From: SURFACTAJJTS IN SOLUTION, VOL. 9Edi!.(!U by K. L. Mittal(Plenum PI!hlishing Corporation. 1989)

INVESTIGA TIONS INTO THE STRUCTURE OF THE ADSORBED LAYER

OF DODECYLSULFATE AT THE ALUMINA-WATER INTERFACE

P. Somasundaran, P. Chandar, N.J. Turro and K.C. Waterman(l)

Langmuir Center for Surface and Colloid SciencesDepartment of Mineral Engineering and ChemistryColumbia UniversityNew York, NY 10027

Ruorescence and electron spin resonance probes have been used along withzeta potentials and adsorption isotherms to study the solid-water interfacial layerformed by dodecylsulfate adsorbed on alumina. In sum, our results indicate that thesurface aggregation (hemimicellization) involves four regions of increasingsurfactant concentration characterized by the following: (I) electrostatic binding ofthe sulfates to the positively charged alumina with minimal aggregation; (II)formation of highly ordered local surfactant aggregates which increase in number,but not in size, as a function of surfactant concentration; «((I) charge reversal at thesurface, and increase in surface aggregate size but not in number; (IV) surfacesaturation, i.e., maximum surface coverage as limited by the available surface areaor maximum monomer activity achieved as a result of solution micellization. It isconcluded that the hemimicelles are considerably more viscous at the microscopiclevel (for motions of solubilized probes) than the corresponding micelles(approximately 100cP for SDS hemimicelles compared to 10 cP for SDS micelles).In addition, the ordering of the alkyl chains appears to be such that the positionsnear the solid surface are more tightly packed than those near the air-waterinterface. The micropolarity of SDS hemimicelles is comparable to that of SDSmicelles, that is, the polarity inside a hemimicelle is considerably lower than that ofwater. Both microviscosity and micropolarity remain constant after the onset ofhemimicellization (Regions (( through IV).

INTRODUCTION

The adsorption of surfactants and polymers on solid-liquid interfaces is a subject ofconsiderable importance in such fields as detergency: lubrication, separation,microelectronics, enhanced oil recovery and as models of biological macromolecule

adsorption.2 Conventional techniques for investigating the solid-liquid interfacial regionhave involved mostly macroscopic measurements of adsorption isotherms, zeta potentials,

wettability and calorimetry.3 By use of such methodologies, it was indicated that surfactant

339

adsorption is enhanced by the aggregation of surfactants on the surface.4 Such aggregates

have been termed "hemimicelles."5 Although conventional techniques do provide someinformation about the structure of the adsorbed layer, spectroscopic techniques can yield amore detailed picture of such layers. Our spectroscopic investigations to date haveinvolved addition of sensitive molecular probes which are present at sufficiently low

concentrations so as not to perturb the system to any significant extent.6-9 Since one cannever be sure that the probe is providing accurate information about the substrate underinvestigation, we sought to confirm the indicated data by the use of several different typesof probes as well as classical, macroscopic measurements. As will be seen below, data fromvarious techniques are consistent with each other and yield a more complete structural

model for the adsorbed layer than was previously proposed,4 though refin~ments willundoubtedly appear as different or more sophisticated techniques are applied to the

problem.

..4

Fluorescence probes have been used extensively in the recent years to investigate

the structure of such microenvironments as micelles, cyclodextrins and surfaces.IO Severalgeneral types of fluorescence methodologies have been used. Among these are studies ofprobes whose vibronic structure depends upon the environment of the probe, probes whoselifetimes (quantum yields) vary with different conditions, and probes which show excimer(exciplex) emission. Pyrene shows fine structure which is markedly dependent on thesolvent. In particular, the ratio of the intensities of the first and third vibronic emissionbands (11/1) of pyrene is a sensitive indicator of the micropolarity of the probe. I I Since

pyrene is hydrophobic, it is solubilized in micelles and, as such, has been used as a polarityprobe of micelles. 12 In addition to these features, pyrene shows excimer emission which is

dependent on the ability of two pyrene molecules to come in contact. The extent of excimerformation then provides an indication of the fluidity of the environment of the probe andthe local concentration of the probe. As will be seen below, determinations of thefluorescence decay patterns of the pyrene monomer and excimer emissions can alsoprovide information on the homogeneity of the probe environment. Dinaphthylpropane(DNP) forms different amounts of intramolecular excimers as a function of mobility of thetwo naphthyl moieties. Therefore, as with pyrene, a determination of the relative amount ofmonomer to excimer emission intensities (Im/le> provides an indication of the microscopic

fluidity surrounding the probe, in this case independent of the local probe concentration.

pyrene Dlnaphthylpropane (DNP)

Spin probes have been used extensively in electron spin resonance (ESR)investigations of microscopic environments such as micelles.13 The most commonlyemployed spin probes involve nitroxides, many of which are commercially available. Thesestable radicals show three ESR absorptions due to hyperfine coupling to the nitrogen

..

340

nucleus (spin =1). In solution, a molecule rotates with a characteristic time constant (Tc)

such that the observed spectrum will be isotropic under conditions of rapid rotation andincreasingly more anisotropic as environmental factors affect its ability to rotate freely.Hence the degree of anisotropy is an indication of the microscopic viscosity (fluidity) of theprobe's environment. The nitrogen hyperfine coupling constant (AN) of the nitroxides has

been empirically correlated with polarity.14 Therefore, by measuring the splitting betweenESR spectral peaks of a spin probe, one can determine the micropolarity of theenvironment of the probe. [n our investigations, we employed spin probes which arethemselves surfactants so that the behavior of the probe can be expected to closelyresemble the surfactant system under investigation. The probes we used are all derivativesof commercially available doxylstearic acids shown below:

m=1 n=14 16-0

m=5 n=10 12-0

~~N.O .CH3(CH1)m (CH1) coon

D

m=12 n= 3 5-0

DOXYL-Stearlc Acid Spin Probes

RESULTS AND DISCUSSION

In studying the general behavior of surfactants adsorbed at interfaces, we chose tostudy the sodium dodecylsulfate (SDS)/alumina system. Detailed experimental proceduresused in these studies have been discussed elsewhere.6-9 At neutral pH, the aluminasurface is positively charged enabling adsorption of the anionic surfactant (dodecylsulfate).The adsorption isotherm6 of SDS on alumina at pH 6.5 (0.1 M NaCI) is typical of anionicsurfactants adsorbed on positively charged oxides (Figure I) and consists of four regions:At low SDS concentrations (Region I), the isotherm is linear. At an adsorption density of

3x 10-13 mol/cm2 , a sharp transition occurs such that adsorption increases dramatically asa function of surfactant concentration (Region II). After a linear increase in the adsorptionin Region II, the isotherm curves in Region III and becomes a plateau at critical (solution)

micelle concentration (CMC) in Region IV. The zeta potential6 behavior follows a similardivision of regions (Figure 2): In Region I, the potential is relatively constant and positive;in Region II, the potential changes rapidly till the isoelectric point is reached at the onsetof Region III; Region III involves a net negative potential which plateaus in Region IV.The proposed interpretation of these effects involves electrostatic binding of the anionichead group to the alumina surface in Region I; patchwise association of the SDS tail groupsin Region II; decreased adsorption ability due to surface charge reversal in Region III; andplateau adsorption due to either complete surface coverage (note that the onset of RegionIV could correspond approximately to a completed monolayer or bilayer) or a valuelimited by the relatively constant activity of the surfactant monomer in solution above theCMC.

341

"i

IIQ~.1_--~~~. -- m- --((:f,~

-->e60.; 40~ 20I-

~ 0I-0-200..-40 m41--60 n 'C1&1 .. . . . , , .,.1 ... , .. ,.1 '" , . ,. ,I . . ... ,..N-80 Lr'J

0 10-5 10-4 10-3 10-2RESIDUAL DODECYlSUlFATE. moles/liter

Figure 2. Zeta potential of alumina with adsorbed SDS as a function of SDS concentration

at pH 6.5 and 0.1 M NaCI.

342

Microviscositv by Steady State Methods

The spectroscopic probes employed report the microscopic viscosity at the siteswhere the probes lie within the hemimicelle. The micro viscosity determined probablyinvolves a time average of many positions within the hemimicelle. With DNP, a calibratedviscosity based on the 1m/Ie values of standard solutions of ethanol/glycerol yielded a value

of 90-120 cp for alumina adsorbed SDS, indicating a considerably more viscousenvironment than the SDS micelle (8 cp by this method).6-7 With the spin probes l6-D, 12-

D and 5-D, high values of the micro viscosity in the hemimicelle are indicated,9 thesevalues being of the same magnitude as determined by DNP fluorescence. In addition, thereare no spectrosco~c indications of microviscosity changes with increasing SDSconcentrations (Regions II-IV) suggesting that the structural features of the hemimicelleremain constant along the adsorption isotherm.

A particularly interesting feature in the case of spin probes is that the carboxylatesof the spin probes bind tightly to the alumina surface such that the nitroxide moieties senseenvironments corresponding to different distances from the alumina surface. What is foundis that in contrast to SDS micelles where all three spin probes indicate similar

microviscosities.14 the SDSfalumina hemimicelles vary in fluidity within the hemimicelleas a function of distance from the alumina surface: the closer to the surface the nitroxide is.the more viscous the environment it senses. This suggests that the chain segments near thepolar head are tightly packed while those further from the surface are more flexible asshown pictorially in Figure 3.

l'

-+

~ "

Figure 3. Schematic representation of binding of 5-DQXYL (top) and 16-DQXYL(bottom) stearic acid probes to alumina in the presence of SOS.

143

Microoolarity by Steady State Methods

As with the microviscosity determinations, we find that both fluorescence and spinprobes show no change in micropolarity at SDS concentrations higher than that at theonset of hemimicellization. Pyrene fluorescence measurements indicate that for theSDS/alumina hemimimicelles, 13/1 I changes dramatically from a value characteristic of

pyrene in an aqueous environment (0.6) to a value similar to that of pyrene in an SDS

micelle (1.0) as shown in Figure 4.6 This indicates that there is little surface aggregation atSDS concentrations corresponding to Region I, while in Regions II-IV, the structure of thehemimicelles remains constant. With 16-D, a nitrogen hyperfine coupling constant of 15.0Gwas determined in SDS/alumina slurries.8 This value indicates that the micropolaritysensed by this probe is somewhere between that of ethanol (AN=14.6G) and water

(AN=16.0G). With 12-D and 5-D, the ESR spectra are too anisotropic to obtain nitrogen

hyperfine splitting constants in SDS hemimicelles. The micropolarity measurementsdetermined by both pyrene fluorescence and spin probe hyperfine splitting indicate that thcSDS hemimicelle is considerably less polar than an aqueous environment and resemblcsthe polarity of methanol. This suggests that there is only limited penetration into thehemimicelle by water. These results are consistent with the concept of hemimicellizationfor the surfactant adsorption; that is, association of alkyl chains which serve to excludewater from the adsorbed layer.

PYRENE IN ADSORBED LAYER SDS/ALUMINA.IM NoCl, pH 6.5

50S MICELLE \.1M HoC A""""".""""""""" WATER to.IM HoCI)"...M

.5

0 .". . . . ... ... ... . . ..., ... . . ...1 ... . . 0 10-5 10-4 10-3 10-2

RESIDUAL DODECYLSULFATE . mole./lile,

Figure 4. Ratio of emission intensities for first and third vibronic bands of pyrene as afunction of residual SDS concentration in the presence of alumina.

A22re2ation Numbers and Microviscositv bv Time Resolved Fluorescence

The decay pattern of pyrene monomer and excimer fluorescence after flashexcitation depends on the nature of the pyrene environment. In homogeneous systems,

344

pyrene monomer and excimer show single exponential decays with parallel slopes. Incompartmentalized solubilization (such as in micelles) the monomer emission becomesmultiexponential, and the monomer and excimer slopes are no longer parallel as typicallyshown in Figure Sa. This can be rationalized as follows if ones assumes that the rate oftransfer between compartments is slow on the time scale of the measurements: In somecompartments, there will be only one pyrene such that there will be no decay of themonomer via excimer formation. In other compartments, there will be more than onepyrene such that excimer formation will be r.aPid and show integral dependence on thenumber of pyrenes within the compartment. Assuming this model and using Poissonstatistics, one can derive the following equation for the time dependence of the monomeremission: 16

Im(t) = Im(O)exP(-kot + <n>(exp(-ket) - I))

<n> - N[pyreneJads/[SDSJad~

where Im(t) is the emission intensity at time t, ke is the rate of excimer formation, <n> is

the average occupation number and N is the aggregation number. Therefore, an analysis ofthe decay of the monomer emission leads to a determination of the average aggregate sizeand rate of excimer formation.

With pyrene in the SDS adsorbed layer in Regions 1\ and III of the adsorptionisotherm, the monomer emission shows multiexponential decay with a distinctly differentslope than seen with the excimer emission (Figure 5b). This indicates that, as withmicelles, the adsorbed surfactant forms compartments (aggregates) in this SDSconcentration range. [n Region [V, the decay of the monomer is nearly monoexponentialand has approximately the same slope as that of the excimer indicating that at the highersurface coverages, pyrene sees a relatively homogeneous environment. We suggest thatthis latter environment is that of a contino us lamellar phase (monolayer or bilayer)adsorbed on the surface.

The rate of excimer formation. ke. is slower in the adsorbed layer of SDS than in

SDS micelles consistent again with the hemimicelle being more viscous than the micelle.This viscosity factor alone is not "likely to account for the change in ke as a function of

adsorption density (see Figure 6) since the spin probes and DNP measurements of themicroviscosity did not indicate any changes after the onset of hemimicellization. Thisdecrease in ke with increase in adsorption density can be attributed to an increase in

micelle size which results in slower encounter rate between pyrenes in the same

compartment.

The aggregation number, N, also shows differing behavior as a function ofadsorption density (see Figure 7). With adsorbed SDS, N varies only slightly as a functionof SDS adsorption density in Region II; however. in Region (II, N increases markedly with

345

>-t-

(/)Zwt-~0-0

~

0 100 200 300 400 500 600

TIME nsec

Figure 5a. Typical pyrene monomer (A and B) and excimer (C) decay profiles in SOSmicellar solution; [SOS] = 8.2xIO-2 M, [NaCl] = O.IM; Monomer emission in the absence(A) and presence (B) of excimer formation.

3

~ 2u;zw.-z

0 10

.J

()0 100 200 300

TIME, nsec400 500 600

Figure 5b. Typical pyrene monomer (A and B) and excimer (C) decay profiles inSDS/alumina adsorbed layer; adsorption density = 1.3xIO-11 mol/cm2 , [NaCI] = O.IM,pH=6.5; Monomer emission in the absence (A) and presence (B) of excimer formation.

346

.

6..': 5.0= 4.,..

~

3

2

110-12 10'"-10-11 10-10

Adsorption Density, mof/cm2

Figure 6. Variation of encounter rate (ke> for pyrene excimer formation in SDS/alul

adsorbed layer as a function of adsorption density.

347

adsorption density. This suggests that at low surface coverages (Region II), hemimicellesform at a relatively constant size (approximately 120 SDS/hemimicelle) with the new SDSmolecules going mostly to form more hemimicelles. In region III, new SDS hemimicellesare not likely to form on the surface since positive surface sites are no longer available (asindicated by the change in sign of the zeta potential). Thus, addition of more SDSmolecules leads to an increase in the existing hemimicelles rather than the number of such

species.

CONCLUSION

By combination of spectroscopic and bulk property techniques, we have shown thatsurface aggregation of SDS on alumina results in a non-homogeneous distribution of SDSon the surface. These aggregates are similar to micelles in that they compartmentalize toform a relatively non-polar environment, but differ in that they are considerably moreviscous than the corresponding micelles. SDS hemimicelles appear to vary in microviscosityas function of the distance from the alumina surface with the alkyl chain segments closer tothe surface being less fluid than those near the water interface. The adsorption isotherm ofSDS on alumina can be divided into four regions as shown schematically in Figure 8. In thefirst region, aggregation is minimal and adsorption is essentially electrostatic. In thesecond region, aggregation occurs to give hemimicelles of about 120 SDS molecules. In thethird region, surface charge reversal occurs and adsorption continues by addition of moreSDS molecules to existing aggregates rather than by increasing the number of aggregatesas in the second region. In the fourth region, a plateau is reached where the surfaceappears to be covered with a completed monolayer or bilayer. In this region, spectroscopicproperties become similar to those in homogeneous solution.

REGION INO AGGREGATION

I

REGION 11

NUMBER OFAGGREGATESINCREASES-120-1~O MOLECULESPER AGGREGATE

!

REGION mSIZE OF AGGREGATESINCREASES>160 MOLECULESPER AGGREGATE

. . -. . .~ -.- . - . - -.. -. - . -. . . . . - .

Figure 8. Schematic representation of the four regions of adsorption of SOS on alumina.

348

ACKNOWLEDGEMENTS

KCW acknowledges PHS grant CAO7957 (National Cancer Institute. DHHS). NJTthanks NSF and ARO for generous support of this research. PS and PC acknowledge NSF

and DOE for financial support.

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