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A small-angle neutron scattering study of sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles

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A small-angle neutron scattering study of sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles
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  Journal of Colloid and Interface Science 257 (2003) 154–162www.elsevier.com/locate/jcis Small angle neutron scattering study of sodium dodecyl sulfate micellargrowth driven by addition of a hydrotropic salt P.A. Hassan, 1 Gerhard Fritz, and Eric W. Kaler ∗ Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA Received 25 January 2002; accepted 18 September 2002 Abstract The structures of aggregates formed in aqueous solutions of an anionic surfactant, sodium dodecyl sulfate (SDS), with the additionof a cationic hydrotropic salt,  p -toluidine hydrochloride (PTHC), have been investigated by small angle neutron scattering (SANS). TheSANS spectra exhibit a pronounced peak at low salt concentration, indicating the presence of repulsive intermicellar interactions. Model-independent real space information about the structure is obtained from a generalized indirect Fourier transformation (GIFT) technique incombination with a suitable model for the interparticle structure factor. The interparticle interaction is captured using the rescaled meanspherical approximation (RMSA) closure relation and a Yukawa form of the interaction potential. Further quantification of the geometricalparameters of the micelles was achieved by a complete fit of the SANS data using a prolate ellipsoidal form factor and the RMSA structurefactor. The present study shows that PTHC induces a decrease in the fractional charge of the micelles due to adsorption at the micellar surfaceand consequent growth of the SDS micelles from nearly globular to rodlike as the concentration of PTHC increases. © 2003 Elsevier Science (USA). All rights reserved. Keywords:  Micelles; Small angle neutron scattering; Sphere-to-rod transition; Hydrotropes; Quasielastic light scattering 1. Introduction The addition of electrolytes to micellar solutions of ionicsurfactants can cause the formation of rodlike micelles, andthe properties of such micelles depend strongly on the mole-cular structure of the counterions [1–10]. The self-assemblyof surfactant molecules in aqueous solutions is governed bya balance between opposing forces. The main driving forceis reduction of the unfavorable water–hydrocarbon interfa-cial energy, while counteracting forces arise from the stericand electrostatic interactions that result from bringing thecharged head groups into close proximity and the associ-ated diffuse layer of counterions. The electrostatic contri-bution to the free energy of formation of micelles can beinfluenced by the addition of electrolytes or cosurfactants.Conventional electrolytes influence the electrostatic free en-ergy by mixing with the counterions while cosurfactantsmay influence the free energy by a decrease of the surface * Corresponding author.  E-mail address:  kaler@che.udel.edu (E.W. Kaler). 1 Permanent address: Novel Materials and Structural Chemistry Divi-sion, Bhabha Atomic Research Centre, Mumbai, India. charge density. Typically, addition of substantial quantitiesof inorganic electrolytes is necessary to induce the forma-tion of rodlike micelles [5]. However, only millimolar quan-tities of hydrotropic salts are sufficient to induce the transi-tion from spherical to rodlike micelles [7]. Appropriate hy-drotropic salts contain aromatic counterions that adsorb onthe surfaces of the micelles and thereby decrease their sur-face charge density. In addition, the co-ion released by ion-ization of the hydrotropic salt increases the ionic strengthand screens electrostatic interactions.There are many studies of the formation and propertiesof rodlike micelles formed by cationic surfactants upon ad-dition of aromatic counterions such as salicylate, tosylate,and naphthalene carboxylate [9]. Analogous reports of an-ionic surfactant micelles are very limited, despite their in-dustrial uses, although we have recently reported a sphere-to-rod transition for sodium dodecyl sulfate (SDS) micellesin the presence of a hydrotropic salt,  p -toluidine hydrochlo-ride (PTHC), at less than equimolar ratios of salt to surfac-tant [11]. The hydrotropic salt PTHC, being a strong elec-trolyte, dissociates in water to form  p -toluidinium cation(PTH + ) and Cl − anion. The cationic nature of PTH + fa-cilitates the adsorption of the ion to the negatively charged 0021-9797/03/$ – see front matter  © 2003 Elsevier Science (USA). All rights reserved.PII: S0021-9797(02)00020-6  P.A. Hassan et al. / Journal of Colloid and Interface Science 257 (2003) 154–162  155 SDS micelle surface. The adsorption and orientation of thePTH + on SDS micelle surface was confirmed from previ-ous NMR studies [11]. Note that the neutral analog of thissalt,  p -toluidine,is highly insoluble in water and in the pres-ence of SDS micelles and so does not induce any significantmicellar growth. The present report details the changes instructure and interactions of SDS micelles with the additionof the hydrotropicsalt PTHC as revealedby SANS measure-ments. 2. Materials and methods Sodium dodecyl sulfate (electrophoresis grade) was pur-chased from Fisher Scientific and  p -toluidine hydrochloridewas obtained from Aldrich. All chemicals were used as re-ceived. Stock solutions of SDS and PTHC were prepared inD 2 O (Cambridge Isotopes) and mixed in varying ratios.Small angle neutronscattering (SANS) experimentswereperformedusing the NG-7 spectrometer at the Cold NeutronResearch Facility of the National Institute of Standardsand Technology (NIST) in Gaithersburg, MD. The incidentneutron wavelength was 6 Å. Samples were held in quartzcells with 2-mm path lengths and maintained at 25 ◦ C.Measurements were made at different sample-to-detectordistances to cover a scattering vector ( q ) range of 0.005to 0.6 Å − 1 . The scattering spectra were corrected forbackground scattering, empty cell contributions, sampletransmission, and detector efficiency, and the corrected datawere radially averagedand placed onan absolutescale usingNIST protocols and calibration standards. The incoherentbackground was determined by Porod extrapolation of thedata in the high- q  region and was subtracted before dataevaluation. Corrections due to finite instrumental resolutionwere taken into account throughout the data evaluation. Inthe model scattering curves, the calculated intensities weresmeared by convoluting with a Gaussian resolution functionat each scattering vector. The quality of the fits is assessedfrom the reduced  χ 2 values [12]. 3. Data analysis There are two general approaches to the analysis of SANS data. One is the model-independent indirect Fouriertransformation (IFT) [13] of the experimental scatteringcurve to obtain the pair distance distribution function(PDDF)  p(r) , and the second is the direct modeling of thestructure with an appropriate form and comparison of thecalculated scattering curve for the model with the experi-mental data. The IFT methodcan be appliedto any spectrumand it transforms the scattering curve into real space infor-mation. Interpretation of the results is limited to the caseswhere interparticle interactions can be neglected. However,this method has been extended to a generalized indirectFouriertransformation(GIFT)[14]methodwhereina modelfor the interparticle structure factor is assumed and is incor-porated into the IFT. The GIFT method separates the formfactor (intraparticle) and the structure factor (interparticle)scattering and uses the same model-free approach to calcu-latethePDDFthatcorrespondsonlytotheformfactor.Inthecase of monodisperse, spherical scatterers the PDDF,  p(r) ,is related to the differential scattering cross section per unitvolume  (dΣ/dΩ)  by(1) dΣ(q)dΩ = NS(q) 4 π ∞   0 p(r) sin (qr)qrdr, where  N   is the number density of the micelles and  S(q)  isthe interparticle structure factor. Considering the instrumen-tal smearing effects due to finite resolution,the experimentalscattered intensity,  I(q) , can be related to the cross sectionby(2) I(q) =    R(q)dΣ(q)dΩdq, where R(q) is theresolutionfunction.Structuralinformationabout the scatterers can be assessed from the  p(r)  functionbecause it is directly connected to the convolution squareof the scattering contrast profile. One important parameterreadily obtained from the  p(r)  function is the maximumdimension of the scatterers.In the case of cylindrical particles, when the length ismuch larger than the diameter, it is possible to obtain thecross section PDDF,  p c (r) , by separating the effects of the length and cross section. Again, from the  p c (r)  themaximum diameter of the micelle can be assessed.Informationfrom the PDDF can be supplementedto con-struct a suitable model that can be fitted to the experimentalscattering curve. Because the model independent analysisshows that the micelles undergo a transition from globularto rodlike objects, a prolate ellipsoidal form factor in con- junction with a suitable structure factor is used to model theexperimental data directly.The differential scattering cross section per unit vol-ume  (dΣ/dΩ)  for a system of monodisperse interactinganisotropic particles can be written as [15](3) dΣ(q)/dΩ  = NP(q)S   (q), where  S   (q)  is the orientationally averaged structure factor.For an ellipsoidal micelle with semimajor axis  a  andsemiminor axis  b , the form factor  P(q)  is [15](4) P(q)  = 1   0  F(q,µ)  2 dµ,F(q,µ) = v(ρ m  − ρ s ) 3 j  1 (u)u, (5) u = q  a 2 µ 2 + b 2 ( 1 − µ 2 )  0 . 5 , where  v  is the volume of the micelle,  ρ m  and  ρ s  are the co-herent scattering length densities of the micelle and solvent,  156  P.A. Hassan et al. / Journal of Colloid and Interface Science 257 (2003) 154–162 respectively, and  j  1 (u)  is the first order spherical Besselfunction. The orientationally averaged structure factor  S   (q) is related to the structure factor for an equivalent solution of spherical micelles  S(q)  by [15](6) S   (q) = 1 + F(q,µ)  2 | F(q,µ) | 2   S(q) − 1  . The interparticle structure factor  S(q)  specifies the cor-relation between the centers of different particles.  S(q)  hasbeen evaluated analytically for charged spherical particlesby Hayter and Penfold using the mean spherical approxima-tion [16]. Later these results have been extended to dilutecharged colloidal dispersions by a rescaled mean sphericalapproximation(RMSA) procedure[17]. We used the RMSAexpressions together with the Yukawa form of the potentialbetween the micellar “macro-ions” to account for the inter-particle interactions.The number of free parameters required to fit the datawere restricted as follows. The micelle number density canbe calculated from the concentration of micelles and thevolume occupied by one surfactant monomer, assuming novolume change on mixing. The surfactant monomer volumehas been estimated by Tanford [18]. With the additionof hydrotropic salt a small but appreciable change in thevolume fraction of the micelle is expected as the salt issolubilized into the micelle surface. The volume occupiedby the hydrotrope molecules is included in the volumefraction of the micelles, reflecting the assumption that allthe hydrotrope molecules are solubilized in the micelles.Because of the penetration of water molecules into theheadgroup region of the micelles, the scattering lengthdensity of the headgroup region is not significantly differentfrom that of solvent. Thus the minor radius ( b ) of theellipsoid is initially held at the length of the hydrocarbonchain (16.7 Å) at low salt concentrations. At higher saltconcentrations, where there is no peak observed in thespectra, the quality of the fits was improved by use of aslightly higher value of the minor radius. The maximumvalue of the minor radius used is 18.2 Å, which is onlyslightly different from the initial value.The interparticle structure factor  S(q)  is governed by thevolume fraction and surface charge of the micelles and theionic strength of the medium. The ionic strength is fixedby the concentration of unassociated surfactant moleculesand the counterions arising from the added hydrotropes. Inthe RMSA procedure, it is assumed that ellipsoidal micellesbehave as a rigid equivalent sphere with diameter  σ   = 2 (ab 2 ) 1 / 3 interactingthrougha screenedCoulombpotential.Thus the two adjustable parameters used to fit the dataare the semimajor axis ( a ) of the ellipsoid and the surfacecharge of the micelles. A similar approach has been usedin the evaluation of SANS measurements of other micellarsolutions[4,15,19–21].Anyaccountingofthepolydispersityof the micelles and/or elliptical cross section of the micelleswill introduce additional parameters and is not done here,although the question of cross section is discussed below. 4. Results and discussion Figure 1 shows the SANS spectra of 50 mM SDS inpresence of various amounts of PTHC. The molar ratio of PTHC to SDS in the sample ( x PTHC ) is varied from 0 to0.6, beyond which the solution becomes turbid due to thestrongly associative nature of the salt and surfactant. We be-lieve that the formation of a catanionic salt near the equimo-lar ratio is responsible for the appearance of turbidity in thesolution beyond  x PTHC  =  0 . 6. In the case of cationic mi-celles such as cetyl trimethyl ammonium bromide (CTAB)with the addition of sodium salicylate [7], no such turbid-ity is observed even at a very high ratio of salt to surfac-tant. This is probably due to the greater length of the hy-drocarbon chains in CTAB (C 16 ) than in SDS (C 12 ). Thelarger asymmetry of the hydrocarbonchain lengths in oppo-sitely charged surfactant–additivepairs prevents the precipi-tation of catanionic salt. This is also evident from the differ-ence in aggregation behavior of cetyltrimethyl ammoniumalkyl sulfonate (CTAC n SO 3 ) surfactants [22]. For  n  =  6or 7 isotropic viscoelastic phases exist at low surfactant con-centrations, whereas for  n  =  8 the aggregates are vesicles.A similar turbid region is observed in the phase behaviorof the SDS–octyl trimethylammonium bromide (C 8 TAB)–water system [23]. Addition of a larger hydrophobiccounte-rion such as hydroxynaphthalene carboxylate (HNC − ), thenaphthalene analog of salicylate, to CTAB also produces aturbid phase near the equimolar ratio, while no such phase isobserved with salicylate [24].The spectra in Fig. 1 clearly distinguish two differentregimes:  x PTHC  0 . 3, where a correlation peak occurs, and x PTHC  >  0 . 3, where the peak vanishes. The presence of apeak is an indication of strong repulsive intermicellar in-teractions and this effect is captured by  S(q) . As  q  →  0, S(q)  becomes proportional to the osmotic compressiblity of the solution, which is quite low. At low salt concentrations( x PTHC  0 . 3), the scattering curves show correlation peaksindicating the presence of a repulsive electrostatic interac-tion. At high values of   q ,  S(q)  →  1, and the intensity isdominated by  P(q) .With increasing  x PTHC , there is a broadening of theinteraction peak suggesting a screening of the electrostaticinteraction. The strength of this repulsive interaction is setby the surface charge and the ionic strength of the medium.Addition of a salt increases the ionic strength and hencedecreases the range of electrostatic interactions. Similareffectsare observedin ionicmicelles in the presenceof largeamounts of inorganic electrolytes, but they are seen hereafter the addition of a small amount of PTHC because of incorporation of the PTHC into the surface of the micellesand a concomitant decrease in the surface charge densityof the micelles. There is also a slight shift of the peak tolower  q  as  x PTHC  increases, suggesting an increase of theaverage intermicellar distance, which would correspond toan increase in the aggregation number of the micelles.  P.A. Hassan et al. / Journal of Colloid and Interface Science 257 (2003) 154–162  157Fig. 1. Scattering intensity,  I(q)  as a function of scattering vector  q  for 50 mM SDS with various amounts of added salt PTHC. The molar ratio of salt tosurfactant is indicated as  x PTHC . When  x PTHC  is above0.3, the peaks in the spectra vanish.The disappearance of the correlation peak by addition of afew millimoles of the salt clearly indicates a decrease of theintermicellar interactions. This is due to the adsorption of the counterion on the surface of the micelles and subsequentneutralization of the charge on the micelles. This surfacecharge decrease also lowers the average area occupied bythe surfactant head group, and closer head group packingmakes rodlike micelles more energetically favorable [25].The SANS spectra at high- q  regions virtually merge for allsalt concentrations,indicatingthat the smallest dimensionof the micelles remains approximately the same for all valuesof   x PTHC .We begin our analysis of the scattering spectra by the IFTmethod. Figure 2 shows the results of IFT calculations with-out considering interparticle interactions, and, as expected,the oscillations of the calculated  p(r)  curves clearly demon-strate that intermicellar interactionscannotbe neglected.For x PTHC  0 . 3 the  p(r)  function goes through a negative min-imum and shows strong oscillations that vanish at very longdistances. The presence of these oscillations is a clear sig-nature of the presence of intermicellar interactions [26]. Thestrength of these oscillations decreases for  x PTHC  =  0 . 4 anda rapid increase in the maximum dimension of the aggre-gates is found for  x PTHC  =  0 . 5 and 0.6. The  p(r)  functionfor  x PTHC  = 0 . 5 and 0.6 resembles that of a mixture of shortrods and long rods with two well-separated distributions.However, because it is unlikely there is a bimodal distrib-ution of micellar sizes, this feature probably arises from thecontribution of weak intermicellar interactions. Fig. 2. Pair distance distribution functions,  p(r) , obtained from the IFTof the SANS spectra without considering any interparticle interactions for50 mM SDS at different values of   x PTHC . Since the IFT evaluations indicated the presence of inter-actions, GIFT evaluations were carried out incorporating asuitable model of the structure factor. There are different ap-proximate closure relations for evaluating the structure fac-tor using the Ornstein–Zernike equation. A commonly usedclosure relation for the analysis of SANS data of micel-lar solutions is the rescaled mean spherical approximation(RMSA) [16,17].Anotherwidelyusedclosure relationis thehypernetted-chain (HNC) approximation, which is accuratewhen long-rangeinteractions are important.A more thermo-  158  P.A. Hassan et al. / Journal of Colloid and Interface Science 257 (2003) 154–162 Fig. 3. One representative pair distance distribution function  p(r)  obtainedfrom GIFT analysis with the interparticle interaction described by theRMSA closure (50 mM SDS, no salt). The solid line is the  p(r)  calculatedby a convolution square of the contrast profile for homogenous mondispersespheres having radius 19.5 Å, while the dashed line is calculated for apolydisperse population of spheres with an average radius of 17.9 Å and15% polydispersity.Table 1Comparison of the results from different closure relations used in the GIFTevaluation of SANS spectra (50 mM SDS, no added salt)Closure relation Radius (Å) Charge ( e )  D max / 2 (Å)RMSA 19 30 21HNC 19 35 21RY 19 32 21 D max  is the point where the  p(r)  function goes to zero. dynamicallyself-consistent closurerelation is that of Rogersand Young (RY) [27], which mixes the different approxima-tions valid for short- and long-range interactions.The  p(r)  functionfor x PTHC  = 0 evaluatedwiththe GIFTmethod when the interactions are taken into account usingthe RMSA procedure is shown in Fig. 3. In the structurefactor calculations only the charge on the micelle and itsdiameter are varied. To check the accuracy of the RMSAprocedure, the results of RMSA calculations are comparedwith those of the HNC and RY closure relations (Table 1).The values are not significantly different for RMSA or RYclosures, so the simpler RMSA closure is used to reducecomputational complexity.The shape of the  p(r)  function in Fig. 3 is similar tothat expected for globular particles. The contrast profile of the scattering objects can be directly assessed from the  p(r) bya deconvolutionprocedure.The p(r)  functionscalculatedusing the convolution square of an assumed contrast profilefor monodisperse or polydisperse homogeneous sphereswere compared with the  p(r)  obtained from GIFT analysis(Fig. 3). The solid line in Fig. 3 indicates the best fit witha monodisperse homogeneous sphere with radius 19.5 Å,whilethedashedlinecorrespondstothefit withpolydisperse Table 2Results from the GIFT analysis of the SANS spectra of samples with  x PTHC 0 to 0.3, using RMSA closure for structure factor x PTHC  r mono  (Å) Mean  r  (Å) Polydispersity (%)  D max / 2 (Å)0 20 18 15 210.1 21 19 15 220.2 22 19 20 240.3 24 17 35 30 r mono  is the optimum radius from the convolution square of contrast profileassuming mondisperse spheres and mean  r  and polydispersity are theparameters assuming polydisperse homogeneous spheres.  D max  is the pointwhere the  p(r)  function goes to zero. homogeneous spheres having a mean radius of 17.9 Åand 15% polydispersity (assuming a Schultz distribution of particlesize). Not surprisingly,the observed p(r)  functionisfitted better with a polydisperse sphere model. For  x PTHC  0 . 3, the shape of the  p(r)  function remains the same asthat shown in Fig. 3 except that the maximum dimensionof the micelles increases slightly. The results of the averagesize and polydispersity and the maximum dimension aresummarized in Table 2.For  x PTHC  = 0 . 4 the shape of the  p(r)  function is similarto that expected for cylindrical particles [26]. In this case p(r)  is similar to that for spheres at small values of   r  butthen shows a pronounced maximum. Beyond the maximumthere is an inflection point, after which the function decayslinearly to zero. The onset of a linear decay in  p(r)  isclear indication of the formation of elongated micelles.A similar profile is observed for  x PTHC  =  0 . 5 with ahigher maximum dimension for the micelles. However, nonumericallystable solutioncouldbe obtainedfromthe GIFTevaluation for  x PTHC  =  0 . 6. This could be because of thefailure of the model used for the structure factor in the caseof such elongated micelles. The results, however, clearlydemonstrate that the micelles are elongated and increase inlength as  x PTHC  increases.For anisotropic micelles, the cross sectional structure canbe directly assessed from an IFT of the spectra at high  q .The cross sectional PDDF,  p c (r) , for a cylindrical particlehaving length  L  much greater than the diameter  d   is relatedto the scattering cross section by the relation [26](7) dΣ(q)dΩ = 2 π 2 Lq ∞   0 p c (r)J  0 (qr)dr, where  J  0  is the zeroth order Bessel function. Since the crosssectional PDDF is calculated from the high- q  region of thespectra, the effect of interparticle interaction is negligible,and the low- q  portion of the spectra is not included inthe analysis. For cylindrical particles, the low- q  cut-off isdetermined from a Guinier plot of ln (I(q)q)  vs.  q 2 becausethe portionof the scattering curvecorrespondingto the crosssection leads to a linear decay on a Guinier plot (see Fig. 4inset).Figure 4 shows the  p c (r)  function for  x PTHC  =  0 . 6and the corresponding fits with monodisperse and poly-
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