strongly ordered as the free amine. A simple quantitative model, relating the variation of contact angles with concentration to the adsorption isotherm, correctly represents the wetting data. Acknowledgments.-It is a pleasure to ack-
nowledge helpful discussions with Professors R. S. Hansen and F. E. Bartell. We are grateful to the American Petroleum Institute for a Grant-in-Aid which contributed materially to the equipment used.
COSSTITUTITE RELATIONS I N THE TTETTISG OF LOW ESERGY StTRF.QCESASD THE THEORY OF THE RETRACTIOS METHOD OF PREPARING ilfOSOLAYERS1 B Y ELAINE G. SHBFRIN AND
WILLIAM
A.
ZISX4N
U . S . S a v a l Research Laboratory, Washington 65, D . C. Receiaed November l i , I969
Earlier systematic studies of the angle of contact (0) exhibited by drops of liquid on plane, polid surfaces of low surface energy have revealed a regular linear variation in cos e JTith the surface tension ( Y L Y ) of a large variety of liquids; this led to the cqncept of the critical surface tension of spreading (re)and its use in characterizing the aettability of organic solids and of high energy surfaces coated with adsorbed organic films. Effects of the nature and packing of the atoms or organic radicals in the organic surface in determining the wetting of the solid are summarized. Simple and useful correlations have been found between y o and the constitution of low energy solid surfaces. It is concluded that usually atoms more than a few atom diameters below the surface have no influence on wetting. The "retraction method" of preparing monomolecular films from solutions on solids is shown to be a direct consequence of the above constitutive law of wetting. The same analysis can be applicd to a pure liquid also, and it results in the explanation of the behavior of the autophobic liquids a t room temperature and of the process of depositing a monolayer on a solid by retraction from the melt over a range of temperatures.
Introduction organic liquids. This has led to the useful concept Since the tendency for a drop of liquid to spread of the rritical surface tension for spreading ( y c ) over a plane solid surface increases as the contact for each series, as defined bj7 the intercept of the V the cos 6 = 1 angle e decreases, the coiitact angle provides a straight line plot of cos e vs. ~ L with useful, inverse measure of wettabilit'y. Providing axis. This concept also has proved useful in apsuitable precaut,ions are talien to employ smooth, proximately describing the spreading behavior of a clean, solid surfaces and pure, well-defined mate- much greater variety of liquids. Close correlations haye been discoTered between rials, t'hert: is no difficulty in obtaining reliable, reproducible measurements of 6. When the Eolid yc and the constitution of the solid surface."14 low energy surface is plane, smooth and non-po- The purpose of this paper is to summarize the rous,z the s10'~~ly advancing and receding contact constitutive law found to describe the wettability angles are equal. In all cases here, reference is to of low energy surfaces and to show that it leads to a rational explanation of the retraction method of the slowly advancing c,oiitactangle. preparing condensed monolayers a t the solid-air Systematic studies have been r e p ~ r t e d ~ -earlier '~ for the contact angles of a wide variety of interface. pure liquids on low energy solid s u r f a c e ~ * - ~ ~ The , ~ ~ Constitutive Law of Wettabi1ity.-The and high energy surfaces.11112 These have re- striking regularities in the contact angles exhibited vealed significant regularities in the wettability of by pure liquids on lorn energy surfaces are exorganic crystals and polymers as well as of high emplified by the data for the homologous series of rnergy surfaces modified by t,he adsorption of liquid n-alkanes on several types of fluorinated solid monolayers of oriented organic molecules. A rec- surfaces. In Fig. 1 is plotted cos 6 for each alkane tilinear relation has been established empirically on a given solid against the surface tension ~ y ~ v ) hetween the cosine of the contact angle and the of the liquid; each curve thus represents the wetsurface tension ( y ~ v for ) each homologous series of ting behavior of a single surface. Cur\ e A of Fig. 1 is a plot of the data for the n-alkanes (from pen(1) Presented at t h e Spring Meeting of t h e Division of Colloid tane to hexadecane) on smooth, clean polytetraChemistry of the Ameriran Chemical Society in Boston, Xassachusctts, April 8, 1959. fluoroethylene (Teflon), and it exemplifies the ( 2 ) H, W. Fox a n d W. A. Zisman, . I Coll. . Sci., 5, 514 (1950). rectilinearity of the cos 0 vs. ~ L plot. V The lower (3) (a) 1%W.Fox a n d 'iv Zisman, ibid., 7 , 109 (1952); (b) 7,428 V the wetting liquid, the smaller 6 is and the ~ L of (1952). the more wettable the surface; below a critical (4) E. G. S i a f r i n a n d W. A. Zisman, i b i d . , 7 , 166 (1952). (%j)F. Shuliiian a n d W. -4.Zisman, ibid., 7 , 465 (1952). value of the surface tension of the liquids, denoted (tj) 1I. W. Fox, E. F . IIare and W.A. Zisman, ibid., 8, 194 (1953). as yo, the contact angle is zero. (7) A. €I. I2 lison, €€. W. Fox a n d 1". A. Zisman, THISJ O U R S A L67, , Cur\-e R of Fig. 1 presents recent datal5for n new 622 (i953). a n d W. A. Zisman. ibid., 68,236 (1951). ( 8 ) E. F. HFre, E. G. commercial copolymer of tetrafluoroethylene and (9) A. H. ELlison a n Zisman, ibid., 68, 260 (1954). . Zisman, ibid., 68, 503 (1954). (10) A. H. I
