maximum polyether group immersion in the solution would be favored. Neutron scattering or small angle X-ray scattering experiments may provide more information about the shape of these micelles. Wetting of polyethylene Certain silicone surfactants are known to wet polyethylene extremely efficiently. The ability of different surfactants to wet a low energy surface is compared in the data on the wetting of Parafilm (paraffin wax film) given in Table 3. Interestingly, fluorocarbon surfactants which lower the surface tension of water to as low as 16 mN m-l do not wet the surface as efficiently as SSl and SS2. Evidently surface tension is not the only criterion governing the wetting of this low energy surface. In fact, similar conclusions that the spreading of a liquid on a solid is not a simple function of surface tension were reached some time ago [ 11-141. TABLE 3 ability of
surfactants” relative
System
factor
water Surface (mN m-‘)
1.0 SSl ss2 surfactant, FSAb Fluorocarbon surfactant, Nonionic Tergitol NP-10” Fluorocarbon surfactant, Sodium sulfate
2.3
20.5 23.5
1.8
16.8 31.1
1.4 1.2
concentration 0.1%. from DuPont “Obtained from Carbide Corporation.
44.3
291
The availability of polyethylene powder made it possible to test wetting in a different way. The fate of a small amount of polyethylene powder placed on the surface of various surfactant solutions was studied. In the case of SSl, wetting was seen to begin at the periphery of the powder aggregate. In this process the wetted particles get dislodged from the mass in a very vigorous way. The concentration at which this process occurs is at about, and above, 0.005% SSl. In contrast to this, SS3 did not exhibit rapid wetting behavior. In this case intense shaking was needed to wet the powder. SS2 which is structurally similar to SSl did not exhibit these properties at the 0.005% level. At higher concentrations ( > 0.03% ), however, SS2 did show mildly vigorous wetting of the powder. In contrast to SSl and SS2, conventional nonionic surfactants did not wet the polyethylene powder efficiently. A typical fluorocarbon surfactant, Fluorosurfactant FSB (DuPont product) at 0.1 and 1% levels (J+~=16.5 mN m-l) also did not wet the polyethylene powder placed on the surface of the solution. The reasons for the unique wetting behavior of SSl and the differences in the behavior of various silicone surfactants are not clear at present. It appears from all the available results that the structure itself of the SSl molecule plays a governing role in determining its superior properties. Some mechanistic possibilities involved in this process are examined below. Spreading of a liquid on a solid surface involves the replacement of solidgas interface by solid-liquid and liquid-gas interfaces. This will occur spontaneously if the spreading coefficient, S, defined by S=YSg-&‘sl-Y,p
(2)
is positive. The criterion for immersion of the solid powder in a liquid is different from that of spreading. The process of immersion involves the replacement of solidgas interface by solid-liquid interface. Therefore, thermodynamically, immersion of solid powder will occur if Y*l< Ysg
(3)
Wetting of a solid surface by a surfactant solution is complicated by the fact that the interfacial tension values in Eqns (2) and (3) are influenced by surfactant adsorption and changes in them with time. In the case of powders, spreading of the solution on the solid may be necessary for adsorption of the surfactant to occur at the solid-liquid interface (or vice versa). Note that the equations imply that if the conditions for spreading [ Eqn (2) ] are favorable (i.e., JJ,- yBl- yipis positive), then the conditions for wetting immersion [ Eqn (3 ) ] must be favorable (7, - yslis positive). The process of wetting/spreading in surfactant-solid systems, as mentioned above, is a dynamic phenomenon. The spreading process involves a number of kinetic processes which themselves involve molecular events. There is clearly
292
an interplay between advancement of the leading edge of the liquid and adsorption occurring at the newly wet solid surface. This in turn can result in a momentary reduction in surfactant concentration near the leading edge which can be corrected by diffusion of surfactant molecules from the liquid-air interface and the subsurface region of the trailing film. The rate of spreading, therefore, can be expected to be governed by any one of the subprocesses involved. Interest in the details of the spreading process has recently increased and there is now evidence that, at least in some instances, the leading edge of the spreading fluid is actually a thin satellite film. Hardy [15], in his classical study of spreading of polar liquids on glass and steel, postulated that the leading edge is an almost invisible film of liquid ( - 1 ,um in thickness) and he referred to this film as the primary spreading entity. He also postulated that the surface tension of the primary film is always higher than that of the spreading fluid. According to Hardy, this film, in order to attain equilibrium, pulls the drop, leading to the formation of a secondary film. This type of mechanism may exist in the present system. It is reasonable to expect that, because of surfactant adsorption at the solid-liquid interface, the primary film will have a higher surface tension than the rest of the drop. A study of the spreading process by various techniques such as ellipsometry, interference microscopy, high speed microscopy, etc. should provide better insight into the presence of such films and, in turn, into the unique spreading characteristics of SSl. A number of indirect tests including kinetic and equilibrium measurements of adsorption, kinetics of wetting of powder, and dynamic surface tension were conducted to examine the role of various factors on wetting. The results obtained showed that SSl at the 0.005% level (ylp=21.5 mN m-l) wets the surface in - 8 min. In contrast to this, SS2 did not wet the powder until its concentration was raised to about 0.03% ( ylln= 24 mN m- ’
) wets the powder in - 2.7 min. Note that the surface tension of the latter solution is only slightly lower than that of the 0.03% solution which wetted polyethylene much more slowly. The difference between the two evidently involves the more rapid diffusion and adsorption which can occur at the higher concentration. The same reasoning can also account for other differences in wetting behavior which evidently involves molecular kinetics. For example, when SSl was used at the 0.003% level where its surface tension (23.5 mN m- ’ ) is identical to that of a fast wetting, 0.1% SS2 solution, wetting by SSl occurred only after - 1 h. This again shows that when the fluid
293
surface tension is below a critical value required for wetting, the availability of surfactant molecules and rapid diffusion control the wetting. Another experimental illustration is that at the 0.01% level, SSl gives complete wetting in less than 1.2 min, whereas SS2, at an order of magnitude higher concentration, takes over 2 min. The higher mobility of SSl molecules over that of SS2 is evident from the kinetic results for their adsorption at the liquid/air and solid/liquid interfaces (See Fig. 5 and Table 4). The dynamic surface tension results show that any depletion of SSl molecules from the liquid/air interface will be replenished by an almost instantaneous diffusion of molecules from the bulk solution. Note that in the adsorption studies (Table 4)) unlike the wetting studies, the powder samples were well mixed with the solution by intense shaking using a wrist action shaker. Interestingly, kinetic measurements of adsorption at the solid/ liquid interface clearly showed that the attainment of equilibrium corresponded to complete wetting of the powder.
20.0
0.0
40.0 TIME,
60.0
min.
Fig. 5. Dynamic surface tension of aqueous solutions of silicone surfactants SSl, SS2 and SS3. TABLE 4 Kinetics of adsorption of SSl and SS2 on polyethylene powder (solution agitated) Adsorption (mol g-l)” Time (min) 0
1 3 10 15 20
SSl
ss2
0
0
1.3*1o-6 2.9*10-6 2.9*1o-6
0 1.7*1o-6
2.9*10-6
2.0. 1o-6 2.0-10-s
“Initial surfactant concentration 0.02%.
294
As mentioned earlier, the leading edge of the spreading film has a lower surfactant concentration than the bulk of the spreading drop. The fast kinetics of adsorption of SSl at interfaces can be expected to help replenish the leading edge quickly with SSl molecules. Results of equilibrium adsorption at the solid/liquid interface are shown in Tables 5 and 6. As expected, adsorption is found to increase with an increase in concentration and to attain a constant or almost constant value for both SSl and SSZ. The presence of a plateau is consistent with monolayer adsorption rather than multilayer condensation. Interestingly, the equilibrium adsorption value for the two surfactants is similar in the plateau region. In the low concentration range, however, more SSl than SS2 is adsorbed. These results are consistent with the surface wetting results in the sense that complete wetting with SS2 occurs at a much higher concentration than that for SSl. An estimate of the area occupied by SSl and SS2 molecules at the interface can be made from the plateau adsorption values. On assuming a surface area TABLE
5
Adsorption of SSl on polyethylene /fin size), 1 g solid, 20 g solution Initial concentration
powder (solution
agitated).
System: Polyethylene
powder (6
agitated).
System: Polyethylene
powder (6
Adsorption (mol g-‘)
(%) 0.003 0.01 0.02
6.65*10F7 2.14.10-s 3.07.10-s
0.03 0.1
3.44*1o-6 3.44.1o-6
0.3
3.68.10V6
TABLE
6
Adsorption
of SS2 on polyethylene
powder (solution
/cm size), 1 g solid, 20 g solution Initial concentration
Adsorption (mol g-‘)
(70) 0.005 0.01 0.03 0.1
5.4. 1o-7 1.3-10-6 2.5-1o-6 2.9.10-s
0.2 1.0
3.8.10V6 3.8-1O-6
295
Polyether
Conventional Non-Ionic Sulfactant
-
7
WATER
J
TIME
$i<;Cg;
/,////// POLYETHYLENE
/,//
/
Fig. 6. A schematic depiction of transfer of surfactant molecules from the liquid-air interface to the polyethylene surface. The progressive advance of SSl solutions can be likened to “molecular zippering” of the polyethylene-water interface. Conventional surfactants surface exposing hydrophobic patches which impede spreading.
tend to lie flat on the
of 1.1 m2 g-l for polyethylene powder (average particle size 6 pm, density 0.93 g cmp3), the area occupied by an SSl molecule is estimated to be about 50 A”. This would be in line with the projected area of the hydrophobic head group of the SSl molecule. Interestingly, the adsorbed layer, on the basis of this calculation, appears to be more compact at the solid-liquid interface than at the liquid-air interface (area per molecule 68 A”). In the case of SS2, the packing area at the solid-liquid interface is almost the same as that of SSl but the reasons for this are not clear at present. It is interesting that the rapid wetting of a powder by SSl is not restricted to polyethylene. Preliminary experiments showed that similar rapid wetting of hydrophobic silica also occurred in SSl solutions, but in this case at still
296
higher concentrations of the surfactant (i.e., 0.01% SSl, yla=20.5 mN m-l). Note that the critical surface tension (23 mN m-l ) for the wetting of an alkane type surface containing methyl groups is appreciably lower than that for a polyethylene surface (31 mN m-’ ). As mentioned earlier, the critical surface tension criterion is not the only one governing the wetting/spreading characteristics of surfactant solutions. The unique structure of SSl must play a central role in determining its rapid wetting/spreading behavior. We believe that the unique ability of SSl solutions to spread on hydrophobic surfaces is related to its compact structure which in turn facilitates a transfer of surfactant molecules from the liquid-air interface to the solid surface. A depiction of possible events at the liquid interface during spreading is presented in Fig. 6. Note that the dynamic advancing angle of the satellite film is shown, on a molecular scale, as being obtuse - a real possibility in our view. Because of their compact nature, efficient adsorption and packing SSl molecules are readily transferred to the polyethylene interface facilitating progressive advance of the liquid film in a process which can be likened to “molecular zippering” of the polyethylene/water interface. This behavior can be contrasted to that observed with conventional surfactants (and other silicone surfactants) in which the more cumbersome hydrophobic groups impede the molecular transfer and wetting process (Fig. 6). On this basis, it is understandable why “contaminant surfactant molecules” (advantitious or deliberately added) can interfere with the molecular processes involved in the wetting action of SSl. Direct determination of the thickness of the adsorbed surfactant layer and that of the leading edge of the spreading film would provide considerable insight into the spreading process. Determination of the advancing dynamic contact angle by high speed video-microscopy could also provide valuable information on the mechanism of spreading. These aspects are currently being examined.
REFERENCES
2 3 5 6 I 8 9 10 11 12
E.G. Schwarz and W.G. Reid, Ind. Eng. Chem., 56 (1964) 26. D.L. Bailey and N.Y Snyder, U.S. Pat., 3.299.112,1967. J.A. Zabkiewicz, R.E. Gaskin and J.M. Balneaves, BCPC Monogr., 28 (1985). R.D. Kulkarni and P. Somasundaran, AIChE Symp. Ser. 150,71 (1975) 124. M.J. Schick, J. Colloid Interface Sci., 18 (1963) 378. J.K. Thomas, ACS Monogr., 181 (1984). N.J. Turro and Y. Tanimoto, Photochem. Photobiol., 34 (1981) 173. S.S. Atik, M. Nam and L.A. Singer, Chem. Phys. Lett., 67 (1979) 75. G.S. Hartley, Trans. Faraday Sot., 37 (1941) 130. N.K. Adam and G.E.P. Elliott, J. Chem. Sot., (1962) 2206. J.L. Moilliet, and B. Collie, Surface Activity, Van Nostrand, New York, 1951. M.K. Bernett and W.A. Zisman, J. Phys. Chem., 63 (1959)
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A. El-Shimi and E.D. Goddard, J. Colloid Interface Sci., 48 (1974) 242. P.G. de Gennes, C.R. Acad. Sci., Paris, Ser. II, 298 (1984) 111. W.B. Hardy, Collected Works, Cambridge Univ. Press, Cambridge, 1936, p. 38,667. W.A. Zisman, Adv. Chem. Ser., 43 (1964) 1.