THE EFFECT OF POLAR-NOS-POLAlt SOLT’TES ON THE IY,lTER IYETTABILITY OF SOLID SURF-kCES SUI33IERGE~I~ IN OIL’ BY WILLARD 11. Basco~rAUI) C. 11. SIXGLE:TEHI~Y U.S. Natxd Research Laborutory, It‘ashinyton 26, D.C. Received J u l y J, 1901
A study has been madc of the effect of polar-non-polar solutes on the contact angles ruhibited by water drops on polytetranuoroc,tliylciie, polyethylene :rnd staiiiles steel t . ~ r f i subincrgrd ~e~ in dcranc, isopropvlbiphenyl and bis-( 2-ethylhexyl)-scbacatr. The solutcs invcstigirtcd induded the oil-soluble diiionylnaphthalene sulfonates and sodium dodecyl sulfatc. A parabolic, relation exists between the cosine of the cont:ict angle, COY e, rbnd the oil-water interfacial tension, yaw, and this is consistent with thr Young-I)uprcl rqiiatio11 for the oil-water-solid line of intcrsrction. The experimental data rtlso provide a measure of thr work required for oils to displace watcr from the submerged surfaces. This work generally is greatrr for ib polytetrafluororth~-le~c surface than that necessary to displace water from polyethylene, becaii~cthe polytctraf~~iorocthylerie has a greater interfarial energy against the oils than does polycthylcnc. Analytic treatment of the data gave n nnmcriral indication of the various oil-polymer iiitrrfimal energies. This treatment suggests that the polar-non-polar solutes are adsorbed a t both the watw-polymer and oil-water interfacrs, but arc not significantly adsorbed a t the oil-polymer interfaces. The work t o displace m t r r from dean steel surfacrs submerged in the pure oils is high because of the polar interaction b e h e e n the water and the inrtal oxide surface, but in the oil-sulfonate solutions the solute is adsorbed as a monolayer a t the metal-oil boundary t o expose a non-polar surface so that the work of water displacement is low.
Introduction The adhesion of a liquid to a solid surface submerged in a second liquid is of technological importanco. Advances in this area of surface chemistry v o d d contribute to the understanding of problcms such as thc detergency process or the recovery of oil from porous subtcrrancan rock by water displaccmtmt. Thc relative wcttability of solids is of critical significance to icc adhesion in lubricatcd systcms2 and to thc displac~ementof oil or water from metal s ~ r f a c c s . ~ The inhcwnt difficulty in the study of these problems is that thc intcrfacial energy bctwcen a solid and a liquid cannot be mcasurcd directly, Instead, attention must, bc givcri to the directly measurable quantitie+-I hc interfacial tension bctwceii the two liquids arid the contact angle onc liquid exhibits on the solid siirfacc submcrged in the sccond liquid. Izarlicr inwstigatioiis have been made of systems of pure liquids on clean metal ~ u r f a c c s . ~Unfortunately, the high interfacial ciicrgics involved favored adsorption of small quanti1ips of polar impurities or the orientation of tliv liquids themsclves a t the various intcrfares, which makes intcrprt.tation of the results difficult without furthcr information about, thc nature of thc adsorbate. The prcscnt investigation examines the effect) of small changes ill thc conccntrntion and chrmioal composition of amphipathio materials on oil-water interfacial tensions and on contact angles at the oil-watcrsolid intersection. Thc o b s c r v ~ lrelation brt~rceii thcse two expcrimenlal qiiant itics permits discushion of the prok)al)lc magnitude of the solid-liquid interfacial cncrgics and of Ihc way in which they arc affected by adsorbed solute. (1) This nark U R R prcsmtrd in p a r t a t t h e 1’38tli National Meeting of t h e 4nirriiun Chrmiral Soriety. 4tlantir Citv N J , 1959 (2) 11. R Baker, W. I> Bascorn a n d C. R Singlrteriy. accepted for Dublicution in J Colloid Srt , 1962 (3) II. R B a k r r , P B. Leach. C. R Singletrrry a n d W. A Zisman. “Surfac e Chemical Methods of I)ispl,tcing \Vater a n d / o r 011.9 a n d SnlvaKing f l o o d e d l:qiiipiiient,” S a d Research Laboratory Report KO.SGOb, r e b r u n r y 21, 1961 (4) (a) F. E. Baitrll and P. 11. Cardwell, J . A m Chem. Scc , 64, 1530 (1942); (b) F. E. B a r t e l l a n d C. W.Bjorklund, J . P h y s . Chem , 5 6 , 463 (10.52).
Experimental Materials and Procedures The polyincr surfaces employed hrre have been tlrscribed elsewhere.6 The stainless steel cylinders uscd had one face highl\. polished. Each specimen was clertncd twtwccn cxprriments by washing with hot solvrnt and then abrading all sides using an aIumiiiiL-wrtrr slurry on a nirtallographic whcrl, folloac~dt)v thorough rinsing with hot tap water and thm distilled water to remove adhrring alumina particles. This p r o r d u r c gavr a rlertn metal oxide surface free of adsorhrd orgiiriic contaminiition. Th(A organic d v e n t s decane, isopropylbiphcnyl and bis(2-ethvl-heq I)-schcate wcrr oht:iincd from commerrial sources. The drranr was wished firht with arid and then wth watrr, drird, and finally prrcolated through F l o r i d adsorlwnt. The rcsiilting inatrrial did not spread on ih clean surfncr of witrr madc slightly acid or alkrtlinr, which indic:Ltrd t h r ahsencc of polar impiirities. Thc isopropylbiphenyl was prrcolatrtl rrpcatedly through adwrhent until this liquitl also s h o w d no tentlcncv to spreitd on acid or alkalinc watrr. Thr dicstcr oil w w molecularly distilled ~ n thrn d pwcolatrd through I~lorisilto give a clcar, colorless liqiiid that lirid physic:d propwtics identical with thosc of B spwally syiit hesizcd sample of tht, same rstcr.6 The dinon\ Inaphthalme sulfonatcs were of hi .h purity; t h t w proprrtics in non-polar solvrnts7 and the mebods usrd for t heir prrparation* h:Lw been described. The sodium doclcq 1 siilf:ite was thrit used bv Bernett arid Zisinan O It s h o w d no minimum in plots of surfset1 trnsiori or i n t w fiicisl trnsmn against concentration, indicating no d(~tertable lauryl alcohol contamination.’O Contact angle measurements werc madc on the solid surfaces sut)incrgrd to :t dcpth of nbout 2.5 rm. in oil or oil solution i n ttn optical cell. W:rter d r o p of approumatclv 0.03 ml. volunir wrrc formed in the oil from a micropipet and allowed to srttlc under gr:tvit) to the surface. The angle of contacj through thv \\ atrr drop was mcasurcd rising a trlescopti goniomc%er l 1 The o i l and water usrd previously nery cquililmttecl 1)y sliiihing with each other and cciitrifrigirig. T‘ndrr thrsc. circrimstanrrs the initial advancing cont:wt ariglrs, mritsurcti through the drop, n wr constant nftrr trn scc. arid wcrr rcprodiiciblc within lcss than two drgrecs. Vurthcr advance of the drops on the sitbmcrged surfirws b\r itdding watrr from the micropiprt wts not fourid to alter the roiitiLct angles. 13 ding angles were formed by wthdrrtwing water from thr drop and w r r not sigriificantly diff rrrnt from the advancing angles proridcd that -
(1901).
(5) W. I) Bascon1 and C . R. Pinglrterrv, s h d , 66, 1083 ( G ) C AI Bricd, I1 I’. Tiiddrr C 11. Murphy a n d W.A Zisrnun, I d Bng. Chcm 39, 184 (1917) (7) S. Kaufiiian a n d C. R. Singletorry, J . Colloid Sci, 12, 4G5 (19Z7). (8) S. Iiarifman a n d C. R Singletcrry, tbid , 10, 139 (1983). (9) 31. IC Bernctt a n d W. A. Zisman, J P h y s . C h a m , 6S, 1241
(1059).
(10) G. n 3111~sand 1, S h e d o v s k y , zbzd , 48, 57 (1941). (11) 11. W.l’ox a n d W. A. Zisman, J . C o l l a d Sct., 5, 514 (19.50).
Feb., 1962
WATERWETTABILITY OF SOLID SURFACES IMMERSED IX OIL
the surface was not dclibcratcly roughed and that one miii. was allowed for re-equilibration. When thc two liquid phases had not been mutu:dly saturatrd the contact angle was obsrrved to chrmgr with time rn a result of delayed adsorption of solute a t the oil-water intcrfacr and of solubilization effects that tended to diminish the drop size. Steel sprcinivns required equi1il)ration with the atiditivrcontaining oil for at least 15 hr. to obtain stable contact anglcs, h i t the length of time the organic polymers werr in contact with the o i l solutions had no effect on the contact angle. This behavior suggests that the solutes were not appreciably adsorbrd a t the oil-polymer intcrfare. The iinusual Icngth of time required for thc nirtal oxide surf:tcc of the sterl specimens to rrach adsorption equilibrium with the sulfonate soaps has bcrn shown to bc linlctd to the presence of watcr in the oil-so:tp ~ o l u t i o n . ~Water appears to c>ffectn slow enhancement of sulfonate adsorption over that observed from anhydrous sulfonate soap solution. The mc:isurcmtbnt of contart :inglcs near 180" wa~b cxtrcmrly difficult. In gcmmil, thc rcproducil)ilitv of the contact angles \\as found to bc f 2 ' , but for drops rxhibiting angles greater than 175" the distance of srparation between watcr and the solid surface near the contarting edge of t h r drop is less than the rcsolving power of the tclcscopc-goniomrter. This error amounts to about 3' for a contact angle mcnsurcdas 1 Z o , but becomes much lrss than t h r rcproducibility for angles smaller than this. Thrrc is, howvcvcr, no qurstion that the watcr drops which exhibit w r y high contact anglcv actually make cont:tct with the solid. It is possiblp to focus the tc,lcscope on the flattcned area bcne:ith the natcr drop and observes thc watw displacing the oil from the solid surface, cvcn nhcii the contact anglr is near 180". The hydrodynamic mtl surface cBheinica1 imp1ic:ttions of this displacement are discussd t \\ here.2 For oils which have densities vrrv close to that of Hatrr, such as isoprop\.Ibiphrn?.l,it wts torind that cont:ict :tnglrs betwwn 175 and 140' were difficult to reproducr. Thc intrrfarial tensions bc.twrrn mater and the oil solut ions of r h t h dinon) Inaphthalrnc~sulEonatP soaps werr dctcrminrd t)\ thc pcwlent drop mrthod.12 7'111s tcchniquc., which is usc,ful in drtcrting changes of intvrfaci:il tens!on with timr, shorsed that for thc siilfonatc's !9,5c; of t h r aging process at thv intwfacc' took plaw mithin thc first minute. T h r intcr1aci:tl tensions Ir)rtwm isopropylbiphrny-1 and t h r aqucous solutions of sodium dodccyl sulfate wwc detrrmined by thc drop volumr mrthoti. The w :iter drops wrre drlivercd from microbiircts aq drscrihcd by Gaddim13 and the prccautions and corrcctions suggested by H a r k i d ' were applied.
loor-
237
_ POLYETHYLENE 0 his (2-ETHYL HEXYL1 0 SEBACATE
.
SEBACATE
-
-70"
STAINLESS STEEL bis 12-ETHYL HEXYL1 SEBACATE
' AISOPROPYL BIPHENYL A ISOPROPYL BIPHEYYL A ISOPROPYL BIPHENYL
DECANE
10
DECANE
1
. CD W
*ab\
-050'
\\
I I
3 I
0 0
OIL- WATER INTERFACIAL TENSION, yaw, dyne /cm, Fig. 1.-Cos e as a function of Y,,,, for water drops on solid surfaces submerged in oil solution* of the dinoriylnaphthalcne sulfonates.
( ilion-polar surface" includes stainless steel surfaces in the oil solutions because thc solutes prcsciit adsorb on this metal oxide surface to givc a moiiolaycr comparable in wett ing properties with the nonpolar polymer surfaces. Thc effect of changes in yo,, on the contact angle for a given oil-polymer pair was determined by using sulfonates having diffcrcnt cations tjo obtain diff crent oil-watcr interfacial tmsions. These values of yawwerc plot tcd against the caosinc of the observed contact anglc. The effect of a change in the oii-solid interfacial teiisioii on the relationship bctwetn cos 0 aiid yaw was determined by employing different)oil-polymer combinations. The polymers choseii were polytetrafluoroethylcne and polycthylcnc; they were studied uiidcr solutions of the diiioriyliiaphthalciic sulfonatcxs in decane, bis-(2-clhylhexyl)-sebacate Results and isopropylbipheiiyl. Thc results on polytetrafluoroethyleiie (l"l'l?E) The purpose of this study was to investigate the variables that delc~rmiiicthe contact angle, e, a aiid on polycthylenc arc plotted in Fig. 1. For both water drop mill exhibit oil a solid surface submerged polymeric surfaces there is a systematic dccrcasc in in oil. These variablrs are the three condensed the contact angle, Le., cos 0 becomes less negative, phase intcdacial tcrisions which art a t the oil- with dccrrasing oil-water interfacial tension. The water-solid line of intersection. The cxplicit, decrease iii contact aiigle for a givcn changc in rclationship bctiwcn these quantities mid the coii- yawgciicrally is grcater for polytetrafiuoroethylenc tact aiigle, meusurcd through thc drop, is givcii by than for polycthylcne. That is, the perfluoropolymcr is thc more readily wct by water whcri subthe Young -1luprc cyuatioii merged in thesc oil solutions, which is the r'cvcrsc of the relative wettability of thcsc polymer surfaces in air.Q Of thesc quantities oiily yon and the contact anglc ill1 of the dinonyliiaphthaleiie sulfonatc soap can be measured expcrimentally. In order to dc1c.r- solutions were a t a concentration of 1.0 wt. yo. mine the dcgrw to whivh changes in each of these It geuerally \vas found that greater water solubility interfacial tensions affect Ilic contact angle, each of thc sulfonate was associated with lower iiitcrshould bc altered iiidcpcndciitly but, whrii yo," and facial tctision. The vnlucs of yaw were easily rcyosarc specified cannot be varied arhitrarily. prodiiciblc bccansc the soaps ivcr(' present> in In this investigation, attention was focused on amoiints n ~ l above l their critical micello (wicciithe relation b r t w e i i yo,. atid cos 0 for water drops tration7 and small diiftwnccs in concentration did on non-polar srirfaccs in oil solution. Thc tc3rm iiot produw sigiiificaiit dif'ft.rt~riwsi n yo,%. ( 1 2 ) .J 11. Antlrras C A. 1Idusrr a n d \I n Turkc Tho data. ob1 aincd for staiiilcss stwl surfacrs in 42, 1001 (103s). the sulfonate solutions (lcig. 1) arc in many ways ( i d ) N. K Adam, ' Tlic Physics and Chemistry of Surfaces," 2nd similar to data obtained for the polymer sured., Oxford Univ Press, London, p 379. faces. This is corisistcnt with the fact that the (14) W. D Ilurkins and 1'. E. U r o l r n J . A m Chem. Soc., 41, 499 (19193. dinonylnaphthalcnc sulfonates are adsorbed from
WILLAEDL). Basco;vr AKD C. 12. SIXGLETERRY
238
Ym, DYNESICM
OIL/WATER INTERFACIAL TENSION, OOO------ - --
-020; P; -0401
(C)
"^I I
i
s -a601 v)
IIYOW,
Vol. 66
In tho absence of polar-non-polar solutes in the liquid phases the contact angles obtaincd on the polymer siirfaccs in the thrce oils all were greater than 175". On a clean stainlcss steel surface submerged in the oils, the watcr drops all gave contact anglrs less than 10"; by making extraordinary efyorts to eliminal c polar impurities it was possible to obtain contact angles very near zero degrcts. The relationship between cos 0 and yon at highcr values of thc oil-water interfacial tension than can he obtaiiird with the dinonylnaphthalene sulfoiiateb was dctermined by mcnsuririg the (witact angles formed by water drops containing difrererit coiicciitrations of sodium dodecyl sulfate (KaLS). Obs~rvationswere made on polytctrafluoroethylelle and polyethylcnc submerged in isopropylbiphenyl ; thc data are indicated in k'ig. 2A. The experimental data also provide an index of the relative adhesional tendency of water and oil to these non-polar surfaces, the reversible work of displacing unit area of water, W r , . 1 6 This quantity is defincd as the work involved in the disappearance of oiic crn. of water-solid interface with simultaneous formation of one cm.2of oil-water interface and onc cm.2of oil-solid intcrfuce. A summation of the energy cihangcs gives an expression for the work of displacement
Fig. 2.--Siirface chemical behavior of aqueous soditiin dodecyl suffiite solutions in isopropylbiphenyl/po~~m~r R ~ S IVO = YoYo,, - ? \ i s (2) terns: A, cos 0 as a function of y",,.; B, work of displacement as a function of Y ~ , , ~ C, ; cos e us. 1 / ~ , , ,(0 ~ = PTFII;, a = The value of Min for any system can be calculated polyethylene). from the cxpcrinient~aldata by substituting equa-
+
.
I
'-
PTFE f2-ETHYL HEXYLI SEBACATE
o bIs
E
POLYETHYLENE STAINLESS STEEL ( 2 - E T H Y L H C X Y L I 10 bls ( 2 - E T H Y L H E X Y L I SEBACATE SEBACATE
btS I
\ 0 V)
' A ISOPROPYL BIPHENYL
g 30
rDECPNE
A ISOPROPYL BIPHENYL IA ISOPROPYL BIPHENYL OECANE
0 DECANE
tion 1 into the expression for l l ' ~to obtain wu = (COS e 1) (3) In Fig. 2H and 8 thc work of displacement calculated from the experimental data is plotted as a function o f . yo,, . The vertical lines extending from each point repremit the probable error in TVu and were calculated taking a probahlc error of k 0.2 dyne/ em. in yon.and f 2' in 0. Generally, the work required to displace water from polytet,rafluoroethylene is larger than that required to displace water from polyethylene.
+
Discussion The Oil-Water-Polymer Systems.-In order to interpret the ohscrved relatioilship between cos 6 and yo,,, consideration must be given to the relative orders of magnitiidr of the interfacial energies acting at the oil-water-Lolid line of intersection and __ .0 5 IO 15 to the effect that polar-non-polar solutcs h a w on these energies. It is assumcd that, in the absence OIL/WATER INTERFACIAL TENSION, Yow , dyne/crn. of sivfacc a tivc solutrs, the interfacial energy existFig. 3--The work of displtwcm(wt as a function of yo,, for water drops on solid suxfsces sut)mcrged in oil solutions of ing bet M (1e1i water and the organic polymrr surfacc is comparable in magnitude to that existing at thc the dinonSlnsphthalenc sulfonatc soaps. interfacc bc~t\wriiwater and thc organic oil. It also oil solution on mctal oxitlr siirfaccs to form rlosc~ is assumed that both these cncrgies are Iargc in compavkcd moriolaycrs that havc surfaccb propcrtics parison with the interfacial energy a t the organic comparable with thosc of the surface of polyeth- liquid-organic solid boundary. These assumptions y l e t ~ e . ~It should be noted that the rholitact angles are reasonable because intcraction between the relob1aiiied for a givrn oil solution on th(. sulfonate atiwly polar wattr molecules and a non-polar orfilms fall hetwrrn thosc ohtained on polytctra- ganic. liqiiid suc.h as dcc.anc should riot he grcatly fluoroethylrnc and thosc obt aiiird on polyethylcnc diffrrcnt from that between wat cr and an organic for 1hr samr solutioi1s. That is, tho sonp monolayrr solid +iich as polyrt hylcne. Thc iiit crfnc*ialcncrgy adsorbed at the oil-oxide intcrfacc has a n a t r r brtwccn dccaiir and polyethylene is much smallcr wet,tabiliiy intrrmcdiate bctwccn that of thc two (la) I1 Trtwnillich ' Colloid and Capillary Chemistrs ," Methuon and Co , London, IYZG p 159. polymers in thr5c oil solutions. 61
than the other two because methylene groups preponderate in both. It is cxprcted that the solutes employed will be strongly adsorbed a t both the oil-water and watersolid hundaries but that little adsorption of solute will occur at, the interface between the oil and the organic solid. Adsorption a t the oil-water interface involves ionic intrractions of the polar head of the solute molecule with the ac~ucousphase. Similar iiiteractioiis are possible when the molecule is a t the water-organic polymer interface. I'owkes and IIarkinslGhave measured the film pressure of solutes adsorbed a t the water-parafin interface and Hernett and Zisman" have demonstrated the adsorption of polar-non-polar solutes a t the water--polyethylene and \~ater-polytetrafli~orocthyleiie interfaces. The interfacial energy at] the boundary lietmcen a non-polar organic liquid and a non-polar organic polymer, howevrr, is not likely to be of sufficient, magnitude to favor the specific adsorption of surface active solute. The contact anglrs of near 180' obtained on the submerged polymcr surfares in the absence of surface active solutes, and the parabolic relatioiiship observed betwcen cos e and yow,are consistent with the preceding assumpl ions concerning the relative magnitudes of thc interfacial energies. In ordcr to obtain contact angles near 180°, ie., cos 0 = - 1, the numerator of the Young-Dupre equation (equation 1) must be negative and nearly equal to the denominator. In other words, thc waterpolymer interfacial tcnsion, yus, must exceed the oil-polymrr interfacial tcnsion, yos, by an amount nearly equal to the oil-water iiit erfacial tcnsion, yow. h decrease in the contact anglo as a result of decreasing you requircs that in eq. 1 the numerator must take less negative values, i.e., yns must decrease. Thus the progrcssivc decrease in the contact angle with increased adsorption at the oil-water interface must be the result of simultaneous adsorption of solute a t the water-polymer interface. It is possible to examine the data more critically by rewriting the Young-Duprc equation in the form
This equation will be linear (in l/yow)if yoSand yws/ yoware constant. Bccause the surface energies of the oil aiid polymcr are small and comparable, it is unlikely that, the solutrs are adsorbed a t the oilpolymer interface, so that yos may reasonably be expected to bc constant for a given oil-polymer combination regardless of thr solute present. The term yus/you, on the othcr hand, can be constant only if adsorption a t the rcspcctivc interfaces changcs the iiitcrfacial energies by proportional amounts. I n Fig. 2C and 4 the data are plotted as cos e against, I/yow. Of thr elcvrn solventsolid syslcrns inr-cstigatchd, only one shows iniqnestioiiahle drpart urcs from linearity. I.'or the systrms that do givc a linear rr1:~tion be1 w e n cos e and l/yo,,, Ihc slope of the linc represents an estimate of the value of yos, the oil-solid interfacial (10) f hI I'orrkes a n d W D. Ilatktns ( 1 0 10).
.I A m Chcm Soc 62, 3377
I/?-.
Fig. 4.-Cos e us. 1/y0,,. for wbter drops on solid surfacos submerged in oil Folutions of tho diiionylnaphthaleiic sulfonated soaps: 0 , polytetrafluoroethylcnc; A, polyethylene; 0, &inless steel.
tcnsion. Therefore, to the extent, the assumptions can be justified, t,his arialyt,ic t reatment provides a novel means of obtaining numerical values of the interfacial tmsioii between non-polar solids and liquids. I n the oil-polymer systems, the data obtained on polymer surfaces in the oil solutions of t,he dinoiiylnaphthaleiic sulfonates (Fig. 4) either are clearly liiicar when plot,ted as cos 0 against) l/yow or arc consistent n d h a linear plot. but h c k enough p0int.s to confirm linearity. The slopes of these lines are taken as estimaks of yosaiid are listed in T:ible I. Thc interfacial t'ensioiis between polytct~rafluoroetli~lenc and bis-(2-ethylhcxyl)scbaoate or isopropylbiphcnyl arc an order of magnit,udc greater t,hari t,he interfacial tensions bet.ween polyetmhylcnc and these same liquids. There is only a small difference in t,he value of yos for the two polymers in decane. The data for t,he polymers in isopropylbiphenyl using the aqueous solutions of sodium dodecyl sulfate also give slopes t,hat, indicate a much largcr value of yos for polytetrafluoroethylene than for polyethylene against this aromatic liquid (Fig. 2C). TARLE I Ix,rwAcI.iL rrENSIONS, EQ.4 A S D 1cIG. 4)
ESTIMATEI) OIL-SOLID
(FROM
For iwpropylbiplicnyl
d ne/cm. $0 r bis-(2etliylhcxy1)sebacate
For decane
2.7 0.4
2.2 0.4
0 . :3 .2
yo$,
Solid surface
PL'FE I'olyethvlenc Sulfonate monolaycr on stainless steel
1.2 TABLE
COMPAI~ISON OF YLV -
y o RITII
.4
11
Two ESTIMATES O F yOsFOR
VARIOUS ~)II.--POI,YMEH P A I R S Estinintod valrie of -dynes/cni.---O!i-polynier yrv - i C . From From interface dynes/cm. Yig. 4 eq. 6
Iuoprop~lbiphenyl-~~ol~-
tetrafluoroethylene Isopropylbiphenyl-polycthylme
17
2.7
2.8
3
0.4
0.2
polytt~tr:~flnoro~!thyl~!ri~! I 3 T k ( 2-c?thvlhcsgl)-sc~):i~~t(~polyct hylcne 1 I )ecanc-polytetrsfluoroct.h ylcne 6 I)ccarie-polycth~lcne -8
2.2
1.8
0.4
0. I
3
.4
.2
.o
WILLARDI>. B a s c o ~AXD C. R. SINGLETEI~RT
240
The interfarial eiicrgy between a iion-polar liquid and a non-polar solid may be considered the result of a differciice in their chemical miistitutioii. More specifically, it is tlic result of differ eiices in t,hc magnitude of the uiisat isficd molecular dispersion forces a t the surfaces of the two phases. Thc best iiidiccs of thc magnitude of these uiisatisficd forces are thc surface tension of the liquid ( ~ L v arid ) thc critical surfacc teiisionI7 of the solid (-yc), and it may be expwted that the solid-liquid intcifacial teiisioii will be greater as the differcncc betwecn thcsc two quantities is greater. Critical surface teiisioiis of the solid surfaces of interest here arc: polytctrafiuoroethylene, 18 dyries/cm.,18 diiioiiyliiaphthalciie s d fonatc monolayer on stainless steel, 29 dyiies,lcm.,j and polyct hylcne, 32 dyncs/cm. The surface tcnsioiis of the model oils employed are: decane, 24 dyiics/cm., bis-(2-ethylhexyl)-seba(,atc, 31 dyncs/cm., and isopropylbiphcnyl, 35 dynes, mi. The diffcrciiccs in the surface tension of the liquid and the criiical surface tension of the solid, yc, for various oil-polymcr pairs are listed in column 2 of Table 11. Reference to clolumn 3 of this table shows that the interfacial tensioiis estimated from Fig. 4 do fall into the geiieral order to be expected from column 2, although therc is iio simplc proportionality \{ hen the value of yoS is less than 0.3 dyne/cm. Thc estimated values of yos listcd iii Tablc I for polyethylene against the various oils arc small quantities, a result of the similarity in rhcinic8:tl constitution of 1hcse organic liquids aiid the surface of the hydrocarboii polymer. l’herc do riot appear to be any expcrimental data available for thc iiiterfacial tciisioii betweeit liquid polycthylciie aiid other hydromrbon liquids with which to make comparisons sincc such pairs of liquids usually arc miscible in one another. This miscibility is evidciice in itself that they would have only a small intcrfacial cwergy. Thc valucs of yoSfor polytetrafluoroct hylcnc. against tlic hydrocarboii liquids (Table I) arc smaller by a factor of threc or four than the esperimciital values reported in the literature for the interfacial tension between pairs of liquids having compnrable diffcrciiccs in chclmical composition. [;or cxamplc, a value of 12.3 dynes’cm. has bccii measured for thc iiit crfacial tension bctivccn amethyliiapht halcne and a high molecular weight fluorocwbon19 as compared to 2.7 dynes/ cm. for isopropylbiphcnyl against po1;lrtctrsfluoroethylciic. Jsrvis and ZismanZohave estimated the intci*facial t eiimii for a pcrfluoroalkane against a polyethylene glycol fluid as 5.7 dyncs/cm.aiid against hcxadecane as 8.8 dyncs/’cm. ‘These values arc eon(17) The criticid ~ i i i f a r rtrnsion, y c is tlir suifrlrr t m s i o n of a Ilquid t h a t nil1 just sprrail 01i :I non-polar solid curfni c (II. 11’ I’ou a n d 1%’ A. Zibmnn, .I Colloid S c a , 6 , 314 (1950), C 0 . Shnfiin a n d W..1 Zivnun, J . I’lrij8 Cliem , 64, 519 (lOb0)) T h e .calnc of isless than the actiral s u i f n i c ciic rgy of t h e solid by tiir qriantitl of ttir. intrrfni ial pni.lgv bcturen tlie solid a n d tile l i q i i i d t h a t just sprrads With ruitablv 1 h o w n spiiv1 s u i a l l SO tliat ttir vliticul surfarc trnrion may be t i h e n 11sd 1cuson.ibli~n p l ) i i ) Y l n i , L t l o n of t h r surface enrigy of the crdid. IT. I). Ras~i,ina n d C 11 Sinplcteity, % b i d ,66, 1683 (IOCrl) (18) E:. Q. Shafiin a n d W. A. Zisinan. % b i d , 64, 510 (1HDO). ( I R ) 1‘. R I . I’owhes n n d Vi. &I. Sawyer, J . Chein. Phzls , 20, 1b50 (19,52). (20)
N. L.
Jarris a n d
W.A. Zisman, J . I ’ h w .
Chem , 68, 737 (1938).
VOI. 66
siderably greater thaii the 2.2 dynes/cm. obtained for polytetrafluoroctliyleiie against the diester oil and the 0.3 dynesjcm. obtained against decanc. The smallcr values for the interfacial tciisioiis at 1he oil-polymer boundaries compared to thc valucs for thc iiiterfacial tension bctwcii liquids having similar differenccs in chcmical constitution that the overlying oil alters the apparent ncrgy of thc polymcr. This n-odd be the case if thc molecules of the oil at the polymer surface are less mobile t haii the oil molecules further removed from thc surface. E’or oil-polymer pairs that are similar iii r h o m i d mistitution, such as polyethylene and decaiica, it8 is possible thut therc is a penetration or solution of the oil into the amorphous polymer surface. It also is conceivable, since the solid surfaces arc riot smooth 011 an atomic scale, that there is an cntanglemrnt of the moleculcs of tlic organic liquid with the polymer. A weak adsorption of oil or solute molecules, particularly at the surface of po1ytctr:ifluorocthglcrie, cannot be cxcludcd. Studies of the heat of immcrsioii of this polymcrZ1indicate thc presence of a small number of relatively active adsorption sites. 111 any event, oil displacement will be from a surface that is comprised of polymer molecules and a few rclativcly immobile molccules of the oil itself. Because of the immohilizcd oil molecules, I he solid surface will have properties less unlike the overlying organic liquid, arid the apparelit oilpolymer interfacial energy will be less than if all tho oil molecules could be displaced. Girifalco and Good have proposcd an equation2? YaI,
= 7n
f
Yh
- %(Ya*/b)1/2
(3)
which cspresscs the interfacial tension between two liquids, ?ah, in terms of their individual surface tensions, 7% and Yb. This exprcssion was arrived a t hy analogy from the Berthclot relation for the intcrxtion bctwecri like and unlike molecules. The term 4 is an empirical factor which corrects for systems that dcviatc from thc simple Bcrthelot relation. Computed values of Tab are particularly sensitive to small altcratioiis in C$, and this correction term approaches oiic (i.~. , systems miform to the Ikrthelotl relation) only ivheii tlie liqnids are mutually soluble. Listed in ciolumri 4 of Table I1 are values of yos for the oil-polymrr pairs invcstigatrd her(>, calculated using equat iori 3 and employing yc as thc surfacc tcnrion of thc solid phase and asuming 4 = 1. Conhideriiig thc uncertainties introduccd by using the critical surface tciisioii, v d ~ i (of ~ syoncalculated in this way arc iii surprisinglv good agrcc~mcntwith the values estimated from Fig. 1 provided C$ is taken as unity. If instead, a value. of O.O.5 is used for 4, as suggested by Girifalco :md Good22for noli-polar but rnutu:dl.y insoluble liquid pairs, thc eomputcd ~ ~ a l u of e s yo? are considerably greater than t he values est imatcd (’ rcsultq of these cbalculations arc 111 that 1 h r vuliie of yos cstimatcd (’ w 3 t t ahilit y tf:it:h expresses the interfacial tension between oil and n polymer surface more or less modified by immobilizcd oil molccules. (21) 4. C Zrttleinoyrr. Chem. Rela., 19, 937 (19%) ( 1 2 ) L 4.Girifatco a n d 12 J Good, J . Phus C h e m , 61, 001 (1957)
For the polymer surfaces studied here. sonic infcrcnces concerning the relative values of yus and you may be derived from a consideration of cq. 1, 2 aiid 4. If the data give a lincar plot of cos 6 us. 1/yow,i.?., if yoa arid y n s / y o uarc constant in eq. 4, it follows from eq. 2 that a plot of I~’I)US. yOwalso must be linc:m. IVlien of a constant yos for solutions of d the same oil is justifiablt, the d W l ) plot from linoarity may hc unambiguously associated with departurcs from clonstancyof ywa/you. ITnfort uiiately, the valuo of 11’1, is extremely sensitive to wrors in the measurement of either COS 0 or yow from which it is calculated. Thcse uncertainties for tho pr nt work are indicated for each datum point, in s. 213 aiid 3. More precise data obi-iously arc necessary for a decisive test of the assumption that thc rat,io yus/you. is constant. Xevcrthelcss, for all of the present data involving polyincr surfaces, :I lirmir plotJis possible within the limits of error indicated. For staiiilcss steel thc points for isopropylhiphenyl are easily compstilile with a linear plot and those for decane are approximatcly so, but thosc for bis-(2-cthylhcxy1)-scbacate deviate well beyond the espcrimental uncrrtainty. Since the solid surfaces in this case arc adsorbed monolayers of polar organic compounds, thc assumption of a (*onstant yos is questionable. The diester is subject to hydrolysis iii t,hc prescim of water and a catalyst; it is possible that varying amounts of hydrolytic products adsorb to modif:{ the surfacc energy of the soap monolayer,and so yos. The measurements with sodium dodecyl sulfate were taken for the specific purpose of extending the wettability study into the range of higher oilwater interfacial tensions. The plot of W D us. yous(Fig. 213) (’a11 be associated reasonably with a straight line for all values of yowbelow 23 dynes/ (am., but the points for the more dilute solutions and for pure water do not, fall on this line. Since yos may be expected t o be (*onstant,the doviation is attributed to a relatively slower incrtwc in y u s than in yo,, as the soap concentration is decreased in this range. Ijeyond you values of 2.5 dynes/ em., WI) is close to zero arid yousarid ywschange in such a way that their differciic+crather than their rtitio is nearly constant and this difference approaches the value of yos. The Oil-Water-Metal Systems.-The conclusions arrived :It in considering the relative wettability of submerged polymer surf:ms may be applied directly to the data obtained on st:iiiiless steel surfaces suhmergcd in oil solutions of thc diiioiiyliiaphthalrne sulfonates. The sulfonate molecules are adsorbed on stainless steel as monolayers with thcir polar hoads on the metal oxide surface and t heir hydrociarl)on portion oriented away from this s11rf:lce.~ 1;ilms atlsorbtd from isopropylhipheiij 1 solutions t h a l prclviously h:rvc 1)cen sat urated with water ha\,(%sui face propert ivs that are independent of t h c sulfoiiiltc cation. T h e caritical surface teiision monolayers is 29 dynes (mi.; this is ~o\verthan that of polyet hylene a i d is presumed to indicate that a bubbtaiiti:il fraction of the surfuve of the sulfonate film is occupicd by riicthyl groups
which give a lower polarizability of the surfacc than is found for polycthylcne. Thus, these films have surface properties between thosc of polytrt rafluorocthylcnc and polyethylene. In the absence of solute, tho low contact angles surfnces submerged in the oils h thc oil-metal interfact. being a boundary betwen the highly polar metal oxide and the non-polar oil; the value of yo%in the Young-Dupre equation must be high. When thc surface of the metal specimen is coated with an adsorbed soap film, the relationship between cos 6 and you is comparable with that obtainrd for the polymer surfaces in t hesc same oil solutions. An analysis of the data according to cy. 4 provides some indication of the intjcrfacial energy a t the boundary betwecn the sulfonate monolayers and the oils (Uig. 4, Table I). For the monolayers against decane the estimated value of yoais 0.4 dyne/cm. A low value of yos for sulfonate monolayer interface is consi similarity bctween the atomic groups in the monolayer surfitce and the rharactcristic atomic groups of thc oil molecule. On the other hand, the end groups of the soap monolayers differ suficiently from the molecular configuration of iwpropylhiphcnyl to give a larger oil-solid interfacial energy. The value of yos for the sulfonate monolayers in isopropylbiphenyl is between t,hc values of yos for the polymer surfaces in this liquid, as \\-auld be cxpcctcd from a comparison of the values of ~ 1 , Y - y ~for . the three oil-solid pairs. The results obtained on stainless steel surfaces in the diester oil solutions of the sulfonate soaps raise a question as to whether the monolayers formed by the individual soaps are in this cas(’ identical in surface cnergy. The relation bctweri cos 0 and yowis not parabolic, the work of displaccmcnt, goes through a pronounced maximum when plotted as a function of yo,%,and there is considerable curvature in thc plot of cos 0 against 1/ you.. As suggested in the foregoing dikcussion, the diester oil may be hydrolyzed at the nietal surface and the resulting hydrolysis products mixed with the adsorbed monolayer. ‘l’hc hydrolysis and the adsorption of hydrolyzed fragments of diestcr oils on clean metal surfaces has been demonstrated p r ~ v i o u s l y . Unforlunat ~~ cly, it has not, been possible to isolatc films adsorbed from the solutions of the sulfonates in this oil to study their surface propcrtics in tiir 1)ccause diester- sulfonate solutions do riot retract cleanly from the films they deposit 011 stainless steel.
Conclusions (1) The work rcquircd to displace w:itcr from a ion-polar solid surface by a non-polar organic liquid will be least when the surface tension of the liquid and the critical surface tension of the solid surface arc as nearly the same as possible. ( 2 ) I)inoiiylnaphth:~lciir sulfoiiatcs present in t hesc systcms :m ac-lsorhedonto polar solid surfacvs to form close packed films that arr comparahlr Lvith thc surfaces of noli-polar solids. They also (23) E. 1’. IIaw arid IV. A. Zlbinnn, J P h y s Chem , 69, 3\35(1933).
AI~YEH H.
242
are adsorbed ovcr the entire surfacc of thc water drop, although solute adsorption at, the water-oil interface may not exactly parallel that at the watersolid interface. (3) While the analytic treatment of the data
Vol. 66
SAMUEL
docs not lead to an assumpt,ion-free detcrminatioii of yes, there is good reason for believing that it furnishes a useful relative index of t~hc interfacial energy of a non-polar liquid against a nonpolar solid surfacc.
THEORY OF RADIATION CHEMISTRY. 17. GENEIULIZED SPUR DIFFUSION MODEL BY ARYEHH.
SAMUEL
Stanford Research Institute, Menlo Purk, C'ulijoi-nza Ileceivod J u l y 10, 1061
The sharp-boundary modcl of hIagwl for diffusion and rcrombination of rtdicals and ions produccd by ionizing r a t h t i o n is extended to thc case of a sphcricrtlly symmetrical spur. The papw includes samplc calculations ?nd asymptotic solutions for the high- and low-background regimes; the importance of thc transition region bctwcen t h e w is emphasized.
I n the first paper of this series, Magee' constructed a simple, but general, model of geometrical effects in radiation chemistry. I"eat,ures of t.his model were a definit'e volume of the expanding track and a discontinuity in the value of the concentration a t the track boundary. A closed-form solution of the diff usion-recombination equations was obtained, and a number of valuable relationships were deduced. Chief among these was the definition of "low-background" and "high-background" regimes. Since the appearancc of t'hat paper, the study of track effects in radiation chemistry has becn devoted largely to specific syst,ems, iiot'ably water. (A recent paper by Ivanov3 is based partially on the Magee model.) I n this paper we re-examine the original model for two reasons: (1) It applies to all states of aggregation and to radicals as well as ions (provided termination is second order). (2) It identifies the irradiation conditions having background ratios near unity as those most useful for determination of diffusion and recombiiiation parameters. I n the low-background rcgime, yields do not depend on dose rate; in the high-background regime, they do not depend on track paramctcrs; but in t,he region of transit'ion betweeii the two regimes they depend on dose rates and track parametors. To develop the Magec model into one which can be applied t.0 physical systems, four steps appear to be necessary. 1. The Magcc modcl, which applies t,o cylindrical tracks, must be ext'endcd to include t,raoks made up of spherically symmetrical spurs, independent of each other, and corresponding to individual primary ionizations or cxcit,at,ions. Such tracks are typical of ionizing radiat,ions of low linear energy transfer. 2. The model must be extended to include .J. L. Mupee, J. A m . Chem. Sor., 73, 3270
Apprndix: ADI-3217 (see footnote 10 in Muqce's [)&per: plictvcoiiy ~ i r i c cnow S1.28 froni Photoduplication Srryice, Librarv of Conarcss, Wasliiiigton 2.5, I). C.). (2) A very coirililt~t(~ review is a i r e n hy A. K u ~ i p c r ~ r i u nill n , "Actions Chimiquos e t 13iologiques des Rtrdiittionn," >f. tlalssinsky, ed., Vol. 8 . Musson e t Cie., Paris, 1961, gp. 85-166. (3) 1'. I. I v a n o v , Atomnoya E n e r y . , 7 , 73 (1DBD); Reactor S C I . ,12, (1)
128 (1960).
(1951);
reactions of active species with a homogeneously distributed suhstratc. These correspond to many act'ual cases of radical and ion reactions. 3. The dependence of yields on dose rate must be obtained as a funct'ion of the background ratio (which wc shall call P / K as ill ref. 1). We may writ'e (3 0: in,Ivhcrc C: is the yield of products per unit dose (molecules/100 c.v.), I is the dose rate, and n is 0 in the low-tlackground region and - 1/2 in tmhehigh-background region. To obtain track parameters from st'udies of the t'ransitiori region, we must bc able t,o find P / K from an cxpcrimental detcrmiiiat'ioii of the change of n with 1. 4. Since many of the experiments at' high dose rates are performed using pulsed radiation, the model must be suitably adapted. This involves making the background concentration, 710, a variable which is zero at. the beginning of a pulse and increases toward it's steady-stat,c value. (yo is taken as a constant in ref. 1 and in this paper.) This paper deals with the first of these four steps. Diffusion and bimolecular recombiiiat,ioil in spherically symmetrical spurs are considered. The model applies primarily to systems in which the only reactions involve active species, e.g., TI atoms in pure hydrogen or ion pairs in rare gases, in any state of aggregation. I t also applies to other systems in which radical-substratc reactions arc slow compared t o diffusion and recombination. Spur Diffusion Model.-The notation of ref. 1 is followed as much as possible (IC = recombination rate constant, D = diffusion const.ailt.) The spur volume v is taken as v
=
(vu2/3
+ Ill)"?
(1)
The background concwitrntion of active spccic.s is y. The number of particltts initially formed iii the spur is wo. The initial number present, is S" = w'o
+
VO?/O
(2)
The t,imc depcndcncc of the numbcr N of active particlos is dY - -- - k.Y(.Y - l ) / u dt
+ !/ d v / t l l
As in rcf. 4, N ( N - I ) is substituted for N2 h( 4 ) A . 11. Suirmrl nnd .I. I.. >laper.. J . Chem. I'hys., 21, 1080 (1
