Symposium on Enjlanced Oil Recovery205th National AmeriC:ln Chemical Society i\leeting

March 28 . April 2, 1993, Denve~ Golorado ( 1-h. / ~ t. - ( 3 <::>./1:(5 Ifl/. Pd. CHr::-H-/ Uc-f. 3~ t(), 1'11.5.:( YJBEHAVIOR OF HYDROPHOBICALL Y ~10DIFIED A1~D SYNTHETIC WATER-SOLUBLEPOLYMERS IN SOLLiION A'olD IN THE PRESENCE OF SODIU~1 DODECYI:SULFATE

C. 1\laltesh and P. SomasundaranLangmuir. Center for Colloids and Interfaces

Henry Krumb School of M~nesColumbia University, New York, NY 10021--

(212854-2925)'

INTRODUcnON

efficient in pumping out the slug. In this study, surface activity and viscosity changes of some

polymeric surfactants are investigated using techniques of fluorescence spectroscopy, visc_ometry and

surface tension. In addition, modifications in the interfacial behavior of polymers and surt-actants

due to complexation are discussed.

Fluorescence spec~roscopy has been widely used to study microheterogenous systems

(1 ),(2).

polymer-surfactant aggregation.

of a pyrene emission spectrum is dependent on the polarity of its environment and as"such is called- .

1>01 a ri tv Darameter here. It varies from 0.5-0.6 in water to 2: 1 in hydrocarbons. This a~pect is used

excimer forming capability of pyrene is utilized here to gain information about polymer

conformation by covalently attaching it to the polymer chain. Excimer formation will depend upon

excimer to monomer intensities (IJIm), implying that the polymer is coiled, and low or. zero Iflm

depicting it to be relatively stretched. This ratio is referred to here as the coilin2 index.

RESULTS A!~D DISCUSSION

Hydroxyethyl cellulose (HEC). is commonly used in polymer flooding. 1\ilodification of this

polymer by the incorporation of very small amounts « 1 wt. %) of alkyl groups results in drastic

. .changes in its solution properties(3). Figure 1 illustrates the changes:.inOthe polarity parameter

of pyrene due to the addition of HEC and hydrophobically modified hydroxyethy.l cellulose

(HMHEC . While the polarity parameter of pyrene is insensitive to polymer conceptrations in the

case of HEc, a sharp increase in IIItIl( (decrease in polarity) is observed when the HMHEC

concentration exceeds a critical value (500 ppm) and reaches a plateau around 2000 ppm

Fagure 1: Aggregation of IIEC (0) & H1\IIIEC (0) as detected by pyrene fluorescence measurements- .

This indicates the association behavior of the hydrophobic groups attached to ~he polymer,

which is significant above 500 ppm polymer. The value of the polarity parameter in HylHEC

aggregates is much lower than that usually obtained for micelle solubilized pyrene." ~ss ordered

packing <?f hydrophobic groups due to the large macromolecular chains a nached to them and

resultant water penetration could be a possible reason for this behavior.

interactions were detected betWeen H.\tIHEC ~d SDS, possibly due to association be~een the

surfactant and the hydrophobic groups (jf the polymer. The concentration dependence ofreduced.--

viscosity of HMHEC in ,vater and in SDS micellar solution is sho\vn in figure 2.

40C WATER0 0.01 MSOS35

~ 30-''0

~ 2S

20.I

I'" .8 15u~Q'"' 10~

c'-',, 0

5

00 1000 2000 3000 4000

HMHEC CONC.. ppm

5000 6000

Fagure 2: Reduced viscosity of H.\IlIEC in water and in micellar SOS solutions..

In micellar solutions, the viscosities are lower than that in water indicating the_absence of

aggregates or bridging of the polymer chain through the micelles. At higher pol}mer

concentrations, however. the viscosity is comparable to that in \vater, suggesting similar pol}mer

structure in these solutions. We suggested this to be Que to the fact that at high polymer

concentrations, the polym~r bound surfactant ino~lculesare insufficient in nuwb~r to form micelles

linked polymer structure.

The solution behavior of a comb-like pol}mer was studi~d using fluor~scence ~pectroscopy

and surface tension( 4). The polymer was DAPRAL GE 202, commercially available from Akio

Chemie America with a molecular weight of 20000 (figure 3).

The aggregation behavior of DAPRAL is identical at the t\vo values of pH studied (figure

4). At low polymer concentrations, the value of the 1,"/1.. ratio corresponds to that of water. Above

(CH2-CH2-0~-8HF"agure 3: Molecular structure or DAPR.,\L G E 202

intramolecular.

-~

t:

~

~f

Yagu~ 4: Effect or pilon the aggregation or Dt\PR.\L as detected by pyrene fluorescence

concentrations intermolecular aggregates will form. Interestingly, th~re is no eff~ct of pH on .the

between them.~

The surface tension decreased with iQcre:1sing polymer concentration and the addition of the-, . .;:".-

salt was found to markedly increase the surface activity of the polymer (figure 5). A 1 wt. % (10000'. .. . .

ppm) solution can attain surface t~.?sions as lo\v as 30 to 38 dyne/cm depending upon the salt

concentration. In the presence of 0.5 M NaCI, the surface tension shows a discontinuity at a, "

concentration around 1000 pprn above which the surface .tension rernams relatively constant. The

solutions above a concentration of 1000 ppm were cloudy, but there was no visible precipitation.. ..

It is possible that, in the absence of the salt, the aggregates of .polymer . formed are

.polydisperse' and many in number and aggregation and adsorption at .the air-water interface take

place simultaneously. An electrolyte, however, can promote the formation of polymer aggregates- .

which are of uniform size and shape and a...concentration analogous to th~ critical ~icellization

concentration of surfactants is hence obtained.

In order to und~rstand the prjncipl~s underlying polymer-surfactant interaction, a model

system consisting of the non ionic polyethylene oxide and the anionic sodium docJecyl sulfat~ was

studied(5). The probe. pyrene. was attached to both ends of the polyethylene oxide chain using

an esterification procedure(6). Figure 6 depicts changes in the surface activity of sodium.-

dodecylsulfate (SDS) and the conformation of pyrene-labeled polyethylene oxide (PyPEOPy) as a

result of the interactions bet\veen SDS and PEG. The critical micellization concentration of SOS.'-

at an ionic strength of 0.5 Msodium sulfate in the absence of any PyPEOPy is =1.5 x 10""kmol/mJ..' - .

In the presence of 20 ppm PyPEOPy, three regions of interactions' between PEG and 50S are

observed(7)(8). At low surfactanlconcentrations, Region I, there are no interactions bet\veen

the polymer and the surfactant. Aboye a certain surfact~nt concentrarion, the critical aggregation

concentration (CAC), there is binding of the surfactant onto the polymer in the form of aggregates

(Region II).

51q ~.i ~ I

,

41,4-,!,,

c.. ~a.. .~

~

0.24\\

1 :;

j! 0.20_E

~0.16 --

,-4\

\

E%E~I

Z0~Z\oJ--

IoJU~It:~~

r

b '..6. \l .-

0.046 I / 1 :. '" :

to 'Y. PyPEOPy-SDS 1

0 y. SDS ALONE i ,26 "..'.'..""'."'" '6.6.;..1 ..0.00

10-6 10-5 10-4 10-33

SOS CONCENiRAnON, kmol/m.

ragure 6: Interactions bet\veen PyPEOPy and 50S as detected ;by surf:lce tension. and nuor~scence

spectroscopy

Once the polymer is saturated with surfactant, excess surfactant resumes its normal solution

behavior and at higher surfactant concentrations polymer-bound aggregates : coexist with free

surfactant micelles (region III). The coiling index of the pyrene-labeled polyethylenc= oxide

(PyPEOPy) indicates considerable changes in the conformation of the polymer on its association

with the surfactant. Due to binding of the sur~actan_t to the polymer, the polymer coil contracts and

the value of the coiling index (IJIm ratio) increases_. Above a ceFtain 50S concentration the coiling

index value drops sharply to zero suggesting stretching o~t of the polymer chain. The mechanism

46

41

36

31

xw

O. 12 ~<-'z

0.08 ~~

of interaction bet\veen PEa and SDS is the bincting of the ionic head of th~ surfactant to the ether

oxygen of the polymer. This mode of binding can be expected to decrc:ase the hydrophilicity of the. ---. --

polymer and cause it to-coil\ As the binding increases. a stagc: ,...ill be reach~d \...here repulsion.. . .

bt=tween the bound surfactantmolt=cul~s will caust= the polymer coil to expand. It can be seen from--.

the figure that the SOS concentration at which PyPEOPy is most coil~d corres.ponds to the end of

the region of constant surface activity. This suggests that the point -at which the pol."1ner gets

saturated with surfactant also causes a change in the conformation of the polym~r. In oth~r \vords!

the maximum in I/lm corresponds to the maximum amount of surfactant accomI}1odated by the

polymer.

It is well established in literature that sodium dodecylsulfate adsorbs on a1urnina(9) and

that there are significant interactions bet\veenpo.lyethylene oxide anct.~odium dodecylsulfate in bulk

solution(lO). It has also been shown previously that PEO does not adsorb on alumina(ll).

In order to determine possible effects of 1he int~ractions bet\veen polyethylene oxide and sodium" -

dod~cylsulfate art the adsorption of polyethylen~ oxide on alumina, alumina preadsorb~d with

varying amounts of sodium dodecylsulfat~ was contacted witJ1 pyren~-lab~t~d polyeth~icn~ oxid~ for

The solids were then separated by centrifugation and fluoresc~nc~ emission ofa fLxed duration.

the pyrene-labeled polymer was recorded for ~oth the sup~rnatant and the solid. The adsorption

of sodium dodecylsulfate on alumina and the coiling index of the pyrene-labefed poly~thylene o~ce

at the alumina/water interface are depicted in figure 7. The shape of the surfactant adsorption.

isotherm is characteristic of the adsorption of ionic surfuctants on charged oxides und~r conditions

of constant pH and ionic strength( 12). Four regions designated :, II, III and IV'are e\;dent.

An abrupt increase in the slope of the isotherm marks the onset of region II. T\\'o-dimensional

surfactant aggregates or solloid$ (surface coll()ids) begin to form in this region and as the surfactant

concentration is increased these aggregates grow in size.

The presence of preadsorbed sodium dodecylsulfate (SDS) has caused the polymer to adsqrb

o.~~UCOI

~o I0 IV 1,)=Ir'tS--c., ,

,IIlQI'

/-3

10

EuQ-..

~

%0 .

~ 10-

0.25~'.-

;#0 ~ o E0... "-

;:!.

X\oj

0.15 ~t.:IZ::;

0.10 O'-:, U

,d

Q.~

~~a~

O.Q~10-1

"A pH - 6.50\I / \ 0.5 M No2S0.

0 \ PYRENE-LASElED.,Q 'O \ PEa: 6000-7500 M.W.:

~ \ INITIAL CONC. 15 ppin

""""""""~8" '...'O.00 -10-5 10-S 104 . 10-3 '-':

50S RESIDUAL CONC., M

F"1gW'e 7: Interactions between PyPEOPy and SDS at the alumina-water interface

and the soltoids have altered the conformation of the adsorbing polymer. It was also found that

there was near complete adsorption of polyethylene oxide (PEa) once solloids were formed at the

interface.

The effect of preadsorbed PEG on the adsorption of SDS 'on silica was alsO-studied(13).

Polyethylene oxide adsorbs strongly on silica~ but sodium dod~cylsulfate does not adsorb on the

similarly charged silica The adsorption of PEa on silica is of the high affinity type and is shown

in figure 8.

F~R 8: Adsorption of polyethylene oxide on silica

\-,\

u '-t-~I~

In the present investigation, increasing amo~.nts of SOS \\~re contacted with silica ha\'ing

preadsorbed polyethylene oxide on it. The polymer concentration chosen \vas such that there \vas. ';-.-- .

no residual polymer left-in s0lution thus avoiding any interference from interactions between PEG.. .. - .

and SDS in bulk solution. The adsorption of sodium dodecylsulf3te onto silica in the presence of""'.;

preadsorbed polyethylene oxide is plotted in figure 9. Preadsorbed PEO has c.learly dra\vn SOS to. .

the interface. Since SDS does not adsorb on bare silica, the isotherm-obtained can be said to be

CONCLUDING REi\l.\RKS

Binding of the dodecyl sulfate surfactant to the polyethylene oxide was found to cause

considerable changes in the polymer conformation; which is a significant parameter in controlling

the rheological properties of the system.-It has been demonstrated that polymer-!utfactant

interactions can be used to force adsorption of either the surfactant or the polymer onto surfaces

where they normally do not adsorb. This could be important in 'determining the adsorption

characteristics and loss of polymers and surfactants on resJ».fVoir minerals. Hydrophobically

modified polymers offer great promise in terms of combining the benefits of high molecular weight

polymers and surfactants.

Acknowledgements: Financial support for this work was from the D~partrnent of Energy (DE-.-\C22---

92BC14884), National Science Foundation. BP America Res~arch and Aqualon Corporation.. ..

REFERENCES

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6. Cuniberti, C. and Perico, A.. ~~~.. 1:!, 369 (1977).

7. Jones, M.N., LColloid Interf. Sci., .:f..,1 36 (1967).

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..1 Koksal, E., Ramachandran, R., Somasundaran~ P. and ~laltesh, C., Powder Tech:, ~ 251(1990). .

12. Somasundaran, P., Middleton, R., and Viswanathan~ K.V., in Structure ilnd P~rformanceRelationshig in Surfactants. Rosen, M.J., (ed), ACS Symposium S~rjes, Washington D.C.,270 (1984). .

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