JOURNAL OF COLLOID AND INTERFACE SCIENCE 201, 223–232 (1998)ARTICLE NO. CS985418

The Structure of the Mixed Nonionic Surfactant Monolayerof Monododecyl Triethylene Glycol and Monododecyl

Octaethylene Glycol at the Air–Water Interface

J. Penfold,* ,1 E. Staples,† I. Tucker,† and R. K. Thomas‡

*ISIS Facility, Rutherford Appleton Laboratory, CLRC, Chilton, Didcot, Oxon; †Unilever Research, Port Sunlight Laboratory, Quarry Road East,Bebington, Wirral; and ‡Physical and Theoretical Chemistry, Oxford University, South Parks Road, Oxford, United Kingdom

Received October 3, 1997; accepted January 6, 1998

adsorption at the interface, and more recently Nikas et al.The structure of the mixed nonionic surfactant monolayer of (6) have used a more fundamental approach, aimed at re-

monododecyl triethylene glycol and monododecyl octaethylene moving the phenomenological nature of the regular solutionglycol adsorbed at the air–water interface has been determined

approach.using specular neutron reflection. Using partial isotopic labeling

To understand the surface chemical properties in mixtures(deuterium/hydrogen) of the alkyl and ethylene oxide chains ofit is important to know the interfacial composition, whicheach type of molecule, the distribution and relative positions ofwill in general be different from the bulk composition, andthose labeled fragments have been obtained. The frustrationto know the factors which control that composition over acaused by the packing of the triethylene and octaethylene glycol

headgroups results in a change of the surfactant structure com- wide range of solution compositions and concentrations. Wepared to the pure monolayer of either surfactant. Compared to have shown previously that neutron reflectivity, in combina-the pure monolayer the alkyl chain distributions of both surfac- tion with hydrogen/deuterium (H/D) isotopic substitution,tants are more extended, the triethylene glycol group is less hy- has the selectivity needed to study the adsorption of mixeddrated, and the octaethylene glycol group is less extended and surfactants over a wide concentration range (7–11), and formore hydrated. q 1998 Academic Press

giving detailed structural information (12). This providesKey Words: nonionic surfactants; adsorption; air–water inter-the opportunity to correlate structure and composition at aface; surfactant mixtures; monolayer structure.level not so readily available in techniques such as surfacetension measurement, ellipsometry, sum frequency spectros-copy, and second harmonic generation. Of these other tech-INTRODUCTIONniques, sum frequency spectroscopy in particular has been

The study of surfactant mixtures is of considerable interest used to study the adsorption of mixed surfactants at inter-for both practical and theoretical reasons (1) . Surfactant faces (see for example Ref. 13), providing information onmixtures are used in many industrial, technological, and do- packing density and orientational order in such mixed mono-mestic applications. This is because mixtures can enhance layers.performance or synergy or because commercial surfactants We have chosen here to describe an investigation of aare impure; that is, they contain mixtures of different alkyl nonionic mixture of monododecyl triethylene glycol,chain lengths, different isomers, and in the case of nonionic C12EO3, and monododecyl octaethylene glycol, C12EO8,surfactants different ethylene oxide chain lengths. Adsorp- which mix ideally and have identical alkyl chain lengths buttion and micellization in mixed surfactants have been exten- different ethylene oxide chain lengths. Rosen et al. (2) , insively studied using a range of techniques, such as surface their pioneering surface tension work, extensively studiedtension (2), and have been the result of considerable theoret- the adsorption of C12EO3/C12EO8 mixtures at the air–waterical effort (3–6). The ideal solution approach (3) and the interface. In an earlier and related study (10) we used neu-subsequent adaptation of regular solution theory (4) , where tron reflectivity to determine the composition of C12EO3/the departure from ideality is characterized by an interaction C12EO8 mixtures at the air–water interface from below toparameter, b, have formed the basis of the theoretical de- Ç100 times the critical micellar concentration (cmc) of thescription of the solution behavior in mixed surfactants. This mixture and obtained good agreement with the work of Ro-approach was adapted by Holland (5) and others (2) for sen (2). This work provided the first experimental verifica-

tion of the abrupt change in surface composition at the cmc1 To whom correspondence should be addressed. due to the changes in distribution of the two surfactant spe-

223 0021-9797/98 $25.00Copyright q 1998 by Academic Press

All rights of reproduction in any form reserved.

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224 PENFOLD ET AL.

FIG. 1. Neutron reflectivity for 5 1 1005 M 30 mol% C12EO3/70 mol% C12EO8 (a) in nrw, (n) dC12hEO3/dC12hEO8, (l) dC12hEO3/hC12hEO8,(s) hC12hEO3/dC12hEO8; and (b) in D2O (n) hC12hEO3/hC12hEO8, (l) dC12hEO3/hC12hEO8, (s) hC12hEO3/dC12hEO8.

cies between the bulk solution and the monolayer as a result where r(Q) is the one-dimensional Fourier transform ofr(z) , the average scattering length density profile in theof the onset of mixed micelle formation at the cmc as pre-

dicted by Nikas et al. (6) and the regular solution theory direction normal to the interface,(4, 5) . The calculations of Nikas et al. (6) were for aC12EO6/C12EO8 mixture, and our results for C12EO3/C12EO8

r(Q) Å *`

0`

r(z)exp( iQz)dz [2]are in qualitative agreement.We describe here the determination of the structure of the

monolayer of a 30 mol% C12EO3/70 mol% C12EO8 nonionic andsurfactant mixture at the air–water interface, at a surfactantconcentration of 51 1005 M. Two different isotopic labeling

r(z) Å ∑i

Ni (z)bi . [3]schemes have been used. Simple labeling of the alkyl chainsand a more complex scheme in which the alkyl chain ispartially labeled and the ethylene oxide chains are labeled The neutron refractive index, n , is defined asprovide detailed descriptions of the mixed monolayer, whichare compared with previous neutron reflectivity measure- n Å (1 0 l 2 ∑

i

Nibi /2p) , [4]ments of the structure of the monolayer of the individualnonionic molecules, C12EO3 and C12EO8.

where Ni is the number density of species i , and bi is itsNEUTRON REFLECTIVITY scattering length.

In the context of using neutron reflectivity to study surfac-Specular neutron reflection provides information about tant adsorption at interfaces the key feature is that the neu-

homogeneities normal to an interface or surface, and its tron scattering properties of H and D are markedly different,theory is described in detail elsewhere (14). The basis of a and hence H /D isotopic substitution can be used to manipu-neutron reflection measurement is that the variation in specu- late the neutron refractive index profile at an interface. Thislar reflection with Q ( the wave vector transfer defined as Q is particularly important in determining structure or investi-Å 4p sin u /l, where u is the glancing angle of incidence gating surfactant mixtures, whereby selective deuteration ofand l is the neutron wavelength) , is simply related to the particular components or fragments can be highlighted orcomposition or concentration profile in the direction normal isolated. This enables adsorbed amounts in complex mix-to the interface. In the kinematic approximation (15) the tures and detailed surface structure to be determined. It isspecular reflectivity, R(Q) , is given by this selectivity which makes the neutron reflectivity method

so powerful. It has been exploited in determining surfactantstructure at the air–water interface in unprecedented detailR(Q) Å 16p 2

Q 2 Ér(Q)É2 , [1](16, 17) and in measuring the composition of mixed surfac-

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225STRUCTURE OF MIXED NONIONIC SURFACTANT MONOLAYER

tant systems adsorbed at the air–water interface. In de- TABLE 2Scattering Lengths and Volumes of the Labeled Surfactanttermining structure and adsorbed amounts in mixtures the

Fragments Used in this Studymethod relies upon the ability to combine reflectivity profilesfrom solutions of the same chemical but of different isotopic

Scattering length Volumecompositions. This, of course, then assumes that there is no(1 1005 A) (A3)

isotopic dependence of the structure or adsorbed amount,and this has now been well established for a range of systems D2O 19.2 30.0

H2O 01.7 30.0(18), including that reported here.C12H24 01.4 350The reflectivity profiles can be analyzed by two differentC12D24 127.0 350but related methods. In the first method, a structural modelC6H13 08.7 149

is assumed for the interface and the reflectivity is calculated C6D13 125.9 149using the optical matrix method (19, 20). To obtain the (OCH2CH2)8OH 35.2 505

(OCD2CD2)8OH 368.5 505structure of the adsorbed layer the model is then optimized(OCH2CH2)3OH 14.5 190to fit simultaneously the reflectivity profiles for the different(OCD2CD2)3OH 139.5 190isotopically labeled combinations. Alternatively, a more di-

rect method based on the kinematic approximation (15) (seeEq. [1]) is used to analyze the reflectivity profiles for the

r Å b1

A1t/ b2

A2t, [6]different isotopically labeled combinations.

At the simplest level, neutron reflectivity can be usedto determine adsorbed amounts in single-component and in

where b1 and A1 are the scattering lengths and area/moleculemulticomponent mixtures straightforwardly, with good accu-of each component, respectively, and r and t are the scatteringracy, and over a wide concentration range (for concentra-length densities and thicknesses from the model fits to thetions below and well in excess of the cmc) at the air–waterdata. This approach has now been successfully demonstratedinterface. For a deuterated surfactant in null reflecting wateron a range of different mixed surfactant systems (7–11).(nrw; water with a refractive index of unity, a 92 mol%

In this paper we will use the more direct method (15),H2O/8 mol% D2O mixture) , the reflectivity arises only forbased on the kinematic approximation, to analyze the re-the layer of deuterated surfactant at the interface. The re-flectivity data for the C12EO3/C12EO8 mixture in terms offlectivity can then be analyzed by assuming that it arisesthe structure of the mixed monolayer at the air–water inter-from a single layer of uniform or homogeneous compositionface. This approach has been used extensively to determine(19). Using the optical matrix method (20, 21), this yieldsthe structure of the adsorbed surfactant layer at the air–a scattering length density and thickness of the layerwater interface. It has been used recently, for example, todetermine the structure of the cationic surfactant, hexadecyl-

r Å btA , [5] trimethyl ammonium bromide (C16TAB) monolayer to a res-

olution of two methylene groups (17), and to determine thestructure of the mixed surfactant monolayers of the nonionicwhere b and A are the scattering length and area/moleculesurfactant hexaethylene glycol monododecyl ether, C12E6of the surfactant at the interface, and r and t are the scatter-and C16TAB (12). Considering the mixed monolayer ofing length density and thickness from the model fit to theC12EO3/C12EO8, in terms of the contributions from the dif-data. A similar approach is used in the case of mixtures,ferent components, we can separately label the two nonionicwhere each component is selectively deuterated in turn andsurfactants and the solvent. The scattering length densitythe resultant reflectivity is treated as arising for a single layerprofile can be written asof homogeneous composition (19). For a binary mixture

this gives (21)r(z) Å bC12(8)nC12(8) (z) / bC12(3)nC12(3) (z) / bsns (z) , [7]

TABLE 1where nc12(3) , nc12(8) , and ns refer to the C12EO3 and C12EO8Area/Molecule for C12EO8 and C12EO3 in 5 1 1005 Msurfactants and the solvent. From Eqs. [1] and [7] we have30 mol%/70 mol% Mixture of C12EO3/C12EO8

Area/molecule Area/moleculefor C12EO3 (A2) for C12EO8 (A2) R(Q) Å 16p 2

Q 2 [b 2C12(8)hC12(8) / b 2

c12(3)hc12(3)

This study (full labeling) 69 { 2 117 { 10/ b 2

s hss / 2bC12(8)bC12(3)hC12(8)C12(3)This study (chain only labeling) 72 { 2 123 { 10From Ref. (10) 68 { 2 130 { 10 / 2bsbC12(8)hsC12(8) / 2bsbC12(3)hsC12(3) ] , [8]

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226 PENFOLD ET AL.

FIG. 2. Partial structure factors for 5 1 1005 M 30/70 C12EO3/C12EO8, (a) (l) Q 2hC12(3) , (s) Q 2hC12(8) ; (b) Q 2hss ; and (c) (l) Q 2hC12(8)s , (s)Q 2hC12(3)s . The solid lines are calculated curves, as described in the text and for the parameters in Table 3.

where hii are the self-partial-structure factors given by A01 in a single measurement at a fixed angle of incidenceof 1.57 using a neutron wavelength band of 0.5 to 6.8 A,where the different neutron wavelengths are distinguishedhii(Q) Å ÉnP i (Q)É2 [9]by ‘‘time of flight.’’ The data were normalized for the inci-dent beam spectral distribution and detector efficiency andand hij are the cross-partial-structure factors given byestablished on an absolute reflectivity scale by reference tothe reflectivity from the surface of pure D2O, using standardhij(Q) Å Re{nP i (Q)nP j(Q)}. [10]procedures (23). A flat background, determined by extrapo-lation to high values of Q (Q Ç 0.3 to 0.5 A01) , was sub-n i (Q) is the one-dimensional Fourier transform of ni (z) .tracted from all the measured reflectivity profiles. This hasThe self-partial-structure factors relate to the distributionsbeen shown to be a valid procedure (24) providing that thereof the individual components, whereas the cross-partial-is no small-angle scattering from the bulk solution, whichstructure factors relate to the relative positions of the differ-is the case for the measurements described here.ent components at the interface. We have shown elsewhere

High-purity water was used for all the measurements(16) that simple analytic functions describe these partial(Elga ultrapure) and the D2O was obtained from Fluoro-structure factors.chem. All glassware and the Teflon troughs used for thereflectivity measurements were cleaned using alkaline deter-EXPERIMENTAL DETAILSgent (Decon 90) followed by copious washing in ultrapure

The specular neutron reflection measurements were made water. All the measurements were performed at 298 K, com-on the SURF reflectometer (22) at the ISIS pulsed neutron pared to a cloud point of 348 K for C12EO8 (25).source, using the ‘‘white beam time of flight’’ method. That All the isotopes of C12EO3 and C12EO8 were synthesized at

the laboratory of R. K. Thomas’ group at Oxford, and theis, the measurements were made in the Q range 0.048 to 0.5

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227STRUCTURE OF MIXED NONIONIC SURFACTANT MONOLAYER

TABLE 3Results from Partial Structure Factor Analysis of C12EO3/C12EO8, C12EO8, and C12EO3 Surface Layers

5 1 1005 M 30/70 5 1 1005 M 30/70C12EO3/C12EO8 C12EO3/C12EO8 1.8 1 1004 Ma 5.5 1 1005 Mb

(Full labeling) (Chain only) C12EO8 C12E3

sC12(3) — 19.5 — 16.5sC6(3) 15.0 — — 14.0sC12(8) — 18.0 15.0 —sC6(8) 15.0 — — —se(3) 15.0 — — 15.5se(8) 15.0 — 19.0 —ts 7.5 8.0 9.0 6.0dC6(3)s 16.0 — — 13.0dC6(8)s 16.0 — — —dC12(3)s — 11.0 — 10.0dC12(8)s — 11.0 11.0 —de(3)s 3.5 — — 2.5de(8)s 01.0 — 2.5 —dC12(3)C12(8) — 0.0 — —dC6(3)e(8) 16.0 — — —dC6(8)e(3) 12.0 — — —dC6(3)e(3) — — — 10.5dC6(3)C6(8) 1.0 — — —de(3)e(8) 6.0 — — —dC12(3)e(3) — — — 8.0dC12(8)e(8) — — 10.5 —

a From Ref. (31).b From Ref. (32).

details of the preparation, purification, and characterization are were deuterium-labeled. Six combinations were then measured:dC12hEO3/hC12hEO8 in nrw and D2O, hC12hEO3/dC12hEO8 ingiven elsewhere (18, 21, 26). The structure of the mixed mono-

layer of C12EO3 and C12EO8 at the air–water interface was nrw and D2O, dC12hEO3/dC12hEO8 in nrw, and hC12hEO3/hC12hEO8 in D2O. More detailed structural information wasmeasured at a surfactant concentration of 5 1 1005 M (at the

cmc) and a solution composition of 30 mol% C12EO3/70 mol% obtained using a more complex labeling scheme, where theethylene oxide group and the outer C6 of the alkyl chain (theC12EO8. The structure was measured using two different label-

ing schemes. In the first instance a simple labeling scheme was C6 furthest from the head group) of each surfactant were deute-rium-labeled. A total of 15 isotopic measurements were thenused; the alkyl chain of the two surfactants and the solventmade: hC12dEO3/hC12dEO8 in nrw, hC12hEO3/hC12hEO8 inD2O, hC12dEO3/hC12hEO8 in nrw and D2O, dC6hC6hEO3/dC6hC6hEO8 in nrw, dC6hC6hEO3/hC12dEO8 in nrw and D2O,hC12dEO3/dC6hC6hEO8 in nrw and D2O, dC6hC6hEO3/hC12hEO8 in nrw and D2O, hC12hEO3/dC6hC6hEO8 in nrw andD2O, and hC12hEO3/hC12dEO8 in nrw and D2O. The abbrevia-tions used here and throughout are as follows: CD3(CD2)11

(OCH2CH2)3OH, dC12hEO3; CD3(CD2)5(CH2)6(OCH2CH2)3OH,dC6hC6hEO3; CH3(CH2)11(OCD2CD2)3OH, hC12dEO3; CH3(CH2)11

(OCH2CH2)3OH, hC12hEO3; and similarly for C12EO8.

RESULT AND DISCUSSION

Figure 1 shows the measured reflectivity profiles for 30mol% C12EO3/70 mol% C12EO8 at a concentration of 5 11005 M for the simple labeling scheme, where only the alkylFIG. 3. Volume fraction profiles for 5 1 1005 M 30/70 C12EO3/chains of the two nonionic surfactants and the solvent haveC12EO8, showing the solvent (---) and alkyl chain ( —) distributions for

C12EO3 and C12EO8. been deuterium-labeled. In Fig. 1a the reflectivity profiles for

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228 PENFOLD ET AL.

FIG. 4. Partial structure factors for 5 1 1005 M 30/70 C12EO3/C12EO8, (a) (l) Q 2hC6(3)s , (s) Q 2hC6(8)s ; (b) Q 2hC6(3)e8 ; (c) Q 2he3e8 ; (d) Q 2hC6(8)e3 ;(e) Q 2he3s ; and (f ) Q 2he8s . The solid lines are calculated curves, as described in the text and for the parameters in Table 3.

the isotopic combination of dC12hEO3/hC12hEO8, hC12hEO3/ and the extent of the adsorbed layer; the significant variationin reflectivity between the different isotopic combinations isdC12hEO8, and dC12hEO3/dC12hEO8 in nrw are plotted. The

data presented are all background-subtracted, using the proce- an indication of the sensitivity of the technique. The gradientsof the profiles are a measure of the thickness of the adsorbeddure described previously (24). These profiles on their own

principally provide information about the adsorbed amount layer and suggest that both alkyl chains are of similar thick-

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229STRUCTURE OF MIXED NONIONIC SURFACTANT MONOLAYER

where Rf and RQ are the exact and kinematic reflectivity fora perfectly smooth interface between two bulk phases, Qc isthe critical value of the wave vector for that interface, andR and Robs are the corrected and the measured reflectivities.The general factors determining the errors in the derivationof the partial structure factors have been discussed in detailelsewhere (27), and the procedures for minimizing thosesystematic errors have been adopted here. The partial struc-ture factors obtained for the reflectivities shown in Fig. 1are illustrated in Fig. 2. The self-terms (see Figs. 2a and 2b)provide information about the distributions of the individualcomponents, whereas the cross-terms (see Fig. 2c) provideinformation about the relative positions of the individualcomponents, as described earlier. The surfactant self-termsare described as a Gaussian distribution, and the solvent as

FIG. 5. Volume fraction profiles for 5 1 1005 M 30/70 C12EO3/ a tanh distribution,C12EO8, showing the solvent (---) , alkyl chain (outer C6 only) , and ethyl-ene oxide chains of C12EO3 and C12EO8.

ni (z) Å niexp(04z 2 /s 2i ) [12]

ness, whereas the adsorbed amount is determined mainly by ns (z) Å nsoF12/ 1

2tanS z

z DG . [13]the absolute level of the reflectivity. From these data we canextract the adsorbed amounts of each component and hence

The partial structure factor for a Gaussian distributionthe surface composition, using Eq. [6]. The area/molecule forgiven by Eq. [12] is thenthe C12EO3 and C12EO8 and hence the surface composition are

in good agreement with our previously published results (seeTable 1). The reflectivity profiles for the isotopic combinations hii(Q) Å 1

A 2i

exp(0Q 2s 2i /8) , [14]

of dC12hEO3/hC12hEO8, hC12hEO3/dC12hEO8, and hC12hEO3/hC12hEO8 in D2O provide information about the solvent distri-

where Ai is the area/molecule of component i and si is itsbution and the degree of overlap of the solvent and the surfac-width. The solid lines in Fig. 2a are calculated using Eq.tant distributions; see Fig. 1b. These profiles are all lower than

that for pure D2O at higher Q (extrapolating toward the valueof pure D2O value at the lower Q values), which is consistentwith the surface layer having a neutron refractive index lowerthan that of D2O. This is indicative of the overlap betweenthe solvent and the surfactant distributions at the interface.

A more detailed structural analysis of this reflectivity datais made using the kinematic approximation and extractingthe six partial structure factors described in Eq. [8] . Theseare obtained by a least-squares solution of the six simultane-ous equations (Eq. [8]) which describe the reflectivity forthe six different isotopically labeled combinations, using thescattering lengths for the different components listed in Ta-ble 2. The reflectivity given by the kinematic approximationin Eq. [1] is approximate and without correction would leadto errors in the partial structure factors. This has been dis-cussed elsewhere by Lu et al. (16), where it was shownthat the expression derived by Crowley (15) can be used toconvert the experimental data to a more exact expression.The correction, applied to the measurements with a D2Osubphase, is

FIG. 6. Volume fraction profiles for (a) 5.5 1 1005 M C12EO3; andR Å RQ /

(Robs 0 Rf)(1 0 Rf) F1 0 (1 0 Q 2

c /Q 2)2 G2

, [11] (b) C12EO3 in 5 1 1005 M 30/70 C12EO3/C12EO8; solvent (---) , alkylchain ( —), and outer C6 of the alkyl chain (rrr) .

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230 PENFOLD ET AL.

[14] for the parameters listed in Table 3. Previous measure- is largest on the surfactant–surfactant cross-term (dc12(3)c12(8))and is responsible for a larger uncertainty in that term.ments using more detailed labeling schemes (16) and com-

The most convenient way of representing the structureputer simulation (28, 29) provide strong justification for theobtained from the partial structure factor analysis is as ause of Gaussian distributions. The solid line in Fig. 2b is anumber density or volume fraction distribution. The volumecalculation for a tanh solvent profile, where nso is constrainedfraction distributions shown in Fig. 3 are for the results ofto be the bulk solvent number density, and the width of thethe simple labeling scheme described in Figs. 1 and 2, anddistribution, z , is listed in Table 3.in Table 3. From this simple labeling scheme we see inThe cross-partial-structure factors provide informationFig. 3 that the alkyl chain distribution of the two nonionicabout the relative positions of the individual components,components entirely overlap and that there is, consistent withwhich are to good approximation independent of any modelother measurements (30, 31), considerable overlap betweenassumptions (23). It has been shown (24) that if nc12(3) (z)the alkyl chain and the solvent distributions. Included forand nc12(8) (z) are exactly even distributions about their cen-comparisons are the previously measured structural parame-ters and ns (z) is an odd distribution, then the followingters for 5.5 1 1005 C12EO3 at an area/molecule of 37 A2

results hold,(30), and for 1.8 1 1004 M C12EO8, measured at an area/molecule of 60 A2 (31). In the mixed monolayer measured

hC12(3)C12(8)Å{(hC12(3)C12(3)hC12(8)C12(8) )1/2cos QdC12(3)C12(8) here the area/molecule of the two components is much

larger, 69 A2 for C12EO3 and 117 A2 for C12EO8; however,[15]the mean area/surfactant molecule in the mixed monolayer

hC12(i )SÅ{(hc12(i )c12(i )hss )1/2sin QdC12(i )S , [16] is Ç44 A2 . The widths of the alkyl chains for both C12EO8

and C12EO3 are systematically larger than the previous mea-surements for pure C12EO8 and C12EO3 monolayers. This iswhere dij is the distance between the centers of the twoin part due to the overestimate of the width, arising fromdistributions. Whereas the widths of the individual distribu-the contribution from the ethylene oxide groups that havetions have contributions from structural and capillary wavenot been separated out here, but were in the previous pureroughness, the separations have been shown (30) to be inde-monolayer measurements. Even accounting for this differ-pendent of roughness. The cross-partial-structure factors, de-ence, the widths in the mixed monolayer are larger. Whetherscribing the relative positions of the surfactant and solventthis is due to increased roughness or conformational changes,distributions at the interface, shown in Fig. 2c, have beenthe simple labeling scheme used so far does not enable usanalyzed using Eqs. [15] and [16]; the solid lines in Fig.to make the distinction. Furthermore the most interesting2c represent fits using the parameters listed in Table 3. Thechanges in structure may well be expected to be associatedthree separations are not independent, andwith the packing of EO3 and EO8 chains, and no informationis available from the simple labeling on their conformations.

dC12(3)C12(8) Å dC12(3)S 0 dC12(8)S . [17]To address these deficiencies further measurements were

made with a more detailed labeling scheme, where the ethyl-The values obtained for these separations (see Table 3) are ene oxide groups were additionally labeled. To provide

consistent with this additional constraint. In this simple label- greater sensitivity to changes in the conformation of theing scheme we have ignored the small but finite contribution alkyl chains the outer C6 of the alkyl chain was labeled,for the hEO3 and hEO8 groups. The error introduced in this rather than the whole alkyl chain (similar measurementsway has been considered elsewhere (27). It results in an were previously made for C12EO3 alone (30)) . There areerror of °1 A in the definition of the alkyl chain widths. Its now five labeled components in the mixed monolayer, givingcontribution to the surfactant–solvent cross-terms measured rise to a minimum of 15 reflectivities (see Experimentalhere has been calculated to be negligible. However, its effect Details) and 15 partial structure factors,

R(Q) Å 16P 2

Q 2

b 2C6(3)hC6(3) / b 2

C6(8)hC6(8) / b 2s hss / b 2

eo3heo3 / b 2eo8heo8

/ 2bC6(3)bC6(8)hC6(3)C6(8) / 2beo3bC6(8)heo3c6(8) / 2beo8bc6(3)heo8c6(3)

/ 2bC6(3)bshC6(3)s / 2bC6(8)bshC6(8)s / 2beo3bsheo3s / 2beo8bsheo8s

/ 2beo3beo8heo3eo8 / 2bC6(3)beo3hC6(3)eo3 / 2bC6(8)beo8hC6(8)eo8

. [18]

The labeled combinations measured were not the ideal errors associated with these cross-terms are correspondinglylarge, they have been excluded from the analysis and discus-combination and, for example, provide only a very indirect

estimate of the cross-terms hC6(8)eo8 and hC6(3)eo3 . Because the sion. Seven of the partial structure factors obtained (the

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231STRUCTURE OF MIXED NONIONIC SURFACTANT MONOLAYER

cross-terms that were most directly measured) are shown in consistent with observations made for the C12EO3/C12EO8

mixture.Fig. 4, and the parameters obtained from the analysis, usingthe approach described earlier for the simpler labelingscheme, are summarized in Table 3. The volume fraction SUMMARYdistributions for the differently labeled components areshown in Fig. 5. The results from the more complex labeling We have used partial labeling of the alkyl chain and label-scheme provide a more detailed picture of the structure of ing of the ethylene oxide chains in the mixed nonionic mono-the mixed monolayer and enable a direct comparison with layer of C12EO3/C12EO8 to determine the changes in thethe earlier results for a pure C12EO3 monolayer (30), where structure of each component surfactant compared to thata similar labeling scheme for the C12EO3 was used. Upon found in the pure surfactant monolayers. Constraints im-closer inspection of the parameters in Table 3 the results posed by packing the EO3 and EO8 chains together havefrom the more detailed labeling are broadly consistent with resulted in a change in the conformation of the EO8 chain,those from the simple labeling scheme. There is, however, which has in turn altered the solvent distribution and resulteda marked difference in the structure of the individual compo- in a change in the conformation of the alkyl chain of bothnents in the mixed monolayer compared to the structure of molecules. Having demonstrated that the reflectivity fromthe pure monolayers ( the results for C12EO8 (31) and C12EO3 such detailed labeling is measurable in a mixed monolayer,(32) alone are included for comparison in Table 3). it will now be interesting to monitor the changes observed

here as a function of composition and concentration in orderThe situation is different from that observed in an earlierto determine the conditions under which the dissimilarity instudy for C12EO6/C16TAB, where it was concluded that theEO chain lengths influences the structure of the monolayer.structure was simply dominated by the mean packing at theSimilar measurements with identical EO chain lengths butinterface. It seems that the frustration caused by trying todifferent alkyl chain lengths are also envisaged.pack the EO8 and EO3 chains does result in a real disruption

or alteration to the structure of the two surfactants in themixed monolayer, compared to that in the pure monolayers.In the mixed monolayer the outer C6 chain–solvent separa- REFERENCEStion of the C12EO3 increases, whereas the whole alkyl chain–

1. Scamehorn, J. F., (Ed.) , ‘‘Phenomena in Mixed Surfactant Systems,’’solvent separation is similar to that of the pure componentACS Symp. Ser. 311, Am. Chem. Soc., Washington, DC, 1986.monolayer. This suggests that the alkyl chain conformation

2. Rosen, M. J., and Hua, X. Y., J. Colloid Interface Sci. 86, 164 (1982).has changed; that is, the chain has become more extended. 3. Clint, J. H., J. Chem. Soc. Faraday Trans. 1 71, 1327 (1975).This is illustrated in Fig. 6, where the C6 and C12 volume 4. Rubingh, D. N., in ‘‘Solution Chemistry of Surfactants’’ (K. L. Mittal,

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Lu, J. R., Langmuir 9, 1651 (1993).ing of the pure monolayer of C12EO8 are not available.8. Staples, E., Thompson, L., Tucker, I., and Penfold, J., Langmuir 10,For C12EO3, the C6 alkyl chain and EO3 chain dimensions

4136 (1994).in the mixed monolayer are similar to those found in the pure 9. Penfold, J., Staples, E., Thompson, L., Tucker, I., Thomas, R. K., andmonolayer. In contrast, the EO8 distribution is noticeably Lu, J. R., Langmuir 11, 2496 (1995).

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12. Penfold, J., Staples, E., Cummins, P., Tucker, I., Thomas, R. K., Simis-The width of solvent distribution in the mixed monolayer ister, E. A., and Lu, J. R., J. Chem. Soc. Faraday Trans. 92, 1549 (1996).larger than that observed for a pure C12EO3 monolayer, but

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15. Crowley, T. L., Lee, E. M., Simister, E. A., and Thomas, R. K., Physicain the conformation of the ethylene oxide chain for C12EO6B 173, 143 (1991).and C12EO8, due to changes in solvent quality as a result of

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232 PENFOLD ET AL.

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