the composition of mixed surfactants and cationic polymer/surfactant mixtures adsorbed at the...

COLLOIDS A AND

Colloids and Surfaces SURFACES ELS EV I ER A: Physicochemical and Engineering Aspects 128 (1997) 107-117

The composition of mixed surfactants and cationic polymer/surfactant mixtures adsorbed at the air-water interface

J. Penfold a, E. Staples b I. Tucker b A. Creeth b j. Hines b L. Thompso n b, p. Cummins b, R.K. Thomas c, N. Warren c

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

c Physical Chemistry Laboratory, Oxford Universio,, South Parks Road, Oaford, UK

Received 1 September 1996

Abstract

The use of specular neutron reflection and small-angle neutron scattering (SANS) to determine the composition of mixed surfactants and polymer/surfactant mixtures, absorbed at the air-water interface, in micelles and polymer or micellar complexes, is described.

For the characterisation of adsorption at the air-water interface by neutron reflectivity, the emphasis is predominantly on the concentration regime where the surface is in equilibrium with a bulk aggregated phase; that is, for concentrations greater than the cmc of the mixture. This is a regime that is difficult to access by standard experimental techniques. The SANS measurements of micelle composition are an important complement to the reflectivity measurements m the understanding of mixed surfactant adsorption.

Recent results on the anionic/non-ionic surfactant mixture of sodium dodecyl sulphate (SDS) and hexaethylene monododecyl ether (C12E6) in the absence and presence of the cationic polymer and on the non-ionic mixture of monododecyl/triethylene glycol (C12E3)/monododecyl octaethylene glycol (C12E8) illustrate the power and advantages of neutron scattering techniques. © 1997 Elsevier Science B.V.

Keywor&v Polymer/surfactant mixtures; Specular neutron reflection

1. Introduction

The composition of mixed surfactants at inter- faces and in bulk phases, and its evolution with concentration, is of both practical and theoretical interest [1 ]. The industrial, domestic and techno- logical uses of surfactants predominantly involve mixtures. This is because mixtures enhance aspects of performance, synergy, or because commercial surfactants are impure. That is, they contain mixtures of different chain lengths and isomeric forms [2].

Polymers in aqueous solutions are extensively

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used in a wide range of applications as viscosity modifiers, stabilisers, deposition aids and in con- trolling foam behaviour [3]. In many instances the required behaviour is achieved by manipulation of the effective solubility of the polymer in surfac- rant solutions, and invariably involves mixed surfactants.

For a complete understanding of the properties and behaviour of such complex mixtures it is important to have a knowledge of micellar, monomer and surface composition over a wide concentration range, from below to well in excess of the cmc of the mixture. The ability to calculate

108 J. Penfoldet al. / Colloids Surfaces A: Physicochem. Eng. Aspects 128 (1997) 107 117

and predict surface and micellar compositions which mix ideally [4] and which show departures from ideal mixing has generated much theoretical interest [5-7]. There are, however, few experimental techniques which can unambiguously provide such information and especially for the surface composition at concentrations in excess of the cmc. In the presence of polymers the interpretation of techniques such as surface tension are further complicated by pre-micellar complex formation in bulk and at the interface, and the resulting varia- tions in surfactant activity render the Gibbs equa- tion difficult to apply.

In recent experiments we have demonstrated that the specular reflection of neutrons [8] is a powerful technique for the study of surfactant adsorption at the air/water interface. The vastly different scattering powers of hydrogen and deuterium for neutrons means that H/D isotopic substitution can be used to manipulate the neutron refractive index profile at an interface and particular features or components can be highlighted or suppressed. It provides a direct and absolute method of determining adsorbed amounts in single [9] and mixed monolayers [10-14] over a wide concentration range from below to well in excess of the cmc. Not only can the adsorbed amounts be evaluated, but the detailed surface structure can be determined [15]. In contrast, SANS, in combination with H/D isotopic substitution can be used to determine micelle compositions and structure [12], and provides an important complement to the neutron reflectivity data.

2. Experimental

The neutron reflectivity measurements described in this paper were made for the surfactant mixtures of C12E3/ClzE s (at bulk compositions of 50:50 and 30:70 mole ratio, and in the concentration range ~10 -5 to 10- /M) [10], of SDS/CazE6 (for bulk compositions of 70:30 and 50:50 mole ratio) in the concentration range 10 -4 to 10 2 M, in the composition range 10:90 to 90:10 at a concentration of 10- /M and in 0.1 M NaC1) [12], for a 70:30 mole ratio of SDS/C12E 6 in 0.1 M NaC1 and added cationic copolymer of poly(dmdaac)/poly(acryl-

amide) [16]. Some measurements were made with polymer concentrations up to 250 ppm, and for different polymer charge densities (different dmdaac/acrylamide ratios). Neutron reflection and SANS measurements were made with different concentrations of deuterated and protonated surfactants, and protonated polymer (see later for details). The protonated surfactants were obtained from Nikkol (C12E3, C12E6, C12E8) and BDH (SDS). The deuterated SDS (d-SDS) was obtained from MSD isotopes and the alkyl chain deuterated non-ionic surfactants (C12E3, C12E 6 and C12E8) were synthesised and purified by methods described previously [17] at Oxford and Unilever. The chemical purity of the surfactants was assessed by surface tension and thin layer chromatography. The poly(dmdaac)-poly(acrylamide) copolymers (MW~ 100 000) were supplied by Allied Colloids, and the charge densities measured conductrimetri- cally. Deuterium oxide (D20) was supplied by CDN and high-purity water (Elga Ultrapure) was used throughout. The glassware, quartz cells and PTFE troughs used for the SANS and neutron reflection measurements were cleaned using alka- line detergent (DECON 90), followed by copious washing in high-purity water.

The neutron reflection measurements were made on the reflectometers CRISP [18] and SURF [19] at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory. The measurements were made using a single detector at a fixed glancing angle of incidents of 1.5 ° and neutron wavelengths in the range 0.5 to 6.5 ~,. The absolute reflectivities were calibrated with respect to D20, as previously described [20]. The background, arising from inco- herent scattering from the bulk solution, was determined from the reflection signal obtained in the limit of high valves of wave vector transfer, K (~c = 47r sine 0/2, where 0 is the angle of incidence, and 2 is the neutron wavelength) and subtracted from the full K range. This has been shown to be a valid procedure providing there is no small-angle scattering from the bulk solution [13] and has been verified by making off-specular measurements (that is, either side of the specular reflection).

The SANS measurements were made on the LOQ diffractometer [21 ] at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory.

J. Pen/bld et al. / Colloids Surfaces A: Phvsicochem. Eng. Aspects 128 (1997) 107 117 109

The measurements were made using the white beam time-of-flight method, to provide a J,- range of 0.01 to 0.15/k -1

The surface tension measurements were made using a Kruss K10 maximum pull tensionmeter with a platinum ring, as described previously [16].

The specular reflection of neutrons gives information about inhomogenities normal to an interface and its theory has been described in detail elsewhere [8]. The basis of the neutron reflection method is that the variation in specular reflection with wave vector transfer, ~c, is simply related to the composition or density profile in the direction normal to the interface. In the kinematic or Born approximation [8] it is simply proportional to the square of the Fourier transform of the scattering length density profile, p(z), where p(z)=2i rti(,7)bi,

and n~(z) is the number density of the ith nucleus

and bi its scattering length. The essence of the application o f the technique for the study of surfactant adsorpt ion is the ability to manipulate the scattering length density or neutron refractive index profile (where the neutron refractive index is defined as n = 1 -)~2p(2)//27E) at the interface. For a deuterated surfactant in nrw (92mo1% H20/8 tool% D 2 0 has a scattering length o f zero, that is, a refractive index identical to air) the specularly reflected signal arises only from the adsorbed surfactant layer at the interface. The most direct procedure for determining the surface concentra t ion o f the surfactant is to fit the measured reflectivity by compar ing it with a profile calculated using the optical matrix method [22] for a single structural model. Typically, in the determination o f the surface concentrat ion, it is sufficient to assume that the surfactant is in the

g

[-.

C oJ L_I C O

10 -1 I - I

10-2

IU 3

10 -L,

I :J /

I

(b)

(c)

10-5 1 I I

0 .2 .t, .6

Mole Fract ion SOS

10-5 I I I i

lO-t* 10-3 I o - Z i0-1 T o t a l Surfacfant Conc~frafion (M}

Fig. I. Regular solution theory calculation for SDS/Ct2E6/0 .1 M NaC1 (with an interaction parameter, [L of -3.0) showing (a) the concentration dependence of micelle composition, (b) the micelle composition at the cmc and (c) the composition dependence of the cmc. The inset shows the effect of surfactant concentration on the SDS and Cx2E 6 m o n o m e r concentrations for a 70:30 mole ratio mixture in 0.1 M NaC1. The data points (O) are the mieelle compositions measured by SANS [12].

110 J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 128 (1997) 107 117

form of a single layer of homogeneous compositions. The parameters obtained for such a fit are the scattering length density (p), and the thickness (~) of the layer. The area per molecule in the adsorbed layer is then

A = b/pz, ( 1 )

where b is the scattering length of the adsorbed molecule. The principal errors in the determination of the adsorbed amount are due to calibration, background subtraction, or due to the assumptions of too simple a model. These have been discussed in detail elsewhere [9], and correspond to an error of + 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

mixture. By selective deuteration of each component in turn the surface excess of each component can be evaluated. For a binary mixture Eq. (1) then becomes [23]

p = b l / ( A l z ) +b2 / (A20 (2)

where bi, Ai, are the scattering lengths and areas per molecule of each component. Measurements are made with either component deuterated and additional measurements are made with both components deuterated (and provide a self-consistent determination of the adsorbed amounts). In some cases measurement errors are reduced by adopting a labelling scheme that maximises the signal. For example, measurements with both components deuterated, and with only one component

0.8

¢.- o

"~,~ 0.6 fo

~J o

~7

0.t~ t~

0.2

I I I

0 I I I 10 -5 10-4 10-3 0.01

Surfacfanf Conc. (M)

Fig. 2. Mole fraction in surface monolayer for an equimolar mixture of C12E 3 and C12E8 as a function of concentration for C12E8 (©) a n d C12E3 ( 0 ) [10] .

Z PenJold et al. ,/Colloids Surfaces A: Physicochem. Eng. Aspects 128 (1997) 1 0 7 - l l 7 I I I

C

° ~

tO l _

L L

0

~U U tlB

0.8

0.6

0./.

0.2

I I

I I

10 -4 10-3 0.01

Surfactant Cone. (H)

Fig, 3. Surface mole fraction for a 70:30 mol% mixture of SDS/C12E6 in 0.1 M NaC1 as a function of surfactant concentration, for SDS (_--') and Ct2E6 (O) [12].

deuterated. The scattering lengths for each of the components used on this study are summarised in Table 1.

For the measurements of the SDS/C12E6 mixture in the presence of the cationic polymer poly(dmdaac)/poly(acrylamide), in the absence of deuterated polymer, it assumed that the scattering length of the polymer is sufficiently close to zero to be neglected. Typically, in the measurements reported here this contributes an uncertainty of <5% [16].

We have shown elsewhere [12] that SANS can be used effectively to measure mixed surfactant micellar compositions, at concentrations close to the cmc to good accuracy (_< 1%). The scattered

intensity from dilute mixed surfactant micelles in the limit of small ~c can be expressed as

Iota, Ni V~(pi: - p s ) 2 ( 3 ) i

where N is the micelle number density, V the micelle "dry" volume, Pip the micelle scattering length density, Ps the solvent scattering length density, and the summation is over all micelle compositions. For a binary surfactant mixture, by making two different measurements with both surfactants protonated and one deuterated in O20, the ratio of the two scattering intensities, extrapolated to ~c = 0, gives a direct estimate of the micellar composition.

112 J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 128 (1997) 107-117

3. Results and discussion

3.1. Surfactant mixtures

Regular solution theory, RST, [5,6] is a convenient framework in which to discuss the composition of mixed surfactant micelles and of surface adsorbed layers. At concentrations close to the cmc the micelles and surface layer are rich in the most surface-active component and as the concentration increases the composition tends towards the bulk composition. This evolution with concentration is shown for the micelle composition of SDS/ClzE 6 in 0.! M NaC1 (with an interaction parameter, /~--~ -3.0) in Fig. 1, shown also in the inset in the variation in monomer composition and illustrates a similar trend. The abrupt change in monomer composition at the mixture cmc is also mirrored in the surface composition. This has also recently demonstrated theoretically by Nikas et al. [7] using a more detailed theory based on a two-dimensional gas approach intended to avoid the phenomeno- logical nature of theories such as RST. The abrupt change in monolayer and monomer composition with concentration at the mixture cmc is rational- ised in terms of the changes in distribution of the two surfactant species between the bulk solution and monolayer due to the onset of mixed micelle formation at the cmc. (The monolayer composition merely reflects the changes in monomer composition due to mixed micelle formation.) The surface concentrations and compositions for the equimolar mixture of C12E 3 and ClzE8 a re shown in Fig. 2 and demonstrate clearly the trends pre- dicted by RST and Nikas et al. [7]. In contrast to the situation for SDS/C12E6/0.1 M NaC1, the composition of the surface for C12E3/ClzE s at 100 times the cmc is significantly different from the bulk composition. The composition of the surface is still heavily biased towards the more surface- active component, C l z E O 3. The correspondence of the bulk and surface composition at high concentrations is not universal. It arises in the case of SDS/C12E6/0.IM NaC1 [12] and of C16TAB/ClaE6/0.1 M NaC1 [ 11 ] because geometri- cal and packing constraints are not dominant, in particular the surface pressures of the pure compo-

nents, identifiable with the limiting monomer concentrations in the mixture, are comparable. The evolution of surface composition for a 70:30 mol% mixture of SDS/C12E6/0.1 M NaC1 with concentration is shown in Fig. 3.

We have shown that the variation of micelle composition with concentration (see Fig. 1), measured by SANS, is consistent with the predictions of RST.

3.2. Polymer/surfactant mixtures

The surface tension data plotted in Fig. 4 are for 50ppm 1:I poly(dmdaac)-poly(acrylamide) copolymer with SDS and with the surfactant mixture 70:30 SDS: C12E 6 in 0.1 M NaC1 [16]. The surface tension data for the copolymer-SDS mix-

60

\

$0

30

10 -6

(a)

n ° _ ~

10 -5 I0 -4 10-3 10 -2 0.I

Log ( C o n c e n t r a t i o n / N o [ L-l)

'T 55

5o

~" ~s

t.o

10 -5 1o -~. 1o -3

Log (Concentration/mot [-1)

Fig. 4. Surface tension for (a) SDS 50 ppm 1:1 poly(dmdaac)- poly (acrylamide) copolymer ( • ) (lines through the data points are a guide to the eye), without polymer (-); (b) 70:30 mole ratio SDS/C12E 6 in 0.1 M NaCI and 50 ppm 1:1 poly(dmdaac)- -poly(acrylamide) copolymer ( l l L without polymer (-) [16].

J. Pen[bid et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 128 (1997) 107 117 ] 13

0.8

0.6

0.4

0.2

I0-4

t -

O .. i-. l .a

{11

0

QJ

- - i l . , f l

1

10 -3 10 -z

Surfacfanf Cone. (H)

Fig. 5. Surface composition as a function of surfactant concentration for 70:30 mole ratio for S D S CI2Eo in 0.1 M NaCI (-), and with 50 ppm 1:1 poly(dmdaac) poly(acrylamide) copolymer (- --); SDS (@) and CLzE 6 (Z,) [16].

ture are typical of those obtained for other polymer-surfactant mixtures [24]. The initial reduction in surface tension in the presence of low surfactant concentrations results from the adsorption of the hydrophobic polymer-surfactant complex that forms prior to the occurrence of micelles bound to the polymer. The initial plateau region can be identified with "bound" micelles and the final plateaux with the formation of free micelles. For the mixed micelles in the presence of the polymer the surface tension data are very different in profile. First, in the absence of polymer, the surface tension is much lower, reflecting the greater surface activity of the non-ionic surfactant. The slight increase in surface tension in the presence of the polymer is consistent with the surface tension being domi- nated by the adsorption of the surfactant. The

higher surface tensions are consistent with a reduced monomer activity resulting from surfactant adsorption into the polymer. It is in principle possible to quantify surface tension changes in terms of adsorbed amounts. However, due to the changes in surfactant activity it is difficult to separate the contributions from surfactant and polymer in all but the simplest cases.

Neutron reflectivity, as discussed earlier, provides a convenient method for determining surfactant and polymer adsorption at the interface. The surface composition for 70:30 SDS/C~2E6 in 0.1 M NaC1 with and without 50 ppm 1:1 poly(dmdaac)- -poly(acrylamide) are compared in Fig. 5.

The addition of polymer results in an enhanced adsorption of the SDS and a reduction in the adsorption of the C12E6. The surface composition

114 J. PenfoM et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 128 (1997) 107-117

is weighted more substantially towards the anionic surfactant. The same enhancement is seen across a wide bulk composition range, from an SDS-rich to a C12E6-rich solution. This is shown in Fig. 6, where the surface composition with and without 50 ppm 1:1 poly(dmdaac)-poly(acrylamide) is plotted as a function of bulk surfactant composition at a concentration of 10 -2 M. Throughout these changes in surface composition the total amount of surfactant at the interface remains essentially constant.

Fig. 7 shows the variation in adsorbed amount of both the SDS and C12E6 at different surfactant concentration and at a mole ratio of 70:30 in the presence of 50 ppm copolymer with varying polymer charge density. The surface composition evolves towards a surface richer in SDS as the

charge density in the polymer increases. In the absence of deuterated polymer it is possible to estimate the amount of polymer at the interface from a combination of the measurements in nrw and D20 subphases [16]. Fig. 8 shows the variation in copolymer (1:1) volume fraction at the interface with surfactant concentration for a 70:30 mole ratio SDS: C12E 6 mixture.

A gradual decrease in the amount of polymer at the interface reflects the evolution of the charge density of both the surface and surfactant complex. Additional measurements [16] have verified that the copolymer is not surface active on its own. Indeed there is no copolymer at the interface unless SDS is present. That is, mixtures of the copolymer and C12E 6 only do not result in any copolymer adsorption at the interface.

Q l,O

t-

O

c- ii

Q#

0

Q~

¢0 'L -I

if)

0.8

0.6

O.t+

0.2 m

i I ~ I , I , I

0 0.2 0.4 0.6 0.8 I

l l ' I ' I l I '

SOS Mole Ratio

Fig. 6. Surface composition at a surfactant concentration of 10 -3 M as a function of bulk composition for SDS-C12E 6 in 0.1 M NaC1 (-), and with 50 ppm 1:1 poly(dmdaac)-poly(acrylamide) copolymer (-- -); SDS ( e ) and CI:E 6 (~) .

J. Penfold et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 128 (1997) 107-117 115

e-

l l

Q/

)--

(U

~c V )

0.8

0.6

0 . ~

0.2

0

I I

~t 'r 1

,i •

¢b !

I I

0.5 1

Potymer Charge Density

Fig. 7. Variation of surface composition with polymer charge density for 70:30 SDS/ClzE 6 in 0.1 M NaC1 and 50 ppm poly(dmdaac) - poly(acrylamide) copolymer; SDS (0) and C12E6 (O) (the lines are guides to the eye only), and surfactant concentrations (7) 5 x 10 4 M, (Zx) 10 -3 M and (O) 2x 10 -3 M.

There have been relatively few applications of neutron reflectivity to the studies of polymer-surfactant mixtures reported in the literature. However the work of Purcell et al. [24] on mixtures of SDS and poly(vinylpyrrolidone) (PVP) and of Lu et al. [25] on poly(ethylene oxide) (PEO) and SDS mixtures provides an interesting contrast to the work reported here. For the SDS-PVP mixtures Lu et al. [25] found that at low SDS concentrations (for concentrations <3 x 10 -3 M) the addition of PVP enhanced the adsorption of SDS, whereas at higher SDS concentrations the addition of PVP suppressed the adsorption of SDS at the interface. For the SDS-PEO mixtures [25] both the surfactant and polymer are surface active and

the adsorption pattern results from the competitive adsorption (from the relative surface activities) of the two components. The addition of the poly- (dmdaac)-poly(acrylamide) copolymer to the SDS-C16E6 surfactant mixture results in a change in the surface composition towards one richer in SDS, whereas the total amount of surfactant adsorption is unaffected. This is due in part to the change in the relative activities of the two surfactants resulting from the formation of p o l y m e ~ surfactant complexes in bulk solution and also due to the adsorption of polymer-surfac tant complexes at the interface. The surfactant concentration and composition dependence of the surface composition suggest, however, that it is the change in

116 J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 128 (1997) 107-117

f -

O

t J /1D ¢..

U . QJ

E O ;> L I11

E O

EL

0.3

0.2

0.1

I

10 -/+ 10 -3 10 -2

Surfacfanf c0nc (H)

Fig. 8. Variation of copolymer volume fraction with surfactant concentration for a 70:30 mole ratio mixture of SDS/Ci2E 6 in 0.1 M NaC1 [16].

relative activities of the two surfactants due to the formation of copolymer/surfactant complexes in bulk that is the dominant contribution. Further measurements with deuterated copolymer will provide a better estimate of the amount of polymer at the interface and establish more definitely the contribution of the adsorption of copolymer surfactant complexes at the interface to the change in interfacial surfactant composition. The assumption or inference from other work [23] is that the addition of the copolymer does not significantly alter micellar structure. Partial deuterium labelling in combination with

neutron reflectivity enables the surface or interfacial structure in surfactant mixtures to be evaluated [26]. The correlation of structure and composition in mixed surfactants is a key aspect of mixed surfactant adsorption and is currently the subject of investigations [11,26]. Measurements to evaluate the effect of the copolymer at the interface on the structure of the SDS-ClzE 6 monolayer are currently being performed. These measurements should provide an important insight into the role of the copolymer at the interface in determining the composition of the mixed monolayer.

J. Penfold et al. / Colloids Surfaces A: Physicochem. Eng. A,7)ects 128 (1997) 107 117 l 17

References

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[2] L.J. Thompson, J. Colloid Interface Sci. 163 (1994) 6. [3] E. Goddard, K.P. Ananthapadmanabhan (Eds.h

Interaction of Surfactants with Polymers and Proteins, CRC Press, Boca Raton, 1993.

[4] J.H. Clint, J. Chem. Soc. Faraday Trans. 1 71 (1995) /377.

[5] D.N. Rubingh, in: K.L. Mittal (Ed.), Solution Chemistry of Surfactants, vol. 1, Plenum, New York, 1979.

[6] P.M. Holland, Colloid Surf. 19 (1986) 171. [7] Y.F. Nikas, S. Pruvvada, D. Blankschtein, Langmuir 8

(1992) 2680. [8] J. Penfold, R.K. Thomas, J. Phys: Condens. Matter 2

(1990) 1369. [9] E.A. Simister, R.K. Thomas, J. Penfold, R. Aveyard, B.P.

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

[10] J. Penfold, E. Staples, L. Thompson, I. Tucker, Colloids Surf. 102 (1995) 127.

[11] J. Penfold, E. Staples, P. Cummins, J. Tucker, I. Thompson, R.K. Thomas, E.A. Simister, J.R. Lu, J. ('hem. Soc. Faraday Trans. 92 (1996) 1773.

[12] J. Penfold, E. Staples, L. Thompson, J. Tucker, J. Hines, R.K. Thomas, J.R. Lu, Langmuir 11 (1995) 2496.

[13] E. Staples, L. Thompson, I. Tucker, J. Penfold, R.K. Thomas. J.R. Lu, Langmuir 9 (1993) 165l.

[14] E. Staples, L. Thompson, I. Tucker, J. Penfold, Langmuir 9 (1994) 4136.

[15]J. Penfold, E. Staples. P. Cummins, 1. Tucker, L. Thompson, R.K. Thomas, E.A. Simister, J.R. Lu, J. Chem. Soc. Faraday Trans. 92 (1996) 1539.

[16] A. Creeth, E. Staples, L. Thompson, 1. Tucker, J. Penfold. J. Chem. Soc. Faraday Trans. 92 (19961 589.

[17] J.R. Lu, M. Hromodova, R.K. Thomas. J. Penfold, Langmuir 9 (1993) 2417.

[18] J. Penfold, R.C. Ward, W.G. Williams. J. Phys. E. Sci. lnstrum. 20 (1987) 141l.

[19] D. Bucknall, J. Penfold, A. Rennie, R. Richardson, J. Webster, A. Zarbakhsh, Proceedings of ICANS Xlll , PSI Proceedings 95-02 vol. 1, 1995, p. 123.

[20] E.M. Lee, R.K. Thomas, J. Penfold. R.C. Ward, J. Phys. Chem. 93 (1989) 381.

[21] R.K. Heenan, R. Osborn, H.B. Stanley, D.F.R. Mildner, M. Furusaka. J. Appl. Cryst. to be published.

[22] J. Penfold, in: P. Lindner, T. Zemb (Eds.), Neutron, X-ray and Light Scattering, Elsevier, New York, 1991.

[23] J. Penfold, R.K. Thomas, E.A. Simister, E.M. Lee, A.P,. Rennie, J. Phys. Condens. Matter 2 t 19901 SA4121.

[24] I. Purcell, R.K. Thomas, A. Howe, J. Penfold, Colloids Surf. A: Physicochem. Eng. Aspects 94 (1994) 125.

[25] J.R. Lu, J.A.K. Blondel, D.J. Cooke, R.K. Thomas, Prog. Colloid Polym. Sci., 100(1996) 311.

[26] J. Penfold, E. Staples, P. Cummins. 1. Tucker, R.K. Thomas, E. Simister, J.R. Lu. J. ('hem. Soc. Faraday Trans. 92 (1996) 1539.

the composition of mixed surfactants and cationic polymer/surfactant mixtures adsorbed at the...

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