Langmuir 1998, 14, 2139-2144
2139
Structure and Composition of the Mixed Monolayer of Hexadecyltrimethylammonium Bromide and Benzyl Alcohol Adsorbed at the Air/Water Interface J. Penfold,*,† E. Staples,‡ I. Tucker,‡ L. Soubiran,‡ A. Khan Lodi,‡ L. Thompson,‡ and R. K. Thomas§ ISIS Facility, CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K., Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, U.K., and Physical and Theoretical Chemistry, Oxford University, South Parks Road, Oxford, U.K. Received September 30, 1997. In Final Form: January 15, 1998 The structure and composition of the mixed monolayer of hexadecyltrimethylammonium bromide and benzyl alcohol adsorbed at the air-water interface has been measured by using specular neutron reflection. The alkyl chain conformation of the hexadecyltrimethylammonium bromide at a surfactant concentration of 2 × 10-3M is not significantly altered by the addition of benzyl alcohol. However, the addition of benzyl alcohol draws the hexadecyltrimethylammonium bromide alkyl chain closer to the aqueous subphase: a similar change is observed in the mixed monolayers of hexadecyltrimethylammonium bromide-hexaethylene glycol monododecyl ether and of sodium dodecyl sulfate-dodecanol. The distribution of the benzyl alcohol molecules in the interface is found to be centered on the third methylene group from the headgroup of the alkyl chain of the hexadecyltrimethylammonium bromide cationic surfactant.
Introduction Solubilization in surfactant and mixed surfactant systems has been extensively studied,1,2 and involves numerous measurements on normal alkanes and alcohols. More recently these studies have been extended to more complex molecules, including, for example, synthetic perfume compounds.3 Understanding the nature of solubilization at a molecular level is important in the widespread industrial applications of solubilization, which include detergency, oil recovery, pesticides, cosmetics, and food science. The role of short- and medium-chain alcohols as solubilizates or cosurfactants in microemulsion formation is also central to understanding microemulsion formation and stability. Their role has been rationalized in terms of their ability to decrease interfacial tension,4 increase the fluidity5 of the surfactant monolayer, and adjust the spontaneous curvature6 (the effective hydrophilic-lipophilic balance, HLB). With linear alkyl chain surfactants the mesophases that form can be rationalized with very simple geometric arguments based upon the area per molecule and the relative dimensions of the headgroup and alkyl chains.7 The effects of adding a shortchain alcohol are therefore expected to be closely linked to its location within the interfacial region. The presence of the alcohol is also expected to be reflected in the change in the structure of any adsorbed surfactant molecule. In previous work in this area the role of the cosurfactant in the monolayer or bilayer properties have been inferred †
Rutherford Appleton Laboratory. Port Sunlight Laboratory. § Oxford University. ‡
(1) Nagarajan, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 391. (2) Christian, S. D., Scamehorn, J. F., Eds. Solubilisation in surfactant aggregates; Dekker: New York, 1995. (3) Tokooka, Y.; Uchiyama, H.; Abe, M.; Christian, S. D. Langmuir 1995, 11, 725. (4) Aveyard, R.; Binks, B. P.; Meed, S. J. Chem. Soc., Faraday Trans 1 1987, 83, 2347. (5) Di Maglio, J. M.; Dvolaitzky, M.; Taupin, C. J. Phys. Chem 1985, 89, 871. (6) Strey, R.; Janstrand, J. J. Phys. Chem. 1992, 96. (7) Mitchell, D. J.; Ninham, B. W. J. Chem Soc., Faraday Trans. 2 1987, 77, 601.
from a variety of indirect measurements, such as surface tension,4 X-ray scattering,8 and ESR.9 Neutron reflectivity, in combination with hydrogen-deuterium isotopic labeling, provides the opportunity to directly determine the effect of a cosurfactant on monolayer structure and to determine the position of the solubilizate in the adsorbed layer.10 Lu et al.11 have used this approach to investigate the coadsorption of sodium dodecyl sulfate, SDS, and dodecanol, and for determination of the structure and composition of dodecane layers spread on aqueous solutions of dodecyl-, tetradecyl-, and hexadecyl trimethylammonium bromide, CnTAB,12 Ward et al.13 have used the complementary technique of sum-frequency spectroscopy to investigate the coadsorption of sodium dodecyl sulfate and dodecanol at a hydrophobic surface. More recently Bell et al.14 have used both neutron reflectivity and sum-frequency spectroscopy to obtain a detailed picture of the structure of a monolayer of the cationic surfactant, hexadecyltrimethylammonium bromide, in the presence of p-tosylate anions. The change of the conformation of the surfactant molecules when bromide counterions are replaced by p-tosylate ions and the location of the counterion at the interface have both been determined. Penfold et al. have used neutron reflectivity in a simillar way to study the composition and structure of mixed monolayers of hexadecyltrimethylammonium bromide, C16TAB, and hexaethylene glycol monodododecyl ether, C12E6, and of SDS and C12E6 at the air-solution15-17 and hydrophilic solid-liquid18 interfaces. (8) Szleitfer, I.; et al. J. Chem. Phys. 1990, 92, 6800. (9) Barsal, V.; Shah, D. O.; O’Connell, S. J. Colloid Interface Sci. 1986, 75, 462. (10) Thomas, R. K.; Penfold, J. J. Phys.: Condens. Matter 1990, 2, 1369. (11) Lu, J. R.; Purcell, I. P.; Lee, E. M.; Simister, E. A.; Thomas, R. K.; Rennie, A. R.; Penfold, J. J. Colloid Interface Sci. 1995, 174, 441. (12) Lu, J. R.; Thomas, R. K.; Binks, B. P.; Fletcher, P. D. I.; Penfold, J. J. Phys. Chem. 1993, 99, 4113. (13) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1997, 101, 1594. (14) Bell, G. R.; Bain, C. D.; Li, Z. X.; Thomas, R. K.; Duffy, D. C.; Penfold, J. J. Am. Chem. Soc. 1997, 119, 10227. (15) Penfold, J.; Staples, E.; Cummins, P.; Tucker, I.; Thomas, R. K.; Simister, E. A.; Lu, J. R. J. Chem. Soc., Faraday Trans. 1996, 92, 1549.
S0743-7463(97)01074-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/24/1998
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In this paper we report the use of neutron reflectivity, in combination with deuterium-hydrogen isotopic labeling, to investigate the coadsorption of benzyl alcohol and C16TAB at the air-water interface. By selective deuterium labeling the benzyl alcohol and C16TAB, the composition of the adsorbed layer has been obtained. From measurements with partial labeling of the alkyl chain of the C16TAB the change in conformation of the alkyl chain in the presence of the benzyl alcohol has been determined and the position of the benzyl alcohol at the interface located. The motivation for this work is that benzyl alcohol can be regarded as a model of a perfume molecule.3 Neutron Reflectivity Specular neutron reflection gives information about inhomogeneities normal to an interface or surface, and its theory has been described in detail elsewhere.10 The basis of a neutron reflection measurement is that the variation in specular reflection with Q (the wave vector transfer defined as Q ) 4π sin θ/λ, where θ is the glancing angle of incidence and λ the neutron wavelength) is simply related to the composition or concentration profile in the direction normal to the interface. In the kinematic approximation10 the specular reflectivity, R(Q), is given by
interface and the reflectivity calculated exactly using the optical matrix method.20,21 The model is then optimized to fit simultaneously the reflectivity profiles for the different isotopically labeled combinations. Alternatively a more direct method, based on the kinematic approximation,22 is used to analyze the reflectivity profiles from the different isotopic combinations by separating out the contributions from the different components of the interfacial layer. This latter approach has now been extensively applied to the determination of the structure of the adsorbed surfactant layer at the air-water interface and has, for example, been used to determine the structure of the C16TAB monolayer at the air-water interface to a spatial resolution to the extent of two methylene groups.23 The scattering length density profile can be written in terms of the contributions from the differently labeled components in a surfactant layer, for example, in the simple case where the headgroup, alkyl chain, and solvent can be separately labeled; it can be written as
F(z) ) bcnc(z) + bhnh(z) + bwnw(z)
(4)
where c, h, and w refer to surfactant chains, headgroup, and water. Equations 1 and 4 then give
(1)
16π2 2 [bc hcc + bh2hhh + bw2hww + 2bcbhhch + Q2 2bcbwhcw + 2bhbwhhw] (5)
where F(Q) is the one-dimensional Fourier transform of F(z), the average scattering length density profile in the direction normal to the interface
where the hjj are the self-partial structure factors given by
R(Q) )
F(Q) )
R(Q) )
2
16π |F(Q)|2 Q2
∫-∞ F(z) exp(iQz) dz
(2)
∑i ni(z)bi
(3)
+∞
F(z) )
hjj(Q) ) |nˆ j(Q)|2
(6)
and hij are the cross partial structure factors given by
where ni is the number density profile of species i and bi is its scattering length. In the context of surfactant adsorption the key features of the neutron reflectivity method are that the neutron scattering properties of H and D are sufficiently different and that at the air-liquid interface H/D isotopic substitution can be used to manipulate the neutron refractive index profile of surfactant adsorbed at that interface. This is particularly important for surfactant mixtures, where by selective deuteration particular components or fragments can be highlighted or isolated. It is this selectivity which makes the neutron reflectivity method so powerful. The effectiveness of the method in determining the surfactant structure depends on being able to combine reflectivity profiles from solution of the same chemical but different isotopic composition. This, of course, assumes that there is no isotopic dependence of the structure or adsorbed amounts, and this has now been well-established19 for a range of systems, including that reported here. In terms of structure the reflectivity profiles can be analyzed by, principally, two different methods. In the first method, a structural model is assumed for the (16) Penfold, J.; Staples, E.; Cummins, P; Tucker, I.; Thomas, R. K.; Simister, E. A.; Lu, J. R. J. Chem. Soc., Faraday Trans. 1996, 92, 1773. (17) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2496. (18) Penfold, J.; Staples, E.; Tucker, I.; Thompson, L. Langmuir 1997, in press. (19) Lu, J. R.; Lee, E. M; Thomas, R. K.; Penfold, J.; Flitsch, S. L. Langmuir 1995, 9, 1352.
hij(Q) ) Re{nˆ i(Q) nˆ j(Q)}
(7)
The nˆ (Q) is the one-dimensional Fourier transform of ni(z), and the self-partial structure factor relates to the distributions of the individual components, whereas the cross partial structure factors relate to the relative positions of the different components. We have shown elsewhere24 that simple analytical functions can be used to describe these partial structure factors. At a simpler level the technique can be used to determine adsorbed amounts in single component and multicomponent mixture monolayers straightforwardly over a wide concentration range (from below to well in excess of the critical micelle concentration (cmc)) at the air-liquid interface. In a mixture, with each component selectively deuterated in turn, the resultant reflectivity for each combination can be analyzed by treating the reflectivity arising from a single layer of homogeneous composition.20 Using the optical matrix methods,20,21 this yields a scattering length density and thickness for the layer for a binary mixture which gives25 (20) Penfold, J. Neutron X-ray and light scattering; Elsevier: New York, 1991. (21) Heavens, O. S. Optical properties of thin films; Butterworth: London, 1955. (22) Crowley, T. L.; Simister, E. A.; Lee, E. M.; Thomas, R. K. Physica B 1991, 173, 143. (23) Lu, J. R.; Li, Z. X.; Smallwood, J.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1995, 99, 8233. (24) Lu, J. R.; Hromadova, M.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1994, 98, 11519. (25) Tucker, I.; Penfold, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 9, 651.
Mixed Monolayer of C16TAB and Benzyl Alcohol
F ) b1/A1τ + b2/A2τ
Langmuir, Vol. 14, No. 8, 1998 2141
(8)
where bi, Ai are the scattering lengths and area/molecule of each component, respectively, and F and τ are the scattering length densities and thicknesses from the model fits to the data. This approach has now been amply demonstrated and applied to a range of different mixed surfactants.15-17,26 Experimental Details The specular neutron reflection measurements were made on the SURF reflectometer27 at the ISIS pulsed neutron source, using the “white beam time-of-flight” method. That is, the measurements were made in the Q range of 0.048-0.5 Å-1 at a fixed angle of incidence of 1.5° using a neutron wavelength band of 0.5-6.8 Å, and at the point at which the different neutron wavelengths are separated by time of flight. The data were normalized for the incident beam spectral distribution and detector efficiency and established on an absolute reflectivity scale by reference to the reflectivity from a surface of D2O using standard procedures.28 A flat background, determined by extrapolation to high values of Q, was subtracted from all of the measured reflectivity profiles. This has been shown to be a valid procedure28 providing that there is no small-angle scattering from the bulk solution, which is the case for the measurements described here. High-purity water was used for all of the measurements (Elga Ultrapure), and the D2O was obtained from Fluorochem. All of the glassware and Teflon troughs used for the reflectivity measurements were cleaned using alkaline detergent (Decon 90) followed by copious washing in ultrapure water. The measurements were all performed at 303 K. All isotopes of C16TAB were synthesized by Thomas’ group at Oxford. The details of the preparation, purification, and characterization are given elsewhere.19,29 The benzyl alcohol and deuterated benzyl alcohol were obtained from Sigma and used as supplied. Five isotopic species of C16TAB 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, hC10dC6hTAB, respectively. The measurements were made in D2O and in null reflecting water, nrw (water index matched to air; a 92 mol %/8 mol % H2O/D2O mixture has a neutron refractive index identical to that of air).
Results and Discussion Adsorption of C16TAB at the Air/Water Interface in the Presence of Benzyl Alcohol. Reflectivity measurements with 2 × 10-3 M dC16dTAB in nrw and benzyl-h alcohol (ba-h) in the concentration range of 1.027.0 g/L were used to assess the effect of the benzyl alcohol on the amount of C16TAB adsorbed at the interface. At 2 × 10-3 M C16TAB and 14 g/L added benzyl alcohol, a series of measurements in nrw, with both components alternatively deuterium labeled, were used to determine the amounts of C16TAB and benzyl alcohol, respectively, at the interface. Figure 1 shows the variation in neutron reflectivity for 2 × 10-3 M dC16dTAB in nrw with added benzyl-h alcohol. The reduction in reflectivity with increasing benzyl alcohol concentration is indicative of a decrease in the amount of C16TAB at the interface. The amount of C16TAB at the (26) Penfold, J.; Staples, E; Thompson, L; Tucker, I. Colloids Surf. 1995, 102, 127. (27) Bucknall, D. G.; Penfold, J.; Webster, J. R. P.; Zarbakhsh, A.; Richardson, R. M.; Rennie, A. R.; Higgins, J. S.; Jones, R. A. L.; Fletcher, P. D. I.; Roser, S.; Dickinson, E. Proceedings of ICANS-XII; PSI Proceedings 95-02; 1995; p 123. (28) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381. (29) Simister, E. A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowski, A. J. Phys. Chem. 1992, 96, 1383.
Figure 1. Variation of neutron reflectivity with added benzyl alcohol for 2 × 10-3 M dC16dTAB in nrw: (b) no benzyl alcohol, (O) 14 g/L benzyl-h alcohol, and (4) 27 g/L benzyl-h alcohol. The solid lines are calculated reflectivities for a single layer of uniform composition of thickness τ and scattering length density F of (b) 20.6 Å and 4.8 × 10-6 Å-2, (O) 20.8 Å and 3.2 × 10-6 Å-2, and (4) 18.9 Å, and 2.3 × 10-6 Å-2.
Figure 2. Surface excess of (mol cm-1) C16TAB at the airwater interface in for 2 × 10-3 M C16TAB as a function of benzyl alcohol concentration. Table 1. Scattering Lengths and Volumes of Labeled Fragments Used in This Study
D2O H2O benzyl-d alcohol benzyl-h alcohol C16D33 C10D21 C6D12 C16H33 C6H13 C10H21 N(CH3)3Br N(CD3)3Br
scattering length (×10-5 Å)
volume (Å3)
extended length (Å)
19.2 -1.7 95.3 22.4 324.8 246.5 125.9 -17.0 -8.7 -27.8 2.50 96.1
30 30 173 173 426 278 149 426 149 279 141 141
21.7 14.1 9.1 21.7 9.1 14.1 5.0 5.0
interface was evaluated by using eq 8 and the appropriate neutron scattering lengths for the different components from Table 1 and is illustrated in Figure 2, where the variation in C16TAB surface excess (mol cm-1) is plotted as a function of benzyl alcohol concentration. At the benzyl alcohol concentration of 14 g/L measurements were made with both the C16TAB and benzyl alcohol deuterium labeled, to provide an estimate of the amounts of C16TAB and benzyl alcohol at the interface. Measurements were made for the combinations of dC16hTAB/benzyl-h alcohol, dC10hC6hTAB/benzyl-h alcohol, dC16dTAB/benzyl-h alco-
2142 Langmuir, Vol. 14, No. 8, 1998
hol, dC16hTAB/benzyl-d alcohol, and dC10hC6hTAB/benzyl-d alcohol. The resulting reflectivities were evaluated by using eq 8, and they provide a consistent estimate of the adsorbed amounts of both components. A least squares solution of eq 8 for all measured combinations gave an area/molecule for the C16TAB and benzyl alcohol of 58 ( 3 and 54 ( 3 Å2, respectively. A reflectivity measurement of 14 g/L of benzyl-d alcohol (ba-d) in nrw gave an area/ molecule for the benzyl alcohol in the absence of C16TAB of 29 ( 1 Å2. In contrast the area/molecule for C16TAB in the absence of benzyl alcohol at a concentration of 2 × 10-3 M was measured to be 42.5 ( 0.5 Å2. The total adsorption for the mixture is less than the adsorption of the two individual components, which is indicative of a negative synergy. It could be argued that this is directly associated with the disruption caused by the packing of the C16TAB and benzyl alcohol together and illustrates the importance of the subsequent measurements of the structure of the mixed monolayer. Structure of the C16TAB Layer at 60 Å2 and the Effect of Benzyl Alcohol on the Structure. The structure of the C16TAB layer was measured at a surfactant concentration of 2.75 × 10-4 M in the absence of benzyl alcohol and at 2 × 10-3 M in the presence of 14 g/L benzyl alcohol. These concentrations were chosen so that the structural comparison was made at approximately the same C16TAB area/molecule The structure was measured by using the six labeled combinations of dC10hC6hTAB, hC10dC6hTAB in D2O and nrw, hC16hTAB in D2O, and dC16hTAB in nrw. For the partially labeled alkyl chain of the C16TAB we can express the reflectivity as
R(Q) )
16π2 [bc12hc1c1 + bc22hc2c2 + bc22hc2c2 + bs2hss + Q2 2bclc2hclc2 + 2bc1bshc1s + 2bc2bshc2s] (9)
where c1, c2 refer to the C10 group furthest from the headgroup and to the C6 group adjacent to the headgroup. The self-partial structure factors hc1c1, hc2c2, and hss describe the distribution of the two labeled fragments of the alkyl chain and of the solvent. The cross-terms hc1s, hc2s, and hc1c2 contain information directly about the relative positions of the two labeled fragments of the alkyl chain and the solvent. Throughout these measurements we have assumed that the C16TAB headgroup makes a negligible contribution to the reflectivity. Its scattering length relative to any of the labeled fragments (see Table 1) makes this a good assumption. For the measurements in the presence of benzyl alcohol, the scattering length of benzyl-h alcohol was considered to be sufficiently close to zero compared to the other labeled components that it was essentially invisible (Table 1 lists the scattering lengths and volumes of the different labeled fragments used in these measurements). The self-consistency of the partial structure factors obtained justify these assumptions. We have shown in earlier work that many of the systematic errors in the analysis can be substantially reduced by ensuring first that the measurements are consistent with respect to surface coverage30 and, following Crowley,31 that the errors due to the approximation in the kinematic approximation can be corrected. Both procedures have been adopted with the data presented in this paper. The partial structure factors have been extracted by a least squares (30) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993, 97, 6062. (31) Crowley, T. L. Physica A 1993, 195, 354.
Penfold et al.
solution of the six equations (see eq 9) which described the six measured reflectivities for the different isotopically labeled combinations that were used. There are two types of partial structure factors in eq 9, the self-terms hii and the cross-terms hij. We have shown elsewhere24 that the self-terms, which describe the distribution of the individual components of the surfactant, can be described as a Gaussian distribution
n ) nio exp(-4z2/σ2)
(10)
which gives
hii(Q) )
πσ12nio2 exp(-Q2σ12/8) 4
(11)
and nio is related to the surface excess by
Γi ) 1/A1 ) σinioπ1/2/2
(12)
For the solvent we use a tanh profile
(21 + 21 tanh(ξz))
ns ) no
(13)
when z is the distance in the direction normal to the interface, ξ is the width parameter, and no is the bulk number density of the solution. The corresponding partial structure factor is
hss(Q) ) no2(ξπ/2)2 cosech2(ξπQ/2)
(14)
The cross-terms provide information about the relative positions of the different components of the interface. When the surfactant components are even distributions about their center and the solvent is an odd distribution, then
hij ) ((hiihjj)1/2 cos(Qδij)
(15)
his ) ((hiihss)1/2 sin(Qδis)
(16)
With this approach eqs 15 and 16 can be written explicitly in terms and eqs 11 and 14. The six partial structure factors in the equations can then be described by six parameters, σc1, σc2, ξs, δc1s, δc2s, and δc1c2 (where no and the area/molecule of the surfactant are, of course, known). In addition the three separations are not independent, in that
δc1c2 ) δc1s - δc2s
(17)
This provides an important test of self-consistency of the analysis and the data. The result of this analysis for 2.75 × 10-4 M C16TAB and 2 × 10-3 M C16TAB + 14 g/L benzyl alcohol are summarized in Table 2 and compared with earlier results from Lu et al.23 for 2.75 × 10-4 M C16TAB (but with a slightly different labeling scheme). The results are also represented as volume fraction profiles for the different components at the interface in Figure 3. The results for the structure at 2.75 × 10-4 M are in good agreement with the previous measurements of Lu et al.:24 the parameters are all within the quoted errors. Slight differences might be expected as slightly different labeling schemes were used. The widths of the individual components have a large contribution from capillary waves,24 and the conformation of the outer part of the
Mixed Monolayer of C16TAB and Benzyl Alcohol Table 2. Structural Parameters Obtained from Kinematic Analysis value at given surfactant conc param
2.75 × 10-4 Ma
σc1 (Å) σc2 (Å) ξs (Å) δc1s (Å) δc2s (Å) δc2c2 (Å) σba (Å) δc1ba (Å) δc2ba (Å) δbas (Å) area/(mol of C16TAB) (Å2) area/(mol of benzyl alcohol) (Å2) a
From Lu et al.24
2.75 × 10-4 Mb
11.0 ( 2 11.0 ( 2 5.5 ( 0.5 9.0 ( 0.5 4.0 ( 0.5 5.0 ( 1.0
12.0 12.0 5.0 10.0 3.0 6.0
60.0 ( 3
56 ( 3
2 × 10-3M + 14 g/L benzyl alcohol 13.0 13.0 6.0 8.0 3.0 6.0 13.0 ( 2 5.0 ( 0.5 2.0 ( 0.5 4.0 ( 0.5 58 ( 3 55 ( 3
b
This study.
Langmuir, Vol. 14, No. 8, 1998 2143
observed in the structure of the mixtures of SDSdodecanol11 and of C12E6/C16TAB,15 where in those cases the addition of dodecanol to SDS and C12E6 to C16TAB draws the ionic surfactants closer to the solvent. The conclusion from previous studies23,24 that the alkyl chains are, on average, strongly tilted away from the surface normal but that the part of the alkyl chain next to the headgroup is less tilted is borne out by this work. However, contrary to the earlier suposition the addition of benzyl alcohol does not significantly alter the structure of the C16TAB monolayer at the resolution of the labeling scheme used here. This suggests that at an area/molecule of 60 Å2 the monolayer structure is already dominated by the surfactant packing and that the benzyl alcohol does not significantly alter that. With this in mind it is important to determine where the benzyl alcohol is located at the interface, and we report in the last section measurements made to determine the location of the benzyl alcohol relative to the solvent and C16TAB alkyl chain. Position of the Benzyl Alcohol in the Adsorbed Monolayer. We have made neutron reflectivity measurements on the neutron reflectivity for the labeled combinations of dC10hC6hTAB/ba-d/nrw, dC10hC6hTAB/ ba-h/nrw, hC10dC6hTAB/ba-d/nrw, hC10dC6hTAB/ba-h/ nrw, hC16hTAB/ba-h/D2O, hC16hTAB/ba-d/nrw, and hC16hTAB/ba-d/D2O to determine the position of the benzyl alcohol in the C16TAB layer. From dC10hC6hTAB/ba-d/ nrw we have
R(Q) )
16π2 [bc12hc1c1 + bba2hbaba + 2bc1bahc1a] (18) Q2
and from hC16hTAB/ba-d/D2O
R(Q) )
Figure 3. Experimentally determined volume fraction distributions of the different components of the C16TAB at the air-water interface for (a, top) 2.75 × 10-4 M (from ref 24) (b, middle) 2.75 × 10-4 M (this study), and (c, bottom) 2 × 10-3 M + 14 g/L benzyl alcohol. The solid line is the outer C10 of the alkyl chain (C6 in the case of a); the dashed line, the inner C6 of the alkyl chain; and the dotted line, the solvent. The dotteddashed line is the total volume fraction (excluding the headgroup which was not measured in this study).
alkyl chain23,24 also suggests that the labeled C10 might not be expected to be significantly different from the C6 labeled in the earlier work.23 The structural parameters obtained for 2 × 10-3 M C10TAB + 14 g/L benzyl alcohol (at essentially the same area/molecule for the C16TAB as the measurements without benzyl alcohol) are similar to those obtained for the measurements without added alcohol. There are however some systematic differences. The chain distributions are slightly broader and the outer part of the chain is closer to the solvent distribution when benzyl alcohol is present. A simillar trend has been
16π2 2 [bs hss + bba2hbaba + 2bbashbas] (19) Q2
Using hC16hTAB/ba-h/D2O to obtain hss, hC16hTAB/bad/nrw to obtain hbaba, and dC10hC6hTAB/ba-h/nrw to obtain hc1c1 in combination with eqs 18 and 19 the cross-terms hc1ba and hbss can be obtained. A similar expression for hc2ba can be derived from hC10dC6hTAB/ba-d/nrw. hc1ba, hc2ba, and hbas then provide an estimate of the position of the benzyl alcohol at the interface relative to the solvent distribution and the C10 and C6 labeled groups of the C16TAB alkyl chain. The partial structure factors hc1ba, hc2ba, and hbas, plotted as Q2hc1ba, Q2hc2ba, and Q2hbas are shown in Figure 4. The solid lines are fits using eqs 15 and 16 and the parameters in Table 2. The experimentally determined volume fraction profile for the C16TAB and benzyl alcohol are shown in Figure 5, and the structure of the mixed monolayer, showing the realtive positions of the C16TAB and benzyl alcohol, are illustrated schematically in Figure 6. The results suggest that benzyl alcohol is centered on the third to fourth methylene group from the C16TAB headgroup. Bain et al.14 have used a simillar experimental approach to determine the position of the p-tosylate ion in the C16TA+ monolayer. Unlike benzyl alcohol, the tosylate ion is not surface active on its own, but the center of the p-tosylate distribution was found to be coincident on the fifth to sixth methylene group from the headgroup of the C16TAB alkyl chain; suggesting that it is behaving like a cosurfactant in the C16TA+ monolayer. Lu et al. have previously measured the structure of the mixed monolayers of SDS/dodecanol11 and dodecane/CnTAB,12 and Penfold et al.15 have measured the structure of the mixed monolayer of C16TAB/C12E6. In the C16TAB/
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Figure 5. Experimentally determined volume fraction profiles for benzyl alcohol and the different labeled components of C16TAB at the air-water interface for 2 × 10-3 M C16TAB + 14 g/L benzyl alcohol. The solid line is the outer C10 of the alkyl chain; the dashed line, the inner C6 of the alkyl chain; the dotted line, the solvent; the dotted-dashed line, the total volume fraction; and the double-dotted-dashed line, benzyl alcohol.
Figure 4. Partial structure factors (a, top) Q2hc1ba, (b, middle) Q2hc2ba, and (c, bottom) Q2hbas for 2 × 10-3 M C16TAB + 14 g/L benzyl alcohol. The solid lines are fits to eqs 15 and 16 for the parameters in Table 2.
C12E6 mixture the two surfactant distributions were totally coincident at the interface, whereas the dodecane distribution was centered on the alkyl chain distribution for C14TAB and coincident with the outer C6 of the C16TAB and the dodecanol distribution relative to the SDS is intermediate between those two extremes. The position of benzyl alcohol at the interface relative to the C16TAB molecule suggests that it is acting very much like a cosurfactant and that it is optimally packed within the layer. The benzyl alcohol distribution is located within the most ordered region of the C16TAB alkyl chain distribution, the part that is least tilted and essentially orthogonal to the surface. This could well explain why the addition of benzyl alcohol appears to have little effect on the structure of the C16TAB alkyl chain. In contrast Lu et al.12 have observed that the incorporation of dodecane into the CnTAB alkyl chain region results in an increase in the extension of the C12TAB alkyl chain. For example, for C16TAB the addition of dodecane increases the alkyl chain extent from 17.5 to 20 Å. Partial labeling of the alkyl chain showed this to be due to a change in the
Figure 6. Schematic representation of the structure of the mixed C16TAB and benzyl alcohol monolayer, showing the relative positions of the C16TAB and benzyl alcohol molecules.
conformation of the chain and not just an increase in roughness. The same partial labeling showed that the distribution of dodecane at the interface was coincident with the outer C6 of the C16 alkyl chain. From the measurement of the structure of the mixed SDS/dodecanol monolayer11 the dodecanol is displaced toward the air interface relative to the SDS. Although some of the differences between our results for C16TAB and benzyl alcohol and those for SDS/dodecanol can be attributed to the different molecular structures, the differences in structure between the two cosurfactants are more significant. The benzyl alcohol is a smaller, more polar molecule than the dodecanol. It is hence likely that the cosurfactant will have an increasingly larger effect on the monolayer structure as it is made more hydrophobic (for example, by replacing the benzyl alcohol with 2-phenylethanol), and such measurements are now planned. LA9710740
