Langmuir 1992,8, 2084-2086
2084
Experimental Observations of Langmuir Film Formation J. M. B. Fernandes Diniz* Chemistry Department, Reading University, Reading RG6 2AD, U.K.
D. A. Armitage, R. G. Linford, and V. H. Pavlidis Chemistry Department, Leicester Polytechnic, Leicester LE1 9BH, U.K. Received March 23, 1992.I n Final Form: June 15, 1992 The resulta of a study of the time that should be allowed prior to the compression of a Langmuir film or the deposition of a Langmuir-Blodgett film are presented. The concept of a “stability isotherm” of surface pressure against time is introduced. The resulta obtained from a Lauda Langmuir film balance suggest that neither compression nor deposition should commence until the last stage of the stability isotherm is attained, i.e., until a conatant surfacepressure is reached. A more rapid equilibrationhas been attained when large volumes of cast material are subdivided. Furthermore, this technique enables the solubility or insolubility of the film solvent in the subphase to be determined. 1. Introduction This experimentally based study addresses some questions often raised by the practising experimenter in the Langmuir/Langmuir-Blodgettarea and recommends a procedure to improve the film casting process. The study was motivated by the observation of surface pressure readings, in a Lauda Langmuirfilm balance, prior, during, and after an octadecanoic acid solution was cast onto purified water. The investigation was only possible after modifying the original Lauda software. Armitage incorporated a subroutine which allowed the concurrent recording of surface pressure and time, before film compression commenced in a normal ns/a, isotherm. The behavior of the Langmuir film was recorded immediately before casting and the subsequent profile of surface pressure versus time was followed. This behavior is referred to asa “stability isotherm” in this article, following Armitage’s suggestion. A stability isotherm is composed of four different regimes. The first one is the initial baseline. This is followed by a casting peak (or peaks, when the sample is subdivided). The third, and most important section is the decay of the transient casting peak to the equilibrium value that forms the fourth section. This section may have two different profiles. In a gaseous-like film the surface pressure converges toward the baseline. In a liquidlike film the surface pressure approaches a nonzero value. These data are important in a variety of purposes, one of which is in determining the time the experimenter has to wait, after casting a film onto an aidliquid interphase, before compression may be started. This particular detail has not been thoroughly addressed in recent references to experimental technique.lV4 2. Experimental Section The purified water used tu subphasein these experiments was obtained through a reverse osmosis system (Millipore MilliRO 15)of polyamide membrane, followed by passage through a Milli-Q system (both provided by Millipore Corp.). This is a preassembled wall-mounted unit which consists of a series of four identical headhousing assemblies and a fiial filter. The first four stages use expendable cartridges, namely of activated 2.1.
* Author to whom correspondence is to be addressed.
(1)Gallety, G. S.; Guiseppi-Elie,A. Thin Solid Films 1985,132,163. ( 2 ) Pethica, B.A. Thin Solid Films 1987,152,3. (3)Mingins, J.; Owens, N. F. Thin Solid F i l m 1987,152,9. (4)Taylor, D.M.; Oliveira, 0. N., Jr.; Morgan, H.Thin Solid F i l m
1989,173,L141.
carbon,two ion-exchange units (containinga strong acid/strong base mixed-bed deionizer that removes dissolved inorganic contaminants, each cartridge having an ion-exchange capacity of 6840ppm as CaCOd, and an Organex fiiter to remove organic impurities. The resistivity of water inside this system was 17 MSl. 2.2. A solution of octadecanoic acid (Aldrich, 99+ %) in diethyl ether (Aldrich, 99.9% GLC) was prepared with a concentration of 100.1mg of octadecanoic acid dissolved in 100.0 mL of solvent. Volumes of this solutionwere cast using two different sizes of Hamilton microliter syringes, 100 rL and 250 rL. 2.3. All experimentswere performed at 15.0 O C inside a Class 500 Clean Room with a Lauda Langmuir f i i balance, Model FW 2, controlled by an Amstrad PC1512 DD computer. 2.4. The trough was thoroughly cleaned by the following procedure: (i)the tankwas emptied,(ii)200mL of acetone (BDH, HPLC grade) was poured into the trough, followed by purified water until the rim of the trough was covered, (iii) the tank was emptied and rinsed 3 times with pure wakq (iv) the manufacturer’s automatic surface cleaningof the trough was undertaken. 2.5. The trough was cleaned, as described above, before every experiment. 2.6. The f i i was allowed to spread up to the maximum area available (927 cm2)and the barrier was not moved throughout the experiments.
3. Rssults and Discussion Figure 1shows a typical stability isotherm which was obtained by the dropwise addition of 150 pL of octadecanoic acid solution at 15.0 “C,added in one portion. On an expanded timescale, not shown here, the rise of the peak shows oscillationswhich reflect the dropwise addition process. The maximum amplitude of fluctuation in surface pressure that was found in regions I and IV was 0.1 mN/ m, which is well within the tolerance value specified by the film balance manufacturer. Larger values indicate contamination. The top of the casting peak which forms the second part of the stability isotherm provides the true time origin of the decay process. The casting process was performed in a dropwise fashion and the peak maximum, which corresponds to the addition of the last drop, is therefore taken as the start of the stage 111decay process. For a gaseous-likeisotherm, the end of the decay process is when the baseline value is reached. Figure 2 shows the comparison between the decay profile for a single addition of 0.150 mL (Figure 2a) and the effect of using three portions of 0.050 mL from the same solution of octadecanoic acid. The time allowed to elapse between each portion was the minimum that could be achieved in
0743-7463/92/240S2084$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8,No. 9, 1992 2085
Letters
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Figure 1. Gaseous-like stability isotherm, obtained by casting 0.150mL of octadecanoic acid solution (1.001mg/mL in diethyl ether) at 15.0O C , added in one portion. Stages I-IV are described in the text.
Figure 3. Liquid-like stability isotherms for 0.200 mL of octadecanoic acid solution (1.001mg/mL in diethyl ether) at 15.0 O C , added in one portion.
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normal operation, i.e., it was the time required to refill the syringe (=708). The decay times are for the former, 1470 s, and for the latter, 460 s. Figure 2 shows that the base-
line is reached much later for the one-aliquot process than for the multiple-stageaddition. These results suggest that if it is necessary to use a large volume of cast material, it would appear to be advantageous to subdivide the volume into smaller fractions. It is interesting to note that the decay time associated with stability isotherms may provide useful information concerning the presence of surface contaminated water. For 0.050 mL of octadecanoic acid solution cast onto surface contaminated water, the corresponding decay time obtained was 2070 s, whereas for the noncontaminated water the decay time was 210 s (10% of the former situation). A different stability isotherm is shown in Figure 3. In this case 200 p L of octadecanoic acid solution was cast in a single stage. There is an excess of octadecanoic acid molecules to cover the available area with a monolayer. As a consequence of this, the equilibration pattern of this liquid-like ordering process is markedly different from the ones previously shown. The stage IV plateau corresponds, as expected, to a nonzero value. An interesting
2086 Langmuir, Vol. 8, No. 9, 1992
--
0.1
Letters Keeping the nature of stability isotherms in mind, two experiments were designed. In the first experiment, drops of pure solvent (diethyl ether) were cast on top of the water surface. The resulting curve is represented in Figure 4. In the second experiment, purified water was cast on the surface of water contained in the trough. The curve obtained is represented in Figure 5.
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minimum is observed during the decay stage. The shape of stage IV also suggests that the system evolves in such a way as to maximize the surface excess concentration of the film monolayer at the bottom. In other words, the system minimizes the number of octadecanoic acid molecules that will form an incomplete second layer. In a different way, this experiment confirms the expectation that, once compressed beyond the collapse pressure, the film will regain its gaseous-like state, provided that a surface expansion process takes place to a sufficient extent.
4. Conclusions Figure 4 and Figure 5 seem to suggest the following conclusions: (a) The nature of the gaseous-like stability isotherms is related to the spreading of the liquid solution dropped onto the water surface, and it is not a real surface pressure measurement. (b) The main profile of the stability isotherm is associated with the evaporation of the solvent; thus the decay time in gaseous-likestability isotherms corresponds to a period of time of solvent evaporation that is possible to monitor by surface pressure readings. As Cadenhead and Kellner have pointed out? surface (Volta) potentials are more sensitive indicators of solvent retention than surface pressure readings. (c) This technique allows a precise control of the solubility or insolubility of the film solvent. (d) The liquid-like stability isotherms present a first part (before the minimum) that is solvent-evaporation dependent, and a second part (after the minimum) that is controlled by molecular diffusion. (e) Ether seem effectivelyto behave as a water-insoluble solvent in this context. (5) Cadenhead, D.A.; Kellner, B.M.J. J. Colloid Interface Sci. 1974, 49, 143.
