HYDROGEN BONDING BETWEEN ALCOHOLS AND MOTOR OIL DISPERSANTS NICHOLAS
E.
GALLOPOULOS
General Motors Cor)., Warren, Mich. This infrared spectrophotometric study demonstrates that hydrogen bonds form between ashless dispersants and alcohols. Of the two ashless dispersants studied, the polyaminosuccinimide is a better proton acceptor than the copolymer of polymethacrylate and N-vinyl-2-pyrrolidone. However, the proton-accepting capacity of the former depends on the molecular weight of the alcohol. The contribution of ashless dispersants to engine cleanliness is probably enhanced b y their ability to form hydrogen bonds. However, how much of the dispersant action occurs by hydrogen bonding is not known.
HYDROGEN is one of the mechanisms proposed to BONDING
explain the ability of metal organic or ashless motor oil dispersants to keep engines clean. Bascom (7) suggested that metal organic dispersants solubilize polar compounds, and that the latter stages of this solubilization are due to hydrogen bonding. Infrared spectroscopic evidence of hydrogen bonding in these systems was offered by Kaufman ( 6 ) . The action of both high and low molecular weight ashless dispersants has also been linked to hydrogen bonding. Stewart (8) postulated that hydrogen bonding is the dominant mechanism in the action of the high molecular weight ashless dispersants. This mechanism was based primarily on the infrared studies of Fontana and Thomas ( 5 ) who demonstrated that similar high molecular weight ashless dispersants form hydrogen bonds with the polar solid, silica. However, evidence that these dispersants form hydrogen bonds with polar liquids as well is not available. I t has also been proposed, but not experimentally shown, that hydrogen bonding is involved in the action of some low molecular weight ashless dispersants (4). This paper reports an infrared spectrophotometric study which proves that ashless dispersants form hydrogen bonds with polar liquids, such as alcohols, and discusses the role of hydrogen bonding in the action of dispersants. Experimental
Apparatus and Procedure. Hydrogen bonding between alcohols and dispersants was investigated by recording the differential spectra of test and reference solutions prepared by dilution from stock solutions of alcohol and dispersant. Alcohol concentration in the test solutions was constant a t 0.0343, 0.0317, 0.0315, and 0.0248 mole per liter for ethanol, I-octanol, 2-octanol, and I-hexadecanol, respectively; however, the dispersant concentration varied from zero to 0.0800 gram per ml. in 0.0200 gram-per-ml. steps. The reference solutions did not contain alcohol, but were the same as the test solutions in all other respects. Alcohol concentrations were selected so that primarily unassociated (free) alcohol would be present. Dispersant concentrations are comparable with those encountered in compounded motor oils. T o obtain the differential spectra 1.0-mm. matched KBr cells were placed in a Perkin-Elmer Model 337 spectrophotometer. Recording speed was approximately 1.2 cm.-' per minute, and the slit program was set a t 8. Sample temperature was about 94' F. Materials. Two dispersants were used: a low molecular weight ashless dispersant which, from its infrared spectrum and 36
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
the patent literature (Q), is identified as a polyaminomonoalkenylsuccinimide (PAMAS) ; a high molecular weight polymeric dispersant and viscosity index improver which, from its infrared spectrum and the patent literature (Z), is identified as a copolymer of a polyalkylmethacrylate and iV-vinyl-2-pyrrolidone (PAM-VP). Both are commercial products commonly used in the formulation of motor oils. The PAM-VP and PAMAS were supplied as 40 and 50% solutions in a mineral oil, respectively. The alcohols were absolute ethanol, reagent grade 1-0ctanol and 2-octanol, and practical grade 1-hexadecanol. The two solvents were: spectrophotometric grade carbon tetrachloride; a white mineral oil, free of inhibitors, which was percolated through a column of silica gel before it was used. Results
Alcohol molecules in dilute solution in nonpolar solvents are not hydrogen bonded. The infrared spectral evidence for this state is a sharp band at about 3630 cm.? which is characteristic of the unassociated (free) 0-H vibration. When a proton acceptor is added to such dilute solutions, hydrogen bonds may form between the proton acceptor and the alcohol molecules, and the formation of these bonds alters the 0-H vibrational band ( 3 ) . The spectral changes due to hydrogen bonding between the ashless dispersant PAMAS, a proton acceptor, and 1-octanol are shown in Figure 1 as a function of dispersant concentration. The dotted trace is of a solution of the alcohol only; the sharp band at 3630 cm.-' is due to the free 0-H groups, and the weak broad band with a maximum at about 3500 cm.-' is due groups from dimeric alcohol species. The solid to 0-H traces are of solutions of the same alcohol concentration, but of progressively increasing dispersant concentrations of 0.02, 0.04, 0.06, and 0.08 gram per ml., respectively. The solvent was carbon tetrachloride. Two spectral alterations are immediately apparent: A new broad band centering at about 3375 cm.-l appeared and increased in intensity as the concentration of the dispersant was increased; the free hydroxyl band at 3630 crn.-' progressively decreased in intensity as the concentration of the dispersant was increased. The development of the broad band is due to the bonded 0-H vibration, and in this system, it is the necessary and sufficient condition for the identification of hydrogen bonding. Its increasing intensity with dispersant concentration indicates that increasingly more alcohol molecules were hydrogen bonded. In addition, the width of the band suggests that several different types of hydrogen bonds were formed. This
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FREQUENCY (CM-’) Figure 1. Effect of hydrogen bonding between 1 -0ctanol and PAMAS on the vibration of 1-0ctanol
is a reasonable conclusion: since the dispersants are multifunctional and contain both oxygen and nitrogen atoms with unshared pairs of electrons capable of forming hydrogen bonds with the alcoholic hydroxyl group. The intensity of the free 0-H band decreased as the concentration of the dispersant increased, because as more of the alcohol was hydrogen bonded, less free alcohol was left. Results, such as those in Figure 1, can be discussed conveniently in terms of changes in some measure of band intensity. Thus, the intensity of the free 0-H band was converted with the aid of calibration curves, to molar concentration of free alcohol. Conversion of the intensity of the bonded 0-H band to concentration of bonded alcohol was not possible. However, the trend of increasing band intensity with increasing dispersant concentratioi~was established by measuring the area of the band between 3500 and 3100 cm.-l with a planimeter. As this intensity measurement lacks a fundamental significance, plots showing how it changes with dispersant concentration will not be shown. Whenever, the intensity of the free 0-H band decreased, the intensity of the bonded 0-H band increased as shown in Figure 1. This simultaneous change in the intensity of the 0-H bands is the proof of the occurrence of hydrogen bonding in this system. I n all cases, band intensity was measured with the base line technique (7). At least duplicate determinations were performed, the results were averaged, and the average values were used to draw the curves shown in the figures. However, all the actual data are also plotted. Discussion
Intermolecular Hydrogen Bonds between Ashless Dispersants a n d Alcohols. T h e first aim of this study was to ascertain whether hydrogen bonds were formed between ashless dispersants and polar liquids. Alcohols were selected to represent polar compounds found in automotive crankcases, not because they are the most important, but because the
0-H
stretching
intensity changes of the 0-H vibration could be measured conveniently. Figures 2 and 3 show that as the concentration of dispersant increased, the concentration of free alcohol decreased. As mentioned previously, this decrease was accompanied by an increase in the intensity of the bonded 0-H band; therefore, hydrogen bonds formed between all four alcohols and either PAMAS or PAM-VP. The hydrogen bonding between alcohols and PAMAS supports the postulated mechanism of action of low molecular weight ashless dispersants (4). Similarly, the hydrogen bonding between PAM-VP and the alcohols complements the work of Fontana and Thomas ( 5 ) , who showed the existence of hydrogen bonds between silica, a solid rich in hydroxyl groups, and PAM-VP. Both of these observations are in agreement with Stewart’s (8)view of how high molecular weight polymeric dispersants contribute to engine cleanliness. Proton-Accepting Capacity of PAMAS and PAM-VP. The data in Figures 2 and 3 also measure the proton-accepting capacity of the dispersants because the negatives of the slopes of the lines are equal to the moles of alcohol hydrogen bonded by each gram of dispersant (Table I). The slopes obtained from Figures 2 and 3 were divided by 0.5 and 0.4, respectively, to account for the fact that the PAM-VP and PAMAS were solutions of the active ingredient in a mineral oil. Table I shows that the proton-accepting capacity of PAMV P was independent of the molecular weight of the alcohol, whereas the capacity of PAMAS decreased as the molecular Table 1.
Proton-Accepting Capacity of PAMAS and PAM-VP Moles of Hydrogen-Bonded Alcohol per G r a m of Dispersant X 106
Dispersant PAMAS PAM-VP
Ethanol 19.2 12.7
1-0ctanol
VOL. 6
15.0 13.7
NO. 1
2-Octanol 15.0 12.2
1-Hexadecanol 10.0 12.5
MARCH 1 9 6 7
37
0
ETHANOL 1-OCTANOL 2-OCTANOL 0 I-HEXADECANOL 0
0 A
ETHANOL I-OCTANOL A 2-OCTANOL 0 I-HEXADECANOL 0 0
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CONCENTRATION OF PAMAS, grn/rnl
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CONCENTRATION OF PAM-VP, grn/rnl
Figure 2. Effect of PAMAS concentration on the concentration of free alcohol
Figure 3. Effect of PAM-VP concentration on the concentration of free alcohol
weight of the alcohol increased. This difference may be indicative of the differences in their molecular structure, of differences in their state of aggregation, or both. However, additional information is necessary to ascertain the cause of the differences. Table I also shows that when compared on a weight basis, PAMAS appears to be a better proton-acceptor than PAM-VP. However, ignorance of the exact purity of these two dispersants weakens this conclusion. Comparison of the dispersants on a weight basis is justified from the viewpoint of formulation of motor oils. However, the comparison reveals nothing about either the number or the strength of proton-accepting sites per equivalent weight of dispersant. Effect of Solvent, Although carbon tetrachloride is very favorable experimentally for this type of study, mineral oil is more important, as it is the solvent in which the dispersant must work. Therefore, one experiment was performed with I-octanol and PAMAS dissolved in mineral oil to see if this change in solvent would materially affect the results. Figure 4 shows that hydrogen bonding occurred in this system just as it did when carbon tetrachloride was the solvent. Furthermore, the hydrogen bonding capacity of PAMAS computed from the slope of the line in Figure 4 is about the same as when carbon tetrachloride was the solvent. Apparently, information developed with carbon tetrachloride as the solvent would be valid for mineral oil systems as well.
However, the mineral oil and carbon tetrachloride systems differed in one respect. The spectra of 1-octanol in mineral oil contained a bonded 0-H band even in the absence of dispersant. The shape of this band, especially its width, suggested that more than dimerization or polymerization due to lower solubility of the alcohol in the oil was involved. I t appears that despite the percolation of the oil through a column of silica gel, polar compounds were still present, and formed hydrogen bonds with the alcohol molecules. Role of Hydrogen Bonding in Dispersancy. From the evidence presented in this paper and in the work of Fontana and Thomas ( 5 ) , and Kaufman ( 6 ) , the role of hydrogen bonding in dispersancy can be described qualitatively. I t is reasonable to assume that dispersants will form hydrogen bonds with hydroxylic compounds found in engine crankcases. This action will contribute to engine cleanliness by preventing the reactions of such sludge, varnish, and rust precursors as the alcohols, acids, and hydroxy acids, and by minimizing the agglomeration and deposition of sludge particles. How much of a dispersant’s action depends on hydrogen bonding is not as clear. Quantitative data linking hydrogen bonding and peptization are unavailable, and those relating it to solubilization are incomplete. As the proton-accepting capacity of the ashless dispersants is low (Table I), it appears that any substantial solubilizing action of these dispersants would occur via other processes. A comparison of this paper’s data with Kaufman’s (6) lends some support to this contention.
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PRODUCT RESEARCH AND DEVELOPMENT
Conclusions
Hydrogen bonds apparently form between the hydroxylic hydrogen atom of the alcohols and the oxygen and nitrogen atoms of the ashless dispersants when the solvent is carbon tetrachloride or mineral oil. PAM-VP, a high molecular weight polymeric dispersant, was a somewhat worse proton acceptor than the low molecular weight dispersant PAMAS. However, the proton-accepting capacity of PAMAS was dependent on the molecular weight of the alcohol. No such dependence was observed for PAM-VP. The white mineral oil used in this study, even though purified, contained polar materials which formed hydrogen bonds with 1-octanol. From these observations, and from the fact that both PAMAS and PAM-VP are effective dispersants when used in the type of service for which they are intended, it is concluded that hydrogen bonding is only one of the processes by which ashless dispersants contribute to engine cleanliness.
3.0
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Acknowledgment
Appreciation is expressed to Glenn L. Collver for his assistance during the experimental work. Literature Cited
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CONCENTRATION OF PAMAS, g d m l
Figure 4. Hydrogen bonding between 1 -0ctanol and PAMAS in mineral oil
I n the solution containing 0.02 gram of PAMAS per ml. and 0.0343 mole per liter ethanol, only about 6% of the ethanol present was hydrogen bonded by PAMAS. By contrast, Kaufman (6) has sho\vn that in a system containing comparable amounts of dispersant and alcohol, sodium dinonyl naphthalene sulfonate solubilizes about 5070 of the alcohol present. Unfortunately, the degree of solubilization due to hydrogen bonding is not known. T h e information available now, indicates that although hydrogen bonding is significant in dispersancy, it may be only of secondary importance in solubilization. However, a firm conclusion must await quantitative data relating hydrogen bonding to the peptization and solubilization aspects of dispersancy for both ashless and metal organic dispersants.
(1) Bascom, I V . D., Kaufman, S., Singleterry, C. R., “Colloid Aspects of the Performance of Oil-Soluble Soaps as Lubricant Additives,” World Petrol. Congr., 5th, N . Y.,1959, Section VI,
PaDer No. 18. (2) Bauer, L. N . , Healy, R. B., Stringer, H. R. (to Rohm & Haas Co.), Australian Patent 216,911 (July 21,1958). (3) Bellamy, L. J., Hallam, H. E., Trans. Faraday SOL.5 5 , 220-4
,/ 1.0,5“,0 \,.
(4) California Chemical Co., Oronite Division, “Deposit and \Year Control by Metal Salt and Organic Lubricating Oil Additives,” pamphlet. ( 5 ) Fontana, J. B., Thomas, J. R., J . Phys. Chem. 6 5 , 480-7 (1961). (6) Kaufman, Samuel, J . ColloidSri. 17,231-42 (1962). (7) Potts. W. J., Jr., “Chemical Infrared Spectroscopy,” Vol. 1, Chap. 6, TYiley, Sew York, 1963. (8) Stewart, W.T., Stuart, F. A , , Miller, J. A,, “Synthesis and Structure-Performance Relationships of a Series of Polar Substituted Polymethacrylate-Type Ashless Detergents,” Petroleum Chemistry Division Preprints, Vol. 7, KO. 4, p. B-19, 142nd Meeting, ACS, Atlantic City, September 1962. ( 9 ) Stuart, F. A., Anderson, R. G. (to California Research Corp.), U. S. Patent 3,131,150 (April 28, 1964). RECEIVED for review September 6, 1966 ACCEPTEDJanuary 18, 1967 Division of Petroleum Chemistry, 150th Meeting, ACS, Atlantic City, N. J., September 1965.
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MARCH 1967
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