Neutron and X-ray Reflectivity from ... - ACS Publications

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Langmuir 2003, 19, 752-761

Neutron and X-ray Reflectivity from Polyisobutylene-Based Amphiphiles at the Air-Water Interface Philip A. Reynolds, Duncan J. McGillivray, Elliot P. Gilbert,† Stephen A. Holt,‡ Mark J. Henderson, and John W. White* Research School of Chemistry, The Australian National University, Canberra, ACT 0200, Australia Received August 5, 2002. In Final Form: November 14, 2002 We have performed X-ray and neutron reflectivity experiments on amphiphile films spread at the airwater and air-saturated ammonium nitrate solution interfaces as a function of film compression. We have examined ca. 750, 1100, and 1700 molecular weight monodisperse polyisobutylene amide (PIBSA), 1100 imide (PIBSIM), and also palmitic acid for the pure amphiphiles and binary mixtures. We have used deuterated and hydrogenous components so that a given film may be examined in up to seven contrasts. All of the films form a monolayer, with molecular area determined by tail group size, which increases slightly in thickness on compression for PIB derivatives. There are differences in film thicknesses as shown by a dependence on compression and substructure; this is understood in terms of the increasing tendency of the tail to adopt a coiled conformation as molecular weight increases. PIBSA films consist of packed micrometer scale disks containing up to 10% open water (polynya) area. Palmitic acid-containing films contain a greater polynya area. All films lose up to half of the original amount of amphiphile from the monolayer when compressed. The almost reversible loss of material followed the trend palmitic acid < 750 amide < 1100 amide < 1100 imide, empirically in order of hydrophile-lipophile balance (HLB) number. This pattern suggests an amphiphile loss into submicrometer aggregates at the interface, rather than dispersion into the aqueous phase. Binary mixtures exhibit differential competition of the components to remain in the monolayer with the trend: palmitic acid >750 amide > 1100 amide > 1100 imide, opposite to the aggregation tendency as expected. The preference to remain at the surface may be physically related to more effective packing of the headgroup molecules on the surface when head and tail areas are better matched in size. Mixtures also spontaneously segregate laterally and form micrometer scale domains. This tendency also follows the HLB number, with better mixing for those components with similar HLB values. Lateral segregation is encouraged by film compression. Surprisingly, properties of films on water and 50 wt % ammonium nitrate subphase show little difference with respect to these trends.

1. Introduction Polyisobutylene (PIB)-based polymer amphiphiles with succinic anhydride derivative headgroups (polyisobutylene amide, PIBSA) are effective in the stabilization of high internal phase water-in-oil emulsions.1,2 Some of us have begun to report the results of a small angle scattering program to directly study the microstructure of such emulsions.3-6 We believe that understanding the surface activity of these amphiphiles at the air-aqueous interface will shed light on the behavior of the same amphiphiles in emulsions at the oil-aqueous interface. In this paper, we present the results of investigations into the surface properties of thin films of amphiphiles at the air-aqueous phase interface using the techniques of * To whom correspondence should be addressed. Fax: (61)2 6125 4903. E-mail: [email protected]. † Present address: ANSTO, Private Mailbag 1, MENAI, NSW 2234, Australia. ‡ Present address: ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K. (1) Cox, A. R.; Vincent, B.; Harley, S.; Taylor, S. E. Colloids Surf. A 1999, 146, 153. (2) Tomlinson, A.; Danks, T. N.; Heyes, D. M.; Taylor, S. E.; Moreton, D. J. Langmuir 1997, 13, 5881. (3) Reynolds, P. A.; Gilbert, E. P.; White, J. W. J. Phys. Chem. B 2000, 104, 7012. (4) Reynolds, P. A.; Gilbert, E. P.; White, J. W. J. Phys. Chem. B 2001, 105, 6925. (5) Reynolds, P. A.; Gilbert, E. P.; White, J. W. Submitted for publication. (6) Reynolds, P. A.; Gilbert, E. P.; White, J. W. Submitted for publication.

X-ray and neutron reflectometry, together with surface balance measurements. We have studied four PIBSA amphiphiles and palmitic acid, both as pure films and in a variety of amphiphile binary combinations on water and almost saturated ammonium nitrate solution subphases. Previous surface studies on polyisobutylene and n-alkyl succinic anhydride-derived amphiphiles have focused mainly on the diethanolamine form employing Langmuir balance in concert with ellipsometry on films compressed at the air-water interface.1,2,7-9 There have also been phase studies of the concentrated amphiphiles7,10 and a Langmuir balance study at the oil-water interface.11 The PIBSA amphiphiles studied here were of welldefined molecular weight (high monodispersity), established purity, and a well-controlled headgroup structure.4 The PIBSA amphiphiles were initially used individually to investigate the nature at the air-water interface of pure films as a function of film compression, time, and spreading methods. To study the effects of both molecular weight and headgroup variation, we then investigated three binary (7) Chattopadhyay, A. K.; Ghaicha, L.; Oh, S. G.; Knight, J.; Moaddel, T.; Friberg, S. E. J. Indian Chem. Soc. 1993, 70, 353. (8) Ghaicha, L.; Leblanc, R. M.; Chattopadhyay, A. K. J. Phys. Chem. 1992, 96, 10948. (9) Ghaicha, L.; Leblanc, R. M.; Chattopadhyay, A. K. Langmuir 1993, 9, 288. (10) Friberg, S. E.; Moaddel, T.; Chattopadhyay, A. K. Liq. Cryst. 1994, 16, 453. (11) Ghaicha, L.; Leblanc, R. M.; Villamagna, F.; Chattopadhyay, A. K. Langmuir 1995, 11, 585.

10.1021/la0206920 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002

Reflectivity from Polyisobutylene-Based Amphiphiles

Figure 1. PIBSA amphiphile, typical structure.

mixtures of amphiphiles: (i) different molecular weights, same headgroup; (ii) different headgroup, same molecular weight; and (iii) both molecular weight and headgroup different. We used the ability of neutron reflectometry to distinguish different components of a mixture through the use of selective deuteration. We were thus able to produce the same film with either one, or the other, or both amphiphile components “highlighted”. These results are complemented with X-ray reflectometry measurements, which have a finer spatial resolution than neutrons and which provide constraints on modeling the reflectivity results. The key experiments were then repeated on a subphase of almost saturated ammonium nitrate solution, to examine the effect on both pure and mixed films of a highly concentrated (50% w/w) ionic solution. These results strongly suggested that the hydrophile-lipophile balance (HLB) of the amphiphile governs the mixing and competition in these films. To confirm this, we experimented on all useful isotopic combinations of a further two sets of binary mixtures. These were of both low and high molecular weight PIBSA with palmitic acidsan amphiphile distinctly more hydrophilic than any of the PIBSA amphiphiles. These confirmed the utility of the HLB scale in predicting mixed film properties. 2. Experimental Section 2.1. Amphiphile Preparation. Pure PIBs of high monodispersity in three stages of polymerization were prepared by living carbocationic polymerization of isobutylene (or isobutylene-d8 for deuterated PIB) through a 2-chloro-2,4,4-trimethylpentane initiator in the presence of methyl chloride.12 Note that each deuterated PIB molecule still contained 17 hydrogen atoms at the tip of the tail due to the use of a protonated initiator. These polymer chains were then reacted with maleic anhydride and 2-hydroxyethanolamine to give the amphiphile species polyisobutylene N-(2-hydroxyethyl)succinamide, PIBSA4 (Figure 1). The purity of all species was checked by IR and 1H and 13C NMR. All were mixtures of the two expected amide isomers, differing slightly only in PIB attachment point to the headgroup, except the 1700 deuterated sample. This was found to contain ca. 30% unreacted deuterated PIB-succinic anhydride. While this latter mixture was useful for studies of chain length effects under compression, the mixture of headgroups meant that use of this component in studies of mixed films was not useful. The succinimide headgroup amphiphile, PIBSIM, was produced from the PIB succinic anhydride by condensation of 2-hydroxyethanolamine during 32 h of reflux conditions with anhydrous magnesium sulfate in o-xylene, followed by chromatographic separation of the product. The characteristics of the various hydrogenous amphiphiles produced are described in refs 4 and 5. For our purposes here, we note that the three stages of polymerization correspond to 6, 14, and 24 isobutylene monomers and total tail contour lengths of 16, 32, and 52 carbon-carbon bonds. The deuterated materials were of similar size. Estimates from comparison of all six vapor phase osmometry molecular weights gave deuterated material contour lengths of 14, 32, and 46. We will use rounded molecular weights of 750, 1100, and 1700 to label the three amphiphiles in both deuterated (D) and hydrogenous (H) forms for clarity, but note that more exact estimates based on the contour lengths above have been used in (12) Balogh, L.; Faust, R. Polym. Bull. 1992, 28, 367.

Langmuir, Vol. 19, No. 3, 2003 753 all calculations. Palmitic acid-h31 (Aldrich) and -d31(CIL) were used as received. 2.2. Experimental Equipment. The surface balance measurements of the films were performed on a NIMA trough, of total open area 560 cm2, in concert with the neutron or X-ray measurements. The films were spread by microsyringe from ca. 2 µM solutions in redistilled toluene on either Millipore water or D2O (Fluorochem) or mixtures thereof to form air contrast matched water (ACMW) as the experiment demanded, in amounts corresponding to about 0.8 monolayer at full trough expansion. When the ammonium nitrate solution subphase was used, only Millipore undeuterated water was used in making up the subphase since adjustment to contrast match with air is not physically possible. All compression/expansion rates were 40 cm2 min-1. Cleanliness of solvents was tested on the NIMA trough by ensuring no increase in surface pressure on compression. Mixtures of amphiphiles were made before spreading to ensure homogeneity. Most measurements were made at 25.0 ((0.2) °C, controlled through the use of circulating water. When ammonium nitrate solutions were used, the pressure sensor was rapidly degraded by crystallizing ammonium nitrate, even though the bulk of the trough remained clear of crystals. The only observations that we could make on these samples were that the isotherms resembled those on water in shape but were about half of the surface pressuresexpected due to the lower surface tension of the salt solution. Brewster angle optical microscopy observations of some spread films were made at the Rutherford-Appleton Laboratory using the BAM microscope (BAM2+, Nanofilm technologie GmbH) mounted over the same NIMA trough. X-ray reflectometry measurements were performed on the angle dispersive reflectometer at the Research School of Chemistry, Australian National University.13 A background determined by offsetting the detector arm of the reflectometer by 0.5° from the specular direction was subtracted from all data. Neutron measurements were performed on the time-of-flight reflectometer SURF at the Rutherford-Appleton Laboratory, United Kingdom, at a fixed angle of 1.5°.14 Background for neutron data was fitted as a parameter in our modeling and was controlled by the flat background at high QZ. The scale factor of SURF was determined by the measurement of the reflectivity of a sample of D2O, allowing the comparison of the modeled parameters with known reflectivity profiles. For the X-ray instrument, because we can reach the critical edge, an absolute scale is known; nevertheless, Millipore water was used to check instrument alignment. This scale factor was used to place all subsequent reflectivity measurements on an absolute scale. 2.3. Data Collected. (i) Initial neutron measurements were on the pure amphiphiles spread on water at 20 °C. Reflectivity from a film of 1100 (D) amide was measured at seven points along a compression/reexpansion curve on both ACMW and D2O subphases (Table 1). The isotherm is shown in Figure 2, illustrating the plateau reached on compression, the hysteresis, and the film relaxations observed while taking the reflectogram at the various fixed trough areas. Note that the relaxation at the seventh point is indeed upward in pressure. If further cycles of compression and expansion are performed, the isotherm eventually attains a limit cycle behavior, i.e., it retraces almost the same path in every cycle. Films of 750 (D) amide and 1700 (D) amide were also measured at a number of compression/expansion points on ACMW (Table 1). The D2O subphase was not measured for these, since, as we shall see, the extra information obtained would have been negligible. (ii) Films of 750 (D) amide, 1100 (D) amide, and 1100 (D) imide were measured on ACMW using a revised three point compression/expansion protocol at 25 °C (Table 2). A higher temperature was used than in the first set, since, as we shall discuss, some anomaly in the 750 (D) amide data appears at 20 °C. The amount of material added was adjusted so that the first point measured (13) Brown, A. S.; Holt, S. A.; Dam, T.; Trau, M.; White, J. W. Langmuir 1997, 13, 6363. (14) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Blake, T. D.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899.

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Table 1. Model Fits to Neutron Reflectivity Data from Single Amphiphiles Under Compression at 20 °C (a) 1100 (D) Amide on ACMW and D2O area (cm2)

Π (mN m-1)

thickness (Å)

SLD (ACMW) (× 10-6 Å-2)

SLD (D2O) (× 10-6 Å-2)

proportion

area (molecule/Å2)

520 400 350 300 250 200 250

0 11.1 16.3 21.3 24.1 31.2 17.5

22.7 (3) 25.3 (2) 26.1 (2) 26.9 (2) 28.3 (2) 29.4 (2) 26.7 (3)

6.62 (6) 6.44 (4) 6.44 (4) 6.51 (3) 6.47 (3) 6.00 (3) 6.26 (4)

6.77 (3) 6.64 (2) 6.76 (2) 6.72 (2) 6.65 (2) 6.56 (2) 6.48 (3)

1.00 0.83 0.75 0.67 0.59 0.45 0.54

71 65 63 61 58 60 63

(b) 750 (D) Amide on ACMW area (cm2)

Π (mN m-1)

thickness (Å)

SLD (× 10-6 Å-2)

proportion

area (molecule/Å2)

490 390 343 292 255 196 250 499

1.3 6.9 11.5 14.6 16.8 22.2 15.8 0

13.1 14.3 16.0 17.5 19.8 20.7 19.1 10.6

6.0 6.0 5.4 5.0 4.6 4.5 4.5 5.8

1.00 0.88 0.78 0.67 0.61 0.47 0.56 0.80

75 69 68 67 65 63 69 96

(c) 1700 (D) Amide on ACMW area (cm2)

Π (mN m-1)

thickness (Å)

SLD (× 10-6 Å-2)

proportion

area (molecule/Å2)

491 399 349 199 249 497

11.1 12.7 13.1 15.4 13.8 10

27.2 27.5 27.5 31.0 28.8 24.6

3.42 4.03 4.32 5.22 4.95 3.80

1.00 0.97 0.91 0.70 0.78 1.02

190 (7) 159 (5) 149 (5) 110 (3) 124 (4) 189 (8)

Table 2. Additional Pure PIBSA Amphiphile Measurements and Neutron Reflectivity on ACMW Combined with X-ray Reflectivity Data at 25 °C 750 (D) amide trough area (cm2) Π (mN m-1) thickness (Å) neutron tail SLD (× 10-6 Å-2) X-ray tail SLD (× 10-6 Å-2) X-ray headgroup SLD (× 10-6 Å-2) proportion of spread material area (molecule/Å2) water molecules (tail)

483 0.4 12.20 4.0 8.9 10.8 1.19 121 5.2

200 34.5 20.2 4.17 9.27 11.7 0.85 70 4.9

1100 (D) amide 334 0.0 15.0 3.2 8.78 10.0 0.81 123 10.6

Figure 2. Typical isotherm showing measurement points: 1100 deuterated amide on ACMW. was at 400-500 cm2swhen a noticeable pressure develops. This ensured that the second point at 200 cm2 was well into the plateau region of the isotherm. Less material was needed at 25 °C as compared to the 20 °C data to reach the onset of significant film pressure, especially for the 750 amide. The experiments were complemented by the use of X-ray measurements to attempt to

444 0.5 21.5 5.52 8.7 10.2 1.16 89 6.8

200 30.2 30.2 6.12 8.9 10.7 0.82 57 3.7

1100 (D) imide 440 0.5 21.1 5.58 8.78 11.5 1.14 90 6.9

353 0.8 20.4 5.39 8.82 10.8 0.86 96 7.9

200 18.7 28.9 5.73 9.3 10.8 0.73 64 8.6

343 0.5 20.8 5.23 8.4 10.0 0.82 97 8.6

define the head region and provide an estimate of the water content in the tail region. (iii) Mixed films of 750 amide/1100 amide, 750 amide/1100 imide, and 1100 amide/1100 imide were measured on ACMW and also D2O for the first pair at 25 °C. The mixtures applied were calculated, using previous isotherm measurements, to produce equal areas of a monolayer of each component. Extensive use was made of isotopic substitution to highlight various factors of interest. In each pairing of amphiphiles, three measurements were made on a given subphase: (i) one protonated and one deuterated amphiphile; (ii) with the reverse combination of deuterated and protonated; and (iii) finally with both amphiphiles deuterated. Generally, a mixture is described as hd/A where the first letter refers to the first amphiphile in the mixture (d ) deuterated, h ) hydrogenous), the second letter refers to the second amphiphile in the mixture, and the letter after the slash refers to the subphase (A ) ACMW, D ) D2O). Finally, all three pairings were measured using X-ray reflectometry. This means that for every pairing there are up to seven different data sets at a single compression. These can be constrained together in the modeling so that the number of physically and chemically reasonable model solutions to fit the data is further reduced. Selective substitution of one member of the amphiphile pair allowed the separate calculation of the amount of each component that is present at the surface and thus allowed competition at the surface to be measured. However, with this variety of data, as we shall see, even more quantitative information can be


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Table 3. Neutron and X-ray Reflectivity Fits for PIB Amphiphile Mixtures on Water at 25 °C 1100 amide/1100 imide (cm2)

trough area π (mN m-1) thickness (Å constrained) separate -dh -hd -dd neutron SLD (× 10-6 Å-2) -dh/A -dh/D -hd/A -hd/D -dd/A -dd/D X-ray SLD (× 10-6 Å-2) headgroup SLD (× 10-6 Å-2) mole factors amide mole factors 750 proportion of spread material area (molecule/Å2)

425 0.8 22.3

200 29.2 29.4

391 0.3 22.8

2.33 2.3 2.02 2.07 5.52 5.97 9.20 10.1 0.53

2.47 2.56 1.67 2.15 5.65 6.05 8.71 11.2 0.59

2.09 2.5 1.86 2.7 5.20 5.88 9.18 10.7 0.53

1.16 86

0.73 64

1.02 89

750 amide/1100 amide 439 1

200 31.3

372 0.7

17.2 21.6 18.4

25.8 29.9 27.8

22.1 21.7 19.0

1.61 1.52 3.34 3.9 4.37 4.67 8.87 11.0

2.02 1.86 3.35 3.54 4.60 5.02 8.99 11.5

1.23 1.36 3.20 3.63 3.93 4.64 9.15 9.9

0.41 1.28 103

0.48 0.93 65

0.41 1.01 94

750 amide/1100 imide 425 0.6 18

200 33.2 27.1

406 0.5 18.3

1.5

1.70

1.41

2.8

3.18

2.97

3.2

4.16

3.52

8.83 10.8 0.49

9.05 10.6 0.49

9.15 9.8 0.46

0.89 144

0.82 74

0.95 129

Table 4. Neutron Reflectivity for PIB Amphiphiles on Saturated Ammonium Nitrate at 25 °C 750 (D) amide trough area (cm2) thickness (Å) neutron tail SLD (× 10-6 Å-2) proportion of spread material area (molecule/Å2)

483 20.25 2.92 1.44 100

200 21.84 3.6 0.79 75

483 16.88 3.19 1.31 110

1100 (D) amide 200 22.28 3.48 0.78 76

483 26.23 4.62 1.29 87

200 29.46 5.14 0.67 70

483 21.55 4.78 1.10 103

1100 (D) imide 200 28.37 5.16 0.65 72

483 21.01 4.89 1.10 103

200 28.33 5.28 0.66 71

483 20.1 4.63 0.99 114

200 28.48 5.11 0.64 73

Table 5. Neutron Reflectivity From Palmitic Acid and PIBSA Amphiphile on Saturated Ammonium Nitrate at 35° palmitic trough area (cm2) thickness (Å) neutron SLD (× 10-6 Å-2) proportion of spread material area (molecule/Å2)

483 20.9 4.33 1.60 65

200 21.5 2.95 .46 50

483 20.5 3.36 1.22 46

750 amide 200 21.2 3.29 .51 45

483 20.0 3.65 1.77 81

200 21.7 4.49 .98 61

483 17.3 3.90 1.64 87

1100 amide 200 21.7 4.40 .96 62

483 26.6 4.65 1.32 86

200 29.6 5.24 .69 68

483 26.9 4.82 1.38 82

200 28.9 5.27 .67 70

Table 6. Neutron Reflectivity From PIBSA Mixtures on Saturated Ammonium Nitrate at 25° 1100 amide/1100 imide trough area (cm2) thickness (Å constrained) -dh -hd -dd neutron SLD (× 10-6 Å-2) -dh -hd -dd mole fraction amide or 750 proportion of spread material area (molecule/Å2)

483 22.9

1.46 1.97 4.78 0.42 1.17 97

200 29.0

0.96 2.07 4.78 0.32 0.61 76

483 21.9

1.15 1.79 4.64 0.39 1.08 105

200 29.2

0.91 2.03 4.72 0.31 0.61 77

obtained. The results are shown in Table 3. Two of the films were held under compression at 200 cm2 for 8 h, and reflectivity measurement was recorded continuously. The gradual loss in observed intensity was small and could be explained by evaporative loss of water, which lowered the liquid height away from perfect alignment with the collimation slits. Thus, over our shorter periods of measurement here, changes must be ascribed to pressure cycling not temporal decay of films. (iv) Many of the above experiments were repeated using a subphase of almost saturated ammonium nitrate solution in H2O (53.4% w/w at 25 °C). This has a scattering length density (SLD) of almost zero (0.74 × 10-6 Å-2) and still provides excellent contrast for the deuterated amphiphile films. Specifically, we repeated the three pure amphiphiles 750 (D) amide, 1100 (D) amide, and 1100 (D) imide and the mixtures 750 amide/1100 amide, 750 amide/1100 imide, and 1100 amide/1100 imide, each in three isotopic combinations. All were measured at four points in a standard compression/expansion cycle (Tables 4 and 6). (v) Mixtures involving palmitic acid were measured on a subphase of almost saturated ammonium nitrate solution in H2O at 35 °C. The higher temperature was required due to the higher

750 amide/1100 amide 483

200

483

26.2 22.1 21.6

29.5 28.0 26.2

21.3 23.1 20.6

1.42 2.45 4.08 0.51 1.55 94

1.64 2.05 4.28 0.56 0.81 74

1.51 2.48 4.06 0.46 1.47 99

750 amide/1100 imide

200 28.9 25.5 1.58 4.24 0.78 76

483

200

483

200

25.1 23.5 19.4

25.6 26.6 23.9

27.1 21.5 20.0

27.8 28.9 24.2

1.30 2.02 3.22 0.51 1.10 132

1.79 1.98 3.82 0.57 0.66 90

1.31 2.14 3.39 0.54 1.19 122

1.71 2.01 3.83 0.55 0.67 89

melting point of bulk palmitic acid as compared to PIB amphiphiles. We measured the three pure amphiphile films 750 (D) amide, 1100 (D) amide, and palmitic acid (D) (Table 5) and the mixtures 750 amide/palmitic acid and 1100 amide/palmitic acid, each in three isotopic combinations and four cycle points (Table 7). 2.4. Modeling. The reflectometry data were modeled using the optical transfer matrix method.15 This method divides the interfacial region into a series of layers, each of which is associated with a thickness (τ, Å), SLD (Å-2), and roughness between layers (σ, Å). These parameters are then refined through minimization of the difference between the data and the calculated model (represented by a χ2 goodness of fit value). The modeling software allows any one of these parameters to be refined separately or constrained between similar data sets.16 To provide additional constraints on the modeling procedure, neutron measurements (15) Penfold, J. In Neutron, X-Ray and Light Scattering; Lindner, P., Zemb, T., Eds.; North-Holland: Amsterdam, 1991; p 223. (16) Brown, A. S.; Holt, S. A.; Reynolds, P. A.; Penfold, J.; White, J. W. Langmuir 1998, 14, 5532.

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Table 7. Neutron Reflectivity From Palmitic Acid/PIBSA Mixtures on Saturated Ammonium Nitrate at 35° palmitic/1100 (cm2)

trough area thickness (Å) -dh -hd -dd neutron SLD (× 10-6 Å-2) -dh -hd -dd mole fraction palmitic proportion of spread material area (molecule/Å2)

483

200

483

palmitic/750 200

483

200

483

200

15.3 20.8 23.4

19.3 24.6 23.2

22.1 19.6 19.8

20.2 24.8 22.9

18.9 17.4 13.0

24.0 17.4 21.8

19.8 15.7 15.3

20.0 17.3 23.7

2.86 3.78 4.55 0.65 1.51 53

4.58 2.68 5.09 0.82 0.69 48

2.02 3.76 4.19 0.67 1.17 68

4.16 3.07 4.65 0.79 0.62 53

2.49 1.68 4.85 0.75 1.32 62

2.61 2.24 3.73 0.75 0.71 48

1.51 1.70 3.96 0.68 1.27 65

1.88 2.85 2.99 0.59 0.62 55

3. Results and Discussion

Figure 3. Neutron reflectivity modeling of 1100 amide/1100 imide mixtures on ACMW as compared to the data. + is hd/A, O is dh/A, and X is dd/A. were often made on several different combinations of deuterated and hydrogenous amphiphiles, on both D2O and ACMW (zero SLD). In modeling the neutron data, we have assumed fixed interlayer roughnesses of 4 Å, in accordance with the value used to fit our X-ray data. All data were modeled until χ2 was in the region 0.6-2. A value of about 1 implies that there is no further information in the data to be extracted. The model that gave the most physically reasonable description of the interface had two main features. The first, described as a polymer “tail” region, was observed in both the neutron and the X-ray data sets. The thickness of this layer was constrained to be the same in each of the data sets, with the SLDs allowed to vary in the multiple data set fitting process. The second was a thin layer of high density observed only in the X-ray data, which was assigned as being the solvated “head”. This layer was not observed in these neutron experiments due to the much lower resolution of the neutron experiments and the low contrast of the hydrogenous headgroup. This layer was fixed at a 5 Å thickness, because the layer is at the very extreme of the resolving ability of the X-ray experiment. The modeling remains sensitive to the product of the thickness and the density of the layer. Neutron measurements performed on a D2O subphase also required about a 5% scale decrease or a small decrease in the SLD of the D2O subphase to achieve an acceptable χ2. Use of the scale factor determined by reflection from D2O gave χ2 values of about 10. Such small apparent scale changes can arise in many ways, changes in specular/offspecular ratios or small defects in the modeling, but we will not speculate further. Thus in a typical simultaneous refinement of an X-ray and two neutron reflectograms (ACMW and D2O), there are eight parametersstwo neutron backgrounds, one tail length, three tail densities, one headgroup density (thickness assumed as 5 Å), and a parameter associated with the apparent lateral organizational change from H2O to D2O. A sample of the goodness of fit of a typical model to the data is shown in Figure 3 for three of the seven data sets used at this compression to illustrate the effect of the three differently deuterated pairs. The reflectivity has been multiplied by QZ4 to highlight the deviation from Fresnel scattering from a sharp interface.

To understand the reflectivity results, we need to consider both the mesoscopic and the nanoscopic film structure. 3.1. Film Mesoscopic Structure. We cannot assume that we are producing uniform, perfect films. Observation of 1100 amide films on water by Brewster angle microscopy (BAM) revealed that for lower surface doses than used here the amphiphile aggregates into disks of about 1015 µm diameter, which are mobile on the water surface. At the dose and compressions used here, the surface appears to consist of such immobile disks, almost closepacked together with noticeable open areas (polynyas) between them. If we use 750 amide, we see the same packed disk morphology, but the disks are less clearly defined. Segmented domains assembled as disks have been reported in Langmuir monolayers with textures dependent upon the nature of the acid.17 It is known that the lateral coherence length for neutrons in the SURF reflectometer is 10-100 µm19 so, for example, disks of 15 µm radius could affect the specular/offspecular ratio. These open areas of the films have consequences in the neutron reflectivity. All five single amphiphiles were measured as spread films using deuterated amphiphiles on an ACMW subphase, so that all coherent scattering can be attributed to the amphiphile located at the surface. For a perfect film, this would allow the direct calculation of the amount of amphiphile that is present at each compression and the molecular area occupied at the surface by each molecule. Results from fitting to these neutron and some X-ray data are presented in Tables 1 and 2. The entries are self-explanatory, except “proportion” is the fraction of spread material detected by the specular scattering determined from the SLD and thickness of the film. That this exceeds one in some entries reflects systematic error. However, for an imperfect film consisting of areas of amphiphile and polynyas, the average molecular area has little physical meaning. The SLD for the pure amphiphiles, packed as efficiently as in a crystal, can be estimated from data for a range of crystals related in structure to the PIB amphiphile. The SLDs estimated in this way are 1700(D) amide, 6.5 × 10-6 Å-2; 1100 (D) amide and imide, 5.9 × 10-6 Å-2; 750 (D) amide, 4.5 × 10-6 Å-2; and palmitic acid (D), 6.32 × 10-6 Å-2. If we assume that this calculated value is the SLD in the islands of material, the proportion of islands to total area may be estimated. On water, the proportions are 0.53 and 0.80 for 1700 (D) amide at 20 °C as spread and then compressed to an area of 200 cm2 (Table 1); 0.94 and 1.04 for 1100 (D) amide at 25 °C and as spread and compressed (Table 2); 0.91 and (17) Johann, R.; Vollhardt, D. Mater. Sci. Eng. C 1999, 8-9, 35. (18) Kirton, G. Ph.D. Thesis, Australian National University, 2000. (19) Richardson, R. M.; Webster, J. R. P.; Zarbaksh, A. J. Appl. Crystallogr. 1997, 30, 943.


0.97 for 1100 (D) imide at 25 °C as spread and then compressed (Table 2); and 0.89 and 0.93 for 750 (D) amide at 25 °C as spread and then compressed (Table 2). Similar numbers can be deduced from other Tables. It is mostly true that as spread films have an island proportion close to but less than one. This proportion increases as the film is compressed. We would not expect a surface film to pack as efficiently as in a crystal, but these numbers suggest significant open space between areas of amphiphile film. A raft of hexagonally close-packed disks has a ratio of 0.907 for disk to total area. As in dendrimers, the highest molecular weight PIBSA is least flexible, with poorest coverage. If we consider the 35 °C data (Table 5), we see that at the third point (reexpansion to 480 cm2) the observed SLD for pure 1100 (D) amide is 0.82 of the calculated value, rising to 0.89 on recompression to 200 cm2. For 750 (D) amide, the respective figures are 0.87 and 0.98, while for palmitic acid (D) they are 0.53 and 0.52. The palmitic acid film appears much less efficiently packed than the PIBbased films. Given its melting point of 63 °C, this film may have more irregular shapes of the islands of film, with larger open area, as observed in some dendrimer films.18 A more direct measure of this open space is afforded by comparing neutron reflectivity data for films on ACMW and D2O (Tables 1a and 3). In both Table 1a for pure 1100 (D) amide and in Table 3 for PIB amphiphile mixtures, we consistently see that the difference between films measured on ACMW and those on D2O is that the latter has an SLD 0.0-0.6 × 10-6 Å-2 larger than the former. This is explained as the contribution from D2O in the polynyas in the latter, whereas in the former the ACMW in the polynyas does not contribute. These numbers imply open water areas of 0-10% of the total area. The wide range is most likely a result of real variability in area caused by inconsistency in the initial spreading of amphiphile. Last, from Table 2, we see that when we compare neutron reflectivity on ACMW and X-ray reflectivity data we can also extract information about water, since neutrons are insensitive to ACMW but X-rays are scattered as well by water as amphiphile. Quantitative fitting of the X-ray reflectivity shows that all of the amphiphiles incorporated about five molecules of water for each amphiphile molecule or about 5% of the film volume. It is likely that the systematic errors are larger than the statistical ones, and further trends are difficult to discern. This associated water, although forced by the modeling to be averaged throughout the molecule, is most likely the result of a slight immersion of the polymer tail into the subphase, together with 5% of intervening polynyas. Such immersion has been observed for other polymer amphiphile solutions but was interpreted as a small water content of uniform films.20-22 We have three conclusions. (i) The PIB-based films contain up to 10% open water area. (ii) The amphiphile islands contain negligible water in the deuterated tail area and are packed almost as densely as they would be if crystalline. (iii) When measuring on ACMW, the SLD derived from fitting the reflectivity data for PIB amphiphile films is underestimated by about 5%. We have (20) Saville, P. M.; Reynolds, P. A.; White, J. W.; Hawker, C. J.; Frechet, J. M.; Wooley, K. L.; Penfold, J.; Webster, J. R. P. J. Phys. Chem. 1995, 99, 8283. (21) Saville, P. M.; White, J. W.; Hawker, C. J.; Wooley, K. L.; Frechet, J. M. J. J. Phys. Chem. 1993, 97, 293. (22) Saville, P. M.; Gentle, I. R.; White, J. W.; Penfold, J.; Webster, J. R. P. J. Phys. Chem. 1994, 89, 5935.

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Figure 4. Thickness of surface film vs area per molecule in the surface film. Diamond, 750 amide; square, 1100 amide; triangle, 1700 amide.

not corrected for this in later discussion, since it varies between about 0 and 10%. For films containing palmitic acid, the correction may be more but can be estimated from the fully deuterated films. We have also omitted standard uncertainties in most tables since this systematic effect is a greater source of error. 3.2. Single Amphiphile Films. The surface pressure for these amphiphile systems begins to rise at a comparatively large average area per molecule. This area varies from 190 Å2 for the 1700 amphiphile to 70 Å2 for the 750 and 1100 amphiphiles and is significantly larger than the expected area based on the headgroup crosssection from crystal studies (ca. 30 Å2). However, the increase in pressure is caused by interactions between amphiphile islands in the presence of large polynyas. Even if we adjust these figures for polynya area, the variation in estimated true molecular area in the amphiphile islands varies from 100 Å2 (1700) to 60 Å2 (1100). This implies that the major interactions between amphiphile molecules are based on the size of the tail. Both the 750 and the 1100 amphiphiles show a limiting true surface area per molecule at high compression of about 55-60 Å2, which corresponds to the cross-sectional area of a fully extended vertical PIB chain. That for the 1700 is 88 Å2 reflecting some chain coiling. That the tail is larger than the head is also reflected in the structure of emulsions made from the same amphiphiles.3-6 These form as water-in-oil, and the water droplets and oil phase both contain structures in which the amphiphile tail is on the outside of a positive curvature, i.e., tail larger than head. The thickness of the measured amphiphile layer is plotted as a function of the average surface area occupied per molecule at the interface in Figure 4. The area per molecule is not calculated from the amount spread and the trough area, since some of the spread material is lost from the monolayer, as explained below. Account has also been taken of this loss such that the horizontal axis is the area occupied per molecule actually in the monolayer at the interface. Polynya fraction has not been taken into account, but this is a small effect, which changes little on compression. At low surface pressure, the three PIBSA amide amphiphiles reach limiting thicknesses of 10 (750), 22 (1100), and 26 Å (1700). We can compare these values to three estimates of molecular thicknesssfully vertically extended (33, 45, and 59 Å); randomly coiled valence angle chain23 (sphere of diameter 20, 23, and 27 Å); and densely packed spheres (diameter 13, 16, and 19 Å). From extensive previous work, we expect values much shorter than for (23) Eyring, H. Phys. Rev. 1932, 39, 746.

758


Figure 5. Comparative isotherms 1700 amide (dotted); 1100 amide (long dash); 750 amide (short dash); 1000 imide (continuous line). Compression rate is 40 cm2 min-1.

the fully extended chain. This is for two reasons, the higher molecular weight molecules are beginning to coil,24 while the smaller molecular weight molecules, although straighter, tilt over relative to the normal to the interface. On compression, the 750 becomes significantly more extended and reaches 22 Å, the relative extension of 1100 is less (-up to 30 Å), while the 1700 hardly changes, extending from 26 to 32 Å. This reflects the well-known increasing stability of coil structures as the molecular contour length increases. From the footprint at high pressure and the film thickness, we can calculate molecular volumes in the film as 2800 Å3 in 1700 (D), 1600 Å3 in 1100 (D), and 1300 Å3 in 750 (D). The corresponding values estimated from crystal data on related compounds are 2300, 2000, and 1240 Å3. The relatively good agreement indicates that our assumptions are reasonable, since although these volumes have been used previously in calculation of polynya fraction, the argument is not circular. There is an interesting difference in detail at low compressions between the results in Tables 1 at 20 °C and Table 2 at 25 °C. At low compressions, higher SLDs are observed at 20 °C than the whole molecule averages, particularly for the 750, but less so for the 1100 amphiphiles. These high SLDs are 6.0 × 10-6 Å-2 for 750 amide as compared to 4.5 × 10-6 Å-2 for the whole molecule and 6.6 × 10-6 Å-2 and 5.9 × 10-6 Å-2 for the 1100 amide. These differences indicate that the initial spreading at 20 °C did not form a good monolayer, rather some areas of multilayer. We note that for the 750 (D) at 20 °C, 50% more material has to be added than at 25 °C to get a significant surface pressure at large area and that on compression the SLD falls rather than rises as is expected and is observed at 25 °C. This is symptomatic of the presence of multilayer and its subsequent reduction by mechanical treatment of the film. Last, the high values of SLDs derived from the fitting routine are probably an artifact of using a physically innappropriate single layer model. That raising the temperature 5 °C cures this problem is not surprising given the evidence of multilayer formation at interfaces of PIBSA amphiphile in emulsions3,25 and the observed sensitivity of this to temperature. As compression proceeds, the surface pressure of the amphiphile layer reaches a plateau value for each of the amphiphiles (Figure 5). This value appears to be headgroup-dependent at approximately 42 mN m-1 for the amide amphiphiles but only 29 mN m-1 for the 1100 imide. The plateau suggests that the surface has reached an equilibrium situation. This plateau has previously been explained as a signature of the formation of a liquid(24) Polymer Handbook; Brandup, J., Immergut, E. H., Eds.; John Wiley and Sons: New York, 1966. (25) Reynolds, P. A.; Gilbert, E. P.; White, J. W. Unpublished data.

Reynolds et al.

condensed phase or as a continuous loss of material from the surface layer through the mechanisms of either overlayer formation or dissolution into solution on compression. From the decrease in SLD, it can be seen that amphiphile material is indeed lost from the surface layer on compression, with as little as 50% of the original amphiphile material present as a monolayer at the highest compressions. This lost material is not accounted for in the fitting/modeling routine by use of more and/or diffuse layers. Here and throughout, we have used the deuterated film on ACMW to calculate this quantity and later for mixtures, using both deuterated components. This loss of material is also found for these mixtures of amphiphiles. On reexpansion, the lost material reappears in the monolayer film. The question of the location of this lost material is of particular interest. The low degree of solubility of these insoluble polymer amphiphiles would suggest that the material is not lost by dissolution into the aqueous subphase as either molecules or micelles. If the material was dissolved into the subphase, it would be expected that lower molecular weights would be the most soluble and amide more soluble than imide. The opposite is observed. If we consider Table 2, then we note a headgroup and molecular weight dependence. On compression, less imide remains than amide and less 1100 than 750. Also, if the material lost was in an adjacent volume to the film, it should (in principle) be detectable by reflectometry as another layer of material. The relatively quick recovery of material to the surface layer on decompression suggests this: repeated compressions retrace the same compression/expansion path following a recovery time in the order of minutes. However, such a layer is not observed in any of our reflectometry measurements. This does not preclude the possibility that the dissolved molecules are dispersed in the subphase in a layer of the order of 300 Å, too extended for the resolution at low Q values of the reflectometer, but we advance a more plausible possibility below. We suggest that the lost material is used to form “overlayers” after compression. These have been observed in other insoluble polymer systems.20,26 In addition, there are theoretical studies predicting buckling transitions and resulting three-dimensional topography of surface layers in some circumstances under compression.27,28 Here, we do not observe the very large degree of hysteresis (almost irreversibility even at very low rates of compression and expansion), which is there associated with such superstructure formation. The hysteresis in our case can be minimized by slow compression. Probably because of roughness or the scale of the overlayer, we postulate its invisibility to the reflectometer although it may exist at the submicrometer scale either above or below the surface. It is useful to compare our results with earlier Langmuir trough work.1,2,7-9,11 A variety of both PIB and n-alkyl tails have been attached to variously substituted succinic acid anhydride heads and studied but only during a single compression. There are some differences between n-alkyl and PIB members. All of the PIB members, however, display similar isotherms with the observed molecular areas determined partly by PIB coil size and partly, for larger headgroups, by the requirement that the whole headgroup contacts the water surface. In more detailed (26) Hatta, E.; Hosoi, H.; Akiyama, H.; Ishii, T.; Mukasa, K. Eur. Phys. J. 1998, 2, 347. (27) Diamant, H.; Witten, T. A.; Gopal, A.; Lee, K. Y. C. Europhys. Lett. 2000, 52, 171. (28) Diamant, H.; Witten, T. A.; Ege, C.; Gopal, A.; Lee, K. Y. C. Phys. Rev. E 2001, 63, 061602.


studies, including ellipsometry (on a diethanolaminesubstituted headgroup PIBSA derivitive), Chattopadhyay et al.7,8 obtained similar molecular areas on condensation but smaller areas at collapse. The result can be understood since those authors were unable to monitor material loss from the film, as we have done in our neutron and X-ray reflectometry studies of our similar system. The ellipsometer, giving only a single number (the thickness for an assumed refractive index), was unable to model simultaneously film thickness and content. Nevertheless, Chattopadhyay et al. obtained similar film thickening on compression and found evidence for new structures forming in the plateau region. We conclude that our more detailed and direct study of three amphiphiles embraces their results. Because of the range of compounds that we have been able to study, a number of key surface chemical properties for this class of PIB derivatives have become apparent. Tables 4 and 5 show the results for films of pure amphiphile spread on saturated ammonium nitrate at 25 and 35 °C. Comparison with the results on water show only neglible differences. Considering that the saturated solution is about 50 wt % of salt, this relative invariance was unexpected. 3.3. Mixtures of Amphiphiles. Given the similarities in the general features of the single amphiphile behavior, we decided to examine the competition between different combinations of tail length and headgroup. We focused initially on (i) the difference between the 750 and the 1100 amphiphiles and (ii) the effects attributable to the amide or the imide headgroups for the 1100 amphiphile on water. This series of experiments gave three different combinations of amphiphiles to be tested, namely, 1100 amide/ 1100 imide, 750 amide/1100 amide, and 750 amide/1100 imide premixed in mixtures containing mole fractions of the first component of 0.50, 0.44, and 0.44, respectively. We will discuss the same properties of the film as for the pure amphiphile films, before turning to discuss the properties specific to a mixture: lateral demixing into islands and competition between the amphiphiles to remain in the film at the interface. The π-A isotherms from this set of data show the same general features as those measured for the individual amphiphiles, including limited relaxation of the film on standing and hysteresis, together with a return to the initial state on expansion within a short time. The maximum surface pressure of the mixture is close to the average of the surface pressures of the two individual components of the mixture. The SLD from the dd mixtures is intermediate between those for the pure amphiphiles. We can deduce that the mesostructure is also intermediate and that significant polynya areas remain. It appears that the mixing process does not render the films more fluid or flexible than either pure component. The film structure for the mixture of the two amphiphiles is similar to that of the individual amphiphile film structures. The films again showed a small incorporation of water molecules with the headgroup and the significant loss of material on compression, which is partially recovered on reexpansion. The film thicknesses for 1100 amide/1100 imide mixtures and for the hd combinations highlighting the 1100 component in the two 750/1100 mixtures show the same values, including increase under compression, as for the pure 1100 materials. However, the layer thickness of the dh sample with deuterated 750 amphiphile in the mixture was distinctly thicker than that measured for the pure 750 amphiphile. It seems that either the interactions between the chains of the 750 and 1100 amphiphiles are

Langmuir, Vol. 19, No. 3, 2003 759

causing the 750 to adopt a more extended conformation or the chains are less aligned in the layer. Given the aligning effect of the headgroup-water surface interaction, the former seems more reasonable. We do not see the converse effect; the 1100 chains are not becoming more coiled to shorten themselves. The deuterated/hydrogenous and the hydrogenous/ deuterated pairings can be used to determine both separation of and competition between the amphiphile molecules. If, on the scale of the coherence length of SURF (ca. 10 µm), the amphiphiles remain mixed then the SLD for hd and dh combinations will be half that for both the dd and the pure amphiphile SLDs. However, if there is significant demixing into islands of pure amphiphile, then this ratio will be 0.707 (1/21/2) for our mixtures, which is designed to be almost 50% area of each amphiphile. This derives, in the kinematic approximation, from the reflectivity in a mixed film being proportional to the average of the two SLDs all squared, whereas for a demixed film the reflectivity will be the average of the squares of the two SLDs. For the 1100 amide/1100 imide mixtures, each of these SLDs are slightly less than half of both the dd and the pure amphiphile values at both high and low compression (Table 3). The amphiphiles in this mixture thus remain mixed. The SLD for dh and hd mixtures would be expected to dip below a ratio of one-half, since in a mixed more irregular film we would expect the specular/ offspecular ratio to decrease. For the 750 amide/1100 amide film, these ratios are significantly greater than 0.5. For example, the sum of the hd and dh SLD exceeds the dd by a ratio of from 1.13 to 1.17. This is less than the 1.414 expected for complete demixing but shows that the process has begun. We note that the increase of the 750 molecule thickness in the mixture also implies incomplete demixing. For the 750 amide/1100 imide mixture, this process of demixing is even more advanced, although still not completesthe ratios vary from 1.17 to 1.34. We can conclude that for all three mixtures lateral segregation into islands occurs with the characteristic size 1100 amide/ 1100 imide < 750 amide/1100 amide < 750 amide/1100 imide. We now will discuss the composition of these films. Compression of the layer increases the number proportion of 1100 amide present in the layer as compared to the 1100 imide, from an initial value of 0.53(1), which is close to the 0.50(2) in the material spread, up to a maximum of 0.59(1). On reexpansion, the proportion of amide to imide returns to close to that which was originally spread, 0.53(1). This shows that the greater preference for water of the amide as compared to the imide is only strongly apparent at high pressures. These results agree with the earlier observation that the pure imide amphiphile, when spread, lost more material than the amide on compression (Table 2). In the competition between the 750 amide and the 1100 amide, the 750 amide showed greater preference for remaining on the surface, increasing from a number proportion of 0.41-0.48 on compression (remembering the as spread fraction was 0.44). For the 750 amide/1100 imide combination, the values 0.46-0.49 were found at all compressions. Here also, it appears that material is lost not just under compression but also at low pressures. We can summarize all of the previous results about loss of material by noting (i) that the nature of the headgroup seems more important than the change in molecular weight, (ii) imide loss is more than amide, and (iii) high molecular weight losses are greater than low molecular weight losses. As discussed earlier, our preferred sink for this loss of material is formation of three-dimensional

760


aggregates of amphiphile-rich material on or under the film, rather than dissolution into the subphase. The results from the same mixtures on saturated ammonium nitrate differ in detail only (Table 6). The dh and hd SLDs when compared to the dd and pure material show that demixing into islands is not observed here. So that we could confirm these trends, which appear connected to the HLB, we examined two mixtures with palmitic acid on AN, which is distinctly more hydrophilic than the most hydrophilic PIBSA, 750 amide (Table 7). The as spread mole fraction of palmitic acid is 0.67 for the palmitic/1100 amide mixture and 0.72 for palmitic/750. These higher ratios were chosen so that the total palmitic acid area in the mixed film remained ca. 50%. We notice from Table 5 the significantly smaller footprint of palmitic acid as compared to the PIBSAs. We also note that on reexpansion the original footprint is not recovered in the pure palmitic acid case suggesting sluggish respreading. General features of the mixed films are that the dd SLD values in the mixtures indicate polynya areas intermediate between 10% for PIBSAs and 40% for palmitic acid. On compression, pure palmitic acid films change little in thickness, and the palmitic component in the mixtures behaves likewise. It is noteworthy that in the palmitic/1100 amide mixture we do not see an increase in the palmitic acid thickness over pure palmitic acid. This is a first hint of almost complete lateral segregation of the amphiphiles into different islands. If we turn to this question, we notice that for the palmitic/1100 amide mixture the ratio of the sum of the hd and dh SLDs to that from dd varies from 1.38 to 1.46sexactly matching the 1.414 expected for complete segregation. For the palmitic/ 750 amide at low compressions, these ratios are 0.86 and 0.81, while at high compressions they are 1.30 and 1.58. A reasonable explanation of this is that the high compression causes demixing. For the film composition, we note that when compressed the amides are both driven out of the films, 1100 more than 750. As we predicted on the basis of HLB differences, we observe the same trends as for the PIBSA mixtures in all of the properties but they are much exaggerated, particularly for the palmitic/1100 mixture. 4. Conclusions The use of monodisperse polymer amphiphiles and the ability of neutron experiments to distinguish between isotopically substituted amphiphiles have enabled the behavior of both single and mixtures of amphiphiles to be studied in detail at the air-water and air-saturated ammonium nitrate interface. All of the five amphiphile systems and their mixtures studied form a monolayer on compression of a spread film. This remains relatively constant in structure on further compression, despite the rise in surface pressure to a plateau. There is no formation of observable multilayers nor a final solid, almost incompressible, film. Nevertheless, we observe significant changes on compression and when the amphiphile is varied. The PIB-based films, both as spread and after compressive cycling, contain 5-10% of open water area, while this area is much more for palmitic acid. The amphiphile areas of the film are shown by Brewster microscopy to be disks a few micrometers in diameter. These are composed of a monolayer containing almost close-packed PIB tails. There is negligible water in the bulk of the tail region. For all of these amphiphiles, the tail group size is the factor that determines the area occupied by the amphiphile molecule in the monolayer. The headgroups would prefer

Reynolds et al.

a much tighter packing to ensure their closepacking, but this is not possible. All of the polymer, PIB-based amphiphiles increased the monolayer thickness as the layer was compressed. The polymer chains become more upright on the surface and less coiled, but this is less marked for the longer chains as expected. Palmitic acid does not show this behavior. All of the pure amphiphiles also lost material in the monolayer when compressed, up to half of the original amount of amphiphile. The isotherms show hysteresis, and there is a rapid recovery of material to the layer once the layer is reexpanded on the time scale of seconds to minutes. The loss of material followed the trend 750 < 1100 < 1700 and amide < imide. The 1100 Imide even lost material under no compression. This pattern suggests a loss due to aggregation to submicrometer lumps, rather than dispersal into the subphase. This aggregation explains why there is no limiting area of film at which surface pressures rise very rapidly on compression. When aqueous saturated ammonium nitrate solution is used as a subphase, very similar behavior is observed. Because the subphase is 50 wt % of an ionic salt, this relative invariance is unexpected. There is a slight trend to increased aggregation and tighter packing of molecules in the film. The mixtures of amphiphiles also displayed these same patterns, and both of the isotherms of the mixtures and the structures mostly correspond to the average of the individual amphiphiles. One notable exception is that when the 750 and 1100 are mixed, the apparent length of the 750 is greater than for the pure 750 material. This can be interpreted as either a straightening effect on the 750 molecules of the thicker mixed environment or vertical disordering of molecules of the same conformation. From the neutron measurements and isotopic substitution, we have also been able to identify two types of competition in amphiphile mixtures. The first type is “vertical competition” and is the competition to remain in the measured amphiphile layer at the interface. The preference to remain at the interface for pure films was found to follow the trend of palmitic acid > 750 amide > 1100 amide > 1100 imide, opposite to the aggregation tendency as required. These trends were also found in the mixtures of amphiphiles. Empirically, this series is in order of decreasing HLB number. Physically, the reason for this preference to remain at the surface may be related to the more effective packing of the headgroup molecules on the surface, which would serve as a more effective screen between the solvent and the hydrophobic tail region. More efficient packing is related to both the size of the tail group and the size of the headgroup, with the larger headgroups and smaller tail group sizes favored. This is because in these polymer amphiphiles, the packing at the surface is mostly affected by interactions between the chain regions of neighboring polymer tails. The second effect noticed was a tendency for mixtures to sometimes spontaneously separate laterally and form isolated domains, leading to changes in specular scattering. Such domains are only visible when one component is deuterated and the other protonated. We notice that such domains are favored by larger differences in HLB number and by increased compression of the films. Thus, segregation laterally and vertical aggregation follow the same trends. Our major conclusion is that we can interpret all of the reflectivity data by a simple monolayer model only if we also take into account, where appropriate, the three factors of (i) open water areas in an imperfect film, (ii) loss of


amphiphile from this monolayer to aggregates, and (iii) segregation of mixtures into distinct patches in the monolayer. Acknowledgment. We thank Dr. J. R. P. Webster at the Rutherford-Appleton Laboratory, United Kingdom, for experimental assistance. We also thank Prof. R. Faust, Harvard University, and Dr. George Adamson, ANU, for their parts in the amphiphile synthesis. Travel grants

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through the Australian Government ISTAC/ANSTO Access to Major Facilities Program are gratefully acknowledged. This work was financed by the Australian Research Council under SPIRT and SRF awards joint with ORICA and ICI UK Ltd. We also thank Dr. Deane Tunaley, Dr. Richard Goodridge, and Dr. David Yates of ORICA for useful discussions. LA0206920

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