COLLOIDS A AND

Colloids and Surfaces SURFACES A: Physicochemical and Engineering Aspects 102 (1995) 127 132 E L S E V I E R

The composition of non-ionic surfactant mixtures at the air/water interface as determined by neutron reflectivity

J. P e n f o l d a, , , E. S t ap l e s b, L. T h o m p s o n b, I. T u c k e r b

a ISIS Facility, Rutherford-Appleton Laboratory, Chilton, Didcot, UK b Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, UK

Received 17 October 1994; accepted 12 April 1995

Abstract

Neutron reflectivity has been used to study the adsorption of non-ionic mixtures at the air/water interface. Measurements have been made over a wide concentration range from below to above the critical micellar concentration (CMC), (10 -5-10 -2 M). The data provides a direct qualitative confirmation of the abrupt changes in surface composition at the CMC due to the onset of mixed micelle formation predicted recently by Nikas, Puvvada, and Blankschtein, Langmuir, 8 (1992) 2680. The data for concentrations below the CMC are compared with the earlier results of Rosen and Hua, J. Colloid Interface Sci., 86 (1982) 164.

Keywords: Air/water interface; Non-ionic surfactant mixtures; Neutron reflectivity

I. Introduction

The study of surfactant mixtures is of consider- able interest, for both practical and theoretical reasons [ 1]. In domestic and industrial applica- tions surfactants are usually mixtures; because mixtures enhance performance, synergy, or because commercial surfactants are impure, that is, they contain mixtures of different alkyl chain lengths and/or isomeric forms. For a complete understand- ing of mixed surfactant systems it is necessary to know or to be able to predict the micellar, mon- omer, and surface compositions. The ability to make such predictions for systems which mix ideally, and which show departures from ideal mixing, over a wide concentration range from below to well in excess of the critical micellar concentration (CMC), has generated much theo-

* Corresponding author.

0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03228-2

retical interest. The ideal-solution approach [2] and the subsequent adaption of Regular Solution Theory (RST) [3,4] where the departure from ideality is parameterised by an interaction parameter, fl, has formed the basis of much of the theoretical effort. More recently Nikas et al. [5] have adopted a more fundamental approach aimed at removing the phenomenological nature of theories such as RST.

An important aspect of the nature of surfactant mixtures is the composition and structure of the interface. There are very few experimental tech- niques which will provide unambiguously such information over the wide concentration range, from below to well in excess of the CMC. Neutron reflectivity [6] has emerged in the last few years as one of those techniques. Measurements on a range of surfactants (see for example Refs. [ 7 -9 ] have demonstrated it to be a useful technique to obtain absolute adsorbed amounts [7] and

128 J. Penfold et al./Colloids Surfaces A: Physicochem. Eng. Aspects 102 (1995) 127-132

detailed surface structure [8,9] at the air/solution interface. More recently, measurements [ 10] have shown it to be a particularly powerful technique for the study of surfactant mixtures.

In a recent paper Nikas et al. [ 5 ] have developed a detailed theory, based on the two-dimensional gas approach, to predict surface tension and mono- layer compositions of mixed surfactant solutions. The predicted variation in monolayer composition shows an abrupt change at the mixture CMC, which can be rationalised in terms of the changes in distribution of the two surfactant species between the bulk solution (monomer concentration and composition) and the monolayer due to the onset of mixed micelle formation at the CMC. We have made neutron reflection measurements on an equimolar mixture of monododecyltriethylene glycol (C12EO3) and monododecyloctaethylene glycol (C12EO8) in the concentration range 10-5-10 .2 M (from below to well above the mix- ture CMC), in order to provide an experimental verification of their theoretical prediction.

Rosen and Hua [ 11 ] have used the method of Hutchinson [12] to determine the surface com- position of binary non-ionic surfactant mixtures, and have made an evaluation based on RST. We have compared our neutron reflectivity results for C~2EOa/C12EO8 with the data of Rosen and Hua [ 11 ]; for a system in which the mixing is essentially ideal this represents a direct and simple compari- son of the two methods in a limited concentration range below the mixture CMC.

2. Experimental details

The specular reflection of neutrons provides information about the composition and concen- tration profiles at surfaces and interfaces, and the technique is described in detail elsewhere [6]. The neutron reflection measurements in this paper were made on the reflectometer CRISP [13] at the ISIS pulsed neutron source, Rutherford-Appleton Laboratory, UK, using the white beam (broad range of incident neutron wavelengths) time-of- flight method.

The measurements were made using a single detector at a fixed glancing angle of incidence of

1.5 °. The absolute reflectivities were calibrated with respect to D20 as described previously [ 14]. The flat background was determined from the reflection signal obtained in the limit of high values of scattering vector, Q(Q=4n sin 0/2, where 0 is the glancing angle of incidence, and 2 the neutron wavelength), and subtracted from the data over the full Q range. This has been shown to be a valid procedure providing there is no small-angle scatter- ing from the bulk solution [14]; verified in this case by making additional measurements both sides of the specular reflection by an angular offset of the detector.

For a deuterated surfactant in null-reflecting water, (n.r.w.), (92 mol% H20/8 mol% D20 has a scattering length of zero, i.e. a refractive index indentical to that of air) the reflected signal arises only from the adsorbed surfactant layer at the interface. The most direct way of determining the surface concentration of the surfactant is to fit the measured reflectivity profile by comparing it with a profile calculated using the optical matrix method [15] for a simple structural model. Typically, in the determination of the surface con- centration, it is sufficient to assume that the surfac- tant is in the form of a single layer of homogeneous composition. The parameters obtained from such a model fit are the scattering length density (p), and the thickness (z) of the layer. The area per molecule, A, in the adsorbed layer is then,

A = b/(pz) ( 1 )

where b is the scattering length of the adsorbed molecule. The adsorbed amount (or surface excess, F, expressed in units of 10 -1° tool cm -2) is given by F = 1/NAA, where N A is Avogadro's number. A detailed assessment of the errors in such a pro- cedure is given elsewhere [7], and are for this data + 1,~t 2 at an area per molecule of 50,~ 2.

It is straightforward to extend this method to the determination of the surface composition of a binary mixture. By selective deuteration of each component in turn the surface excess of each component can be evaluated by,

p = bl / (Alz) + b2/(A2z) (2)

where bi and Ai are the scattering lengths and area per molecule of each component.

J. Penfold et al./Colloids Surfaces A: Physicochem. Eng. Aspects 102 (1995) 127-132 129

The surface compositions have been determined by neutron reflectivity for an equimolar mixture of C12EO 3 and C12EO8 in the concentration range 10-5-10 -2 M. The protonated surfactants (h-C12EO,) were obtained from Nikkol. The deuterated surfactants (alkyl-chain deuterated only, d-C12EO,) were synthesised and purified at Unilever by methods described previously 1-16]. The chemical purity of the surfactants was assessed by surface tension measurements and thin layer chromatography. Deuterium oxide (D20) was supplied by MSD and high purity water (Elga Ultapure) was used throughout. The glassware and teflon troughs, used in the neutron reflection meas- urements, were cleaned using alkaline detergent (Decon 90) followed by copious washing in ultrapure water.

For each concentration, reflectivity measure- ments were made with two isotopically label- led combinations, d-C12EO3/d-C12EO s and d- C 1 2 E O 3 / h - C l z E O 8 , in n.r.w. The adsorbed amounts of each component were then evaluated from Eq. (2), using the scattering lengths for the different isotopically labelled species given in Table 1. Fig. 1 shows the reflectivity profiles (back- ground subtracted) for the different isotopically labelled combinations in n.r.w, at a concentration of 5 x 10 -3 M, and indicates the sensitivity in reflectivity to the change in the labelling of the two components.

3. Results and discussion

Fig. 2 shows the adsorbed amount of C12EO3 and C 1 2 E O s and the total adsorption as a function of concentration, evaluated using Eq. (2) for the isotopically labelled combinations described

Table 1 Scattering lengths of isotopically labelled non-ionic surfactants, CI2EO 3 and C12EO 8

Species Scattering length (A-l)

d-C12EO8 2.82 x 10 3 h-ClzEOs 2.16 x 10 -4 d-CI2EO 3 2.61 x 10 3 h-C12EO 3 1.13 x 10 -4

previously. The adsorption of the more surface- active species, C12EO3, is higher than for the C12EO8, and increases with increasing concen- tration upto the CMC of the mixture. There is an abrupt change at the CMC, and beyond the CMC the adsorbed amount of the C12EO3 decreases with increasing concentration. This occurs at a concen- tration of 5 × 10 5 M, compared to the measured CMC (by surface tension) of 5.2 × 10 -5 M. In contrast the adsorption of the C12EO8 is much less and almost constant with increasing solution con- centration. The total adsorption (C12EO3) and C12EO8) is dominated by the C12EO3 adsorption and shows a similar trend with concentration. Fig. 3 shows the mole fraction at the surface as a function of concentration for both components, and the change in behaviour at the CMC of the mixture is clearly seen. The data points for the lowest surfactant concentration (10 -5 M) shows two independent measurements at essentially the same concentration. This is a good indication of the level of error (both statistical and systematic) in these measurements, which is approximately _+ 4% in the surface composition.

Nikas et al. [5] have recently predicted such an abrupt change in the monolayer composition at the mixture CMC for a mixture of C 1 2 E O 6 and C12EO8, and our results for C12EO3 and C12EO 8 are in qualitative agreement with their theory. The abrupt change in monolayer composition with concentration at the mixture CMC is rationalised in terms of the changes in distribution of the two surfactant species between the bulk solution and the monolayer due to the onset of mixed micelle formation at the CMC. The monolayer composi- tion merely reflects the changes in monomer com- position due to mixed micelle formation. Although Nikas et al. [5] have demonstrated this from a detailed theory, based on a two-dimensional gas approach intended to avoid the phenomenological nature of other theories, the same basic trends are inherent in the regular solution approach [17]. Nikas et al. I-5] remark in their paper that "this interesting prediction remains to be tested experi- mentally", and this is largely due to the lack of suitable experimental techniques. Surface tension, for example, is only able to provide information up to the CMC. One of the particular virtues of

130 J. PenfoM et al./Colloids Surfaces A: Physicochem. Eng. Aspects 102 (1995) 127-132

10-3 I I I I

10

10 - 5

-6 10

-4.

0.05 0.1 0.15 0.2

Scattering Vector, O, (in ~ - 1 )

Fig. 1. Reflectivity for equimolar mixture of CIzEO3 and ClzEO8 at a concentration of 5 × 10 -3 M for (0) , d-ClzEO3/d-ClzEO8 and (©), d-C~2EO3/h-C~2EOs. The solid lines are model fits for a single uniform layer.

I E

o o E

I O

x

¢u

tU -+ t / )

2 - I

O

0 I 10 - 5

I I I

[] [] [] 0 D [] [] D D

[ ] • • Q

0 0 0 0 0 0 0 0

0

I I

10 _z, 10 -3 0.01

Surfactant conc. (if)

Fig. 2. Surface excess, F, ( x 10- ao mol cm -2) as a function of concentration for (©) C]zEOs, ( 0 ) C12EO3, and ([3) total.

J. Pen fold et al./Colloids Surfaces A: Physicochem. Eng. Aspects 102 (1995) 127-132 131

o

0.8

0.6

0.~

0.2

I I I

$

8 0 0 0 0 0 0 0

0 0

I I I

10 -5 10 -t~ 10-3 0.01

Surfactant [ont. [M)

Fig. 3. Mole fraction in surface monolayer as a function of concentration for (O) C12EOs, and (0) C12EO 3.

the neutron reflection technique is that it can be used to obtain adsorbed amounts over a wide concentration range from below to well in excess of the CMC, and our results provide the first experimental confirmation of the predictions of Nikas et al. [ 5].

In previous studies on non-ionic/anionic surfac- tant mixtures [10,18] it has been observed that the surface composition tends toward the bulk composition in the limit of high concentrations. This is not universal and arises only in those cases because the interracial and micellar compositions are similar. Notably in the data presented here, even at a .concentration two orders of magnitude larger than the CMC, the surface composition is heavily biased toward the more surface-active com- ponent, C12EO3.

Rosen and Hua [ 11] have used the method of Hutchinson [12] to evaluate the surface tension data for various binary surfactant mixtures, includ- ing the non-ionic mixture of Ci2EO6 and C12EO8 .

The method uses the Gibbs adsorption equation to determine the surface excess and mole fraction of each component from a series of surface tension

vs. concentration curves, in which the concen- tration of the second surface active component of the system is kept constant. Rosen and Hua [ 11 ] have compared their data with the predictions of RST, and they obtain a reasonable agreement. They quote an average error in their determination of surface mole fraction (derived from the two different methods of evaluation) of 6% and a modest spread in interaction parameter of 0.2 _+ 0.1 for the surface monolayer. The errors associated with the determination of the interaction parameter and hence the micellar and surface compositions from surface tension data have been analysed in more detail by us [18], using the method of Bayesian error analysis, and by Hoffman et al. [19], using an alternative method. From Rosen and Hua [ 11 ] the surface interaction parameter is given by,

ln(~Cl2/xC1) /~ - (1 - x ) 2 (3)

where c~ is the solution composition, x the measured surface composition, and C12, C1 the

132 J. PenfoM et al./Colloids Surfaces A: Physicochem. Eng. Aspects 102 (1995) 127-132

Table 2 Comparison of surface mole fraction of C12EO 3 from this study and Rosen and Hua [ 11 ]

Concentration Mole fraction Surface mole fraction (M) in solution

This Rosen and Hua study a [ 11]

8.8 x 10 -6 0.5 0.66 0.60 1.36 x 10 -s 0.5 0.70 0.59 1.62 x 10 -~ 0.5 0.71 0.71

a + 0.04.

concentrations of the mixture and pure solution that have identical surface tensions. For an equimolar mixture of C12EO 3 and ClzEOs at a concentration of 4 x 10 -5 M we obtain a surface composition of 0.74 __+ 0.4 mole fraction of C I 2 E O 3.

Using surface tension values from Rosen and Hua [ 11 ] this gives, from the application of Eq. (3), an interaction parameter of -1.4. An error of 5% in the surface composition alone gives rise to a spread in the interaction parameter from -0.7 to -2.5.

We have interpolated our data for concen- trations below the CMC to provide a direct com- parison with the data of Rosen and Hua [ 11 ]; and the comparison is summarised in Table 2. Given the errors discussed above for the surface tension and neutron reflection data, the surface composi- tions obtained by Rosen and Hua [11] are in reasonable agreement with our neutron results. Furthermore the derived interaction parameter shows (as expected for non-ionic surfactant mix- tures) only a slight departure from ideality; however, the experimental errors discussed above give rise to a spread in values.

References

[1] J.F. Scamehorn, (Ed.) Phenomena in mixed surfactant systems, ACS Symp. Ser. 311, American Chemical Society Washington D.C., 1986.

[2] J.H. Clint, J. Chem. Soc., Faraday Trans. 1, 71 (1975) 1327. [3] D.N. Rubingh, in K.L. Mittal (Ed.), Solution Chemistry

of Surfactants Vol. 1, Plenum Press, NY, 1979. I-4] P.M. Holland, Colloids Surfaces A: Physicochem. Eng.

Aspects. 19 (1986) 171. [5] Y.F. Nikas, S. Pruvvada and D. Blankschtein, Langmuir,

8 (1992) 2680. [6] J. Penfold and R.K. Thomas, J. Phys., Condens. Matt., 2

(1990) 1369. [7] E.A. Simister, R.K. Thomas, J. Penfold, R. Aveyard, B.P.

Binks, P. Cooper, P.D.I. Fletcher, J.R. Lu and A. Sokolowski, J. Phys. Chem., 96 (1992) 1383.

I-8] E.A. Simister, E.M. Lee, R.K. Thomas and J. Penfold, J. Phys. Chem., 96 (1992) 1373.

[9] J.R. Lu, Z.X. Li, R.K. Thomas, E.J. Staples, L. Thompson, I. Tucker and J. Penfold, J. Phys. Chem., 98 (1994) 6559.

[I0] E.J. Staples, L. Thompson, I. Tucker, J. Penfold, R.K. Thomas and J.R. Lu, Langmuir, 9 (1993) 1651.

[11] M.J. Rosen and X.Y. Hua, J. Colloid Interface Sci., 86 (1982) 164.

[12] E. Hutchinson, J. Colloid Sci., 3 (1948) 413. 1-13] J. Penfold, R.C. Ward, W.G. Williams, J. Phys E, Scient.

Instrum., 20 (1987) 1411. [14] E.M. Lee, R.K. Thomas, J. Penfold and R.C. Ward,

J. Phys. Chem., 93 (1989) 381. [15] J. Penfold, in P. Lindner and T. Zemb (Eds.), Neutron,

X-ray and light scattering, Elsevier, NY, 1991. 1,16] J.R. Lu, M. Hromodova, R.K. Thomas and J. Penfold,

Langmuir, 9 (1993) 2417. 1-17] E.J. Staples, L.J. Thompson, I. Tucker and J. Penfold,

Langmuir, 10 (1994) 4136. [18] E.J. Staples, L.J. Thompson, I. Tucker and J. Penfold,

Langmuir, for publication. [19] H. Hoffmann and G. Possnecker, Langmuir, 10 (1994)

381.