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Molecular adhesion interactions between Langmuir monolayers and solid substrates
COLLOIDS AND ELSEV INN
Colloids and Surfaces A: Physicochemical and Engineering Aspects 131 ( 19981 215 224
A
SURFACES
Molecular adhesion interactions between Langmuir monolayers and solid substrates Karlheinz Graf", Hans Riegler b., a blstitutfiir Phvsil,alische Chemie, LbffversitOt Mainz, Welder-Weg ] 1, D-55099 Mainz, Germam" b Max-Planck-lnstitut.li'ir Kolloid- und GrenzfldchenJorschung, Ruthn~:er Chaussee 5, D-124¢¢9 Berlin, Germtmv
Received 12 August 1996: accepted 7 October 1996
Abstract
Substrate monolayer adhesion interactions between differently prepared SiO 2 surfaces and Langmuir monolaycrs were investigated. From the relation between the contact angle and the surface tension of a kangmuir monolayer in the configuration of Langmuir wetting the work of adhesion between the substrate and the monolayer as a function of the molecular packing and the transfer ratios was determined. Thus the molecular work of adhesion of different SiO 2 dimyristoylphosphatidylethanolamine surfaces was quantified. The relation between the local adhesion interactions and the molecular packing and structure is presented and substrate-induced phase transitions are discussed. ~c~, 1998 Elsevier Science B.V. Kevwords. Langmuir wetting; Adhesion: Contact angle
I. Introduction
Knowledge of the adhesion interaction between monomolecular films and solid substrates is of crucial importance in the field of physisorbed monolayers. The adhesion contains contributions from the various possible interfacial interactions (van der Waals, acid base, etc.) and is thus of fundamental interest for the understanding of intermolecular interactions. It is also the key factor for the preparation and stability of Langmuir Blodgett multilayers. Static contact angle measurements are a widely used technique in surt:ace and adhesion science to investigate the interactions at interfaces between different bulk phases [ 1]. In the so-called Langmuir-wetting configuration (Fig. 1 [2]) it can be used to measure
* ('orresponding author. 0927-7757:98:$19.00 ~C, 1998 Elsevier Science B.V. All rights reserved. P l l SI)927-7757(96)03923-4
the interactions between monomolecular organic films (Langmuir monolayers) and planar solid substrates. Already in 1973 Clint and Walker [3] used contact angle measurements to determine the adhesion interaction between Langmuir multilayers of fatty acids with alkyl chains and partially fluorinated alkyl chains. For these systems they found that the interaction is dominated by van der Waals forces. Aveyard et al. [4-6] investigated the relation between the contact angle, the transfer ratio, and the headgroup ionization of li~tty acid Langmuir monolayers. They confirmed Bikerman's hypothesis [7] which states that quantitative Langmuir-Blodgett deposition occurs only at contact angles greater than 90 . Peng et al. [8] studied the transition of Y- to X-type LB-transfer of cadmium fatty acid salts as functions of the dipping time and the number of deposited monolayers. Their data agree with the theory of X-type
K Graf H. Riegler / Collohts Surfaces A: Physicochem. Eng. Aspects 131 (1998) 215-224
216
// ./
/ \
solid substrates. However, these investigations predominantly offered global values (e.g. the work of adhesion per unit area). No detailed information on the relation between the molecular properties (e.g. molecular packing) and the adhesive properties (e.g. the molecular work of adhesion) was obtained because of the largely unknown details on the molecular ordering and structure of the investigated films. We will present here static equilibrium contact angle measurements of Langmuir wetting and a new thermodynamic analysis of the data. We will show that from the data new and valuable information on the interactions between substrate and monolayer as a function of the molecular packing can be derived. For instance, for two different substrate surfaces substrate-induced structural changes in monolayers and the molecular work of adhesion as a function of the molecular packing will be presented and analysed.
2. Theoretical background
Fig. 1. Schematic of the contact angle measurement via laser beam reflection at the meniscus area in a Langmuir wetting configuration. The substrate is inclined for better access to the meniscus area (the equilibrium contact angles are independent of the substrate inclination). The contact angles are derived from the directions of the incoming and reflected beams and the substrate inclination.
film formation of Honig [9] who assumed that molecular detachment and overturn in the submerged outermost deposited monolayer lead to an increase of defects and eventually to the observed X-type deposition. All these and other investigations [10-13] show that contact angle measurements are well suited to investigate and quantify the wetting, adhesion and transfer properties of ultrathin organic films on
In Langmuir wetting (see Fig. 1) a Langmuir monolayer covers both the aqueous subphase and the submerged solid substrate [2]. With respect to interface forces this configuration is equivalent to a conventional wetting situation. In the case of a static equilibrium contact line Young's equation describes the relation between the three surface energies and the contact angle [14]: ~,SV = ~,SL 71_./LV COS 0
( 1)
where ~,sv 7SL and ?LV are the surface energies at the solid/vapour, solid/liquid, and liquid/vapour interfaces, respectively. 0 is the contact angle as defined in Fig. 1. From Eq. (1) the work of adhesion between the monolayer and the substrate surface can be derived as [14]: Wasv = ) ' L V ( 1 --COS O )
(2)
where Wsv is the change of interface energy upon replacing the air/monolayer/water interface by the air/monolayer/substrate interface. According to Gibbs the chemical potential of interfaces, ~, is
IC Grq/~ H. Riegler / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 215 224
related to the surface energy by [14]: 1
d~ = - - d7 F
(3)
where F is the surface excess. For insoluble monolayers the surface excess F can be replaced by the absolute surface concentration of the surfactant, or equivalently, by the area per molecule, Am: (4)
d~= -A m d/
Eqs. ( 1 ) and ( 3 ) can be combined: d(7 Lv cos O) = d) 'sv - d7 sL = YsL dp sL - / - s v d/~SV 15) In the case of equilibrium between all three interfaces (d/Lsv = d p se = d~tLv) one obtains: d() 'Lv cos O ) = ( F SL - F S V ) d # Lv
3. M a t e r i a l s and methods
(/~v __/-SL)
d() 'Lv cos 6)) -
]~V
~7)
For soluble surface active agents this equation was derived by Lucassen-Reynders some decades ago [15 17]. In the case of Langmuir wetting, Eq. (7) can be modified substantially. Because of the insolubility of the amphiphilic molecules in the liquid subphase, their surface excess at the solid/liquid interface, /SL, can be neglected. Hence, to an excellent approximation Eq. (7) can be written as: d(3 'Lv cos O) d;,LV
F sv -/-LV -
(8)
Or, alternatively according to Eq. (4): d("/Lv cos O)
d;. Lv
[18]. Hence, from Eq. (8), by variation of the surface energy yev, and by the simultaneous measurement of the corresponding contact angles, the transfer ratio /SV/FLV and thus the surface coverage at the solid/vapour interface, /-sv, can be determined. This determination of the transfer ratio is much more accurate than the usual method of comparing the area of deposited monolayers with the decrease of the floating monolayer area during the transfer process because it is independent of any errors arising, for instance, from partial monolayer collapse, as long as the conditions for Eq. 18) are valid. Finally, the combination of the results obtained via Eqs. (2) and (8) allows the quantification of the adhesion energy per molecule, V sv = W~aV/I -sv, at the substrate/air interface as a function of the molecular packing.
(6)
and together with Eq. (3): d;,L',
217
ALtov -A~v
(9)
For Langmuir monolayers, the molecular packing at the air/water interface, F Lv, and thus the surface tension, ?LV, can be varied in a controlled and easily measurable way by expansion or compression of the monolayer. Additionally, for many substances the area per molecule at the liquid/vapour interface, Am LV, and thus the absolute value of FLv, is well known, either from the isotherms, or from other measurement techniques
The experiments were performed with a homebuilt all-Teflon Langmuir trough. The surface tension was measured by the Wilhelmy method. The lipid L-c(-dimyristoylphosphatidyl-ethanolamine ( D M P E ) was obtained from Avanti and used without further purification. The subphase was ultrapure water (Milli-Q). As substrates we used silicon wafers with thermally grown oxide layers (400-600A) which were cleaned and prepared with a modification o f the RCA cleaning procedure [19]. After degreasing with organic solvent the wafer surfaces were prepared with either only a basic preparation step, SC-I, or both the basic and, additionally, an acidic cleaning procedure, SC-2. For SC-1 the substrates were placed at room temperature into a quartz beaker filled with a solution of 5 volume parts ultrapure water, l volume part ammonium hydroxide (30wt%, A.C.S. reagent, Aldrich), and 1 volume part hydrogen peroxide (30 wt%, nonstabilized, med. puriss., Merck). The solution was then heated to 8 0 C and kept for 10 min at this temperature. The substrates were then thoroughly rinsed with pure water. They were stored under water and used within 1 h for the contact angle measurements or the additional acidic surface preparation SC-2 was applied. For this the substrates were kept in a
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K. Graf H. Riegler / Colloids Surfaces A." Physicochem. Eng. Aspects 131 (1998) 215 224
solution of 1 volume part hydrochloric acid (32 wt%, puriss, p.a., Fluka) and 1 volume part water in an ultrasonic bath for 5 min and were rinsed with water afterwards. The contact angles were determined from the reflections of a H e - N e laser beam hitting the threephase line area as shown schematically in Fig. 1. In this setup one part of the laser beam is reflected from the substrate surface, the other part comes from the meniscus surface adjacent to the threephase line. By simple geometrical calculations the contact angle is determined to an absolute accuracy of about 0.3 ° from the directions of the two reflected beams relative to the incoming beam and to the substrate surface orientation. The method is sensitive to relative changes of the contact angle of ~ 0.1 o. For easier access to the three-phase line area the substrate is usually inclined at ~40" relative to the air/water interface [20]. The substrate tilt has no influence on the static equilibrium contact angle, which is solely given according to the Young equation by the relative strengths of the surface tensions. Each contact angle value was measured after the surface pressure was adjusted, then the substrate was withdrawn several 100 pm (under concomitant monolayer deposition), the substrate movement was stopped, and finally the meniscus allowed to relax to equilibrium. Our criterion for equilibrium is that the contact angles changed by less than 0.1 ° within 20 rain. All measurements were performed with a subphase of pure water ( p H i 5 . 5 ) at 20°C.
4. Experimental results Fig. 2 shows the isotherm, i.e. surface pressure 0 r = 7 2 . 7 5 m N m - 1 _yLV) VS. the molecular area, of a D M P E monolayer on pure water at 20°C (pH,~5.5). The isotherm can be split into four regions with different molecular ordering [21]. Beyond ~ 80 A2 per molecule the monolayer shows the coexistence of a liquid-like (liquid-expanded, LE) and gaseous-analogue (G) phase. Between 80 and 66 A2 per molecule it is a homogeneous LE phase. At 66 ~2 per molecule (i.e. above G) a phase transition into the coexistence of the LE and a condensed LC phase ( liquid-condensed, LC )
50.0 L-c~-DMPE T = 20°C, pure water 40.0 ~ LC
30.0 Z E
LE/LC
LE
LE/G
20.O 10.0
~c i o.ooI
~ '
30
'
'
'
l
'
'
40
'
'
l
'
'
'
'
50
l
'
' ' ' 1 ' ' '
60
70
'
'
80
'
'
'
I
'
90
'
'
'
100
area [A2/molecule]
Fig. 2. Isotherm of a L-~-DMPE monolayerat 20 C on a subphase of pure water. The ranges with the different molecular ordering are indicated (G, gaseous: LE, liquid-expanded; LC, liquid-condensed). The pressure 7to marks the transition from the LE phase to the LC phase.
is observed. The phase coexistence ends below ~45,&2 per molecule where the monolayer is predominantly in a condensed phase. The variation of the molecular ordering is reflected in the plot of the equilibrium contact angle O between D M P E and SiOz vs. the surface pressure at the air/water interface (Fig. 3). As in the following Figs. 3-7 the full circles are data points for SiOz surfaces prepared only with the basic step SC-I, whereas the open circles represent those with both the basic (SC-1) and the acidic (SC-2) surface treatments. In general, the contact angles increase with increasing surface pressure. The contact angles of the surface with the basic surface treatment are roughly twice those of the surface with both basic and acidic treatments. At pressures corresponding to the LC phase at the air/water interface the contact angles of both systems increase approximately linearly with increasing surface pressure. The region around the phase transition pressure, ~c, is marked by conspicuous variations of the contact angles. The physics behind the contact angle variation become clearer upon plotting the spreading tension, yLv COS O, of the DMPE/SiO2 interfaces vs.
219
K. Grq/i H. Riegler ,; Colloids SurJ~tces A." Physicochem. Eng. A~7~ects131 (1998) 215 224 2.0-
SC-1 I SC-1 + SC-2
1.8-
>,
o o
9"
1.6-
0
1.4
•
SC-1
o
SC-1 + S C - 2
i
%
o o
W 3o.o-i
o
1.2-
o
o3 c-
o
Z 20.0-
0.8-
° o o o
10.0-
0.6 0.012
o
-
8 o i .m.. . . . . . . . . . . . . .
""/i ....20.0 .....25,0i
.
.
.
.
.
.
.
.
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
40.0
Fig. 3. (2ontact angles of D M P E SiO2 systems as a function of the surface pressure. (Q) Data for substrates prepared only with the basic step (SC-I). I i;) Data for adsorption onto surfaces additionally prepared with acidic rinsing (SC-1 + SC-2 ). 80.0 •
LE
L C
70.0
i //
60.0 Z
/ /
50.0
/
o "~
400
. // ///
300
,,"
,c
/
/ /
.i
/'
•
SC-1
o
SC-1 + S C - 2 |
I I
200 20.0
30,0
40.0
LE/LC 0.016
0.018
LC 0.020
0.022
0.024
0.026
Fig. 5. Transfer ratios, Fsv/'~ v, of DMPE onto SiO, as a function of the molecular packing at the air/water interface. 1~-v
o [o]
. . . . . . . . .
0.014
F TM [molecules/A 2]
LO
o
.....
0.0
/~c
LE
o
q)
1,0-
o
E
50.0 "ixv [mN/m]
60.0
7o0
80.0
Fig. 4. Spreading tension of DMPE SiO2 as function of the DMPE surface tension at the air/water interface (substrate sur-
face preparation: (Q) SC-I, ('~"}SC-I +SC-2), the surface energy at the air/water interface, 7Lv (see Eq. ( 8 ) . The plots (Fig. 4) show straight lines o f r o u g h l y unit slope in the L C r e g i m e w i t h m o d u l a t i o n s in the range o f the p h a s e transition pressures (insert o f Fig. 4). A c c o r d i n g to Eq. ( 8 )
(substrate surface preparation: (O) SC-I: ( 5 ) SC-I +SC-2). The vertical lines indicate the LE to LC phase transition on the water at ~z. and the beginning of the steep slope of the isotherm on the water.
the slope directly represents the transfer ratio. At low surface tensions the transfer ratio is close to unity, whereas around 7zc an increased transfer ratio is found. These transfer ratios, as derived from the data of Fig. 4 and Eq. ( 8 ), are shown in Fig. 5. For the solely SC-l-treated SiO2 surface one observes an increase of the transfer ratio of up to 1.7 in the LE region below the phase transition. For the surface which was additionally SC-2 treated an increase in the transfer ratio below the phase transition is also observed, but it is much less pronounced and increases only much closer to the phase transition. In the LC regime the transfer ratios of both surface treatments are approximately unity with a tendency to slightly lower values for the basically treated surface. Fig. 6 presents the adhesion energies per unit area, I,t -SV ~a, as a function of the surface energy at the air/water interface. The data are extracted via Eq. (2) from the experimental data of Fig. 3. Wsav depends strongly on the surface treatment and lies between ~ 7 and 8.5 mN m - 1 with the SC-1 treated surface and between 1.5 and 3 m N m 1 for the SC-1 + S C - 2 treatment. The phase transition regime around lzc is marked by abrupt changes in 14/SV aa, In general, in the LE phase the adhesion energies per area are smaller than in the LC phase. Combination of the data from Figs. 5 and 6 yields
K. Graf H. Riegler / Colloids Surfaces A." Physicochem. Eng. Aspects 131 (1998) 215 224
220
ing at a level of about 0.6 KJ mol - 1. In both cases the abrupt changes of W sv near the phase transition (see Fig. 6) are now largely smoothed by the normalization of the adhesive energy relative to the molecular area instead of the unit area.
10.0SC-1
]
SC-I + SG-2 8.0
E
6.0
E
LC
LE
4.0
5. Discussion 2.0
0.0 20.0
30.0
40.0
50.0
60.0
/I; 7 0 . 0
¢-v [.N/m]
80.0
c
Fig. 6. Adhesion energies, W'~.~',of DMPE-SiO2 as a function of the air/water interface energy },ev (substrate surface preparation: (O) SC-I; (©) SC-1 +SC-2). The vertical line marks the phase transition at nc from the LE phase and the LC/LE coexistence regime at the air/water interface.
4.00 3.503.00 ~"
2.50
2o0
~',~ 150 >
1.00
:
~ sc-1
SC-1 + SC-2
I
-
0.50
"
0.00 . , . , . . . , . , . 0.012 0.014 0.016
, . . . ~ . . .,.,,, 0.018 0.020 0.022
F sv [molecules/A
, . . 0.024 0.026
2]
Fig. 7. A d h e s i o n energies per molecule, V sv = W~,~'/Isv, o f D M P E - S i O 2 at different m o l e c u l a r p a c k i n g densities on the
substrate, /sv (substrate surface preparation: (O) SC-1; (O) SC-1 + SC-2). the work of adhesion per molecule, Vsv, as a function of the molecular packing on the substrate, ysv. The results are presented in Fig. 7. Again one can see the difference between the two substrate surface treatments. For the solely SC1-treated surface one observes a steady decrease of I/sv from 3.5 kJ mol -1 at loose molecular packing (LE phase) to half this value in the densely packed state. Obviously, the acidic treatment strongly decreases the attractive interaction and keeps it largely independent of the molecular pack-
The Langmuir wetting configuration is well suited for the investigation of the relations between the contact angle and the various interfacial energies because it is possible to vary selectively and precisely the interracial energy of one interface (~,ev). Thus it is possible to examine the contact angle O as a function of the surface energy, },ev, of the monolayer phase, and of the substrate surface preparation. The work of adhesion, W~aa v, the transfer ratio, FSV/I+v, and the molecular work of adhesion, V,d, sv can be derived from the data. Eqs. (2) and (8), on which our data analysis are predominantly based, can be used if the conditions for their derivation are valid in the presented cases. In detail this means: (1) the three-phase contact line is in mechanical equilibrium (i.e. validity of Eq. (1)); (2) the surface energy (pressure) measured with the Wilhelmy system at the plane air/water interface is indeed the 7Lv of Eq. (1); (3) the observed contact angle is O of Eq. (1); (4) the chemical potentials of the three interfaces are in equilibrium (validity of Eq. (6)); (5) the adsorption of amphiphilic molecules at the substrate/subphase interface can be neglected
(Fsc~0). It should be remarked here that these conditions do not include the necessity for the monolayers to represent equilibrium systems with respect to surfactant molecule exchange between the monolayer and the liquid subphase. The data interpretation presented is valid for surface pressures below and above the equilibrium spreading pressure as long as the surface energy is defined and measurable and conditions (1)-(5) are fulfilled. In the derivations of Eqs. ( 1 )-(9) bulk quantities (e.g., surfactant solubility, etc.) play no role. We only assume
h2 Grq¢: H. Riegler / Colloids Surfaces A: Physicochem. Eng. Aspects 131 I 1998) 215 224
that the surface excess of the surfactants is identical to the absolute concentration of surfactant molecules localized in a monolayer at the air/water interface. This is a well established perception for the surfactant molecules investigated. We are convinced that the receding contact angles (monolayer adsorption) presented and discussed in this report are virtually the equilibrium contact angles because numerous measurements have shown that they can be reproduced with high accuracy. This can be understood because the energy barriers for adsorption are small. In contrast, for monolayer desorption, energy which might have been dissipated irreversibly upon adsorption, has additionally to be brought up by submerging the substrate beyond its equilibrium value. Indeed, we observed a contact angle hysteresis, but it was quite small, typically less than 0.3 ~', Hence, even if the real equilibrium contact angles were somewhere between the measured adsorption and desorption values, this still would not invalidate the use of Young's equation; it would only lead to larger errors. Compared to the hysteresis of more than 1 typically found in conventional contact angle measurements [22] our hysteresis is remarkably small. This may originate in our better defined surfaces (interlaces). Both interfaces which are most in danger of contamination - the air/water and the substrate/air interface - are already covered and virtually saturated with surface active substances (the monolayer). Hence, they have a much reduced susceptibility to contaminant adsorption and thus surface energy reduction. In the case of bare surfaces, often even minor amounts of adsorbent may influence the contact angle and cause significant hysteresis because differences between the surface energies of different surfaces and not absolute values determine the spreading tension and thus the contact angle [23]. There is no reason for a substantial difference between the local surface energy, 7Lv, of Eq. ( 1 ) and the value measured at the plane air/water interface. Capillary effects can be neglected because the radius of curvature of the meniscus is typically much more than l mm, which corresponds to a capillary pressure of less than 150N m -2 compared to the ambient air pressure of 105N m--'. Also, it is well established that, for the fairly fluid
221
monolayers used in this study, local surface pressure changes equilibrate within less than seconds to uniform pressure values everywhere at the air/water interface (at least for the trough dimensions used in this study). The contact angle measured by the method presented in the materials and methods section represents the O of Eq. (1) because it expresses the macroscopic change of the surface energy component, 7 cv cos O, as demanded by the mechanical force equilibrium due to the Young equation. Any microscopic contact angle te.g., on a molecular scale, if one can be defined l is not relevant in this context. The equality of the chemical potentials at the air/water interface, It Lv, and at the substrate/air interface, tt sv, presumes the exchange of monolayer molecules between the two interlaces locally at the three-phase contact line. At the air/water interface the molecules will be sufficiently mobile laterally. At the substrate/air interface it suffices for the validity of this study if only the first few rows of amphiphilic molecules adjacent to the three-phase line are sufficiently mobile laterally to arrange themselves in equilibrium according to the local energetic environment. We do not claim that ;,sv and F sv arbitrarily close to the three-phase line, i.e. those values which are the topic of the study presented here, are always necessarily identical to the ;,sv and /sv anywhere on the solid substrate surface. The 7sv and F sv may be uniform over larger areas, for example in the case of monolayer transfer in the course of a very slow substrate upstroke (Langmuir-Blodgett transfer). Fluorescence microscopy observations indicate that there is some molecular exchange and mobility on the solid substrate over distances up to several tens of microns away from the three-phase contact line. The assumption of equilibrium between the chemical potentials at the air/water and solid/air interface and at the substrate/subphase interface, it sv, is vindicated by the small contact angle hysteresis which shows the reversibility of the monolayer adsorption onto the solid. The substrate surfitce without monolayer coverage is perfectly hydrophilic ( O = 0 : with pure water). With an adsorbed monolayer it is quite hydrophobic. If a substrate
222
K. Graf H. Riegler / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 215-224
coated with a monolayer is quickly submerged into a subphase (with or without a floating monolayer) and then the substrate is kept stationary, one observes advancing contact angles much larger than the receding (equilibrium) values. This 'static' advancing contact angle then relaxes, which typically takes a long time (tens of minutes to hours), until it eventually approaches the receding value to within less than 1°. This shows that there is monolayer ablation from the solid surface, i.e. surfactant molecule transport into the subphase or, much more likely due to the very low solubility of the amphiphilic molecules, to the air/water interface. All this, the monolayer ablation, the very low surfactant solubility, and the fairly small contact angle hysteresis, strongly indicate that /-sL is very small and can be neglected according to Eq. (8). This is further corroborated by the remarkably linear section-by-section behaviour of the spreading tension vs. surface energy which we have always obtained for many different monolayer/substrate configurations (results to be published soon). As a general result we find that the contact angle depends in a non-monotonic and non-obvious way on (a) the surface tension, ?,Lv, (b) the monolayer phase, and (c) the substrate surface preparation. This is as expected because interracial energies are a function of long-range and local interactions [24,25]. The local interactions are determined by the molecular packing which is altered by phase transitions and chemical modifications of the surfaces. The impact of phase transitions on the interfacial energies is obvious in the data presented in Fig. 3. If the surface is prepared with procedure SC-1 a substrate-mediated phase transition of the monolayer on the substrate is observed at ~ 2 mN m - 1 which is responsible for the kink in the contact angle behaviour at this pressure. If the substrate is prepared with both SC-1 and SC-2, the phase transition occurs at higher pressures ( ~ 5 mNm-1). These phase transitions can also be identified in the plots of W ~ (Fig. 6). The loop in the contact angle and I4~,v behaviour at ~2 mN m-1 for the SC-1 surface preparation is reproducible and observed with advancing and receding contact angles. This peculiar behaviour is not yet understood but it might be responsible for
the dynamic contact angle instabilities observed in this pressure range [26]. The substrate-induced phase transitions can, independently of the kinks in the contact angle behaviour, also be identified from the transfer ratios. In the case of the SC-l-treated surface the transfer ratio is between 1.4 and 1.7 at molecular densities between ~0.0140 and 0.0150 molecules z (Fig. 5). This means that condensation begins at about 71 ~2 per molecule, in the middle of the LE phase region, at ~ 3 m N m 1 below 7rc, in quantitative agreement with fluorescence microscopy studies [2]. For the surface treated with SC-1 and SC-2 the transfer ratio is significantly larger than 1 only very close to ~c, which again is in qualitative agreement with fluorescence microscopy observations. The data of Fig. 5 were derived by using the local slope of the spreading tension, i.e. the slope calculated from the measurement point which belongs to the corresponding molecular packing, / ~ v and the two adjacent measurement points of the next higher and lower molecular packing. The results are virtually identical to those derived (not shown) with the slopes calculated from a wider range (see slopes of Fig. 4). We present here the qocal' data, although they show a larger data scattering because they corroborate the accuracy and meaningfulness of our method. The importance of local interactions is also manifested by the impact of the substrate surface preparation on the contact angle and thus on the monolayer adhesion. The adhesion interaction differs by up to a factor of 5 depending on the surface treatment (Fig. 7). It is not yet clear what is responsible for the different adhesion behaviour. It can only be assumed that the SC-2 treatment increases the degree of surface protonation. Further, it can be expected that the interface between the deposited monolayer and the substrate surface is hydrated. The surface preparation may influence the hydration and/or the degree of dissociation of the silanol and/or the DMPE headgroups. A change of the degree of dissociation of the DMPE headgroups would be in agreement with measurements (to be published) of the work of adhesion of DPPC (dipalmitoylphosphatidylcholine) on the same substrate. It is found that DPPC is much Jess sensitive to different surface
I( Grq[i H. Riegh, r / ColloMs Sur/~wes A: Physicochem. Eng. Aspects 131 (1998) 215 224
treatments. The DPPC headgroups cannot dissociate whereas the D M P E headgroups start to deprotonate at p H i 8 in monolayers [27]. It is also conceivable that the headgroup hydration and/or the degree of dissociation depend on the molecular packing. This would lead to the variation of the molecular work of adhesion as observed (Fig. 7). The measured molecular work of adhesion is in the range of k T ( ~ 2 . 5 kJ mol ~) for the SC-1 treatment or substantially smaller (SC-1 and SC-2 treatment). At first glance this might call into question the stability of the three-phase line. However, any deformation of the straight threephase line, for example by locally ablating a few monolayer molecules and shuffling them back to the floating monolayer, necessitates not only an energy contribution g .,sv ~d. but is also penalized by an unl:avourable increase of the total surface area and local surface bending. The establishment of the contact angle and the (straight) shape of the three-phase line is a cooperative phenomenon.
223
monolayer area loss with the transferred area. By combining both the adhesion and the transfer ratio data we obtain m o l e c u l a r adhesion data. We investigated the work of adhesion, the transfer ratio, and the molecular work of adhesion of a DMPE-SiO2 system. The molecular packing and phase behaviour of D M P E monolayers are well known from independent methods, therefore they are especially well suited in this context to gain insight into structure-property relations. We find that the transfer ratios, the work of adhesion, and the molecular work of adhesion depend on the molecular packing of the floating monolayer, the deposited monolayer and on the substrate surface preparation. The molecular mechanisms governing these dependencies are not yet understood but it can be assumed that hydration and/or headgroup dissociation effects (local pH ) play an important role. The contact angle data confirm quantitatively the substrate-mediated condensation hitherto only observed by fluorescence microscopy.
6. Summary and conclusion The quantification and modification of monolayer substrate interactions is of key importance for the controlled deposition of water-insoluble molecules onto solid substrates (e.g., for the LB technique). In this paper we show that Langmuir wetting is suitable for measuring the relations between the contact angle, the various interfacial energies and the work of adhesion. Interfacial energies are governed by long- and short-range interactions. The latter particularly can be well modified and controlled in the Langmuir wetting setup. Thus we investigated the adhesion interactions as a function of various parameters relevant for the monolayer-substrate adhesion. By using a beam reflection method we could obtain an angular resolution of the contact angle measurements such that details of the relation between the molecular ordering (packing) and the monolayer sttbstrate adhesion interaction could be quantified. We further show that the contact angle data also contain quantitative information on the transfer ratios which are much more precise than those usually derived by comparing the floating
Acknowledgment We thank the Deutsche Forschungsgemeinschaft for financially supporting the investigations (Sachbeihilfe Ri 529/8-1) and Wacker Siltronic G m b H Burghausen for generous donation of the silicon wafers. Helpful discussions with Helmuth M6hwald, Bob Aveyard, and Terry Blake are much appreciated.
References [1] J. Kloubek, Adv. Colloid Interface Sci. 38 (1992~ 99. [2] K. Spratte, H. Riegler, Langmuir 10 ( 19941 3161. [3] J.H. Clink T. Walker, J. Colloid lnlerface Sci. 47 t l~ 1974) 173. [4] R. Aveyard, B.P. Binks, P.D.I. Fletcher, Prog. Colloid Polym. Sci. 84 ( 1991 I 36. [5] R. Aveyard, B.P. Binks, P.D.I. Fletcher, X. Ye, Thin Solid Films 210,,'211 (1992} 36. [6] R. Aveyard, B.P. Binks. P.D.I. Fletcher. X. Ye, Colloids Surfaces A: Physicochem.Eng. Aspects 94 119951 279. [7] J.J. Bikerman, Proc. R. Soc. London, Ser. A 170 (1939~ 130.
K. Graf H. Riegler / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 215-224
224
[8] J.B. Peng, J.B. Ketterson, P. Dutta, Langmuir 4 (1988) 1198.
[9] E.P. Honig, Langmuir 5 (1989) 882. [ 10] G.L. Gaines Jr., J. Colloid Interface Sci. 59 (3) (1977) 438. [11] M.R. Buhaenko, R.M. Richardson, Thin Solid Films 159 (1988) 231, [12] M. Engel et al., Ber. Bunsenges. Phys. Chem. 95(11) (1991) 1514. [13] J.G. Petrov, A. Angelova, Langmuir 8 (1992) 3109. [14] A.W. Adamson, Physical Chemistry of Surfaces, Wiley, New York, 1990. [15] E.H. Lucassen-Reynders, J. Phys. Chem. 67 (1963) 969. [16] M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New York, 1994. [17] T.D. Blake, in: Th.F. Tadros (Ed.), Surfactants, Academic Press, London, 1984, p. 221. [18] F. Bringezu, G. Brezesinski, P. Nuhn, H. M6hwald, Biophys. J. 70 (1996) 1789, and references therein.
[19] W. Kern, Semiconductor Int. April (1984) 94. [20] J.E. Riegler, Rev. Sci. Instr. 59 (10) (1988) 2220. [21]H. Riegler, H. M6hwald, in: O. Wolfbeis (Ed.), Fluorescence Spectroscopy, New Methods and Applications, Springer, Berlin, 1992, p. 111. [22] R.J. Good, in: K.L. Mittal (Ed.), Contact Angle, Wettability and Adhesion, VSP, Utrecht, 1993, p. 3. [23] V.V. Yaminsky, K.B. Yaminskaya, Langmuir 11 (1995) 936. [24]L.-H. Lee, in: K.L. Mittal (Ed.), Contact Angle, Wettability and Adhesion, VSP, Utrecht, 1993, p. 37. [25] F, Brochard-Wyart, J.-M. di Meglio, D. Quere, P.-G. de Gennes, Langmuir 7 (1991) 335. [26] K. Spratte, L.F. Li, H. Riegler, Europhys. Lett. 25 (3) (11994) 211. [27] H. Riegler, J.D. LeGrange, Thin Solid Films 185 (1990) 335.
Colloids and Surfaces A: Physicochemical and Engineering Aspects 131 ( 19981 215 224
A
SURFACES
Molecular adhesion interactions between Langmuir monolayers and solid substrates Karlheinz Graf", Hans Riegler b., a blstitutfiir Phvsil,alische Chemie, LbffversitOt Mainz, Welder-Weg ] 1, D-55099 Mainz, Germam" b Max-Planck-lnstitut.li'ir Kolloid- und GrenzfldchenJorschung, Ruthn~:er Chaussee 5, D-124¢¢9 Berlin, Germtmv
Received 12 August 1996: accepted 7 October 1996
Abstract
Substrate monolayer adhesion interactions between differently prepared SiO 2 surfaces and Langmuir monolaycrs were investigated. From the relation between the contact angle and the surface tension of a kangmuir monolayer in the configuration of Langmuir wetting the work of adhesion between the substrate and the monolayer as a function of the molecular packing and the transfer ratios was determined. Thus the molecular work of adhesion of different SiO 2 dimyristoylphosphatidylethanolamine surfaces was quantified. The relation between the local adhesion interactions and the molecular packing and structure is presented and substrate-induced phase transitions are discussed. ~c~, 1998 Elsevier Science B.V. Kevwords. Langmuir wetting; Adhesion: Contact angle
I. Introduction
Knowledge of the adhesion interaction between monomolecular films and solid substrates is of crucial importance in the field of physisorbed monolayers. The adhesion contains contributions from the various possible interfacial interactions (van der Waals, acid base, etc.) and is thus of fundamental interest for the understanding of intermolecular interactions. It is also the key factor for the preparation and stability of Langmuir Blodgett multilayers. Static contact angle measurements are a widely used technique in surt:ace and adhesion science to investigate the interactions at interfaces between different bulk phases [ 1]. In the so-called Langmuir-wetting configuration (Fig. 1 [2]) it can be used to measure
* ('orresponding author. 0927-7757:98:$19.00 ~C, 1998 Elsevier Science B.V. All rights reserved. P l l SI)927-7757(96)03923-4
the interactions between monomolecular organic films (Langmuir monolayers) and planar solid substrates. Already in 1973 Clint and Walker [3] used contact angle measurements to determine the adhesion interaction between Langmuir multilayers of fatty acids with alkyl chains and partially fluorinated alkyl chains. For these systems they found that the interaction is dominated by van der Waals forces. Aveyard et al. [4-6] investigated the relation between the contact angle, the transfer ratio, and the headgroup ionization of li~tty acid Langmuir monolayers. They confirmed Bikerman's hypothesis [7] which states that quantitative Langmuir-Blodgett deposition occurs only at contact angles greater than 90 . Peng et al. [8] studied the transition of Y- to X-type LB-transfer of cadmium fatty acid salts as functions of the dipping time and the number of deposited monolayers. Their data agree with the theory of X-type
K Graf H. Riegler / Collohts Surfaces A: Physicochem. Eng. Aspects 131 (1998) 215-224
216
// ./
/ \
solid substrates. However, these investigations predominantly offered global values (e.g. the work of adhesion per unit area). No detailed information on the relation between the molecular properties (e.g. molecular packing) and the adhesive properties (e.g. the molecular work of adhesion) was obtained because of the largely unknown details on the molecular ordering and structure of the investigated films. We will present here static equilibrium contact angle measurements of Langmuir wetting and a new thermodynamic analysis of the data. We will show that from the data new and valuable information on the interactions between substrate and monolayer as a function of the molecular packing can be derived. For instance, for two different substrate surfaces substrate-induced structural changes in monolayers and the molecular work of adhesion as a function of the molecular packing will be presented and analysed.
2. Theoretical background
Fig. 1. Schematic of the contact angle measurement via laser beam reflection at the meniscus area in a Langmuir wetting configuration. The substrate is inclined for better access to the meniscus area (the equilibrium contact angles are independent of the substrate inclination). The contact angles are derived from the directions of the incoming and reflected beams and the substrate inclination.
film formation of Honig [9] who assumed that molecular detachment and overturn in the submerged outermost deposited monolayer lead to an increase of defects and eventually to the observed X-type deposition. All these and other investigations [10-13] show that contact angle measurements are well suited to investigate and quantify the wetting, adhesion and transfer properties of ultrathin organic films on
In Langmuir wetting (see Fig. 1) a Langmuir monolayer covers both the aqueous subphase and the submerged solid substrate [2]. With respect to interface forces this configuration is equivalent to a conventional wetting situation. In the case of a static equilibrium contact line Young's equation describes the relation between the three surface energies and the contact angle [14]: ~,SV = ~,SL 71_./LV COS 0
( 1)
where ~,sv 7SL and ?LV are the surface energies at the solid/vapour, solid/liquid, and liquid/vapour interfaces, respectively. 0 is the contact angle as defined in Fig. 1. From Eq. (1) the work of adhesion between the monolayer and the substrate surface can be derived as [14]: Wasv = ) ' L V ( 1 --COS O )
(2)
where Wsv is the change of interface energy upon replacing the air/monolayer/water interface by the air/monolayer/substrate interface. According to Gibbs the chemical potential of interfaces, ~, is
IC Grq/~ H. Riegler / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 215 224
related to the surface energy by [14]: 1
d~ = - - d7 F
(3)
where F is the surface excess. For insoluble monolayers the surface excess F can be replaced by the absolute surface concentration of the surfactant, or equivalently, by the area per molecule, Am: (4)
d~= -A m d/
Eqs. ( 1 ) and ( 3 ) can be combined: d(7 Lv cos O) = d) 'sv - d7 sL = YsL dp sL - / - s v d/~SV 15) In the case of equilibrium between all three interfaces (d/Lsv = d p se = d~tLv) one obtains: d() 'Lv cos O ) = ( F SL - F S V ) d # Lv
3. M a t e r i a l s and methods
(/~v __/-SL)
d() 'Lv cos 6)) -
]~V
~7)
For soluble surface active agents this equation was derived by Lucassen-Reynders some decades ago [15 17]. In the case of Langmuir wetting, Eq. (7) can be modified substantially. Because of the insolubility of the amphiphilic molecules in the liquid subphase, their surface excess at the solid/liquid interface, /SL, can be neglected. Hence, to an excellent approximation Eq. (7) can be written as: d(3 'Lv cos O) d;,LV
F sv -/-LV -
(8)
Or, alternatively according to Eq. (4): d("/Lv cos O)
d;. Lv
[18]. Hence, from Eq. (8), by variation of the surface energy yev, and by the simultaneous measurement of the corresponding contact angles, the transfer ratio /SV/FLV and thus the surface coverage at the solid/vapour interface, /-sv, can be determined. This determination of the transfer ratio is much more accurate than the usual method of comparing the area of deposited monolayers with the decrease of the floating monolayer area during the transfer process because it is independent of any errors arising, for instance, from partial monolayer collapse, as long as the conditions for Eq. 18) are valid. Finally, the combination of the results obtained via Eqs. (2) and (8) allows the quantification of the adhesion energy per molecule, V sv = W~aV/I -sv, at the substrate/air interface as a function of the molecular packing.
(6)
and together with Eq. (3): d;,L',
217
ALtov -A~v
(9)
For Langmuir monolayers, the molecular packing at the air/water interface, F Lv, and thus the surface tension, ?LV, can be varied in a controlled and easily measurable way by expansion or compression of the monolayer. Additionally, for many substances the area per molecule at the liquid/vapour interface, Am LV, and thus the absolute value of FLv, is well known, either from the isotherms, or from other measurement techniques
The experiments were performed with a homebuilt all-Teflon Langmuir trough. The surface tension was measured by the Wilhelmy method. The lipid L-c(-dimyristoylphosphatidyl-ethanolamine ( D M P E ) was obtained from Avanti and used without further purification. The subphase was ultrapure water (Milli-Q). As substrates we used silicon wafers with thermally grown oxide layers (400-600A) which were cleaned and prepared with a modification o f the RCA cleaning procedure [19]. After degreasing with organic solvent the wafer surfaces were prepared with either only a basic preparation step, SC-I, or both the basic and, additionally, an acidic cleaning procedure, SC-2. For SC-1 the substrates were placed at room temperature into a quartz beaker filled with a solution of 5 volume parts ultrapure water, l volume part ammonium hydroxide (30wt%, A.C.S. reagent, Aldrich), and 1 volume part hydrogen peroxide (30 wt%, nonstabilized, med. puriss., Merck). The solution was then heated to 8 0 C and kept for 10 min at this temperature. The substrates were then thoroughly rinsed with pure water. They were stored under water and used within 1 h for the contact angle measurements or the additional acidic surface preparation SC-2 was applied. For this the substrates were kept in a
218
K. Graf H. Riegler / Colloids Surfaces A." Physicochem. Eng. Aspects 131 (1998) 215 224
solution of 1 volume part hydrochloric acid (32 wt%, puriss, p.a., Fluka) and 1 volume part water in an ultrasonic bath for 5 min and were rinsed with water afterwards. The contact angles were determined from the reflections of a H e - N e laser beam hitting the threephase line area as shown schematically in Fig. 1. In this setup one part of the laser beam is reflected from the substrate surface, the other part comes from the meniscus surface adjacent to the threephase line. By simple geometrical calculations the contact angle is determined to an absolute accuracy of about 0.3 ° from the directions of the two reflected beams relative to the incoming beam and to the substrate surface orientation. The method is sensitive to relative changes of the contact angle of ~ 0.1 o. For easier access to the three-phase line area the substrate is usually inclined at ~40" relative to the air/water interface [20]. The substrate tilt has no influence on the static equilibrium contact angle, which is solely given according to the Young equation by the relative strengths of the surface tensions. Each contact angle value was measured after the surface pressure was adjusted, then the substrate was withdrawn several 100 pm (under concomitant monolayer deposition), the substrate movement was stopped, and finally the meniscus allowed to relax to equilibrium. Our criterion for equilibrium is that the contact angles changed by less than 0.1 ° within 20 rain. All measurements were performed with a subphase of pure water ( p H i 5 . 5 ) at 20°C.
4. Experimental results Fig. 2 shows the isotherm, i.e. surface pressure 0 r = 7 2 . 7 5 m N m - 1 _yLV) VS. the molecular area, of a D M P E monolayer on pure water at 20°C (pH,~5.5). The isotherm can be split into four regions with different molecular ordering [21]. Beyond ~ 80 A2 per molecule the monolayer shows the coexistence of a liquid-like (liquid-expanded, LE) and gaseous-analogue (G) phase. Between 80 and 66 A2 per molecule it is a homogeneous LE phase. At 66 ~2 per molecule (i.e. above G) a phase transition into the coexistence of the LE and a condensed LC phase ( liquid-condensed, LC )
50.0 L-c~-DMPE T = 20°C, pure water 40.0 ~ LC
30.0 Z E
LE/LC
LE
LE/G
20.O 10.0
~c i o.ooI
~ '
30
'
'
'
l
'
'
40
'
'
l
'
'
'
'
50
l
'
' ' ' 1 ' ' '
60
70
'
'
80
'
'
'
I
'
90
'
'
'
100
area [A2/molecule]
Fig. 2. Isotherm of a L-~-DMPE monolayerat 20 C on a subphase of pure water. The ranges with the different molecular ordering are indicated (G, gaseous: LE, liquid-expanded; LC, liquid-condensed). The pressure 7to marks the transition from the LE phase to the LC phase.
is observed. The phase coexistence ends below ~45,&2 per molecule where the monolayer is predominantly in a condensed phase. The variation of the molecular ordering is reflected in the plot of the equilibrium contact angle O between D M P E and SiOz vs. the surface pressure at the air/water interface (Fig. 3). As in the following Figs. 3-7 the full circles are data points for SiOz surfaces prepared only with the basic step SC-I, whereas the open circles represent those with both the basic (SC-1) and the acidic (SC-2) surface treatments. In general, the contact angles increase with increasing surface pressure. The contact angles of the surface with the basic surface treatment are roughly twice those of the surface with both basic and acidic treatments. At pressures corresponding to the LC phase at the air/water interface the contact angles of both systems increase approximately linearly with increasing surface pressure. The region around the phase transition pressure, ~c, is marked by conspicuous variations of the contact angles. The physics behind the contact angle variation become clearer upon plotting the spreading tension, yLv COS O, of the DMPE/SiO2 interfaces vs.
219
K. Grq/i H. Riegler ,; Colloids SurJ~tces A." Physicochem. Eng. A~7~ects131 (1998) 215 224 2.0-
SC-1 I SC-1 + SC-2
1.8-
>,
o o
9"
1.6-
0
1.4
•
SC-1
o
SC-1 + S C - 2
i
%
o o
W 3o.o-i
o
1.2-
o
o3 c-
o
Z 20.0-
0.8-
° o o o
10.0-
0.6 0.012
o
-
8 o i .m.. . . . . . . . . . . . . .
""/i ....20.0 .....25,0i
.
.
.
.
.
.
.
.
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
40.0
Fig. 3. (2ontact angles of D M P E SiO2 systems as a function of the surface pressure. (Q) Data for substrates prepared only with the basic step (SC-I). I i;) Data for adsorption onto surfaces additionally prepared with acidic rinsing (SC-1 + SC-2 ). 80.0 •
LE
L C
70.0
i //
60.0 Z
/ /
50.0
/
o "~
400
. // ///
300
,,"
,c
/
/ /
.i
/'
•
SC-1
o
SC-1 + S C - 2 |
I I
200 20.0
30,0
40.0
LE/LC 0.016
0.018
LC 0.020
0.022
0.024
0.026
Fig. 5. Transfer ratios, Fsv/'~ v, of DMPE onto SiO, as a function of the molecular packing at the air/water interface. 1~-v
o [o]
. . . . . . . . .
0.014
F TM [molecules/A 2]
LO
o
.....
0.0
/~c
LE
o
q)
1,0-
o
E
50.0 "ixv [mN/m]
60.0
7o0
80.0
Fig. 4. Spreading tension of DMPE SiO2 as function of the DMPE surface tension at the air/water interface (substrate sur-
face preparation: (Q) SC-I, ('~"}SC-I +SC-2), the surface energy at the air/water interface, 7Lv (see Eq. ( 8 ) . The plots (Fig. 4) show straight lines o f r o u g h l y unit slope in the L C r e g i m e w i t h m o d u l a t i o n s in the range o f the p h a s e transition pressures (insert o f Fig. 4). A c c o r d i n g to Eq. ( 8 )
(substrate surface preparation: (O) SC-I: ( 5 ) SC-I +SC-2). The vertical lines indicate the LE to LC phase transition on the water at ~z. and the beginning of the steep slope of the isotherm on the water.
the slope directly represents the transfer ratio. At low surface tensions the transfer ratio is close to unity, whereas around 7zc an increased transfer ratio is found. These transfer ratios, as derived from the data of Fig. 4 and Eq. ( 8 ), are shown in Fig. 5. For the solely SC-l-treated SiO2 surface one observes an increase of the transfer ratio of up to 1.7 in the LE region below the phase transition. For the surface which was additionally SC-2 treated an increase in the transfer ratio below the phase transition is also observed, but it is much less pronounced and increases only much closer to the phase transition. In the LC regime the transfer ratios of both surface treatments are approximately unity with a tendency to slightly lower values for the basically treated surface. Fig. 6 presents the adhesion energies per unit area, I,t -SV ~a, as a function of the surface energy at the air/water interface. The data are extracted via Eq. (2) from the experimental data of Fig. 3. Wsav depends strongly on the surface treatment and lies between ~ 7 and 8.5 mN m - 1 with the SC-1 treated surface and between 1.5 and 3 m N m 1 for the SC-1 + S C - 2 treatment. The phase transition regime around lzc is marked by abrupt changes in 14/SV aa, In general, in the LE phase the adhesion energies per area are smaller than in the LC phase. Combination of the data from Figs. 5 and 6 yields
K. Graf H. Riegler / Colloids Surfaces A." Physicochem. Eng. Aspects 131 (1998) 215 224
220
ing at a level of about 0.6 KJ mol - 1. In both cases the abrupt changes of W sv near the phase transition (see Fig. 6) are now largely smoothed by the normalization of the adhesive energy relative to the molecular area instead of the unit area.
10.0SC-1
]
SC-I + SG-2 8.0
E
6.0
E
LC
LE
4.0
5. Discussion 2.0
0.0 20.0
30.0
40.0
50.0
60.0
/I; 7 0 . 0
¢-v [.N/m]
80.0
c
Fig. 6. Adhesion energies, W'~.~',of DMPE-SiO2 as a function of the air/water interface energy },ev (substrate surface preparation: (O) SC-I; (©) SC-1 +SC-2). The vertical line marks the phase transition at nc from the LE phase and the LC/LE coexistence regime at the air/water interface.
4.00 3.503.00 ~"
2.50
2o0
~',~ 150 >
1.00
:
~ sc-1
SC-1 + SC-2
I
-
0.50
"
0.00 . , . , . . . , . , . 0.012 0.014 0.016
, . . . ~ . . .,.,,, 0.018 0.020 0.022
F sv [molecules/A
, . . 0.024 0.026
2]
Fig. 7. A d h e s i o n energies per molecule, V sv = W~,~'/Isv, o f D M P E - S i O 2 at different m o l e c u l a r p a c k i n g densities on the
substrate, /sv (substrate surface preparation: (O) SC-1; (O) SC-1 + SC-2). the work of adhesion per molecule, Vsv, as a function of the molecular packing on the substrate, ysv. The results are presented in Fig. 7. Again one can see the difference between the two substrate surface treatments. For the solely SC1-treated surface one observes a steady decrease of I/sv from 3.5 kJ mol -1 at loose molecular packing (LE phase) to half this value in the densely packed state. Obviously, the acidic treatment strongly decreases the attractive interaction and keeps it largely independent of the molecular pack-
The Langmuir wetting configuration is well suited for the investigation of the relations between the contact angle and the various interfacial energies because it is possible to vary selectively and precisely the interracial energy of one interface (~,ev). Thus it is possible to examine the contact angle O as a function of the surface energy, },ev, of the monolayer phase, and of the substrate surface preparation. The work of adhesion, W~aa v, the transfer ratio, FSV/I+v, and the molecular work of adhesion, V,d, sv can be derived from the data. Eqs. (2) and (8), on which our data analysis are predominantly based, can be used if the conditions for their derivation are valid in the presented cases. In detail this means: (1) the three-phase contact line is in mechanical equilibrium (i.e. validity of Eq. (1)); (2) the surface energy (pressure) measured with the Wilhelmy system at the plane air/water interface is indeed the 7Lv of Eq. (1); (3) the observed contact angle is O of Eq. (1); (4) the chemical potentials of the three interfaces are in equilibrium (validity of Eq. (6)); (5) the adsorption of amphiphilic molecules at the substrate/subphase interface can be neglected
(Fsc~0). It should be remarked here that these conditions do not include the necessity for the monolayers to represent equilibrium systems with respect to surfactant molecule exchange between the monolayer and the liquid subphase. The data interpretation presented is valid for surface pressures below and above the equilibrium spreading pressure as long as the surface energy is defined and measurable and conditions (1)-(5) are fulfilled. In the derivations of Eqs. ( 1 )-(9) bulk quantities (e.g., surfactant solubility, etc.) play no role. We only assume
h2 Grq¢: H. Riegler / Colloids Surfaces A: Physicochem. Eng. Aspects 131 I 1998) 215 224
that the surface excess of the surfactants is identical to the absolute concentration of surfactant molecules localized in a monolayer at the air/water interface. This is a well established perception for the surfactant molecules investigated. We are convinced that the receding contact angles (monolayer adsorption) presented and discussed in this report are virtually the equilibrium contact angles because numerous measurements have shown that they can be reproduced with high accuracy. This can be understood because the energy barriers for adsorption are small. In contrast, for monolayer desorption, energy which might have been dissipated irreversibly upon adsorption, has additionally to be brought up by submerging the substrate beyond its equilibrium value. Indeed, we observed a contact angle hysteresis, but it was quite small, typically less than 0.3 ~', Hence, even if the real equilibrium contact angles were somewhere between the measured adsorption and desorption values, this still would not invalidate the use of Young's equation; it would only lead to larger errors. Compared to the hysteresis of more than 1 typically found in conventional contact angle measurements [22] our hysteresis is remarkably small. This may originate in our better defined surfaces (interlaces). Both interfaces which are most in danger of contamination - the air/water and the substrate/air interface - are already covered and virtually saturated with surface active substances (the monolayer). Hence, they have a much reduced susceptibility to contaminant adsorption and thus surface energy reduction. In the case of bare surfaces, often even minor amounts of adsorbent may influence the contact angle and cause significant hysteresis because differences between the surface energies of different surfaces and not absolute values determine the spreading tension and thus the contact angle [23]. There is no reason for a substantial difference between the local surface energy, 7Lv, of Eq. ( 1 ) and the value measured at the plane air/water interface. Capillary effects can be neglected because the radius of curvature of the meniscus is typically much more than l mm, which corresponds to a capillary pressure of less than 150N m -2 compared to the ambient air pressure of 105N m--'. Also, it is well established that, for the fairly fluid
221
monolayers used in this study, local surface pressure changes equilibrate within less than seconds to uniform pressure values everywhere at the air/water interface (at least for the trough dimensions used in this study). The contact angle measured by the method presented in the materials and methods section represents the O of Eq. (1) because it expresses the macroscopic change of the surface energy component, 7 cv cos O, as demanded by the mechanical force equilibrium due to the Young equation. Any microscopic contact angle te.g., on a molecular scale, if one can be defined l is not relevant in this context. The equality of the chemical potentials at the air/water interface, It Lv, and at the substrate/air interface, tt sv, presumes the exchange of monolayer molecules between the two interlaces locally at the three-phase contact line. At the air/water interface the molecules will be sufficiently mobile laterally. At the substrate/air interface it suffices for the validity of this study if only the first few rows of amphiphilic molecules adjacent to the three-phase line are sufficiently mobile laterally to arrange themselves in equilibrium according to the local energetic environment. We do not claim that ;,sv and F sv arbitrarily close to the three-phase line, i.e. those values which are the topic of the study presented here, are always necessarily identical to the ;,sv and /sv anywhere on the solid substrate surface. The 7sv and F sv may be uniform over larger areas, for example in the case of monolayer transfer in the course of a very slow substrate upstroke (Langmuir-Blodgett transfer). Fluorescence microscopy observations indicate that there is some molecular exchange and mobility on the solid substrate over distances up to several tens of microns away from the three-phase contact line. The assumption of equilibrium between the chemical potentials at the air/water and solid/air interface and at the substrate/subphase interface, it sv, is vindicated by the small contact angle hysteresis which shows the reversibility of the monolayer adsorption onto the solid. The substrate surfitce without monolayer coverage is perfectly hydrophilic ( O = 0 : with pure water). With an adsorbed monolayer it is quite hydrophobic. If a substrate
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K. Graf H. Riegler / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 215-224
coated with a monolayer is quickly submerged into a subphase (with or without a floating monolayer) and then the substrate is kept stationary, one observes advancing contact angles much larger than the receding (equilibrium) values. This 'static' advancing contact angle then relaxes, which typically takes a long time (tens of minutes to hours), until it eventually approaches the receding value to within less than 1°. This shows that there is monolayer ablation from the solid surface, i.e. surfactant molecule transport into the subphase or, much more likely due to the very low solubility of the amphiphilic molecules, to the air/water interface. All this, the monolayer ablation, the very low surfactant solubility, and the fairly small contact angle hysteresis, strongly indicate that /-sL is very small and can be neglected according to Eq. (8). This is further corroborated by the remarkably linear section-by-section behaviour of the spreading tension vs. surface energy which we have always obtained for many different monolayer/substrate configurations (results to be published soon). As a general result we find that the contact angle depends in a non-monotonic and non-obvious way on (a) the surface tension, ?,Lv, (b) the monolayer phase, and (c) the substrate surface preparation. This is as expected because interracial energies are a function of long-range and local interactions [24,25]. The local interactions are determined by the molecular packing which is altered by phase transitions and chemical modifications of the surfaces. The impact of phase transitions on the interfacial energies is obvious in the data presented in Fig. 3. If the surface is prepared with procedure SC-1 a substrate-mediated phase transition of the monolayer on the substrate is observed at ~ 2 mN m - 1 which is responsible for the kink in the contact angle behaviour at this pressure. If the substrate is prepared with both SC-1 and SC-2, the phase transition occurs at higher pressures ( ~ 5 mNm-1). These phase transitions can also be identified in the plots of W ~ (Fig. 6). The loop in the contact angle and I4~,v behaviour at ~2 mN m-1 for the SC-1 surface preparation is reproducible and observed with advancing and receding contact angles. This peculiar behaviour is not yet understood but it might be responsible for
the dynamic contact angle instabilities observed in this pressure range [26]. The substrate-induced phase transitions can, independently of the kinks in the contact angle behaviour, also be identified from the transfer ratios. In the case of the SC-l-treated surface the transfer ratio is between 1.4 and 1.7 at molecular densities between ~0.0140 and 0.0150 molecules z (Fig. 5). This means that condensation begins at about 71 ~2 per molecule, in the middle of the LE phase region, at ~ 3 m N m 1 below 7rc, in quantitative agreement with fluorescence microscopy studies [2]. For the surface treated with SC-1 and SC-2 the transfer ratio is significantly larger than 1 only very close to ~c, which again is in qualitative agreement with fluorescence microscopy observations. The data of Fig. 5 were derived by using the local slope of the spreading tension, i.e. the slope calculated from the measurement point which belongs to the corresponding molecular packing, / ~ v and the two adjacent measurement points of the next higher and lower molecular packing. The results are virtually identical to those derived (not shown) with the slopes calculated from a wider range (see slopes of Fig. 4). We present here the qocal' data, although they show a larger data scattering because they corroborate the accuracy and meaningfulness of our method. The importance of local interactions is also manifested by the impact of the substrate surface preparation on the contact angle and thus on the monolayer adhesion. The adhesion interaction differs by up to a factor of 5 depending on the surface treatment (Fig. 7). It is not yet clear what is responsible for the different adhesion behaviour. It can only be assumed that the SC-2 treatment increases the degree of surface protonation. Further, it can be expected that the interface between the deposited monolayer and the substrate surface is hydrated. The surface preparation may influence the hydration and/or the degree of dissociation of the silanol and/or the DMPE headgroups. A change of the degree of dissociation of the DMPE headgroups would be in agreement with measurements (to be published) of the work of adhesion of DPPC (dipalmitoylphosphatidylcholine) on the same substrate. It is found that DPPC is much Jess sensitive to different surface
I( Grq[i H. Riegh, r / ColloMs Sur/~wes A: Physicochem. Eng. Aspects 131 (1998) 215 224
treatments. The DPPC headgroups cannot dissociate whereas the D M P E headgroups start to deprotonate at p H i 8 in monolayers [27]. It is also conceivable that the headgroup hydration and/or the degree of dissociation depend on the molecular packing. This would lead to the variation of the molecular work of adhesion as observed (Fig. 7). The measured molecular work of adhesion is in the range of k T ( ~ 2 . 5 kJ mol ~) for the SC-1 treatment or substantially smaller (SC-1 and SC-2 treatment). At first glance this might call into question the stability of the three-phase line. However, any deformation of the straight threephase line, for example by locally ablating a few monolayer molecules and shuffling them back to the floating monolayer, necessitates not only an energy contribution g .,sv ~d. but is also penalized by an unl:avourable increase of the total surface area and local surface bending. The establishment of the contact angle and the (straight) shape of the three-phase line is a cooperative phenomenon.
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monolayer area loss with the transferred area. By combining both the adhesion and the transfer ratio data we obtain m o l e c u l a r adhesion data. We investigated the work of adhesion, the transfer ratio, and the molecular work of adhesion of a DMPE-SiO2 system. The molecular packing and phase behaviour of D M P E monolayers are well known from independent methods, therefore they are especially well suited in this context to gain insight into structure-property relations. We find that the transfer ratios, the work of adhesion, and the molecular work of adhesion depend on the molecular packing of the floating monolayer, the deposited monolayer and on the substrate surface preparation. The molecular mechanisms governing these dependencies are not yet understood but it can be assumed that hydration and/or headgroup dissociation effects (local pH ) play an important role. The contact angle data confirm quantitatively the substrate-mediated condensation hitherto only observed by fluorescence microscopy.
6. Summary and conclusion The quantification and modification of monolayer substrate interactions is of key importance for the controlled deposition of water-insoluble molecules onto solid substrates (e.g., for the LB technique). In this paper we show that Langmuir wetting is suitable for measuring the relations between the contact angle, the various interfacial energies and the work of adhesion. Interfacial energies are governed by long- and short-range interactions. The latter particularly can be well modified and controlled in the Langmuir wetting setup. Thus we investigated the adhesion interactions as a function of various parameters relevant for the monolayer-substrate adhesion. By using a beam reflection method we could obtain an angular resolution of the contact angle measurements such that details of the relation between the molecular ordering (packing) and the monolayer sttbstrate adhesion interaction could be quantified. We further show that the contact angle data also contain quantitative information on the transfer ratios which are much more precise than those usually derived by comparing the floating
Acknowledgment We thank the Deutsche Forschungsgemeinschaft for financially supporting the investigations (Sachbeihilfe Ri 529/8-1) and Wacker Siltronic G m b H Burghausen for generous donation of the silicon wafers. Helpful discussions with Helmuth M6hwald, Bob Aveyard, and Terry Blake are much appreciated.
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