1188
0. LEVINEAND W. A. ZISMAN
Vol. 61
better for the Speiser, Naiditch and Johnston data.
definite structural and spectroscopic information
validity of the assumed bipyramidal structure for BIOs and the vibrational frequency assignments. In order t o resolve the issue completely, more
Acknowledgment.-The author wishes to express his thanks to J. A. Homan for his extensive assistance in carrying out the experiments.
This conclusion, however, depends upon the is required.
PHYSICAL PROPERTIES OF MONOLAYERS ADSORBED AT THE SOLIDAIR INTERFACE. 11. MECHANICAL DURABILITY OF ALIPHATIC POLAR COMPOUNDS AND EFFECT OF HALOGENATION BY 0. LEVINE'AND W . A. ZISMAN Georgetown University and U.S. Naval Research Laboratory, Washington, D. C. Received M a y 8, 1967
Polar paraEnic compounds and some of their halogenated derivatives were adsorbed as condensed monolayers at the glasslair interface. These films were prepared by both the liquid retraction and vapor-condensation methods of Part I. The state of closest molecular packing of each monolnger was controlled and measured by the contact angle (Omax) with meth lene iodide and the kinetic coefficient of friction (pk). Measurements of the mechanical durability were made with a murtiple-traverse technique. This consisted of making repeated unidirectional traverses with a loaded ball slider on a single trark of the monolayer-covered glass plate a t the relative sliding velocity of 0.01 cm./sec. A plot of the kinetic coefficient of friction (pk) us. the numbcr of traverses ( v ) give8 the rate of wesr (dpkldv) or failure of each monolayer. Mixed films of non-polar solvent and primary amine (or fatty acid) molecules as well as single-component moiiolqrers of aliphatir amino compounds, fatty acids erfluoroalkanoic acidp, telomers of tetrafluoroethylene and chlorinated and w-brominated akanoic acids were studied. &e durability data confirm and extend the conclusions reached in Part I. Thus, for an homologous series of fattv acids, monolaycrs of compounds of 8 4 N 5 12,13 I( N 6 16 and N 2 16 are liquids, solids and crystalline solids, respectively. Monolayers of the erfluoroalkanoic acids and telomers of tetrafluoroet!$% of N 2 12 are plastic and crystalline Rolide. The close-packef, crystalline-like solid structure of some of the monolayers of this investigation enabled these films to withstand many repeated traverses under loads as great as 0000 g. (107,000 Ib./in.*) without any appreciable rise in the kinetic coefficient of friction and without the formation of wear wars detectable by the unaided eye. By correlating the frictional behavior of such films with their constitution and structure i t w m possible to set forth the requirements for a good physic:dly adsorbed boundar lubricant. Even in the presence of a solid film there is always some motd/glasq contact. By resisting enctration, a Polidboundary lubricant decreases the real area of contact bctween the surface a8peritie.Pof rubbing solids a n 8 thris reduces both tho frirtion aud wear to their low& possible values. Film diirItbility measmements r 8 d i l y distinguished between pure films and those containing solvent molecules. A mixed film Betwecn polar and solvent molrrules w 3 formed ~ by the retractioc method only when both were paraffinic. It occurred only in those instances whcre insufficicnt time had heen allowed for the nttainment of ndsorption equilibrium. Such mixed films repremit a transient or possibly a metastable state. It is concluded that the depletion of a boundary luhricatin monolayer by a clean rubbing surfacc through a desorption-adsorption merhanism is an important factor in the mechanica! failure of physically adsorbed films. I n the presence of excess lubricant, the monolayer ia regenerakd. Finally, it is concluded that the measurement of film durability by the multiple-traverse technique is a convenient and revealing mcthod of studyiiig the properties of adsorbed organic films.
Introduction The durability of an organic film on a flat plate can be measured by repeatedly sliding without rolling a loaded ball over the same track until a rapid rise in the frictional force indicates the mechanical breakdown of the film. This method was used by Langmuir2 to demonstrate that a multilayer of calcium stearate on glass is more durable than a monolayer. Akamatu and Sameshima,3 Bowden and Leben14 Hughes and Wittinghams and others used the same method to obeerve the durability of films on steel. We have used this multiple-traverse technique to measure the mechanical durability of adsorbed condensed monolayers on glass of a variety of polar-non-polar paraffinic derivatives. As in Part I , 6 each monolayer studied was deposited (1) A portion of a thesis submitted to the Graduate School of Georgetown University in partial fuEllment for the requirements of Ph.D. in Chemistry. (2) I. Lsngmuir, J . Franklin Inrt., 218, 143 (1934). (3) H. Akamatu and J. Ssmeahima, Bull. Chem. Soe. Japan, 11, 791 (1936). (4) F. P. Bowden and L. Leben, Phil. Trans. Roy. Soc., 9398, 1 (1840). (6) T. P. Hughes and G. Whittmgham, Tram. Faraday S a c , 88, 9 (1842).
(6) 0. Levine and W. A. Ziaman, T ~ r JOURNAL, s 61,1068 (1957).
a t the solid/air interface by: (a) retraction from solution, (b) retraction from the melt, or (c) vaporcondensation. Also, the state of condensation of each monolayer was measured by the contact angle exhibited by methylene iodide. Such monolayers were shown in Part I to obey Amonton's law over a wide range of load and to reduce the coefficient of friction to the minimum value when in the twodimensional solid state. The materials, experimental methods and equipment used, unless otherwise stated, are the same as those described in Part I. Hence, the stainless steel sliding ball used was '/z inch in diameter; the relative sliding speed of ball and glass plate was 0.01 cm./sec.; the monolayers were deposited on soda-lime glass microscope slides; and the friction and wear measurements were all made at 25' and 30-50% R.H. The coefficient of friction observed, therefore, is the kinetic coefficient of friction ( p k ) for the sliding speed used. Results (a) Durability of Condensed Monolayers of Aliphatic Ammo Compounds.-Condensed monolayers of rt-hexadecylamine hydrochloride were
MECHANICAL DURABILITY OF ALIPHATIC POLAR COMPOUNDS
Sept., 1957
1189
TABLE I DURABILITY DATA FOR CONDENSED MONOLAYERS OF FATTY AMINESAND THEIRDERIVATIVES (Data at 25" unless otherwise stated) Compound
lt-CisHa3NH3Cl
Method of reparation of &m
ornu
(degrees)
(0-e"dbg.)
69-70
0.06
Retraction from aq. s o h ; concd. E 1 X 10-4 mole/I.; natural pH
Durability No. of Load W traverses
(i.) ,
1000
0.06
1000
15 30 15 15 21
0.06
5000
5
0.04
1000 5000
20
5000
7000 9000 Retraction from aq. s o h ; 58-59 concd. E 1 X 10-4 mole/l.; natural pH Retraction from aq. soln.; 58-59 concn. E 1 x 10-4 mole/l.; natural pH Vapor-condensation a t 85" 69 Retraction from nitromethane soh. (satd.) Retraction from n-cetane soln.; concn. 3 X 10-4 mole/l. Retraction from n-cetane soh. (satd.); 4 hr. immersion period Retraction from n-cetane s o h (satd.); 20 hr. immersion period
slides in an aaueous solution having a con&ntration of about mole/ liter. The film-covered slide was then withdrawn slowly from the solution, the liquid retracting from the surface leaving an adsorbed film a t the solid/air interface. The condensed condition of this film was confirmed by the contact angle of 69 to 70' exhibited by pure methylene iodide and the value of pk of from 0.05 to 0.06.6 The same value of Pk was obtained even when using loads on the ball slider as large as 9000 g. instead of the maximum of 600 g. usually employed in the experiments described in Part I. Such loads correspond to pressures of 75 and 3'0 kg./mm.2 (or 107,000 and 43,000 Ib./in.2), respectively, over the apparent area of contact of ball and plate in the case of elastic deformation; the ability of a solid monolayer to withstand such large unit load values is good evidence of the great durability of such a film when adsorbed on a polished glass surface. A series of successive unidirectional traverses of the steel ball were made on a 2 mm. path over the film-coated glass plate while the ball was held firmly in one position in its holder, so that the same area of the ball was always in contact with the glass plate. The kinetic coefficient of friction under a 1000 g. load was measured during each traverse. During a series of 15 successive traverses, pk always remained between 0.05 and 0.06. With the ball in the same position, the experiment was repeated over new areas of the coated glass plate using lolids of 5000, 7000 and 9000 g.; again the value of Pk of 0.05 to 0.06 was obtained after 15 or more traverses a t each load. Evidently, a solid monolayer on such a smooth, hard substrate is much more durable than had ever been reported. The pertinent data are recorded in Table I and graphed in Fig. 1. After subjecting the film to these severe durability tests, it was observed that the wear scars on the surface of the glass were barely perceptible even under a magnification of
69
70 70
71 .
0.05
Variation
(.)
Bk
0.05
0.05 .05+ .06 .05+ .05 .06-. .06 0.05 + 0.63 -+
0.07+0.40
0.04+0.04 .05+ .06 .05+ -05 .05+ .08
10
0.05
1000 5000 1000
0.05-0.06
5000 5000
22
.04- -05 .04+ .30 0.064 0.36
0.05
5000
35
0.06-.0.07
10 30
20 20
150X. These wear scars prove that the solid film did not entirely ,prevent contact between the surface asperities of the stainless steel and glass.
0 60
W E 7000 gmr
r
2
040
y.
0
c
5
0.30
Y
0 Y
I-
0.20
0 y. w
2
010
0.05 5
10 VUMBER
15 OF T R A V E R S E S
20 lv 1.
25
30
Fig. 1.-Durability of monolayers of amino compounds.
Similarly, condensed monolayers of n-hexadecyltrimethylammonium bromide on glass, which exhibited a maximum methylene iodide contact angle (Omax) of 58 to 59" and a value of pk of 0.06, were prepared by retraction from aqueous solution, and pk was measured as a function of the number ( v ) of successive traverses of the loaded ball slider. These results in Table I are plotted in Fig. 1. The recorded value of pk for any single traverse is that constant value usually exhibited along the terminal portion of the traverse. Evidence of the lesser durability of this film under a 1000 g. load than that of the film of n-hexadecylamine hydrochloride will be seen from the linear rise in to 0.11 during the first 10 traverses, followed by a
u. LEVINE AND W. A. ZISMAN
llYU
Vol. 61
Similar measurements on a condensed monolayer of hexadecylamine hydrochloride led to essentially similar results and conclusions; the only notable difference was that with loads of 1000 and 5000 g. the film of primary amine was more durable than the condensed monolayer of the quaternary ammonium bromide. These experiments make it evident that one 0 cause of failure of adsorbed boundary lubricating 1s) monolayers is the desorption of molecules from one Fig. 2.-Mechanical desorption of cetyltrimethylammonium rubbing surface and their readsorption on to the bromide monolayer under a load of 1OOOg. other. Because of this desorption mechanism, it more rapid rise to 0.63 in the next 11 traverses. is unnecessary to coat the ball slider with a monoWhen the load was increased to 5000 g., pk increased layer before commencing measurements of the coquickly in the first 5 traverses indicating rapid fail- efficient of friction of the glass plate covered with a monolayer. The same conclusion was reached ure of the film. (b) Desorption a s a Contributing Mechanism by Langmuir2 during his studies of the durability for Film Failure.-An illuminating variation of the of multilayers of soaps. Unless otherwise indicated preceding multiple-traverse experiment is to rotate the remaining experiments to be described here the steel ball slider at the end of each traverse over were done without rotating the ball after each trathe film-coated surface in order to allow a clean verse because the emphasis was on the ability of area of the ball to contact the monolayer during the monolayer on glass to support the load rather the next traverse. The results obtained in measur- than on the mechanism for transfer of film moleing pk as a function of the distance (8) along the cules from glass plate to ball slider. (c) Relation of Method of Preparation to path traversed for a condensed monolayer of hexaFormation of Mixed Films; Their Instability.decyltrimethylammonium bromide ,,e,( = 58') are given in Fig. 2 for 5 successive traverses. Condensed monolayers of octadecylamine were deThese measurements were made using a 1000-g. posited on glass by vapor-condensation at 85" load, and each pass was made over the same 2 mm. and by retraction from solutions in hexadecane and path. During each traverse, pk started with its in nitromethane. The contact angles of methylene highest value and then gradually decreased to a iodide with each film were between 69 and 70', minimum value which was manifested along the which proves that all these films were equally remainder of the path. The minimum value of pk close packed. The results of friction and duraoccurred later in each successive traverse, and its bility measurements with 1000 and 5000 g. loads value gradually rose as the number of traverses are summarized in Table I and graphed in Figs. 3A increased. Microphotographs of the glass surface and 3B. The value of ,uk for the first traverse over revealed that the greatest surface damage occurred each film was the same within the experimental at the beginning of each traverse and became least uncertainty of f 0.01. Durability experiments when pk reached its minimum value. Upon using using 20 successive traverses with a 5000-g. load higher loads than 1000 g., pk increased with 8 led to the same results with monolayers prepared more rapidly in each traverse, took longer to at- either by the vapor-condensation method or by tain its minimum value and the latter rose to retraction from nitromethane, and they agreed with the above-mentioned results obtained with higher values in fewer successive traverses. The radically different and more destructive re- hexadecylamine hydrochloride. sults obtained by rotating the ball slider after each The durability of the condensed monolayer of traverse have a simple explanation. When the octadecylamine prepared either by retraction from clean ball was pressed against the monolayer- nitromethane solution or by vapor-condensation coated glass plate, some molecules of the quater- was the same under a 1000-g. load as that of the nary ammonium compound were able to desorb film prepared by retraction from hexadecane solufrom the plate and to reorient and adsorb on un- tion. However, there resulted a considerable decoated areas of the ball. This depletion of the film crease in durability of the latter monolayer when a on the glass slide caused the real area of contact of 5000-g. load was used (see Fig. 3). Similar durglass and ball to increase, and the coefficient of ability results with a 5000-g. load were obtained friction increased proportionately. As the ball with condensed monolayers of docosylamine and was moved over the surface of the film, it contacted eicosanoic acid prepared by retraction from hexademore densely populated areas of film from which cane solutions (see Table I and Fig. 3 and Table it could extract more molecules to coat vacant ad- 11). The lesser durability under a 5000-g.load of sorption sites on its own surface. Eventually, the films prepared by retraction from hexadecane soluarea of contact of the ball became completely coated tion is caused by the formation of mixed monowith the monolayer. During the remainder of the layers comprising hexadecane and amine (or acid) traverse, pk remained constant at the value for a molecules. The decreased durability of these film-coated surface sliding on a film-coated surface. mixed films results from the decreased surface denThis minimum value of pk during the early traverses sity of polar molecules caused by the presence in corresponded30 that arising from the friction of the film of the hexadecane molecules with a consecondensed monolayers. Such values for various quent loss of adhesional energy between the glass compounds are recorded in the tables of Part I. substrate and the polar amino (or carboxyl) groups. 20
DSTINCF
ALONG ~ . T M
I N MM.
Sept., 1957
1191
MECHANICAL DURABILITY OF ALIPHATIC POLARCOMPOUNDS TABLE I1 DURABILITY DATAFOR CONDENSED MONOLAYERS OF FATTY ACIDS (Data at 25" unless otherwise stated)
Compound
Method of preparation of film
emax
(degrees)
71
(&GOg.)
Retraction from n-cetane soln. (satd.) 1. Vapor-condensation a t 100"
70
2. Retraction from nitromethane
70
.05
soln. (satd.) Vapor-condensation a t 100'
69-70
.05
n-CI2Hz6COOH
1. Vapor-condensation at 100" 2. Vapor-condensation a t 70" Vapor-condensation at 70"
68 69 66-67
.05 .05 .06
n-CllH2&OOH
Retraction from melt a t 50"
62-63"
.10
n-CgH&OOH
Retraction from melt a t 40"
58-59"
.13
n-C,H&OOH
Retraction from melt at 25"
57"
.17
~-CIQH&OOH n-C17H3,COOH
n-Cj5HalCOOH
n-ClsHz7COOH
0.05-0.06 0.05
Durability Load No. of W traverses
(g.1
5000 1000 5000 1000 5000 1000 5000 7000 9000 1000 5000 1000 5000 1000 5000 1000 5000 1000 1000
Test dro consisted of a dilute solution of the bulk film material in methyene iodide (cf. Part I). layer a t solif/liquid octanoic acid interface.
Variation In
(V)
23 10 30 10
30 25 20 15 15 35 27 25 25 25 1 13 1 10 15
Pk
0.06 40.41 0.05+0.05 .06.06 .05+ .05 .05+ -07 0.05 +0.05 .06+ .07 .06+ .06 .06+ .08 0.06 4 0.06 0.06+0.45 0.06+ 0.08 0.07 +0.44 0.09-.0.50 0.36 0.14 + 0.95
>0.4 0.17 + 0.80 0.17 -.0. lib
Durability of mono-
The existence of a mixed film of octadecylamine FROM VPPOR FROM NITROMETHANE (A) and hexadecane was indicated by Bartell and b qms 0 w qm, A qmn , Ruch? in a recent study with the ellipsometer of monolayers adsorbed at the solid/air interface by retraction from solution. Our results establish that whereas wettability and friction measurements cannot discern the difference between such mixed films and films containing only the amine (or acid) molecules, measurements of the film durability by the multiple-traverse technique can distinguish between them readily. It should be noted that mixed films were not encountered in films adsorbed and retracted from nitromethane solutions. This undoubtedly is due to the nearly spherical shape of the nitromethane molecule with a consequent inability of the molecule to adlineate between the adsorbed molecules of amine and form mixed metastable films. A mixed film of hexadecane and amine should be less stable than B condensed film of the amine alone. This means that much longer times of immersion may be necessary to attain adsorption equilibrium in systems where mixed film formation can occur early in the adsorption process. Therefore, the glass plate was immersed in the hexadecane solution of docosylamine and allowed to remain there for various times before separating the film-coated glass from the solution phase by retraction. Whereas the film resulting from a two-thirds hour immersion in the solution exhibited a rise in CCk under a 5000-g. load from 0.06 to 0.09 after 12 traverses and 0.36 after 10 more traverses (see Fig. 3C), that which resulted from a twenty-hour immersion exFig. 3.-Mixed film formation as evidenced by durability hibited an increase in & from 0.06 t o only 0.07 after measurements: A, monolayers of Cl8Ha7NH2from nitro35 traverses. This demonstrates that the mixed methane solution and va or condensation; B, monolayers H ~ C 1 ~ H s ~ 8 0 6from H hexadecane solutions; film of docosylamine and hexadecane obtained after of C I B H ~ T Nand from hexadecane solutionC , monolayers of C~~H~F,NH~ (7) L. 8. Bartell and R. J. Ruch. THIE JOURNAL. 60. 1231 (18561. immersions of # and 20 hours. s 1000
WI!OOO
W.5000
0. LEVINEAND W.A. ZISMAN
1192
VoI. 61
ment of adsorption equilibrium a t the solid/liquid interface. Examination of the glass slide after the 35 traverses on the single component film of docosylamine revealed the presence of a wear scar. As was true with films of hexadecylamine hydrochloride previously described, this indicates that some metal-to-glass contact does occur through a solid film. (d) Effect of Homology on Film Durability; the Fatty Acid Series.-Eight members of the homologous series of fatty acids ranging from 8 to 20 carbon atoms per molecule were studied in order to learn the effect on durability of varying the length of the principal chain of the molecule. The method of preparation of each film is indicated in Table 11, and the results of measurements of pk during the multiple traverses are recorded in Table NUMBER TRAVERSES ( V I . Fig. 4.-Durability of monolayers of fatty acids under a load I1 and plotted in Figs. 4, 5 and 6. In Fig. 4 is included the value of the methylene iodide contact of 1000 g. angle (Om,,) for each film of fatty acid. Evidently, only monolayers with 13 or more carbon atoms per molecule can endure 10 or more traverses under a 1000-g. load. The rapid decrease in durability of the fatty acid monolayer (under a 1000-g. load) as the number of carbon atoms per molecule ( N ) decreases below 14 is made evident by plotting in Fig. 5 the vahie of pk after 10 traverses as a function of N . Inspection of Fig. 5 reveals that monolayers behave like liquids when 8 5 ' N 5 12, amorphous or plastic solids when 13 S N 5 15 and crystalline solids when N 2 16. These conclusions are consistent with the durability data of Fig. 6 which were obtained with a load of 5000 g. Thus, plC remained constant at 0.06 for 20 traverses on films having N = 16 and N = 18 whereas monolayers of compounds having N = 13 and N = 14 began to break down at about the 15th traverse. Films of fatty acids with 8 5 N 5 12 failed on the first traverse (see Table IT). The large difference in durability (under loads of 1000 and 5000 g.) between solid and liquid monolayers agrees with the conclusion of Part I that the films are solid when NUMSER OF C A R B O N ATOMS I N T U E STRAIGHT CHAIN ( N l . N 2 14. The results obtained with films of eicoFig. 5.-Coefficient of kinetic fric,tion after 10 traverses sanoic acid prepared by retraction from hexadecane under a load of 1000 g. for monolayers of fatty acids. solution using short immersion times (see Fig. 3B) show (like films of the amines) the effect of mixed u 5( I I I 1 film formation in decreasing durability. L (e) Comparative Durability of Films Adsorbed 0.4C at the Solid/Air and Solid/Liquid 1nterfaces.-The z 0 durability of a liquid monolayer of fatty acid !! adsorbed at the solid/air interface would be exE 0.3C pected to be less than that of the same film adsorbed u at the interface between the solid and the bulk z liquid phase of the fatty acid, because in the latter 02c case the holes in the film caused by desorption or c other damage could be repaired readily by the 3 OK adsorption of molecules from the adjacent liquid u. phase. However, one would expect the values of :005 pk during the first traverses to be identical. This I t 1 1 I 0 1 I 5 10 I5 20 25 30 mas found to be true in experiments with octanoic NUMBER O F T R N E R S E S ( ' v . acid, which is liquid at 25'. Using a 1000 g. load Fig. B.-Durnbility of monolayers of fatty acids under a load to slide on the monolayer at the solid/air interface, of 5000 g. pk increased rapidly from its initial value of 0.17 a two-thirds-hour immersion was an unstable film. with the number of traverses as shown in Fig. 4. This condition did not persist if sufficient time (20 On repeating the measurements using instead a hours) was allowed before retraction for the attain- monolayer covered by a pool of the liquid acid, p OF
I
w
0
W
'L
I
I
-
MECHANICAL DURABILITY OF ALIPHATIC POLAR COMPOUNDS
Sept., 1957
1193
TABLE 111 DURABILITY DATAFOR CONDENSED MONOLAYERS OF HALOQENATED ALKANEDERIVATIVES (Data a t 25" unless otherwise stated) Compound
FaC( CFz)iaCOOH FaC( CF2)sCOOH HFzC(CF2)isCOOH HFe( CFz)iiCOOH HF&( CF3)iiCHSOH FsC( CHz)&OOH
ema.
Method of reparation of &m
(degrees)
Retraction from n-decane 102 s o h (satd.) 97 Retraction from n-decane soln.; concn. E 1 X 10-4 moles/l. Retraction from %decane 89 soln. (satd.) Retraction from n-decane 88-89 soln. (satd.) Retraction from n-decane 88 soln. (satd.) Vapor-condensation at 100" 78
Durability Load No.of
W
traverses
(P.1
(4
Variation ln Irk
1000
5000
40 24
0.12-0.13
1000
13
0.08-0.09
5000 5000
4 30
(O-IGO g.)
9000
16
0.09
5000
20
0 .os 4 0.10 0.10 -.c 0.42 0.13 0.72 0.16 0.40 0.10 +-0.15 0.10 0.12 0.10-,0.39
0.08
5000
15
0.08+0.40
1000
35 20
0.07-+0.07 .07+ .07
10 11
.07+ .07 .07+ .07 0.07+0.07
0.09
0.07
5000
7000 FaC(CHz)irNHsCl Cl( CFaCFC1)nCFaCOOH
Retraction from aq. soh. (satd.); natural pH Vapor-condensation a t 25"
BrCHa( CHz)&OOH
Vapor-condensation a t 70'
73-74
0.0741.08
9000 1000
5000 77
0.10
42
0.10
1000
5000 1000
5000
-+
-+
--+
20 4 20 25
0.08 0.16 0.13 0.10 0.13 0.32 0.09-.0.85 -+
-+ -+
16 1
0.44
remained constant a t the value of 0.17 during a large number of traverses (see Table 11). (f) Effect on Film Durability of Halogen Substitution.-The effect of substituting covalentbonded fluorine, chlorine or bromine atoms for hydrogen on the durability of a condensed monolayer of a paraffin derivative was studied. The H F I C ( C F 2 \ , COOH (W.5000 qmr) halogenated compounds used have been described I in Part 1 . 6 The method of preparation of each .. condensed film and the results of the measurement 0$ 0 4 0 Omax, Mk and durability are given in Table 111. E The data show the behavior of a condensed 9 030 monolayer of perfluorolauric acid, F3C(CFz)lo- z COOH, when traversed repeatedly by the slidep 0 under loads of 1000 and 5000 g. These results are c 020 to be compared with those obtained with a load of Y 1000 g. in a similar series of measurements on lauric 0 acid (previously given in Table I1 and in Fig. 4). ws 010 Thus a monolayer of lauric acid broke down rapidly under a 1000-g. load after several traverses whereas I I I I I 1 0; I 5 10 15 20 25 30 a monolayer of perfluorolauric acid was so stable NUMBER OF T R A V E R S E S ("1. that after 40 traverses pk was still only 0.10 (see Table 111). Furthermore, the lauric acid mono- Fig. 7.-Durability of monolayers of +alkanoic acids and a #'-a1 kanol. layer under a 5000-g. load failed completely during the first traverse (see Table 11) whereas the perCondensed monolayers of two +alkanoic acids, fluorolauric acid film exhibited a value of /& of orlly HFzC(CF2)&OOH and HF&(CF,)llCOOH, and 0.14 after 14 traverses. Therefore, we can con- a 9'-alkanol, HF&(CF,),,CH2OH, were prepared clude, as in Part I, that the condensed monolayers by retraction from decane solutions. The frictional of perfluorolauric acid were solid whereas the con- and durability behavior of each is summarized densed monolayers of lauric acid were liquid. in Table I11 and graphed in Fig. 7. The ability of During the first traverse on a monolayer of per- each of these films to withstand a load of 5000-g. fluorolauric acid pk was slightly greater under the for a t least 10 traverses is strong evidence for con5000-g. load than under the 1000-g. load. Presum- cluding as in Part I, .that each is solid. A monoably, this is due to the increased area of contact layer of HFzC(CF,)13COOH behaves like a crystalbetween metal and glass a t the higher load. line solid under loads of 5000 g. for 30 traverses Although a liquid monolayer of perfluorooctanoic and loads of 9000 g. for 16 traverses; the results acid (Table 111) is much less durable than a solid should be compared with the behavior of films of monolayer of perfluorolauric acid, it is more dur- the fatty acids of N = 16 and N = 18 (see Fig. 6 I
LL
005tt-i--t
,a
1194
0. LEVINEAND W. A. ZISMAN
Vol. 61
ana Table 11). The lower durability of a mono- veals that the molecules in the present film were layer of HF&(CF2)nCOOH under an applied load more closely packed. Values of Om,, of 73 to 74" of 15000 g. shows that the decrease in chain length reported here and in Part I6 for films of w,w,win going from N = 15 t o N = 13 causes a transition trifluorooctadecylamine hydrochloride also indicate in state of the adsorbed monolayer from crystal- these molecules were not in closest packing. The linity to plasticity. This results from the decrease difficulties encountered in adsorbing this amine as in intermolecular cohesional energy caused by re- a close-packed monolayer on glass from aqueous ducing the length of the aliphatic chain. A solution is believed due to the low solubility of this comparison of the durabilities of HFzC(CF2)ll- substance in water. COOH and HFzC(CF&lCH20H reveals that the Condensed monolayers of the highly chlorinated alcohol monolayer is more plastic than the corre- acid, C1(CF2CFC1)3CF2COOH, and of an wsponding acid monolayer. This is consiatent with brominated alkanoic acid, BrCH2(CH2)sCOOH, the conclusion of Part I where a comparison of the were prepared by vapor-condensation. Results of frictional behavior between the fatty acids and measurements of friction and durability under refatty alcohols revealed that the hydroxyl group ad- peated traverses with various applied loads are heres less strongly to the glass substrate than does given in Table 111 and plotted in Fig. 8. A comthe carboxyl group. parison of the curve of Uk vs. v (obtained with a A comparison of the curves in Fig. 7 of p k v8. v 1000-g. load) for w-bromoundecanoic acid with for monolayers of HFzC(CF2)llCOOH and F3C- those of Fig. 4 for the fatty acids having from 8 to (CF2)loCOOH under a 5000 g. load reveals that the 12 carbon atoms per molecule reveals that the for12-carbon acid with the terininal -CFs group is mer monolayer must be liquid. Thus, p k increases slightly more durable than is the 13-carbon acid in value from 0.09 to about 0.17 after 10 traverses with the terminal -CF2H group. Hence, the and then continues to rise to a value of p k = 0.85 nature of the outermost terminal group does help during the next 6 traverses. The value of pk to determine the durability of a physically ad- 0.17 at the end of the first 10 traverses is to be comsorbed monolayer. The greater durability of a pared with that of 0.35 for the corresponding 11film of the 4-alkanoic acid could be caused either carbon fatty acid, obtained by interpolation of the by the lower shea,ringstress required for slippage be- curve of Fig. 5. This indicates that the substitutween two parallel planes of close packed -CF3 tion of a covalent bonded w-hydrogen atom of an 11groups or by the lower energy of adhesion between carbon fatty acid by a bromine atom produces a more durable monolayer. It is presumed this is such surfaces. A series of measurements of pk vs. v were made caused by an increase in the intermolecular coon a condensed monolayer of HFzC(CF2)13CH20H hesion. This conclusion is consistent with that prepared by retraction from decane solution, dur- presented in Part I to explain the ability of wing which the ball slider was rotated to a clean area bromo substitution to decrease the value of pk obat the end of each traverse. Quite like the results served in a single traverse. Inspection of the pk vs. Y curves of Fig. 10 for of analogous experiments with the aliphatic amino compounds exemplified in Fig. 2, the graph of Uk C1(CF2CFC1)3CF2COOH and F3C(CF2)gCOOH, vs. S exhibited a rise in ,uk during each of the early makes evident the large and beneficial effect on traverses followed by a gradual drop to a much film strength caused by the substitution of covalent lower and steady-state value. Also, there was a bonded chlorine atoms for fluorine. Thus, whereas rise in the steady-state value of pk with Y which perfluorooctanoic acid forms a liquid monolayer, was greater, the larger the load. Thus when a load CI(CF&FC1)3CF2COOH forms a plastic solid of 5000 g. was used, pk rose from an initial value of monolayer. The greater mechanical strength of 0.09 at the end of the first traverse to a value of 0.37 the chlorinated film is attributed, as concluded in at the end of the second traverse. Hence, like the Part I, to the larger energy of intermolecular coparaffin derivatives, desorption of physically ad- hesion. It is noteworthy that the value of p k sorbed molecules from the glaes to the sliding ball under a 5000-g. load is larger after one traverse than is a mechanism contributing to the mechanical over a monolayer of C1(CF2CFC1)3CF2COOH a 1000-g. load. As with the was observed under failure of highly fluorinated paraffin derivatives. film of perfluorolauric acid mentioned previously, Condensed monolayers of w,w,w-trifluorostearic this results from the increased area of contact beacid and of w,w,w-trifluorooctadecylamine,the for- tween the rubbing solids at the higher load. The mer prepared by vapor-condensation and the latter durable nature of a plastic film of Cl(CF&FC1)3by retraction from aqueous solution, were each studied by the multiple-traverse technique without CF&OOH is made evident by the equal values of rotating the ball, and the results are summarized (dpk/dv) of a film subjected to 20 successive travin Table 111. A close-packed monolayer of w,w,w- erses under loads of 1000 and 5000 g., even trifluorostearic acid (e, = 78") behaved similarly though there is an increased area of metal/glass to stearic acid and is therefore believed to form a contact at the higher load. Discussion two-dimensional solid. Thus, with repeated traThis investigation shows that the use of the verses under loads of 1000, 5000, 7000 and 9000 g, p k remained constant at 0.07. These data prove multiple-traverse procedure in conjunction with there was little or no loss in adlineating ability of the adsorption, wettability and friction-measuring molecules of this compound because of the ter- techniques described in Part I , 6 results in a powerminal -CF3 group. Comparison of the emax value ful tool for studying the structure and composition of 78" with the value of, 74" reported in Part 1: re- of adsorbed organic monolayers on solid surfaces.
*
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Sept., 1957
MECHANICAL DURABILITY OF ALIPHATICPOLAR COMPOUNDS
The resolving power and value of these new surface chemical techniques may be illustrated by the reproducibility and self-consistency of the frictional and durability properties for mixed films of nonpolar solvent and primary amine (or fatty acid) molecules and single-component monolayers of aliphatic amino compounds, fatty acids, 4-alkanoic acids, +-alkanoic acids, +’-alkanols, terminally fluorinated octadecyl derivatives and highly chlorinated and w-brominated alkanoic acids. By correlating the constitution, shape, orientation and packing of the molecules comprising the films with their wettability, friction and wear properties, it has become possible to formulate the basic requirements for good boundary lubrication. By the use of the multiple-traverse technique and (when necessary) higher loads, it has been shown that some monolayers of aliphatic polar compounds adsorbed from hexadecane solution are mixed films of polar compound and solvent molecules. Evidently, the long, rod-like, hexadecane molecule can form a mixed film with the aliphatic polar solute because of the ability of the paraffin chains to adlineate or cohere in a parallel molecular array. When the solvent molecules cannot adlineate with the polar molecules, as is the case with the almost spherical nitromethane molecule, mixed film formation cannot take place. Our experiments also demonstrate that the mixed films represent a transient or metastable state the probability of whose formation though high early in the adsorption process, becomes low as film packing increases; if sufficient polar solute is present, the film eventually becomes free of solvent at adsorption equilibrium. With the highly, adsorptive polar compounds discussed here, no mixed films were formed at equilibrium even when solute concentrations of only mole/liter were used. This supplies a long-sought explanation t o the long time required to attain adsorption equilibrium in such systems. Inability to detect a mixed adsorbed film by wettability measurements with methylene iodide results from the fact that the adlineated hexadecane molecules present the same outermost surface of -CH, groups as do the adsorbed polar molecules. It has been shown* that the wettability of an adsorbed monolayer is determined essentially by the nature and packing of the outermost surface atoms or radicals. The inability of measurements of pk over the lower load range 0 to 600 g. to reveal the existence of the two-component film is attributed to the solid state of both the mixed films and the pure films. If an organic monolayer at the interface between two rubbing solids is to cause minimum friction and wear, it must be a solid monolayer. The results of Parts I and I1 permit firm conclusions about the molecular structural requirements in order that a monolayer of polar molecules should be able to adsorb as a two-dimensional solid boundary lubricant. Most important of all, the adsorbed molecules must be able to adlineate in order to at(8) W. A. Zisman, “The Relation of Chemical Constitution to the Wetting and Spreading of Liquids on Solids,” to be published. Presented at O N R Decennial Symposium, Washington, D. C . , March 19, 1957.
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tain a large enough value of the intermolecular cohesional energy to form the most impenetrable solid film possible. A long chain of methylene 0 90
ob
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I I
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s
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1 10 NUMBER
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20
25
I IS OF TRAVERSES
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Fig. 8.-Durability of monolayers of highly chlorinated and w-brominated alkanoic acids.
groups, or covalently bonded chlorine or bromine atoms substituted along the chain are particularly desirable structural features. Next, the polar group of the monolayer should strongly adhere to the substrate. Third, the outermost terminal groups of the adsorbed molecules should comprise a surface having the lowest possible free surface energy. This will assure low adhesion between the monolayers attached to the two rubbing surfaces. Ideal structures for the outermost terminal groups are -CF3, -CF,H and -CH3. Finally, much loss in film durability can result if the conditions are such as to permit the formation of a mixed monolayer. Thus, a monolayer of a polar additive adsorbed from oil solution as a singlecomponent film at the boundary between two rubbing solids immersed in the oil will be more effective in decreasing the wear of the underlying solid surfaces than if adsorbed as a mixed film of polar and non-polar molecules. Bowden and Leben’s2study of the durability of Langmuir-Blodgett monolayers and multilayers on steel showed that a condensed monolayer broke down in a few traverses. The greater durability exhibited by adsorbed monolayers on glass in our investigation is attested by the independence of pk of the load for loads as great as 9000 g. (75 kg./ mm.2, or 107,000 1b./h2). The ability of these films to withstand dozens of repeated traverses of the slider under such high unit loads must be caused by their close-packed, crystalline-like, solid structures. The observed durability of these solid monolayers a t the interface of stainless steel and glass makes untenable the conclusion that they reduce friction and wear because of their low shear strength. Very little shearing takes place within these films during a single traverse on a highly polished surface. Our observations are more in
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IGNACIO TINOCO, JR.,AND MARKP. FREEUN
accord with Hardy’sg early views which stressed the importance of the existence of low adhesion and easy slippage in the interface defined by contact of the outermost terminal groups of the monolayers adsorbed on each of the two rubbing solid surfaces. Examination of the glass slide after many traverses at high loads reveals the presence of slight wear scars, which confirms previous work2 that even in the presence of a solid film, there is always some metal/glam contact. Unquestionably, however, the high resistance t o penetration of the solid monolayers is also essential to decrease greatly the real area of contact of metal and glass. The roughness of the two sliding surfaces would therefore be expected to have some effect on the durability and hence the wear preventive properties (9) W. B. Hardy, “Collected Scientific Papers,” Cambridge Univesity Press, 1936.
Vol. 61
of adsorbed films. Research on this point will be reported later. The results obtained by rotating the ball slider before each successive traverse over the filmcoated slide reveal that desorption from the glass slide to the steel ball takes place quite readily. The depletion of molecules from the slide causes a large increase in the area of contact between surface asperities of the two solids with a proportionate increase in the friction and wear. Where desorption from a rubbing surface is a controlling factor in film failure, it is necessary to have a source of supply of additional polar molecules nearby in order to replenish the film-depleted surface. The importance of film replenishment in keeping friction and wear to a minimum is illustrated in Table I1 by the durability behavior of a film of octanoic acid under a 1000 g. load both in the presence and absence of a surrounding pool of the liquid acid.
THE OPTICAL ACTIVITY OF ORIENTED COPPER HELICES. I. EXPERIMENTAL BY IONACIO TINOCO, JR.,AND MARKP. FREEMAN Contribution from the Department of Chemistry, University of California, Berkeley, California Rscsbbd M a y 8, 1867
The rotation of linearly polarized microwaves by copper helices has been measured as a function of wave length. The optical activity, both along the helical axis eS3,and perpendicular to the helical axis ell, has been obtained, The rotatory dispersion curves can be described by Drude equations; 038 contains positive terms, while e,1 contains negative terms. The optically active absorption wave lengths are simply related to the length of wire in the helix and are nearly equal for the two optical activities.
Introduction The current interest in the optical activity of helical has prompted us to study a model system of macroscopic helices. It is hoped that by measuring the rotation of microwaves by oriented helices whose dimensions can be varied systematically, present theories of the optical activity of helices*+‘ can be tested, and that new insight may be gained. Lindman’ found that the rotatory dispersion of a system of randomly oriented 9 cm. copper helices could be described in the wave length range from 12 t o 34 cm. by a one term Drude equation. That is, the wave length dependence of the optical rotation of the helices had the same form as that observed €or molecules. Recently Winkler8 attempted t o repeat this work and claimed that the optical rotation observed by Lindman was due t o anisotropic scattering and not to optical activity. We have therefore carefully tested for diffraction effects in our apparatus. Furthermore, unlike Lmdman and Winkler, we have studied oriented arrays of heli(1) J. T. Yang and P. Doty, J . A m . Cham. SOC.,79, 761 (1957). (2) C. Cohen and A. a. Szent-Gyorgyi, ibid., 79, 248 (1957). (3) P. Doty and R. D . Lundberg, Proc. Natl. Acad. Sci., U.S.,43, 213 (1957). (4) D. D. Fitta and J. G. Kirkwood, ibid.. 41, 33 (1956). 736 (1956). (5) W. Moffitt, ibid., a, (6) W. Kauzmann, “Quantum Chemistry,” Academic Press, Ino., New York. N. Y.,1957, p. 616. (7) K. F. Lindman, Ann. Phtiaik, 63, 621 (1920). (8) M. H.Winkler, THIEJOURNAL, 60, 1665 (1956).
ces. The optical activity could therefore be measured both for light incident along the helical axis and for light perpendicular to the helical axis. The optical activity of a randomly oriented sample has been shown to be the weighted mean of these two activities. Experimental .The microwave polarimeter had three principal parts: a polarizer, a container for the systems studied and an analyzer. Each of these components was mounted separately on a 2 meter optical bench. The polarizer consisted of a section of x band (3 cm.) wave guide (25.4 cm. long) and a klystron oscillator. The polarization of the radiation was caused by the physical dimensions of this wave guide and the natural polarization of a klystron oscillator. For the type723 klystron (A = 3.0 to 3.65 cm.) theradiofre uency power was injected 10.3 cm. from the analyzer end of &e rectangular wave guide. This end was terminated in a horn and the wave guide was tuned with a shorting stub inserted in the other end. For the wave length region 2.4 to 3.4 cm., a Varian Associates type X-13 klystron was used and the power was injected into the end of the polarizer through a gyrator. For both of these oscillators the wave length was determined by use of an F-R Machine Works type X410A absorption wave meter mounted between the horn and the wave guide on either the polarizer or the analyzer. The highest frequency region (2.1 to 2.44 cm.) was covered by a Varian Associates type X-12 klystron. Thls was coupled to a 2 cm. wave guide by means of an adjustable attenuator and a Tee joint which was connected to the end of the polarizer by a tapered section of wave guide. The wave length was determined by a “magic tee” and reactance type wave meter (De Mornay Bonardi type F715-1) attached to the other leg of the input tee. The (9) I. Tinooo, Jr., and W. G. Hammerle, ibid., 60, 1619 (1956).
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