J . Phys. Chem. 1994,98, 5935-5942
5935
Specular and Off-Specular Neutron Reflectivity of a Low Molecular Weight Polystyrene Surfactant at the Air-Water Interface P. M. Saville, I. R. Gentle, and J. W. White' The Australian National University, Canberra 0200, Australia
J. Penfold and J. R. P. Webster The Rutherford Appleton Laboratory, ISIS Facility, Chilton, Didcot. Oxon OX11 OQX,U.K. Received: February 8, 1994"
Neutron reflectivity measurements were made on films at the air-water interface of a highly characterized low molecular weight polystyrene surfactant, with a single hydroxyl end group. The II-A isotherm is consistent with collapse of a classical fluid surfactant monolayer to a trilayer structure a t high compressions. Before the collapse point, reflectivity measurements indicated a decrease in the average tilt angle of the molecules to the surface normal as the layer was compressed. After the collapse point, the film structure was time dependent and was best represented by a monolayer of constant thickness a t the water interface, covered by a bilayer whose thickness increased with time and a time-dependent diffuse layer above the bilayer. Multidetector reflectivity profiles showed only specular reflection for surface pressures below the collapse point, but in the collapse region, some off-specular reflection was clearly present, indicating the presence of surface texture on the micron length scale in the plane of the interface. This is believed to be the first observation of well-developed off-specular neutron reflection from a Langmuir film.
Introduction The structure of insoluble surfactant phases at the air-water interface has been studied by many techniques. From II-A isotherm measurements with a Langmuir trough,' bends and kinks in the isotherm are indicative of new phase formation.' Other techniques, such as surface X-ray diffraction using synchrotron radiatioq2-5 neutronG9 and X-ray5JoJ1 reflectivity, ellipsometry,'*J3 and second harmonic g e n e r a t i ~ n , ' ~give . ' ~ information about molecular packing in the monolayer as well as thickness and orientational data. Other optical techniques, such as fluorescence16-17and Brewster angle microscopy,18provide detailed pictorial evidence of the shape and growth of new surface phases as monolayers are compressed, while computationall9~20and k i n e t i ~ 2 ' -methods ~~ help to elucidate the mechanisms involved in phase changes. The effects on surfactant behavior on increasing the hydrophobic chain length and hence the magnitude of interchain forces while the head group-substrate interaction is kept constant are of interest and have been studied for fatty acids.25 Through the use of highly monodisperse polymer surfactants, such as those produced by anionic polymerization,26 extensive variations of both the hydrophobic chain length and conformation are possible. For such polymeric systems little research has been done,27v28though there has been much study of polymers with hydrophilic groups as part of the repeat ~nit2~,29 and of the scaling properties30 of these molecular filmsS3' In the present paper, the change in structure of atactic polystyrene, terminated with a single hydroxyl end group (PSOH), films at the air-water interface is studied using neutron reflectivity. This system has an isotherm which is consistent with collapse to a trilayer structure above a critical pressure.32-34 The behavior of the film is studied through the collapse region as well as in the so called "liquid expanded" region of the isotherm. Specular reflection of radiation from the air-film-water interface leads to information about the film thickness and densities in the direction normal to the interface.6 The presence To whom correspondence should be addressed.
* Abstract published in Advance ACS Absfracfs, May 15, 1994. 0022-365419412098-5935$04.50/0
of an in-plane structure at the interface, arising from density fluctuations, in either a regular or irregular pattern, will produce an off-specular component to the scattered radiation in an analogous manner to small angle scattering.3s3s Theoretical descriptions of the off-specular scattering relating to parameters such as size, height, and domain-domain correlations of the surface features are being developed35-38 and promise interesting structural information from these measurements. Both the specular and off-specular reflectivities from a polystyrene surfactant system are presented here, though an in-depth analysis of the off-specular reflectivity will be given later. Experimental Section Polystyrene surfactants, PS-OH, were synthesized from styrene-ds (98+ atom % deuterated) by anionic techniques26 and found by gel permeation chromatography, GPC, to be highly monodisperse (Mw/Mn < 1.1). The conditions produced the atactic polymer, and the "living" chains were terminated by the addition of ethylene oxide to form an alcohol end group. Molecular weights were determined by GPC and the polymer purity and conformation checked with NMR. The molecular weight of the polymer used was 973 g mol-' corresponding to ca. 8 monomer units. This equates to a length of 23.5 A if all the molecular weight is represented by repeat units. We consider that the terminal CH2CH20H group is shorter than the last phenyl group and so does not contribute to the size exclusion. This is borneout by GPC separation of low molecular weight polystyrene surfactant (with repeat units of the order of 1-3 clearly resolved) having retention times identical to those found for PS-H molecular weight standards. The initiator, an n-butyl group, is about 7.3 A long and will contribute as it does in the polymer standards used in calibration. Thus it is reasonable to take 23.5 A as the correct deuterated chain length when fully extended. The N M R spectra show peaks arising from the protons of both the n-butyl group of the initiator and the terminating group corresponding to a molecular formula CH~CH~CH~CH~-(CD~-CDC~DS-),CH2CHzOH. For the surface balance work, great care was taken to ensure the purity of all solvents used, and hence HPLC grade toluene 0 1994 American Chemical Society
5936
The Journal of Physical Chemisfry. Vol. 98, No. 23, 1994
wasdistilled in glass to remove tracesofgrease. Beforeisotherms were run, an excess of the solvent was spread on the surface balance and compressed to ensure that there were negligible impurities in the spreading volumes used with the polymers. The surface balance used for isotherm measurement at the Research School of Chemistry consisted of a small automated Teflon Langmuir trough (maximumsurfacearea -220cm') whichused the rod-in-free-~urface~~ method for surface pressure measurement with a resolution of 0.06 mN m-I. Isotherms were run at compression rates of 0.2 cm' s-I with the trough thermostated to 20.0 0.2 "C. Neutron reflectivity measurements were performed on the CRISP reflectometer6 at the ISIS Facility, Rutherford Appleton Laboratory, U.K. All measurements were made on the fully deuterated polymer spread from dilute toluene solution on subphasesofeither DzOor air contrast matched water (ACMW) to provide contrast variation. A NIMA 601A Langmuir trough (surface area 600 5.n') with Wilhelmy plate surface pressure measurement was used for these measurements. Isotherms obtained on this system were identical to those using the smaller trough. The theory of specular neutron reflectivity has been described previously.69 In this work standard modeling procedures were used to fit specular reflectivity profiles in order to obtain values of thickness and scattering length density. The models were constrained by requiring an adequate fit to the reflectivity from a given film on both DzO and ACMW, and the simplest model which provided a consistent description for each contrast was adopted. More direct methods using the kinematic approximation-' would have required measurements on many more isotopically different species than were possible in this case. Measurements were made initially of the specular reflection only over a wavelength range of 0.5-6.6 A; this gave a range of momentum transfer (in units h ) perpendicular to the air-water interface, Qz,of 0.05-0.65 A-1. Further measurements were made with a onedimensional position sensitive "multidetector". giving neutron reflectivity as a function of both wavelength and reflected angle away from the specular direction. The momentum transferred, Q,in the reflection process can be resolved into its x, y . and z components such that
Q, = ko[ws eo- COS e,
COS
$1
Qy = ko COS 0, sin $ Q, = ko[sin 0,
+ sin e,]
(1)
(2)
(3)
The uncertainties in themomentum-transfer vectors aregiven by the equations AQx Ik, cos eo( 1 - cos A$)
(4)
AQy Iko cos 9, sin A$
(5)
AQ,
2k0 sin Ago
I
(6)
wherethein-planedirectionisin thexyplaneandzis thesurface normal, ko is the propagation number (2s/A). and the angles 80 and 8, are the incident and reflected angles, while the angle $ is the in-plane deflection of the beam from the incident plane (see Figure I). Itcanbeseen from theseequationsthatanyuncertainty in the angle $ due to beam divergence leads to an uncertainty in bothQ,andQ,. In the typical reflectivity experiment, thevertical collimationisgood,leadingtoan errorinQzofonlya few percent. Thebeam, however, is pwrlycollimated in thehorizontaldirection so that a greater flux reaches the single detector. The measured signal is. in effect, the integral over Qv. Typical beam parameters and dimensions of the instrument were as follows: The final slit before the sample was 40 mm (w)
X2.5mm(h),withasamplefwtprint(umbra)of40X 100mm'.
Saville et al.
"
1
Fkgve 1. Ray diagramshowingreflection fromthcintdaa. Forspcular reflection 8, equals 81 and # = 0.
The incident beam (00 = 1 . 5 O ) had a divergence in the vertical direction of A& = 0.07O or about 5% of the incident angle. In the horizontal plane the beam was not as well collimated and the divergence was A$ = 0.9O from the straight-through direction. To intercept the reflected beam,the single or multidetector was situated 1.75 mfrom thesample. Forthesingledetector.40mm (w) X4 mm (h),themeasuredverticaldivergenceofthereflected beamwasAO1=0.1°0rabout7%. Forthemultidetector.40mm X 200 mm, the vertical divergence was A8, I0.06O and A$ = 1.4O. The horizontal divergence for both detectors was A$ = 1.4O. The wavelength resolution, AA/A, was 51% over the A range used, and so the A dependence in Qz comes only from ko. These instrumental parameters gave uncertainties in Q,, Q,. and Q. of the order of AQ, = 0.0003k0,AQy = 0.024k0.and AQ, = 0.0024ko. The neutron background from the instrument without sample was
