Designing Coating Agents for Inorganic Polycrystalline Materials

Chapter 30

Designing Coating Agents for Inorganic Polycrystalline Materials 1

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A. K. Chattopadhyay , L. Ghaicha , Bai Yubai , G. Munger , and R. M. Leblanc

Downloaded by CHINESE UNIV OF HONG KONG on February 24, 2016 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch030

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ΙCI Explosives Group Technical Centre, 701 Boulevard Richelieu, McMasterville, Québec J3G 6N3, Canada Centre de Recherche en Photobiophysique, Université du Québec à Trois-Rivières, Québec G9A 5H7, Canada

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This paper deals with the investigation on the influence of four amphiphile monolayers viz. arachidic acid, a mixture of dioctadecyl amine and arachidic acid (3:1), carboxybetaine and dioleoyl-L-α-lecithin (DOPC) on ammonium nitrate crystallization. The study was carried out by means of surface pressure vs. area, surface potential vs. area and fluorescent microscopy. Results indicated a strong effect on ammonium nitrate crystallization in the presence of carboxybetaine and dioctadecylamine monolayers, whereas an inhibition in crystal growth was observed in the presence of DOPC.

This study involves crystallization behavior of inorganic salts below Langmuir films of organic monolayers for the purpose of investigating the cooperative effects of molecular arrays. A detailed appreciation of recognition factors of inorganic surfaces will lead us to the rational design of new surface active molecules applicable for inorganic surface coating agents, scale inhibitors and rust preventives (1-8). Amongst various commercially available inorganic salts, ammonium nitrate probably occupies the largest portion in consumer market. In order to solve storage and transportation problems associated with ammonium nitrate (AN), there is a need to produce consistently noncaking particles, generally available in globular forms known as prill. It is an industrial challenge for the commercial manufacturers of ammonium nitrate to overcome the caking tendencies between ammonium nitrate particles and their subsequent changes in physical properties with time. This happens due to the multiple crystalline phase transitions in A N that occur with temperature changes and are accelerated in the presence of moisture. It is well known that the phase IV 2N(CH3)3

O=CO=CO=P-O-

D

0

P

C

R

4 =

O L E O Y L

CH2 - CH - CH2 Figure 2.

Molecular structures of the amphiphiles used.

In Film Formation in Waterborne Coatings; Provder, Theodore, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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CHATTOPADHYAY ET AL.

Coating Agents for Inorganic Materiah

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The behaviour of A N crystallization was studied by surface pressure-area and surface potential-area isotherms. The monolayers were spread on aqueous subphase containing AN, where A N concentration was varied between 0-55% by weight. The surface potential-area isotherms were obtained simultaneously with an automated Langmuir trough described elsewhere (14). The surface crystallization of A N was directly observed on the Langmuir trough using fluorescence microscopy (FM). The total surface area of the trough used for this purpose was 200 mm by 15 mm and it was milled to a depth of 3 mm. The fluorescence technique described elsewhere (15) involves addition of a small amount of fluorescent probe to the subphase. Rhodamine B, used as a fluorescent probe, was obtained from J.T.Backer Inc. (USA). The surface crystallization was investigated by F M with 55% aqueous subphase under two different conditions: (a) The rhodamine solution (1% of the monolayer concentration) was injected in the subphase after the monolayer was spread and stabilized. (b) The rhodamine solution was injected in the subphase prior to spreading the monolayer. The results obtained from both methods were similar. S E M pictures of A N crystals grown on the Langmuir-Blodgett films of the surfactants were taken by Hitachi S-2700 Scanning Electron Microscope. The L-B films were formed by transferring the surfactant monolayers onto the silicon slides at a constant surface pressure of 20 mN/m by means of an autoadvancing Teflon barrier. Eight monolayers of each surfactant were deposited by horizontal X-type transfer from A N subphase on the silicon slides and A N crystals were allowed to grow in a desiccator.

Results and Discussion Arachidic acid. The surface pressure-area isotherms of arachidic acid on pure water and on A N subphases at pH 4.5 are shown in figure 3. With the increased A N concentration in the aqueous subphase, the monolayers were changed from condensed state to an expanded one, particularly below the phase transition point at ~24mN/m. However, above the phase transition there was no significant change in molecular surface area of arachidic acid with the increase in A N concentration. Further increase in molecular area was also noticed with the increase in waiting time before the film compression. Infig.4, the differences in compression behavior of arachidic acid films on 55% A N solution are shown. These differences could probably due to the (i) increased spacing between the head groQps due to increased repulsion and (ii) increase in effective head group



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FILM FORMATION IN WATERBORNE COATINGS

- - PURE WATER SUBPHASE 1 . 1 0 % NH4NO3 s o l u t i o n s u b p h a s e 2. 3 0 % N H N 0 s o l u t i o n s u b p h a s e 3. 5 0 % N H N 0 s o l u t i o n s u b p h a s e 4

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.500· .400f .300· < J?00.100· 0 40 50 (%) C20 SURFACE POTENTIAL AS A FUNCTION OF NH NO3 CONCENTRATION 4

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92.

MOLECULAR AREA ( A /molecule ) Figure 3.

Π-Α and A V - A isotherms of arachidic acid at A N solution - air interface. Inset : AV as a function of A N concentration. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)



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Coating Agents for Inorganic Materials

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— Pure water subphase

MOLECULAR AREA ( J?/molecule ) Figure 4.

Π-Α isotherms of arachidic acid as a function of waiting time. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)


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volume due to the association of the head group moiety with NH and N0 " through hydrogen bonding(/6). The surface potential as shown in the inset of fig.3, was found to increase with increased A N concentration. The increase in surface potential of arachidic acid films with increased A N concentration also reveals that there is an increase in surface charge accumulation at the monolayer level.


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The F M micrographs of A N surface crystallization taking place in the presence of arachidic acid monolayers are shown in figure 5. From these micrographs it was evident that the film compactness of arachidic acid monolayers (by viewing the monolayers at various surface pressures) had hardly any effect on surface crystallization. The F M images remained more or less similar at all stages of film compression. This reveals that the -COOH as a polar head group has minimum or no influence on A N crystallization in 2D plane. Dioctadecylamine (DOA). DOA molecules are comprised of two long hydrophobic chains and a secondary amine functional group . Because of its poor hydrophilicity, DOA molecules do not form a stable monolayer. The improvement in their monolayer stability can often be observed by mixing the pure monolayer with some long chain carboxylic acid or alcohol. In order to achieve a stable monolayer, DOA was mixed with arachidic acid at a molar ratio of 3:1. From Π-Α and AV-A isotherms of DOA/ arachidic acid mixed monolayers shown in figure 6, several points are apparent: The Π-Α profiles of the mixed monolayers in the presence of A N in the subphase are completely different from the one obtained from pure water. This indicates that the film undergoes some major changes in the presence of AN. Regardless of the concentration of A N in the aqueous subphase, the Π-Α isotherms exhibit two distinct discontinuities, the first one occurring at ~10mN/m and the second one at ~24mN/m. The second discontinuity can be attributed to the phase transition of pure carboxylic acid present in the film, whereas the first transition in Π-Α profiles could be due to the demixing of the film in the presence of AN. Contrary to the arachidic acid monolayers, the mixed monolayers show a decrease in surface potential in the presence of AN. This indicates a change in physico-chemical properties of the monolayer contributed by the - N H groups of DOA. An opposite trend in surface potential can be attributed to the reversal of monolayer charge through protonation of amine groups of DOA and their subsequent interactions with A N (20,21). The effect of DOA/arachidic acid monolayers on surface crystallization of A N was investigated by F M at different surface pressures (figure 7) with a subphase



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Figure 5.

F M images of arachidic acid spread on pure water and 55% A N solution at surface pressures of 5 mN/m, 15 mN/m and 30 mN/m. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)



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Figure 6.

Π-Α and AV-A isotherms of DOA/arachidic acid spread on pure water and 10%, 30% and 55% A N solution subphases. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)




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Figure 7.

F M images of DOA/arachidic acid spread on pure water and 55% A N solution at 6mN/m,15mN/m and 20mN/m surface pressures. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)



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comprising 55 wt% A N The nucleation and subsequent growth of crystals which appeared visually as domains at the monolayer surface, took place within 15 minutes after compressing the films at a desired pressure. The observed induction time for the domains to form, which was apparently absent in the case of arachidic acid, strongly suggests that there exists a specific interaction between the DOA polar head groups and A N . The size and stability of such domains depend mostly on the monolayer compressibility. Viewed from the top, the crystals appeared to be dendritic in morphology. It must be noted in this regard that no such domains were observed with a subphase containing water only. This rules out the possibility of formation of any domains of amphiphiles caused by surface micellization. Carboxybetaine (CB). Figure 8 shows the Π-Α and A V - A isotherms of pure carboxybetaine. The monolayer at the air-water surface has a fairly large compressibility, as implied by the fact that the surface pressure begins to rise at about 160À and collapses at about 60Â , giving an area of compression around 100À . It can also be seen that the monolayer collapses at relatively higher pressure around 40A , suggesting that a stable monolayer is formed at the air-water surface. In the presence of A N solution subphase, however, the CB monolayers exhibited decrease in collapse pressure as well as molecular area with increased A N concentration. 2

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From the molecular structure of this surfactant, it can be seen that the head group comprises a tether group which bears both positive and negative charges. In the presence of AN, when the monolayer is compressed, the tether group is bent and squeezed out of the subphase which can explain the large decrease in the molecular area. Crystallization of A N under CB monolayers at a 55 wt% A N solution - air interface was observed by means of F M (figure 9). After compressing the film at a desired surface pressure, it also required a certain period of induction time for the flower-like crystal domains to be visible by FM. It should be noted that F M images of Carboxybetaine spread on pure water were similar to those of DO A/arachidic acid monolayers. From F M studies, visually a marked difference between CB and DOA/arachidic acid monolayers was observed with regard to the rate of formation of A N crystals and their growth patterns. The study revealed that CB monolayers accentuated surface crystallization of A N more than DOA/arachidic acid mixed films. Dioleoyl-l-cc-Iecithin (DOPC). Despite having a zwitterionic head group, DOPC showed a different behaviour when compared with carboxybetaine. The major differences between these two molecules stems from the differences in head group size, hydrocabon chains and the nature of ionic groups. DOPC molecules comprise double hydrocarbon chains with large head groups containing phosphonate ions. The presence of A N in the aqueous subphase expands the DOPC monolayer and



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MOLECULAR AREA ( A /molecule ) Figure 8.

Π-Α and A V - A isotherms of carboxybetaine on pure water and A N solution subphases. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)


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Figure 9.

F M images of carboxybetaine spread on pure water and 55% A N solution at 5mN/m, lOmN/m and 20mN/m. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)


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decreases its surface potential values (fig. 10). The association of A N with the polar head groups in the DOPC monolayer has expanding and fluidizing effects, as it is indicated by the enhanced compressibility of the monolayers.


F M micrographs (figure 11) exhibited no surface crystallization of A N under the compressed monolayer of DOPC. This indicates that DOPC, unlike other amphiphilic systems of the present study, inhibits A N crystallization. The results obtained from four different amphiphilic monolayer systems indicate that the crystallization of A N is influenced by the nature of molecules present in the monolayers. The changes in the functionalities of the polar head groups produce significant differences in nucleation and growth pattern of A N crystals as observed by F M . These studies also showed that both D O A and CB with amine functionalities present in their polar head groups induce A N crystallization more favorably than arachidic acid or DOPC molecules. The crystallization of A N in the presence of amine group is due to the stereochemical correspondence of ammonium ions and the protonated amines of the surfactant head groups. At a pH 4.5 (pH of the saturated subphase) both D O A and CB polar groups bear a positive charge. In such events, it is possible that the mode of interaction of A N at the interface occurs through the nitrate ions and the protonated amines head groups of the surfactants. This may provide a degree of stereochemical recognition between the organic-inorganic boundary layer. For proper interpretation of the results, however, further elucidation involving molecular packing as well as structural details such as position, conformation and alignment of the head groups in relation to the disposition of A N crystals is required. DOPC is found to inhibit A N crystallization. Although DOPC contains amine functional groups, the crystal inhibition property of DOPC presumably comes from the presence of phosphonate functionalities which are known to be crystallization inhibitors and habit modifiers when present in various mineral salts (22-23). In spite of the widespread application of phosphonates in industrial processes, the mechanism of their action is not yet fully understood. The S E M micrographs of A N crystals grown on L-B films of two different surfactants viz. DOA and CB (figure 12) clearly demonstrate that DOA influences the crystals to grow in needle shaped o,rhombic forms of phase IV, whereas CB modifies the pattern in the form of plates. In the case of DOA, the molecules comprising N H - groups act as nucleating sites for A N crystals to grow in o,rhombic forms because of the charge and structural similarity. However, in the presence of CB molecules with zwitterionic head groups comprising quaternary amines and carboxylate ions provide a major effect on the habit modification of A N crystals (1619). Such morphological changes in A N crystals grown under L-B films of different surface active agents revealed the importance of surface binding sites. Therefore by +

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Figure 10.

Π-Α and A V - A isotherms of DOPC on pure water and A N solution subphases. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)



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Figure 11.


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F M images of DOPC spread on pure water and 55% A N solution at 15mN/m and 20mN/m. (Reproduced from reference 8b. Copyright 1996 Marcel Dekker.)


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-σ c


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Designing Coating Agents for Inorganic Polycrystalline Materials

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