Adsorption of Nonionic Surfactants, …ps24/PDFs/Adsorption Nonionic...Adsorption of Nonionic...

5
Adsorption of Nonionic Surfactants, Anionic/Nonionic SurfactantMixtures, and Hydrophobically Modified Polymers on Mineralsand its Effect on Their Flotation and Dispersion aun Xu1 and P Somasundaran1 ABSTRACT ControUcd mooificatioo IX mineral/liquid interfacial ~rties by adsorptioo is critical for mineral processing qJeratioossuch as flotatioo, flltrauon, dispenioo and nocculatioo.Ionic/nonionic surfactantmixtures usually show enhanced surface activity and salt tolerance OYerionic surfactants, and this is es~iaUy beneficial for notarial of salt type minerals in which case dissolved multivalent catioos in high ax\centrations usually cause lossof aniooic surfactants by precipitation. Adsorption of nonionic/anionic surfactant mixtures 00 kaolinite has been stUdied with sodiumdodecyfsuHate as the anionic surfactantand oc:uethyleneglycd mooo-n-dodecylether as the nooionie surfactant. Synergistic intel3CtiOO betweenthese surfactants was found to cause adsorptioo of the nooiooic surfactant with its hydrophilic chains protrUding into the oolk, leading to substantial changes in flotability and dispenioo characteristics of kaolinite. Similarly, hydrophobicity changes a~ observed also for silica particles upoo adsorption of nooionic ethoxylated 8\ooOOls owing to possible changes in the orientatioo of the adsorbed surfactant sJx:cies at the interface. Also, based(XI the results ot-.&ined with surfactant adsorptioo, a hydrophobicaUy modified polymer wasused successfully to dispcnegraphite particlesin aqueous solution. converted 10 Na-kaolinite by an ion-exchange method (Siracusa and Somasundaran, 1986). Surface area of the sample. as determined by our ocr measurement using nitrogen. was 8.2 m2/g. Silica: The silica used for this study was Spherosil XOB15 supplied by Rhone - Poulenc. France. It had a particle size dislribution between 40 - 100 microns. The average JX)re size was specified to be 1200 A °. The surface area as determined by ocr nilrogen adsorption was 25 m2/g. Graphite: Graphite JX)wder (size 1-2 ~m) purchased from Aldrich Chemicals was used as received. Surfactant: Sodium dodecyl sulphate of over 99 per cent purity was purchased from Ruka Chemicals. Anionic sodium para - octylbenzenesulphonate. CH3(CH2)7C6H4S03Na. was synthesised in our laboralory (Fu, 1987). Its purity as analysed by high pressure liquid chromatography was greater that 97 per cent. This surfactant will be referred 10 as Cs~S. The nonionic surfactant, octaethylene glycol mono-n-dodecyl ether [CH3(CHvtl(OCH2CH2)sOH or CI2EOsJ. purchased from Nikko Chemicals. lapan. was specified 10 be >97 per cent pure. Two alkylphenoxy JX)lyethoxyethanols (Cs9EO1O &l¥i CaQEOto) of the IGEPAL series products from GAF Corp were used for adsorption on silica. These two nonionic surfactants have the general slrUcture of CnH2n+l-C6H4-(OCH2CH2kOH. or CD~EOm. and are polydispersed with respect 10 the degree of elhoxylatioo. Polymer: The polymer used in this study was DAPRAL GE 202. a maleic anhydride a-olefm copolymer of average molecular weight 20 000 supplied by Akzo Chemie America. As shown in Figure 1. it has a comb-like slrUcture with both hydrophobic and hydrophilic side chains. In a sense. this fKoduct bears features of both common polymers and surfactants. AdsorptiOtl on kaolinite: Three gram samples of Na-kaolinite were cont.:1ed with 15 ml ofO.03M NaCl solution for two hours at room temperature (25 OC) in a Teflon Sloppered glass vial. Following this. 15 ml of the surfactant solution of desired concentration was added &l¥i the contents allowed to stand for 72 hours with periodic manual shaking for one minute after every eight hours. The suspension was then Iransferred 10 a 20 mm INTRODUCTION Modification of solid/liquid interf~ial properties by surfactant and polymer adsorption is of importance for many indusbial processes. In mineral processing,for example, the surface of particles of desiredmineralsneed to be made hydrophobic by adsorption of surfactant so that selective flotation can be obtained. In the manufacturing of aqueous ink and dye solutions, on the otherhand. thehydrophobic surf~ of the particlesshould be made hydrophilicto obtaindispersion. In recent years, adsorption of ionic/oonionic surfactant mixturesat solid/liquidinterface hasreceivedsomeattention(Fu, 1987; Scamehom el aI, 1982; Harwell el ai, 1988), essentially since such mixtures usuallyshow better performance over single surfactants in terms of surface ~tivity, salt tolerance etc (Fu. 1987).But the mechanism for surfactant mixture adsorptionand its modificationof solid/liquid interfacial properties have not yet been investigated in any detail. In addition, special polymers combining the features of commonpolymer and surf~tant have emerged. providing new possibilities for modifying solid/liquid interfactialproperties andthe mechanisms of their effect are also not well understood In this study, modificationof solid/liquid interfacial properties involving different solid substrates has been investigated by adsorption of oonionic surfactants, anionic/oonionic surfactant mixtures and a special polymer.DAPRAL GE 202. Mechanisms of adsorption will be discussed in relalion to changes in surface hydrophobicity, zeta potential. and dispersion behavior of the suspensions. EXPERIMENTAL Kaolinile: Well-crystaUised Georgia kaolinite was purchased from the clay repository at the University of Missouri and Henry KlUmbSchool of Mines.Colwnbia University. New York. NY 10027. USA. FIG I . Molecular stRIcture of DAPRAL GE 202. XVIII Intemationat Mneral PIOc:essi"O Conor~s ~vff'- ~ -,. u.v,-

Transcript of Adsorption of Nonionic Surfactants, …ps24/PDFs/Adsorption Nonionic...Adsorption of Nonionic...

Page 1: Adsorption of Nonionic Surfactants, …ps24/PDFs/Adsorption Nonionic...Adsorption of Nonionic Surfactants, Anionic/Nonionic Surfactant Mixtures, and Hydrophobically Modified Polymers

Adsorption of Nonionic Surfactants, Anionic/NonionicSurfactant Mixtures, and Hydrophobically Modified Polymers onMinerals and its Effect on Their Flotation and Dispersionaun Xu 1 and P Somasundaran 1

ABSTRACTControUcd mooificatioo IX mineral/liquid interfacial ~rties byadsorptioo is critical for mineral processing qJeratioos such as flotatioo,flltrauon, dispenioo and nocculatioo. Ionic/nonionic surfactant mixturesusually show enhanced surface activity and salt tolerance OYer ionicsurfactants, and this is es~iaUy beneficial for notarial of salt typeminerals in which case dissolved multivalent catioos in highax\centrations usually cause loss of aniooic surfactants by precipitation.

Adsorption of nonionic/anionic surfactant mixtures 00 kaolinite hasbeen stUdied with sodium dodecyfsuHate as the anionic surfactant andoc:uethyleneglycd mooo-n-dodecylether as the nooionie surfactant.Synergistic intel3CtiOO between these surfactants was found to causeadsorptioo of the nooiooic surfactant with its hydrophilic chainsprotrUding into the oolk, leading to substantial changes in flotability anddispenioo characteristics of kaolinite. Similarly, hydrophobicity changesa~ observed also for silica particles upoo adsorption of nooionicethoxylated 8\ooOOls owing to possible changes in the orientatioo of theadsorbed surfactant sJx:cies at the interface. Also, based (XI the resultsot-.&ined with surfactant adsorptioo, a hydrophobicaUy modified polymerwas used successfully to dispcne graphite particles in aqueous solution.

converted 10 Na-kaolinite by an ion-exchange method (Siracusaand Somasundaran, 1986). Surface area of the sample. asdetermined by our ocr measurement using nitrogen. was 8.2

m2/g.Silica: The silica used for this study was Spherosil XOB15

supplied by Rhone - Poulenc. France. It had a particle sizedislribution between 40 - 100 microns. The average JX)re size was

specified to be 1200 A °. The surface area as determined by ocrnilrogen adsorption was 25 m2/g.

Graphite: Graphite JX)wder (size 1-2 ~m) purchased fromAldrich Chemicals was used as received.

Surfactant: Sodium dodecyl sulphate of over 99 per centpurity was purchased from Ruka Chemicals. Anionic sodiumpara - octylbenzenesulphonate. CH3(CH2)7C6H4S03Na. was

synthesised in our laboralory (Fu, 1987). Its purity as analysedby high pressure liquid chromatography was greater that 97 percent. This surfactant will be referred 10 as Cs~S. The nonionicsurfactant, octaethylene glycol mono-n-dodecyl ether[CH3(CHvtl(OCH2CH2)sOH or CI2EOsJ. purchased fromNikko Chemicals. lapan. was specified 10 be >97 per cent pure.Two alkylphenoxy JX)lyethoxyethanols (Cs9EO1O &l¥i CaQEOto)of the IGEPAL series products from GAF Corp were used foradsorption on silica. These two nonionic surfactants have thegeneral slrUcture of CnH2n+l-C6H4-(OCH2CH2kOH. orCD~EOm. and are polydispersed with respect 10 the degree ofelhoxylatioo.

Polymer: The polymer used in this study was DAPRAL GE202. a maleic anhydride a-olefm copolymer of averagemolecular weight 20 000 supplied by Akzo Chemie America. Asshown in Figure 1. it has a comb-like slrUcture with bothhydrophobic and hydrophilic side chains. In a sense. this fKoductbears features of both common polymers and surfactants.

AdsorptiOtl on kaolinite: Three gram samples of Na-kaolinitewere cont.:1ed with 15 ml ofO.03M NaCl solution for two hoursat room temperature (25 OC) in a Teflon Sloppered glass vial.Following this. 15 ml of the surfactant solution of desiredconcentration was added &l¥i the contents allowed to stand for 72hours with periodic manual shaking for one minute after everyeight hours. The suspension was then Iransferred 10 a 20 mm

INTRODUCTION

Modification of solid/liquid interf~ial properties by surfactantand polymer adsorption is of importance for many indusbialprocesses. In mineral processing, for example, the surface ofparticles of desired minerals need to be made hydrophobic byadsorption of surfactant so that selective flotation can beobtained. In the manufacturing of aqueous ink and dye solutions,on the other hand. the hydrophobic surf~ of the particles shouldbe made hydrophilic to obtain dispersion.

In recent years, adsorption of ionic/oonionic surfactantmixtures at solid/liquid interface has received some attention (Fu,1987; Scamehom el aI, 1982; Harwell el ai, 1988), essentiallysince such mixtures usually show better performance over singlesurfactants in terms of surface ~tivity, salt tolerance etc (Fu.1987). But the mechanism for surfactant mixture adsorption andits modification of solid/liquid interfacial properties have not yetbeen investigated in any detail. In addition, special polymerscombining the features of common polymer and surf~tant haveemerged. providing new possibilities for modifying solid/liquidinterfactial properties and the mechanisms of their effect are alsonot well understood

In this study, modification of solid/liquid interfacial propertiesinvolving different solid substrates has been investigated byadsorption of oonionic surfactants, anionic/oonionic surfactantmixtures and a special polymer. DAPRAL GE 202. Mechanismsof adsorption will be discussed in relalion to changes in surfacehydrophobicity, zeta potential. and dispersion behavior of thesuspensions.

EXPERIMENTAL

Kaolinile: Well-crystaUised Georgia kaolinite was purchasedfrom the clay repository at the University of Missouri and

Henry KlUmb School of Mines. Colwnbia University. NewYork. NY 10027. USA. FIG I . Molecular stRIcture of DAPRAL GE 202.

XVIII Intemationat Mneral PIOc:essi"O Conor~s ~vff'- ~ - ,. u.v ,-

Page 2: Adsorption of Nonionic Surfactants, …ps24/PDFs/Adsorption Nonionic...Adsorption of Nonionic Surfactants, Anionic/Nonionic Surfactant Mixtures, and Hydrophobically Modified Polymers

Q~ xu ~ P SOMASUNDARAN

0

-tQ

>

E

-20 -

.~

c

.

"0

-30 (l.

0

..

N

-40

-500 2 ,3 5 6 1 8 9 .10

Residual CtztOs Con.. - 10. kmol/m

FIG 3 - Settling rate of kaolinite suspension after adsorptim of SDS +

ClzEDI mixlu~s and the conespooding Uta p>lenlial.

diameter cylinder for the settling tests. The height of thesuspension/supernatant interface from the bottom of the cylinderwas r«.orded as a function of settling time. The interf.:e heightvs time curve showed an initial linear part and the slope wasdefined as settling rate of the suspension. After completion of thesettling test a small portion of the supernatant was decanted.centrifuged to remove any fines and the residual concentralion ofthe surfactant determined. The remaining supernatant and thesediment were then mixed and the resulting suspension was usedfor the zeta potential measurements using a Zeta Meter Model Dsystem. Under certain experimental conditions skin flotation ofkaolinite was observed during the settling test. In such cases. thecontents of the volwnettic cylinder were transferred to aseparatory funnel and the sediment and a major portion of theliquid were drained out. The skin floated fraction retained in thefunnel was removed by washing with water, dried and weighed.This fr~tion provided a measure of the wettability of thekaolinite particles.

Adsorpl~ on silica: 0.2 gram of the silica sample wascontacted with 10 ml of surfactant solutions of knownconcentration for ten lM>un. The solids were then separated bycentrifugation and the supernatant analysed for residual surfactantconcentration. While adsorption of IGEPAL surfactants(CnoEOm) was performed under room temperature. octaethyleneglycol mono-n-dod«.yl ether (Ct2EOa) and sodiumpara-octylbenzenesulphonate (C~S) adsorption was carried outin a waler bath at 500C. thus keeping the temperature above theKrafft point of C8~S.

Dispersion of graphite by polymer adsorptiOfl: A sample of 0.5gram graphite was stirred by a magnetic spin bar in 120 mldistilled waler for 20 minutes. The sample was then stirred on acounterrotation agitator for ten minutes. Two minutes after thisstirring was started, DAPRAL stock solution was added drop bydrop 11) the suspension. At the end of the DAPRAL addition.distilled water was added to make the total volume to 130 ml andthe pH adjusted using IN HCI or NaOH. The sample wasremoved from the counterrotation agitator and stirred manuallyusing a glass rod for a short period of time to make thesuspension homogeneous. The suspension was allowed to settlefor five minutes following which the upper half of the suspensionwas removed by suClion. The per cent solid-seltled was estimatedby measuring the solid content in the lower-half portion.

20

8

N J'Eu~Ec.2"0"0(;:

.!:£"00~

...6 E

"0E

..0

4. ';

C.2"Q...0

2 ~-<

,~'

Suspensions for zeta JX>tential measunnents were prepared by thesame procedure as for dispersion.

Analysis: The anionic sodium dodecylsulphate (SDS) andsodium para-octylbenzenesulphonate (CaOS) were measuredusing a two phase titration method (Li and Rosen. 1981).CI2EOaconcenttation was detenninai using high perfonnance liquidchromatography (HPLC) with a reverse phase CIa column and arefr~tive index detector. The mobile solvent used in HPLC wasa mixture of 90 per cent acetonitrile and ten per cent waterfiltered through 0.4 micron nylon membrane. The residualconcenttations of IGEPAL surfactants were detennined by UVabsorption at 223 run or 275 run using a Beckman DU-8specttopootorneter.Hydrophobicity of silica particles: Hydropoobicity of the silicaparticles after the adsorption of IGEPAL surfactants wasdetennined as follows. Following UV analysis and surfacetension measurements, the supernatant was pourai back into thevial containing the solids to r~nstitute the slurty. The slurry wastransferred to a separatory funnel to which 3 ml of toluene wasadded. The silica-surFactant-toluene dispersion was shaken forfour minutes and then allowed to seule overnight. The bulk 9f theaqueous phase and hydrophilic solids were then allowed tol1owout of the funnel by gravity. The toluene phase and thetoluene-aqueous intcrf~ial layer containing the hydrophobicsolid was rotevaporated in a 10 ml round oouom flask and theweight of the silica was recorded. The relative hydrophobicitywere estimaled as follows:

% Hydrophobicity = (Weight of silica in Ioluem exlr8Cf/rolaJ weight) x 100.

5

00

RESULTS AND DISCUSSION

Effect of surfactant mixture adsorption on kaolinitenotation

Kaolinite surface by itself is hydrophilic and no flotation wasobserved in the absence of surfactant. Adsorption of anionicsodiwn dooecylsulphate renders the surface partially hydrophobicresulting in flotation. Such flotation is, however, drasticallydiminished as adsorption of the nonionic surfactant, Ct2Eaa.increases (Figure 2), showing restoration of hywophilicity to thekaolinite surface.

1 2

Residual c,ztO, Conc.3 4.

.10 kmol/m5

s

FIG 2 . Skin flotation of kaolinite after adso~oo of sodium dodecylsuI~ate (SDS) and CJ2EOs mixture. Initial [SDS] = 1.0 mM. pH = 5,

0.03 M NaC. 25 oc.

602 XVlllln!emationa! Mineral Pro~SSIng Congress!'Yd~Y ?3 . ?8 May 1993

Page 3: Adsorption of Nonionic Surfactants, …ps24/PDFs/Adsorption Nonionic...Adsorption of Nonionic Surfactants, Anionic/Nonionic Surfactant Mixtures, and Hydrophobically Modified Polymers

aquoous solution. These hydroplrilic chains can be expected totransfonn the hydrophobilised kaolinite surface back tohydrophilic, and to decrease the flotation correspondingly (Figure2). Also, the dangling hydrophilic chains would provide sterichindrance to thc flocculation of kaolinite particles. resulting in theobscrved stabilisation of the suspension.

(a) (b)

FKJ 4 . Sdlcmatic representation of the medtanism for adsorption ofsodiwn dooecyl sulJilate (SDS) and SDSK:t7.Eo. on kaolinite. a)

Head-down SDS ad~ to result in hyd~lisation of kaolinitesurface; b) Tail-up adsorp(ioo of Ct2EOs fJ(Kl1 Ct2Eo.JSDS rnixtu~ to

restore hydropililicily to kaolinite surface and also to provide stemhindrance to .wroaching particles.

Stabilisation of kaolinite suspension was also observed for theabove system. As is seen in Figure 3. the settling rate of kaolinitesuspension is markedly reduced as nonionic surfactantconcentration increases. Soch stabilisation is clearly not due toelectrostatic repulsion since the zeta potential of kaoliniteparticles actually showed only a slight decrease in magnitude.

An adsorption mechanism based on above observations isillustrated in Figure 4. The anionic sodium dodecylsulphatemolecules assume head-down adsorption due to its electrostaticinteraction with the possitive sites on the k~linite surface. Thisadsorption. with the hydrocarbon chains facing the aqueousphase. would render the surface hydrophobic. as indicated by theflotation of the particles. In the case of mixture adsorption. somenonionic surfactant mol~ules assume a reversed adsorption atthe interface, leaving the hydrophilic chains dangling into the

Hydrophobicity changes of the silica surface uponnon ionic surfactant adsorption

Figure 5 shows the adsorplion isotherms for two alkylphenoxyJX1lyethoxyethanols (CS<>EOIO and CSOEO40) on silica. It is seenthat in the low concentration region. the surfactant with longerethylene oxide chain (CSOEO40) adsorbs more than that with theshorter ethylene oxide chain (CSOEOIO). This is understandablebecause the longer the ethylene oxide chain. the stronger win bethe cumulative hydrogen oonding bctween the surfactantmolecule and the silica surface. In the higher concentrationrcgion. however. CsQEOlo adsorbs more than CSOEO40 due to thelimited parking area for the ethylene oxide chains at the interface.

The hydrophobicity of silica particles after adsorption ofCSQEOIO and CS~EO40 showed markedly different behaviors(Figure 6). For CS<lEOIO adsorption. silica particles becomeincreasingly hydrophobic till a constant maximumhydrophobicity is maintained. For the other surfactant, CSQEO4o.initial adsorption imparted hydrophobicity to the silica surfaces,but further adsorption actuany reduced the hydrophobicity andcomplete hydrophilicity (zero hydrophobicity) was restored in theplateau adsorption region.

Bilayer adsorplion. as in lhe case of adsorption of surfactantmixtures on kaolinile. could render the surface hydrophilic.However. bilayer adsorption is found not likely to be themechanism in the case of CsQEOIO and CSc)EO40 o'¥isorption.Bilayer adsorption. if any. should be more prominent forCS<lEOI0 than for Cs<lEO40 adsorption because the higher degreeof aggregalion of CsOEOlo would provide stronger hydrophobicdomains lo abstracl the hydrocarbon chains of addilionalsurfaclant molecules to form the second adsorplion layer. On theolher hand, aggregalion of CSOEO40 is virtually impossible due tothe sleric hindrance from lhe long ethylene oxide chains and will

100

80&-t

~'u

:00

.C-o.0...

"0>-

:r

N

E

-0E 10

i-'Wic.

0

c.9:"Q.

~ 10"Q.

60

40

20

010~ -3-t -3 0-2 to 0 10

3kmol/m

10 10 fQ 10

Residual Alcohol Conc. kmol/m'

FKJ 6 . Hydroph~icily of silica particles a(ler ad!O~ioo of alkypitcnoxy

polyelhoxyclhanols. pH 7.7.6, uro ~aCI, 25 °C.FIO 5. AdsOlption ofalkylpilenoxy polyethoxylelhanols on silica.

pH 7 - 7.6, ~ro NaC, 25 OC.

XVllllntemalionai "nera! P~ssing Congress Sydney, 23 - 28 May 1993 603

Page 4: Adsorption of Nonionic Surfactants, …ps24/PDFs/Adsorption Nonionic...Adsorption of Nonionic Surfactants, Anionic/Nonionic Surfactant Mixtures, and Hydrophobically Modified Polymers

10

NE"-"0E

.:i-

.Mc.

0

C.~'Q...0..

~.

10"0..!...V)

.!~Q,0...

(.)

'0

i""

1O

1O

~Individual Surfactant Conc kmol/m

ppmFKJ 7 - AdsocptionolCI2EOa8nd 1:1 CnEo.,(:ltS mixlure 00 sili~..

pH 7.2, o.m M NaC, SO.c.FM) 9 . Effect of DAPRALaxlcenlrauon (XI slalxlity of suspensions of

&rarhile. pH = 7 .0, ~ Naa. 25 OC.resull in less bilayer adsorption. In this regard. if lhehydrophobicity change is caused by bilayer adsorption. silicapanicles afler C..EO10 adsorption should be m<X'e hydrophilicthan those afler C"E040 adsorption. This wu nol observed. Inaddition. the parking .-ea for . single ethylene oxide unil in theplateau adsorplion rcgion is calculated 10 be 9.2 A .2/-{OCH2CHvfor tX>th CaqEO10 and C"EO4o. oomparable with the 8.5A.2/_(OCH2CHv value reponed by Partyka el al (1984) forTriron series Surf~lanl adsorp(ion on ~ same silica. This wouldsuggesl a oomplele adsorption of the entire ethylene oxide chainsin contact with silica for tX>1h C'fEOJO aId C'9EO4o. ~ same

conformation as asswned by JX)lyethylene oxide chains atsilicalliquid interf~ (Char ", oJ. 1988; Howard and McCormell.1961; Hommel tt aI. 1984; KiJ1maM tt aI. 1911). As a resulc.thepossibility of bilayer adsorption is e~cluded.

We proJX)Se the differmt changes in hydrophobicity 10 be theconformational changes of the hydrocarbon chains of theadsorbed surfaclant molecules at the silical1iquid interface. Atlow adsorption densities. the hydrocarbon chains of the adsorbedsurfactant molecules can be considered 10 lie Oat on the surf.:e.This will cive maximum coverage of the silica surf~ withrespect 10 a single hydrocarbon chain and will render the slDfxehydro~bic. As adsorption density ~. however. the flathydrocarbon chains will be pushed up by further adsorption ofthe ethylene oxide chains at the interf~ and the eff~tivecoverage or silica surrace by a single hydrocarbon chain isreduced. This will. in the case of CiQEO4O adsorption. result in athe decrease or the hydrophobicity. When the plateau adsorptionis reached. the surf~ is least covered by the hydrocarbon chainsand correspondingly hydrophilicity is almost completely reslOredto the silica surrxe. For CaOEOIG. the hiCher adsorption densityand the higher decree or aggregation as comlJlred with those inthe case of C.OEO40 adsorption would offset the effect ofconformational change of the hydrocart.>n chains and.consequently. a consistent increase and finally a constantma~imum hydrophobicity of the silica particles were obtained.

>E

:§c.'0Q.0'iN

Zeta potential change or silica particles uponanioniclnonionic surractant mixture adsorption

Sodium para-OClylbenzenesulphonale (CaQS) alone was foundnot \0 adsorb on silica bC£ause both \he surfactant head group and\he substralc are negllively charged. However, as seen in Figure7. significant adsorption of CaQS is induced by the presence of\he nonionic octaethylene mono-n-<k>d«yl ether (Ct2EOa). Sinceadsorption of JX)lyethylene oxide on silica also inducesadsorption of anionic SUrfKtant (Mallcsh and Somasundaran,1992), the adsorption of CiQS may be considered \0 be JX)ssibleby inlerKUOns between its head groups and \he ethylene oxidechains of the nonionic C t2EOa as well as by the hydrocarbonchain-chain interactions.

FIG 8. Zeta pMenual of silica ulQ' adsorplioo of nooicxlic ClzEOa aM1:1 CllEO8JC8QS mixtures. pH 7.2,0.00 M N.a, SOoC.

XVllllntemallOnal Mneral P~Slng COtgess804 Srdney. 23 . 28 Mar , 993

Page 5: Adsorption of Nonionic Surfactants, …ps24/PDFs/Adsorption Nonionic...Adsorption of Nonionic Surfactants, Anionic/Nonionic Surfactant Mixtures, and Hydrophobically Modified Polymers

The co-adsorption of the anionic para-OClylbcnzencsulphonate(C.,S) is round to have a marked effec:t on the zcta polenlial orthe silica parliclcs. Without C.,S. the zeta potential or silicaparticles is sharply reduced in magnitude by the adsorption or thenonionic oclacthylene mono-n-OOdec:yl ether (CI2EO.) (Figure8). With the co-adsorption of C.,S. however. the zeta potential orsilica particles remains negative between -40 and -50 mY. It isproposed that the masking effect or CI2EO. adsorption on silicasurr~e charge is offset by the presence of the charged headgroups or C.QS at silicaniquid interface. Such behaviors due lomixed surfactant adsorption could be userul ror flotation.dispersion and flocculation (X"occsses where zeta polcnlial is animportant parameter that can be manipulaled.

silica, confonnational changes of the hydrocarbon chains of theadsorbed molecules are proposed to be rcsponsible for thediffercnt changes in the hydrophobicity. Thc co-adsorption ofanionic surfw:tant. Ca~S, at silica/liquid interface with nonionicCt2EOa was found to offset the ma.'iking of silica surface chargedue to the nonionic surfactant ad'iorption and thus maintain thesilica zeta potential highly negative. Graphite powders were welldispersed in aqueous media by DAPRAL adsorption due to bothelectrostatic and steric rctXllsions between panicles. This studyshowed that by utilising surfactant or polymer ad'iorptionIX'opcrly, the solid/liquid intcrfacial properties could be adjustedto suit different IX'ocesses such as flotation, dispersion orflocculation.

ACKNOWLEDGEMENTS

The authors acknowledge the support of the National ScienceFoundation. British Petroleum and Nalco Chemical Company.

Stabilisation of graphite suspension by dapraladsorption

Graphite surface is hydrophobic and dispersion of the particles inaqueous media requires modification of the surface properties.Based on the results of surfactant mixture adsorption on kaoliniteand the unique structure of DAPRAL as shown in Figure I.DAPRAL can be predicted to be suitable for stabilising graphiteparticles in aqueous media since upon contact with graphite. thehydrophobic side chains of lhe polymer will be attached tographite surface due to hydrophobic interaction. leaving thehydrophilic ethylene oxide side chains dangling inlo the solutionto stabilise the suspension through steric repulsion. The increaseof DAPRAL concenb"ation is indeed seen to lead 10 stabilisationof the graphite suspension (Figure 9).

CONCLUSION

Solid/liquid interfacial properties such as hydrophobicity,disJx:rsion and zeta potential can be significantly changoo bysurfactant or polymer iKisorption. For the case of iKisorption ofanionic/nonionic surfactant mixture on kaolinte, a reversooadsorption layer of Cl2EOs is proposed and verified by skinflotation and dis~ion exJx:rirnent of the suspension. Foradsorption of non ionic surfactants CsQEOlo and CXOEO40 on

REFERENCES

Clar, K, Gast, A P, and Frank, C W, 1988.LAlIgmuir, 4:989.Fu, E, 1987. Ad50~ioo of aniooic-ooniooic surfaCtaM mixlU~s 00 oxide

minerals, PlIO Thesis, Columbia Univenity, New York.HarwcU, J H, RobeRS. BLand Scamehom, J F, 1988. Colloids aM

Surfaces, 32:1.H~me1. H, Legrand. A P. Tougne, P, Balard, P H, and Papirer. E. 1984.

Macromolecules, 17:1578.Iloward. G J, Ma~eU. P. J Phys Chent. 71 :2974.KillmaM, E, Eisenlauer. J, Kom. M, 1977. J Polym Sci. PolY'" Symp

61:413.Li,Z,andRosen,MJ, 1981. in Anal Chent 53:516.Mallcsh, C, and ScxnasW1daran. P. 1992. J Colloid lllll.r{ac. Sci. 153:298.Panyka. S. Zainj. S, Lindheimer, M. and BRUt, B, 1984. Colloids aM

Surfaces, 12:255.Scamehom, J F, Schechter, R S and Wade. W H, 1982. J Colloid fIller{

Sci,85:494.Siracusa, P A and Somasundaran, P, 1986. J Colloid llllerfac. Sci.

114:184.

XVltllntemationai Mineral PrO<»SSrIg COf'9ress Sydney. 23.28 May 1993 ..