2324
Marianne K. Bernett and W . A. Zisman
Surface-Chemical Properties of Highly Fluorinated Compounds Containing Oxygen in the Aliphatic Chain Marianne K. Bernett and W. A. Zisman* Laboratory for Chemical Physics, Naval Research Laboratory, Washington, D. C. 20375
(Received February 28, 1973)
Publication costs assisted by the Naval Research Laboratory
Two series of liquids with the formulae CF3CF2CF2[0CF(CF3)CF2InOCHCF3, where n is 1, 2, 3, or 4, and CF3CF2CF2[0CF(CF3)CF2l,OCF2CF3, where x > n but not necessarily an integer, were studied for selected surface-chemical properties. Surface tensions were found to be remarkably low (starting from 12.7 dyn/cm a t 25");interfacial tensions with water and selected organic liquids were high. Atomic and bond parachors for fluorine in such highly fluorinated organic compounds were significantly lower than previously established values for alkyl mono- or trifluorides. Spreading ability of drops of any of these liquids on the clean surface of bulk water or the above-mentioned organic liquids, in the absence of functional or hydrophilic adsorbing groups, was determined to be a function of the free surface and interfacial energies, expressed as spreading coefficients or equilibrium spreading pressures. Although in these polymers the oxygen atoms are ether oxygens, they are modified by the neighboring fluorine atoms to be more stable and nonhydrophilic. Thin films of the oxyperfluoropropylenes, spread on horizontal smooth solid surfaces, had a critical surface tension of wetting of 20 dyn/cm at 25". Several of the lower polymers of the oxyperfluoropropylenes exhibited marked ability to displace surface-chemically organic liquids which had previously coated glass or stainless steel surfaces.
Introduction Highly fluorinated organic compounds are distinguished by unusually low surface energies which characterize the liquids by their high surface activity and spreading ability, and the solids by the large contact angles when wetted by numerous liquids. In a 1966 review on the surface chemistry of organic fluorochemicals, Jarvis and Zisman' explained that the extreme surface properties of perfluoroaliphatic compounds arise from the extraordinarily weak field of force existing in the vicinity of covalently bonded fluorine atoms and their neighbors. They also stated that the extent of their surface activity in liquids (or adsorptivity on liquids) was dependent on the organophobicorganophilic balance of the fluorochemical molecule with respect to the solvent (or substrate liquid). Since the time of the review, studies of organic flurochemicals have continued along two main avenues: (i) additional areas in surface chemical properties were explored, such as spreading pressures,2 or the ability to displace liquids from solid ~urfaces.3,~ and (ii) structurally new compounds and polymers were synthesized by various sources and were made available. These studies have progressed to the stage where an investigation of selective surface chemical properties of such a structurally different material seemed highly desirable; it would demonstrate whether the properties peculiar to organic fluorochemicals are altered appreciably by a major molecular modification as, for instance, the incorporation of another constituent in the aliphatic chain. The results of such an investigation can unify the recent advances with the established knowledge and point to further areas of exploration. Experimental Materials Ideally suited for this study was a series of homologous aliphatic polyether fluorocarbons, made available by the Du Pont Co. The four low-molecular weight compounds of The Journal of Physical Chemistry, Vol. 77, No. 19, 1973
this series had the general IUPAC name a(perfluoropro[oxy(perfluoro-l,2pyl) - w - ( 1,1,1,2-tetrafluoroethoxy)poly propylene)] and the formula CF3CF2CF2[0CF(CF3)CF2],OCHFCF3, where poly and n stand for 1, 2, 3, or 4.5 The colorless liquids had a purity of 99.9% for the monomer and dimer, 99% for the trimer, and 98% for the tetramer, and were used as received. Table I lists conveniently abbreviated code names for each liquid, along with selected physical properties. Boiling point, density ( d ) a t 25", and critical temperature (T,) are given as determined by the producer for the now commercially available materiais.5 Another set of seven liquids with the general formula C F & F ~ C F ~ [ O C F ( C F ~ ) C F Z ] ~ O Care F ~ Cpolymers F~ with molecular weights ranging from about 2000 to 7000 (each polymer being a mixture) and distinguished from the OPFP series only by being capped by a fluorine instead of a hydrogen atom on the w terminal. These liquids are known commercially as Krytox 143 fluorinated oils6 and will be referred to here by that name. Code names, viscosities, and densities, as supplied by Du Pont, are shown in Table II. The polymers were used in the experiments without further purification. Surface Tension and Eotvos Constant Surface tensions (yLv)of the OPFP and Krytox series were measured by the ring method with a Cenco du Nouy interfacial tensiometer using the Fox and Chrisman correction factors.? All measurements were carried out at 25" and 45% relative humidity and are given in Tables I and 11. The Eotvos constant ( K ) for the OPFP series was calculated from the equation
K
=
~ L i l ( M / d ) " ~ / (T cT - 6)
(1)
where M is the molecular weight of the respective liquid
2325
Surface-Chemical Properties of Highly Fluorinated Compounds TABLE I: Physical Properties of Oxyperfluoropropylene Liquids
TABLE Ill: Calculated Parachor Values
CF~CF~CF~[OCF(CFS)CF~]~OCHFCF~
1 2 3
OPFP-1 OPFP-2 OPFP-3 OPFP-4
4
104.4 152.3 193.8 224.2
1.658 1.723 1.763 1.792
218.3 263.2 295.2 322
12.7 13.8 14.5 14.9
2.85 3.00 3.20 3.35
a
TABLE II: Physical Properties of Krytox Liquids
~
Compd
Eq 2
OPFP-0' OPFP-1 OPFP-2 OPFP-3 OPFP-4
332.9 514.3 691.4 867.3 1041.3
Reference 8. m . ~
Reference 9.
Quayle's atomic incrementsa 361.9 558.4 755.5
952.5 1149.6
Vogel's bond incremen@
Computed bond increments
345.0 536.4 727.8 919.2 1110.6
326.3 507.5 688.7 869.9 1051.1
d Z 5 = 1.538 g/ml, y ~ \ =, 10.4 ~ ~dyn/
CF~CF&F~[OCF(CF~)CF~]~OCFZCF~ Code
Krytox Ai! Krytox AA Krytox AY Krytox AB Krytox AX Krytox AC Krytox AD a
Viscosityz5, csa
d Z 5 ,g/mla
YLV25, dyn/cm
1.86 1.88 1.88 1.89 1.90 1.90 1.91
16.5 17.2 17.3 17.4 17.5 17.9 18.6
32 74 100 200 300 600
1000
From ref 6
and T("C) is the temperature at which the experiments were performed. Each value of K (Table I) is much higher than the value of 2.1 usually obtained for unassociated liquids and for molecules of roughly spherical structure. Thus, not only do the K values correspond to linear structures, but also the larger K values with larger n indicate these are progressively more elongated molecules; this would be predicted of such increasingly linear polymers even if some random coiling did occur, especially if n becomes greater than 4. Parachors Molecular parachors of the OPFP compounds calculated from molecular weight, surface tension, and density are P = (M/d)y1I4 much lower than those calculated by using Quayle'ss or Vogel'sg values of atomic and structural parachors (Table 111). Earlier observations of parachors for highly fluorinated organic compoundsl0Jl have noted a similar discrepancy, caused by the large fluorine parachors of 25.5 or 26.1 reported by Quayle and Vogel, respectively. Using reckntly published surface tension data on a group of highly purified fully fluorinated n-alkanes,2 we determined the molecular parachors of n-CcFl4 through n-CgFzo. Thlese, in turn, along with Vogel's atomic parachor of 8.6 for carbon,l2 resulted in an average value of 22.5 for the atomic parachor of fluorine, in excellent agreement with earlier observations.10 The (C-F) bond parachor, as obtained by Vogel's type of calculation,l3,14 (2n 2)(C-F) = CnFzn+2 + (2n 2)(C-H), gave an average value of 24.6, which is in fairly good agreement with 25.4 obtained earlier from highly fluorinated compounds,lO but much lower than the values by Vogel which he calculated to be 28.9 from the n-alkyl fluoridesl3JB or 26.3 from the trifluoroacetates.14 Molecular parachors of the oxyperfluoropropylenes as computed using the (C-F) bond value of 24.6, closely agreed with our values calculated from eq 2 (Table 111, last column).
+
+
We can therefore state that atomic or bond parachors for fluorine in highly fluorinated organic compounds are lower than those in the alkyl monofluorides, or even the trifluorides, where the three fluorines are attached to the same terminal carbon atom. Gibling16 had stated that the volume contribution of any atom is not constant but varies according to the nature of the other atoms with which it is linked and that allowance should be made for the interference of the nonlinked atoms. Obviously, having many bulky fluorine atoms attached in close proximit y to a carbon chain alters the volume contribution considerably from that of a single fluorine atom attached to a hydrocarbon chain. Fluorine atoms attached to carbon atoms which, in turn, are linked to an ether oxygen, also greatly increase the stability of the ether linkage. The electronic configuration of the oxygen is altered to change the behavior of the atom from hydrophilic to nonhydrophilic. Critical Surface Tension of Wetting (yc) Duplex films of each OPFP liquid were prepared by permitting several drops of the liquid to spread over the horizontal smooth surface of either an acid-cleaned sodalime glass slide or a freshly flamed disk of platinum and unidirectionally wiping off the excess liquid with clean, Whatman filter paper. The remaining film was thick enough to display interference colors but not thick enough to form a depression when a drop of the wetting liquid was placed upon it. The advancing contact angle (0) of each sessile drop of a pure, freshly percolated liquid was measured on the film by use of a contact angle goniometer;l7 with a few exceptions, reproducibility of 0 was k2". When cos 0 of each member of a homologous series of liquids on a smooth, clean, solid, low-energy surface is plotted against yLv for each of those liquids, a straight line results; the intercept at cos 0 = 1 ( 0 = 0") is referred to as the critical surface tension of wetting (yc) for that particular surface.17 Figure 1 shows the cos 0 us. yLv graph obtained for n-alkanes on a film of OPFP-2 but, since contact angles of any one n-alkane on films of OPFP 2, 3, or 4 were close enough to one another to fall within the range of reproducibility, the graph represents the average of all three oxyperfluoropropylenes. The yc of 20 dyn/cm obtained for these liquid films is slightly higher than the yc of solid polymers whose surface constitutions, although comparable in molecular arrangements, are comprised solely of fluorocarbon groupings, such as in polytetrafluoroethylene (yc = 18.5 dyn/cm),17 polyhexafluoropropylene (rc= 16.2-17.1 dyn/cm),ls and copolymers of the two in various molar proportions (yc = 17.8-19.0 dyn/ cm).l9 Except for the presence of oxygen atoms in the fluorocarbon chain, the surface structure of the oxyperfluThe Journal of Physical Chemistry, Voi 77, No. 19, 1973
Marianne K. Bernett and W . A. Zisman
2326
1.0
0.9 Q
;0.8 0
0.7
0.6
20
22 24 26 y L v (dynes/cm)
Figure 1. Wetting of n-alkanes on thin films of or OPFP-4 spread on glass at 25”.
28 OPFP-2, OPFP-3,
oropropylenes resembles most that of the polyhexafluoropropylene by virtue of the pendant -CF3 group in the repeating unit. It is known that the yc value of a hydrocarbon is increased by the introduction of oxygen atoms into the uppermost surface.20-22 Our results show that the yc value of a fluorocarbon surface also is increased by the introduction of oxygen atoms. We do not know the exact contribution, such as location or orientation, of the fluoroether group in the surface of the adsorbed film at the particular state observed. We assume, though, that ether oxygens bonded to fully fluorinated carbon atoms may act very differently from conventional ether oxygens bonded to hydrogen-linked carbon atoms and therefore modify the properties of compounds. Close proximity of fluorinated carbon atoms to functional groups has large effects upon these groups as shown, for instance, by the greatly increased stability of the whole perfluorodialkyl ether molecule. Spreadability on Bulk Liquids For spreadability studies, the surface of triply distilled water was covered with a duplex film of indicator oil (oxidized petroleum oil) whose surface pressure could be adjusted from about 2 to 11 dyn/cm by observation of the interference colors.23 A small drop of the respective oxyperfluoropropylene specimen on the tip of a clean fine platinum wire was touched to the center of the film-covered water, and the spreading behavior was observed by the action of the indicator oil. OPFP-1 rapidly spread and vanished from the surface. That the rapid disappearance of the drop was caused by the evaporation of the spread monolayer and not by the dissolution of OPFP-1 in the water was ascertained by observing the effect of a drop of OPFP-1 suspended above the water surface coated with the indicator oil; the volatile vapor molecules escaping from the suspended drop adsorbed on the liquid surface and forced away the indicator oil film to create a hole; this action continued until the drop either completely evaporated from the platinum tip, or until it was placed upon the surface, where it then spread and quickly disappeared. OPFP-2 acted less rapidly than OPFP-1, while OPFP-3 spread over the entire water surface, but remaining thicker in the center as indicated by the interference fringes on the advancing periphery. OPFP-4 spread very slowly, and the bulk of the drop remained as a lens that could be contracted or expanded with the surface pressure exerted by the film of The Journal of Physical Chemistry, Vol. 77, No. 79, 1973
the indicator oil. Liquids of the Krytox series did not spread but stayed on the surface as a lens with a well-defined but small contact angle. The results of the Krytox liquids can be compared but not fully equated with those of the OPFP series because the number of monomers, x , is not a well-defined number and the w terminal is chemically somewhat different. However, the generalization can be made that spreadability decreases with increasing n (or x) until eventually a limiting value of n (or x ) will be reached which will prevent spreading. For observation of spreadability on organic liquids, the indicator oil was replaced by fine particles of clean Teflon. All four OPFP liquids spread on substrates of hexadecane and on propylene carbofiate, the speeds decreasing with increasing n. In the Krytox series, all liquids spread on propylene carbonate, although the speed of spreading decreased with increasing molecular weight; a drop of Krytox AD enlarged to a thin lens which could be expanded and contracted by expanding or compressing the surface pressure of the indicator oil. On hexadecane, only the members below the molecular weight of Krytox AB would spread. Harkins Spreading Coefficients, Interfacial Tensions, and Equilibrium Spreading Pressures When a drop of a liquid “b” is placed on the surface of another liquid “a,” the spreading behavior of this system can be expressed by the initial value of the Harkins spreading coefficient S b a = (Ya
- Y b ) - Yatb’
(3) ya and y b are the surface tensions of liquids “a” and “b,” and y a , b ’ is the interfacial tension of “a” and “b,” the prime superscripts indicating that “a” is saturated with “b” and vice versa. When S b a is positive, spreading will occur spontaneously; when S b a is zero or negative, no spreading will occur, and the drop will remain unchanged.24 Table IV presents measured values of y a , y b , and y a ’ b , for a few selected systems. Interfacial tensions were measured by the ring method, and corrected by the method of Zuidema and Waters.25 Excellent agreement was found with compounds against water for which literature values were available.26 Although the ring method for interfacial tension had not previously been reported on systems comprising highly fluorinated compounds of low surface tension and high density against water or organic liquids, it was considered acceptable for this investigation, since the reproducibility of the measurements was never greater than A0.370 (mostly kO.170).As expected from the low solubility of the oxyperfluoropropylenes in either water or organic liquids (
