Chapter 21
Structure of Surfactant Monolayers at the Air—Water Interface Determined by Neutron Reflection
Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0615.ch021
J. R. Lu and R. K. Thomas Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, United Kingdom
The determination of the structure of a soluble surfactant monolayer, hexadecyl trimethyl ammonium bromide (C TAB), using isotopic substitution in conjunction with neutron reflection is described. The roughness of the layer has been determined as has the separation of the various parts of the molecule in the normal direction. The molecule is strongly tilted with respect to the surface normal and the outer part of the molecule is more tilted than the inner part. The roughness is comparable with the projection of the molecule on the surface normal direction. In different C TABs with n = 12 to 18 the thickness of the chain region of the monolayer is found to be almost independent of chainlength at 16 - 17 Å. In monolayers of C E , where E is an ethylene glycol chain of m units, the thickness of the ethylene glycol chain region varies from about 8 Å for m = 2 to 19 Å when m = 8. 16
n
12
m
m
Neutron specular reflection is a recently developed technique for exploring composition and density profiles normal to the surface for a variety of different types of interface (e.g. (i)). The intrinsic spatial resolution of the experiment is not very g;ood but the systematic use of isotopes to label different parts of the layer can increase the resolution so that structure can be probed at the A level (2). There is no restriction to any particular type of interface, the only condition for the feasibility of the experiment being that one of the bulk phases on either side of the interface is transparent to neutrons. In this paper we explore the structure of equilibrium monolayers of surfactants adsorbed at the air/water interface. The main characteristic of an equilibrium monolayer that distinguishes it from an artificially spread monolayer is that the limiting area per molecule is usually much larger than the cross sectional area of the hydrophobic chain part of the surfactant. Thus, for a typical limiting coverage of a soluble surfactant at the air/water interface the area per molecule, A, is about 45 A , whereas A would be 20 - 25 A for a typical spread monolayer. The extra space available in the 2
2
0097-6156/95A)615-0342$12.00/0 © 1995 American Chemical Society In Surfactant Adsorption and Surface Solubilization; Sharma, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
21. LU ET AL.
Structure of Surfactant Monolayers
343
equilibrium monolayer is expected to lead to tilting of the molecule with respect to the surface normal and possibly to a significant incidence of chain defects, orientational disorder of the chains, and disorder in the direction normal to the surface, i.e. roughness (5). At the air/water interface there will be a further contribution to disorder from thermal excitations (capillary waves). Until now, no experiment has been able to characterize all these features for a surfactant monolayer. The first part of this paper describes the extent to which neutron reflection and isotopic labelling can yield this information. In the second part of the paper we compare the structural parameters obtained by neutron reflection for two types of chain in equilibrium monolayers of surfactants at the air/water interface, hydrocarbon chains in the series of alkyl trimethyl ammonium bromides (the C TABs), and ethylene oxide chains in the series of monododecyl ethers of ethylene glycols (C^EQ,). Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0615.ch021
n
Neutron Reflection from C^TAB There are two key structural quantities that can be determined, the width (in the normal direction) of the average distribution of a given fragment i of the surfactant, which we will denote by oj, and the separation between the centres of the distributions of any pair of fragments in the layer, which we will denote 6jj where i and j identify the two fragments. The way the width may be determined is illustrated by the following example for C TAB (3). We suppose that we wish to determine the width of the distribution of the outer butyl group of the hexadecyl chain. This is achieved approximately by measuring the reflectivity of C D C 2H24N(CH )3Br (dC hC hTAB for short) in a 9:1 mixture of H 0 and D 0 . Because protons and deuterons scatter neutrons with opposite phases, an H 0/D 0 mixture of this proportion has a neutron refractive index the same as air and therefore does not reflect neutrons. We refer to this composition of water as null reflecting water (n.r.w.). The head group is also null reflecting because the contributions of the N, C, and Br nuclei cancel out the contribution from the protons, which is of opposite sign, and a similar effect approximately operates for the C ^ I ^ group. The latter can be exactly matched to air by the inclusion of a small amount of C ^ D ^ to make dC 0"C hTAB and this has been done for all the results described below. The reflected signal from dC hC hTAB in n.r.w. is then only from the labelled butyl group. The reflectivity can be written approximately (4) 16
4
2
9
1
3
4
12
2
2
2
,,
4
12
4
12
R(K) = ^-%h
(1)
[f
where K is the momentum transfer defined in terms of the grazing angle of incidence and the wavelength of the neutron (K = 4jisin8/X,), bf is the empirical scattering length of the fragment (known independently), and hff is the partial structure factor of the fragment, given by A = K(K)I
2
ff
(2)
where Wf(ic) is the Fourier transform of the number density profile of the
In Surfactant Adsorption and Surface Solubilization; Sharma, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
344
SURFACTANT ADSORPTION AND SURFACE SOLUBILIZATION
fragment in the direction normal to the surface. Generally, a Gaussian distribution is found to be a good representation of the surfactantfragments,in which case, /iff becomes (3) where A is the area per molecule at the surface and Of is the full width at 1/e of the maximum of the distribution. We have not justified the assumption of a Gaussian distribution because, at this stage, the exact nature of the distribution is not important for the arguments that follow. When neutron reflection is done on C TAB, on either a labelled C group at any position in the chain, or on the labelled head group, the resulting /iff is the same within error, that is, the head group or any chosen C fragmentdistributions all have exactly the same width. The average /iff for these groups is shown as points in Figure 1. The continuous line is the best fit of equation 3 with o = 14 ± 2 A. This dimension is much greater than the fully extended length of the fragments (7 - 1 0 A) and indicates that much of the width is generated by roughness. We can determine the roughness directly as follows. We increase the length of the fragment visible to neutrons, i.e. the length of the deuterated fragment, by studying the series of compounds C H _ 2 C D dTAB as a function of n, in null reflecting water. Since the neutron reflection experiment then only observes the labelled fragment the measured width in each case is the width of the labelled part of the molecule only, not of the whole molecule. The width depends on the relative contributions of the roughness and the average intrinsic projection of the labelled fragment along the surface normal. The latter will dominate when it becomes significantly larger than the roughness, i.e. as the length of the labelled fragment increases. In Figure 2 we plot o for this series of compounds against n . The reason for plotting the square of the width is that the convolution of two Gaussians to give a composite width gives the relation
Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0615.ch021
16
4
4
16H1
n
33
n
2n
2
2
a
2
= (/i/) + w 2
(4)
2
where hi is the projection of the fragment on the surface normal and w the roughness. For the shorterfragmentsthis gives a linear extrapolation to n = 0, where the value of o is the square of the roughness. Because there must also be some contribution to the length from the head group we actually use (/i+l) rather than n , but this only has a slight effect on the extrapolated value of w. For C TAB w is found to be 14 ± 1 A. Values in the range 10 - 14 A have been deduced for other surfactants in a less direct way (5-$). This is, however, the first direct measurement. The slope of the plot of Figure 2 is determined by the increase in the length of the normal projection of the chain as each CD group is added. That a straight line is obtained up to n ~ 10 indicates that the chain remains fairly straight, although the value of the gradient indicates that it must be tilted away from the surface normal. At higher n the plot flattens off indicating that the end part of the chain is on average tilted more strongly away from the surface normal than the lower part. However, Figure 2 is not a sensitive means of determining the chain orientation or any disorder within the chain. This is better done by determining the separation between two fragments of the chain. 2
2
2
16
2
In Surfactant Adsorption and Surface Solubilization; Sharma, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
21. LU ET AL.
345
Structure of Surfactant Monolayers
Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0615.ch021
10
K/A Figure 1. Partial structure factor for the "smallest" unit of C, TAB averaged over measurements on "0"C dTAB, dC "0"C hTAB, and "0^C dC hTAB. The continuous line is calculated using equation 3 with o = 14 ± 1 A and A = 45 ± 2 A 2 6
16
4
12
12
4
(N+1)
2
Figure 2. Plot of the square of the width against the square of the number of labelled carbon atoms in the series C _ H 3_ C D2ndTAB. The straight line is the best linear fit to this region of the curve (see text). 16
n
3
2n
n
In Surfactant Adsorption and Surface Solubilization; Sharma, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
346
SURFACTANT ADSORPTION AND SURFACE SOLUBILIZATION
If we study the reflectivity of a C TAB species labelled with deuterium in such a way that there is one dC fragment and the head group is deuterated, e.g. dC hC dTAB, in n.r.w. the reflectivity is determined by three partial structure factors: 16
4
4
l2
m
= Tjz (6Si*o«e«
+
V>*W«h>
*8*hh +
(5)
where and denote the self partial structure factors of the dC fragment and the head group, and denotes the cross partial structure factor between C and the head. The two self partial structure factors can be determined directly from measurement of the appropriate reflectivity using labelled compounds where either the dC group or the head group is the only deuterated fragment and everything else is null reflecting. Application of equation 1 then gives the self partial structure factor directly. If a third reflectivity measurement is made, where both dC and head group are deuterated, then application of equation 5, with the known two self partial structure factors, gives the cross partial structure factor /i^h . The special feature about this partial structure factor is that it can be written, to a very good approximation, as 4
4
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4
4
*
