The structure of mixed surfactants at the air–water interface

Colloids and Surfaces A: Physicochemical and Engineering Aspects 155 (1999) 11–26

The structure of mixed surfactants at the air–water interface J. Penfold a,*, E.J. Staples b, I. Tucker b, R.K. Thomas c a ISIS Facility, Rutherford Appleton Laboratory, CLRC, Chilton, Didcot, Oxon, UK b Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, UK c Physical and Theoretical Chemistry, Oxford University, South Parks Road, Oxford, UK Received 21 November 1997; accepted 16 February 1998

Abstract The structure of the mixed non-ionic surfactant monolayer of monododecyl triethylene glycol (C E ) and 12 3 monododecyl octaethylene glycol (C E ) and of the mixed cationic–non-ionic surfactant monolayer of hexadecytri12 8 methyl ammonium bromide (C TAB) and monododcyl hexaethylene glycol (C E ) adsorbed at the air–water 16 12 6 interface are described. For the non-ionic surfactant mixture the frustration caused by packing the triethylene and octaethylene glycol headgroups results in a change of the surfactant structure compared to the pure monolayer of either surfactant. In particular the octaethylene glycol group is less extended, and the alkyl chain conformation is altered. Although the structures of the cationic and non-ionic surfactants in the non-ionic–cationic surfactant mixture are similar to those of the pure monolayers at an equivalent area per molecule, the detailed labelling schemes used reveal some changes. In the mixed monolayer the alkyl chains of both surfactants are more extended, and the amount of overlap between the surfactant and solvent distributions is greater. The structure of the C TAB/C E mixture is 16 12 6 contrasted with previously published results for the mixtures sodium dodecyl sulphate (SDS )-dodecanol and dodecyl trimethylammonium bromide (C TAB)-dodecane, which show an increasing change in position of the non-ionic 12 additive at the interface relative to the solvent distribution with increasing solubility of the non-ionic component. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Surfactant mixtures; Air–water interface; Adsorption

1. Introduction The study of the adsorption of surfactant mixtures at interfaces is of considerable interest, for both practical reasons [1] and for the theoretical challenges that their understanding imposes [2]. In most industrial, technological and domestic applications of surfactants, mixtures are used. This is because mixtures can enhance many aspects of performance, synergy [3] or because commercial * Corresponding author. Tel: +44 1235 5681; Fax: +44 1235 5642.

surfactants are inherently impure [4]. Mixed surfactants have been extensively studied using a wide range of techniques [1], such as surface tension [5], and have been the result of considerable theoretical effort [1,2,6–9]. The ideal solution approach [6 ], and the subsequent adaptation of regular solution theory [3,7,8], where the departure from ideality is characterized by an interaction parameter, b, has formed the basis of much of this theoretical effort. More recently Nikas et al. [9] have used a more fundamental approach, aimed at removing the phenomenological nature of the regular solution approach. The rigorous testing of such theories, and the availability of appropriate

0927-7757/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 9 2 7- 7 7 57 ( 9 8 ) 0 03 9 3 -8

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J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

data in the literature to stimulate the development of more sophisticated theories, have been hampered by the lack of suitable experimental techniques. This was remarked upon by Nikas et al. [9] most poignantly in their recent paper on nonionic mixtures. It has been demonstrated in a series of recent papers that neutron reflectivity, in combination with hydrogen/deuterium (H/D) isotopic substitution, has the selectivity to study the adsorption of mixed surfactants over a wide concentration range at the air–water interface [9–14], and to give detailed structural information [15,16 ]. Although the ability to measure interfacial compositions over a wide concentration range (from below to well in excess of the mixture critical micellar concentrations (cmc)) has been shown to be an important advance, it does not address all the assumptions in the recent theoretical developments. In particular, the opportunity to correlate structure and composition, at a level not available in techniques such as surface tension, ellipsometry, sum frequency spectroscopy and second harmonic generation, enables key theoretical assumptions to be tested. The major assumption in regular solution theory is that the excess entropy of mixing is zero [17], and heat of mixing data [18] have been found to be at variance with this assumption. OsborneLee and Schechter [19] have considered the contribution of the non-random arrangement of non-ionic and anionic surfactants in mixed nonionic–anionic micelles and the conformational changes of the ethylene oxide chains of non-ionic surfactants to this excess entropy of mixing. Another aspect of the regular solution approach is that ‘‘residual’’ solvent at the interface is not implicitly included [17], and although it is considered to be accounted for in the standard chemical potentials at the surface and in the net non-ideal interaction parameter, it will contribute the surface free energy of the system. The theoretical approach adopted by Nikas et al. [9] is based on a twodimensional gains approach, intended to avoid the phenomenological nature of theories such as regular solution theory, but is clearly more dependent upon the detailed structure of the monolayer at a molecular level. This paper will demonstrate that neutron reflec-

tivity can be used to determine the important structural features of a mixed surfactant monolayer at the air–water interface required to test and refine the theories of surfactant mixing. It will describe the structures of two different mixtures and compare them with the structure of their pure component monolayers. The non-ionic mixture C E /C E represents a system close to ideal 12 3 12 8 mixing, with identical alkyl chain lengths but with significantly different headgroup geometries. In contrast to cationic–non-ionic mixture of C TAB/C E , although in electrolyte is also close 16 12 6 to ideal, contains both different alkyl chain lengths and headgroup geometries. In the latter systems the role of solution composition and concentration is also more extensively investigated.

2. Neutron reflectivities Specular neutron reflection provides information about inhomogeneities normal to an interface or surface, and its theory is described in detail elsewhere [20]. The basis of a neutron reflection measurement is that the variation in specular reflection with Q (the more vector transfer normal to the surface defined as Q=4p sin h/l, where h is the glancing angle of incidence and l the neutron wavelength) is simply related to the compositional or concentration profile in the direction normal to the surface. In the kinematic approximation [21], the specular reflectivity, R(Q), is given by R(Q)=

16p2 Q2

|r(Q)|2

(1)

where r(Q) is the one-dimensional Fourier transform of r(z), the average scattering length density profile in the direction normal to the interface,

P

r(Q)=

2

r(z)exp(iQz)dz

(2)

−2

and r(z)=∑ N (z)b i i i

(3)

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J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

The neutron refractive index, n, is defined as

A

B

Nb n= 1−l2 ∑ i i (4) i 2p where N is the number density of species i and b i i is its scattering length. In the context of using neutron reflectivity to study surfactant adsorption at interfaces the key feature is that the neutron scattering properties of H and D are markedly different, and hence H/D isotopic substitution can be used to manipulate the neutron refractive index profile at an interface. This is particularly powerful in determining the structure of surfactant mixtures, where by selective deuteration particular components or fragments can be highlighted. The structural determination relies on being able to combine reflectivity profiles from solutions of the same chemical but different isotopic compositions. This assumes that there is no isotopic dependence of the structure or adsorbed amount, and this has been well established for a range of systems, including those reported here [22]. The analysis of the reflectivity profiles for the different isotopically labelled combinations to obtain the structure of the mixed monolayer uses a direct method based on the kinematic approximation [21] [see Eq. (1)]. At the simplest level for a binary surfactant mixture the two surfactant components and the solvent can be separately labelled. The scattering length density can be written as r(z)=b n (z)+b n (z)+b n (z) (5) 1 1 2 2 s s where 1, 2, s refer to the two surfactants and the solvent, n (z) is the number density distribution of i component i and b its scattering length. Eqs. (1) i and (5) give: R(Q)=

16p2 Q2

[b2 h +b2 h +b2 h 1 11 2 22 s ss

+2b b h +2b b h +2b b h ] (6) 1 2 12 1 s 1s 2 s 2s where h are the self partial structure factors given ii by h (Q)=|nˆ (Q)|2 (7) ii i and h are the cross-partial structure factors given ij

by h (Q)=Re{nˆ (Q)nˆ (Q)} (8) ij i j nˆ (Q) is the one-dimensional Fourier transform of i n (z). The self-partial structure factors relate to the i distributions of the individual components, whereas the cross-partial structure factors relate to the relative positions of the different components at the interface. It has been shown elsewhere [23] that simple analytic functions describe these partial structure factors. The surfactant self-terms are described as a Gaussian distribution, and the solvent as a tanh distribution n (z)=n exp i i

A B −4z2

(9)

s2 1

and,

C

A BD

1 1 z n (z)=n + tan s so 2 2 j

(10)

where s and j are the characteristic widths of the i Gaussian and tanh profiles. The partial structure factor for the Gaussian distribution is then

A

B

1 −Q2s2 i h (Q)= (11) exp ii A2 8 i where A is the area/molecule of component i. The i solvent partial structure factor is then h (Q)=n2 ss so

A B

jpQ 2 2

cosech2

A B jpQ 2

(12)

The cross-partial structure factors provide information about the relative positions of the individual components, and the good approximation independent of any model assumptions [24]. If in Eqs. (5) and (6) n (z) and n (z) are exactly even 1 2 about their centre and n (z) is exactly odd then the s following results hold h =±(h h )1/2cos Qd ij ii jj ij and

(13)

h =±(h h )1/2 sin Qd (14) is ii ss is where d , d are the distances between the centres ij is of the two distributions. Although the widths of the distributions of the individual components

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J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

have contributions from both capillary wave and ‘‘structural’’ roughness, it has been shown [25] that the separations are unaffected by roughness. The simplest labelling scheme described in Eqs. (5) and (6) and more complex schemes are used in this study. The more complex schemes used will be described in detail during the discussion of the results. The partial structure factors are then obtained by measuring reflectivity profiles, for the differently labelled combinations, and extracted by a least squares solution of the resulting simultaneous equations. These procedures, and the errors associated are described in more detail elsewhere [15], and are not repeated here. Typical errors for the alkyl chain and headgroup distribu˚ , for the solvent distribution ±0.5 A ˚ tions are ±1 A and for the separations between the different com˚ : for ponents (the cross-terms d and d ) ±0.5 A ij is the cross-terms when d close to zero is error can is ˚ . The be as large as ±1 A resulting partial structure factors are analysed using the expressions described already to provide number density or volume fraction distributions of the individually labelled components.

3. Experimental The specular neutron reflection measurements were made on the CRISP [26 ] and SURF [27]) reflectometers at the ISIS pulsed neutron source, using the ‘‘white beam time of flight’’ method. That is, the measurements were made in the Q ˚ −1 in a single measurement at a range 0.048–0.5 A fixed angle of incidence of 1.5° using a neutron ˚ , and where the wavelength band of 0.5–6.8 A different neutron wavelengths are distinguished by ‘‘time of flight’’. The data were normalized for the incident beam spectral distribution, detector efficiency and established on an absolute reflectivity scale by reference to the reflectivity from the surface of pure D O using standard procedures 2 [28]. A flat background, determined by extrapola˚ −1), was tion to high values of Q (Q=0.3–0.5 A subtracted from all measured profiles, shown to be a valid procedure [29] providing there is no coherent scattering from the bulk solution (and is applicable to these results).

High purity water was used for the measurements ( Elga ultra-pure) and the D O was obtained 2 from Fluorochem. All glass ware and Teflon troughs used for the reflectivity measurements were cleaned using alkali detergent (Decon 90), followed by copious washing in ultra-pure water. All the measurements were performed at 298 K (unless otherwise stated ). All the isotopes of C E , C E , C E and 12 3 12 6 12 8 C TAB were synthesized by R.K. Thomas’s group 16 at Oxford, and the details of the preparation, purification and characterization are given elsewhere [22,30,31]. The details of the isotopically labelled combinations used in the structural determinations of the C E /C E and C TAB/C E 12 3 12 8 16 12 6 mixtures have been described elsewhere [15,16 ]. The structure for the C E /C E non-ionic mix12 3 12 8 ture was measured at a concentration of 5×10−5 M for two different solution compositions, 30 mol% C E /70 mol% C E and an equi12 3 12 8 molar mixture of C E /C E . In contrast the 12 3 12 8 structure of the C TAB/C E mixture was mea16 12 6 sured over a wider range of solution compositions and concentrations, at a solution concentration of 2×10−5 M and compositions 25/75, 50/50, 75/25 and 95/5 mol ratios (all measured in 0.1 M NaBr except for 95/5, 25/75 measured at 30 and 50°C ), 25/75 mol ratio also at 10−5 and 2×10−4 M, 40/60 mol ratio at a concentration of 2×10−4 M, and 95/5 mol ratio (in the absence of electrolyte) also at 2.5×10−4 and 5.0×10−4 M.

4. Results and discussions 4.1. C E /C E non-ionic surfactant mixtures 12 3 12 8 The structure of the mixed non-ionic monolayer of C E and C E was measured at a concen12 3 12 8 tration of 5×10−5 M and two different solution compositions, 30 mol% C E /70 mol% C E and 12 3 12 8 in equimolar mixture of C E and C E . For the 12 3 12 8 30 mol% C E /70 mol% C E the structure was 12 3 12 8 determined using the simplest labelling scheme, where the alkyl chain of each surfactant and the solvent were deuterium labelled to give [following

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

Eqs. (1), (4) and (5)], r(z)=b

n (z)+b n (z)+b n (z) C12(8) C12(8) C12(3) C12(3) s s (15)

where n ,n and n refer to C12(3) C12(8) s C E , C E alkyl chains and solvent 12 3 12 8 16p2 R(Q)= b2 h +b2 h C12(8) C12(8) C12(3) C12(3) Q2

the

C

+b2 h +2b b h s ss C12(8) C12(3) C12(8)C12(3) +2b b h +2b b h s C12(8) sC12(8) s C12(3) sC12(3)

D (16)

The most convenient way of representing the structure obtained from the partial structure factor analysis is as a number density or volume fraction distribution. The volume fraction distribution shown in Fig. 1 are for the results of the simple labelling scheme [described by Eq. (16)] and summarized in Table 1. It should be emphasized at this stage that the widths of the distributions obtained from the data (see, for example, Table 1) have a contribution from capillary wave roughness ˚ [23] at these levels of coverage), (typically 9 A

15

which have to deconvoluted from the data in order to obtain the intrinsic widths (this has not been done for any of the data presented here). Whereas it has been shown that the distances between the centres of the individual distributions (obtained from the cross partial structure factors) [25] are independent of the capillary wave roughness at the interface or surface. Fig. 1 shows that the alkyl chain distribution of the two non-ionic components entirely coincident, and that, consistent with other measurements [25,31] there is considerable overlap between the alkyl chain and solvent distributions. Included in Table 1 for comparison, are previously measured structural parameters for 5×10−5 M C E (at an area per molecule of 12 3 ˚ 2 [25]), and 37 A for 1.8×10−4 M C E (at an 8 ˚ 2 [32]). In12the area per molecule of 60 A mixed monolayers measured here the area molecule of ˚2 the individual components is much larger (69 A ˚ 2 for C E for the 30/70 mol for C E and 117 A 12 3 ˚ 2 for12C 8 E and 176 A ˚ 2 for ratio mixture, and 52 A 12 3 C E for the equimolar mixture), but the mean 12 8 area/surfactant molecule in the mixed monolayer ˚ 2. The width of the alkyl chains for is ca 40–44 A both the C E and C E are systematically larger 12 3 18 8 than the previous measurements for the pure

Fig. 1. Volume fraction profiles for 5×10−5 M 30/70 C EO /C EO , showing the solvent (- - -) and alkyl chain (———) distributions 12 3 12 8 for C EO and C EO . 12 3 12 8

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J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

Table 1 Results for partial structure factor analysis of i E /C E , C E and C E surface layers 12 3 12 8 12 8 12 3

˚) s (A C12(3) s C6(3) s C12(8) s C6(8) s e(3) s e(8) t s d C6(3)s d C6(8)s d C12(3)s d C12(8)s d e(3)s d e(8)s d C12(3)C12(8) d C6(3)e(8) d C6(8)e(3) d C6(3)e(3) d C6(3)C6(8) d e(3)e(8) d C12(3)e(3) d C12(8)e(8)

5×10−5 M 30/70 C E /C E (full labelling)a 12 3 12 8

5×10−5 M 30/70 C E /C E (chain only)a 12 3 12 8

5×10−5 M 50/50C E / 12 3 C E (full labelling)a 12 8

1.8×10−4 Mb C E 12 8

5.5×10−5 Mc C E 12 3

— 15.0 — 15.0 15.0 15.0 7.5 16.0 16.0 — — 3.5 —1.0 — 16.0 12.0 — 1.0 6.0 — —

19.5 — 18.0 — — — 8.0 — — 11.0 11.0 — — 0.0 — — — — — — —

— 17.0 — 16.0 17.0 16.0 6.5 16.0 15.0 — — — —1.0 — 14.0 13.0 — 1.0 5.0 — —

— — 15.0 — — 9.0 9.0 — — — 1.0 — 2.5 — — — — — — — 0.5

16.5 14.0 — — 15.5 — 6.0 13.0 — 10.0 — 2.5 — — — 10.5 — — 8.0 —

aSee main text for description. bFrom Ref. [25]. cFrom Ref. [32].

C E and C E monolayers (see Table 1). 12 8 12 3 Because of the simple labelling scheme there is a small contribution to this width due to finite contribution from the unlabelled ethylene oxide groups, this has been estimated [25] to be ca ˚ . It is difficult to say from the results of the 1–2 A simple labelling scheme whether the increase in alkyl chain width is due to increased roughness or conformational changes in the alkyl chain. Furthermore from the simple labelling scheme there is no information about the packing of the E and E ethylene oxide chains. To provide greater 3 8 sensitivity to changes in the conformation of the alkyl and ethylene oxide chains the structure was determined with the ethylene oxide chains and the outer C of the alkyl chains deuterium labelled 6 (similar measurements were previously made for C E [25]). The five labelled components (includ12 3 ing the solvent) in the mixed C E /C E mono12 3 12 8 layer gives rise to 15 reflectivities and partial

structure factors, R(Q)=

16p2 Q2

[b2 h +b2 h +b2 h c6(3) c6(3) c6(8) c6(8) s ss

+b2 h +b2 h +2b b h e3 e3 e8 e8 C6(8) C6(3) C6(8)c6(3) +2b b h +2b b h e3 C6(8) e3C6(8) e8 C6(3) e8C6(3) +2b bh +2b bh C6(3) s C6(3)s C6(8) s C6(8)s +2b b h +2b b h +2b b h e3 s e3s e8 s e8s e3 e8 e3e8 +2b b h +2b b h ] (17) C6(3) e3 C6(3)e3 C6(8) e8 C6(8)e8 The labelled combinations measured were not the ideal combinations, and provide only a very indirect estimate of the cross-terms h and h c6(8)e8 c6(3)e3 and hence these are not included in Table 1. The volume fraction distributions obtained from the more detailed labelling scheme are shown in Fig. 2, and the parameters are also summarized in Table 1. The results from both labelling schemes provide a

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

17

Fig. 2. Volume fraction profiles for 5×10−5 M 30/70 C EO /C EO , showing the solvent (- - -) and alkyl chain (outer C6 only) 12 3 12 8 (———) and ethylene oxide chains ( · · · ) of the C EO and C EO . 12 3 12 8

consistent description of the structure at the interface. The result from the more complex labelling scheme provide more detailed picture of the structure of the mixed monolayer, and enable a direct comparison with earlier results for the pure C E monolayer [25]. The volume fraction distri12 3 butions for the differently labelled compounds are shown in Fig. 2 for the 70/30 mol ratio mixture, and summarized along with the results for an equimolar mixture in Table 1. The results from the simple and more detailed labelling show that the structure of the individual components in the mixed monolayer is markedly different compared to the structure of the pure monolayers (the results for C E [32] and C E [25] alone are included 12 8 12 3 for comparison in Table 1). The frustration caused by trying to pack the E and E ethylene oxide chains results in a real 8 3 disruption or alteration to the structure of the two surfactants in the mixed monolayer, compared to that in the pure monolayers. In the mixed monolayer the width of the C alkyl chain increases for 12 the C E . The outer C -chain solvent separation 12 3 6 increases, whereas the C alkyl chain-solvent sepa12 ration is similar to that of the pure monolayer.

This suggests that the alkyl chain conformation has changed; that is, the chain have become more extended. This is illustrated in Fig. 3, where the C and C volume fraction distributions relative 6 12 to the solvent distribution for the pure C E 12 3 monolayer and the C E in the mixed monolayer 12 3 are plotted. Similar data for the C E alkyl chain 12 8 distribution in the mixed monolayer suggest that the same is true for the C E : however, the data 12 8 for the identical labelling of the pure monolayer of C E is not available. These changes in alkyl 12 8 chain distribution are accompanied by changes in the ethylene oxide chain conformation. Although the E chain dimensions are similar in the mixed 3 monolayer to that found in the pure monolayer, the E distribution is noticeably narrower in the 8 mixed than in the pure monolayer of C E . 12 8 Furthermore compared to the pure monolayer, the E distribution is less hydrated (that is, it is 3 shifted away from the solvent), whereas the opposite is true for the E distribution. The width of 8 the solvent distribution in the mixed monolayer is larger than that observed for a pure C E mono12 3 layer, but smaller than that seen in the pure C E monolayer. This is consistent with the 12 8 changes in the conformation of the E ethylene 8

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J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

from one mixture with an extreme composition (95 mol% C TAB/5 mol% C E with no added 16 12 6 electrolyte), although the surface composition varied considerably throughout the series of measurements, the total adsorption was roughly constant (mean area/surfactant molecule was ca ˚ 2), see Table 2. 45–60 A The variation of the structure of the mixed monolayer was initially investigated over a wide concentration and composition range using a simple isotopic labelling scheme. In this case C TAB was fully deuterium labelled, the alkyl 16 chain of the C E and the solvent were deuterium 12 6 labelled to give r(z)=b n (z)+b n (z)+b n (z) (18) c c nc nc s s where n , n , n refer to the C TAB, C E alkyl c nc s 16 12 6 chain and solvent, respectively. The reflectivity can in turn then be expressed as R(Q)=

Fig. 3. Volume fraction profiles for (a) 5×10−5 M C EO and 12 3 (b) C EO in 5×10−5 M 30/70 C EO /C EO ; solvent (- - -), 12 3 12 3 12 8 alkyl chain (———) and outer C6 of the alkyl chain ( · · · ).

oxide chain. Indeed the changes in hydration can be directly associated with the changes in the ethylene oxide headgroup packing, due to the asymmetry in the sizes of the two headgroups. The structure of the equimolar mixture of C E /C E was, within error, the same as the 12 3 12 8 structure at the surface composition of 30/70 mol ratio of C E /C E (see Table 1). This is, per12 3 12 8 haps, hardly surprising as the mean area/molecule in the mixed monolayer is similar for both compositions. The equimolar mixture has a surface richer in C E (77 mol% C E compared to 12 3 12 3 63 mol% C E for the 30/70 mixture). 12 3 4.2. C TAB /C E mixture 16 12 6 For the non-ionic–cationic surfactant mixture of C TAB/C E a wider range of solution concen16 12 6 trations and compositions were investigated. Apart

16p2 Q2

[b2 h +b2 h +b2 h +2b b h c cc nc nc s ss c nc cnc

+2b b h +2b b h ] (19) c s cs nc s ncs The results from the partial structure factor analysis are summarized in Table 3 for the different C TAB/C E compositions of 25/75, 50/50, 75/25 16 12 6 and 95/5. At 25/75 measurements were made at two different solution concentrations, 10−5 and 2×10−4 M, and at 95/5 measurements were made without added electrolyte and at two different concentrations, 2.5×10−4 and 5.0×10−4 M. A convenient way of representing the results is either as volume fraction or number density distributions, and the results for 25 mol% C TAB/75 mol% 16 C E at 10−5 M are shown as volume fraction 12 6 distributions in Fig. 4. Consistent with other neutron reflectivity measurements of surfactant structure at the air–water interface there is significant overlap between the solvent and surfactant distributions. The midpoint of the alkyl chain distribution of the non-ionic surfactant is at a greater separation from the solvent than the midpoint of the larger cationic alkyl chain: this point will be returned to later in the discussion on the context of the more detailed labelling. Given the errors in the parameters derived from the partial structure factor analysis

19

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26 Table 2 Area/molecule for different C TAB/C E combinations measured 16 12 6 Composition, Concentration, labelling scheme, temperature

25/75, 10−5 M, full, 50°C 25/75, 10−5 M, chains, 30°C 25/75, 2×10−5 M, full, 30°C 25/75, 2×10−4, chains, 30°C 25/75, 2×10−5, partial 25/75, 2×10−5,partial 40/60, 2×10−4, full 50/50, 2×10−5, chains 75/25, 2×10−5, chains 95/5, 5×10−4, chains, no salt 95/5, 2×10−5, partial, no salt 95/5, 2×10−4, partial, no salt 95/5, 2.5×10−4, chains, no salt

˚ 2) Area/molecule(A

˚ 2) Mean area/molecule (A

C TAB 16

C E 12 6

175 154 140 141 148 132 88 78 60 92 243 92 180

79 100 95 77 94 87 96 124 228 145 167 122 102

54 61 56 50 57 52 51 48 47 56 99 52 65

The errors in the adsorbed amounts is typically ±5%. Where ‘‘full’’ refers to separate labelling of the alkyl chain, headgroup and solvent, ‘‘chain’’ refers to labelling of alkyl chainand solvent only and ‘‘partial’’ refers to a partial labelling of the alkyl chain, the headgroup and solvent. Table 3 Results from partial structure factor analysis of C TAB/C E in 0.1 M NaBr (from simplest labelling scheme used) 16 12 6 Composition/concentration

˚) s (A c s nc t s d cs d ncs d cnc

25/75, 10−5 M

25/75, 2×10−4 M

50/50, 2×10−5 M

75/25, 2×10−5 M

95/5, 5×10−4 M (no salt)

95/5, 2.5×10−4 M (no salt)

17.0

17.0

18.5

18.0

18.5

19.0

18.0 6.0 5.0 10.0 4.0

17.5 6.5 5.5 11.0 4.5

14.0 7.0 6.0 9.5 4.0

15.0 7.0 6.7 8.0 3.0

19.0 5.2 6.5 8.5 1.0

17.0 5.5 5.5 9.5 2.0

(see earlier discussions) there are no dramatic changes in structure of the mixed monolayer with concentration or composition, and this is not surprising given that the total adsorbed amount is essentially constant. This implies that, at this level of resolution, the structure appears to be dominated purely by the local packing constraints. As the composition of the solution, and hence the surface, is changed some systematic changes in the structure are described. As the surface becomes richer in C TAB (from a solution composition of 16 25/75 to 75/25) the extent of the C TAB molecule 16 ˚ ) and increases marginally (17–18.5 A the extent of

˚ ). These the C E alkyl chain decreases (18 to 14 A 12 6 changes are accompanied by changes in the ˚ 2 and of area/molecule of the C E of 95–228 A 12 ˚ 6 the C TAB of 150 to 60 A 2. At the same time the 16 relative positions of the two surfactants at the interface has altered. The distance between the centres of the cationic surfactant and solvent distributions has increased, whereas for the non-ionic and solvent distributions it has decreased (see Table 3). The measurements made at a solution composition of 95 mol% C TAB/5 mol% C E 16 12 6 show a similar structure to those richer in C TAB, except that the extent of the C TAB 16 16

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J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

Fig. 4. Volume fraction distributions for 10−5 M C TAB/C E in 0.1 M NaBr at a mole fraction ratio of 25/75 16 12 6 for: ———, C TAB; - - -, solvent; · · · , C E alkyl chain. 16 12 6

molecule and of the C E alkyl chains both remain 12 6 large. This may well be associated with the absence of electrolyte (the measurements made at a solution composition of 95/5 were made in the absence of electrolyte), and will be the subject of further investigations. At two solution compositions (25/75 and 40/60, and at 25/75 two different temperatures, 30 and 50°C were used) the structure of the surface monolayer was measured using a more detailed labelling scheme, in which the cationic surfactant was fully deuterated and the non-ionic alkyl and ethylene oxide chains and solvent were separately deuterium labelled; such that r(z)=b n (z)+b n (z)+b n (z)+b n (z) nh nh nc nc c c s s (20) where nh, nc now refer to the C E headgroup 12 6 (ethylene oxide chain) and alkyl chain. The reflectivity is now, R(Q)=

16p2 Q2

[b2 h +b2 h +b2 h +b2 h nh nh nc nc c cc s ss

+2b b h +2b b b +2b b h nh nc nhnc nh c nhc nc c ncc +2b b h +2b b h +2b b h ] (21) c s cs nc s ncs nh s nhs The results are summarized in Table 4, and Fig. 5 shows the results from the more detailed labelling

scheme for the mixture 25 mol% C TAB/ 16 75 mol% C E in 0.1 M NaBr at 30°C plotted as 12 6 volume fraction distributions of the individual components. Compared to Fig. 3 the distributions of the non-ionic alkyl and ethylene oxide chains are shown separately. Fig. 5 also shows that the distribution of the entire C E and C TAB mole12 6 16 cules coincide. It is interesting to compare this structure with that obtained for other non-ionic– ionic mixtures. Table 5 summarizes the results from the C TAB/C E , SDS/dodecanol [33] and 16 12 6 dodecane/C E [34] mixtures, and the relative 12 6 positions of the solvent and surfactants for these mixtures are plotted in Fig. 6. With increasing solubility of the non-ionic component there is a systematic shift of the non-ionic component towards the solvent, and this is seen in the separations between the non-ionic and solvent and between the two surfactant components. The structure for the 40/60 and 25/75 mixtures are very similar, given the errors in the different structural parameters (see Table 4 and earlier discussion on relative errors). However, consistent with the earlier discussion this is not entirely surprising given that the total adsorbed amount and surface composition (see Table 2) is very similar between the two solution compositions. For the solution mixture of 25 mol% C TAB/75 mol% C E the surface structure was 16 12 6 measured at two different temperatures (30 and 50°C ). Some systematic changes in the structure with increasing temperature are observed. The width of the alkyl chain distributions of both surfactants increases with increasing temperature, and may be attributed to increasing roughness (either structural and/or capillary wave) or to a change in conformation of the alkyl chain. In conjunction with these changes, the cationic and non-ionic chain-solvent separations (d and d ) cs ncs increased, consistent with a temperature driven dehydration of the surfactants. This is also accompanied by an increase in the non-ionic headgroup– solvent separation, d , and an increase in the nhs cationic–non-ionic headgroup separation, d . All cnh these trends confirm the conclusion about the temperature driven dehydration, and are consistent with a reduced hydrophobic interaction between the solvent and alkyl chains. These changes in structure with increasing temperature are also asso-

21

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26 Table 4 Results from partial structure factor analysis of C TAB/C E in 0.1 M NaBr (from full labelling scheme used) 16 12 6 Composition/concentration

˚) s (A c s nc s nh t s d cs d ncs d nhs d cnc d cnh d ncnh

25/75, 2×10−5 M, 30°C

25/75, 2×10−5 M, 50°C

40/60, 2×10−4 M, 30°C

17.0 16.0 18.5 6.5 6.0 9.0 1.0 3.0 3.0 6.0

19.0 18.0 18.5 6.5 8.0 10.0 3.0 4.0 4.0 8.0

19.0 18.0 20.0 6.5 6.0 10.0 3.0 3.0 4.0 8.0

Fig. 5. Volume fraction distributions for 2×10−5 M C TAB/C E in 0.1 M NaBr at a mole fraction ratio of 25/75 16 12 6 for: ———, C TAB; - - -, solvent; · · · , C E alkyl chain; 16 12 6 – · –, C E ethylene oxide chain; and - · · -, entire C E 12 6 12 6 molecule.

ciated with changes in the surface composition. The surface becomes richer in C E with increas12 6 ing temperature (changing from 0.58 to 0.69 mol fraction of non-ionic), due to the increased surface

activity of the C E relative to that of the 12 6 C TAB. 16 The structures of the two surfactant components in the mixed monolayer are similar to those measured for the pure C E and C TAB monolayers 12 6 16 [23,35]. The structural parameter obtained previously for C E [23] and C TAB [35] are sum12 6 16 marized in Table 6. The alkyl and ethylene oxide chains for C E in the mixed monolayer are more 12 6 extended and more immersed in the solvent, compared to the pure C E monolayer (see Fig. 7). 12 6 Whether this is due to increased roughness or a change in conformation is difficult to conclude without further more detailed labelling, and this will be discussed later. Similar trends are observed for C TAB. However, an additional factor for 16 the C TAB comparison needs to be taken into 16 account. In the mixtures the whole molecule was deuterium labelled (alkyl chain and headgroup), whereas the comparison with the pure C TAB 16 structure is on the basis of the alkyl chain and headgroup distributions being separately identified. This will account for some of the apparent increase in thickness compared to the pure mono-

Table 5 Structural parameters for, 25/75 C TAB/C E at 2×10−5 M, 0.007 M SDS/0.5 wt% dodecanol and 1.4×10−3 M C TAB/dodecane 16 12 6 12 (at saturation of dodecane in the layer) Mixture

C TAB/C E 16 12 6

SDS/dodecanol

C TAB/dodecane 12

˚) d Non-ionic–solvent (A d Ionic–nonionic

6.0 0.0

10.0 3.5

12.0 6.5

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J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26 Table 6 Structural parameters for pure C E (at area/molecule of 12 6 ˚ 2) [35] and C TAB (at areas/molecules ˚ 2) 55 A of 43 and 60 A 16 [23] Surfactant (area/molecule)

˚) s (A c s h t s d cs d hs d ch s c2 d c2s

˚ 2) C E (55 A 12 6

˚ 2) C TAB (43 A 16

˚ 2) (60 A

16.0 16.5 8.0 10.0 1.0 9.0 — —

16.5 14.0 6.0 9.0 2.0 8.0 14.0 12.0

14.0 10.0 5.5 6.8 — 7.0 11.0 9.0

Where C refers to the C furthest from the alkyl chain 2 8 headgroup.

layer. However, there is a inherent contribution ˚ ) from capillary wave roughness, and taking (~9 A that and the headgroup–chain separation [23] into ˚ increase in the apparaccount, accounts for ~1 A ent thickness. By partial labelling of the surfactant alkyl chain, additional information about the reason for the change in thickness of the alkyl chain in the mixture compared to the pure surfactant monolayer can be obtained. With this in mind, measurements were made for the 25/75 mol ratio mixture at a concentration of 2×10−5 M and for the 95/5 mol ratio mixture at 2×10−5 and 5× 10−4 M (in the absence of electrolyte). Measurements were with the half of the alkyl chain of the C TAB furthest from the headgroup (either 16 C or C ) deuterium labelled, for C E ethylene 8 10 12 6 oxide headgroup and solvent deuterium labelled, such that r(z)=b n (z)+b n (z)+b n (z) C8 C8 nh nh s s The reflectivity can then be written as R(Q)=

Fig. 6. Volume fraction distributions for (a) 25/75 C TAB/C E at 2×10−5 M, (b) 0.007M SDS/0.5 wt % dode16 12 6 canol and (c) 1.4×10−2 M C TAB/dodecane: ———, solvent; 12 - - -, C E , dodecanol, dodecane; and · · · , C TAB, SDS, 12 6 16 C TAB. 12

16p2 Q2

(22)

[b2 h +b2 h +b2 h C8 C8 nh nh s ss

+2b b h +2b b h +2b b h ] C8 nh C8nh C8 s C8s nh s nhs (23) The results from the analysis of these data are summarized in Table 7. The parameters obtained

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

23

are consistent with those obtained from the analysis of the other labelling schemes, and the volume fraction profiles in Fig. 8 show the additional information about the distribution of outer C of 8 the C TAB. A similar labelling scheme was used 16 earlier in determining the structure of the pure C TAB monolayer [23], and enables a direct 16 comparison to be made. The width of the distribution of the C TAB alkyl chain and of the outer 16 C are both larger in the mixture than for the 8 pure C TAB monolayer. The distances between 16 the midpoints of the C and solvent distributions, 8

Fig. 7. Volume fraction distributions for: (a) C E (at an ˚ 2); and (b) C E 12 in6 25/75 area/molecule of 55 A 12 6 C TAB/C E mixture at 2×10−5 M; ———, solvent; - - -, 16 12 6 alkyl chain; and · · · , ethylene oxide chain.

Fig. 8. Volume fraction distributions for 2×10−5 M 27/75 C TAB/C E in 0.1 M NaBr, for: - - -, solvent; – · –, C E 16 12 6 12 6 ethylene oxide chain; · · · , C E alkyl chain; ———, entire 12 6 C TAB molecule; and - · · -, outer C distribution of the 16 8 C TAB alkyl chain. 16

Table 7 Structural parameters for C TAB/C E mixtures (from partial labelling at cationic alkyl chain, labelling of nonionic ethylene oxide 16 12 6 chain and solvent) Composition, concentration, electrolyte

˚) s (A c2 s nh t s d c2s d nhs d c2nh

95/5, 2×10−5 M no salt

95/5, 5×10−4 M no salt

25/75, 2×10−5 M

16.0a 15.0 4.0 7.0 2.0 4.0

19.0 19.0 5.0 11.5 2.0 8.0

19.0* 18.0 5.5 9.0 1.0 6.0

aC , not C labelled. 10 8

19.0 19.0 6.0 11.0 1.0 10.0

24

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

d , are similar, but d (the distance between the C8s Cs midpoints of the whole alkyl chain and solvent distributions) is smaller in the mixture. This is consistent with a change in the conformation of the alkyl chain and as a result of the chain solvent overlap being greater (see Fig. 9). For most of the structural measurements made on the C TAB/C E mixture the mean 16 12 6 ˚ −2. However, for area/molecule was ca 50–60 A the 95/5 mol ratio mixture at a concentration of 2×10−5 M (with no added electrolyte) the mean ˚ −2. Measurements at this area/molecule is ca 100 A concentration and composition were only made using the labelling scheme described earlier, where the C TAB alkyl chain is partially deuterium 16

labelled, the C E ethylene oxide headgroup and 12 6 solvent are deuterium labelled. Compared to the structures obtained for the other C TAB/C E 16 12 6 mixtures (see Tables 3–5 and 7) the alkyl chain and ethylene oxide chains have narrowed noticeably and the alkyl chain and ethylene oxide chain to solvent overlaps have increased (see Fig. 10, for a comparison with the structure at 5×10−4 M ). Changes in hydration are observed for both mixtures, compared to their pure component monolayers. For the non-ionic mixture of C E /C E this is not entirely surprising and can 12 3 12 8 (as discussed earlier in the paper) be rationalized as being directly associated with the changes in the ethylene oxide headgroup packing, due to the

Fig. 9. Volume fraction distributions for (a) C TAB (at an 16 ˚ 2) and (b) C TAB area/molecule of 43 A in 25/75 16 C TAB/C E mixture at 2×10−5 M: ———, solvent and alkyl 16 12 6 chain; - - -, C ; and – · –, C . 16 8

Fig. 10. Volume fraction distributions for 95/5 C TAB/C E 16 12 6 (with no added electrolyte) for: (a) 5×10−4 M and (b) 2×10−5 M; ———, solvent; - - -, outer C of alkyl chain; and 8 · · · , ethylene oxide chain.

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

asymmetry in the relative sizes of the two ethylene oxide chains (headgroups). It is typical of surfactant adsorption isotherms that very small changes in the area/molecule result in large changes in surface pressure. As the surface pressure at the cmc is very similar for the pure and mixed surfactants studied here it would be expected, as strong specific interactions are absent, that the area per molecule of the components in the mixture would be very similar to that found in the single component system. The neutron reflection technique provides a direct measure of the surfactant surface excess and, for mixed systems near the cmc, it can therefore be predicted the fractional surface coverage expected on the basis of surface area measured for the pure system. For the C E /C E mixture this 12 3 12 8 correspond to sufficient molecules to cover 105% of the surface, and indicates a slightly more efficient packing in the mixed monolayer, even though there is ideal mixing (the error associated with these measurements is ~5%). For the C TAB/C E mixture the equivalent calculation 16 12 6 gives a coverage ~90%, the packing of the mixture is less efficient. This is perhaps surprising as the inclusion of the non-ionic would reduce the electrostatic headgroup interactions between the cationic molecules. The less efficient packing must presumably be associated with the incompatibility between two rather disparate headgroup geometries. The increased overlap between the alkyl chains and solvent distributions for this mixture could the be attributed to the role of the E headgroup of the 6 C E drawing the surfactants closer to the solvent, 12 6 due to its greater hydrophilicity.

5. Summary It has been demonstrated with two different surfactant mixtures, both close to ideal mixing, that neutron reflectometry is a surface probe with sufficient sensitivity to determine changes in the structure of the surfactants in the mixed monolayer compared to that in the pure monolayer of the same surfactant. In the two mixtures investigated so far, the non-ionic mixture of C E /C E have 12 3 12 8 identical alkyl chain lengths, but significantly different headgroup geometries, whereas the

25

cationic/non-ionic mixture of C TAB/C E have 16 12 6 differences in both the alkyl chain length and headgroup geometries. For both mixtures changes in alkyl chain conformation, compared to the pure monolayer, are observed, and can be attributed directly to the influence of packing headgroups with significantly different geometries. The nature of the interactions between the headgroups in the mixed monolayer are also thought to be responsible for the changes in hydration of the surface layer. These structural changes are important in the context of the application of theories such as Regular solution theory, where a central assumption is that the excess entropy of mixing is zero, and it has been shown that this is clearly not the case even for systems close to ideal mixing. Furthermore the changes in the hydration of the layer are important in the context of the failure of Regular solution theory to consider residual solvent. Finally such detailed structural information will provide important input to the more detailed theoretical approaches [9,19] that are currently being developed.

References [1] J.F. Scamehorn (Ed.), Phenomena in Mixed Surfactant Systems, ACS Symposium Series 311, American Chemical Society, Washington, DC, 1986. [2] E.H. Lucassen-Reynders, Progress in: D.A. Cadenhead, J.F. Danielli ( Eds.), Surface and Membrane Science, Academic Press, New York, 1976, p. 253. [3] M.J. Rosen, in: J.F. Scamehorn ( Ed.), Phenomena in Mixed Surfactant Systems, ACS Symposium Series 311, American Chemical Society, Washington, DC, 1986. [4] L. Thompson, E. Staples, private communiation. [5] M.J. Rosen, X.Y. Hua, J. Coll. Int. Sci. 86 (1982) 164. [6 ] J.H. Clint, J. Chem. Soc. Faraday Trans. 1 71 (1975) 1327. [7] D.N. Rubingh, in: K.L. Mittal ( Eds.), Solution Chemistry of Surfactants, vol. 1, Plenum Press, New York, 1979. [8] P.M. Holland, Colloids Surf. A: Physiochem. Eng. Aspects 19 (1986) 171. [9] Y.F. Nikas, S. Pruuvada, D. Blankschtein, Langmuir 8 (1997) 2680. [10] E.J. Staples, L. Thompson, I. Tucker, J. Penfold, R.K. Thomas, J.R. Lu, Langmuir 9 (1993) 1651. [11] E.J. Staples, L. Thompson, I. Tucker, J. Penfold, Langmuir 10 (1994) 4136.

26

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 155 (1999) 11–26

[12] J. Penfold, E.J. Staples, L. Thompson, I. Tucker, R.K. Thomas, J.R. Lu, Langmuir 11 (1995) 2496. [13] J. Penfold, E.J. Staples, L. Thompson, I. Tucker, Colloids Surf. A, Physiochem. Eng. Aspects 102 (1995) 127. [14] J. Penfold, E.J. Staples, P. Cummins, I. Tucker, L. Thompson, R.K. Thomas, E.A. Simister, J.R. Lu, J. Chem. Soc. Faraday Trans. 92 (1996) 1773. [15] J. Penfold, E.J. Staples, P. Cummins, I. Tucker, R.K. Thomas, E.A. Simister, J.R. Lu, J. Chem. Soc. Faraday Trans. 92 (1996) 1549. [16 ] J. Penfold, E.J. Staples, I. Tucker, R.K. Thomas, J. Coll. Int. Sci. (1998), in press. [17] P.M. Holland, in: J.F. Scamehorn ( Ed.), Phenomena in Mixed Surfactant Systems, ACS Symposium Series 311, American Chemical Society, Washington, DC, 1986, p. 102. [18] P.M. Holland, in: M.J. Rosen ( Ed.), Relation Between Structure and Performance of Surfactants, ACS Symposium Series No. 253. American Chemical Society, Washington, DC, 1984, p. 141. [19] I.W. Osbourn-Lee, R.S. Schechter, in: J.F. Scamehorn ( Ed.), Phenomena in Mixed Surfactant Systems, ACS Symposium Series 311, American Chemical Society, Washington, DC, 1986, p. 30. [20] J. Penfold, R.K. Thomas, J. Phys. Condens. Matter 2 (1996) 1369. [21] T.L. Crowley, E.M. Lee, E.A. Simister, R.K. Thomas, Physica B 173 (1991) 143. [22] J.R. Lu, E.M. Lee, R.K. Thomas, J. Penfold, S.L. Flitsch, Langmuir 9 (1995) 1352. [23] J.R. Lu, M. Hromadova, E.A. Simister, R.K. Thomas, J. Penfold, J. Phys. Chem 98 (1994) 11519.

[24] E.A. Simister, E.M. Lee, R.K. Thomas, J. Penfold, J. Phys. Chem. 96 (1992) 1352. [25] J.R. Lu, M. Hromadova, R.K. Thomas, J. Penfold, Langmuir 9 (1993) 2417. [26 ] J. Penfold, R.C. Ward, W.G. Williams, J. Phys. E. Scient. Instrum. 20 (1987) 1411. [27] J. Penfold, R.M. Richardson, A. Zarbakhsh, J.R.P. Webster, D.G. Bucknall, A.R. Rennie, R.A.L. Jones, T. Cosgrove, R.K. Thomas, J.S. Higgins, P.D.I. Fletcher, E. Dickinson, S.J. Roser, I.A. McLure, A.R. Hillman, R.W. Richards, E.J. Staples, A.N. Burgess, E.A. Simister, J.W. White, J. Chem. Soc., Faraday Trans. 93 (1997) 3899. [28] E.M. Lee, R.K. Thomas, J. Penfold, R.C. Ward, J. Phys. Chem. 93 (1989) 381. [29] E.A. Simister, E.M. Lee, R.K. Thomas, J. Penfold, J. Phys. Chem. 96 (1992) 1352. [30] E.A. Simister, R.K. Thomas, J. Penfold, R. Aveyard, B.P. Binks, P.D.I. Fletcher, J.R. Lu, A. Sokolowski, J. Phys. Chem. 96 (1992) 1383. [31] J.R. Lu, Z.X. Li, T.J. Su, R.K. Thomas, J. Penfold, Langmuir 9 (1993) 2408. [32] J.R. Lu, Z.X. Li, R.K. Thomas, E.J. Staples, L. Thompson, I. Tucker, J. Penfold, J. Phys. Chem. 98 (1994) 6559. [33] J.R. Lu, I.P. Purcell, E.M. Lee, E.A. Simister, R.K. Thomas, A.R. Rennie, J. Penfold, J. Coll. Int. Sci. 174 (1995) 441. [34] J.R. Lu, R.K. Thomas, B.P. Binks, P.D.I. Fletcher, J. Penfold, J. Phys. Chem. 99 (1995) 4113. [35] J.R. Lu, Z.X. Li, R.K. Thomas, E.J. Staples, I. Tucker, J. Penfold, J. Phys. Chem. 97 (1993) 8012.