Silver metal liquidlike films (MELLFs). The effect of surfactants

Optical response of concentrated colloids of coinage metals in the near-ultraviolet, visible, and infrared regions. I. Farbman , O. Levi , S. Efrima. ...
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Langmuir 1991, 7, 267-271

267

Silver Metal Liquidlike Films (MELLFs). The Effect of Surfactants D.Yogev and S. Efrima* Department of Chemistry, Ben-Gurion University of the Negev, P.O.Box 653, Beer-Sheva, Israel 84105 Received December 13, 1989. I n Final Form: June 7, 1990 This is a study of the effect of surfactants on the production and stabilization of silver metal liquidlike films (MELLFs). The main role of the surfactant is in stabilizing the silver MELLFs and improving their properties (reflectivity, “fluidity”). A variety of different surfactants were found to be active, and from those investigated, anionic fluoroalkyl surfactants seem to be the most effective. In the case of anionic surfactants the countercation has a significant effect on the silver MELLF, especially if it is a surfaceactive agent in itself. We report on the effect of the surfactants on the interfacial tension and their effect on the measured reflectivities of the MELLFs and discuss the results in the context of the interfacial colloidal model of silver MELLFs. Introduction The novel silver MELLF (metal liquidlike film) system was introduced in several recent reports.’-7 This is a unique system of an interfacial colloidal film of silver that is located between two immiscible liquids: water, on the one hand, and an organic solvent, on the other hand. This interfacial film exhibits rheological properties resembling those of a liquid, yet it retains the metallic luster and high reflectivity similar to those of a continuous silver film. A series of different quantitative investigations and qualitative explorations indicate that the silver MELLFs are highly condensed colloidal suspensions, confined a t and near the interface of the two liquids that comprise the system. Thus these films are associated with a unique blend of colloids and interfaces with an unusually high concentration of the colloid. The MELLF films are produced by chemical reduction of silver ions in an aqueous phase which is placed over an organic solvent. Usually such a reduction yields granular and dull interfacial films. In order to form a MELLF, a specific recipe involving several components is required. A MELLF is produced when the ammoniacal aqueous solution contains, in addition to silver nitrate, also small quantities of additives such as anisic acid and a surfactant, and only when one of a given set of organic liquids is used. Changing any of these components, by nature or amount, often significantly affects the quality of the film or results in the formation of a granular film, rather than a MELLF. Very recently Gordon et ale8reported on an alternative procedure that produces shiny interfacial silver films, similar, it would seem, to those described above. These films are based on different additives (some metal complexes) and do not contain surfactants. In a separate paper5 the effect of the additives and that of the organic liquid was discussed in detail. In the following the influence of the surfactant on the production, stability, and nature of silver MELLFs is investigated. In ,

(1) Yogev, D.; Efrima, S. J . Phys. Chem. 1988, 92, 5754. (2) Yogev, D.; Efrima, S. J. Phys. Chem. 1988, 92, 5761. (3) Yogev, D.; Efrima, S.; Kafri, 0. Opt. Lett. 1988, 13, 934. (4) Yogev, D.; Deutsch, M.; Efrima, S. J. Phys. Chem. 1988,93,4172. (5) Yogev, D.; Efrima, S. Chemical Aspects of Silver Metal Liquidlike

fact, silver MELLFs can be produced without using any surfactant. However, the MELLFs thus produced are not stable in time and also tend to be easily disrupted by mechanical perturbations. In the absence of a surfactant the MELLFs are generally not as reflective and are thinner than otherwise, as evidenced by their relatively high transmittance of light. In addition, a large fraction of the silver is distributed in the organic phase, as a colloidal suspension. The systems produced by Gordon et ale also do not contain surfactants but are reportedly stable. They ascribe a putative surfactant role to the metal complexes they use as additives. The goal of this study is to investigate in detail the role of the surfactant in a MELLF system produced by our procedure’ and the dependence on its structure. On the basis of the colloidal model we proposed for the silver MELLFl one expects the silver cores to be charged by the adsorption of ions. This charge is important in stabilizing the colloid and is responsible, a t least in part, to the “fluid” properties of a MELLF; i.e., it prevents the sticking of the particles and allows them to move easily one with respect to the other. One would like to know whether the surfactant too has a similar “electric” effect in the MELLF. To answer that we have investigated a variety of surfactants with different headgroups: anionic, nonionic, and cationic. Mixtures of anionic and cationic surfactants were also investigated, as well as the effect of replacing an inorganic countercation of an anionic surfactant with an organophilic cation. We also addressed the question of the more specific interactions that are possible within the film such as the dependence of the activity of the surfactant in MELLF systems on the nature of the polar headgroup (not merely its charge) or the hydrophobic “tail”. Specifically hydrocarbonic chains were compared to perfluoroalkyl chains. We also report on the measurement of the interfacial tensions between the aqueous and the organic phases, in the presence and the absence of a surfactant, in the search for a correlation with the nature of the silver MELLF which is formed, as given, for instance, by the reflectivity. The effect of the MELLF itself on the interfacial tension was also measured and discussed.

Films, submitted for publication. (6) Yogev, D., Kuo, C. H.; Neuman, R. D.; Efrima, S. J . Chem. Phys.

Experimental Section

1989, 91,3222. (7) Yogev, D.; Shtutina, S.; Efrima, S. J. Phys. Chem. 1990,94, 752. (8)Gordon, K. C.; McGarvey, J. J.;Taylor, K. P. J. Phys. Chem. 1989, 93, 6814.

The silver MELLFs were prepared by following the procedure outlined previously.’ The surfactants were investigated in the concentration range of 0.01-0.1 % active material (w/w). We

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0 1991 American Chemical Society

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Table I. Reflectivities, Interfacial Tensions, and Optimal also tried to produce the MELLFs without adding the anisic Surfactant Concentrations of Silver MELLFs Formed with acid, which was found to be so crucial in the past. In addition, Various Surfactants each surfactant was investigated with four different organic liquids: dichloromethane (C12Met), 1,2-dichloromethane (Clzinterfacial Et), chloroform, and carbon tetrachloride. The latter served as concn,* tension,c refleca reference and a control as it invariably gave interfacial granular name tYl)e" T dyn/cm tivity,d 7; films, rather than MELLFs. (Gordon et a1.8 report that they Surfactants That Produced MELLFs formed a film similar to a MELLF also over CC14.) 47 Monflor 32e FC, A + C 0.03 10.1 The surfactants that were investigated are listed below: Mon12 0.1 15.7 Monflor 31f FC, A flor 32, CloF190-p-phenylsulfonatetri-n-butylammonium salt, 33 0.025 16.0 Zonyl FSC FC, C 3OP, (w/w) active solids in 2-(2-butoxy)ethanol, produced by 12 0.01 14.0 Zonyl FSPf FC, A 0.03 21.9 35 Zonyl FSN FC, N ICI; Monflor 31, CioFlsO-p-phenylsulfonate sodium salt, 30 70 34 FC98 FC, A 0.03 26.0 (w/w) active solids in isopropyl alcohol/water (1:2) (w/w), 40 FC 99 FC, A 0.05 20.0 produced by ICI; Monflor 53, nonionic fluoroalkyl surfactant, 21 FC, A 0.1 16.0 FC93 loopr active, produced by ICI; Zonyl FSP, R&HzCH2P(O)FC, A 0.1 16.0 36 FC 120 (ONH1)235$ active solids in water 4 5 % , 2-propanol 20% (w! FC, A 0.03 22.6 42 FC~ 143e w), produced by Du Pont; Zonyl FSA, R ~ ~ H ~ C H ~ S C H ~ C H ~ C O LI 21 HC, N 0.1 22.4 Triton X-100 50°r active solids in water 250C,2-propanol25 (w/w), produced 0.08 28.7 21 octadecyl mercaptan HC, N by Du Pont; Zonyl FSN, R&H2CH~0(CH2CH20),H40% active 0.03 13.0 34 Merpasol DIO 60 HC, A solids in water 3 0 ° C ,2-propanol 3 0 " ~ (w/w), produced by Du Surfactants That Did Not Produce MELLFs 50 % Pont; Zonyl FSC, R&H*CH~SCHZCHZN+(CH~)~CH~SO~Monflor 53 FC, N 0.03 21.5 active solids in water 25"( and 2-propanol 2 5 7 (w/w), produced FC, A 0.01 26.8 Zonyl FSA by Du Pont; FC 128, potassium fluorinated alkyl carboxylates, FC, A 0.03 20.2 FC 128 produced by 3M; FC 99, ammonium perfluoroalkylsulfonates FC, A 0.06 19.0 FC 129 25", (w/w) active solids in water, produced by 3M; FC 98, FC, C 0.1 15.0 FC 134 potassium perfluoroalkylsulfonates 100p~ active, produced by FC 135 FC, C 0.06 16.1 3M; FC 143, ammonium perfluoroalkylcarboxylates looo' active, FC, N 0.1 17.5 FC 170 produced by 3M; FC 120, ammonium perfluoroalkylsulfonates Merpisap AP85W HC, A 0.1 6.8 250C,in 2-butoxyethanol37.5$ andwater 37.5% (w/w),produced HC, A 0.1 5.0 Merpisap AP9OP by 3M; FC 93, ammonium perfluoroalkylsulfonates 25% in 2-proHC, A 0.04 11.0 H-17-5 panol 20', and water 55% (w/w), produced by 3M; FC 129, The type specifications are as follows: FC, perfluoro chemical; potassium fluoroalkyl carboxylates 50'1 in 2-butoxyethanoll4%, HC, hydrocarbon; A, anionic; C, cationic; N, nonionic. This is the ethanol 4 O, ,and water 32 rr ,produced by 3M; FC 134, fluorinated concentration (weight percent) that was used when the interfacial alkyl quaternary ammonium iodides 5 0 " ~in 2-propanol 33% tension was measured. For those surfactants that produced a and water 17"1 (w/w), produced by 3M; FC 135, fluorinated MELLF, this is also the optimal concentration for its production. alkyl quaternary ammonium iodides 50"" in 2-propanol 33% This is the interfacial tension measured between dichloromethane and water 17'[ (w/w), produced by 3M; FC 170, fluorinated and the ammoniacal aqueous solutions containing 0.05 M AgN03, alkyl poly(oxyethylene)ethanols, produced by 3M; Triton X-100, 0.lpI anisic acid, and the surfactant. This is in the absence of a octyl phenoxy poly(ethoxyethanol), produced by Sigma Chemical MELLF. Typical scatter of the results is 0.2 dyn/cm. The organic Co.; Octadecylmercaptan, Weitzman Institute, Israel; Merppaphase was dichloromethane. The typical scatter of the results is 2 % . e Very stable and bright MELLFs. f MELLFs of very poor stability. sol DIO 60, diisooctylsulfosuccinate sodium salt 60 % active solids in water 32' and an unspecified solvent 870,produced by KemResults pen, West Germany; Merpisap APSOP, linear sodium dodecyl benzenesulfonate 85 Yo active solids, produced by Kempen; MerTable I lists the surfactants according to whether or not pisap AP85W, linear sodium dodecyl benzenesulfonates 85 YC they formed silver MELLFs over at least one of the four active solids, produced by Kempen; H-17-5, alkylaryl polyglycel organic phases used. These experiments were carried out ether sulfate, fatty acid alkanolamide mixture 75 7 active solids, with various concentrations of the surfactant and under produced by Kempen. several different conditions. As mentioned previously, All the surfactants were used, as such, without any purification. over CC14 we never obtained a MELLF. (Gordon et a1.8 Most of them, one should note, are mixtures containing a range reported the formation of MELLFs over CCl, using their of hydrophobic chains. procedure.) This table also gives the values of the The procedure for replacing the counterion is demonstrated interfacial tension measured for the ammoniacal aqueous below, where one starts from Monflor 31 and replaces the sodium solution (containing silver nitrate, anisic acid, and the surion with the tri-n-butylammonium cation to obtain Monflor 32. factant) in contact with dichloromethane. These interAn aqueous solution of the surfactant is acidified with 1 N H2SO4to liberate the free acid. This acid is extracted into an organic facial tensions were measured at the surfactant concenphase (chloroform, ClZMet, etc.) and then it is neutralized with trations specified in Table I. These concentrations are tri-n-butyl (or tri-n-octy1)amine yielding the corresponding salt. those that were found optimal in producing a MELLF. Finally the solvent is removed and the surfactant is redisolved We also report the reflectivities of the MELLFs measured in a desirable solvent. In the case of trimethylamine and tetwith a He/Ne 632.8-nm line. The reflectivities give one ramethylammonium hydroxide, which do not desolve in the quantitative measure of the quality of the films. Our organic phases we used, the procedure described above was experience shows that the stability of the MELLFs (the replaced with a simple mixing of these materials with the anionic long-term stability as well as the stability with respect to surfactants, in proper quantitative proportions. mechanical mishandling) roughly goes along with the reThe reflectivities were measured at about normal incidence flectivities. with a 0.5-mW He/Ne laser (Spectra Physics 155A) and a Spectia Table I1 shows those replacements of counterions, and Physics Model 404 power meter. those mixtures of anionic and cationic surfactants that The interfacial tension of the MELLFs was measured by the were tried, noting whether a MELLF was obtained. Again Wilhelmy plate method. We used a Cahn Model 2000 electrobalwe also give the measured values of the interfacial tensions ance with a platinum plate made hydrophobic by a dimethylof the aqueous/CHZClz interface and the reflectivities dichlorosilane treatment, as well as a Lauda tensiometer. (when relevant). The materials we use are AR quality without any further Several aspects of the findings that were summarized purification and the water is of 18-MQresistivity, obtained from in the tables are worth mentioning. The first point is that a Millipore Corp. ionic exchange and filter system.

*

Silver Metal Liquidlike Films

Langmuir, Vol. 7, No. 2, 1991 269

Table 11. Formation of Silver MELLFs with Different Counterions or Mixtures of Surfactants anionic surfactant Merpisap APSOP Merpisap APSOP Merpisap APSOP Merpisap APSOP Merpisap APSOP Merpisap APSOP Monflor 32 Zonyl FSA Zonyl FSA FC 143 FC 143 a

counterion or additional cationic surfactant sodium ion tetramethylammonium trimethylammonium tributylammonium trioctylammonium Zonyl FSC Zonyl FSC lithium ion tributylammonium ammonium tributylammonium

MELLF formationa

concn, %

o. 1 0.1 0.1 0.1 0.05 0.08 0.04 0.01 0.035 0.03 0.04

interfacial tension, dyn/cm 5.0 4.3 7.0 23.2 18.8 8.5 12.9 26.8 24.8 22.6 24.2

reflectivity, 7;

32 34 36 42 32

+ and - denote whether a MELLF was produced or not.

regardless of the surfactant, in the absence of anisic acid (or a similar acid) we were not able to form silver MELLFs. Under such conditions only granular or “creamy” interfacial films were obtained. At times, the aqueous phase became turbid, due to a colloid, which would stay stable for days. In all these cases, the organic phase was always clear, just as was the case when C c 4 was used. The second point is that we found Monflor 32 to be the surfactant that produced the best silver MELLFs compared to those obtained with all the other surfactants. Indeed this surfactant, as it was marketed by ICI, combines a very strong perfluoroalkyl anionic surface agent, a cationic surfactant (the tri-n-butylammonium cation), and a nonionic surface active solvent. On the other hand, Monflor 32 is slow, meaning that it takes 12-24 h (and longer) for a MELLF to develop, when using this surfactant, as compared to 2-12 h for most of the other surfactants. Another experimental observation is that usually the formation of a MELLF is accompanied by the appearance of a turbidity in the lower organic phase, due to the production of a suspended silver colloid. Sometimes a black deposit forms on the bottom of the vessel. In contrast to that, when Monflor 32 is used one can find conditions for which the organic phase remains clear. By increasing the concentration of the Monflor 32 and increasing the quantity of the reductant (hydrazine sulfate), one can obtain thicker silver MELLFs (as judged by their transmittance). The organic phase still remains clear. These thicker films are not as “fluid” as the usual ones, can be torn easily, and are less stable, forming a crust after a few days. In a control experiment only the concentration of the reductant was increased, without a parallel increase in that of the surfactant. In this case the films that were produced were not thicker, but the additional free metal silver appeared as a colloidal suspension in the organic phase. The surfactant FC 143 is also a very active agent in producing high-quality silver MELLFs. Using the same percentage of active material as in the case of Monflor 32, a MELLF is formed within 2 h. However, the organic phase in this case is contaminated with a silver colloid. Upon a 4-fold increase in the concentration of FC 143 a MELLF is produced in a much slower process, >24 h, but the organic phase remains clear. This is in contrast to most of the other surfactants which exhibited optimal surfactant concentrations. Away from these optimal concentrations either the MELLFs were of poorer quality than at the optimal concentration or they did not form a t all. The effect of the addition of salts on the silver MELLFs was discussed recently.’ In short, addition of sodium, barium, or lanthanum nitrate salts causes the silver MELLFs to coagulate and contract, and to lose the re-

flectivity and the “fluid” behavior. Cyanide, in contrast, causes a total disruption of the film, “exploding” it into fragments that fly into the aqueous phase. The same behavior was seen regardless of the surfactant. The overall behavior of the silver MELLFs under the influence of a voltage applied with two electrodes was also the same for all the surfactants studied. The MELLFs turn black, starting from the cathode and spreading toward the anode. Though anionic surfactants are the most effective in inducing the formation of silver MELLFs, the cationic counterion has an important role. While Monflor 31 gives MELLFs of rather poor quality, Monflor 32 forms the best MELLFs. The main difference between these two surfactants is the counterion, sodium in the former and tri-n-butylammonium in the latter. Another example is the surfactant MERPISAP APSOP, which is marketed as the sodium salt. We were not able to form a silver MELLF with this surfactant, even after replacing the sodium with trimethylamine or tetramethylammonium hydroxide. However, when the sodium was replaced with tri-n-butylammonium a MELLF was formed, and a better one was produced by using tri-n-octylammonium. A clear trend is seen here. Notwithstanding the above, a surface active counterion may be detrimental to the production of a silver MELLF. For instance, Zonyl FSC, a cationic surfactant, the anion being tert-butylsulfonate, forms a silver MELLF, but not in the presence of the anionic MERPISAP AP9OP. Table I11 lists the values of the interfacial tensions measured for several surfactants and the four organic liquids that were studied. The entries in the table include the values for the interfacial tension of the pure water/ organic liquid and those of the organic liquid in contact with the aqueous ammoniacal solution which contains silver nitrate, anisic acid, and the surfactant. For two surfactants we also report the interfacial tensions measured in the presence of silver MELLF. The main points to note in Table I11 are as follows: (i) Generally Monflor 32 and DIO 60 are the surfactants that reduce the interfacial tensions to the greatest degree. Of note are the very small values of the surface tension when using CC4, with these two surfactants. (ii) The absolute reduction of the interfacial tension due to the addition of any of the surfactants is, by far, the largest for CC4. However, the values themselves show no clear trend with respect to the interfacial tensions involving the other solvents. Particular attention should be given to E l 4 , as it is the organic phase that never gave a silver MELLF using our procedure of production. (iii) No clear trend is seen for the interfacial tensions as a function of the organic solvent, both in the absence of the silver MELLF and in its presence. Also one does not see any correlation between the quality of the films (ClsMet producing the best) and

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Table 111. Interfacial Tensions between the Aqueous Solution and the Organic Phase. surfactant pure water FC 99 DIO 60 Zonyl FSN FC 98 Monflor 32 Monflor 32 MELLF FC 143 FC 143 MELLF

CHzClz 28.1 17.6 13.0 21.9 25.4 10.2 (18.8) 22.3 (21.1)

CzHrClz 28.4 21.9 9.9 21.3 25.1 13.2 (18.0) 23.9 (24.9)

CHCh 31.0 13.2 12.5 23.3 24.6 17.0 (22.6) 22.0 (22.5)

ccl4 44.6 14.8 1.5 21.2 29.5 4.9 26.6

The aqueous solution contains ammonia, 0.05 M silver nitrate, 0.1 r,' anisic acid, and the surfactant, Le., before the reduction of the silver. The numbers in parentheses denote values measured in the presence of a silver MELLF. The units are dyn/cm and the experimental uncertainty is i O . 1 dyn/cm. The temperature was 20-21 "C. The surfactant concentrations are the optimal concentrations given in Table I. (I

the absolute value of the interfacial tension or its value relative to the other interfacial tensions which were measured. (iv) In the MELLF system comprising FC 143 the interfacial tensions are very close to those measured in the absence of the film. For the case of Monflor 32 the interfacial tensions in the presence of the MELLF are significantly higher than those measured for that interface, just before the film was initiated.

Discussion Though the silver MELLF systems were shown in the past1~5to be sensitive to the chemical environment, there are some aspects to which they are relatively insensitive. We have demonstrated here that silver MELLFs can be produced with a large variety of surfactants, of a very different nature. In fact, one obtains a MELLF even without a surfactant at all. The role of the surfactants is in stabilizing the MELLFs at the interfacial region and improving their quality, though not changing their nature. This is evident from the stability in time of the films formed with surfactants, compared to those that do not contain surfactants. Also, the addition of increasing amounts of Monflor 32 (and reductant) increases the thickness of the films. It was also seen with FC 143 that with increasing quantities the silver increasingly concentrated in the interfacial region. The overall behavior of the silver MELLFs with respect to the addition of ions and applying electric voltages does not depend on the specific surfactant. In a separate study4 it was shown that there is no significant change in the appearance of the MELLFs under an electron microscope for the different surfactants. Also it was shown2 that the Raman scattering was essentially the same, except the obvious difference in the signals from the surfactant itself. The main feature seen in Tables I and I1 is that a large variety of surfactants, though by no means all of them, are effective in stabilizing silver MELLFs. Of the 23 surfactants in Table I, 13 were found to work. Also it is seen that though most of the appropriate surfactants are anionic, also three nonionic and one cationic surfactants gave positive results. It is also true that not all the anionic surfactants worked. No obvious common denominator can be found for the surfactants that showed MELLF activity. Consider, for instance, the interfacial tensions which are the obvious physical parameters which, one would think, should be related to the effect of the surfactants on the MELLFs. Monflor 32, the best surfactant for MELLFs found so far, gives the lowest interfacial tension among those established by MELLF-forming surfactants. In comparison, FC 143, which is also an excellent

MELLF-forming agent, givens one of the highest interfacial tensions. In a note of caution let us stress that the statement that a certain surfactant did not work holds only within the range of experimental conditions which were tried, though it was a rather large range of compositions. Returning to the value of the interfacial tensions, one sees that they do not exhibit any recognizable trend and give no clue to the quality of the MELLF that is produced, if any. The results in these tables also indicate that a surfactant may have a deleterious effect on the silver MELLFs sometimes preventing their formation altogether. It is clear from our results that the surfactant plays only a small role, if any, in the electric charging of the silver colloidal cores. Otherwise one would expect a stronger dependence on the charge of the polar head (anionic, nonionic, or cationic). One can see, for instance, that Zonyl FSC (a cationic surfactant), Zonyl FSN (an anionic surfactant), and several anionic surfactants (FC98,FC 120) give MELLFs with similar reflectivities (33-3696). In addition, the large number of MELLF-active surfactants also indicates that there are no important specific interactions between the surfactant and the silver. For instance, there was no real advantage to carboxylic headgroups as compared to sulfonic headgroups. The mercaptan, which should have a very strong ''head'! interaction with silver, produces only a medium quality MELLF. Also hydrocarbon surfactants gave reflectivities similar to those measured for MELLFs of perfluoroalkyl surfactants. Thus the "tails" are not all important. It is not yet clear how the surfactant stabilizes the MELLFs in the interface. On the one hand there was no clear trend involving the interfacial tensions. Also the polarity of the solvent does not seem to be a crucial ~ a r a m e t e r .On ~ the other hand, we saw that the surfactant is responsible for keeping the silver colloid in the interface. Also Raman spectroscopy2 indicates in some cases that some of the surfactant is located very close, actually adsorbed on the silver cores. Gordon et alasalso see the additives in their extensive Raman measurements, which indicates their close proximity to the silver particles. Electron diffraction studies of silver MELLFs also indicate the presence of the surfactant in the film and its association with silver ions.' All this is a strong indication of an important interaction of the surfactant and the silver, though it is relatively nonspecific. It is possible that the surfactant enshrouds the colloidal cores, thus preventing direct contact and precipitation. It is also possible that the surfactant forms micellar structures in the interface with which a t least part of the silver cores are associated. Such structures would stabilize the colloid against extraction into the organic phase and would explain why the silver MELLF is confined to the interface and to the upper reaches of the organic phase. Indeed the concentrations we use for the surfactants are above typical critical micellar concentrations. Another interesting point that comes out of the results is the significant dependence on the nature of the counterion in the surfactant. The presence of cationic surface active counterions often greatly improves the activity of anionic surfactants in producing high-quality MELLFs. On the other hand, a deleterious effect of cations or cationic surfactants was also demonstrated. Some insight regarding the nature of the outer envelope of the silver cores can be gleamed from the fact that generally when a silver MELLF is formed, a colloid suspension is also found in the organic phase, but not in the water. This indicates a hydrophobic character, and

Silver Metal Liquidlike Films

an attractive interaction with the organic solvent. In fact, when granular interfacial films are formed, one does not have a colloid in the organic phase, though it may appear in the aqueous phase. Also, often, the deterioration of the films is through an "extraction" of the silver from the interfacial region into the bulk of the organic phase.1These facts give credence to the model which assumes that the organic liquid is one of the constituents of the silver MELLF.' The signal of the organic solvent picked up in the Raman spectrum of the film2is a further corroboration of this point. A very active surfactant, such as Monflor 32, seems to alter the interaction of the covered silver cores with the organic liquid. This also is the case when excess amounts of FC 143 are used. The exact details of these changes are yet to be worked out. In this context it is interesting to note that the interfacial tensions measured for the FC 143 system, in the presence and in the absence of the silver film, were very similar. For Monflor 32, however, when the silver MELLF was present the interfacial tension was considerably higher than that measured before the reduction was carried out. This trend was seen for all the solvents. I t could mean that more of the surfactant interacts with the silver and is not "free" to organize in the waterlorganic interface in the usual way, which leads to the decrease of the interfacial tension. Indeed, the amount of silver in the film is larger in the case of Monflor 32 than in the case of FC 143, where some of the silver is suspended in the organic phase.

Langmuir, Vol. 7, No. 2, 1991 271

The case of Zonyl FSN is interesting because though it is formally a nonionic surfactant, with an alcoholic functionallity, there is evidence from the Raman spectroscopy that it has been oxidized into a carboxylate in the silver MELLF.2 To summarize, it was shown that the surfactant has an important role in the stabilization of silver MELLFs and in determining their quality. Though anionic surfactants are the most effective in promoting silver MELLFs, in tandem with the anionic additives which are necessary for producing MELLFs, cationic and nonionic surface agents also work, indicating that their charge is not a crucial parameter. Also the interaction with the silver is not very specific in the sense that many surfactants, with widely different structures, are active in stabilizing silver MELLFs. The nature of the detailed interactions is not yet clear, as well as the position of the surfactants within the MELLFs. We did not find a simple relation between the interfacial tensions and the activity in producing silver MELLFs. We are performing second harmonic generation studies to answer some of these questions.

Acknowledgment. We wish to thank Mr. Leslie Morgan of B & S Durbin, Ltd., for supplying us with Monflor 31 and the procedure for transforming it to Monflor 32, and Professor N. Gerti of the Cazali Institute and Profesor R. D. Neuman of Auburn University for enabling the surface tension measurements.