-4RADIOACTIVE TRACER STUDY OF THE iIDSORPTIOS OF FLUORINATED COMPOUNDS ON SOLID PLANAR SURFACES. 11. C,Fl,S02N(C2H&)CII2COOH BYJOHN P. RYAN,R. J. KUNZAND J. W. SHEPARU Contribution S o . 114 f r o m Central Research Department, Minnesota Mining and Afanufactui i n g Coiijpanj, St. Paul, Minnesota Recewed November 1 1 1969 ~
h C-14 labeled fluorochcmical acid ( C,FI,SOZS(CzH6)CHzC*OOH) was adsorbed from an n-decane solution onto solid planar surfaces. Platinum, quartz, glass and aluminum were studied. Rates of adsorption and solution isotherms were determined. Physical adsorption took place on platinum, quartz and glass, while extensive chemisorption took place on aluminum. Pretreating the samples a t 450" in a nitrogen atmosphere gave the most reproducible results. 1)esorption of the adsorbed film orrurred at an appreciable rate from platinum, uartz and glass, depending upon solvent polarity. Desorption from aluminum was murh slower. N o desorption from aquminum was observed with non-polar decane, and very little with mater. Use of adsorption data to calculate roughness factors is discussed. Values of about 1.0 were obtained for platinum, quartz and glass. An interesting exchange was observed between the adsorbed molecules and those in solution on all surfaces except aluminum. This phenomenon was reported previously for adsorbed films of perfluorooctanoic acid.
Introduction I'revioiis studies' with carbon-1 4 tagged perfluorooctanoic acid o n planar glass aiid aluminum surfaces gal-e evidence of chemisorption. This made determination of roughness factor TTalues for these surfaces uncertain. ];or platinum and quartz substrates physical adsorption appeared to take place and reasonable roughness factor values were obtained. As an extension of this work sonic (3-14 tagged S-ethyl-S-perfluorobctaiiesulfonylglycine, a weakel- acid than perfluorooctanoic acid, 11as prepared This acid was expected to be less reactive and thus provide more reliable data for calculating roughness factor l-alum To ob1 ain roughness factor 1 alues (or surface area) from solution adsorption data, tn o assuniptions are made: ( I ) thc surface is occupied hy a closely pxcked, oriented, monomolecular film ; ( 2 ) the area occupied by each molecule is the same as on an aqueous substrate. The latter assumption is necessar) because molecular area d u e s obtained from film balance mcasurcments are used in the calculatic~iis. Other investigators4 haye shown that surface area values obtained 011 ponders using long chain hydrocarbon carboxylic acids and esters are in good rtgreement 11ith i-alucs obtained by indcpendent met hods. Experimental Materials. A. Labeled Acid.-The radioactive solutions were prcpared from a sample of tagged arid purificd h y rccr~stalliration:mtl vnmum sublimation. T h e niatrrid h:td a inelling point of l ( i 3 i 1O and carbon, fluorinr anti nitrocren :inalvsc.s showed vcrv agrcenirnt with the . good calculated ralles. The suerific activitv of thc acid (0.92 millicurie/L'.) was determi~etf by couniing weightless samples of t h i 'compound in a windowless Q-gas counter. This value w:ts checked b y measurements of toluene solutions of the acid in a liquid scintillation coiintcr. ( I ) J. W. Shepard and .I. P. Ryan, T A MJOTJRNAL, 60, 127 (l9S6l. ( 3 ) J. W. Sliepard a n d .J. T'. Ryan, i b i d . , 63 1729 (1959). (3) We a l e indebt,ed to Mr. T. N. Lahr of t h e Niiclear Research Section f o r t h e synthesis of the tagged compound. (4) a '. D , Harkiris and D. hI. Gans, THISJOURNAL.36, 86 (1932): W . W. Ewing, J . A m . C h e m . Soc.. 6 1 , 1317 (1939); H. A. S m i t h and J. F. Fuaek. t t i d . , 68, 329 ( 1 9 4 t i ) ; E.1% Greenhill, T r a m Faraday S o c . , 45, 625 (1049): W , Ilirst a n d .I. I -
A -
x
1
i__-LA-i
5
15 min.
80 6 hr. 14 hr. 22 hr. Time in desorption cell. Fig &-Decane desorption of C8FliFOJ(C2H )CH2C* OOH from planar surfaces; temp , 2 9 ; no agitation.
0.47 0.08 1.04 80
0.86 1.10 0.80
d
$
60
m
0.85 1.12 1.14
2.7 4.2 4.5 7.3
0.79 0.81 0.90 1.14 3.3 4.1 4.2 4.4
Z S
40
20 0 5 15 min. 60 6 hr. 14 hr. 22 hr. Time in desorption cell. Fig. 7.-Desorption of C8F17SOJ( C2Hs)CI12C*OOII from quartz; temp., 29"; no agitation. 100 80
: 8 60
d
1.12 6.2 21.3 65.0
D. Exchange.-When a quartz, glass or platinum surf:tce containing an adsorbed film of CsFI7SO2J(CJL,)CHnC*OOH is immersed in pure n-
c2s 40 z 20 0 5 15 30 min. 1 hr.
6 hr. 14 hr 22 hr Time in desorption cell. Fig. 8.--l>esorption of CgF,iSO,x(C~H,)CII~C*OOI1 from aluminum; temp , 29"; no agitation.
Jorm 1'. RYAN,R. J. KEN
528
Exchange results for both acids on platinum surfaces are shown in Fig. 11. The rate of exchange is considerably greater for perfluorooctanoic acid. No exchange was observed for the chemisorbed N-ethyl-Tu'-perfluorooctanesulfonylglycine on aluminum. Data obtained with perfluorooctanoic acid on aluminum also indicated a chemisorption process but in this case exchange was observed. Comparison of the exchange rates of the two acids on the various surfaces indicates that when the 0 5 15 30 60 type of adsorption is the same, i.e., physical Time in exchange cell (min.). Fig. 9.- Exchange rate of C8FnSOzP\'(CZHL)CHZC*OOH on (platinum) or chemisorption (glass), the rate for perfluorooctanoic acid is much faster than for the quartz; temp., 29". N-ethylglycine compound. When the octanoic acid film has been deposited via chemisorption and the S-ethyl acid zlia physical adsorption (glass), the rates are approximately the same. E. Contact Angles.-Advancing contact aiigles using hexadecane and methylene iodide were used to verify the difference between exchange and desorption. In exchange, where radioactive molecules were replaced by non-radioactive molecules of the same compound, the contact angle did not change. On the other hand, in desorption, where X-ethylglycine derivatil e molecules were replaced 0 5 15 30 60 by solvent molecules, the contact angles decreased Time in exchange cell (min.). an amount depending upon the extent of desorpFig. 10.-Comparison of exchangp rates of C8F17S02X (C,H,)CH&*OOH and C7Fl&C*OOH on glass: temD., tion. 29"; __ , exchange with- non-active s o l h o n s of t'he Contact angles n ere also determined for coniacide; - - -, desorption in pure decane. plete coverage of the N-ethylglyciiic acid on various substrates. These 1-alues, along with i d u e s previously obtained with perfluorooctanoic acid, are given in Table 11. The extremely high T-alues on alumiiium were unexpected. Further TI ork is under m y to elucidate the phenomenon. TABLE I1 --Ar)rancing contact angle (degrees)-Perfluoro- ---iV-Ethj lglycine-octanoic Methylene If exadecane Hexademno iodide
Substiate
15 60 Time in exchange cell (min.). Fig. 11 -comparison of exchange rates of C8F17S01N (CzII,)CH,C*OOH and C,Fl,C*OOH on platinum; temp., 290; -_ , exchange with non-active solutions of the acids; - - - -, desorption in pure decane.
0 5
decane, the adsorbed film is slowly removed (about 20% in one hour). I n contrast, if a sample containing an adsorbed layer of the tagged acid is immersed in a solution of the non-tagged glycinc compound, the activity changes a t a greater rate (40-80Cj, iii 15 minutes). Interaction between the molecules ill solution and tliose 011 the surface resulted i i i a n exchange phenonmion. Contact angles shoved no depletion of the adsorbed film. The rate of exchange for an adsorbed film on quartz is shown in Fig. 9. The values reported are the per cent. decrease of tagged acid for a given immersion time in Ihe non-tagged acid solution under static conditions. Desorption data in pure n-decane are shown for reference. Obviously the presence of the compound in solution has a pronounced effect. Data for exchange on glass are presented in Fig. 10. Curves obtained for perfluoro6ctanoic acid are prewitcd for comparisoii.
I'latiniim Quartz Glass Aluminum
G3 67 67 110
b3
72 74
90 93 93 160
Discussion The surfaccs used in this study were composed of oxide films with the possible exception of platinum. This work is part of a program to study adhesion on surfaces encountered in industrial processes. As such, it is these surfaces that are of interest. The pretreatment used was designed to give reproducible results \\ ith a wick \-ariety of surfaccs. IVater (wiltact angles showed the cfl'ectivciicbs of the clcaning procedure (Table 111). clD\ASCIKG
TABLE 111 CONTACT L\NGLE 1 \ L L h C 1 O R A N D NON-PRETREATED Sr RFACES
\$'AI'cR
TREATED
Surface
I'IXE-
-Water contact angle value, degreeBefore treatment After pretreatment
Platinum Glass Quartz Aliiminnm
.Is i b qhoii-ii 1)y contact
85 '77
< 10
47 95
I5 10
mglc
28
1
a l ~ wtlw , 11retre:lt-
iiiciit coiitril)utes significantly t o reino\xl of prc-
adsorbed contaminants, chiefly organic in nature. rated the slight temperature drop caused by the Previous experiments using flaming treatments immersion of the sample could ha\-e caused tempogave lowei contact angles but reproducibility was rary precipitation. Considerable care is required when working with relatively iiisoluble compounds. very poor. Tingles and I>aiiielg have discussed adsorption The autoradiographic technique as an aid to normal from solution 011 various metal substrates. They radiochemical counting techniques has proven pointed out that reliable surface area values could iiir-aluble in this work. Rideal and TadayonIO have studied the effect of not be obtitined from data oii substrates which have thick permeable oxide layers capable of reacting polar liquids in causing the carboxyl group of stearic with the adsorbate While perfluorooctanoic acid acid molecules buried in a parafin substrate to data pro^ ided reliable estimates of roughness come to the surface. IJ’hen water was placed on a factors for quartz and platinum substrates, reac- solidified mixture of paraffin and stearic acid, the tion with subsequent buildup of adsorbate on contact angle decreased as a fuiiction of time, glass and aluminum surfaces precluded placing indicating reorientation of the stearic acid moleany degrets of certainty on roughiiess factor values d e s . Some correlation of solvent desorption effor thcse surfaces. The data for the K-ethylgly- ficiency for perfluorooctaiioic acid films with solcine compound used in this research provides rea- ieiit polarity was possible.2 With the N-ethylsonable estimates for roughiiess factors on platinum, glycine compouiid, desorption efficiency again apquartz, arid glass, but the surprising reactivity peared to correlate with solvent polarity. Solufound with aluminum makes the calculated rough- bility seemed to be of secondary importance. The ness factor Talues meaningless. The greater re- dipole moments of the solvents and the solubility activity of the ;“\;-ethylcompound relative to per- of the glycine acid in these solvents are preseiited fluorooctaiioic acid, the difference in the exchange along with desorption data in Table IT. properties, and the unusual wetting properties of TABLE IT’ the S-ethjdglyeine compound film on aluminum are indeed perplexing. Additional in\ estigation of the SOLVENT CHARAC TERISTICS A N D DEbORI’TION EFFI( I character of this interaction is warranted. FOR N-ETHYLGLYC INE F I L m ON VARIOY‘S PT’BSTRATES It shoulcl be iioted that 11hen using a cornpound So1ubil;ty 76 of film desorbed In Dipole a t 29 5 nun-.moment as r e l a t i d y ixisoluble in decane as the S-ethyl( X 10-6 Plati- Alumi(DeSolrent byes) mole/l.) Quartz Glass num num glycine co npouiid, slight variation in temperature 2 8 -130,000 99 98 83 19 can alter the solubility noticeably. This was borne Acetone out in the cases of platinum and aluminum where Ilistillecl water 1 8 8 81 91 47 3 localized ‘ precipitation” occurred when these sub0 220 53 19 56 0 strates TWR dipped in saturated solutions. I n Benzene 0 4 9 8 8 0 both case‘< it \vas possible that the “saturated” Decane solutions \yere rc.ally supersaturated owing to slight The interaction of the polar S-ethylglj cine cointemperature fluctuations during the solution preparation procsew. In the case of platinum the amount pound in solutioii a i t h the molecules in the adremoved ily physical adsorption Tvas so slight that sorbed film could also provide a mechanism for the thc solutioii concentration was not lowered enough exchange process. As has been suggested, the to redissolve the “precipitated” solute. I n the adsorbate molecules in the decane could interact to case of a1 miinuin, where extensive chemisorption bring about the overturning arid subsequent detook placv, the solution n-as depleted enough to sorption of the film molecules. Further work is planned in the areas of exchange redissolve the precipitate. Tf the aluminum dip solutioii v w c ‘ truly saturated and not supersatu- and contact angle us. coverage.
----
(8) E. D. ’lingle, T r a n s P o r a d a y Soc., 46, 93 (1950). (9) S. G. Daniel. tbzd , 47, 1345 (1951)
(10) E. Rideal a n d J. Tadayon, PTDC. Rou. SOC.( L o n d o n ) ,2 2 5 8 , 346 (1954).
