J. Phys. Chem. 1992, 96, 1383-1388
1383
Comparison of Neutron Reflection and Surface Tension Measurements of the Surface Excess of Tetradecyltrimethylammonium Bromide Layers at the Air/Water Interface E. A. Simister, R. I(.Thomas,* Physical Chemistry Laboratory. South Parks Road, Oxford, OX1 3QZ UK
J. Penfold, Rutherford-Appleton Laboratory, Chilton, Didcot, Oxon, OX11 ORA UK
R. Aveyard, B. P. Binks, P. Cooper, P. D. I. Fletcher, J. R. Lu, and A. Sokolowskit School of Chemistry, University of Hull, Hull, HU6 7RX UK (Received: July 3, 1991)
Neutron reflection and surface tension (ring and plate methods) have been used to study the adsorption of four deuterated isotopic species of tetradecyltrimethylammoniumbromide (C,,TAB) at the air/water interface at 298 K. The critical micelle concentration (cmc) was found to be (3.7 0.1) X lo-' M for three of the four isotopic species, in good agreement with earlier determinations. The fourth species showed a slight surface tension minimum but was otherwise similar. The close similarity of the surface tension curves indicated that the surface properties of C14TABare independent of deuteration, a result of importance for the interpretation of neutron reflectivity. Neutron reflection has been used to measure the surface concentrations of the fully deuterated and chain-deuterated isotopes of CI4TABfrom 3 X lo4 to 4.5 X lov3M. The area per molecule at the cmc (A) was found to be 46 f 2 AZand continues to decrease above the cmc. The y-ln c plots gave a limiting area per molecule at the cmc, which depended on both the method used to determine y and the method of analysis of the data. The simplest approach using a Pt-Ir ring and a straight line plot gave a value of 60 A2, in agreement with some earlier determinations, but, using a plate and a polynomial fit to the data, the value was found to be in close agreement with the neutron measurement, i.e., 45 f 2 A*. The implications of the difficulty in determiningA from surface tension measurements are discussed in detail.
*
Introduction Neutron reflection is a new method for studying structure at interfa-.' It is particularly effective for the study of the air/water interface. Its application to the study of surfactants at this interface raises the possibility of answering a range of important questions concerning, for example, how the structure of a given surfactant layer might depend on coverage, temperature, ionic strength, and counterion or how it might vary with different types of head group and chain. Since preliminary work on decyltrimethylammonium bromide (CloTAB)2and tetramethylammonium dodecyl sulfate at the air/water interface,' the technique has significantly improved, making it now worthwhile to undertake a systematic study of a given series of surfactants. In this and subsequent papers we present the results of the application of the technique to some surfactants in the C,TAB series. The determination of the structure of a surfactant layer by neutron reflection depends on being able to assume that the structure is invariant between different deuterated isotopic species. It is therefore essential to examine whether or not there is any variation in the surface properties of a surfactant when it is isotopically substituted. This is best done by surface tension measurements. The more initial constraints that can be used in analyzing reflectivity profiles, the better the quality of the structural information. One such constraint is the surface excess, which can be determined independently from surface tension measurements via the Gibbs isotherm. The question then arises whether the accuracy obtainable in such an independent measurement is sufficient for it to be useful. Finally, all surface measurements on surfactant systems are susceptible to errors introduced by trace quantities of impurities. It is important to establish how the neutron technique is affected by such impurities. The purpose of the present paper is to examine these problems in detail for one surfactant, CI4TAB,before the structural analysis of this surfactant and others presented in subsequent papers. On leave from Institute of Organic and Polymer Technology, Technical University of Wroclaw, SO 370 Warsaw, Poland.
Experimental Details Deuterated tetradecyl bromide and deuterated trimethylamine were obtained from Merck, Sharp, & Dohme, the protonated materials from B.D.H. All the solvents used in the preparation were of the highest purity commercially available (all B.D.H. Aristar grade). The method of preparation was that outlined by Voeks and Tartar! Primary and secondary amines were removed from the trimethylamine by reaction with acetic anhydride. Excess trimethylamine was then distilled under vacuum into a mixture of the tetradecyl bromide and anhydrous methanol, and the mixture refluxed with stirring for 24 h under vacuum. The temperature was initially held at 0 OC and then raised to room temperature. Solvent and excess trimethylamine were removed by pumping through a liquid nitrogen trap. Excess tetradecyl bromide was removed from the residue by extraction with anhydrous ether. The material was recrystallized from acetone to which a small amount of anhydrous ethanol was added. This was found to be a very important stage in the purification. The salt was heated to about 60 "C with about 10 times its weight of acetone, and ethanol added very slowly until solution was just complete. The proportion of ethanol required was about 5%. Previous preparations have used ethanol for recry~tallization,~ but the main impurity, tetradecyl bromide, is not very soluble in ethanol or methanol whereas it is completely soluble in acetone. C14TAB, on the other hand, is rather soluble in the alcohol but only weakly soluble in acetone. Even with this improvement in the recrystallization procedure we found it necessary to recrystallize the material at least twice and preferably three times. We discuss this further in the Results. We prepared four isotopic species, the fully protonated, the fully deuterated, the chain deuterated, and the head-group deuterated, by the same method. (1) Penfold, J.; Thomas, R.K. J. Phys. Condens. Marrer 1990, 2, 1369. (2) Lee, E. M.; Thomas, R.K.; Penfold, J.; Ward, R.C. J . Phys. Chem. 1989, 93, 38 1. (3) Penfold, J.; Lee, E. M.; Thomas, R. K. Mol. Phys. 1989, 68, 33. (4) Voeks, J. F.; Tartar, H. V. J . Phys. Chem. 1955,59, 1190. ( 5 ) Venable, R. L.; Nauman, R. V. J . Phys. Chem. 196468, 3498.
0022-3654/92/2096-1383$03.00/0Q 1992 American Chemical Society
1384 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
We also made measurements on two commercial samples of protonated C14TABfrom Sigma and Merck, Sharp & Dohme. The purification procedure was similar to that given above except that an initial extraction of impurity was made with heptane and ether and the final recrystallization was made from an acetone/heptane/ethanol mixture. As for the laboratory prepared material, three recrystallizationswere found to be necessary to obtain constant surface properties. The neutron reflection measurements were made on the reflectometer CRISP at the Rutherford-Appleton Laboratory (Didcot, U.K.). The procedure for making the measurements has been fully described previo~sly.~-~ The surface tension measurements were made on a Kruss K10 or K12 surface tensiometer using a platinum-iridium ring or roughened platinum plate at 298 K. The ring, plate, and all glassware were cleaned in fresh chromic acid and then rinsed with ion-exchanged water (Millipore). The ring or plate was then flamed. The instrument was calibrated with pure water. Using the ring method the surface tension is determined from the maximum force exerted on the ring without detachment of the meniscus. Correction factors6were used to obtain the final values. With such a tensiometer the relative accuracy is extremely high, better than OS%, and the only source of error is the assumption that wetting of the ring or plate is complete.
Neutron Reflection The theory of the neutron reflection method has been described elsewhere,' and the basis of its application to the measurement of surface concentration has also been discussed in outline.' The basis of the reflection experiment is that the variation of specular reflection with momentum transfer K (related to angle by K = 47r sin e/A) is related to the composition profile in the direction normal to the interface. Furthermore, the scattering properties of H and D are such that with water of the appropriate composition, null reflecting water (nrw), and a deuterated surfactant, approximately the only part of the system which contributes to the reflectivity is the surfactant adsorbed at the surface.' At typical subcmc concentrations of surfactant the homogeneous solution makes no detectable contribution to the reflectivity over the range of K normally used. The reflectivity depends then only on the amount of surfactant in the surface layer, regardless of its extent and regardless of any anomalous structuring of the water, because the latter is exactly matched to air. Thus the experiment determines the absolute surface concentration, which is not the same as the Gibbs surface excess. In other cases, where the concentration of solute is much greater or where the contrast matching is not as well defined, it will still be possible to determine the absolute surface concentration, but this may become model dependent and may need to be defined for each case. The normal procedure for determing the surface concentration of a surfactant is to fit the measured reflectivity profile by comparing it with a profile calculated for a simple structural model. Typically, in the determination of the surface concentration, the surfactant will be assumed to form a single layer of homogeneous composition. Possible errors in the use of reflection for the determination of surface concentration are then errors in the measurement of the reflectivity profile, errors in the calculation, and errors arising from the assumption of too simple a model or the wrong model altogether. There are two measurement errors other than the effects of impurities. They are first that other scattering processes such as incoherent scattering give rise to a background which has to be subtracted before the final reflectivity profile is obtained, and second that the intensity at the detector is calibrated by comparison with the reflectivity from D20.Since the background comes almost entirely from the bulk solution, it is easily determined in a separate measurement either on the solvent alone or, better still, on a solution of surfactant where both solvent and surfactant are ( 6 ) Zuidema, H. H.; Waters, G.W. Ind. Eng. Chem. 1941, 13, 312. (7) Crowley, T. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K., Physica B 1991, 8173, 143.
Simister et al. contrast matched to air. It is in any case insignificant (-6 X lod for runs on nrw) in comparison with the more intense parts It introdum no error into of the reflectivity profile (-2 X the measurement of surface concentration, although it puts an important limit on the resolution attainable in the determination of structural parameters. The calibration of CRISP with D20 is necessary only if the reflectivity profile is not measured down to low enough angles to observe total reflection. The procedure can be shown to be valid by measuring the reflectivity over the whole range of momentum transfer. However, experimental errors in the calibration arise because it is difficult to align samples perfectly in the beam. This gives an error in the measured reflectivity of about lo%, which transforms into an error of about 5% in the surface coverage. This is the main error in the use of neutron reflection to determine surface concentration. In passing, we note that the calibration and background subtraction procedures are a source of considerably less uncertainty than in the only other method of determining the absolute surface concentration, that using radiotracers.* When a reflectivity profile has been fitted by a model of a uniform layer the two parameters obtained are the scattering length density of the layer, ps, and its thickness T . The area per molecule at the surface is then
nibi
A=C-
i Ps7
where ni and bi are the number and scattering length of atom i in the molecule. Although there may be a significant range of values of T that will fit a given reflectivity profile, the set of values of p s required to make the fit a good one exactly compensates for the change in T so that A is independent of the uncertainty in the thickness determination. A similar compensation is found when roughness is introduced or when the composition profile is best described by a distribution.2 The only circumstances where the surface coverage could be model dependent is (i) when the adsorbed species contains a mixture of species with positive bi with a significant number of species with negative bi, (ii) when the scattering length density profile normal to the surface has a more complex structure, (iii) when the roughness becomes unusually large, or (iv) when the layer is nonuniform on a microscopic scale in the plane of the surface. (i) and (ii) are not relevant here, and (iv) seems unlikely for a monolayer of a soluble surfactant at such a high concentration, although such inhomogeneities have been observed at low concentrations9). We will show below that (iii) contributes no error unless the surface is unusually rough. The calculation of reflectivity for a given structural model can be done exactly using the optical matrix method,1° and therefore no error is introduced in the calculation. Approximate methods of analysis can be used to show the validity of the arguments of the previous paragraph, that the measurement of surface concentration is model independent for simple systems? However, in this paper we use the matrix method. The main error is then the alignment, or calibration, error, and this will typically be f 2 A2 in the area per molecule at derived values of 50 A2.
Results Reflection. The surface concentration can be determined directly and absolutely using neutron reflection as described in the previous section. Just as with surface tension the results of the experiment can be affected by the presence of surface active impurities. Possible surface active impurities in CI4TABare of two types, resulting either from the solid sample or from contamination of the troughs and glassware. A possible residual impurity from the preparation is unreacted CI4Brand CI4amines, (8) Tajima, K.; Muramatsu, M.; Sasaki, T. Bull. Chem. Soc. Jpn. 1970, 43, 1991. (9) Aranoto, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R., J. Colloid Interface Sci. 1984, 98, 33. (10) Born, M.; Wolf, E. Principles ofoprics, 5th ed.;Pergamon: Oxford, 1975. (1 1) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactanr Systems; National Bureau of Standards; Washington, DC, 1971.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1385
TetradecyltrimethylammoniumBromide Layer
1
3XlOL)
bl
P 0
I
4
3~10’~ y 1u-
oh0
0
oh
0:20
’
K / P
b o
Figure 2. Neutron reflectivity profit@ of fully deuterated CI4TABin null reflecting water at different concentrations: (0)4.5 X lo-’ M, (A) 3.5 X M, (X) 1.0 X lo-’ M, and ( 0 )3 X lo-* M. M, (+) 2.5 X T = 298 K. The incoherent background has been subtracted.
Co . O
105-
TABLE I: Parameters Determined from Neutron Reflection c/lO-’ M
OM
00
0
h/A
Figure 1. Neutron reflectivity from fully deuterated CI,TAB at 3 X lo-’ M and 298 K (a) showing the effects of successive recrystallization, (b) showing the effects of cleaning the surface by foaming. In both cases the reflectivity at low K drops as the surface impurity is removed.
which will also be surface active, may be generated by gradual decomposition of the C14TAB. Both these impurities will be deuterated when the chain of the parent C14TABis deuterated. In the reflection measurement such impurities will make a contribution to the reflectivity similar to that of the main surfactant. Impurities arising from contamination of the trough or the water will almost always be protonated. These will depress the reflectivity if they displace the main surfactant from the surface and otherwise have no effect. In the first case the apparent area per molecule will generally be smaller than the value for the pure surfactant, and in the second case it will be higher. Figure l a shows the change in reflectivity from the fully deuterated C14TABwith successive recrystallization after the initial preparation, at 3 X lo-’ M (cmc = 3.7 X lo-’ M). From these profiles the area per molecule after the first recrystallization was 26 A2 but had reached a limiting value of 48 A2 after the third recrystallization. The impurity in this case was undoubtedly unreacted deuterated C14Br,and it is also clear that recrystallization eventually, though surprisingly inefficiently, eliminates it. The surface tension measurements on the fully deuterated C14TABshowed a minimum, indicating the presence of an impurity. These measurements were made some time after the first set of reflectivity experiments and indicated that there had been some decomposition of the fully deuterated material in the interim period. A reflection measurement ’ust before the surface tension measurements gave an area of 43 significantlylower than the value of 48 A2 given above. Three months later we remeasured the reflectivity and found indeed that the area per molecule had dropped further to 33 A2 at the same concentration, indicating that the concentration of surface active impurity was increasing with time. However, using the standard technique of foaming the solution and sucking off the foam to remove any surface active impurity restored the reflectivity (Figure lb) to the same value obtained in our first experiments. Further foaming made no
i2,
pa/lOd
4.5 3.0 1.0 0.3 0.1
4.7 4.1 3.3 2.4 1.5
4.5 3.7 3.5 3.0 2.5 1.9 1 .o 0.3
3.5 3.5 3.4 3.1 3.3 2.8 2.8 2.3
A-2
r/ 10-10
r/A A/A2 dC IddTAB 20 f 2 44 f 2 20 48 18 67 f 3 16 1 O O f 10 16 f 3 160 f 20
dC IdhTAB 19 f 2 19 19 19 17 17 16 17 f 3
44 f 2 45 46 49
50 60 64 74 f 8
mol cm-2 3.8 f 0.2 3.5 2.5 1.7 1 . 1 f 0.15 3.8 f 0.2 3.7 3.6 3.4 3.3 2.8 2.6 2.3 f 0.2
difference to the reflectivity, indicating again that we had reached the limit of 48 Azat 3 X lO-’ M determined in the first experiment. The presence of surface active impurity in the fully deuterated species of CI4TABwas confirmed by the surface tension measurements (see below), but on separate occasions the reflection experiment was able to observe the surface free of impurity. Parallel measurements on the chain deuterated isotope, for which the surface tension showed no minimum, also give an area at 3 X lo-’ M of 49 f 2 A2. The same experiment was also done on the head deuterated isotope. However, because of the small signal from the layer the error in the neutron measurement increases from f 2 to *6 A2. Nevertheless the neutron value is 52 A2.
Figure 2 shows the variation of reflectivity with surfactant concentration for chain deuterated C14TAB. The measurements cover a range of concentration from about 0.lcmc to above the cmc at 1.5”. The surface concentrations determined from these reflectivity profiles are given in Table I together with the results for the fully deuterated C14TAB. At high surface coverages the reproducibility between different runs and the agreement between the two different isotopes is better than f 3 A2 for the area per molecule. The fit to an individual reflectivity profile gives the mean thickness of the layer with an accuracy of about 10% (this will be discussed in the following paper), but the error in the area per molecule is always better than f 2 A2,as illustrated in Figure 3. At low concentrations this error increases because of the much lower reflectivity and, together with the problem that we are more vulnerable to contamination by surface active impurities from the
Simister et al.
1386 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
TABLE Ik Fitted Parameters When Roughwss Is Included M dCdTAB) (Calculations Done for 3 X 71.4 ps J 104 A-2 rllA z2/A AIA~ 0 0 41.9 19.5 4.1 2 2 41.6 18.5 4.35 47.1 5.35 4 4 15.2 2 0 41.9 19.0 4.21 18.0 4.5 4 0 41.3 6 0 46.9 15.5 5.26
55 \
\
\
\ \
\
CMC
\
I
\
O
0.15
0.w)
\
0
0.20
dk' Figure 3. Sensitivity of reflectivity to changes in surface coverage. Points are the observed profile for 3 X lo-' dC,4dTAB in nrw. The areas per molecule for the calculated profiles are (i) continuous line, 48 A2, (ii) dashed line, 50 A2,and (iii) the lowest line, 60 Az.
. i
E
45-
y\
z
5
I
40 -
35 -
