Acid-Soap Fom1ation in Aqueous Oleate Solutions

K. P. A~A~lH.~PAD~IA~ABHA:-.." A:-.'D P. SO~I.~SU~D.~R.~~

School of Engineering and Applied Scie1lC~o Columbia (;"niversir:... .\"ew Yorko .\"c""° YrJrl.: 100:7

Received October ~. 1986: accepted April 6. 1987

Surfactants. have been conside~ in the past to Conn prcmiceUar multimcn such 3S dime~ :l.Dd acidsoap. ia-a prc~ous publication. we presented ~ thcnnodynamic mode! :or the surface tension dependenceon su~t concentration taking into account pnmiceUar aareption and we considered the possibilityof fonnation of dimcn in alkaline pow1ium olcate solutions. tn this paper. the surface tension da~ forpow1ium oleate solutions at tWo pH valu~ 'oS ~alJ,"Zcd using the a~~e model and the :)Cser-.ed differ-ences !'rom predictions of the model arc attributed to the fonnation of acid soaps in ~e neutnl pHregion. The formation constant for acid soap is =mated from this 3nal~-sis to be 1.75 x 10' (km01el

mJ)-I. C 1911 Acadcasic PI-. IK.

.iP:~

-:'.

t

cinity) can e:lhance the fonnation of the acidsoap. Thus the formation of acid soap shouldbe even more favorable than that of the doubtycharged dimer. In this paper. the possibility offormation of acid soap in oleate solutions isinvestigated by analyzing the surface tensionbehavior of oleate solutions in the neutral pHrange. The treatment also suggestS. in general.a method to estimate the surface activity of a ~complex molecule such as acid soap. ~

I:.. "TROD\.' Crt o.'ii

THER.~fOD~AMIC ASPECTS 1:~

A thermodynami~model. taki~1 into ac- .count the fonnation Of pre micellar ~tes, tfor the SUrfaCe tension of surfactant solutions ;was present~d in Ref. (I). A brit'ct- outline of ;the model and the equations "~l~vant to the ~discussion5 in thi5 paper are given below.

Consid~':" the anionic surfacUDt RH inaqueous solution in the presence of an indit'.ferent electrol~"te Mx.. Species to be considered '

in tbis system are RH. R -, R:~. R:H-, M"',X-, H-, OH-. and H:O. Chemical equilibriasuch as

Associative interactions betWeen surfactantspecies in aqueous solution to fonn premicel-lar complexes such as dimers and acid soapshave been discussed by many workers (1-18).Our investigation (1) of such interactions inpotaSSium oleate solutions using surface ten-sion measurements in the alkaline pH rangeyielded some additional evidence suggestingthe fonnation of doubly charged dimers in thesystem. Analysis of the surface tension datausing a thermod:--namic model which takesinto account the fonnation of premicellar ag-gregates also ~;elded the fonnation constantfor the doubly charged oleate dimer.

The major driving force for the formationof dimers is the so-called hydrophobic bondinghetween t~,e hydrocarb~n chaill.s. While in thecase of the doubly charged dimer ele..u-osLaticrepulsion between the ionic heads can hinderits fonnation. such repulsion will be absentfrom the case of the acid soap (see Fig. I fora schematic representation of the dimer andthe acid soap). Furthennore. possible H bond-ing between the ion and the neutral molecule(possibly through a water molecule in the ..i-

RH-R- - H-

and the rele...ant thd:mlod...namic constantSsuch as ~:- ~ .

K = (R-\ (H -\. / .'RH 'a "( J "( ) l .', At p~nt ~;th L"nion Carbide Coli/.. T3n';.1o-.-n. :-"1.

1040021-9797/88 S3.00C~I ~ 1911 by 4cademic PI-. I~All n.,.u of ~UCtiOft iD my farm ~ ~arrh :...J-.w."C,.:",.;.uldI~...s...,~. Vol !::.~

ACo.SOAP FOR.\tA TtON IOS

"~-. . "'CMAI.-c...:. . "~l.lt. . "'i.E.(.) (.) (.)

.:..- -~ ~

.,,° ..]:0 . . GO. ..0-~- ~:..A:~-c..A'. . "1.£.(-) (0) (.:

FIG. 1. R~ntation of (a) 41imcr and (b) acid soap.

where the tenus in the bracketS represent theacti..;ties of the species involved. can be wrinenfor this S)"stem. The Gibbs equation relatingthe che~;ca1 potential of the ..-arious species(P,) t.:.. :urface ten.c:ion ("Y) is given by

'. -4-r - L rpl£i. [1]i

{R~-}K.s=~. [7]

{R2H-}Kad8 {R-}{RH;-' [8)

Note that when there is no association. i.e..Kad a 0 and K.s = 0 and = = O. a is equal tounity. Under conditions of complete associa-tion. on the other hand. z tends to cx; and abecomes i. Thus the value of a. betWeen 1and!. reflects the degree of ~atibn. Note ""also that the slope of the surface tension vs log 'i

CT curve is proportional to the product of r Tanda.

where r is the surface excess of species i. Con-Sdering the various chemical equilibria andmau balance in the System. the surface tensionchanges under constant pH and high ionicmength c~nditions can be related to thecbemi.: :e,tial ora single species by equa-boos ( i ;

[2}-iic: I ~

EXPERI~fE~IAl

.:\1alerials

Oleic acid (>99.9% as per manufacturer'schromatographic analysis) purchased fromApplied Science Laboratories was in I-g am-poules sealed under nitrogen atmosphere.Stock solutions of potassium oleate were pre-pared by saponifying the oleic acid'. ~ith KOt!.Special ~ ~-as taken to prevent tfie Ox.idatiQ.Dof oleate solutions by preparing them usingdeaerated ~'ater and storing them under ni-

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[3]

[4]

{5]

[6)

--106 A."A~"THAPADMA~ABK"~ A~ SOMASUNDARA~

trogen atmosphere in the refrigerator. All theother chemicals used were of a.r. grade pur-chased from FiSher Scientific Co.

the decrease in 0..,/0 loa CT above a ccnaiQ'. " - .

dimerization at pH 11.4.a marked slope change at pH 9.4, -cannot be taken as definite e"idenceabsence of any aggregation under theseditions. Possible reasons forthe dependence of surface tension on

.\.ferhods

Surface tension was meas~ using theWilhelmy plate technique ~th sand-blastedplatinum as the sensor ( I).

below using the thermodynamicpresented earlier. The slope of the "(

RESLLTS A~"D DISCL'SSION. .-. .

Results obtain~for the surface tension ofpotassium oleate ~lutions at pH 9.4 as afunction of surfactant concentration are givenin Fig. 2. For the purpose of comparison, sur-face tension results at pH 11.4 are also in-cluded in the same figure. The pH in theseSystems W3~ maintained at a constan! valueusing the KOH/K:HPO. buffer. It can be seenthat the surface teDSlon at pH 9.4 is lower thanthat at pH 11.4 under all the tested oleate lev-els. Most interestingly. the observed decreasein 0..,/6 log CT at pH 11.4 with an increase inoleate above a certain level is not evident atpH 1 ~.4 at least up to oleate concentrations (2-3 kmGie/m) almOSt equal to CMC. Note that

.., -- -the product rT X a (see Eq. [2}). Since

not expected to decrease with an. oleate concentration. ---

o"Y/o log Cor at pH 11.4 was attributed --crease in a. At pH 9.4, ~ . '--

0 log c,. does not exhibit such a decreasepremice1lar relion. The -- -'~~--a cenain oleate level at pH 9.4 suggeststhe product. r T X a. - ' - - ~~- ~- .-

case. This can happen under ," - -(i) absence of any association (a - 1) andattains a constant saturation value.increase in rT is compensated by ain a. Assuming case i to be valid for

!...

,:St~

t~ PH..n'4 '. ""

PM.9.3~\;

~,T~

."f.

ezE

z~'"Z\01...\01

~

~

"'-.i\...1,_,.

.',"..\:\.

K-OLEATEI . 10-1kmol/m] KNO]T . 2.4.0.C "

zo.

10-1 '0-' 10-5 10-.CONCENTRATION OF K-OLEAiE. ~"'ol/",'

FIG. 2. Surface tension IS a funCtion of oleate concentration at pH 9.4 and 11.4.

"-"- .C"- .../~.x-. Va 1:1. ~ I. ~ 19A

~

~I

rsot-

I

:~

A.aD-SOAP FOR.\(." TION 101

TABLE!

Computed Values of K. tor DitTerentSituation Areas per Molecule

~porlDGl8Culefot-.nIia8 (A I 1.1'-*1,.1)-0

202530

.7.1 X 10'

5.4 X 10'6.1 X 101

~-stem. the ...alue ofrT at pH 9.4 is~.7S x 10-6mole/m:. The corresponding value of area permolecule of oleate is 35 A:. This value is foundto be identical to the saturation area per mol-ecule at pH 11.4 (19). The same value of areaper molecule at pH II.~ and 9.4 is. however,c ry to what can be e~pected on the basisot t:--pe of monolayers at these pH levels.While the monolayer will be fully ionized atpH 11.4, it can be expected to be only partiallyionized at pH 9.4. The area per molecule atpH 9.4 sIlould therefore be lower than that atpH 1 1.4; "'tie lower values obtained for surfacetelJiion at. pH 9.4 compared to the corre-spo~ng v81ues at pH 11.4 under all the testedol~;.~ levels also suggest this. In other words,t';:.. value calculated assuming a = I is lowerthan the expected value at pH 9.4. Based onthese considerations. it can be concluded thatthe observed constancy ot. o..,/~ log Cy in thepremicellar region at least up to about 2-3X 10-3 kmole/m3 oleate is the result of thecompensatory effects of the increase in r T andthe decrease in a.

Determination of a requires a knowledgec" ~ value of r T at different levels of oleate.&c..use of the partially ionized nature of themonolayer at pH 9.4, the saturation value ofr T should correspond to an area per moleculein the range of 20 to 35 A:. The former is foran unionized fatty acid monolayer and thelatter is for an ionized oleate monolayer at pH11.4. The results of monolayer studies reportedin the literature, however. show that the lowest~- ,er molecule for an unionized oleic acidfr.-, .olayer is only 30 A: (10). Using the valueestimated for the dimerization constant (Kct>at pH 11.4 (i). the acid-soap formation con-~tant (Kid) was computed for different r T val-ues corresponding to different molecular areasin the range 20 to 30 A z. The dependence ofKad on the value selected for the area per mol-ecule is illustrated in Table I. The value cor-: )nding to an area per molecule of 30 Az

--sents the maximum ~ble value of Kad.The minimum value of Kid should of coursebe higher than Kcs (5 x 103 (kmole/m3)-I).

Since the monolayer is partially ionized at pH

- - ~ - - .

9.4 the K8d value corresponding to the geo-metric average of the maximum and the min-imum values of K8d (1.75 x 10' kmole/mJ) issuaested to be a reasonable estimate of theacid-soap fonnation constanL Based on theavailable infonnation on K., pKIDI, ~ (1. 11)and the data obtained in the current study.constants for the various chemical equilibriain oleate solution are

R~ i:t RH pK. - 7.6

RH;:t R- + H- pK. - 4.95

2R- ~ Ri- p~= -3.7R- + RH;:t R2H- pK8d - -S.25

Activities of various oleate species at anytotal oleate concentration can now be com-puted as a function of pH. As an example, thespecies distribution obtained for 3 X 10-'kmole/mJ total oleate is sh°wn.~~ Fig. 3. The100owin. featu~ are to be no~: (a) the pHof the onset of precipitation o~f>leic acid forCT = 3 x 10-' kmole/mJ is 7.9; (h) the acti..-itiesof oleate monomer (R -) and dimer (R:2) re-main almost constant above the pH of precip-itation of oleic acid and decrease rapidly belowit: (c) the activity of acid soap increases ~thincreases in pH in the (")Ieic acid precipitc:.tionrange and decreases above it. Similar diagramscan be computed for other oleate concentra-tions.

SVRf'ACE ACTIVln" Of ACID SOAPAND ION DIMER

As discussed above. the observed depen-dence of surface tension on total oleate con-centration can be considered to indicate the

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108 A~A~IHAPADMA~A8HA~ A~D SOMASu~DA~~~

IK-O\.E;.r£ 3.:'_",,1 i

II

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I

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formation of premicellar aggregates such asdimers and acid soaps. The role of those spe-cies will actually be determined by their surfaceactivities in comparison to that of the mono-meric species. The surface acti,,;ties of thesecomplexes, on the basis of charge and molec-ular size considerations. can be different fromthose of the monomers. ~"hile it is possible toestimate the surface acti..ities of a homologousseries of surfactanu by considering the chainlength and the free energy for transfer of-CHz- groups from the bulk solution to theinterface. the peculiar shapes of these com-plexes make it difficult to estimate their surfaceacti..ities. For example. in the case of the acidsoap. the molecular size has doubled eventhollgh the effective chain lenath has not nec-essarily doubled. Therefore a method involv-ing the surface area of the molecule rather thanthe chain length is proposed here to estimatethe surface activity of these complexes. Theunderlying assumption in this estimation isthat the free energy change invol"'ed in thetransfer of a hydrocarbon chain from theaqueous phase to the liquid-air interface isproponional to the surface area of the species.Estimates based on this approach show thatthe surface activity of the acid soap can beJ~..ClJilotJgItd 1-I-s.-.~. Vol I::' ~o. I. MardI I'..

/-.

~ -3

:;.\.. .0 -...;--=u -12c

.,.~=.~ -1 .,

..

-~..-

/

) t- ~2.~

Z 4 6 a :0 :Z 1

FIG. 3. Oleate species distribution as a function of pHcomputed using the values of Kid (1. 7S x 10') and x.. (5x 10') estimated during the CU,Tent investiption.

J,50 ' J ' t

J(~J1.. ._I/~ ' / ' ::

I. .

e I ~ .... -:-~ /~ _: '. .E4..;- .~ . .i : "', .~. ,(' / '1- ~ .' '. / IVi ; -. '#~ / / .. ~z . ~ ~ / I... ' ~ 'po 3,,1 . .. . I,,~ -. I ~ \oJ .. Iu . '"c ~ ". I ... ,

a K-OLEAT! :::JNC.20. __1/",3

'

2 ~ 10.FIG. 4. Surface tension of oleate solutions as a funcUoa

of pH at three different oleate levels.

se.."en orders of magnitude higher than that 0(the oleate ion and five orders of magnitudi,'higher than that of the neutral oleic acid rno1ecule. Consequently, oleate solutions can bexpected to be most surface active under co~ditions ~"hen acid soap is present in ma.~irnum'amounts. The observed minimum in the suiface tension of oleate solutions (see Fig. 4) aIKthe ma."timum in the hydrophobicity or..number of oleate-treated oxide and silicatlminerals in the pH range of 7 to 8 (21) are itaccordance with the above considerations. j

A precise comparison oftbe surface acti4of the dimer ~ith that of the monomer is difficult because of the presence of the ionlgroups at the opposite ends of the dimer. Aisorption of the dimer at tbe solid-liquid uterface can, howeve~. be considered to be IIvored since such adsorption would lead to h!drophilic groups toward both the hydrophjJ8.solid and the solution. Available results for thdependence of the wenability of solids as weas the flocculation/dispersion characteristics (hydrophilic particles on surfactant concentrition do show them to become hydrophilic athigh concentration (22). These observatio~are in accordance with the possible adsorptioa."of doubly charged dimers at the solid-liquidinterface.

109AOc.SOAP FORMA T10~

SL'MMARY

: tion of acid-soap oleate dimer has~n ::. .estigated by anal~"zing the dependenceof surface tension on total oleate concentra-tion. While the surface tension vs log concen-tC3tion curve obtained at pH 11.4 shows a de-~ in slope ~ith an increase in oleate abovea ~rtain 'fe~'el in the p!emic"ellar range. thecuJe at pH 9.4 does tiJt exhibit such a de-CTe35e. The decrease in slope at 11.4 has beenat:";.- .~ to the formation of doubly chargeddi:::. :1 the solution. Comparison of the .."a1-ues ,,(the slopes at pH 9.~ to those at pH Il_~indicates the presence of a significant associ-ation at pH 9.4 also. {Ising the surface tensiondata. the acid-soap formation constant hasbeen estimated for the present system. Olc:atespecies distribution has also been computedas a function of pH using the available ther-mc~""amic constants and those determinedd~: ;he present study. The treatment fur-ther pro~ides a methQd to estimate the surfaceacti\ity of complex :nolecules such as oleateacid soap.

ACK.~O~"LEDG~\

The suppon or the ~ment or EneflY. ~ationa1Scie::.:~ Foundation. Amcxo Oil Production Co.. Che'TOn0-;: - Research Co.. EoUon Resc2rch and Enlincerinc

Co .. Research and Development Co.. Marathon OilCo.. Snell Development Co.. Sohio. Texaco Inc.. 3ndL"nior: Oil Ct'. orCalifomia is &rattrully acknowl~.

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(196').9. 8anp. L B.. Ph.D. thesis. ~flT. 1964.

10. Mukerjee. P.. AdIo' Colloid IrIl&"r[Qt'~. xi. 1.241 (196-:).1 i. Jun.. R. F.. ~f.s. thesis. L'ni..ersity of Melbourne.

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Chmr. 52. 21 (1948).17. Malik. \\'. V_. Sri,'astava. S. K.. and GupQ. D.. J,

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(1977).19. Ananthapacimanabban. K. P.. D. ,nJ. sa. thesis. Co-

lumbia L'ni'~ty, ~-- "ork.I980.10. "Insoluble ~fonolaym at Liquid-Gas Interfaces"

(C. C. Gaines. Ec1.).lnters ience. ='o"ew York. 1966.21. .",nanthapadmanabhan. K. P.. Somasunc1aran. P.. and

Healy. T. \\... Trans. AI.\/E 266. ~3 (1980\.22. Somasundann. P.. and Lee. L. T.. Sip. Sci. Tech-

/fol. 16(1()\. 14~S (1981).

~1911'--"C, ,.-/~s.--y. VaL I::':'io.