JOURNALOPcou.oID AND INTERFACE SCIENCE 183, 243-248 (1996)AR11CLE NO. 0539

Effect of Temperature on the Mobility of Nitroxide Probes inCyclohexane and at the Alumina-Cyclohexane Interface

rS. KRlSHNAKUMAR AND P. SOMASUNDARAN I

Langmuir Center for Colloids and Interfaces, Henry Knunb School, Columbia University, New Yort, New York lOOl?

Received February 5, 1996; accepted April 30, 1996

Nitroxide spin probes have been widely used to study the behav-ior of surfactants in solution as well at solid-liquid interfaces. Inthis study the effect of temperature on the mobility of nitroxidespin probes in cyclohexane and at the alumina/cyclohexane inter-face is investigated in the presence of anionic aerosol-aT (AOT).The probe mobility in AOT micelles in solution is markedly depen-dent on the amount of solubilized water. The variation with tem-perature of the mobilities of adsorbed doxyl stearic acid with thenitroxide probe at the 5th, 10th, and 16th positions from the-COOH group has shed some light on the mechanisms of interac-tion of the probe with the solid surface as well as the molecularstructure of the adsorbed layer. The results indicate that the ad-sorption of the probe occurs mainly through interactions of the-COOH group with the surface while the - NO group interactsonly weakly with the surface. The adsorption of AOT causeschanges in probe mobility with the changes being most significantwhen the nitroxide group on the stearic acid chain is farthest fromthe anchoring group. It is suggested that the adsorption of thesurfactant causes an orientational rearrangement of the probe mol-ecules. The relatively low-temperature effect on the probe mobilityobserved in the presence of surfactant is attributed to the compact-ness of the adsorbed layer which is found to be stable even atelevated temperatures. Q 1996 Academic Pres.o, In.:

Key Words: ESR; doxylstearic acid; nitroxide; non-aqueous;solid-liquid interface.

INTRODUCTION

the particles to aggregate. The dispersi ve action of the surfac-tants can be due to electrostatic repulsion and! or a reductionof the attractive van der Waals forces via modification ofthe particle surface due to the presence of the adsorbed layer(5,6). The electrostatic repulsion depends on the nature ofthe surfactant and its tendency to dissociate and developresidual charges on the particle surface. In contrast, the alter-ation of the van der Waals energy depends on the extent ofsurfactant adsorption and the microstructure of the adsorbedlayer, i.e., the conformation and packing of the molecules atthe solid-liquid interface. Indirect information on molecularorientation at the interface can be obtained from a detailedanalysis of the adsorption isotherm as well as from measure-ments of thermodynamic parameters pertinent to the adsorp-tion process. For example, Kipling and Wright in their workon the adsorption of fatty acids on different substrates de-duced molecular orientation relative to the substrate surfacefrom measurements of heats of immersion (7). Mills andHockey discussed the possible variations in molecular con-formations with adsorption from the heats of adsorption dataas a function of surface coverage of n-fatty acids on silica(8). Such studies while providing some insight into molecu-lar conformation are still inconclusive owing to the indirectnature of their observations. A number of spectroscopic tech-niques are now available that can be applied to obtain directin situ information on the structure of the adsorbed layers.We have recently used fluorescence, electron spin resonance(ESR), Raman, infrared, and nuclear magnetic resonance(NMR) to study adsorption phenomena and to correlatechanges in the adsorbed layer microstructure with variationsin macroscopic properties such as wettability and hydropho-bicity of the dispersions (9-11).

Spin probing using ESR spectroscopy is a sensitive tech-nique which involves the incorporation of a suitable para-magnetic moiety into the system under investigation andmeasuring its response to changes in its environment. Thistechnique has been successfully applied to study biologicalmembranes (12) and membrane mimetic systems such asmicelles or reverse micelles (13-15). This has also beenemployed to study the exchange kinetics and solubilizationsites in water-in-oil microemulsions (16). Information on

The study of colloidal dispersions in nonaqueous mediais of great importance to a number of industrial applicationssuch as ceramic processing (1), paints, inks, pigments (2),electrophoretic image processing (3), and lubrication (4).Efficiency of these processes is dependent on the ability tocontrol the dispersion stability by adjusting parameters suchas adsorption density and molecular conformation.

Dispersions in nonaqueous liquids are usually stabilizedby adsorption of surfactants or polymers on the particles toprovide repulsive forces to counter the natural tendency of

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244 KRISHNAKUMAR AND SOMASUNDARAN

where ~o = peak to peak distance of the central line ingauss, 1011- t = ratio of the central to high field line peakheights, and 1011+ I = ratio of the central to low field linepeak heights. For anisotropic spectra in the slow regime theapplication of the above equation leads to erroneous valuesof the correlation times. In such cases T values were esti-mated using the following method of calculation (22).

- S)b,T =a(,.where a and b are constants given in literature (23) and S

= A,,! A" where A", is one-half the separation of the outerhyperfine extrema and A, is the rigid limit value for thesame quantity obtained from a frozen spectrum of the sampleunder consideration.

the micropolarity and micro viscosity of the probe can beobtained from the ESR spectra and used to deduce structuralinformation on its environment. This technique has beensuccessfully employed to the study of adsorbed polymersand surfactants (17, 18). We have also shown, using ESR,that adsorption of water causes conformational changes inthe adsorbed aerosol-aT layer at the alumina-cyclohexaneinterface ( 19). This observation was consistent with changesobserved in the stability of these dispersions. Variations inprobe mobility with temperature have been used in the pastto understand structural changes in membranes and vesiclesas well as to study diffusion of molecules in micelles andin adsorbed layers (20,21).

In this study we use a series of nitroxide probes to under-stand the nature of interaction governing the adsorption andmicrostructural evolution of anionic aerosol-aT (AOT) onalumina in cyclohexane. The effect of temperature on themobility of probes in solution and at the solid-liquid inter-face was monitored in the presence and the absence of thesurfactant.

Sample Preparation

The alumina is dried by heating at 200°C for 6 h andcooling under vacuum to room temperature. Half a gram ofthe dried alumina is conditioned with 5 ml of a 10-4 MIliter solution of the probe in cyclohexane for 6 h. The slurryis then transferred to a narrow quartz capillary tube andintroduced into the ESR cavity. For samples with adsorbedsurfactant, the probe is initially adsorbed onto the aluminaand the solvent removed by drying under vacuum. The solidsare then contacted with AOT solutions of different concen-trations for 6 h and the spectra recorded.

EXPERIMENTAL

Materials

Linde alumina of approximately 0.3 JJ.m with a BET sur-face area of 14 m2/g, purchased from Union Carbide, wasused as the substrate for the adsorption studies. The surfac-tant used, sodium bis (2-ethylhexyl) sulfosuccinate (aerosol-OT), was purchased from Fisher Scientific and is purifiedby solubilization in methanol and recrystallization by solventevaporation. Cyclohexane of spectroscopic grade was alsopurchased from Fisher Scientific and was dried by storing inmolecular sieves. The amount of water present after drying,measured using Karl Fisher titration, was less than 5 mM.The spin probes used were a series of doxyl stearic acids withthe paramagnetic nitroxide free radical attached to differentpositions along the stearic acid chain (n = 5, 10, 16). An-other probe, 5-doxyl dodecane which has no terminal-COOH group was also used to investigate the effect ofthis functional group on probe adsorption (Fig. I). All theprobes were purchased from Aldrich Chemicals and used asreceived.

~UL TS AND DISCUSSION

Probes in Solution

A careful analysis of line broadening features of dIe ESRspectra can provide information on dIe rotational mobilityof the nitroxide probe from which the characteristics of itsenvironment can be deduced. A nitroxide radical tumblingisotropically in a non-viscous fluid will yield a characteristicsharp dIree-line spectrum. Immobilization of dIe radicalcauses line broadening and dIe resultant spectrum wouldhave lost most of its details. If the molecule is rotating slowlydIen an intermediate spectrum is obtained.

Figure 2 shows the temperature variation of the rotationalcorrelation time ( T B) of 7 -doxyl stearic acid in cyclohexanein dIe presence and absence of aerosol-aT micelles. Therotational correlation times are shown on a log scale againstdIe reciprocals of the absolute temperature. The first pointof interest is dIat at all temperatures the probe mobility islower in dIe micellar solution. This has been reported pre-viously and used to detect the onset of micellization in avariety of surfactant-solvent systems (15). Such an increasein TB is due to dIe incorporation of the probe molecules inthe AOT reverse micelles, thereby restricting their rotationalmotion. The probe moves along with the micelle and themobility measured now is a reflection of the micellar mobil-

Methods

All ESR spectra were acquired using a Micronow 8300X-band spectrophotometer, equipped with a temperature-controlled cavity, at a modulation frequency of 100kHz.For isotropic spectra in the fast motion regime the rotationalcorrelation times are calculated directly from the spectrumusing the equation

TB = 6.25.10-10 Mo£(loIl_I)I12 - (/011+1)112)8,

245TEMPERA 11JRE EfFEcr ON Nn'ROXIDE SPIN PROBES

(b)(a)

~ , 'X o-r y

CH3- ~CH3

(c)FIG. 1. Structure of (a) aerosol-OT, (b) n-ooxyl stearic acid, (c) 5-doxyl dodecane.

ity in solution and the rotation of the probe within the mi-celle. The overall mobility is observed to increase (corre-sponds to a decrease in TB) with temperature in both theabsence and presence of the AOT micelles. Such an increasein probe mobility may be attributed in both cases to changesin the medium viscosity. We have estimated also the activa-tion energies (Ea) for the probe rotation from plots of logTB against lIT for the various cases and the values obtainedare -16 and -19 kJ Imol in the absence and presence of

AOT micelles respectively. The similarity of these valuessuggests that nature of the probe rotation is similar in thetwo cases, and the lower probe mobility in the micelle isindeed due to the slower reorientational motion of the bulkiermicelle in which it is incorporated.

The micelle size as well as the structure of the water in themicellar core is known to vary with the amount of solubilizedwater. Figure 3 illustrates the variation of To with tempera-ture in micellar solutions of AOT at different amounts ofsolubilized water (wo = [HP1/[AOT]). At room tcmpera-

E (klimal).0 -25. -14.50 -13.5

It ~~'FIG. 2. Effect of temperature on the mobility of 7-doxyl stearic acid

in cyclohexane and in AOT micellar solution in cyclohexane. Also shownare the activation energies (E.) of probe motion under different conditions.

l/T'lOOO

FIG. 3. Variation of probe mobility with temperature in AOT micellesat different levels of water content.

246 KRlSHNAKUMAR AND SOMASUNDARAN

E (kJ/mol)a5D % -32.7

lOO = -40l6D = -24.25DD = -3.7

..I-U.3

3~

{1/T) .10'

FIG. 4. Effect of temperature on the mobility of various probes adsorbedat the alumina-cyclohexane interface.

ture (T = 299 K) the mobility values decrease with increasesin the water content. The micelles swell as they solubilizewater, become bulkier, and hence their rotational motion isslowed down. For all the samples the mobility is seen toincrease with temperature. The increase in mobility withtemperature can be attributed to a change in the mediumviscosity as well as increased exchange kinetics of the probebetween the micelle and the bulk solution. The rate of in-crease in mobility is higher for the sample with the leastamount of water. The Ea values for these cases are indicatedin Fig. 3 and show that the activation energy for the probemotion decreases with increases in water content of the mi-celles. In the dry micelle the probe is presumably rigidlybound to the surfactant molecules via the trace amounts ofwater present. Earlier studies have also indicated that at lowwater contents the water in the reverse micelle is structuredaround the head groups of the surfactant molecule (24). Inthis case the probe rotation within the micelle is not signifi-cant and the probe mobility more or less accurately reflectsthe rotation of the micelle. However with increases in watercontent the micellar core becomes loose and the probe bind-ing to the micelle becomes less rigid, and they start to haverotational freedom within the micelle.

creases in the order 5 < 10 < 16. This can be explained bya model for probe adsorption wherein the -COOH groupsinteract strongly with the alumina surface while the -NOgroup interacts only weakly with the surface. Thus the fartheraway the -NO is from the -COOH group the lesser itsinteraction with the surface, and hence 16-doxyl stearic acidexhibits higher mobility. All the doxyl stearic acid probesshow an increase in probe mobility with temperature. Thisincrea.~e in mobility is due to a weakening of the interactionof the -NO group with surface. The E. values shown in Fig.4 indicate a low value for 16-doxyl corroborating the fact thatthis is more loosely bound to the surface. It should be notedthat in all these cases there was no probe desorption detectedeven at the highest temperatures studied. It is to be noted thatthe 5-doxyl dodecane shows a different behavior. It showsvery high mobility at the solid-liquid interface, and in thiscase the probe could be detected in the supernatant as well.This suggests that this probe adsorbs weakly onto the aluminasurface, reiterating the fact that the -NO groups interactonly weakly with the surface and cannot promote adsorption.The lower value of E. estimated in this case further supportsthis fact. The 5-doxyl dodecane becomes immobile only atlower temperatures (below freezing point of cyclohexane) ascan be seen from Fig. 4.

The adsorption of aerosol-OT causes interesting changesin the probe mobility. Figure 5 shows variation in probemobility as a function of AOT concentration at the alumina!cyclohexane interface. While there is no significant changein the mobilities observed for 5- and 10-doxyl there is adramatic increase in the mobility of 16-doxyl stearic acid.This can be schematically explained as shown in Fig. 6where possible probe conformations in the presence and ab-sence of AOT is shown for 5- and 16-doxyl stearic acid

.c.-E~c0i!~0uC0~

~~

Probes at the Alumina/Cyclohexane Interface

The spectrum of nitroxide probes adsorbed at the solid-liquid interface displays line broadening features characteris-tic of hindered motion. 5-, 10-, 16-Doxyl stearic acids and 5-doxyl dodecane were adsorbed onto the alumina surface fromcyclohexane and the corresponding ESR spectra analyzed toestimate their mobility at the solid -liquid interface. Figure 4depicts the variation of the interfacial mobilities of the differ-ent probes with temperature. It is observed that at room tem-perature, the mobility of the doxyl stearic acid probes in-

Concentration of AOT, moln

FIG. 5. Variation in probe mobility at the alumina-eyclohexane inter-face on surfactant addition.

TEMPERATURE EFFECT ON NrrROXIDE SPIN PROBES 247

doxyistearic acid

.J-}; ~(~OH OH OH OH OH OH OH

AOT(d)(c)

\!v- JOH OH OH OH OH OH OH OH OH OH OH OH OH OH

FIG. 6. Schematic diagram illustrating possible changes in probe confonnation on surfactant addition (a) 5-doxyl on alumina surface, (b) 5-doxylon alumina at monolayer coverage of AOT, (c) 16-doxyl on alumina, (d) 16-doxyl on alumina at monolayer coverage of AOT.

adsorbed layer is compact and stable even at elevated tem-peratures.

CONCLUSIONS

Temperature variation of mobility of nitroxide probes incyclohexane and at the alumina-cyclohexane interface wasstudied using ESR. The temperature dependence in solutionshowed the probe binding to a micelle to vary as a functionof the water content. The probe was found to be rigidlybound to the micelle at low concentrations of water and tobecome more loosely bound at higher water concentrations.

Adsorption of different nitroxide probes at the alumina-cyclohexane interface was also investigated. The probe ad-sorption was clearly shown to occur by interactions of thesurface species with the -COOH group of the probe mole-cule. The - NO group while incapable of promoting adsorp-tion by itself interacts weakly with the surface. The degreeof interaction depends on the location of the -NO grouprelative to the anchoring -COOH group. with the interac-

cases ( 18). The probe molecules are proposed to be pushedinto a perpendicular orientation by the adsorbing AOT mole-cules. This is reflected as an increase in probe mobility inall three cases. However in the case of 16-doxyl the -NOgroup escapes the adsorbed layer of AOT which is approxi-mately 10 -CH2 chains long which explains the significantincrease in mobility shown by the 16-doxyl stearic acid. Inthe other two cases the -NO group is still buried in theadsorbed AOT layer and continues to be rnotionally re-stricted.

The effect of temperature on the mobility of 5-doxyl inthe presence and absence of surfactant is shown in Fig. 7along with the activation energies involved. The activationenergy estimated from the temperature dependence of themobility is less in the presence of the surfactant, indicatingthat the probe is less rigidly bound to the surface in thepresence of the surfactant. A similar observation is made forthe case of 16-doxyl stearic acid where also the probe bind-ing to the surface is less in the presence of the adsorbedsurfactant (Fig. 8). These results also indicate that the AOT

t

m..

~.3

..-.q

E. (kJ5D5D+AOT

,...

J:

4;~

FIG. 7. Effect of temperature on the mobility of 5-doxyl stearic acidat the alumina-cyclohexane interface in the absence and presence of AOT.

FIG. 8. Effect of temperature on the mobility of 16-doxyl stearic acidat the alumina-cyclohexane interface in the absence and presence of AOT.

/mol)- -32.7c -10.7

248 KRlSHNAKUMAR AND SOMASUNDARAN

tion weakening with increasing distance between these twogroups. The interaction is also weakened by increases intemperature. An examination of the temperature dependenceof probe mobility suggests that the 16-doxyl is indeed boundweakly to the surface compared to the 5-doxyl. Addition ofaerosol-OT causes measurable changes in probe mobility,depending on the position of the -NO moiety. The ad-sorbing surfactant weakens the probe interaction with thesurface and this fact is further illustrated by the temperaturedependence of probe mobility in the presence of the surfac-tant. The surface pressure developed by the adsorbing AOTmolecules is shown to cause the probe to orient perpendicu-lar to the surface.

{

ACKNOWLEDGMENTS

The authors acknowledge the support of this work by the National ScienceFoundation (NSF-CfS-93-11940) and MMRRI, New York:.

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