336
L. J. HEIDT,M. G. MULLIN,W.B. MARTIN,JR.,AND A. M. J. BEATTY
tricarballylate. Several possible orientations at the organic liquid-air interface can be postulated for these solutes, the most plausible suggesting random orientation away from the interface for all three fluorocarbon chains. However, it also is possible that only two of the fluorocarbon groups are oriented out of the interface and the third group remains behind in the solvent. In all instances the lowest value of A obtained for each system is consistent with the areas measured by the Stuart-Briegleb ball models arranged in the proposed configurations. General Discussion.-Although it was thought that a closer packing of the fluorocarbon groups could be achieved by increasing the number or length of the organophobic fluorinated alkane substituents, the results show that other factors also play an important role. Since the lowest value of A obtained is a measure of the closeness of packing obtained, it is concluded that the dicarboxylic esters in polar solvents adsorbed as the most condensed monolayers in the systems studied. Factors that contribute to this close packing are: (1) structure and solubility of the organophilic constituent; either a long straight-chain hydrocarbon group, or a hydrocarbon constituent made more soluble by chlorine substitution seemed to be the most effective; (2) type and position of the polar connecting group; (3) fluorination, length and number of organophobic chains; a terminal hydrogen in an otherwise fluorinated alkyl chain increases solubility in polar solvents and decreases adsorptivity ; longer chains decrease solubility and often lead to a closer packed monolayer, but too many chains attached in close proximity to the same organophilic portion of the molecule will interfere sterically with each other.
Vol. 66
Possibly the molecules cannot adsorb from solution at the interface in a close-packed orientation because the average adsorption lifetime is too short. Equally large areas per molecule a t about the same film pressures were reported long ago1’ for butyl alcohol in aqueous solutions; in order for A to be small enough to indicate close packing, the film pressure F had to be about three times higher than the highest values we were able to attain. Close packing may be prevented in these organic solutions due to a more marked competitive adsorption than occurs in aqueous solutions. The most surface active compounds studied did orient to cause large surface tension decreases, but in no case was the depression as great as one might expect from a liquid completely covered with perfluorinated alkyl groups. Eevertheless, some of the new solutes reported were more surface active in organic solvents than any previously reported compounds, as evident by the much steeper initial slopes of the graphs of surface tension us. conceiitration and adsorption us. concentration. These and similar surface active agents should prove interesting in their ability to adsorb and modify the surface composition and other surface properties of organic liquids. Where the solubility is too low to Dermit the use of these compouii-ds as surface aciper recordings of the optical density were kept within suitablc limits by attcriuatiori of the light beam passing through tile rcfercnce compartment t1.y )lacing therein appropriate ficr(!eris of square-movcn s1nJ e strands , ‘ of brass wire.D
The evaluation of mas ctlrricd out by notiug that for only the ferrous arid ferric species All =
€62
+
~
C
= S
~ $ 2
+- e ~ +a ~
€3~3‘
=
A,
+
CBCI’S
(2)
wh(Arc c$ and c3a = c3’s rcyxwclit.the fcrric inadvcrtciitly prescnt, and added to t!hc solution, rcspcct i v ~ l y . W o used s in this way bzcwse the values of csa were obt,ained by dilution of a solut,ion containing the maximiim value of c3&and the other solutes :it tho coriceiitr:Lt,ioris prcv:iiling in t,he ferrous (!I)
I,. .I. Iiridt :in,i 1). If. I l m l t ~ y ..I. < ) / I / . S o r . . l m . . 43. 760 (1953).
11lM
F
A
400 390 380 370
21 0 47 7 103 205 391 693 1140 lG40 2110 2320 2400 2450 2430 2330 2230 1!160 2600
17 2 37 9 83 2 1i 2 397 552 941 1420 1840 2080 2200 2290 2320 2290 2230 2040 2840
X0
354 3.20 330 320 314 310 304 300 294 2!)0 280 254
Solutioil
7
I3
C
E
19 4 42 4 00 0 181 350 609 1010 1520 1950 2240 2370 2490 2510 2490 2420 2240 2990
18 4 41 9 !I3 1 186 362 634 1050 I550 2040 2200 2400 2510 2530 2480 2410 2210 2710
22 0 48 9 I06 221 431 i54 121U 1860 2480 2810 3000 3140 3180 3130 3070 2910 2960
It should be recognized that EC = &,e, so that td and €2 arc weighted average valucxs. In the case of thc ferric species the e, represent mostly different ferric sulfate complexes, namely, [Fe(S04)n]3--2n where n = 1, 2 or 3. The vsriatioiis in c3 brought about by the changes i n c2 arid in the conceiitraiion of sulfuric acid Probably are produ(’(‘d mostly by changes the bulk cormlitration of sulfate and not by ally irltpraction betmccyi the ferrous and ferric The evaluation of c? follom.eda circui~ouspruce(jure Which is PrePe11ted in Table Luckily, one of thc concentrated ferrous s~lfat(’ SolLitions prepared for photolysis displayed an absorptioll spectlum having no maximum in regiol; of the absorption peak of ferric iron a t about 3000 A. This is shown in Fig, 2, so as a first approximatiori this solutioii \+as assumed to contain 110 ferric iron. Evcn in this solution, howexr, thc values of e2 were the same within 1% in thre? dijfereiit reaction vc~ssclsonly at, 2840 and 2860 A. The pertinent data are prcscntcd in Table IV, Part
,1. At wave lcngths outside llic raiige 2840 to 2860 the valucs of €2 were less rc>produciblc. At longer wavc lengths tlic optical drnsities of thc. solution in the reaction vc~ssclsw r c too small and at sh0rtc.r wave ltwgths t hcy cit1ic.r ch:ingcd too rapidly with wavc length or exceeded the rango of tlic chart paper. I n order to obtain better values for €2 in this sohitioil a t n.tlve lengths oulsidc the range 2840 to 2860 A., w’ prc%p:wcdfor iis(: xith 1h r :it)mrt)nnc2c~ wlls :iiiotlwr soliii ioir o f f ( w o i r y +iilf:ii(, of :il)oiii ih c ~
QUAXTUM YIELDSOF FEILIWUS TO F~iznrcSULFATE
Fct,., 1962 TABLE
389
I\’
1)at.a pcrtsiiiirig to thc evaluation of the molar absorptiv-
it,ics, e,of the ferrous sulfate in thc different suliatc solutions a t 25”. The explanation of the table is given in the text.. Part A : Data pertaining to the ferric frce solution C which a a s 0.70 N in Fc804and 2 df in &Soh. rnp
ltenction vessol 13
284 284 284
I) E B I) E
286 286 286
Sol’n. depth 1, in em. 2.76 2.70 2.67 2.76 2.70 2.67
A 0.334 ,331 ,328 ,223 .221 ,210
A/1 01
em
0.121 .122 ,123
t2
cr
0.173 ,178
0.174
. 175 .I16 ,117 .117
.0809 ,0817 ,0821
0.117
l’art 11: I h t s pcrtnining to thc eviduatio1I of the trncc :mount of ferric, c30, insdvcrtcntly in the solution 0.688 d l in I?eSO4and 2 M i n I12S04. The solution was about thc mnie composition as solution C so the values of 22 a t 281 and 286 mp obtnincd for solution C were nssigncd to this solution. nip
28.1 286
iz
ezc2
0,174 0.120 .I17 .080
ii
t
0.291 ,285
1.00
105 x
facao
1.00 0.171
fa
2200 ,178 2240
105
x
can
Ea0
7.78 7.82
7.80
l’art C: Data pert,ilining to the cv:iluatiori of tz at. 300, 304 :md 310 nip in the above solution 0.688 iM in FcS04and 2 M i n I12S04 which contained no added ferric iron but in which ca = cyn = 7.80 X ilf as dctcrmined above.
~ i p
300 300 301 304 310 310
63 f3C3 A ac2 f2 ?2 2530 0.197 1.045 5 . 0 0 0 . 0 1 2 0.017 0.013 2530 ,197 0 . 2 0 3 1.00 ,006 ,009 2508 ,196 1.023 8.00 ,009 .013 ,009 2508 ,196 0.199 1 . 0 0 .003 ,004 2404 ,188 ,988 5 . 0 0 ,009 ,013 ,008 2404 ,188 .190 1.00 ,002 ,003
“Best” values chosen for fZ 0.018 .011 .011
2900 3000 3100
A. Fig. 2.--hbsorbancc values, A, for our ferrous solution containing the least amount of ferric iron which happened to be less than the amount measurable. The values are for a water solution 0.70 M FeS04 and 1.9 211 HzS04 in the three different reaction vessels having solution light paths of 2.76, 2.70 and 2.67 cm. The A values adjusted for these small differences in depths would reduce the spread of the data at every wave length and this was taken into account in the calculations of the molar absorptivitics, e?, of the ferrous iron as indicated in Table IV. Evidence for the lack of ferric iron in the solution is indicated by the abscncc of an absorbance masimum in the region of 3000 A. where ferric iron in sulfate solution has an absorption peak. .The values of €1 calculated from the data at 2840 and 2860 A . are conaidered to be accurate within onc per cent. and equal 0.174 and 0.117, respectively.
same composition. This solution, however, inadvertently contained a significant amount, c30, of ferric, which was determiiied as outlined in Table IV, Part B. The values of E~c:!andosubsequently of EZ outside Net quantum yields based on ihe light of 253i A. the range 2840 to 2860 A. next were determined in absorbed only by the ferrous sulfate remained co11thc manner outlined in Table IT7,Part C. Eventually the values of €2 given in the last stant in any given solution as d s o is illustrated in column of Table IV, Part C were selected as the Fig. 3 . Table V presents the values of dg arid dn together “best” values because their use combined with the values of c2 and E J gave, within the limits of error, with pcrtinent informatioii. It should be noted in reasonablc values for c30 in all of our solutions. this table that the light intensity incident on thc Our best estimates of the values of €2 are given: photolyzed solutions did not vary greatly. The inbetween 300 and 400 mp there is in every case a tensity available to the ferrous sulfate, howevw, peak a t about 384 mp. In the case of solutions decreased several-fold during the course of photolyA, B, C and F, €2 is about 0.03 a t this peak and no sis of a given solution because of iiicreases in the greater than its value a t 300 mp outside the range fraction of light absorbed by the ferric sulfate. Figure 4 displays the independence of dn upon 380 to 390 mp; in the case of solution E the corresponding values of EZ are about three times larger. the concentration of ferrous sulfate in our solutions. At 2537 A , howcvcr, €2 is about 12.5 and 7.2, re- The apparent slight increase in dn is partly thc result of our assumption thatl the actinometer and spectively, in the corresponding solutions. It secms worthwhile to point out that our charts ferrous sulfate solutions undcr invcstigation nbalso revealed an absorption peak for ferrous sulfate sorbed the same fractioii of light of 2537 A. inridriit at about 2360 A. which remained unch,znged in our up011 them. This is strictly truc, however, only solutions lip to 2 111 i i i sulfuric acid but became when both solutions have the same optical density. sharper whcn the acidity was iricrcased to 6 111. Actually our most dilute ferrous sulfatci solution This effect produced the decrease ill E~ of about two- had ir;itially a slightly lower opticxl density at Sold a t 2537 A. 2337 A. than thc actiiiomcttr whitiori and ihc Gross quantum yields arc in terms of gram atoms fraction of light absorbed by the ferric iroii was of ferrous convertcd to ferric per niolc of quanta OS small. Thus, initially, slightly lcss light, than we a11 the light of 2537 A. absorbed by the solution. have calculated actually was absorbed by the more Thc dccrcase i n these yicldr :LS the rcactiori pro- dilute solutions phoiolyzed so both & and dn :wtug r ~ - r d i< illii\tr;it(d i i i IGa 3. opt i c 8 : i l ( h i :illy arc 1:irgvr i h:ui r : i l r i i l : ~ t ( ~ ( l . ‘I’hc1
3 io
L. J. HEIUT,AI. G. AIULLIN, W. I3. MAETIN,Jrt., AND A. 31. J. BwrrrY
TABLE V Gross and net quantum yields for the oxidation of ferrous t o ferric and the accompanying reduction of water to gaseous hydrogen under an inert atmosphere by light of 2537 A. absorbed by solutions of the compositions given in Table 11. The solutions w r c at 25' when photolyzed. The values of Ig are expressed in moles of quanta of 2537 k incident per second upon a square cm. of the photolyzed solution. The amount of ferrous converted to ferric was a t the most 1.4%.
0.40
3 0.32 .-
3
6
+ I
3
P
VOl. 66
%
-c,
d
Soh.
% 0.24
+E
+ti
1081s
A 0.080t00.285 0.334 f 0 0'24 3 . 5 13 .278to .410 .3% f .023 2 . 1 C .304to .455 .428 f ,034 1.8 D .308to .4OO .386 ZIZ ,018 1 . 7 E .183to .SO4 .737 f .OB6 2 . 7 F .091 to .I32 ,155 f ,005 4 . 5
-I2
:
-0
a
m
8
60.18
2637 h. abs. by No. of Fe(11) cxpts.
24to80 78to98 79 to99 82 t o 9 7 26to77 56 t o 8 1
47 38 21 35 27 8
contrast to ccrous pcrchlorate solutions arc not mcasiirably sensitivc to light of 2537 A,2b The ratio of hydrogen produced to ferrous con1 I I L--. -_8 verted to ferric was dctcrmined in solution C which 40 80 120 was 0.70 M in ferrous d f a t e arid 2 M in sulfuric I O 5 X Fe(II1). acid and under an atmosphere of carbon dioxide Fig. 3.-Typical results obtained for gross, &, and net, &, instead of helium. The aniount of hydrogen proquantum yields during the course of the photolysis of the ferrous sulfate solutions a t 25'. The results are for solu- duccd was varicd ovcr a range of about threefold tion A which was 0.10 M in FeS04 and 2.0 M in E12S04. The by varying the cxtent (time) of photoly upper points represent dll and fall on the horizontal line The procedure for determining the amount of within the limits of error. The lower points falling away hydrogen was a tracer technique employing a mass from the horizontal line represent the values obtained for &. The constancy of $., which is based only on the light spectrograph. Briefly, a measurcd amount of hcabsorbed by the ferrous iron, is strong evidence that in these lium was added to the systcm containing all the solutions the ferric iron acts mainly as an inner filter. solution photolyzed and the products of photolysis. The hydrogen and added helium, as well as thc carbon dioxide and water vapor, wcre allowed to equilibrate for about an hour. Then a measured fraction of thc system containing a finger and closed off cvacuatcd sampling flask was isolated from the rest of thc system and the carbon dioxide and water vapor in this part mcre frozen out by immrrsing the fiiigcr in liquid nitrogcn. Finally, the sample flask I I 0.2 Awas opened and the gases entered it by passing 0.4 0.8 0.0 through thc cold finger. After allowing sufficient Moles of Fe(I1) per liter. time for equilibration, tht: sample flask was closed Fig. 4.-Plots showing that the net quantum yields in our off, and the ratio of €12 to He in the flask was desulfate solutions A, B, C and D, which were 2.0 N in sulfuric termined. acid, are nearly independent of the concentration of the ferrous sulfate over the range 0.10 to 0.80 M . The apparent h calibration curve for thc analyscs was conslight increase in +,, probably is the result of a small error in structed from the rcsults of ratios of knowi mixturcs evaluating the light of 2537 1.absorbed by the system, as of hydrogcn arid helium in the range of those found in explained in the text. the analyses. Allowance >vas made for thc consities at 2537 A. of all thc photolyzed solutions, tribution He++ by mcasuring the ratios of the 2 however, evcntually became greater than the opti- and 4 mass to charge peaks in pure €IC blanks. cal density of the actinometer solution as the Drs. Klaus Ritmann and Gottfricd Dcffner of this ferrous was convertcd to ferric iron because of the Laboratory kindly helped with thcsc analyses. About 10+ molc of hydrogen should have bcen grcatcr values for than for €2 a t 2537 A. This effect a t first decreased the crror in both the gross produced by these photolyses. The amounts oband net quantum yields and eventually changcd the tained by the analyses were about 40% lomcr. The diffcrence is belicvcd due to failure of the sign of the crror as the rcaction progrvssed. Figure 5 shows that an increase in the sulfuric hydrogen formed in the solution to come into equiacid brings about a significant incrcasc in &. The librium with its gas phase and to preferential adlinear relationship between & and the concentra- sorption of the gaseous hydrogen compared to the tion of sulfuric acid is quite surprising since it does dilutcnt carrier gas hclium on the surfaces of thc not lead to a linear rclationship hctwwn l/& and system espccinllv on thc carbon dioxide and watcr l/(H+) as in the case of cerous perchlorate solu- snow in the cold firigcr through which the mixturc tiomZb We plan to carry out experiments designed of hydrogen and helium passed on its way to the l o revcal the reason for this difference. It should sampling flask for subsequent analysis with the be not,ed, however, that cerous sulfatc solutions in mass spectrograph. O
8
00
0
08
OC
I
L
I
QTJANTTJJI YIELDSOF FERROUS TO FERRIC SULFATE
Feh., 10G2
341
The over-all reaction in oiir ncidic solutions call bo r t ~ p r w ~ r ~ wilhiri lrd thr limits of crror by the cYl”.Ltlull
IWI)
+ TI+ -1-
light
-
I+(III)
+ 1/z irnjB)
Our measured net quantum yield of aboiit 74% for light, of 2,537 A. absorbed by ferrous in 6 M H2S04is l o be compared to yields of about, 0.1% and of less than 0.001% for the conversion of ccrous to ceric in ptrchloric and sulfuric acids, respectivt:ly,2b
Discussion We hnd nnticipntcd a maximum value of about 0.4 for & based on extrapolation to infinite acidity and sulfate concentration of the linrar plots obtaincd for l/&iis. l/(II*) a t 2 M and less sulfuric acid. The iinrxpc dly largc valuc of 74% for 4, in 6 M sulfuric acid we believe is due to thc existi ence of significant amounts of a new kind of ferroils sulfatc complex a t the larger concentrations of sulfuric acid. In 2 &I and less sulfuric acid the ferrous sulfatc complcx appears to exist mainly as an ion pair of the type Fc(H20),S0., in which there are more than two (n > 2) layers of waler molecules between the ferrous and sulfate ions. More than two layers of water appear to be necessary to keep the elcctrostatic forcrs between the oppositely charged ferrous and sulfate ions from squeezing out the intervening water. This concept is based on conductivity,’O”$b s o l ~ i b i l i t y , isopiestic12 ~~~~~ and freezing point13 measurements of aqueous solutions of 2-2 sulfates ~ of compared to 2-1 c h l o r i d e ~ l ~in~ ,particular C:LSO,,arid MgSOI comparcd to CaC12, MgClt and I?eCl2which 1iar.c been found to be weak and strong clertrolytes, respectively. Absorption spectra measiirrments of CuSOd in watcr15also support the hypothesis that the 2-2 sulfutc is a wcak clcctrolytc. That the associated ferrous sulfate in dilutr arid exists as ion pairs of the kind first proposed by 13jerrum16is supported by 1hc luck of any measur-
_L
0
2
4
-
1
I
6
Moles of H2SOIper liter. showing the linear dependence of the net quantum yields upon the concentrations of sulfuric acid in solutions l
