Detergency of Alkaline Salt Solutions 11. Lowering of Interfacial Tension

FOSTER DEE SNELL, 130 Clinton St., Brooklyn, N. Y.

Interfacial tensions were determined by falling drops of water in oil and by rising drops of oil in water. The latter are lower. The oil is a mixture of mineral and saponifiable oil containing accurately controlled amounts of free fa t ty acid f rom 0.16 to 2.12 per cent, or it is benzene containing 0.1 per cent free fatty acid.

The technic in which benzene containing 0.1 per cent free fa t ty acid rises through the detergent solu- tion is recommended.

The “neutral” point of soap at p H 10.2 should govern prediction of its behavior with alkaline salts. It is suggested that alkalinity available above p H 8.0 and above 10.2, expressed as sodium oxide, is a better method of comparative commercial expression of one function of builders than is total sodium oxide.

There is a general parallelism between the inter- facial tensions obtained and the p H of the alkaline salt solution, although that is not the only factor. Data show that a relatively highly alkaline soap builder such as metasilicate can partially replace soap, so f a r as interfacial tension measurements are concerned.

HE tension existing a t the interface between two solutions may be expressed semi-quantitatively by the T number of drops formed when a given volume of one

solution is discharged into the other at a given rate. The number of drops increases with decreasing interfacial tension but is not a linear function. For more exact comparison the drop numbers must be calculated to an expression of the interfacial tension in dynes per centimeter.

Such a method has been employed for determination of the wetting power of a series of solutions of alkaline salts, com- monly used for addition to soap for detergent purposes, against various water-insoluble liquids. The degree of al- kalinity of these salt solutions has been previously deter- mined ( l a ) as shown both by the initial pH and COE of the solu- tion and by the total available alkali present. Sodium chlo- ride was used for comparison as a neutral sodium salt. Inter- facial tension determinations are one method of indicating the detergent value of such salt solutions and the relationship existing between the degree of alkalinity and the wetting power when used with soap.

This type of method was used by Donnan (2) to measure the interfacial tensions of solutions of different concentrations of sodium hydroxide against a commercial rapeseed oil. He counted the drops of oil discharged from a pipet into the sodium hydroxide solution. Hillyer (6) obtained drop numbers in this way for a series of materials (including saponin, sodium oleate, and sodium hydroxide) against kerosene or pure cottonseed oil. He also studied oleate, palmitate, and stearate soaps a t different concentrations. Harkins and Zollman (5) studied the effect of added acids, bases, and salts on the interfacial tension of sodium oleate solution by meas- urement against benzene.

The effect of the alkaline builder is interpreted as manifesting itself in three ways in the order of de- creasing importance: by reaction with fa t t y acid in the dirt to form soap, by repression of the hydrolysis of soap by the hydroxyl ion to form micellar soap, and to a much lesser extent by repression of the ionization of soap by the sodium ion to f o r m micellar soap. Of themselves alkaline salts do not possess the de-

tergent property of lowering the interfacial tension of water against pure benzene. W h e n added to soap solutions they do lower the inlerfacial tension against pure benzene. With or without added soap they lower the interfacial tension against a nonaqueous mixture containing free fatty acid. In the presence of soap, neutral salts also lower slightly the inter- facial tension against either pure benzene or a non- aqueous medium containing free fa t ty acid.

Of the materials studied, sodium hydroxide causes the greatest lowering of interfacial tension when added to soap. Sodium orthosilicate is next, fol- lowed by sodium metasilicate.

Elledge and Isherwood (3) used the same method for meas- uring the effect of soap with added builder on the interfa- cial tension of the solutions against benzene.

Millard (8) also used benzene as the nonaqueous solution into which a known volume of aqueous solution was discharged. His apparatus consisted of a pipet controlled by displacement of air from a vessel into which mercury was allowed to drop slowly. The pipet and flask of benzene into which it dipped were surrounded by a water bath maintained a t 40’ C. His measurements were on the effect of addition of various alkaline materials to a 0.03 per cent soap solution. Of the series he studied, sodium hydroxide added to soap solution gave the lowest interfacial tension, with sodium carbonate next. Two silicates were included in his series, the more alkaline one having an alkali ratio of 2Kaz0:3SiOz. Drop numbers for sodium silicate were also determined by Richardson (9). He used a silicate having the composition Naz0:2.83SiOz and found the interfacial tension of a solution containing 0.05 per cent of this salt and 0.10 per cent soap to be greater than that of a similar solution containing sodium carbonate.

Shorter (10) studied the rise of nonaqueous droplets through soap solutions in which he varied the ratio of fatty acid to alkali from 0.951.0 to 1.43:l.O. I n general, his results were higher, the greater the amount of alkali present, whether his nonaqueous liquid was benzene or benzene containing 5 per cent free oleic acid. In the case of benzene he measured the effect of soap, hydrolyzed and unhydrolyzed. When working with benzene containing free fatty acid, he had, in addition, soap formed a t the interface, although the excess alkali was insufficient to form much soap, relative to the amount al- ready present and to the amount of fatty acid available. Shorter and Ellingsmorth (11) presented similar data and

1051

I N I) U S T I t 1 A L A N D E N G I N E E H I N 1; C l l E >I I Y T H Y \<,I. 21. No. 9

DTBT-TIGTIT SWING Ilanrnrnc~ MILL

concluded that "tlie effect oS i( given addition of alkali is the greater the smaller tlie constant concentration of tlie soap." They suggested that t.liis is diic to ail increase in tlie colloidal nature of the soap solution.

It will be helpful in visualizing the results iu whiclr soap is used if the i~li value of a "neutral" soap is recalled. This is approximately 10.2 for a 0.1 per cent solution, viiries hut sliglitly over ii comiderable range OS concentratiou, and may be spoken of as tlie neutral point for soap. Addition of a more alkaliue buffer salt rcndera such a solution more alkaline. Addition OS auy biifler salt whiclr gircs solutions having a lower pH renders the soap solutioti more acid. This is the case with sodium silicates OS a ratio 1:3.8G, or greater, niodified soda and borax. Therefore, soap solutions can be considered as acidified by borax and rendered alkaline by soda ash.

EXPBRIXBNTAI. ~ h T l I O I j 8

For these determinations a method similar to Millard's wa8 first adopted.

The Trauhc stdugrnornet,er used consisted of a capillary t ihe sealed into n pipet. The mpillnry was flattened out at the end in order to provide n larger dropping area. The radius of the ti0 v a s 4 mm.. and the volume of the pbet w m 6.02 cc. a t 20' C. -rho time roiuired to empty tbe pipit was tipproximately 6 minuteu. The stem of the pipet was graduated above and below the bulb. The stalagmometer dippedjnto a glnss dish containing the oil against which t,he determinatron >vas being made. The dish was fitted with a the-hole st,opper through which the stal- agmometer, a thermometer, and a &ASS tube to provide for dis- placement of air were inserted. The whole WRS surrounded hy a glass water bath maintained at 40' * 0.5" C.

Since benzene was used as the nonaqueous liquid io the majority of previous experiments, one series of deterininat,ions was carried out against that substance. These measure- ments illustratc the detergent effect on an unreactive and inert substance. Such determinations, however, are not repre- sentative of actual washing conditions since, in general, the oil or grease present in dirt contains a small amount of acid, and this is quite generally believed to be of the nature of fatty acid. Theresore, a factor is present in actual washing which is not represented by interfacial tension determinations against benzene the reaction between the alkaline salt and fatty acid t.o form soap. A furthor difference is the presence of saponifiable oils as well as unsaponifiable oils in dirt. I n order to more elosely approximate actual conditions, a series of oils was made up composed of equal volumes of a light refined

nririeral oil and a food-grade cuttoilseed oil to which varying amounts of oleic acid were addcd. The cottonseed oil originally contained 0.543 per cent free fatty acid c a l c u l a t e d as oleic. The oleic acid added was a commercial grade which sliawed 92.34 per cent free oleic acid on analysis. The free fat,ty acid present in the dilferent niistures is drown iii Table 1. Bfter s t a n d i n g f o r some time, thc free fatty acid eontent of the oil mixtures changed slightly so that they could not he used to give results which would he strictly comparable Tvith determiria.. tions made somc weeks earlier.

This means that, when one solution as discharged into the otlicr, the iiiterfacitii tension was so low that drops were too close together to count or that actual drops mere not formed. In this case thc interfacial tension \vas assumed to be less than one dyne per centimeter. This i s in accordance wit,h the results of IIarkins and %oilman (;i) who found that with interfacial tensions of less tliari one dyne per ceiitimeter enuilsification takes place so readily that two

1iiyei.a caiiriot be separated. In later cxperimeiits a mixture of Lenaeue with oleic acid

made up to contain 0.1 per cent Sree fatty acid was adopted in place of the various oil mixtures. Wlrile this did not approxi- mate as eloscly tlic actual composition of tile oils in dirt, it had the advantage of being easily duplicable and of having less t,eridency to change over a. period of time. The benaeneoleic

In some cases streaming o c c u r r e d .

SYORAGF; BIN FOR MILLED SODIUM MI:T.~SILICATE I>IAECTLY IINDI:,, Sw-1xu H M M E 1 , M I L L

acid mixture was suitable for comparison of the materials under consideration, as the free fatty acid content was sufficient to bring out the differences between them and yet was small enough so that the minimum amount of streaming occurred.

TABLE I. PILEE FA%TY ACID CONTENT OF 011. ~IIXTWREB FnsE PATTI. .4cm

ddded Total Dresent % % OIL MLXTDRP SP. Gn.

I 0.8754 1.85 2.12 I1 0.8756 0.0 0.27 VI 0.8760 0.05 0.32 VI1 0.8833 0.0 0.19 VI11 0.8831 0.0 0.16

s<:,,l,l!,,,t><.r, 1932 I S D U S T R I A L A N D E li 0 IN E 13 I< I 3 ( 2 C: H E M I S T K Y

The interfacial tensions, expressed in dyries per centimeter, as determined against the series of oil mixtures and against bensene with the Traube stalagmometer are shown in Table 11.

In these determinations t.he fatty acid pres- ent in tlie test was always in excess of t h e alkaline salt, since the aqueous solution was being liberated under the surface of the oil, and the amount of akaline salt solution was relatively milch less than the amount of oil p r w ent. This by no means iridicatcs that excess fatty acid would be prcseirt in tile interface. it does mean that a somewhat greater amount of alkali would be present in the interface than would be the case if the oil were in large excess hecaiise of the large reserve supply available to ditTuae into the interface and thus prevent local exhailstion of alhnii by reaction. Under actual conditions the oil or dirt is dispersed isi the aqueous detergent solution, and the alkaline salt should bc in excess of any acidity in the dirt to be removed. An apparatus in which drops of oil rise tliruugh tlie solution of detergent would tlierdore off& a comparison under conditions more closely approximating those oc!curriiig in actual use. Then, instead of drops of detergent, solution fornied and dropping through a largc excess [if oil, drons of oil reurcserrtinn dirt would be formed and worild rise in a large eacess OS detergent solution.

PH"

11.85 S I 2 10.8 10.7 10.65 10.1 10.(1 9 .a:> 7 .2

10.2 5 . 8

11.9 11.8 11 1 11.ll I 1 o 10. ,4 Ill .i 111 2 o x

#a .At 2.: > - ,

T u obtairi ti wtnpariwu uf this sort,, a Uinnnig pi1,ct war einploytxl. 'i'liis apparatus is :i modification of the pipet used by 1)onnan (S) and xas developed by tho Am(, vwan Society for 'Testing Materials ( I ) for measuring tlie eniiilsify ing tendency of lubricating oils.

The mpi1I:wy t,ip is 1 nim. in diamdar and used in s i r d n. way that the drop forms only on the inner edge nf the tip. The pipet has IL viihimr oE 2 ec. nnii is grdunted to 0.01 co. The rate of formation of drops is regulated by the rate of f los of a he:%vy uil contained io gin upper comp:wtmmt nod contrnllcd by R stop- cock. A rate of two drops per minute was selected. The tip of the pipet is oirved so that t.he drops oi oil are dolivered near the hot,tom 01 the aqueous solution and rive through It. This wpa- ratus was dipped into a dish containing 50 co. of the detergent solution tind the whole was maintained at a temperature of 409 * 0.5" C . by a wnter bath.

The results of the determinations with this appnrutiii a r k

shown in Table $11.

I h p numbers were calciilrited to expression i n dyiies per eentioiet,er, accordiiig to the law of Tate, is-hidi is expressed by the eipi:ition:

>ny 27Tl Y ~ --

where 7 = int,erfacid tension

The rnilss of tiie drop was calcnlrit~xl f r w i t.lie drop iimnber by dividing the volume of the pipet liy tire drop iiumber and inidtiplying by tlie iliiFcreiice in rlimsity ixkiveen tiie aqueoiis solution and tlie Tlrt: specific griri ty of the dilute aqucous solnt,ions wa i l r n e d t,o lie the same as t,hat Of water. The specific gravities oil tlre iliffiwnt, oil inixtiires are inclodad in Table 1.

ii.85

10.7

10.1 in.0

10.2

11.2 10.8

10.65

9.35 7.2

5.6

iiiTxoi:-l- S<>",<:M "LXIiE 28.88<s.o < I 0 < i . n < i . o < i . o

X . W I 1. 18 i . s o 5.30 8.45 8.98 LO 29 X9 <1.0 R.20 2.15 6.41 6 .71

1.58 29.8!1 <1.0 a.Ci2 2.77 R . 6 i 7.26 2 9 . 8 o < 1 . 0 2.92 1 . 1 3 9 . 2 1 5 . 4 4

diiiiiiailiinte. 1':l.Xti 29.89 6.Rd 9.00 X.78 11 40 11.58 2U.86 3.41, 6.49 5.46 7.72 7 . 7 2 LD.li!I 7.61 11.51 10.76 12.18 13.30

water

4.71 9 .51

10.75 10.46 D 7 2

13.51 12,:ii 1 2 . 3 s 17.81

. .

. . .. ..

. .

. .

. . .. . .

In the ileterrninations with the Traube stalagiriometer, correction was made for the shape of the drop, using the formula oi Harkins and llronn (4). They express the func- tion of the ratio of the tip radius to the cube root of the drop volume by #(r/J"'a), and the 'Me fnnnrda is modified hy tt1eoi to:

IN D U S T I$ I A L A N D I3 N G 1 N 1.: I.: I1 I !4 (; C 11 F: hl I S T 11 Y Vol. 24., No. 9

? = . nw w(iiL)

Values for + ( r / V i ' ) corresponding to cnicu1ati:d values for T/W are read directly from a table prepared by them. In the case oi the Ilimmig pipet the radius of the tip is 0.5 mm., which is so small that the valucs for T/V"* did not appear on the Harkins and JIrown table. The drops wore assumed to approach tlie shape of the ideal drop, a d no correction nas

inediutn and for hydroxyl ion to diSfuse through tlie similar film oE wrtcr to meet :urd form soap in the iiiterfa,ce.

Another diikrence between results with the two pipets is that in one case a much larger reserve supply of fatty acid than of alknline salt is availalile, nlrereas iii t.lie other the rcscrve supply of alkaline salt is in excess nhcn reaction talies place between t.hc two. This factor is not present in the results against benzene, and in that case the values with the two pipets fire in close agreement.

In Talh 1V arid Reure 1 are eiven the deviations of results i/ j, applied to the Eormula. ohtaincd rrith tlie Ihnmig stalapriometcr using two different

solutions, d e n the rate OS discharge is varied from one t,o sixty drops per nrinute. The variable size of the drop duringfonna- tion makes considerat,ion of this factor estremely complex. The data sliow that under no conditions st.udied is equilibrium attained, nor do they indicate that equilibrium can be at- tained.

Coiirarirsox OF & h i i o n s

The \rnlnes obtained with the Traube pipet are, ill general, The re- i,igher t~lan those obtained witi, the 1)inIrnig pipet,

suits not be expected to agree since &lie conditions

FIG,,,%*: 1

equilihriurn are so wi-idely diffcn:nt,. 'lk results of s i i c l i interfacial tension oimisurernents dcpend largsly on tlie extent to w-hicli eqiiilibriurn is rcaclied in tlie interface.

Tlie orifice of tlre Dirnnrig pipct is smaller than that of tho Traulx: pipct. The rate of dischrtrge from the Uinnnig pipet was two drops per niinnk. The rate from the Tranhc pipet was directly proportional to tlre nunibcr of drops, since with this pipet tho time of clischnrge wris coiistarit as is cus- tomary in the use of this app:xatus. With the Traube stal- agmometer tlie period of exposure of the drop decreases :is the drop number increases. Thus the nvcrage are8 exiiosirid varies as an inverse Eunction oE the drop number, and the ex- tent to which equilik~riutn is attained would he lessened w i t h t h e more efficient b u i l d e r s . T h e d r o p numbers with the 1)immig pipet are larger and the time for determination is longer so that i t may safely be as- sumed that eqnilibriinn is more closely appro:iohed with that apparatus. With that pipet the timeof Eorma- tion nE eikclr drop is coii- stant. Tlie &rea exposed is therefore varying, inoreas- ing with decrease of inter- facial tension. This tends to accentuate the differences between the different. aque- ous solutions. W h e n t h e drop of oil enlarges rapidly on t h e p i p e t a n d r i s e s q u i c k l y f rom it, there is less opportunity for fatty acid to diffuse through the stagnant film a t the inter- face oE the n o n a q u e o u s

I n the vieioity of two drops per minute, or 30 seconds per drop, plotting these data shows that the curve descends a t a rate of :ipprwirniitcly 0.06 dyne per centimeter per second. This iiiilicntes that the value of 10.80 y would be 10.85 Y if rim at 2V seconds per drop, or 10.75 y if a t 31 seconds per drop. Thn cume is therefore descending sumciently slowly so tlmt diiplicritioii of deterniinat.ions within 0.5 per cent is to be expcctad and 'ix-as obtained.

Thr? results do not indicate an equilibrium condition. nor one dujrlioahle otlm than by apparatus of approximately the same size orifice discharging a t the same rate. The re- sults d the Iliimnig detenninations are probably more ac-

curate than those with the Trauhe pipet and a truer designatbni of the interfa- cia1 tensions existink in ' com- mercial use of the salt soh- tions as builders.

RATIO OF BGILDER AND SOAP

1Jsage as to the propor- tion of buildcr to soap varies in different laundries. A ratio of 1:3 was adopted for this stiidy as that most commonly used. In some cases, p a r t i c i i la r I y with the more highly a l k a l i n e builders, a larger propor- tioir of the alkaline salt is used. Equal parts of soap a n d b u i l d e r h a v e been recommended. Determina- tions of tlre interfacial ten- sion against benzene plus 0.1 per cent free fatty acid, using solutions containing 0.05 per cent soap and 0.05 per cent builder, were the re fore made with a restricted

103,

these data on sodium hydroxide to indicate the relative importance of interfacial tension in t.he process of detergency. From laundry practice i t is to be concluded that a buffered salt is more satisfactory. This was eonfinned by data subs- quently. Similarly, sodium chloride was included as showing the effect of a nonalkaline salt giving a high concentration of the sodium ion.

1 N 1) 0 S T K I A I, A N D E N G I N E E 1% I N G C If E iM I S T 1% Y vug. 21, No. 9

r Sodium orthosilicate has recently been prepared in crya- talline form (7). If it becomes available commercially at a price within the range of other soap builders, these inter- facial tension data support the conclusion previously indicated by data on its alkalinity in solution that, of the series studied, this salt would be superior as a soap builder.

EFFECT OF PARTIAL NEUTXAL~ZATION

One would predict that the interfacial tension would iii- crease as the builder was progressively neutralized and would give approximately the same value when completely neu- tralized as is given bv sodium chloride. if i t did not react with

~

soap present. In Table VI1 are given such results for sodium metasilicate

SODIUM M~ASILICATP: PREMIXING TANKS Eqaipned with stesm ooi!s2 '+e bottoms, and turbine stirreis ~ o r n p l e t e l ~

mounted on soale. facilitating weighing. additions. and withdrawale.

Of the salts, sodium nietasilicatc causes the greatest lower- ing when added to soap in the presence of free fatty acid. As these conditions rcpresent a logical parallel to those under which a builder is used commercially, such tests should fomi one basis of comparison of alkaline salts for laundry use. Under these conditioiis sodium carbonate is second to sodinm metasilicate, although in t.he absenee of soap it shown greater lowering of interfacial tension than the metasilicate.

SOVIGM ~K~l%osILlcATE

In the determination of initial and available alkalinity of these materials, sodium orthosilicate was later introduced into the series as a silicate having a higher ratio of sodium oxide to silicon dioxide than the metasilicate, and therefore likely to be even niore satisfactory as a soap builder so far as dogrec of alhaliuity was coneerned. The pH 7.aIue and titra- tion curve of sodium orthosilicate corifiimed this supposition. The orthosilicate was thercrore similarly int.roduced into this series of interfacial t,nssion dcterminations. The v a l u ~ s for w 0.03:3 per cent solution of this salt (both alone and added t,o a 0.1 per cent soap solution) against benzene, bermene and 0.1 per cent free fatty acid, and the various oil mixt.ures are shown in Table VI, in comparison with similar results for sodium mr;tasilicat,e. In all cases the interfacial tension measurements for sodium orthosilicate are lower, rorrespond- ing to its greater alldinity.

and sodium carbonate, with correlating data needed for comparison.

With the builders alone there is a rise in interfacial tension and then a lag as neutralization proceeds. When com- pletely neutralized, the values are intermediate between those for the neutral salt, sodium chloride, and for water.

When builder is present with the soap, the results are better tlian those for soap alone until over 50 per cent neutralization has occurred. The interfacial tension then rises since the partially neutralized builder is helow the neutral point for soap and therefore lowers its efficiency as shown by the value when 75 per wnt neutralized. When 100 per cent neutralized, the interfacial tensions of 7.82 y and 7.57 y approximate that of 7.50 y for borax and soap against this oil. This again demonstrates the detrimental action of a builder below the neutral point of soap, pH 10.2. Sodium oleat,e with sodium chloride is much superior to sodium oleate with the acidified alkaline buffers.

I). 1% sudium "icatr 5 . 2 3 . . . . . . . . . . . . , . . , . . (0.0416)

0 (133% tiodium mete- *i i iehie and 0.1% 6". ~Jium olcutc CI.O 2.73 3 . 9 3 ( i . 27 7 . 8 2

0.033% sodium carboe- &e atid 0.1% siidirim

<1.0 2.35 3.30 3 . 4 7 7.57 olcstr 0.033% sudiiim, ohloride

and 0.1% sodium oleate . . . . . . . . . . . . . . . . . . . water . . . . . . . . . . . . . . . . . . . 1%65

3 . SZ

* With Diininig Pqmt. b Figures in pP.renthesos ,,re C"H VsIUeS.

This series illustrates the function of the builder in pro- tecting the soap from acidity of the soil. Values for C,,, of the solutions without soap (the only ones available) are inserted to show that the correlation is only a general one, and that other factors than C,,, enter in. At least one such factor is the effect of sodium ion on micelle formation.

bIETliODS OF EXPltESSIOA OF VALUI? OF SOAP RUlLDEh S"l.uTr~rr"

Coniparative values of soap builders are often expressed nenreiie + numerically in terms of the sodium oxide content. Such an

ncn- 0.1% Oil Oil Oil Oil Oil B. F. A.C I 11 V I VII vi11 expression is of little or no value. comparison of the fihwrcs _- D W ~ P ~ T cerctirnriar-------. for soilium carbonate and modified soda show this most

certainly not directly comparable to the large differences in thrir lowering of interfacial tension. Even sodium chloride, and map 8.01 4 .29 <1 .0 C1.0 <1.0 <1.0 < I 0

and sosp 6.54 <1.0 < I . " 1.m a nonalkaline salt, can be expressed in analytical terms for comparison as having a high sodium oxide content.

~ ~ ~~--I\-T*nl~ni:i.~i. TraaioN A E I I N X T : ~--.

Sodiiiru oitllorilicnl~e 28.86 6 . 8 1 <1.0 <1.0 <1.0 3 .70 48;:; readily, Tile small differences in sodiuni oxide content are Sodium mrlsaiiiriiie 29.89 10 .60 <1.0 3.20 2.15 6 ~ 4 1 Sodium ort,hosilioate

Sodium metasilicate

With Dimmip stalagmometer. k Free fatty acid.

September, 1932 I S D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 1057

An important factor is estimation of the level below which sodium oxide is relatively ineffective. As sodium ion i t is effective at any pH level within reason. The corresponding hydroxyl-ion concentration is more important and more effec- tive in its use as a soap builder. Either pH 8.0 or p H 10.2 can be selected as a satisfactory level of comparison. Both are useful. In general the latter is much preferable. lieither is perfect.

I n Table VI11 are given such values in usual terms and in terms of sodium oxide above p H 8.0 and above 10.2. The clearer expression of the last two, particularly of the final one, is evident.

TABLE VIII. COMPARATIVE SODIUM OXIDE CONTENTS ABOVE DIFFEREXT PH LEVELS CALCULATED FROM 0.66 PER CENT

SOLUTION TOTAL NarO ABOVE Kat0 A B O V E

SALT XazO PH 8.0 PH 10.2 % % %

Sodium hydroxide 7 7 . 5 7 7 . 5 7 5 . 2 Sodium metasilicate 29.6 2 8 . 2 1 7 . 8 Trisodium hosphate 26. 9 . 4 8 . 9

Modified soda 50.1 1 7 . 4 0 Borax 2 9 . 6 1 7 . 4 0 Sodium chloride 53 .0 0 0

Sodium cargonate 5 8 . 5 2 9 . 1 9 . 4

0 Theoretical, 24.3.

PROPOSED COMP.4R.4TIVE METHOD

Benzene alone is not a suitable standard for comparative interfacial tension measurements because the factor of reac- tion between the builder and fatty acid at the Interface is absent. There is always a question as to the accuracy of duplication of oil mixtures in which refined saponifiable oils are used. The necessary factors can be measured against ben- zene to which a known amount of refined oleic acid has been added. The series reported here against this mixture is not complete because many builders had been eliminated experi- mentally before its use was adopted.

I n the absence of a better method it is recommended that for comparative measurement of the effect on interfacial tension of soap or other alkaline detergents, alone or in com- bination, the following method be used:

Determine the drop number at a standard temperature with benzene containing 0.1 per cent free fatty acid against a large ex- cess of the detergent solution. For such determination use the Dimmig pipet discharging at two drops per minute. The de- sirable temperature is 40' C. unless stearates are present, in which case 50" or 60" C. should be adopted. Calculate the drop number to interfacial tension in dynes per centimeter by the law of Tate without correction for deformation of the drop.

No data are available at present to show the duplicability of results between different laboratories. Duplicates at different times in the same laboratory have been consistently obtained.

COWLUSION Of themselves, sodium hydroxide, alkaline salts, and sodium

chloride do not possess the detergent property of lowering the interfacial tension of water against pure benzene. When added to soap solutions, they lower the interfacial tension against pure benzene. With or without added soap all ex- cept sodium chloride substantially lower the interfacial ten- sion against a nonaqueous mixture containing free fatty acid.

The factors involved are, in their order of decreasing importance: reaction of the alkali or alkaline salt with fatty acid in the dirt to form soap, repression of the hydrolysis of the soap by the hydroxyl ion (presumably to form micellar soap), and repression of the ionization of soap by t.he sodium ion (presumably to form micellar soap).

Of the series studied, sodium hydroxide causes the greatest lowering of interfacial tension when added to soap. Of the alkaline salts studied, the effect of sodium orthosilicate is greatest. Of the salts at present commercially available, sodium metasilicat'e has the greatest value on the basis of its effect on interfacial tension.

ACKNOWLEDGMEKT These results are published by permission of the Provident

Chemical Works, a subsidiary of the Swann Corporation. The laboratory work was largely carried out by Elbert L. Jung and prepared for publication by Beatrice F. Grey.

LITERATURE CITED (1) Dimmig, Proc. Ana. SOC. Testing M d e r i u l s , 23, 363-9 (1923). (2) Donnan, 2. phuaik. Chem., 31, 42-9 (189!3). (3) Elledge and Isherwood, J. ISD. EBG. CHEY., 8, 793 (1916). (4) Harkins and Brown, J . Am. Chem. SOC., 41, 499 (1919). (5 ) Harkins and Zollman, Ihid., 48, 69-80 (1926). (6) Hillyer, Ibid. , 25, 511-32 (1903). (7) Kracek, J. Phys . Chem., 34, 1583-98 (1930). (8) Millard, IND. EXG. CHEM., 15, 810 (1923). (9) Richardson, Ibid., 15, 241 (1923).

(10) Shorter, J . Soc. Dyers Colourists, 31, 64-9 11915). (11) Shorter and Ellingsworth, Proc. Royal SOC. (London). A92, 231-

(12) Snell, ISD. ESG. CHEM.., 24, 76-80 (1932).

REcEIvEn January 20, 1932. Presented before the Division of Physical and Inorganic Chemistry a t the 83rd Meeting of the American Chemical Society, New Orleans, La., March 28 to April 1, 1932.

47 (1916).

Use of Lime in Salt Solution for Removing Hydrogen Sulfide from Natura l Gas

The problem of removing hydrogen sulfide from natural gas has become within the last few years one of major importance to the natural gas and petroleum industry. Hydrogen sulfide is probably the most toxic gas associated with natural gas, and from the standpoint of health and safety it is important that it be removed if the gas is to be used for domestic purposes. Hydrogen sulfide is under certain conditions an active corrosive agent and very destructive to equipment. The annual loss caused by its corrosive action on equipment amounts to hundreds of thousands of dollars.

The largest percentage of natural gas produced in the United States comes from wells which also produce oil. This associated gas invariably carries with it light fractions of petroleum, which when separated from the gas condense to the liquid form a t ordinary temperatures and pressures. Commercial application is made of this natural condition, and %et gas" is processed to obtain natural gasoline by employing the principles of ab- sorption, adsorption, or compression and subsequent expansion accompanied by cooling. If hydrogen sulfide is not first re- moved, some of it may oxidize to form free sulfur and water.

The free sulfur will dissolve in the gasoline and cause the product to be unsuitable for commercial use.

0. H. Schoenwald and G. M. Ford of the Wentz Oil Co., Ponca City, Okla., working in cooperation with refinery engineers of the U. S. Bureau of Mines, have developed a method of re- moving hydrogen sulfide from natural gas, based on the reaction of hydrogen sulfide with lime. This report has been published as U. S. Bureau of Mines Report of Investigations 3178. Treat- ment of the gas t o remove the hydrogen sulfide was accom- plished by scrubbing the gas with a solution of sodium chloride and calcium hydroxide. The salt solution appreciably increases the solubility of lime in water up to a certain percentage, and thus a smaller gallonage is required t o remove a small amount of hydrogen sulfide. The treating plant consisted essentially of a scrubbing device for contacting the gas with the solution and a series of tanks for making up or settling the treating solution. The removal of hydrogen sulfide from natural gas before the gasoline was extracted had a beneficial effect on the chemical treatment required for the extracted gasoline before it could be marketed.