ELSEVIER Physica B 198 (1994) 110 115

The study of surfactant adsorption by specular neutron reflection

J. P e n f o l d a'*, R .K . T h o m a s b, J .R. L u b, E. S t ap l e s c, I. T u c k e r c, L. T h o m p s o n c

a ISIS Science Division, Rutherford Appleton Laboratory, Didcot, Oxon, UK b Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, UK

c Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, UK

Abstract

The specular reflection of neutrons is now a well-established technique for the study of surfactant adsorption at the air-liquid interface. In combination with isotopic substitution it provides a powerful method for determining adsorbed amounts and the surface/interfacial monolayer structure. The application of the technique for the determination of adsorption isotherms will be described, with particular emphasis on multi-component systems. Recent results for nonionic and mixed nonionic-cationic surfactants will be discussed, and compared with theoretical predictions.

1. Introduction

The specular reflection of neutrons gives information about inhomogeneities normal to an interface, and its theory is described in detail elsewhere [1 3]. The basis of a neutron reflection experiment is that the variation in specular reflec- tion with wave vector transfer, Q (where Q is de- fined as Q -- 4~ sin 0/2, 2 is the neutron wavelength and 0 the glancing angle of incidence), is simply related to the composition or density profile in the direction normal to the interface. In the kinematic or Born approximation [2] it is proportional to the square of the Fourier transform of the scattering length density profile, p(z), where O(z) = Y~i ni(z) bi and ni is the number density of the ith nucleus and b~ its scattering length. The neutron scattering lengths of H and D are sufficiently different that for the study of problems in surface chemistry H/D isotopic substitution can be used to manipulate the scattering length density or neutron refractive in- dex profile at an interface. This is the essence of the application of the technique to the study of surfac-

* Corresponding author.

tant adsorption at the air-liquid interface. The neutron reflection measurements reported here were made on the reflectometer CRISP [4] at the Rutherford Appleton Laboratory (UK), and the procedure for making the measurements is de- scribed in detail elsewhere [5, 6].

The measured reflectivity profiles can be ana- lysed by two different methods. In the first and most commonly used method a structural model (which may be simply a single uniform layer) is assumed for the interface, and the reflectivity is calculated exactly using the optical matrix method [7]. The other method is based on the kinematic approximation, described eai'lier, and is more direct. Both methods of analysis have been used for the data presented in this paper.

Neutron reflection in combination with H/D iso- topic substitution has now been used to determine the adsorbed amount and for the determination of the interfacial structure of a range of surfactants adsorbed at the air-liquid interface. Particular em- phasis has been placed on the study of nonionic [8-11] and cationic [12,13] surfactants, and surfactant mixtures [6, 14]. In this paper we will describe in more detail the application of neutron reflection specifically to the determination

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J. PenfoM et al./ Physica B 198 (1994) 110-115 111

of adsorbed amounts of surfactants adsorbed at the air-liquid interface. Particular emphasis will be placed on the study of multi-component systems. Recent results on non-ionic, cationic and non- ionic-ionic surfactant mixtures will be used to dem- onstrate the power of the technique in this area of application.

2. Determination of adsorbed amount

For the determination of surfactant adsorption at the air-aqueous-solution interface the scattering properties of H and D are such that with null reflecting water, N R W (water of the appropriate composition to be refractive index matched to air), and a deuterated surfactant, the only part of the system which contributes to the reflectivity is the surfactant adsorbed at the interface. The normal procedure for determining the surface concentra- tion is to fit the measured reflectivity profile by comparing it with a profile calculated using the optical matrix method [7] for a simple structural model. Typically, in the determination of the sur- face concentration, it is sufficient to assume that the surfactant is in the form of a single layer of homo- geneous composition. The parameters obtained from such a fit are the scattering length density of the layer Ps and it thickness r. The area per surfac- tant molecule in the adsorbed layer at the surface is then

b A = - - , ( ] )

psZ

where b is the total scattering length of the adsor- bed layer, and the adsorbed amount, or surface excess, F (expressed in units of 10- lO mol c m - 1), is given by F = 1 /ANA, where NA is Avogadro's num- ber. Possible errors in the use of neutron reflection for the determination of surface concentration arise from errors in measurement, due to calibration or background subtraction, or errors due to the as- sumption of too simple a model. These sources of errors have been discussed in detail elsewhere [15]; the main sources are calibration errors, typically < _ 2 ~2 at an area per molecule of 50 ~2.

Figure l(a) shows the neutron reflectivity for cationic surfactant octadecyl trimethyl ammonium

1.0 0.5

o

ol 0.05

o.oi w 0.o05 _J LL IJJ

0001. ' ' ' ' ' ' ' ' ' 0.0~ 0.~0 0.15 0.20 0.25 0.30 0.35 0./,0

W A V E V E C T O R TRANSFER, Q (A °-1)

G " I

o X

v

".-,1"

O

0.016

0.01/,

0.012

O.010

0008

0OO6

0.004

0002

0 0.05

+ + + ÷ -I.

÷.÷÷ I ÷÷ I, ~ ÷÷ I

Q® 0@

,QO O O • I I I I I I I I

0.10 0.15 0.20 0.25

W A V E V E C T O R T R A N S F E R , 0 ( A 0 -1 )

Fig. 1. Top: neutron reflectivity, R, for dClahTAB NRW, for surfactant concentrations in the range 3x 10 -4 M to 6 x 1 0 - 6 M. Bottom: RQ 4 for dC18hTAB in NRW at concentra- tions of ( + ) 3 x 10 -4 M and (O) 6 × 10 -6 M.

bromide (dClshTAB) in null reflecting water in the concentration range 3 x 1 0 - 4 M to 6 x 1 0 - 6 M . The fiat background at high Q arises from incoher- ent scattering from the bulk aqueous subphase. The reflectivity data, plotted as R Q 4 in Fig. 1 (b) for the extremes of the concentration range, clearly show from the position of the intensity maximum the thinning of the layer with decreasing concentration, and the reduction of the adsorbed amount from the height of the interference fringe.

The neutron surface excess for the nonionic sur- factant monododecyi hexaethylene glycol (C12E6) is shown in Fig. 2, and is typical of such surfactant adsorption isotherms. For the limited range of sur- factants studied by neutron reflection to date the plateau seen in the isotherm occurs at about the critical micellar concentration, CMC, for the

1 1 2 J. Per~lbldet a l . /Phys i ca B 198 (1994) 110 115

i

35

"~ 30 s o 2'.5

x ff'l 20

Ixl 1.5

W 1.0 hi

U

05 i J L i i i J

5 lo 5o loo 5o0 zooo

c12e6 CONCENTRATION (re, x10 -6)

Fig. 2. Surface excess for CI2E 6 from neutron reflection; the solid line is a smooth curve through the data points.

nonionic surfactants [8, 11] and above the CMC for charged surfactants [13, 15]. The neutron sur- face excesses have been compared with the surface tension surface excesses, and are in good agreement when the surface tension data are correctly evalu- ated [15]. It has, in these studies, been clearly demonstrated that neutron reflection provides an effective and accurate method of determining ad- sorbed amounts. It has highlighted some of the difficulties associated with methods such as surface tension, and has enabled measurements to be ex- tended to concentrations above the CMC, where the adsorbed layer is in equilibrium with a bulk aggregated phase of micelles.

The use of neutron reflection to obtain accurate values of adsorbed amounts has, for example, in some recent work [16] enabled the temperature dependence of nonionic surfactant adsorption at the air-liquid interface to be investigated. It was expected that the temperature-driven dehydration of the ethylene glycol chain would cause a change in the conformation, resulting in a consequent increase in packing and reduction in the area/ molecule. Conflicting results have been obtained from surface tension data, where both increases and decreases in the adsorption with increasing temper- ature have been reported [17, 18]. Figure 3 shows the area/molecule obtained for the nonionic surfac- tants with a C12 alkyl chain and different ethylene glycol chain lengths (from C12E3 to C12E12) as a function of temperature from neutron reflection data. The data show that at the CMC there is only

-3

100

80

6 0 "

~ 1 L I ~ [ i _ , l

4 0

20

0 20

J

J

Temperature (c)

Fig. 3. Temperature dependence of area/molecule for (V) C 1 2 E 3 , ( A ) C 1 2 E 5 , (A) C 1 2 E 6 , ( Q ) C I 2 E s a n d (Q) C 1 2 E 1 2 ,

a small reduction in the area/molecule over a wide temperature range. This suggests that at this con- centration the packing is already dominated by inter-molecular forces, and that changing the ethy- lene glycol hydration only subtly alters the packing and structure of the adsorbed layer.

Isotopic labelling has been extensively used to determine the structure of adsorbed surfactant layers using the partial structure factor approach, and it is important to establish that adsorption is independent of isotopic content [8, 13]. The surfac- rant self-partial structure factor, h,, can be de- scribed by a Gaussian distribution [13] such that

1 hii(Q) = ~2 exp ( - QZa2/8). (2)

A plot of ln(h,) against Q2[ l l ] provides an alter- native and direct way of evaluating A and ai (the extent of the layer) as the intercept is - 2 In A and the slope is - 02/8. Figure 4 shows a set of such plots for the alkyl chain partial structure factor, hcc, the ethylene glycol chain, hoe, and the whole surfac- tant molecule, haa, for the nonionic surfactant CIzE 6 at its CMC. The different isotopes have a common intercept, and this demonstrates that the adsorbed amount is independent of isotope. This is consistent with surface tension data for different isotopic labelling of both the surfactant and solvent [8] which also indicate that the adsorption is not dependent upon the isotopic content.

J. Penfoldet al./Physica B 198 (1994) 110 115 113

- 8 . 0

- 8 . 5

- 9 . 0

~ -9.5

-10 .0

- 1 0 . 5

-11 .0

11.5

q:

0.01 0.02 0.03 0.04 0.05 0.06

Q~(~-~)

Fig. 4. In hii against Q2 for C12E6 at the CMC for (©) ethylene glycol chains, (&) alkyl chains and ( + ) the whole molecule. The solid lines are fitted curves to Eq. (8) for width parameters a of 16.5, 16.0 and 22.0/~, respectively, and an area/molecule (inde- pendent of isotope) ratio of 55,~2.

The data obtained using the position sensitive detector can be evaluated in a different way in the context of the kinematic approximation. R(Q) has been shown to be directly proportional to the square of the surface excess, and the measured intensity from the position sensitive detector integ- rated under the specular peak and over the Q range of the instrument gives directly the excess

F = c R(Q) dQ , (3)

Qm~n

where F is the surface excess (as defined previously) and c is an experimentally determined constant. This approach provides an effective method for systems with concentrations well in excess of the CMC [6], and for which there is a small angle scattering contribution, from micelles in the bulk subphase, under the specular peak which can be effectively subtracted. It also provides a reliable method of determining low levels of excess which would give rise to unacceptably large errors using the methods described earlier. It is an important extension of the technique for complex systems

where components with low adsorption exist, and the bulk phase is a concentrated dispersion.

3. Multi-component systems

It is straightforward to extend the methods de- scribed in the previous section to the determination of surface excesses in multi-component systems by selective deuteration of each component in turn. For example, for a two-component surfactant monolayer, Eq. (1) becomes [19]

bl + bzA1/A2 p, = , (4)

A~r

where bi ,2 , A1, 2 are the scattering lengths and areas/molecule of each component. In calculating the area/molecule of the deuterated component the effective scattering length of each component is used, to allow for the nonzero contribution of the protonated component (although in principle the nondeuterated components could be index match- ed to air). For measurements with the position sensitive detector, evaluated using Eq. (3), the area/molecule of the deuterated component is cor- rected for the contribution of the protonated com- ponent by

A ~ = Ad 1 + bdApj , (5)

where Ad, p and bd, p refer to the area/molecule and scattering lengths of the deuterated and protonated components and A~ is the corrected area/molecule. The contribution of the protonated component (and hence the correction) is typically of the order of a few percent. Additional measurements with both components deuterated have provided a veri- fication of such an approach.

For surfactant mixtures where nonideal mixing is expected micellar and surface compositions can only be estimated from measurements such as sur- face tension by the application of a theory such as regular solution theory [20]. This assumes the existence of a "regular solution" with ideal entropy of mixing. The departure from ideality is character- ised by an interaction parameter, fl, which accounts for the excess free energy of mixing.

114 J. Penfold et al./ Physica B 198 (1994) 110-115

10

N I

r...,

~q

to

~ x x ×

I 10 -5 1 0 - 4

+ +

I t 10 -3 10 - 2 1.0

Concentration(M)

Fig. 5. Surface excess for 65 mol% C12E3 35 tool% SDS4).I M NaCI, ( + ) SDS, ( × ) C12E 3, (*) total surfactant excess; solid lines are predictions of regular solution theory for an interaction parameter fl of - 2.42.

Staples et al. [6] have used neutron reflectivity to determine the surface concentrations of one com- position (65 mol% of C ~ 2 E3) of the nonideal mixed nonionic-anionic surfactant system of C 1 2 E 3 - s o -

dium dodecyl sulphate (SDS) in 0.1 M NaC! and have contrasted the findings with the predictions of regular solution theory. The measurements have been made over a wide concentration range from the CMC to 300 times the CMC using the position sensitive detector, and have been evaluated using Eq. (5). The neutron reflection results (shown in Fig. 5) are compared with those obtained from the application of regular solution theory [18] to sur- face tension data for the same system, and demon- strates that the concentration dependence of the adsorption for this binary mixture is consistent with regular solution theory (for an interaction parameter, fl, of - 2.42 and which is typical of the values found in such mixtures [20]). Furthermore, the results establish that neutron reflectivity can be reliably used to determine adsorbed layer amounts when the adsorbed layer is in equilibrium with a concentrated dispersed bulk phase (lamellar in this case). At concentrations close to the CMC the agreement was poor due to the presence of dodecanol at the interface from impure SDS. As found in previous neutron reflection studies [14]

I E 0

-6 E

0

'o

o 0-"

0

I I I I

I I I I

0.2 0.4 0.6 0.8

CleTAB mole froction

Fig. 6. C16TAB surface excess for C16TAB ClzE6-0.1 M NaBr mixture f o r ( © ) 2 × 1 0 5 M a n d ( e ) 3×10 4M.

at concentrations above the CMC ( > 2 times the CMC) the dodecanol is removedfrom the surface and solubilised into mixed micelles.

As part of a wider study on the structure of mixed surfactant monolayers, we have used specu- lar neutron reflection to determine the adsorbed amounts of the mixed cationic-nonionic surfac- tants of Ct6TAB and C l z E 6 in 0.1 M NaBr [21] at concentrations above and below the CMC of the mixture. Figure 6 shows the surface excess for Cx6TAB for different C16TAB mole fractions at two surfactant concentrations, 2 x 10-5 and 3 x 10-4 M, above and below the CMC of the mix- ture. Above the CMC, when the monolayer is in equilibrium with the micellar phase, ideal mixing is observed, whereas below the CMC a marked de- parture from ideality is seen. The nonideal mixing is seen for both the C16TAB and C 1 2 E 6. The surface excesses obtained from surface tension data are in good agreement with the neutron reflection data. The preferential adsorption of C16TAB at low C16TAB mole fractions corresponds to a regime where the repulsive interactions between the cationic headgroups will be less dominant. The presence of a bulk dispersed phase clearly alters the surfactant mixing at the interface.

In the final example we demonstrate the ability to investigate complex multi-component systems, where in a recent study Staples et al. [22] have investigated the adsorption from mixtures contain- ing various monodisperse alkyl ethoxylate nonionic

J. Penfold et al./ Physica B 198 (1994) 110-115 115

SDS C12Q1"1 C12E3 C12E5 C12E8'12

Fig. 7. Surface and solution concentrations for the mixture SDS-dodecanol-C12E3 C12Es-C12Es-C12E12 at a surfactant concentration of 3 × l0 -3 M and in 0.1 M NaCl.

surfactants together with SDS and dodecanol. The aim of this work was to clarify the role of dodecanol and to demonstrate how the partitioning effects ob- served give rise to changes in the surface chemistry of a "mixed" system relative to that of a "pure" system. The measurements on the complex sixfold mixture of dodecanol, C12Ea, C12Es, C12E8, C12Elz and SDS, here the distribution of ethoxylates and dodecanol mimic a typical commercial ethoxylate, such as Synperonic A5, demonstrate the capabilities of neutron reflection in measuring the individual adsorption of the various components in a highly complex mixture. The data (see Fig. 7) show that the SDS is almost eliminated from the interface.

The most important feature is that there is a clear preferential partitioning to the interface of the short EO length components. The total adsorption of the complex mixture is about 40% higher than an "equivalent" mixture containing only SDS and CtaE5, largely due to the preferential adsorption of the dodecanol. This preferential adsorption of dodecanol at the interface has reduced the average ethoxylate chain length at the interface to 3.6 com- pared to the bulk average of 5.0. The concentration dependence of similar nonionic -atomic mixtures has been investigated and can only be explained by highly nonideal mixing [22] in both the monolayer and micelles. In particular, the surface concentra- tion of dodecanol increases with increasing surfac- tant concentration, contrary to expectation.

However, it has been argued [22] that the surface concentrations of the most surface active com- ponents can increase or decrease with increasing surfactant concentration depending on the com- position of the solution relative to the composition which results in the CMC minimum for the mixture. Furthermore, it has been pointed out that this cannot be predicted quantitatively from existing theories.

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