Modification of metal surfaces with difluorocarbene and heptyl nitrene

Modification of Metal Surfaces with Difluorocarbene and Heptyl Nitrene DOUGLAS A. OLSEN ~ A~D A. JE A N OSTERAAS 2 Research Center, Ashland Chemical Co., Minneapolis, Minnesota 55420

Received December 11, 1968 Difluorocarbene and heptyl nitrene have been shown to irreversibly modify the surfaces of several metals as established by wettability measurements. The critical surface tension values thus obtained are shown to have a strong dependence upon the atomic radius of the underlying metal substrate. Several possible surface structures are discussed; however, the results suggest bonding between the difluorocarbene (or heptylnitrene) biradieal and the metal substrate.

In previous publications the authors have reported that a variety of substituted carbenes and nitrenes will react with polyethylene surfaces to give what may be insertion products on that surface (1, 2). In the work reported here difluorocarbene (: CF2) and heptyl nitrene (: NCTHI~) are shown to react with several metal surfaces. It has been found that the surface energy of the treated metals as measured by critical surface tension determinations is dependent upon the nature of the underlying metal substrate. Carbenes may be defined as divalent carbon intermediates in which the earbene carbon is linked to two adjacent groups by covalent bonds, possesses two unbonded electrons, and may be thought of as a biradical. Methylene is considered to be the parent earbene. The carbene form of nomenclature, although not conforming to the established rules of the International Union of Pure and Applied Chemistry, has been nevertheless widely accepted because of its clarity and consistency. Additional arguments for its acceptance have been given by Kirmse (3). ~Present address: Applied Science Division, Litton Systems, Inc., Minneapolis, Minnesota 55427. 2 Present address: Research Division, GouldNational Batteries, Inc., Minneapolis, Minnesota 55414.

Nitrenes likewise possess two unbonded electrons and are analogous to carbenes. In the work reported here we have used the carbene and nitrene system of nomenclature. One of the main problems associated with the study of metal surfaces is that metals possess high-energy surfaces (on the order of 500 to 2000 dynes/em) (4) and are easily subject to contamination. Surface contamination by atmospheric gases is impossible to avoid since the kinetic theory of ideal gases states that the rate ~ at which gas molecules strike a surface is given by (5): -

P __=-~,

[1]

where p is the gas pressure, m is the molecular mass, k is Boltzmann's constant, and T is the absolute temperature. Simple calculation from Eq. [1] shows that in air at atmospheric pressure there are about 10~3 molecular impacts per square centimeter of surface per second. As a typical surface has about 1015 adsorption sites per square centimeter complete monolayer coverage occurs in about 10-s second. To keep a surface effectively free of adsorbed gas (i.e., less than 0.1 monolayer for several minutes) a vacuum of about 1 0 - 9 mm Hg is necessary (6). In the systems discussed here high vacuums are not practical. Thus in the work discussed below the

Journal of Colloidand Interface Science, ¥o1.32, No. -1, January 1970 12

MODIFICATION OF METAL SURFACES term "clean" metal surfaces refers to those surfaces from which organic laboratory contamination has been removed. EXPERIMENTAL PROCEDURE 1. Preparation of Difluorocarbene and Heptyl Nitrene Precursors. Sodium chlorodifluoroaeetate was used as the :CF2 precursor. The sodium salt was prepared by adding NaHCO3 to the free acid (Genera] Chemical Div., Allied Chemical Corp.) until CO2 evolution ceased. The C1F2. CCOONa could then be used immediately or dried for storage. Heptyl azide (NaCTH15) was used as the heptyl nitrene precursor and was prepared according to the method of Lieber et al. (7). 2. Preparation of Metal Surfaces. The metal specimens used in this work had to be thick enough for surface grinding procedures and also of sufficient thickness to withstand buckling and warping when heated at 300-400°C for several hours. These constraints dictated the choice of metals. The following commercial grades were used: copper (electrolytic, 99.95%), lead (extruded, 96% Pb, 4% Sb), magnesium (AZ31BH24, 96% Mg, 3% A1, 1% Zn), zinc (AC41A-XXV, 95% Zn, 4% A1, 1% Cu), and aluminum (7075, 90.2% A1, 5.5% Zn, 2.5% Mg, 1.5% Cu, 0.3% Cr). The other metals were obtained from A. D. Mackay, Inc., New York, with purities as follows: iron (99.9 %), gold (99.9 %), silver (99.9%), molybdenum (99.9%), and nickel (99.5 %). The general procedure for preparation of a surface was to first surface grind and polish the metal to IIMS values of 6.0 to 9.0 microinches (except for lead, 25-40 microinches, and molybdenum, 13 microinehes). The polished surface was then protected from oxidation by a film of light machine oil. At the time of use the metal surface was washed in a detergent followed by copious rinsing in water for as long as several hours in some cases. When a zero water contact angle was observed uniformly over the entire metal surface (i.e., the water break test) (8) the surface was considered to be free from organic contaminants and traces of detergents. Since detergent adsorption can seriously affect surface measurements several

13

detergents were studied. For stainless steel surfaces it appeared that anionics were removed most easily. Inclusion of i~lorganics such as metasilicates and polyphosphates left behind high surface energy residues which gave spurious water break tests. Of the several detergents studied Concentrate RBS-25, a proprietary formulation (R. Borghgraef, Brussels, Belgium), used in a 5 % solution was the most easily removed and was used throughout this work. 3. Exposure to Carbene or Nitrene Vapors. In a typical preparation of a difluorocarbene modified metal surface about 10 gm of C1F2CCOONa was placed in an open top pyrolysis chamber consisting in the most simplified form of a Petri dish on a hot plate (1, 2). The metal substrates were exposed to :CF2 vapors by placing them over the open top of the pyrolysis chamber. Unless noted otherwise the pyrolysis temperature was 305°C with a diffusion distance between the precursor bed and the metal substrate of 0.5 cm. Prior to exposure to difluoroearbene vapors, the metal surfaces were allowed to come to 305°C. The degree of reaction was determined by observing the advancing water contact angles of the cooled specimens versus time of exposure to :CF2 vapors. Complete monolayer coverage by :CF2 of stainless steel surfaces occurred in less than 1 minute whereas coverage of lead surfaces required an exposure of about 1 hour. The reaction of the metal substrates with heptyl nitrene generated from heptyl azide was much more rapid than for difluorocarbene. A 90 second exposure at 375°C with a diffusion distance of 1.4 cm provided complete monolayer coverage by :~NCTI-I15. 4. Contact Angle Measurements. Advancing contact angles were measured directly using an Eberbach telescopic cathetometer equipped with a circular protractor. The procedure was essentially that described by Sehonhorn and Ryan (9). For wettability measurements the following liquids (10) were used in combinations that gave the best spacing of experimental data points: o-xylene, nitroethane, octadecyl isocyanate, acetic anhydride, propylene glycol, phenerole, aeetophenone, benzaldehyde, aniline, quinoline, ethylene glycol, methylene iodide,

Journal of Cdloid and Interface Science, Vcl. 32, No. 1, J a n u a r y 1970

14

OLSEN AND OSTERAAS

formamide, glycerin, water, and 4M CaC12 in water. The liquids were Eastman White Label Grade except for aniline, which was freshly distilled prior to use, and octadecyl isoeyanate, which was awilable in our laboratory in research quantities. The CaC12 was A. R. grade. The literature values (11, 12) for the surface tensions of the liquids were verified by capillary rise measurements.

Equation [5] thus shows that there is a theoretical basis for studying adsorption kinetics through the use of contact angle measurements. The adsorption of difiuorocarbene (or its dimers) onto stainless steel surfaces was studied first. CIF2CCOONa was used as the precursor. The pyrolysis chamber was the same as discussed in a previous section. Pyrolysis temperatures of 195°C, 305°C, and RESULTS AND DISCUSSION 450°C were used with a diffusion distance Equation [1] also serves as the original of 0.5 cm. Prior to exposure to carbene basis for rate equations applicable to sur- vapors, the stainless steel plates were allowed faces. Langmuir (5) used Eq. [1] to derive the to come to the pyrolysis temperature of following rate equation the apparatus. After cooling of the metMs, the contact angles of water on the substrate d X _ ~#N (~ + ~ ) N X [2] were measured versus exposure time. The dt No No ' results for three different temperatures are where X is the fraction of surface sites shown in Fig. 1. covered, a is the sticking coefficient, # is The data of Fig. 1 present what appear the rate of supply from Eq. [1], v is the rate to be classical Langmuir adsorption kinetics, of dcsorption from a fully covered surface, i.e., low contact angles at the left indicate N is Avogadros number, No is the number that bare substrate is exposed and, after of molecules adsorbed per square centimeter complete coverage of the substrate, a conof surface, and t is the time. For carbene- stant contact angle is obtMned. The shape treated metal surfaces the reaction appears of the curve showing data at 195°C apto be irreversible so the rate of desorption proaches that of the other curves as the u is probably small. Thus Eq. [2] simplifies abscissa is compressed. Once the plateau has been reached it is not possible to deterto (13): mine from contact angle measurements dX whether further surface reaction is taking - k(1 - X), [3] dt place. The critical surface tension (~¢) of where k = a # N/No and has the units of treated stainless steel in the plateau region is 21 dynes/cm. This value of 21 dynes/cm reciprocal time. To express Eq. [3] in terms of contact compares favorably with the 7~ of 18 dynes/ angles an expression derived by Shafrin 120 and Zisman (14) can be utilized, viz., I 1 I I f

X = cos O~

-

cos Oo

[4]

COS 01 - - COS Oo' ILl

where Or, 0o, and 01 are, respectively, the instantaneous values, the contact angle on a bare surface, and the contact angle on a completely covered surface. This equation was originally derived for monolayers of carboxylic acids. However, the derivation is quite general and should be applicable here. Substitution of Eq. [4] into Eq. [3]

{2C

[5]

I 5

I I0

I 15

2O

l--

,~ sc 505°C

oo 4c I

0.1

I

I

0.2 TIME

gives :

dO~ _ k (cos O~ -- eosO~) dt -- sin O,

4o

0.:5

I

0.4

(Min)

FIG. 1. W a t e r c o n t a c t angles v e r s u s t i m e of e x p o s u r e of s t a i n l e s s steel s u r f a c e s to difluorocarbene vapors.

Journal of Colloid and Interface Science, Vol. 32, No. 1, January 1970

MODIFICATION OF METAL SURFACES cm for polytetrafluoroethylene (12); however, it was fortuitous t h a t stainless steel was studied first. As will be discussed below, the surface reactions of :CF2 on metals are dependent on the nature of the metal° I n all the following results the various metals were exposed to : CF2 generated from the pyrolysis of C1F2CCOONa at 305°C. T h e measured water contact angles versus the time of t r e a t m e n t with :CF2 vapors are shown in Fig. 2. The data shown in Fig. 2 at least qualitatively obey Eq. [5] in t h a t at low contact angles large values of dO~/dt are predicted and observed; as the observed contact angles approach those of the plateau region zero rates are predicted and are observed.

It is also apparent that the plateau values for the water contact angle are dependent on the nature of the metal substrate. The nature of this dependency is not understood; however it is evidently systematic since a graph of the eosine of the plateau values of the water eontaet angles versus the atomic radius of the metals would show a linear dependenee. Such a graph is not shown since the critieal surface tension values of the various plateaus were also determined. T h e results are shown in Fig. 3 and Table I. T h e critical surfaee tensions thus obtained were also plotted versus atomic radius (15). As shown in Fig. 4 a straight line is obtained. The linear dependency of the critical surface /oo

I

I

I

I

I

l

i

l

l

l

I

I

I

[

I

I

i

I

1.0 0.8 0.~ Pb

0.4

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o

Mo Zn

-0.2 SS

-0.4 I

20

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30

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40

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50

60

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80

SURFACE TENSION

I

90

I

I00

II0

(Dynes/Crn)

FIG. 3. Wettabilities of treated metal surfaces from the plateau regions of Fig. 2. TABLE I CRITICAL SURFACE TENSION OF DIFLUOROCARBENE-TREATED METALS Metal

Stainless steel Nickel Iron Copper Zinc Molybdenum Brass Aluminum Magnesium Lead

"Yc (dynes/

era)

21 22.5 23c 24 25 27 28 28.5 34.5 36.5

Atomic radius (A)a

Work functionb

1.244 1.260 1.276 1.339 1.386

4.1-5.0 4.2-4.7 4.80 3.7.4.2 4.12

1.429 1.598 1.704

3.6-4.4 3.60 3.8

Reference 15. b Reference 22. Estimated from Fig. 2.

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A

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TIME (Min) FIG. 2. W a t e r c o n t a c t a n g l e s v e r s u s t i m e of e x p o s u r e of s e v e r a l m e t a l s u r f a c e s t o d i f l u o r o -

carbene vapors.

tension values versus atomic radius (Fig. 4) implies a variation as discussed below in the surface density of packing of the adsorbed :CF2 molecules or the resulting dimers. Somewhat similar results have been obtained for stearic acid monolayers adsorbed on freshly polished metal surfaces (16). Timmons and Zisman stated t h a t (16): "A graph of contact angle [instead of cos 0 or ~'c as in this study] versus covalent radius [instead of atomic radius] showed a linear dependence in the region of large covalent radii and a constant contact angle for small covalent radii. The knee of the eurve oc-

Journal of Colloid and Interface Science, Vol. 32, .No. 1, January 1970

16

OLSEN AND OSTERAAS The possibility of (: CFs)n bonding directly to the metal cannot be ignored since it has been shown that CF2 = CF2 can insert into Sn--Sn bonds (17) and Sn-h~[n bonds (18). One of the products of the second investigation is assigned a structure of the form:

/

Z 0 ~4 O3 Z Ld ~0 hi (.3 <

o Mg

i

AI No

c~

Zn

F ~ - - M I n(CO)4

Cu

b

d 22 < p.--

GF

(col4m~g

18

1.0 ATOMIC

R,~DIUS

(~)

Fis. 4. Dependence of the critical surface tension of the difluoroearbene-treated metal surfaces versus the atomic radii. curred at 1.31 A. Assuming a hexagonal close packed array and that a stearic acid molecule will always adsorb directly over one of the metal atoms when possible, it can be seen that, because of the relatively large dimensions of the acid molecule with respect to the substrate atoms, the acid molecules will not completely close pack as long as the cross-sectional radius of the acid is less than twice the atomic radius of the substrate metal. Using this model for the adsorption process, it is an evident inference that the closest packing of adsorbed molecules will occur when the cross-sectional radius of the acid molecule is exactly twice the atomic radius. Assuming the crosssectional area of stearie acid to be 21 ~2, the cross-sectional radius is 2.59 A. The radius of substrate atom, then, which will create closest packing (and therefore the maximum contact angle) is 1.29 A, which is ahnost exactly the value at which the maximum contact angle is observed." Their interesting observation indicates the possibility of similar results for : CF2 adsorbed on metal. The graph of Fig. 4 would have to be extended. The only possible remaining substrates are beryllium and carbon, which may not be enough additional points to give the information desired. However, if the graph were to be extended and a knee observed, it would imply that the :CF2 or its dimer bonds directly to the metal atoms in the substrate.

In the preceding discussion of metal surfaces, the effects of adsorbed atmospheric gases and crystallinity were ignored. Because of the adsorbed gases there will be a thin metal oxide layer. However, the structure and orientation of a thin oxide film in its early stages of development, such as on the freshly polished metals studied here, is directly related to the structure and orientation of the underlying metal (19). Another possibility which can be suggested to explain the data of Fig. 4 is that the monolayer of :CF2 or its polymers are "semitransparent" to the underlying highenergy metal substrates. Normally, as shown by Zisman and his co-workers (20), condensed adsorbed organic monolayers consisting of 10 or more methylene groups per molecule are able to shield out effectively any forces due to the underlying metal surface. We wondered, therefore, whether the same would also be true for an adsorbed condensed monolayer of :CF2. As a first approximation of the surface forces of the underlying metal substrates the surface tensions of the metals at their melting points were used (21). These values were plotted versus the critical surface tension values of Table I. The rationale for this plot is that if the forces associated with the metal surface are not being shielded out by the monolayer, then high critical surface tension values would be observed on the higher energy metal surfaces. The results are shown in Fig. 5. As can be seen there is a very good negative correlation. Thus the idea of a :CFs layer "transparent" to the underlying metallic forces is probably erroneous; however, the dependence upon the atomic radius persists since the liquid surface tensions

Journal of Colloid and Interface Science, V o l . 32, N o . 1, J a n u a r y 1970

MODIFICATION

OF

METAL

SURFACES

E

17

I 20001-

I

t

I

I

I

1

I

I

]

i

I

I

I

I

I

LO

~

°c~

0.8 OE

~

eZn

SO0 F 600 F

oMg

o 0.2

~oo[ ::) O3

500

I

20

I

I

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24

P

28

1

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52

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I

56

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I

40 Fe (SS)

CRITICAL

SURFACE

-0.2

TENSION

FIG. 5. Correlation between the surface tension of the various liquid metals and the critical surface tension of the difluorocarbene modified solid surfaces. 5E

I I P I I I I 50 40 50 60 70 80 90 SURFACE T E N S I O N (Dynes/Cm)

Fie.

7. W e t t a b i l i t i e s

TABLE i

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30

,,<

28

b

[

Z 0

¢.)

~: 22 o I

WORK

I

4.0

38 40 43 47 54

•

~24

i 2O 5.0

"Yc (dynes~era)

Stainless steel Copper Zinc Aluminum Magnesium

32

~ 26 (D

treated

II

Metal

54

w o

nitrene

C R I T I C A L S U R F A C E T E N S I O N OF M E T A L S U R F A C E S T R E A T E D BY ~ I E P T Y L ~NTITRENE

o

z

of hepty]

surfaces.

I

FUNCTION

5.0 (ev)

FzG. 6. Correlation between the observed critical surface tension and the work function of the metal substrate.

J

I

Z 55 w t,W 5¢ 0 < LI.. rY 4~ o3

~ 4c

I

o AI

oq

t.[

used in Fig. 5 have been shown to be dependent upon atomic volume. T h e relationship, however, is not obvious. Finally it should be noted t h a t there is weaker positive correlation between the critical surface tension values of the :CF2treated metals and the work function of the various metals (22, 23) as shown in Fig. 6. To explain the correlation of Fig. 6 the following conjecture can be offered. The work function (¢) is an inverse measure of the ability of an electron to escape from a surface, i.e., it can be thought of as a poten-

[

I

1.3

I

1.4 ATOMIC

I

}.5 RADIUS

I6 (,&)

FzG. 8. Dependence of the critical surface tension of the heptyl nitrene treated metal surfaces versus the atomic radii. tial barrier at the metal surface. If instead the electron comes from an adsorbed molecule outside the surface, the potential barrier is now a potential well. Thus when an adsorbed: CF2 (or CF~ = CF2) gives up electrons it should be bonded most strongly to metals with a high work function; this might

Journal of Colloid and Interface Science, Vol. 32, 1~o. 1, J a n u a r y 1970

18

OLSEN AND OSTERAAS

appear to be the case since ~/~ decreases (indicating stronger bonding?) as ~ increases. The foregoing results are not restricted to the difluorocarbene treatment of metals. The surfaces of several metals were also exposed to heptyl nitrene generated b y the pyrolysis of heptyl azide at 375°C for 90 seconds with a diffusion distance of 1.4 cm. The critical surface tensions (~,~) for the treated metal surfaces were determined (Fig. 7) and are given in Table II. As shown in Fig. 8 a linear dependence of the critical surface tension of the heptyl nitrene treated metal surface versus atomic radius also results. Thus, in summary, it has been shown that difluorocarbene and heptyl nitrene react irreversibly with metal surfaces as established by critical surface tension values. The values obtained have also been shown to be dependent upon the atomic radius of the metal substrate; the nature of this dependence is not fully understood. I t appears, however, t h a t bonding between the difluorocarbene or the heptyl nitrene and the metal substrate is a definite possibility.

J. Phys. Chem. (English Transl.) 34, 206 (1960). 5. LiNOMUIR, I., J. Am. Chem. Soc. 40, 1361

(1918). 6. See, for example, MYxTJni, H., "Solid Surfaces and Interfaces," p. 42. Dover, New York, 1966. 7. LIE~Ea, E., C~io, T. S., AND RAMACttANDIRA RAO, C. N., 3. Org. Chem. 22,238 (1957). 8. SPRING, S., "Metal Cleaning," p. 151. Reinhold, New York, 1963. 9. SCItONHORN, H., AND RYAN, G. F. W., J. Phys. Chem. 70, 3811 (1966). i0. OLSEN, D. A., MORAVEC, 1~. W., AND OSTDRAAS,A. J., dr. Phys. Chem., 71, 4464 (1967).

ACKNOWLEDGMENTS

ii. LANGE, N. A., "Handbook of Chemistry," 10th ed., p. 1652. McGraw-Hill, New York, 1961. 12. Fox, H. W., AND ZISMAN, W. A., dr. Colloid Sci. 5,514 (1950). 13. See also DUSHMAN, S., revised by G. L. Gaines, "Scientific Foundations of Vacuum Technique," 2nd ed., p. 413. Wiley, New York, 1962. 14. SttAFRIN, E. G., AND ZISMAN,W. A., Monomol. Layers Symp. Philadelphia 1951, p. 129 (1954). 15. The atomic radii used here are metallic radii of ligancy 12; PAULINO,L., "Nature of the Chemical Bond," 3rd ed., p. 403. Cornell University Press, Ithaca, New York, 1960.

This paper is based on work conducted at the Research Center, Ashland Chemical Co., Minneapolis, Minnesota. This paper is published with the approval of Ashland Chemical Co. Notice is hereby given that portions of this paper are covered by U. S. patents and patent applications of Ashland Chemical Co.

16. TIMMONS, C. O., AND ZISMAN, W. A., dr. P h y s . Chem. 60,984 (1965). 17. BEG, M. A. A., AND CLARK, H. C., Chem. & Ind. (London) 1960, 140. 18. CLA~K,H. C., ANDTsiI, J. H., Chem. Commun. (London) i~lo. 6,111 (1965). 19. KUBASCHE-WSKI, O., AND HOPKINS, B. E.,

REFERENCES 1. OSTERiAS, A. J., AND OLSEN, D. A., Nature 221, 1140 (1969). 2. OLSEN,D. A., AND OSTERiiS, A. J., dr. Appl. Polymer Sei. 13, (1969). 3. KIICMSE,W., "Carbene Chemistry," Chap. 1. Academic Press, New York, 1964. 4. For estimates, see: (a) BoNm, A., Chem. Rev. 52,417 (1953); (b) BELoou~ov, B. V., Russ.

20. 21. 22.

23.

Journal of Colloid and Interface Science, ¥ol. 32, 1Wo.I, January 1970

"Oxidation of Metals and Alloys," Chap. 1. Butterworth, London, 1962. ZISMAN,W. A., Advan. Chem. Set. 43, 1 (1964). GnossE, A. A. V., dr. Inorg. Nuel. Chem. 26, 1349 (1964). Data from UHLIG, H. It., Ann. N. Y. Aead. Sci. 58, 843 (1954). The referee has noted that the Fermi energy of a metal might be used rather than the work function; see, for example, BEWlG, K., AND ZISMAN, W . A . , dr. Phys. Chem. 68, 1804 (1964).