THE JOURNAL OF
PHYSICAL CHEMISTRY (Registcred in
U. S. Patent Ofice)
(0Copyright,
19GO. by the Anierican Choniical Society)
WETTIXG OF SOLIDS BY SOLUTIOKS AS A F U S C T I O S OF SOLUTE ADSOKPTIOS'?' BY It. J. RUCH fiistitutc JUT
A o i k
.4SU
L.
s. BdRTJ:I,L
Kescurch atLd Department of Chciiiislry, Zozua Slule C'iiicosily, i l tiles, lolocz Iieceiaed Xonember l l * 195.9
The wetting of flat plati~iumand chromium slides of small surface arca by aqueous solutions of decylaminc \>as measured as a function of the adsorption of the amine and the surface tension of the solutions. Adsorption a t the solid-solution intcrface was memured ~ns i t u by an optical polarimetric method and wetting was determined concurrently by contact angle measurements employing the captive bubble technique. Adsorption and wetting results for platinum were sharply defined but those for chromium R-ere somewhat obscured by erratic kinetic effects. Multilal er adsorption was observed in all cascs, and isotherms on platinum in basic solutions exhibited steps. The observed contact angles of the solutions on platinum rose from 0" to a maximum of about '30" as adsorption increased, and then fell, sometimes to O", as adsorption proceeded further. -4model is proposed to esplain the metting results which provides a senii-quantitative scheme for computing the behavior of the contact angles from the adsorption isotherms.
Introduction Becaust of tlhe ability of molecules on a surface t,o rriask the force fields of underlying molecules, the wetting of surfaces may be strongly influenced by adsorptioii. When orientational effects are present, the dependence of wett,iiig upon surface excess of adsorbate may be complex. While the problem is understood in a qualitative way, aiid, indeed, is utilized in a wide variety of commercial processes, no theories seem to have been proposed which have allowed the direct calculation of contact angles as a function of adsorption from simple observables. The difficulty in measuring adsorpt'ion on small surface areas or of wetting on powders has greatly impeded progress. Several iriforniat'ive studies have heen madc., iicverthelese, which circunivent,ed sonie of tlie diff icultics h y utiliziiig polished specimens for \vct,t8iiignieasurcnieiit.s :md crushed specimens for adsorption as, for csample, by Gaudin a i d co-\vorlters.~ In the preseiit study ail optical polurimet8ric met,hod'-" was used to measure adsorption in situ
on flat polished surfaces of small area. Coiitact angles mere measured on the same surfaces by the captive bubble technique. The systems investigat,ed, namely, aqueous solutioiis of n-decylamine on polished platinum aiid chromium, exhibited coiltact, angles which rose to a maximum and then fell as the coiiceiitrat8ionof adsorbate was increased. The purpose of the present iiivestigation was, first, to deTelop the optical methwl for studying adsorption of small molecules 011 Bat surfacek, and secondly, t.o elucidate the structural basis of wetting of solids by solutions. Experimental Details
3 9 , 4 8 1 (18Yo:.
-.
Materials.-Thc same piece of pl:ttinnni, 20 X 10 X 2 mm., was used for all of the adsorption cqcrinients 011 platinum. Thc plat~iriuni was polished wit,h wet I3c:hrManning emcry polishing p:qier followed til- Lincie A on :L ~~olishing wheel covcred with Buehlcr Ltd. finest quality microcloth. In exporimerits on chroniiiini, sepamte chromium plated st,cel slides having an arca of about 2 cm.2\ywo cut from wmmercial ferrotype plates for each adsorption trial. Prior to an adsorption trial, the platinum and chromium slides were cleaned by hand polishing with I inde (1) W o r k ~ i ' a s performed i n tile Aines Laboratiiry of tlie U. S. A or Linde B, flushing in a stream of distilled water, and wiping dry with a benzene dampened Kleenex. The platinuin Atomic Energy Commission. slide was flamed in a 31eker burner t,o a bright red glow and (2) Based on a dissertation by R . J . Ruch t o t h e Graduate School, cooled for two minutes after the disappearance of the glow Iowa S t a t e University. in partial fulfillment. of t h e requirements for t h e before i t was placed in the adsorption medium. The degree of Doctor of Philosophy, 1939. chromium slides were passed through t>heflame of a Meker (3) A. RI. .>nudin, "Flotation," 2nd Edition, RIcCraiv-Hill Book hiirner six t,imes and cooled 35 seconds. The surfaces arere ('(I., rnc., K ~ T x o r k , N. Y . , 1'337. p ~ ) 176, . 468. :Lasnmed to tie free of impurities and suitable for adsorption (I) l'. Drurle, d n n . p h y s i k . Chen..36, 632 (18861; 36, 805 (1889); ( 5 ) L. Tronstad, 7 ' ~ ~ sI.'aroduy . Suc., 29, 202 (IUJY).
\ I > )A.
Itothuii, Nci-. Scs. lrmtr.8 16, 20 (1945).
It. J. RUCHAND L. S.BARTELL
5 14
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Ir 3 5 -
I
I
101 '
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-
LOG CONCENTRATION MOLES/LITER,
Fie. 1. -6urfac.e tension of aoueous solutions of dervlamii; hydrochloride: 0 (upperLabscissa); pH 10.2; 28"; rystem buffered with 0.05 M 4aJCO3and 0.05 M NaHC03. (lower abscissa): pH 6.0-6.2: 28.5"; no buffer, acid or 6ase add(,d to system
!-
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pH.93
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7'
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CChCENTEAT OWMOLES/LI'ER
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Vol. 64
Air bubbles for contact angle measurements were formed with a micro-syringe attached to a 0.1 ml. pipet having a flat Teflon tip. The bubbles were viewed with a low-power microscope having a filar micrometer eyepiece. The microscope was mounted in a graduated rotating object-holder and contact angles were measured directly. Procedures.-The concentration of solutions used for adsorption and for surface tension measurements was increased by pipetting stock solutions of decylamine hydrochloride to known quantities of the adsorption medium. No corrections were made for adsorption on container walls. The error for the most dilute solutions used in adsorption would have been about lOy0if unimolecular adsorption had occurred and the glass had a roughness factor of one. The pH in the basic region was controlled \\ith sodium carbonate-sodium bicarbonate buffer or by using 0.1 S potassium hydroxide and a Radiometer Model T T T l:t automatic titrator. Doubly distilled water from the still had a pH of approximately 6 and was used Tithout buffrr, acid or base in adsorption trials a t this pH value. Solutions used for surface tension measurements were stirred for three minutes after the addition of the amine h l drochloride. Readings were taken over a 10 to 20 minute period until the drift of the readings had fallen to a point where the average deviation of the last five to seven readings was about 0.1 dyne/cm. I n studies of adsorption, a t least 15 minutes and often more than one hour was allowed for equilibrium to be reached. Optical measurements mere taken after ( , R and S,respectively, can be for adsorption. In many cases it v a s riecweary inferred, at any concentration, from the adsoryticii for air huhbles to remain in coiitact with the isotherm. The amine molecules in the first layer chromium surfaces for a minute or longer before the w e presumed t o he attached to the solid principally 1)uhl)les heciinie attached and measuremeiits could by their polar groupc: such that their hydrocarbon be made. On platinum adhesion of the air 1~ul)hlec: chains are exposed to the adsorption medium. The t o the surfwe nsunlly tork place in .-ecwidq. molecules iii the second layer are postulated to adsorb on those of the first iii the opposite otientation, Proposed Model of Wetting Tlir revealing shape of the adsorption isotherms with polar groups directed toward the adsorption and c l o d y cornxlated hehavior of the contact niedium. The eff ecti1-e boundary tensions are angles siiggeqtr a Finzple model of the wetting proc- considered t o be the area average of the individual ws. The nmdel. illurtratetl schematically in I'ig. tensions of the regions pregent. Representative regions of each type are assumccl to be of sufficient 10, inakes ucc of the familiar J-oung cquatioii size that edge effects can be ignored. Equation 1 -is i - 391. = YLA COS 6 (1 1 then becomes The contact angle 8 can he calculated if the appro- P ( - , n 4 - m r ) R!Y\f4 - yur) S ( r a 4 - ynr'i = priate boundary tensions, yi, are known. The sur-,Ls ('OS B ( 2 ) is measured diface tension of the solution, rectly. In t,he proposed model the other tensions where the subscripts W, ;\I and D refer to a d s d i e d are derivable from the adsorption isotherm as fol- water, monolayer of amine and double molecular layer of amine, respectively. lows. The adsorbed multilayer of water iq assiimed to be In basic mlutions it is considered that the siirface of the solid i q divided into regions varioudy essentially liquid-like, and hence (ywa - y~v1,Jbe-
+
+
318
xl-
F;,
=
YOUNG'S
y
LA
cos9
EQUATION
TPPPP P+R+S= I 0 = WATER
+--'POLAR-NON-POLAR MOLECULE WzWATER M-MONOLAYER D=DOUBLE MOLECULAR LAYER
MODIFIED
Fig. 10.-1'1
YOUNG'S
EOUATION
oposed model for calculating contact angles.
comes simply the surface tension of water, YWA. The value of 72 dynes/cm. was taken. The value of (YMA - Y M L ) may be eLtimated from the contact angle of water on various types of hydrocarbon surfaces with the aid of 'I-oung's equation. The surface involved should in many respects be intermediate hetween close-packed methyl surfaces such as are found in crystalline hydrocarbons, and methylene surfaces found in polyethylene. Alternatively they should resemble monolayers of n-octadecg lamine, although the packing is not as compact in the shorter compound. The contact angles of water or1 these surfaces of ill", 9.2" and 102" reported by Zismaii, et suggest that a plausible value for ( Y X ~ A- YML) is - 15 dynes/cm., the average for the three surfaces. The hydrated surface of the double adsorbed layer may be considered to have a composition similar to that of concentrated ammonium hydroxide. The interfacial tension between such a surface and water should be low. Hence, (YDA YDL) was taken for the purpose of cemparisori as 57 dynes 'cm., the surface tension of concentrated aminoniuni hydroxide. The remaining variables, P , R and S, are not uniquely defined hy the isotherm, but may be estimated t o within small limits if the asymptotic value correspoading to AE,, the monolayer reading, is assumcd to be known. Plausible values, AE,, representing adsorption of molecules in the first layer, were inferred from curves constructed to connect smoothly the initial portions of the isotherms with the asymptotic line, AE,. The dotted lines in Fig. 2-4, and the dot dashed line in Fig. 5 correspond to these estimates of AE, for an assumed asymptote of 0.55". The fraction P then equals ( A E , - AE,)/AE,, S equals ( A E - AE, AE,, and R equals 1 - (P S). In calculations, I E values were taken from the smooth curves drawn through experimental points. ~
1
.
~
+
(11) H. W. F o x and W. A. Zisman, J . Colloid Sei., 7 , 428 (1952). (12) E. G . Sliafrin a n a W. A . Zisman, %hid.,7, 100 (1952).
~
3
h comparison between calculated and obwrved contact angles is presented in Fig. 7 and 8. The solid calculated curves are based on a value of AE, of 0.55". In Fig. 8 the effect of varying the choice of AE, from 0.55 to 0.65" is hh0v-n. On the whole the agreement betxyeen experiment and calculation is surprisingly good considering the simplicity of the model. S o attempt, lias beeii made to vary the parameters of the model arbitrarily t o obtain an optimum fit. *is discussed above, the structure of the filni adsorbed from acid solutions appears lcss well defined than the film at p l I 10. -Iccordingly, the wetting model cannot be expected to apply as accurately to acidic films. X rote applicatioii of the model, assuming AEn, = 0.33", gives a better fit, as shown in Fig. 9, than might have been anticipated, but a fit inferior to that a t pH 10. The faster increase of 0 in the experimental acidic curws probably results from a faster blanketing of thtx surface by the haphazardly tipped amine ions than by the more nearly vertical amine molecules, at 'I gii e11 n n n i h r of molecules per square centimeter. When slides with films thicker than a monolayer were withdrawn from the adsorption medium it was sometimes observed that the film, in air, was thinner than in solution, but ney-er thinner than a monolayer. If the receding solutio11 quantitatively removed the molecules beyond the firqt layer, the quantity ( Y D A - YDL) would perhaps correspond more nearly t o the surface teiision of a hydrocarbon than to that of an ammonia solution. Wcttiiig curves iiidicate no great need for such a correction, and direct optical measurements, difficult to compare precisely in two different meclin, iiitlicntc. oiily partial removal. I t caiinot he espccted, ho\T;e\ er, that the stability in air of an amine laver with itb polar group outward, would be v ~ r great. y Finally, it may be noted that thr model does not account for h j steresiq. I t perhapi TT o d d haT P been preferable to have ax-era& the ten sin118 ' around the periphery of the drop rather than takiiig a direct area average. That such an ax-erage might be different, and account a t least partidly for the hysteresis, can be explained a.; follon~. The periphery of an advancing droplet m u i d lciid to move spontaneously ox-cr the high energy pitches and come to rest on lnw energy patchc. oi nioiio!ayer. The periphcry CJf :i receding drop n oulcl tciid to be held back on high energy patches of adhorhcd witm or amine double molecular layer, a i d 10x1 contact angle. It is clenr that th(. mngiiititde of the hpsterwis would depend upoii the ficwnrti-ic diqtribution of such patehe.. S o qiin:itii 'itivr treatniciit of this has betw deyised. '
~
Conclusions It is possible, wing optical methods, to nieasurc concurrently both adsorption isothcrms from aqueous solutions and contact angles of the solutiolis on small flat metal slides. Isotherms of n-decyl. amine on platilium have a stepprd shape corresponding to the adsorption of a first layer xitli functional groups dnwn, and of a second layer with functional groups up. Decylaniinr hydrochloride also exhibits multilayer adsorptioii but is riot as
strongly ordered as the free amine. A simple quantitative model, relating the variation of contact angles with concentration t o the adsorption isotherm, correctly represents the wetting data. Acknowledgments.-It is a pleasure t o 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,oiitact angle. 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 a s 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. I n 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
