Langmuir 1995,l I, 4167-4169 4167

Notes

Surface Activity of Fluorinated Poly(propy1ene oxide) Derivatives

P. A. Stevenson,+ D. A. R Jones, 'J . Lin,? and L. A. M. Rupert*$§

Shell Research Ltd, Thornton Research Centre, P.O. Box 1, Chester, CH1 3SH, England, Shell Research Ltd.,

Sittingbourne Research Centre, Sittingbourne, Kent ME9 8AG, England, and Koninklijke lShell Laboratorium,

Amsterdam, P.O. Box 38000, 1030BN Amsterdam, The Netherlands

Receiued December 21, 1994. In Final Form: June 12, 1995

Introduction The possibilities of reducing the surface tension of

hydrocarbons by using surfactants are rather limited: this is in contrast to aqueous systems where numerous compounds are able to do s0.l In fact only surfactants based on poly(dimethylsi1oxane) (PDMS) or fluorocarbons show any surface activity in hydrocarbon liquids. The PDMS-based surfactants reduce the surface tension of, for example, hexadecane from 27.6 to about 25 mN/m,2,3 whereas partially fluorinated diesters give a further reduction in the surface tension of hexadecane to 23-24 mN/m and that of ethylbenzene from 28.6 to 21 mN/m.4,5 Similar results were more recently obtained by Katritzky and co-workers6 using surfactants which contain a fluo- rocarbon tail, a central cyclic structure, and a polar head group. However, all these effects are dwarfed by the effect of phenylcarbonylpoly(hexafluoropropy1ene oxide) and phenylpoly (hexafluoroproylene oxide) which apparently reduce the surface tension of xylene from 28 to 10 mN/ m.7-9

This reduction of the surface tension of xylene to 10 mN/m by fluorcarbon-based surfactants is remarkable and the mechanism by which this happens deserves further investigation. The need for such an investigation is supported by the current interest for derivatives and practical applications of poly(hexafluoropropy1ene oxide); cf. refs 9-12. Interestingly, Sauer and Dee12noticed that the surface tension of this polymer, as measured using the Wilhelmy plate technique, is lower than that theo- retically predicted and have attributed this difference to a specific orientation of the CF3 groups at the polymer-

' Shell Research Ltd., Thornton Research Centre. * Shell Research Ltd., Sittingbourne Research Centre. 5 Koninklij keWhell Laboratorium. (1) van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physicochemical

properties of selected anionic cationic and nonionic surfactants; Else- vier: Amsterdam, 1993.

(2) Ellison, A. H.; Zisman, W. A. J. Phys. Chem. 1956, 60, 416. (3) Jarvis, N. L. J. Colloid Interface Sci. 1969,29, 647. (4) Jarvis, N. L.; Zisman, W. A. J. Phys. Chem. 1960, 64, 150. ( 5 ) Bernett, M. K.; Jarvis, N. L.; Zisman, W. A. J. Phys. Chem. 1962,

(6) Katritzky, A. R.; Davis, T. L.; Rewcastle, G. W.; Rubel, G. 0.;

(7) Ishikawa, N.; Sasabe, M. J. Fluorine Chem. 1984,25, 241. (8) Daikin Kogyo KK, J P Patent J86044534-B, 1982. (9) Abe, M.; Morikawa, K.; Ogino, K.; Sawada, H.; Matsumoto, T.;

(10) Klassen, J. K.; Mitchell, M. B.; Govoni, S. T.; Nathanson, G. M.

(11) Hung, M-H.; Farnham, W. B.; Feiring, A. E.; Rozen, S. J . Am.

(12) Sauer, B. B.; Dee, G. T. J. Colloid Interface Sci. 1994, 162, 25.

66, 328.

Pike, M. T. Langmuir 1988, 4 , 732.

Nakayama, M. Langmuir 1992,8,763.

J. Phys. Chem. 1993, 97, 10166.

Chem. SOC. 1993, 115, 8954.

air interface. In light of the critical surface tension for a layer of CF3 groups of 6 mN/m and the critical surface tension for CF2 of 18 mN/m,13 the above reduction in surface tension of xylene would mean an almost perfect layer of CF3 groups on the surface. In comparison with the critical surface tension of poly(hexafluoropropy1ene) of 16.2 mN/m,13 the results with the fluorinated polyethers are indeed surprising.

In this paper we report the results of our efforts to get insight into the origin of this large reduction in surface tension of xylene by phenylcarbonylpoly(hexafluoro- propylene oxide) and phenylpoly(hexafluoroproy1ene ox- ide) surfactants. To this end we have measured the surface tensions of phenylcarbonylpoly(hexafluoropropy1ene oxide)/ xylene systems, using the Wilhelmy plate and pendant drop technique, as well as the contact angles.

Experimental Section Materials. The hexafluoropropylene oxide surfactants were

synthesized by an anionic oligomerization of hexafluoropropene oxide and a subsequent Friedel-Crafts acylation of benzene similar to the literature p r ~ c e d u r e . ~ For l a and l b the intermediate oligomers of hexafluoropropylene oxide were

PhC-CF 0-CFzCF O-CF2CF2CF3 "t 7Fl L J r l

l a , n = 2 l b , n = 3

fractionated by distillation to obtain individual oligomers with n = 2 and n = 3. For the synthesis of a further compound (2) the intermediate oligomers were not fractionated but directly reacted with benzene. All crude products from the Friedel- Crafts acylations were purified by distillation. On the basis of the analytical data,14 it can be concluded that l a and lb are pure compounds with n = 2 and 3, respectively. However, 2 is a mixture containing l a and l b as minor components and, most likely, the free acid derivatives of the hexafluoropropylene oxide oligomer as major component. Analar Grade xylene (mixed isomers) and a solution of dichlorodimethylsilane were obtained from BDH and used as received.

Surface Tension Measurements. Surface tensions were measured using either the pendant drop techniqueI5 or the

(13) Zisman, W. A. In Contact angle, wettability and adhesion; Fowkes, F. M., Ed.; American Chemical Society: Washington, DC, 1964.

(14) la.'H-NMR(inCDC13,ppm): 7.7(2H), 7.70(1H), 8.10(2H);lit.7 7.4-7.8 (3H), 7.9-8.2 (2H). I3C-NMR (in CDC13, ppm): 99.8-123.9, 128.6, 129.9, 131.8, 134.8, 184.5-184.9. IgF-NMR observed (in CDC13, ppm from CFC13): -76.2 to -78.2, -124.9, -126.3, -137.7, -141.3 to -141.7.1it.7(correctedtoCFC1~)-78.8to-79.0, -128.2, -129.7, -145.2. IR (cm-1): C=O at 1711; Ph at 1600, 1583, 1452. Anal. Observed: C, 30.2; H, 0.8; Calcd: C, 29.93; H, 0.7. Mass: m l e 722, calcd 722. lb. 'H-NMR (in CDC13, ppm): 7.5 (2H), 7.7 (lH), 8.0 (2H). 13C-NMR (in CDC13,ppm): 99.8-123.9,128.6,129.9,131.9,134.9,184.5-184.9. I9F- NMR (in CDC13, ppm from CFC13): -74.3 to -78.5, -125.0, -126.5, -137.7, -141.3 to -141.8; IR (cm-'): C=O a t 1711; Ph a t 1600, 1583, 1451. Anal, Obsd: C, 28.6; H, 0.7. Calcd: C, 28.39; H, 0.7. Mass m l e 889, calcd888.2.'H-NMR (inacetone-d6, ppm) 7.6,7.8,8.1; Quantitative '3C-NMR (in acetone-&, ppm): 79.0,99.4-123.7, 129.8-136.2 (minor peaks), 159.8. '9F-NMR obs. (in acetone-de, ppm from CFC13): -80.0

at 1775. Anal. C, 22.2; H, 0.4. Calculated based on the oligo- (hexafluoropropylene oxide) acid: C, 21.75; H, 0.12. Mass positive ion pobe CI m l e 889,723,557, and others; negative ion probe CI m l e 827, 661, 495 and 329 for hexafluoropropylene oxide acid.

(15)Andreas, J. M.; Hauser, E. A,; Tucker, W. B. J. Phys. Chem. 1938,42, 1001.

to -84.7, -129.6, -131.1, -140.9, -144.5 to -145.2; IR (cm'): C=O

0743-7463/95/2411-4167$09.00/0 0 1995 Amer ican Chemical Society

4168 Langmuir, Vol. 11, No. 10, 1995

Table 1. Effect of Compounds la, lb, and 2 on the Surface Tension of Xylene As Measured by Using the

Wilhelmy Plate Technique"

Notes

u (mN/m)

com- concn clean glass silanized glassb pound (wt %) advancing receding advancing receding xylene - 28.0 la 0.30 27.2 lb 0.30 27.0 2 0.002 26.7

0.02 9.2 0.10 8.7 0.30 6.6 0.50 7.7

28.1 24.5 25.6 27.5 26.6 13.4 25.1 25.6 13.9 24.0 25.4 13.3 11.1 25.3 12.7 15.7 23.3 12.6 17.8 22.2

u (mN/m)

corn- conen clean platinum silanized platinum pound (wt %) advancing receding advancing receding

xylene 28.8? 26.0 27.6 la 0.30 26.1 27.1 lb 0.30 27.7d 26.0 26.6 2 0.23 21.0c 25.8c

0.46 18.8? 23.3c

a Using a Cahn 322DCA. * The fact that a low surface tension has been observed for xylene by using a silanized glass plate is due to the change in critical surface tension ofthe plate, which becomes about 24 mN/m.13 Therefore the same situation arises as with the adsorption of the impurity in compound 2. Using the Kruss K10 apparatus a t 25.0 "C. Using the Kruss K10 apparatus a t 21.3.T

Wilhelmy plate method.16 The Wilhelmy plate measurements were done using a Cahn 322DCA or a Kruss K10 with thin plates ofglass or platinum dipping into a dish containing the test solution (20 mL). The glass slides (microscope cover slips) were taken from a pack and either cleaned by flaming (clean glass) or coated with organosilane (silanized glass) by immersion in dichlorodi- methylsilane solution followed by rinsing with acetone. Platinum plates were silanized as above. In each of the tests with the Cahn, the slide was driven a distance of 3-4 mm into the solution at a constant ra te (4 m d m i n ) , held stationary for 30 s and then withdrawn. Surface tensions were obtained in both directions (advancing and receding).

Contact Angle Measurements. Contact angle measure- ments were made for small (diameter ca. 1 mm) droplets ofxylene and xylene + surfactant on clean glass using a Erma G1 contact angle meter. The contact angle was calculated using the expression17

tan(8l2) = 2h/A (1)

where 6' is the contact angle between the drop and the surface ofthe glass, h is the maximal height ofthe drop and A the contact diameter of the drop.

Results and Discussion The surface tensions of various solutions of the three

surfactants (la, lb, 2) have been measured using the Wilhelmy plate technique using several types of plate. In at least one of the previous publications on these systems a Wilhelmy plate apparatus equipped with a glass plate has been used,g whereas in the other publication the plate material has not been ~pecified.~ In Table 1 the results of the present measurements are collected.

The hysteresis between the advancing, i.e. dipping the plate into the liquid, and the receding, i.e. pulling the plate out of the liquid, process reflects factors such as (i) surface roughness of the plate, (ii) heterogeneity of the plate, (iii) purity of the system under investigation, (iv)

~ ~~~

(16) Padday, J. F. In Surface and Colloid Science; Matijevic, E., Ed.;

(17) Bartell, F. E.; Zuidema, H. H. J. Am. Chem. SOC. 1936,58,1449. Wiley: New York, 1969; Vol. 1, p 124.

direction in which the plate is moving, and (v) the speed at which it is moving.'* From the measurement on xylene in the absence of the additives it follows that all possible contributions to the hysteresis effects were well under control. Consequently, the hysteresis observed for 2 reflects the fact that it is a mixture. In the following analysis and discussions the surface tensions obtained with the receding plate will be used. From Table I it can be seen that, by using clean glass plates for 2, surface tensions are obtained which are similar to those reported in ref 7 for compounds when n is 2 or 3, i.e. 12.1 and 9.9 mN/m, respectively. Surprisingly, the results seem to depend on the plate material, i.e. clean glass, silanized glass, clean platinum, or silanized platinum. Moreover, the pure single component compounds la and lb show hardly any effect on the surface tension at a concentration of 0.30 wt % which is well above the minimum concentra- tion for which maximum reduction in surface tension has been reported in refs 7 and 8. Apparently, the low surface tensions observed for 2 are the result of a component (or components) in 2 other than l a or lb. Since the pure l a and lb should be identical to the surfactants used in refs 7 and 8, it seems as if the reported surface tension reduction^^-^ are caused by components in the reported surfactants similar to those present in 2. In light of the fact that the initial reductions in surface tension in refs 7 and 8 are observed at 10-30 times higher surfactant concentrations than in the present study, it is likely that the fraction of the active component in those studies is a similar factor lower than in 2.

An indication of the origin of the low surface tensions might be found in the observation that, in contrast to xylene and solutions of la and lb in xylene, solutions of 2 in xylene do not fully wet clean glass. Apparently, the active component(s) in 2 adsorbs on the glass plate and creates an outer surface layer of CF3 and CF2 groups. This in turn changes the wetting characteristics of the plate. At 0.002 wt % of 2 the concentration of the active component is so low that the adsorption process is diffusion limited and the surface tension observed with the ad- vancing plate is higher than with the receding glass plate (see Table 1). For measurements using the Wilhelmy plate technique, the liquid-air surface tension OLV can be calculated from the experimental data by using eq 2, where

F - ( W - B)g 2(L + t ) OLV cos8 =

F is the force on the plate, W ' is the apparent weight of the plate, B is the buoyancy correction, g is the gravita- tional constant, L is the width ofthe plate, tis the thickness of the plate and 8 the contact angle.lg Most liquids wet a glass plate fully and cos 6' = 1, but in this particular case where the solution of 2 does not wet the plate completely, i.e. it behaves as a so-called autophobic liquid,13 the correction with the contact angle has to be incorporated.

Contact angle measurement for a 0.02 wt % solution of 2 on clean glass gives, in contrast to the negligible angles observed for xylene and l a in xylene, a value of 60". Therefore, the experimentally determined average surface tension of 13.2 mN/m has to be corrected to 26.4 mN/m. This value of the surface tension is similar to the results obtained for the pure single component compounds la and lb. It appears that silanizing the glass plate o r changing over to a platinum plate reduces the amount and, perhaps, the orientation of the active component of

(18) Myers, D. Surfaces, interfaces and colloids: principles and

(19) Jordan, D. 0; Lane, J. E. Aust. J . Chem. 1964, 17, 7. applications; VCH: New York, 1991.

Notes Langmuir, Vol. 11, No. 10, 1995 4169

Table 2 . Effect of Compounds la, lb, and 2 on the Surface Tension of Xylene Using the Pendant Drop

Technique" compound concn (wt %) ab (mN/m)

la lb 2

At 25.0 "C. f 0 . 5 mN/m.

27.6 0.23 27.2 0.23 28.0 0.23 26.3

2 adsorbed on the plate. Consequently, different degrees of wetting and higher surface tensions are obtained (see Table 1).

Independent support for the proposed effect of wetting has been obtained from surface tension measurements using the pendant drop technique. Table 2 shows the surface tension of xylene solutions of la, lb and 2. At concentrations similar to those reported in refs 7 and 8, i.e. 2.3 wt %, a surface tension of about 10 mN/m should have been found. Surface tensions are measured instead which are close to the surface tension of xylene itself. The value obtained for 2 (26.3 mN/m) is similar to the corrected value obtained from the Wilhelmy plate technique (26.4 mN/m).

The above results suggest that the phenylcarbonylpoly- (hexafluoropropylene oxide) surfactants used in refs 7 and 8 and the phenylpoly(hexafluoropropy1ene oxide) surfac- tants used in ref 9 probably contained a similar component as in 2,20 which adsorbed onto the plates of the Wilhelmy plate equipment, and apparently, by ignoring the cor- rection for the changes in wetting characteristics, too low surface tensions have been reported. Therefore, caution should be taken in the investigation of the behavior of poly(hexafluoroethy1ene oxide) using a Wilhelmy plate technique.

Acknowledgment. Dr. B. Taylor, Dr. S. J. Hammond, and Mr. M. A. Selby (all from Sittingbourne Reseach Centre) are gratefully acknowledged for NMR, mass spectroscopy, and elemental analyses.

LA9410265

(20) Interestingly, poly(hexafluoropropy1ene oxide) acid is a byproduct in the synthesis ofthe phenylpoly(hexafluoropropy1ene oxide)g and seems also to be present as one of the components in Z.'*