Journal of Colloid and Interface Science 247, 397–403 (2002)doi:10.1006/jcis.2001.8041, available online at http://www.idealibrary.com on

Comparison of the Coadsorption of Benzyl Alcohol and Phenyl Ethanolwith the Cationic Surfactant, Hexadecyl Trimethyl Ammonium Bromide,

at the Air–Water Interface

J. Penfold,∗,1 E. Staples,† I. Tucker,† L. Soubiran,† and R. K. Thomas‡∗ISIS Facility, CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K.; †Port Sunlight Laboratory, Unilever Research, Quarry Road East,

Bebington, Wirral, U.K.; and ‡Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, U.K.

Received January 9, 2001; accepted October 15, 2001

A comparison of the coadsorption of benzyl alcohol and phenylethanol with the cationic surfactant, hexadecyl trimethyl ammo-nium bromide, C16TAB, at the air–water interface is made usingthe specular reflection of neutrons. The phenyl ethanol is more sur-face active than the benzyl alcohol, and competes more effectivelywith the C16TAB for the interface. The structure of the C16TABcomponent in the mixed monolayer is compared with the structureof the pure C16TAB monolayer at an equivalent area per molecule.The addition of the aromatic alcohol subtly alters the conforma-tion of the C16TAB and draws it closer to the aqueous subphase.The center of the alcohol distribution is located in the interfaceadjacent to the C6 group of the C16TAB alkyl chain closest to theheadgroup. Compared to the benzyl alcohol, the more hydrophobicphenyl ethanol is slightly farther away from the headgroup, andhas a greater impact on the conformation of the alkyl chain of theC16TAB. C© 2002 Elsevier Science (USA)

Key Words: mixed surfactants; coadsorption; interfaces.

INTRODUCTION

Understanding the impact of the solubilization of cosurfac-tants, alcohols, or alkanes on surfactant and mixed surfactantsystems is important in the context of many of the applicationsof surfactants, in detergency, cosmetics, oil recovery, and pes-ticides. Cosurfactants result in a decrease in interfacial tension,an increase in monolayer fluidity, and an alteration of the spon-taneous curvature of the monolayer. These effects are importantand can have widespread implications.

The addition of a cosurfactant or alcohol can have a significanteffect on the phase behavior of lamellar phase dispersions (1).The addition of cosurfactants, such as model perfume moleculeslike benzyl alcohol or limonene, results in a reduction in the Lα

to Lβ phase transition in dichain cationic surfactant lamellarphases (1). The increased flexibility of such mixed membranesresults in a different response to external stimuli, such as shear(2); this is of significance in terms of formulation and processing.

1 To whom correspondence should be addressed.

397

The molecular origins of the changes in phase behavior, and inmechanical properties, are not well understood. The effect of acosurfactant on surfactant mesophases can be rationalized withsimple geometric arguments, based on the area per moleculeand the relative dimensions of the headgroup and alkyl chain(3). Hence, the location of the cosurfactant within the interfacialregion and its impact upon the structure of the monolayer are ofimportance.

The interaction of synthetic perfumes (such as benzyl acetateand limonene) and of model perfumes (such as phenyl ethanol)with surfactants is important in the context of the widespreadapplications in detergency, fabric conditioners, and cosmetics.The bulk properties of such mixtures and how perfumes inter-act with surfactants in aqueous solution are important, and suchstudies have been undertaken (4). However, an important aspectof product performance in such systems is the delivery of per-fume to the surface or interface, and an understanding of thesurface or interfacial properties is of vital importance. The ad-sorption properties are crucially linked to the structure of themixed adsorbed layer, and to the location of the cosurfactant inthe monolayer.

The effects of perfume-like molecules on bulk phase behaviorand on interfacial characteristics provide the motivation for thisstudy.

We have previously shown that specular neutron reflection, incombination with D/H isotopic substitution, is a powerful tech-nique for the study of the composition and structure of mixedsurfactant monolayers adsorbed at the air–water interface andother interfaces (5–13). This has now been extensively demon-strated on a range of different mixed surfactant systems (5–11),in surfactant/alcohol mixtures (12) and surfactant/alkane mix-tures (13).

More directly related to this study was the work of Bain et al.(14), who used both neutron reflectometry and optical tech-niques to study the structure of hexadecyl trimethyl ammonium,C16TA, tosylate monolayers. Using partial D/H labelling of theC16TA and tosylate ions, they were able to show that the para-and ortho-tosylate counterions behaved differently. The para-tosylate is located partially in the hydrophobic chain region,

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whereas the ortho-tosylate behaves more like a conventionalcounterion. This difference sheds light on the role of the para-tosylate to invoke micellar growth in C16TA tosylate solutions,as discussed by Bain et al. (14). Following the same approachwe have recently investigated the effect of benzyl alcohol onthe structure of the hexadecyl trimethyl ammonium bromide,C16TAB, monolayer (15). The addition of benzyl alcohol doesnot significantly alter the conformation of the C16TAB, but con-sistent with the observations for other mixtures, sodium dodecylsulphate, (SDS)/dodecanol (11) and C16TAB/monododecyl hex-aethylene glycol, C12E6 (7), the C16TAB alkyl chain is closer tothe aqueous subphase in the mixed monolayer. The distributionof benzyl alcohol molecules in the interface is found to be cen-tered on about the third methylene group from the headgroupof the C16TAB, similar to that observed for the para-tosylatecounterion.

In this paper we report an extension of an initial studyon the structure of the C16TAB/benzyl alcohol mixed mono-layer. The benzyl alcohol has been replaced by the more hy-drophobic phenyl ethanol; it is expected that the phenyl ethanolwill have a greater effect on the C16TAB monolayer. Neu-tron reflectivity measurements of both the composition andthe structure of the mixed C16TAB/phenyl ethanol monolayerwill be described, and the results compared with those previ-ously obtained for the C16TAB/benzyl alcohol mixed mono-layer.

EXPERIMENTAL DETAILS

Specular neutron reflection provides information about in-homogeneities normal to an interface or surface, and has beendescribed in detail elsewhere (16). The basis of a neutron re-flection experiment is that the variation of specular reflectionwith Q (the wave-vector transfer perpendicular to the surfaceand defined as Q = 4π sin θ/λ , where θ is the glancing angleof incidence, and λ the neutron wavelength) is simply related tothe composition or concentration profile normal to the surface.In the kinematic approximation (16) the specular reflectivity,R(Q), is given by

R(Q) = 16π2

Q2|ρ(Q)|2, [1]

where ρ(Q) is the one-dimensional Fourier transform of ρ(z),the average scattering length density profile in the direction nor-mal to the surface,

ρ(Q) =∞∫

−∞ρ(z) exp(i Qz) dz [2]

and

∑

ρ(z) =

i

ni (z)bi , [3]

ET AL.

where ni (z) is the number density profile of species i , and bi itsneutron scattering length.

H and D have vastly different neutron scattering lengths, andD/H isotopic substitution can be used to manipulate ρ(z); thisis the essence of the use of the technique to determine adsorbedamounts in mixtures and to determine surface structure to a res-olution ∼2 A. At the simplest level D/H isotopic substitution isused to highlight the surface layer and enables the determinationof adsorbed amounts in complex mixtures over a broad rangeof concentrations (4–12). For a deuterated surfactant in null re-flecting water, nrw (a 92 mol% H2O/8 mol% D2O mixture witha refractive index of unity), the specular reflectivity arises onlyfrom the adsorbed layer. Assuming that the adsorbed layer canbe treated as a single layer of homogeneous composition (17),then application of the optical matrix methods (18) to analyzethe reflectivity profile gives a scattering length density, ρ, and alayer thickness, d. The adsorbed amount is then given by

A =∑

b

ρd, [4]

where A is the area per molecule of the adsorbed surfactant, and∑b the sum of scattering lengths for the surfactant molecule.

For a binary mixture it is straightforward to extend this ap-proach to obtain the adsorbed amount of each component suchthat (17)

ρ = b1

A1d+ b2

A2d, [5]

where bi , Ai are the scattering length and area per molecule ofeach component in the binary mixture. By deuterium labelingeach component in turn the adsorbed amount of that componentcan be determined, whereas measurement with both componentslabeled gives an estimate of the total adsorption. This approachhas been successfully applied to a range of different surfactantmixtures (5–13).

Within the kinematic approximation, the method of partialstructure factors (19) provides a direct method for determina-tion of the distribution and relative positions of the differentdeuterium-labeled components at the interface. This approachhas been used extensively by us to determine the structure ofsurfactant monolayers (21) and mixed monolayers (7, 22) to aresolution ∼1–2 A.

Substituting ρ(z) = ∑i ni (z)bi into Eq. [1], the kinematic ap-

proximation for the specular reflectivity gives

R(Q) = 16π2

Q2

∑i

b2i hii , [6]

where the self and cross partial structure factors are one-dimensional Fourier transforms of the number density distribu-tions of the different labeled components. The self-terms (hii )

contain information about the distribution of the individually la-beled component, and the cross-terms (hi j ) about their relative

COADSORPTION AT THE

positions at the interface, and the details of this approach havebeen extensively described elsewhere (20, 21).

The specular reflection measurements were made on theSURF reflectometer (23) at the ISIS Pulsed Neutron Source,using the “white beam time of flight” method at a fixed geome-try. That is, the measurements were made in a Q range 0.048 to0.5 A−1 at an angle of incidence of 1.5◦, using a wavelength bandof 0.5 to 6.8 A, and where the different neutron wavelengths aresorted by the time of flight. The data was normalized to the in-cident beam spectral distribution and the variation of detectorefficiency with λ, and the data is established on an absolute re-flectivity scale by reference to the reflectivity from the surfaceof D2O using standard procedures (24). A flat background, de-termined by extrapolation of the data to a high value of Q, wassubtracted from all the measured reflectivity profiles. This hasbeen previously shown (24) to be a valid procedure, providingthere is no bulk scattering from the subphase solution, which isthe case for the measurement described here.

High-purity water was used for all the measurements (ElganUltrapure) and the D2O was obtained from Fluorochem. All ofthe glassware and Teflon troughs used for the reflectivity mea-surements were cleaned in alkaline detergent (Decon 90), fol-lowed by copious washing in ultrapure water. All measurementswere made at 25◦C. The phenyl ethanol and deuterated phenylethanol were obtained from Sigma, and used as supplied.

The different isotopically labelled C16TAB’s were synthe-sised by R. K. Thomas, and details of the synthesis, purification,and characterization are given elsewhere (17, 23). Five differentisotropically labelled C16TAB’s were used in the experiments,C16D33N(CD3)3Br, C16D33N(CH3)3Br, C16H33N(CH3)3Br,C10D21C6H12N(CH3)3Br, and C10H21C6D12N(CH3)3Br,which we refer to as dC16dTAB, dC16hTAB, hC16hTAB,dC10hC6hTAB, and hC10dC6hTAB, respectively. Measure-ments were made in D2O and null reflecting water, nrw (waterindex matched to air, a 92 mol%/8 mol% D2O/H2O mixturewhich has a neutron refractive index identical to that of air).

RESULTS AND DISCUSSION

(1) Effect of Aromatic Alcohol on the Adsorption of C16TABat the Air–Water Interface

At a C16TAB concentration of 2 × 10−3 M neutron reflectiv-ity measurements were made for dC16dTAB in nrw with phenylethanol (h-pe) in the concentration range 1.0 to 14 g/l. Assum-ing that the contribution from the h-pe to the reflectivity is neg-ligible (�b for h-pe is much less than �b for dC16dTAB, seeTable 1), then using Eq. [4] (described earlier) we can evaluatethe amount of C16TAB at the interface. The results are sum-marised in Fig. 1, where the area per molecule obtained forC16TAB is plotted as a function of phenyl ethanol concentra-tion. For comparison the previous result for benzyl alcohol (15)is also shown. For both alcohols the area per molecule of the

C16TAB at the interface increases linearly with alcohol concen-tration. For phenyl ethanol the slope is steeper, consistent with

AIR–WATER INTERFACE 399

TABLE 1Scattering Lengths of Labelled Fragments Used in This Study

Component Scattering length (×10−5 A)

D2O 19.2H2O −1.7C16D33 324.8C10D21(c1) 246.5C6D12 (c2) 125.9C16H33 −17.0C6H12 −8.7C10H21 −27.8N(CH3)3Br 2.50N(CD3)3Br 96.1Phenyl-d-ethanol 115.0Phenyl-h-ethanol 21.6Benzyl-d-alcohol 95.5Benzyl-h-alcohol 22.4

the greater hydrophobicity of the phenyl ethanol compared tobenzyl alcohol. The phenyl ethanol is more surface-active, andcompetes more effectively with the C16TAB for the surface. Ata phenyl ethanol concentration of 9 g/l an additional neutronreflectivity measurement was made, using hC16hTAB and d-pe,to estimate the amount of phenyl ethanol at the interface in equi-librium with the C16TAB. The area per molecule of the phenylethanol was found to be 49 ± 3 A2, and compares with 55 A2

for benzyl alcohol measured at a higher concentration of 14 g/l.The area per molecule of the C16TAB in the presence of 9 g/lphenyl ethanol has increased from ∼42 A2 to ∼60 A2. Assum-ing that the area per molecule of phenyl ethanol in the absenceof C16TAB is similar to that of benzyl alcohol, the total ad-sorption of the C16TAB/phenyl ethanol mixture is less than thesum of the adsorbed amounts for the individual pure component

FIG. 1. Area per molecule (A2) for 2 × 10−3 M C16TAB at the air–water

interface with (�) phenyl ethanol and (�) benzyl alcohol as a function of alcoholconcentration.

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400 PENFOL

monolayers. This is also found in the C16TAB/benzyl alcoholmixture, and is indicative of a negative synergy. This is not al-ways the case for competitive adsorption between different sur-factants, and situations arise where either a positive or a negativesynergy exists. This is in contrast, for example, to the situationfor SDS/dodecanol (11) and C16TAB/dodecane (13), where thetotal adsorption for the mixture is greater than the adsorption forthe individual components.

It is difficult to make a quantitative analysis of the adsorptionof the C16TAB/alcohol mixture in the usual framework of the-ories such as Regular Solution Theory (26), due to the absenceof a critical micellar concentration, cmc, for either the benzylalcohol or phenyl ethanol. This was discussed by Staples et al.(27) in the context of C12E5/dodecanol mixtures at the air–waterinterface. In this case the solubility of dodecanol was too low fora micellar phase to form, and approximations to account for thispredicted a strong variation of the interaction parameter withalcohol content.

The negative synergy observed here is assumed to arise froma disruption of the packing of the C16TAB monolayer by thealcohol. Detailed measurements of the structure of the surfactantmonolayer and its changes due to the coadsorption of the benzylalcohol or phenyl ethanol will provide direct evidence to supportthat hypothesis.

(2) Structure of the Mixed C16TAB/Phenyl Ethanol Monolayer

The structural measurements were made at a C16TAB con-centration of 2 × 10−3 M and a phenyl ethanol concentration of9 g/l. Under these conditions the surface has a roughly equimolarcomposition, and as such provides the maximum sensitivity inthe neutron reflectivity measurements to changes in the C16TABconformation, and to the location of the phenyl ethanol in theinterface. Two different sets of structural measurements weremade to determine the conformation of the C16TAB molecules,and to establish the location of the phenyl ethanol at the interface.

First, assuming h-pe is closely matched to zero scatteringlength, measurements with differently labelled fragments ofC16TAB in nrw and D2O were used to determine the structureof the C16TAB in the presence of phenyl ethanol. Measurementswere made using six different isotopically labelled combinationsof dC10hC6hTAB, hC10dC6hTAB in D2O and nrw, hC16hTABin D2O, and dC16hTAB in nrw. From Eq. [6] we can write thereflectivity as

R(Q) = 16π2

Q2

[b2

c1hc1c1 + b2c2hc2c2 + b2

s hss + 2bc1c2hc1c2

+ 2bc1bshc1s + 2bc2bshc2s], [7]

where c1, c2 refer to the C10 of the C16TAB alkyl chain farthestfrom the headgroup, and to the C6 group adjacent to the head-group. The self-partial structure factors hc1c1, hc2c2, and hss de-scribe the distributions of the two labelled fragments of the alkyl

chain and of the solvent. The cross-terms hc1s, hc2s, and hc1c2

ET AL.

contain information directly about their relative positions at theinterface. The chain self-terms are analyzed as Gaussian func-tions and the solvent as a tan h function, as previously described(20). Assuming that the cross-terms arise from a combination ofodd and even functions (20), the cross-terms can be expresseddirectly in terms of the displacements between the centers of therelevant distributions. For the cross-term between two even dis-tributions (for example, the two alkyl chain fragments) and forthe cross-term between an odd and an even distribution (wherethe solvent is described as an odd distribution) we have

hi j = ±(hii h j j )1/2 cos (Qδi j ) [8]

hiδ = ±(hii hss)1/2 sin (Qδis). [9]

The partial structure factors described in Eq. [7] are charac-terized by six parameters: the width of the alkyl chain fragmentsand solvent σc1, σc2, and ζs and their relative positions, δc1s, δc2s,and δc1c2 (the bulk solvent density, no, and the area per moleculeor adsorbed amount of the surfactant are known). The ± signin Eqs. [8] and [9] reflects the phase uncertainty arising fromEq. [1], and in practice this does not present a problem for suchsystems. There is also an additional relationship that

δc1c2 = δc1s − δc2s. [10]

In these structural measurements, it is assumed that the con-tribution from protonated headgroup and phenyl ethanol is smallcompared to that from the deuterated fragments (see Table 1).In Table 2 the structural parameters for C16TAB in the presenceof phenyl ethanol are compared to those for the pure C16TAB

TABLE 2Structural Parameters for Surfactant Monolayer, Obtained from

Partial Structure Factor Analysis

Parameter(dimensions of A, 2 × 10−3 M 2 × 10−3 Munless otherwise 2.75 × 10−4 M C16TAB +14 g/l C16TAB + 9 g/l

stated) C16TABa benzyl alcohola phenyl ethanol

σc1 12.0 ± 2 13.0 ± 2 12.0 ± 2σc2 12.0 ± 2 13.0 ± 2 14.0 ± 2ζs 5.0 ± 0.5 6.0 ± 0.5 7.0 ± 0.5δc1s 10.0 ± 0.5 8.0 ± 0.5 8.0 ± 0.5δc2s 3.0 ± 0.5 3.0 ± 0.5 1.0 ± 0.5δc1c2 6.0 ± 1.0 6.0 ± 1 7.0 ± 1σba(pe) 13.0 ± 2 14.0 ± 2δc1ba(pe) 5.0 ± 0.5 4.0 ± 0.5δc2ba(pe) 2.0 ± 0.5 2.0 ± 0.5δba(pe)s 4.0 ± 0.5 4.0 ± 0.5Area/molecule 60 ± 3 56 ± 3 60 ± 3[C16TAB (A2)]Area/molecule 55 ± 3 49 ± 3

[benzyl alcohol(phenyl ethanol) (A2)]

a From Ref. (15).

A

COADSORPTION AT THE

monolayer and the previous measurements for a C16TAB/benzylalcohol mixed monolayer.

The most convenient way of presenting such structural in-formation is as volume fraction distributions and in Fig. 2 wecompare the distributions obtained for the C16TAB monolayer,the mixed C16TAB/benzyl alcohol monolayer, and the mixedC16TAB/phenyl ethanol monolayer.

Figure 2 shows the main structural features of the C16TABmonolayer derived from the analysis in the form of volumefraction distributions. It shows the distribution of the solvent,the C10 of the alkyl chain farthest from the headgroup, and theC6 of the alkyl chain adjacent to the headgroup and their relativepositions. The headgroup is not shown as this was not specif-ically measured. The overlap and relative positions of the twolabeled fragments (C10, C6) of the C16 alkyl chain provide in-formation about the conformation of the chain. The structureobtained is consistent with previous measurements of C16TABand other cationic surfactants (15, 21).

A second series of structural measurements were made todetermine the position and extent of the phenyl ethanol dis-tribution relative to the C16TAB in the monolayer. Follow-ing the procedure adopted for the C16TAB/benzyl alcoholmixture, measurements were made with the labelled com-binations, dC10hC6hTAB/d-pe/nrw, dC10hC6hTAB/h-pe/nrw,hC16hTAB/h-pe/D2O, hC16hTAB/d-pe/nrw, and hC16hTAB/d-pe/D2O. From the combination dC10hC6hTAB/d-pe/nrw wehave

R(Q) = 16π2

Q2

[b2

c1hc1c1 + b2pehpepe + 2bc1bpehc1pe

], [11]

and from the combination hC16hTAB/d-pe/D2O,

R(Q) = 16π2

Q2

[b2

s hss + b2pehpepe + 2bpebshpes

]. [12]

Using the combinations hC16hTAB/h-pe/D2O to obtainhss, hC10hTAB/d-pe/nrw to obtain hpepe, and dC10hC6hTAB/h-pe/nrw to obtain hc1c1, in combination with Eqs. [11] and[12] the cross-terms hc1pe and hpes can be obtained. A simi-lar expression for hc2pe can be derived using the combinationhC10dC6hTAB/d-pe/nrw. hc1pe, hc2pe, and hpes then provide anestimate of the position of the phenyl ethanol at the interfacerelative to the solvent and the C10 and C6 labelled groups of theC16TAB alkyl chain. The parameters from this analysis are alsosummarised, in Table 2 as σpe, δc1pe, δc2pe, and δpes. The phenylethanol and benzyl alcohol distributions are included in Fig. 2.

The errors quoted in Table 2 for the structural parameters,derived from the partial structure factor analysis, arise from acombination of systematic errors in the measurements (28), thestatistical quality of the data, and the quality of the model fitsto the individual partial structure factors. The values quotedin Table 2 take all these factors into account, and errors of

±2 A for the widths of distributions and ±0.5 A for the sep-arations between the different components are typical. The

IR–WATER INTERFACE 401

FIG. 2. Experimentally determined volume fraction distributions as a func-tion of z (in A) for the different components of C16TAB at the air–water interface.(a) 2.75 × 10−4 M C16TAB (from Ref. (15)) : (-) solvent, (. . .) outer C10 (C6

for a), (–) inner C6 of alkyl chain. (b) 2 × 10−3 M C16TAB +14 g/l benzyl

alcohol, (·-·) benzyl alcohol. (c) 2 × 10−3 M C16TAB + 9 g/l phenyl ethanol,(- · -) phenyl ethanol.

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402 PENFOL

differences in the structural parameters for the C16TAB/phenylethanol (benzyl alcohol) mixed monolayer are compared to thosefor the pure C16TAB monolayer. The changes in the parameterswhich define the conformation of the C16TAB molecule and itsrelative position compared to the solvent (δc1s, δc2s, and δc1c2) aresignificant compared to the quoted errors. When comparing theC16TAB/benzyl alcohol and C16TAB/phenyl ethanol measure-ments the changes are now relatively subtle. The only significantchanges are associated with the position of the alcohol in the in-terface and the conformation of the C16 alkyl chain.

The position of the alcohol in the mixed monolayer, relativeto the C16TAB, is determined primarily from δc1pe(be), δc2pe(be),and δpe(ba)s, and there is a small but systematic change on goingfrom benzyl alcohol to phenyl ethanol. The results for the ben-zyl alcohol/C16TAB mixture have been previously discussed,and the distribution of the benzyl alcohol was found to be cen-tered adjacent to the third and fourth methylene groups from theheadgroup of the C16TAB alkyl chain. The results for phenylethanol, as discussed above, are similar, and the small but sys-tematic decrease in δc1pe, compared to δc1ba, is interpreted as thephenyl ethanol distribution being centered farther away from theC16TAB headgroup; that is, farther up the alkyl chain. This isexpected for the more hydrophobic phenyl ethanol.

The widths of the distributions of the different individualcomponents are dominated by capillary wave roughness (20),and in general are similar (within error) for C16TAB and forthe C16TAB/benzyl alcohol and C16TAB/phenyl ethanol mix-tures. However, there is a small but systematic increase inσc2 and ζs compared to the C16TAB monolayer for the mixedC16TAB/phenyl ethanol monolayer. When this is taken into ac-count with the other variations observed (the relative positionsof the two labelled parts of the alkyl chain and the solvent (δc1s,δc2s), and the relative positions of the two labelled fragmentsof the alkyl chain with respect to each other (δc1c2)) it is in-dicative of a change in the alkyl chain conformation. Althoughsome of the changes are within the experiemntal error, the vari-ations in δc1c2, δc2s are significant. In detail, in the case of theC16TAB/benzyl alcohol mixture there were small but systematicchanges in the conformation of the C16TAB alkyl chain with theaddition of benzyl alcohol, in that the overlap of the solvent andhydrophobic chain distributions increased. In the case of thephenyl ethanol/C16TAB mixture the chain/solvent overlap wasfurther increased. The increase in the thickness of the C6 adja-cent to the headgroup suggests that the alkyl chain conformationhas altered; that is, the region closest to the headgroup is moreextended or less tilted. The more hydrophobic phenyl ethanol hasa greater impact on the conformation of the C16TAB alkyl chain,and can be related directly to the change in its impact on mem-brane fluidity, spontaneous curvature, and the Lα/Lβ transition.

These results are consistent with the observations for a num-ber of other different mixtures, and similar trends in structurewith cosurfactant of increasing hydrophobicity or surface activ-ity are observed. For example, a comparison of the result for

the mixtures C16TAB/dodecane (13), SDS/dodecanol (12), andC16TAB/C12E6 (8) showed that the midpoint of the alkyl chain

ET AL.

distribution of the cosurfactant (dodecane, dodecanol, C12E6)was increasingly farther from the solvent interface, and from theother component, with increasing hydrophobicity (from C12E6 tododecane). In the case of the C16TAB/dodecane mixture, partialdeuterium labelling of the C16TAB alkyl chain provided infor-mation on the change in conformation of the alkyl chain whendodecane is added. The changes were interpreted as a decreasein the mean tilt of the chain from 65 to 45◦; that is, the chain hadadopted a more upright configuration. The increased packinghas resulted in a more ordered monolayer. This is in contrast tothe situation reported here for the mixed C16TAB/aromatic al-cohol layer, where the conformational change of the alkyl chainis more localized. This will result in a less efficient packing,and is likely to be a contributing factor to the negative synergythat is observed.

SUMMARY

We have used specular neutron reflectivity in combinationwith D/H isotopic substitution to determine the structure of theC16TAB/phenyl ethanol mixed monolayer at the air–water in-terface. On the basis of the volume fraction distributions de-rived from the data we have compared the structure with a pureC16TAB monolayer and a mixed monolayer of C16TAB/benzylalcohol. From these measurements we have shown, as pos-tulated, that the addition of a more hydrophobic cosurfactant(phenyl ethanol) to the C16TAB surfactant monolayer has agreater impact on the structure of that monolayer.

REFERENCES

1. Soubiran, L., unpublished results.2. Penfold, J., Staples, E., Tucker, I., and Soubiran, L., in preparation.3. Mitchel, D. J., and Ninham, B., J. Chem. Soc., Faraday Trans, 2 77, 601

(1987).4. Tokuoka, Y., Uchigama, H., Abe, M., and Christian, S. D., Langmuir 11,

725 (1995).5. Penfold, J., Staples, E., Thompson, L., and Tucker, I., Colloids Surf. 102,

127 (1995).6. Penfold, J., Staples, E., Thompson, L., Tucker, I., Hines, J., Thomas, R. K.,

and Lu, J. R., Langmuir 11, 2496 (1995).7. Penfold, J., Staples, E., Cummins, P., Tucker, I., Thomas, R. K., Simister,

E. A., and Lu, J. R., J. Chem. Soc., Faraday Trans. 92, 1549 (1996).8. Penfold, J., Staples, E., Cummins, P., Tucker, I., Thomas, R. K., Simister,

E. A., and Lu, J. R., J. Chem. Soc., Faraday Trans. 92, 1773 (1996).9. Hines, J. D., Thomas, R. K., Garrett, P. R., Rennie, G. K., and Penfold, J.,

J. Phys. Chem. B 101, 9215 (1997).10. Hines, J. D., Thomas, R. K., Garrett, P. R., Rennie, G. K., and Penfold, J.,

J. Phys. Chem. B 102, 8834 (1998).11. Hines, J. D., Thomas, R. K., Garrett, P. R., Rennie, G. K., and Penfold, J.,

J. Phys. Chem. B 102, 9708 (1998).12. Lu, J. R., Purcell, I. P., Lee, E. M., Simister, E. A., Thomas, R. K., Rennie,

A. R., and Penfold, J., J. Colloid Interface Sci. 174, 441 (1995).13. Binks, B. P., Crichton, D., Fletcher, P. D. I., MacNab, J. R., Li, Z. X., and

Penfold, J., Colloids Surf. A 146, 299 (1999).14. Li, Z. X., Bain, C. D., Thomas, R. K., Duffy, D. C., and Penfold, J., J. Phys.

Chem. B 102, 9473 (1998).15. Penfold, J., Staples, E., Tucker, I., Soubiran, L., Lodi, A. K., Thompson,

L., and Thomas, R. K., Langmuir 14, 2139 (1998).16. Thomas, R. K., and Penfold, J., J. Phys. Condens. Matter 2, 1369 (1990).

A

COADSORPTION AT THE

17. Simister, E. A., Thomas, R. K., Penfold, J., Aveyard, R., Binks, B. P.,Fletcher, P. D. I., Lu, J. R., and Sokolowski, A., J. Phys. Chem. 96, 1383(1992).

18. Heavens, O. S., “Optical properties of thin films,” Butterworths, London,1955.

19. Tucker, I., Penfold, J., Thomas, R. K., Lu, J. R., and Staples, E., Langmuir9, 651 (1995).

20. Simister, E. A., Lee, E. M., Thomas, R. K., and Penfold, J., J. Phys. Chem.96, 1373 (1992).

21. Lu, J. R., Li, Z. X., Smallwood, J., Thomas, R. K., and Penfold, J., J. Phys.Chem. 99, 8233 (1995).

22. Penfold, J., Staples, E., Tucker, I., and Thomas, R. K., J. Colloid InterfaceSci. 201, 223 (1998).

IR–WATER INTERFACE 403

23. Penfold, J., Richardson, R. M., Zarbakhsh, A., Webster, J. R. P., Bucknall,D. G., Rennie, A. R., Jones, R. A. L., Cosgrove, T., Thomas, R. K., Higgins,J. S., Fletcher, P. D. I., Dickinson, E., Roser, S. J., McLure, I. A., Hillman,A. R., Richards, R. W., Staples, E. J., Burgess, A. N., Simister, E. A., andWhite, J. W., J. Chem. Soc, Faraday Trans. 93, 3899 (1997).

24. Lee, E. M., Thomas, R. K., Penfold, J., and Ward, R. C., J. Phys. Chem. 93,381 (1989).

25. Lu, J. R., Lee, E. M., Thomas, R. K., Penfold, J., and Flitsch, S. L., Langmuir9, 1352 (1995).

26. Holland, P. M., Colloids Surf. 19, 171 (1986).27. Staples, E. J., Thompson, L., Tucker, I., and Penfold, J., Langmuir 10, 4136

(1994).28. Lu, J. R., and Thomas, R. K., Nucl. Instrum. Methods A 354, 149 (1995).