water interface studied by neutron reflection

Journal of Colloid and Interface Science 325 (2008) 114–121

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Structure of adsorbed layers of nitrophenoxy-tailed quaternary ammonium surfactants at the air/water interface studied by neutron reflection Xu Huang a , Yilin Wang a,∗ , Chuchuan Dong b , Hsin-Hui Shen b , R.K. Thomas b,∗ a b

Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 20 March 2008 Accepted 22 May 2008 Available online 29 May 2008

The adsorbed layers of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (C10 TAB) and N,N,N  ,N  -tetramethyl-N,N  -bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide [(C10 )2 C6 ] at the air/water interface have been studied by neutron reflection. The coverage of the surfactants was obtained over the concentration range from critical micelle concentration (CMC) to CMC/100. The area per C10 TAB molecule changes from 50 ± 3 to 390 ± 60 Å2 over this concentration range and the area per (C10 )2 C6 molecule changes from 139 ± 3 to 288 ± 10 Å2 . The overall thicknesses (single uniform layer) of the surfactant layers at CMC are about 19 and 16 Å for C10 TAB and (C10 )2 C6 respectively. The distributions of the C10 chains show that the chains of both surfactants are tilted away from surface normal, with the tilt increasing in the outer part of the layer. The distribution of C10 chains in (C10 )2 C6 is narrower than that in C10 TAB, indicating that the alkyl chains of (C10 )2 C6 are more tilted. For both surfactants, the broad nitrophenoxy distribution may indicate significant positional disorder of the nitrophenoxy groups along the surface normal direction and their intermixing with alkyl chains in the adsorbed layer. © 2008 Elsevier Inc. All rights reserved.

Keywords: Neutron reflection Surface coverage Adsorbed layer structure

1. Introduction Interfaces play essential roles in numerous physical and chemical phenomena, e.g., solvent extraction, emulsification, phase transfer catalysis [1–3], and biomimetic membranes [4,5]. In the last two decades or so, several surface-specific techniques capable of investigating structural aspects of wet interfaces have been developed. Prominent among these are second harmonic generation (SHG), sum frequency spectroscopy (SFS) and neutron/X-ray reflection, each accessing different characteristic features of the interface. In some of these new developments, spectrally active molecules are frequently utilized as probes of the interface. Combined with surface-specific techniques, these probes can be particularly useful for assessing the nature of the interfacial environment. For example, it has been proposed that the thickness and roughness of liquid/liquid interfaces can be probed by the combination of SHG spectra and so-called ‘molecular rulers,’ which are surfactants containing the solvent-sensitive chromophore based on the 4-nitrophenoxy group [6–10]. The 4-nitrophenoxy group is a sensitive indicator to solvent polarity. Changes in the length of the

*

Corresponding authors. Fax: +86 10 82615802. E-mail addresses: [email protected] (Y. Wang), [email protected] (R.K. Thomas). 0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.05.043

© 2008

Elsevier Inc. All rights reserved.

spacer between this group and the surfactant head group generate molecules that span different interfacial widths. Depending on the spacer length, the chromophore is exposed to different chemical environments and its spectral response can therefore be used as a probe of the interfacial polarity at different levels in the interface. The limitation of this otherwise attractive method is that the introduction of the chromophore alters the nature of the surfactant to an extent that it is no longer representative of a typical surfactant probe. For example, if the chromophore at the hydrophobic terminus of the molecule is at all hydrophilic, the conformation adopted by the modified surfactant may be quite different from a conventional surfactant. This is a distinct possibility in that the nitrophenoxy-group is both polar and contains oxygen atoms, which always tend to be hydrophilic. This situation is exacerbated by the fact that SHG does not give any direct information about the thickness of the adsorbed layer, the degree of immersion of the surfactant in water, or the conformation of the alkyl chains, so that there is no internal check in the experiment of where the chromophore is located in the interface. Neutron reflection gives direct measurements of coverage and interfacial structure and, in order to give a more solid structural basis to the applicability of the molecular ruler idea in SHG measurements, we have carried out a thorough structural investigation of two different solvatochromic surfactants at the air/water interface using isotopic labeling to optimize the resolution of the experiment.

X. Huang et al. / Journal of Colloid and Interface Science 325 (2008) 114–121

115

rification of hhC10 hTAB and (hhC10 )2 hC6 have been described elsewhere [18,19] and the various deuterium labeled surfactants were prepared similarly. At least three recrystallizations of the crude surfactants from acetone/methanol were carried out to obtain satisfactory purity as assessed from the absence of a minimum in the surface tension around the critical micelle concentration (CMC). 2.2. Surface tension

Fig. 1. Chemical structures of C10 TAB and (C10 )2 C6 .

Neutron reflection has been successfully applied to probe internal structure and distribution of surfactant molecules along the surface normal direction and it is particularly effective at the air/water interface. Isotopic substitution (hydrogen/deuterium) can be used to create differences in the neutron refractive index at the surface that highlight different structural features [11]. The technique has been widely applied to amphiphilic systems, e.g., Li et al. [12,13] have employed selective deuterium labeling to study the two branched chains of sodium bis(2-ethyl-1-hexyl) sulfosuccinate, Li et al. [14] have investigated the position of the p-tosylate counterion at the air/water interface in monolayers of the hexadecyltrimethylammonium cation (C16 TA+ ), and Penfold et al. [15] have studied the coadsorption of phenyl ethanol with the cationic surfactant C16 TAB at the air/water interface and found that the addition of the aromatic alcohol subtly alters the conformation of the C16 TAB and draws it closer to the aqueous subphase. Here, we apply neutron reflection to determine the coverage and structure of layers of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (C10 TAB) and N,N,N  ,N  -tetramethylN,N  -bis[10-(4-nitrophenoxy)-decyl]-1,6-hexanediammonium dibromide [(C10 )2 C6 ] adsorbed at the air/water interface. The chemical structures of single chain surfactant C10 TAB and gemini surfactant (C10 )2 C6 are presented in Fig. 1. 2. Materials and methods 2.1. Materials The protonated organic materials and inorganic salts for the synthesis of the surfactants were from Sigma and Aldrich and were of analytical grade. All the organic solvents were dried and distilled. Deuterated dimethylamine and trimethylamine were obtained from D. Styrkas and Aldrich respectively. Deuterated 4aminophenol was synthesized by H/D exchange from 4-aminophenol hydrochloride according to the general procedure of Werstiuk and Kadai [16], and then was oxidized to [d4 ]-4-nitrophenol with Oxone [17]. Chain-deuterated 10-bromodecanol was prepared by reduction of perdeuterated decanedioic acid with LiAlD4 to give decane-1,10-diol followed by partial bromination of the decane1,10-diol. Four isotopic species of the single chained ammonium bromide − surfactant, O2 N–C6 H4 OC10 H20 N(CH3 )+ 3 Br , O2 N–C6 D4 OC10 H20 N+ − + − (CD3 )3 Br , O2 N–C6 H4 OC10 D20 N(CD3 )3 Br , O2 N–C6 D4 OC10 D20 N− (CD3 )+ 3 Br , and four isotopic species of the gemini surfactant, − (O2 N–C6 H4 OC10 H20 N(CH3 )+ (O2 N–C6 D4 OC10 H20 N2 )2 C6 H12 2Br , + − − (CD3 )2 )2 C6 D12 2Br , (O2 N–C6 H4 OC10 D20 N(CD3 )+ 2 )2 C6 D12 2Br , and + − (O2 N–C6 D4 OC10 D20 N(CD3 )2 )2 C6 D12 2Br , were synthesized and employed in the neutron reflection experiments. These formulae are abbreviated to hhC10 hTAB, dhC10 dTAB, hdC10 dTAB, ddC10 dTAB, (hhC10 )2 hC6 , (dhC10 )2 dC6 , (hdC10 )2 dC6 , and (ddC10 )2 dC6 , respectively. The methods of preparation and pu-

Surface tension measurements of the surfactants were carried out by the drop volume method at 25.0 ± 0.1 ◦ C [20]. The adsorption amount of surfactant was calculated according to the Gibbs adsorption equation,

Γ =−



1 2.303nR T

dγ d lg C

 .

(1)

T

Here γ is the surface tension in mN m−1 , C is the surfactant concentration, Γ is the saturated adsorption amount in mol m−2 , T is the absolute temperature, R = 8.314 J mol−1 K−1 , dγ /d lg C is the maximal slope in each case, and n is a constant dependent upon the number of individual ions comprising the surfactant (n = 2 for single the chain surfactant and n = 3 for the gemini surfactant). Then the minimum average surface area per surfactant molecule ( A min ) is obtained from the saturated adsorption by A min =

1

(2)

NAΓ

where N A is the Avogadro constant. 2.3. Neutron reflectivity Measurements were performed on the SURF reflectometer at ISIS, Rutherford Appleton Laboratories, Didcot, UK, using a neutron wavelength band of 0.5–6.8 Å at a fixed incident angle of 1.5◦ , to provide a momentum transfer, κ = 4π sin θ/λ, where θ is the glancing angle of incidence and λ is the neutron wavelength, in the range 0.048–0.5 Å−1 . The data were normalized to the incident beam spectral distribution and the variation of detector efficiency with wavelength, and put on an absolute reflectivity scale by scaling to the reflectivity from the surface of D2 O using procedures described previously [21]. A flat background, determined by extrapolation of the data to a high value of κ , was subtracted from all the measured reflectivity profiles. This has been shown previously to be a valid procedure for removing the incoherent scattering background from the bulk solution [22]. High-purity water was used for all the measurements (Elga Ultrapure), and the D2 O was obtained from Fluorochem. All of the glassware and Teflon troughs used for the reflectivity measurements were cleaned in alkaline detergent (DECON 90), followed by repeated washing by ultrapure water. All measurements were made at 25 ◦ C. 3. Modeling neutron reflectivity profiles Specular neutron reflection provides information about inhomogeneities in the direction normal to an interface [23]. The neutron reflectivity R ( Q ) is given in the kinematic approximation by R(Q ) =

 ρ ( Q )2

16π 2 

κ

2

(3)

where κ is the momentum transfer as defined above and ρ ( Q ) is the one-dimensional Fourier transform of the scattering length

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density profile in the direction normal to the interface, is given by

ρ (z), which

∞

ρ(Q ) =

ρ (z) exp(i Q z) dz

ρ ( z) =



ni ( Q ) =

A=

ni ( z)b i

(5)

b

(6)

ρτ



where b is the sum of the scattering lengths of the nuclei in the surfactant molecule. When fitting the reflectivity profile with a uniform layer model, ρ and τ can be individually varied over a wider range than their combination to give A, so that A is always accurately determined even though the assumption that the layer is uniform may not be a good one. The kinematic model offers a more direct means for determining the distribution and relative positions of the different deuterium-labeled components at the interface. Since isotopic labeling can provide a set of different reflectivity profiles for a single chemical structure, the fitting of a set of isotopic compositions to a single structure model greatly reduces any uncertainty in the interpretation of the layer structure. This method has been extensively used to determine the structure of surfactant monolayers [13,27– 31] and mixed monolayers [14,32,33] to a resolution of ∼2 Å. In terms of the distributions of different components in the monolayers of C10 TAB and (C10 )2 C6 , the scattering length density profile can be written as

ρ ( z ) = b  n ( z ) + b c nc ( z ) + b h nh ( z ) + b w nw ( z )

(7)

where , c, h and w refer to nitrophenoxy groups, C10 chains, quaternary ammonium headgroups (with –(CH2 )6 – spacer for the gemini surfactant), and water. Substituting Eq. (7) into Eq. (3) gives



R ( Q ) = 16π 2 / Q



2

b2 h + b2c hcc + b2h hhh + b2w hww

+ 2b bc hc + 2b bh hh + 2b bw hw  + 2bc bh hch + 2bc bw hcw + 2bh bw hhw

(11)

 2 h ii = ni ( Q ) ,

(8)

and h i j are the cross-partial structure factors, containing the information about the relative positions of the fragments at the interface, given by

h ii =

1 A 2i



exp −

σi2 Q 2



(12)

8

where σi is the full width of the i fragment layer at 1/e of its maximum height and A i is the area occupied by the fragment. The nitrophenoxy group, C10 chain, and headgroup can be satisfactorily characterized by Eq. (10) with different σi values. For the purposes of expressing the reflectivity analytically, the water distribution can be represented by a tanh distribution as

 hww = n20

ζπ

2

2

 csc2

ζπ Q



2

(13)

where n0 is the number density of bulk water and ζ is the width parameter for the tanh profile (in practice a more realistic water distribution was used in conjunction with numerical Fourier transformation). The cross-partial structure factors can be obtained from self-partial structure factors when the number distribution of components i and j are both even about their centers by h i j = ±(h ii h j j )1/2 cos Q δi j

(14)

where δi j is the separation of the centers of the two distributions. When one distribution is exactly even and the other exactly odd, then h i j = ±(h ii h j j )1/2 sin Q δi j .

(15)

Thus, Eqs. (14) and (15) offer a method to determine the separation between pairs of distributions. Further constraints can be made by

δh = δc + δch ,

(16)

δw = δc + δcw ,

(17)

δcw = δch + δhw .

(18)

If Eqs. (14) and (15) are incorporated into Eq. (8), an approximate expression for the reflectivity can be expressed by





R ( Q ) = 16π 2 / Q 2 b2 h + b2c hcc + b2h hhh + b2w hww

+ 2b bh (h hhh )1/2 cos Q δh

(9)

h i j = Re ni ( Q )n j ( Q ) .

where ni ( z) is the number density distribution of group i normal to the interface. For soluble surfactants, which form a relatively low density layer at the surface, it has been found that the Gaussian distribution is a good representation of the number density profile of a layer consisting of a fragment of amphiphile [28], especially when the length of the surfactant chain is relatively short. For a Gaussian distribution

+ 2b bc (h hcc )1/2 cos Q δc

where the h ii are the self-partial structure factors, describing the distributions of the individually labeled fragments, given by



ni ( z) exp(i Q z) dz

−∞

where ni ( z) is the number density profile of species i and b i is its empirical scattering length. Since hydrogen and deuterium have opposite signs in neutron scattering lengths, H/D isotopic substitution is particularly useful for highlighting the surface layer and for manipulating ρ ( z), which is the essence of the utilization of the neutron reflection technique to determine surface structure with a resolution of ∼2 Å. The simplest example of the effect of isotopic substitution is that the scattering length density of the aqueous solvent can be matched to that of air by mixing 9 mol of H2 O and 1 mol of D2 O (null reflecting water, NRW). Any reflection of neutrons from the surface of such a solution can only result from the adsorbed layer, i.e., the technique is surface specific. Assuming the adsorbed layer is homogeneous [24], the optical matrix method [25,26] can be used to analyze the reflectivity profile and derive a scattering length density ρ and a layer thickness τ for the layer. The area per molecule of the adsorbed surfactant, A, is given by



∞

(4)

−∞

and

The ni ( Q ) are the one-dimensional Fourier transforms of the number density distribution of group i in the direction normal to the interface, and are given by

(10)

+ 2b bw (h hww )1/2 sin Q δw + 2bc bh (hcc hhh )1/2 cos Q δch + 2bc bw (hcc hww )1/2 sin Q δcw

 + 2bh bw (hhh hww )1/2 sin Q δhw .

(19)

The 10 terms in Eq. (19) suggest that 10 independent measurements should be necessary for determining the structure of the layer at interface. However, consideration of Eqs. (16)–(18) shows that the separations are not independent, so Eq. (19) contains only

X. Huang et al. / Journal of Colloid and Interface Science 325 (2008) 114–121

four self-partial structure factors and three unknown separations. However, it is not necessary to measure one reflectivity profile to obtain each unknown parameter because a limited set of reflectivity measurements may contain enough information to extract the structural parameters, provided that the most sensitive contrasts are chosen [34]. We have therefore performed six neutron reflection measurements for the single chain surfactant C10 TAB, which are ddC10 dTAB, dhC10 dTAB and hdC10 dTAB in NRW, dhC10 dTAB, hdC10 dTAB and hhC10 hTAB in D2 O. For the gemini surfactant (C10 )2 C6 , we combined the –(CH2 )6 – spacer and the two quaternary ammonium groups as the modified headgroup. The reflection profiles for (C10 )2 C6 are studied from their C10 TAB counterparts, which are (ddC10 )2 dC6 , (dhC10 )2 dC6 and (hdC10 )2 dC6 in NRW, (dhC10 )2 dC6 , (hdC10 )2 dC6 and (hhC10 )2 hC6 in D2 O. The sets of reflectivity profiles were fitted simultaneously by least squares using numerical Fourier transformation, which allowed us to use a more realistic model of the water distribution than outlined above [35]. In this model the water is space filling up to a certain level in the surface and then decays as a half Gaussian. To calculate the space filling condition requires an estimate of the volumes of the different surfactant fragments. These were taken to be 200, 250, 150 and 400 Å3 for the nitrophenoxy group, C10 chain, trimethylammonium (for C10 TAB) and tetramethyl-1,6-hexanediammonium (for (C10 )2 C6 ) headgroups, respectively. While some of these may have significant errors, the only aspect of the fitting procedure that is affected by these errors is the water distribution and the effect on this is not large because the space filled by these fragments is quite small. In the fitting procedure the position of the water interface is defined as the position of the Gibbs plane for zero adsorption of water. In the fitting, the calculated reflectivity was corrected by an expression derived by Crowley [36], which corrects for the inaccuracy of the kinematic model at low momentum transfer. There are seven adjustable parameters in the fitting procedure. These are the widths of the distributions of the nitrophenoxy group (σ ), C10 chain (σc ) and headgroup (σh ), the area per molecule ( A), and the distances between the nitrophenoxy and head groups (δh ), between the C10 chain and head groups (δch ), and between the head groups and the zero adsorption plane of water (δhw ). For convenience of the display, but not part of the fitting process, a small flat background, the same for all samples, has been added to any data plotted in the results section. Although isotope effects are expected to be small, they do nevertheless occur, either as genuine isotope effects or as variations in sample quality. These effects usually show up as small differences in coverage when differently deuterium labeled species are studied in NRW. We know from experience that the variation of the structural parameters with coverage is weak, i.e. small changes in coverage do not affect the structure within the relatively low resolution of the experiment. An improvement in the overall fit can then be obtained by first optimizing the fit using the constraint of the same area per molecule for all six samples and then allowing a further minimization of the NRW fits at constant average coverage. Both methods have been used here and no difference was observed between the structures with and without the additional minimization. The advantage of the extra minimization is that it ensures that all profiles contribute to the structure with equal weight. 4. Results and discussion 4.1. Surface coverage The surface tension curves of the fully hydrogenated surfactants hhC10 hTAB and (hhC10 )2 hC6 are shown in Fig. 2. The CMC values of the two surfactants are identified to be 6.6 mM for hhC10 hTAB and 0.6 mM for (hhC10 )2 hC6 from the breaks

117

Fig. 2. Variation of the surface tension of hhC10 hTAB and (hhC10 )2 hC6 with concentration at 25.00 ± 0.05 ◦ C.

in the curves. The limiting surface tensions at the CMC (γCMC ) are unusually high at 53 and 54 mN m−1 for the single chained and gemini surfactants respectively. For the corresponding surfactants with terminating methyl groups these values would be in the region of 42 mN m−1 . The difference can be attributed either to the presence of the much higher surface energy nitrophenoxy group at the outer surface or destabilization of the underlying chain layer by the polar nitrophenoxy group. The surface area per surfactant molecule ( A) values of the two surfactants derived from the slope of surface tension curve before the CMC are 94 Å2 for hhC10 hTAB and 256 Å2 for (hhC10 )2 hC6 , respectively. These values have been calculated using prefactors of 2 and 3 in the Gibbs equation, respectively. The surface coverage of deuterated and partially deuterated surfactants in NRW can be obtained directly from the neutron reflectivity experiment. Isotopic differences between deuterated and protonated samples are generally found to be small for this type of amphiphilic layer [37]. The observed reflectivity profiles of ddC10 dTAB and (ddC10 )2 dC6 at five different concentrations in NRW are shown in Fig. 3 together with the best fits of a uniform monolayer to the data using the optical matrix method. Three parameters are obtained on fitting the model, which are the thickness of the layer τ , the scattering length density of the layer ρ , and the area per molecule (proportional to 1/surface coverage) A, and these parameters are listed in Table 1. The fits at CMC/100 are not as well as those at higher concentrations, since the reflection intensities are relatively weak at such low concentrations, which may induce larger error in the experiments. The area per molecule of ddC10 dTAB is about 50 Å2 at the CMC, and rapidly increases below about CMC/10 to 390 Å2 at CMC/100. The value of A at the CMC is comparable with that of C12 TAB, which is 48 Å2 [38]. Thus, the introduction of the nitrophenoxy group has little effect on the surface coverage for the single chain surfactant. However, the area per molecule of (ddC10 )2 dC6 is 139 Å2 , which is significantly larger than that of the nearest equivalent cationic gemini surfactant, 1,6-bis(Ndodecyldimethylammonium)hexane bromide C12 C6 C12 (95 Å2 ), at its CMC [39,40]. It is also unusual that the area per molecule of a gemini surfactant should be larger than twice that of its single chain counterpart, since the introduction of the spacer group connecting the two monomers together, especially one only 6 carbon atoms long, will make the gemini molecule have a greater tendency to self-assemble and to pack more tightly at the air/water interface. However, (ddC10 )2 dC6 has a strong tendency to form small aggregates in the premicelle region because the nitrophe-

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X. Huang et al. / Journal of Colloid and Interface Science 325 (2008) 114–121

Fig. 3. Neutron reflectivity profiles of (a) ddC10 dTAB and (b) (ddC10 )2 dC6 in NRW at different concentrations. (1) CMC, (!) CMC/3, (P) CMC/10, (e) CMC/30, and (E) CMC/100. The continuous lines are fits using the single uniform layer model with the parameters given in Table 1. Table 1 Fitted parameters for ddC10 dTAB and (ddC10 )2 dC6 in NRW at 25 ◦ C using the single uniform layer model Surfactant

Concentration

τ (Å)

ρ (10−6 Å−2 )

A (Å2 )

ddC10 dTAB

CMC 1/3 CMC 1/10 CMC 1/30 CMC 1/100 CMC

19 ± 1 19 ± 1 17 ± 1 12 ± 1 10 ± 1

4.1 ± 0.1 4.0 ± 0.1 3.7 ± 0.1 2 ± 0.1 1.0 ± 0.1

50 ± 3 51 ± 3 61 ± 4 160 ± 20 390 ± 60

(ddC10 )2 dC6

CMC 1/3 CMC 1/10 CMC 1/30 CMC 1/100 CMC

16 ± 1 16 ± 1 16 ± 1 14 ± 1 14 ± 1

3.8 ± 0.1 3.65 ± 0.1 2.25 ± 0.1 2.15 ± 0.1 2.0 ± 0.1

139 ± 3 150 ± 3 235 ± 5 281 ± 10 300 ± 20

noxy groups have a tendency to replace water molecules close to the aliphatic chain, providing an exothermic component to the surfactant aggregation [18]. It is therefore possible that better solubilization of these molecules favors their remaining in solution rather than adsorbing at the surface. On decreasing the concentration to CMC/100, the area per (ddC10 )2 dC6 molecule increases to 288 Å2 , a smaller relative change than that of ddC10 dTAB. There are large discrepancies of surface area per surfactant molecule ( A) between the values determined by surface tension and neutron reflection methods. The value obtained from neutron reflectometry is independent of any model and requires no assumptions. Apart from experimental accuracy (approximately ±5% at the highest coverages) this must be regarded as the correct value. The value obtained from surface tension data requires an assumption to be made about state of association of the surfactant

and counterions in solution, i.e. the value of the Gibbs prefactor in the Gibbs equation. Either ion pairing between counterion and surfactant or preassociation of surfactant molecules below the CMC would change the value of the Gibbs prefactor. A large discrepancy has also been found previously for simpler geminis [39,40] and in all assessments of area per gemini surfactant it seems to be generally accepted that the coverage must be much higher than obtained by an analysis of the surface tension data based on complete dissociation of the surfactant into monomers and ions at all concentrations [41]. Any association of surfactant and counterion in the bulk solution is in principle accessible using electrical conductivity and Zana [42] has shown such association in short chain geminis, where the CMC is high enough for association to become important. However, the surface tension is likely to be much more sensitive to this effect than the electrical conductivity and association is therefore the likely explanation of the discrepancy for the present system. For the purposes of the present work the accurate internal calibration of the coverage in the reflectometry experiment makes our conclusions about the surface structure independent of the problem with surface tension. However, it is interesting to consider why association might become more likely in surface active molecules. It was shown by Kirkwood and Westheimer [43] some 70 years ago that the presence of low dielectric material in a doubly charged molecule can depress very strongly the second ionization. They used the simple model of two charges embedded in an ellipse of low dielectric, itself embedded in water, to explain a pattern of ionization constants in dibasic alkane dioic acids that deviates strongly from simple electrostatics. More recently, Kornyshev et al. [44] have done parallel calculations for two charges in an infinite cylinder of low dielectric constant and have shown that the effect of low dielectric material in the vicinity of a charged group may strongly suppress ionization. The more general nature of this calculation suggests that incomplete ionization may be a more widespread phenomenon in surfactant systems than has been supposed and may even extend to some singly charged systems. The thicknesses of the surfactant layers at the CMC are about 19 and 16 Å for ddC10 dTAB and (ddC10 )2 dC6 respectively. Comparing these values with their fully extended chain length determined from the CPK model, which is about 20 Å, we conclude that the hydrophobic chains of (ddC10 )2 dC6 are more tilted away from the surface normal because of their looser packing at the interface, although it must be borne in mind that there is also a capillary wave (thermal fluctuation) contribution to the layer thickness, which will be discussed in the context of the adsorbed layer structure. 4.2. Adsorbed layer structure At the concentration of CMC, the neutron reflectivity profiles were fitted using the kinematic approximation and partial structure factors of the different components of C10 TAB and (C10 )2 C6 to describe the adsorbed layer structure. Each set of six reflectivity profiles of different substituted species of C10 TAB and (C10 )2 C6 has been fitted simultaneously by least squares using Eqs. (10)– (13). The observed reflectivity profiles with the best fits to the data for C10 TAB and (C10 )2 C6 are shown in Figs. 4 and 5, respectively. The fitted parameters are listed in Table 2, providing the area per molecule ( A), the width distributions of nitrophenoxy group (σ ), C10 chain (σc ) and headgroup (σh ), and the distances between nitrophenoxy group and headgroup (δh ), between C10 chain and headgroup (δch ), and between headgroup and water (δhw ). Using these parameters, the structure of the layer can be displayed in terms of the volume fraction profiles, where the number density distributions of each component of the surface layer have been converted to volume fraction units as a function

X. Huang et al. / Journal of Colloid and Interface Science 325 (2008) 114–121

119

Fig. 4. Simultaneous fits of six neutron reflectivity profiles of different labeled C10 TAB isotopes at CMC. The continuous lines are the best fits using the parameters in Table 2. (a) In NRW (1) ddC10 dTAB, shifted up by a factor of 10 for clarity, (!) dhC10 dTAB, and (P) hdC10 dTAB, shifted down by a factor of 10; (b) in D2 O (1) dhC10 dTAB, (!) hdC10 dTAB, shifted down by 10, and (P) hhC10 hTAB, shifted up by 10.

Fig. 5. Simultaneous fits of six neutron reflectivity profiles of different labeled (C10 )2 C6 isotopes at CMC. The continuous lines are the best fits using the parameters in Table 2. (a) In NRW (1) (ddC10 )2 dC6 , shifted up by 10 for clarity, (!) (dhC10 )2 dC6 , and (P) (hdC10 )2 dC6 , shifted down by 10; (b) in D2 O (1) (dhC10 )2 dC6 , (!) (hdC10 )2 dC6 , shifted down by 10, and (P) (hhC10 )2 hC6 , shifted up by 10.

of the distance normal to surface (z), using the volumes of the fragments given above. We compare the volume fraction distributions of C10 TAB and (C10 )2 C6 at their CMC in Fig. 6. The maximum volume fraction reached for the single chain is about 0.7 at the CMC, which is about 10% higher than the gemini. As with all other surfactants [11] there is a significant amount of empty space/roughness in the layer and the gemini is the more open/disordered structure of the two. These fits show first of all that the values of the area per molecule for different isotopic species of C10 TAB and (C10 )2 C6 are consistent with each other within error (±3 or ±5 Å2 respectively). The most significant difference between the two surfactants is the width of the distributions of the C10 side chains σc , which is about 14.0 Å for C10 TAB, but only 11.5 Å for (C10 )2 C6 , although the intrinsic length of the alkyl chain does not change. The width parameters that describe the distribution of nitrophenoxy and cationic quaternary ammonium headgroup in the layer, σ and σh , are considerably larger than their maximum extended lengths, which implies either large positional disorder in the layer and/or significant roughness. As shown elsewhere [13,45], roughness makes a significant contribution to the observed width distribution for each fragment. The relationship between the intrinsic thickness of the fragment in the direction normal to the surface (l z ) and the roughness (ω ) can be approximately expressed by

Table 2 Structural parameters of C10 TAB and (C10 )2 C6 obtained from the best simultaneous fit to the neutron reflectivity data

σ 2 = l2z + ω2

(20)

where the value of l z depends on contributions from the extended component length, different chain conformations and the average

C10 TAB (C10 )2 C6

A (Å2 )

σ (Å)

σc (Å)

σh (Å)

δh (Å)

δch (Å)

δhw (Å)

51 ± 2 139 ± 3

19.0 ± 1.5 19.0 ± 1.5

14.0 ± 1 11.0 ± 1

12.5 ± 1 12.5 ± 1

10.0 ± 1 9.0 ± 1

6.5 ± 1 6.5 ± 1

0.0 ± 1 0.0 ± 1

orientation with respect to the surface normal. There are two possible contributions to the roughness of the surface, static disorder and capillary waves. On the other hand, the values of fragment separation δ depend only on the intrinsic dimensions normal to the surface and it is possible to use this parameter to derive quantitative information about the roughness. In the present case, there are two δ values containing information about the orientation of the chains, δh and δch . For simplicity, we assume that there is a single average chain orientation and that the chain is fully extended. The distance from the center of the quaternary ammonium headgroup to its adjacent CH2 , the length of the C10 hydrocarbon chain and the length of the chromophore are then 1.75, 12.5 and 7.0 Å, respectively. Thus, the calculated distance from the quaternary ammonium headgroup to the center of the C10 chain will be about 8.0 Å, and the distance from the center of the C10 chain to nitrophenoxy group will be about 9.5 Å. The average tilt angles of the inner part (adjacent to the headgroup) and the outer part of alkyl chain (adjacent to nitrophenoxy group) from the surface normal, θ1 and θ2 , can be derived by comparing the calculated distances with the observed values of δch and δc (δc = δh − δch ). Hence, the values of θ1 are 35 ± 5◦

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Fig. 6. Volume fractions distributions as a function of distance normal to surface ( z) for the different components of (a) C10 TAB and (b) (C10 )2 C6 at the air/water interface. (Black) Nitrophenoxy group, (red) C10 chain, (green) quaternary ammonium headgroup (including the –(CH2 )6 – spacer for (C10 )2 C6 ), and (blue) water. The total volume fractions are shown as dashed lines and the dotted line marks the point of zero adsorption for water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to be better solvated by the organic phase. However, here, where there is no solvation by an organic phase, the strongly polarized tail may induce the molecule to incline further away from the surface normal in order to solvate the polar groups into the adsorbed layer rather than stretch into the air. This is further substantiated by consideration of the widths of the fragments, to which we now turn. The roughness contribution to the thickness of the adsorbed layer can be estimated by subtracting the contribution of the projection of the C10 alkyl chain of about 10 Å along the surface normal from the overall width using Eq. (20). This gives a roughness of the C10 alkyl monolayer of about 10 ± 3 Å for C10 TAB. Considering the errors in the determination of the width distribution and the tilt angle, this roughness is comparable with values determined for Cn TAB at the CMC [30,38]. Allowing for the slightly larger tilt of the C10 alkyl, the similarly estimated roughness for (C10 )2 C6 is about 7 ± 3 Å, implying that the alkyl layer of the gemini surfactant is less rough. However, the discussion of the distribution of the nitrophenoxy group below indicates that the distribution of orientations of the chains may be too wide for the simple arguments about separation of roughness to be other than very approximate. If the roughness of the interface determined from σc is attributed entirely to capillary waves there should be an equivalent contribution to the width σ of the nitrophenoxy group. Removing the roughness contributions of 10 and 7 Å determined above for the chains in the two surfactants gives intrinsic widths of the nitrophenoxy group of 16 and 17.5 Å, to be compared with the maximum extended width of about 7 Å, i.e. there is an unusually large contribution of positional disorder to the nitrophenoxy group distribution. This suggests that the nitrophenoxy groups spend appreciable time within the layer of C10 alkyl chains or even in the aqueous region. These results are consistent with our observations from NMR measurements for the two surfactants [18] in aqueous solution, which suggest that in micelles the aliphatic chains partially overlap the aromatic groups at the end of the hydrophobic tails. Extensive mixing of the nitrophenoxy groups and alkyl chains, but with the structural parameters determined above, would mean that a range of different chain conformations would have to contribute to the structure. 5. Conclusions The surface coverage and structure of adsorbed layers of

C10 TAB and (C10 )2 C6 at the air/water interface have been in-

Fig. 7. The adsorbed monolayer of C10 TAB and (C10 )2 C6 at air/water interface, illustrating the mean separation and orientation of the fragments.

for C10 TAB and 41 ± 5◦ for (C10 )2 C6 , while the values of θ2 are 68 ± 5◦ for C10 TAB and 72 ± 5◦ for (C10 )2 C6 . The significant difference between θ1 and θ2 shows that the part of the chain next to the headgroup is, on average, oriented much more closely to the surface normal than the outer part of the chain. Also, the chain fragments in the gemini surfactant (C10 )2 C6 are slightly more tilted than those of C10 TAB. This is probably a direct result of the greater area available per chain in the surface layer of the gemini surfactant. A schematic diagram representing these differences is given in Fig. 7. The previous results from the polarization-dependent SHG measurements suggest that the nitrophenoxy group at the end of the molecule is inclined only 34◦ from the normal at the water/octane interface [9], suggesting that the chromophore itself adopts a more upright orientation in order

vestigated by the combination of neutron reflection and isotopic labeling. With a decrease in the surfactant concentration from CMC to 1/100 CMC, the area per surfactant molecule changes from 50 to 390 Å2 for C10 TAB, and from 139 to 288 Å2 for (C10 )2 C6 . It is thought that the greater tendency of the (ddC10 )2 dC6 molecules to solubilize in bulk solution causes (ddC10 )2 dC6 molecules to be more loosely packed than C10 TAB at the air/water interface. The thicknesses of the surfactant layers at the CMC are about 19 and 16 Å (uniform layer) for C10 TAB and (C10 )2 C6 , respectively, and the distribution of C10 chains in (C10 )2 C6 is narrower than that in C10 TAB, implying that the alkyl chains of (C10 )2 C6 are more tilted away from the surface normal. The unusually broad distribution of the sublayers formed by the nitrophenoxy groups in both C10 TAB and (C10 )2 C6 should indicate that the chromophores significantly immerse in the layer of C10 alkyl chains, though they are probably randomly oriented. This study indicates that the introduction of a nitrophenoxy group in the end of the surfactant alkyl chain causes significant deviations in molecular orientation from that of the surfactants without nitrophenoxy group. Thus this special orientation should be considered when this kind of surfactants is used as molecular rulers to study liquid–liquid interface structures by SHG technique.

X. Huang et al. / Journal of Colloid and Interface Science 325 (2008) 114–121

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