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The study of nonionic surfactant micelles by small angle neutron scattering
PHYSICA II
Physica B 180 & 181 (1992) 537-538 North-Holland
The study of nonionic surfactant neutron scattering
micelles by small angle
J. Penfold”, E. Staplesb and P.G. Cumminsb “ISISScience Division, Rutherford Appleton Laboratory, Chilton, ‘Unilever
Research,
Port Sunlight Laboratory,
Didcot, Oxon OX11 OQX, UK Quarry Road East, Bebington. Wirral, UK
Small angle neutron scattering (SANS) has been used to study the structure of nonionic surfactants in a variety of different circumstances. The nature of the adsorption of the alkyl polyoxyethylene oxide nonionic surfactants C,,E, on ludox silica sols is described. The evolution of miceller geometry of the nonionic micelles of C,,E, and mixed cationic nonionic micelles is discussed. and the nature of nonionic micelles in less polar solvents is described.
1. Surfactant
adsorption
In recent papers [I, 21 we have described the use of SANS to characterise the nature of the adsorbed layers of the nonionic surfactant C,,E, on ludox silica ~01s. In the condition where the silica particle is index matched to the solvent; the resultant SANS arises only for the adsorbed layer, and provides a sensitive measure of the nature of the adsorbed layer. The SANS results were interpreted on a layer of means thickness -40 A, with a uniform density which depends upon the fractional coverage arising from small islands of bilayers (see fig. 1). At sufficiently low values of pH the adsorbed layer remains intact as the lower consolute boundary of the system is approached and the accompanying onset of attractive interparticle interaction between the coated silica sol particles have been characterised using modern liquid state theories
PI. Recent
results
on C,,E,
[3] suggest
2cQr
that
a small
I
O.ll 0
0 02
OOL
006
008
Fig. 1. Scattered intensity, 1(Q) (in cm-‘), for 8% ludox TM silica so1/4% C,,E,/H,O (0.39)/Dz0 (0.61) at 27°C and a pH = 9.2. The solid line is a model fit for a mean layer thickness of (45 ? 5) 8, and a scattering length density corresponding to an 80% coverage of surfactant. 0921.4526/92/$05.00
0
1992 - Elsevier
Science
Publishers
change in EO length has a significant effect on the temperature and pH dependence of the adsorption of the nonionic surfactants. The more vigourous arrangement of the C,,E, surfactant compared to C,,E, on the surface of the silica sol with increasing temperature, and the enhanced adsorbed layer thickness are consistent with a smaller fractional number of EO’s being in contact with the surface. 2. Shear flow alignment micelles
of nonionic surfactant
SANS in combination with shear flow alignment has been used extensively by us to study a wide range of dilute anisotropically shaped micelles [4, 51, and especially nonionic [6] and mixed nonionic-cationic [7] surfactant micelles. In the nonionic C,,E, we have observed the evolution of micelle size with temperature: with increasing temperature the rod length initially increases and eventually decreases as the lower consolute boundary is approached. The increase in rod length is attributed to the dehydration of the EO; whereas the subsequent decrease and accompanying increase in rod flexibility are associated with modifications to the EO-EO interaction (see fig. 2). A common feature of these shear flow studies is that at low shear the degree of anisotropy is greater than expected, implying an increase in the effective rod length. This is attributed to hindered rotation [5]; as although in terms of interactions these systems are dilute, they are in semi dilute regimes and in isotropic solution entanglements will hinder rotational diffusion. As alignment is imposed the hindering is reduced, and the expected shear and concentration dependence is observed (see fig. 3). At low concentrations the concentration and temperature dependence of the anisotropy changes markedly and we postulate that this is associated with a critical concentration for rod formation.
B.V. All rights
reserved
J. Penfold et al.
53x
I Nonionic surfactan: micelles by SANS
Wovevector transfer.
Q i k’,
Fig. 4. Scattering cross-section (in cm-‘) for 5% C,?E,/ D,Oiglycerol at 30 “C for (+) h-glycerol, (0) d-glycerol and A 20% glycerol. n 40% glycerol. solvents. where as a complement to the earlier nonionic studies the emphasis has been on the modification of the hydrocarbon-solvent interface. The initial results for the scattering from C,2E,/wateri glycerol mixtures have been interpreted in terms of a polydisperse core plus shell model of the micelle (81. The addition of glycerol causes an increase in the micelle aggregation number and decreases in the EO hydration (see fig. 4); which is probably associated with the closer proximity of the lower consolute boundary. as the same trends are seen at fixed glycerol content and increasing temperature where the intermicellar interactions become attractive.
polar (Tc -T)
(‘C)
Fig. 2. Effective rod length as a function perature (T, - T) for 1% C,,E,/D20/0.5 shear gradient. G. of 5000s ‘.
of reduced M NaScN
temat a
References
Fig. 3. Effective for C,,E,/D,O 0.001 M. 3.
Surfactants We have
on
the
rod length as a function of shear gradient for (.O) 0.02M (+) 0.002M and (0)
in less polar solvents
recently
nature
of
obtained nonionic
some surfactant
preliminary micelles
results in less
[1] P.G. Cummins, E. Staples and .I. Penfold. Chem. 94 (1990) 3740. [2] P.G. Cummins. E. Staples and J. Penfold, Chem. (1991) in press. [3] P.G. Cummins, E. Staples and J. Penfold, Chem. (1991) submitted. [4] J. Penfold, J. Appl. Cryst. 21 (1988) 770. [5] J. Penfold, E. Staples and P.G. Cummins. Adv. Sci. 34 (1991) 451. [6] J. Penfold, E. Staples, P.G. Cummins and R.K. Langmuir 5 (1988) 1195. (71 J. Penfold, E. Staples and P.G. Cummins. (1991) submitted. [8] J. Penfoid et al.. unpublished result.
J.
Phys.
J.
Phys.
J.
Phys.
Coil. Int. Heenan. Langmuir
Physica B 180 & 181 (1992) 537-538 North-Holland
The study of nonionic surfactant neutron scattering
micelles by small angle
J. Penfold”, E. Staplesb and P.G. Cumminsb “ISISScience Division, Rutherford Appleton Laboratory, Chilton, ‘Unilever
Research,
Port Sunlight Laboratory,
Didcot, Oxon OX11 OQX, UK Quarry Road East, Bebington. Wirral, UK
Small angle neutron scattering (SANS) has been used to study the structure of nonionic surfactants in a variety of different circumstances. The nature of the adsorption of the alkyl polyoxyethylene oxide nonionic surfactants C,,E, on ludox silica sols is described. The evolution of miceller geometry of the nonionic micelles of C,,E, and mixed cationic nonionic micelles is discussed. and the nature of nonionic micelles in less polar solvents is described.
1. Surfactant
adsorption
In recent papers [I, 21 we have described the use of SANS to characterise the nature of the adsorbed layers of the nonionic surfactant C,,E, on ludox silica ~01s. In the condition where the silica particle is index matched to the solvent; the resultant SANS arises only for the adsorbed layer, and provides a sensitive measure of the nature of the adsorbed layer. The SANS results were interpreted on a layer of means thickness -40 A, with a uniform density which depends upon the fractional coverage arising from small islands of bilayers (see fig. 1). At sufficiently low values of pH the adsorbed layer remains intact as the lower consolute boundary of the system is approached and the accompanying onset of attractive interparticle interaction between the coated silica sol particles have been characterised using modern liquid state theories
PI. Recent
results
on C,,E,
[3] suggest
2cQr
that
a small
I
O.ll 0
0 02
OOL
006
008
Fig. 1. Scattered intensity, 1(Q) (in cm-‘), for 8% ludox TM silica so1/4% C,,E,/H,O (0.39)/Dz0 (0.61) at 27°C and a pH = 9.2. The solid line is a model fit for a mean layer thickness of (45 ? 5) 8, and a scattering length density corresponding to an 80% coverage of surfactant. 0921.4526/92/$05.00
0
1992 - Elsevier
Science
Publishers
change in EO length has a significant effect on the temperature and pH dependence of the adsorption of the nonionic surfactants. The more vigourous arrangement of the C,,E, surfactant compared to C,,E, on the surface of the silica sol with increasing temperature, and the enhanced adsorbed layer thickness are consistent with a smaller fractional number of EO’s being in contact with the surface. 2. Shear flow alignment micelles
of nonionic surfactant
SANS in combination with shear flow alignment has been used extensively by us to study a wide range of dilute anisotropically shaped micelles [4, 51, and especially nonionic [6] and mixed nonionic-cationic [7] surfactant micelles. In the nonionic C,,E, we have observed the evolution of micelle size with temperature: with increasing temperature the rod length initially increases and eventually decreases as the lower consolute boundary is approached. The increase in rod length is attributed to the dehydration of the EO; whereas the subsequent decrease and accompanying increase in rod flexibility are associated with modifications to the EO-EO interaction (see fig. 2). A common feature of these shear flow studies is that at low shear the degree of anisotropy is greater than expected, implying an increase in the effective rod length. This is attributed to hindered rotation [5]; as although in terms of interactions these systems are dilute, they are in semi dilute regimes and in isotropic solution entanglements will hinder rotational diffusion. As alignment is imposed the hindering is reduced, and the expected shear and concentration dependence is observed (see fig. 3). At low concentrations the concentration and temperature dependence of the anisotropy changes markedly and we postulate that this is associated with a critical concentration for rod formation.
B.V. All rights
reserved
J. Penfold et al.
53x
I Nonionic surfactan: micelles by SANS
Wovevector transfer.
Q i k’,
Fig. 4. Scattering cross-section (in cm-‘) for 5% C,?E,/ D,Oiglycerol at 30 “C for (+) h-glycerol, (0) d-glycerol and A 20% glycerol. n 40% glycerol. solvents. where as a complement to the earlier nonionic studies the emphasis has been on the modification of the hydrocarbon-solvent interface. The initial results for the scattering from C,2E,/wateri glycerol mixtures have been interpreted in terms of a polydisperse core plus shell model of the micelle (81. The addition of glycerol causes an increase in the micelle aggregation number and decreases in the EO hydration (see fig. 4); which is probably associated with the closer proximity of the lower consolute boundary. as the same trends are seen at fixed glycerol content and increasing temperature where the intermicellar interactions become attractive.
polar (Tc -T)
(‘C)
Fig. 2. Effective rod length as a function perature (T, - T) for 1% C,,E,/D20/0.5 shear gradient. G. of 5000s ‘.
of reduced M NaScN
temat a
References
Fig. 3. Effective for C,,E,/D,O 0.001 M. 3.
Surfactants We have
on
the
rod length as a function of shear gradient for (.O) 0.02M (+) 0.002M and (0)
in less polar solvents
recently
nature
of
obtained nonionic
some surfactant
preliminary micelles
results in less
[1] P.G. Cummins, E. Staples and .I. Penfold. Chem. 94 (1990) 3740. [2] P.G. Cummins. E. Staples and J. Penfold, Chem. (1991) in press. [3] P.G. Cummins, E. Staples and J. Penfold, Chem. (1991) submitted. [4] J. Penfold, J. Appl. Cryst. 21 (1988) 770. [5] J. Penfold, E. Staples and P.G. Cummins. Adv. Sci. 34 (1991) 451. [6] J. Penfold, E. Staples, P.G. Cummins and R.K. Langmuir 5 (1988) 1195. (71 J. Penfold, E. Staples and P.G. Cummins. (1991) submitted. [8] J. Penfoid et al.. unpublished result.
J.
Phys.
J.
Phys.
J.
Phys.
Coil. Int. Heenan. Langmuir