C12E12 at the Air-Water Interface
J. Phys. Chem. B, Vol. 101, No. 49, 1997 10339
TABLE 4: Structural Parameters for the C12En Series at
Their cmcs
overlap of the two chains was leading to greater mixing between
water and alkyl chain. However, this involves an assumption
that there is constant hydration throughout the ethoxylate chain,
which is not necessarily the case. It should be remembered
that the C12En compounds are more soluble in hydrocarbons
than in water, and in this case the ethoxylate group is not
hydrated at all. The distribution of the headgroups for C12E12
agrees well with that predicted by Sarmoria et al., suggesting a
tethered polymer-like behavior for the E12 group. Indeed, even
beyond the limit of the predictions of Sarmoria et al., the whole
C12En (n e 8) series shows a reasonable agreement between
calculated and observed headgroup distributions, although it
should be borne in mind that the calculations assumed that the
widths of the distributions were independent of the surface
coverage, an assumption which is reasonable for polymeric
surfactants but not for C12En.
parameters
C12E3
C12E6
C12E8
C12E12
A/Å2
σc/Å
σh/Å
δch/Å
36 ( 2
55 ( 3
62 ( 3
72 ( 3
16.5 ( 2
15.5 ( 2
8.0 ( 1
10.0 ( 1
6.0 ( 1
16.0 ( 2
16.5 ( 2
9.0 ( 1
10.0 ( 1
8.0 ( 1
15.0 ( 2
19.0 ( 3
10.5 ( 1
11 ( 1
15.0 ( 3
21.5 ( 4
12.0 ( 2
13.0 ( 2
12.0 ( 2
δ
cw/Å
êw/Å
9.0 ( 1
the dodecyl chain on the surface normal is estimated to be 12
Å. Since the fully extended length of C12 is 16.7 Å this implies
that the chains are tilted at angles such that cos θ ) 0.72. If
all the chains are assumed to be at the same angle this would
correspond to a tilt of 45° away from the surface normal.
It is interesting to determine the distributions of the ethoxylate
group as a function of the number of ethoxylate units and to
examine to what extent the in situ headgroup distributions
behave like tethered polymer chains. An obvious difference
between the adsorbed monolayer at the air-water interface and
the true tethered chain at the solid-water interface is the effect
of roughness which tends to broaden the width of the monolayer
at the air-water interface. Contributions from roughness from
different sources will act to form a finite thickness for the wall
at the air-water interface, which must be taken into account
before comparison with other results. Sarmoria et al.9 have done
simulations on anchored ethoxy groups using the rotational
isomeric state model, and these calculations suggest that a
polymer-like square root dependence of the extension of the
EO units has set in by about n ) 10. Sarmoria et al.9 predict
an ethylene oxide layer thickness of 18 Å for the grafted E12,
which compares reasonably with our value of 19 Å at the cmc
after the removal of the contribution of the capillary wave
roughness. In their simulation, Sarmoria et al. assumed that
there was no significant effect of surface coverage on the
distribution of the anchored polymer. Our results show that
this is a reasonable assumption for E12, but as can be seen from
Figure 3, this is not so for the short En. As n decreases, surface
coverage is increasingly concentration dependent and so is the
layer thickness. Apart from the value of 21 Å at the cmc, the
thicknesses at the three lower concentrations are all between
17 and 18 Å. This suggests that the distribution of the
ethoxylate head does conform to the prediction of terminally
anchored polymer chain behavior. However, it should be
emphasized that the accuracy of our current results for the E12
fragment would be much improved by performing neutron
reflection experiments with head deuterated surfactant.
Acknowledgment. We thank the Engineering and Physical
Science Research Council for support.
References and Notes
(1) Israelachivili, J.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc.,
Faraday Trans. 2 1976, 72, 1525.
(2) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. J. Chem.
Soc., Faraday Trans. 1990, 86, 3111.
(3) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Thompson, L.;
Tucker, I.; Penfold, J. J. Phys. Chem. 1994, 98, 6559.
(4) Lu, J. R.; Li, Z. X.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir
1993, 9, 2408.
(5) Lu, J. R.; Hromadova, M.; Simister, E.; Thomas, R. K.; Penfold,
J. J. Phys. Chem. 1994, 98, 11519.
(6) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.;
Penfold, J. J. Phys. Chem. 1993, 97, 8012.
(7) Lu, J. R.; Li, Z. X.; Smallwood, J.; Thomas, R. K.; Penfold, J. J.
Phys. Chem. 1995, 99, 8233.
(8) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Penfold, J. J. Chem. Soc.,
Faraday Trans. 1996, 92, 403.
(9) Sarmoria, C.; Blankschtein, D. J. Phys. Chem. 1992, 96, 1978.
(10) Teo, H. H.; Swales, T. G. E.; Domszy, F.; Heatley, F.; Booth, C.
Makromol. Chem. 1983, 184, 861.
(11) Bucknall, D. G.; Penfold, J.; Webster, J. R. P.; Zarbakhsh, A.;
Richardson, R. M.; Rennie, A. R.; Higgins, J. S.; Jones, R.; Fletcher, P.
D.; Thomas, R. K.; Roser, S. J., Dickinson, E. Proceedings of International
Conference on Advanced Neutron Sources XIII and ESS-PM4 PSI
Proceedings 95-02; 1995, 1, 123.
(12) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem.
1989, 93, 381.
(13) Born, M.; Wolf, E. Principles of Optics; Pergamon: Oxford, 1970.
(14) Lekner, J. Theory of Reflection; Nijhoff: Dordrecht, 1987.
(15) Crowley, T. L. Physica 1993, A195, 354.
(16) Lu, J. R.; Lee, E. M.; Thomas, R. K. Acta Crystallogr. 1996, A52,
11.
(17) Lange, H. Kolloid Z. 1965, 201, 131.
(18) An, S. W.; Lu, J. R.; Thomas, R. K.; Penfold, J. Langmuir 1996,
12, 2446.
(19) Glass, J. E. J. Phys. Chem. 1968, 72, 4459.
(20) Couper, A., Eley, D. D. J. Polym. Sci. 1948, 3, 345.
(21) Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J.; Richards, R. W.
Polymer 1996, 37, 109.
(22) Henderson, J. A.; Richards, R. W.; Penfold, J.; Thomas, R. K.;
Lu, J. R. Macromolecules 1993, 26, 4591.
(23) Tanford, C. J. J. Phys. Chem. 1972, 76, 3020.
(24) Takahashi, Y.; Sumita, I.; Tadokoro, H. J. Polym. Sci. 1973, 11,
2113.
Conclusions
The extent of overlap between the alkyl chain and the
headgroup increases with the size of the ethoxylate group, an
observation clearly supported by the values of the parameters
listed in Table 4. While the thickness of the dodecyl chain
changes very little, the thickness of the headgroup increases
substantially with size of the ethoxylate. In parallel, the distance
between the centers of the chain and ethoxylate distributions
also increases, but not as rapidly as it should if the overlap
between chain and head were constant. Conventional wisdom
would say that each ethoxylate group is always hydrated with
two water molecules, and this would then suggest that increased
(25) Sears, V. F. Neutron News 1992, 3, 26.
(26) Cooke, D. J.; Lu, J. R.; Lee, E. M.; Thomas, R. K., Pitt, A. R.;
Simister, E. A.; Penfold, J. J. Phys. Chem. 1996, 100, 10298.
(27) Schwartz, D. K.; Schlossman, M. L.; Kawamoto, E. H.; Kellogg,
G. J.; Pershan, P. S.; Ocko, B. M. Phys. ReV. 1990, A41, 5687.
(28) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys.:
Condens. Matter 1994, 6, A403.