.:(r~ric~ ON
AI,
EMULSIONS, WITH EXAMPLES OF INTEREST L“; ‘‘HE SPRAYING O F TREES - -I l O W L A N D
MAILCUS WOODMAN
Xobertson’, Clayton2) Seifriz3, Woodman4 and others, have shown that i 3 some emulsion systems inversion can be accomplished simply by alteration of the phase-volume ratio. Instances of dual emulsions existing a t the same phase volume ratio with the same emulsifier present in the same amount, are, however, of much rarer occurrence : Clayton2 noted a tendency of margarine emulsions to invert completely when subjected to vio!ent agitation; Seifriz3 has shown that standing, with subsequent reshaking, of emulsions containing certain petroleum oils causes inversion, whilst Woodman4 demonstrated this standing effect to hold in the system cresylic acid-water-gelatine, and also that an actual prediction of the emulsion type formed by shaking together the phases may be made according to the mode of shaking employed. The present paper consists of observations on other systems giving dual emulsions a t the same phase-volume ratio, special attention being paid to the significance of the results in the preparation of spraying emulsions. The first example was a sample of B.D.H. petroleum ether A.R., b.p. I O O - IZOOC., guaranteed to conform to the standard of punty given in “The B.D.H. Book of A.R. Standards (1926).” Ten C.C. of this were vigorously and intermittently shaken in a cylinder with I O C.C. of 0.5% potassium oleate solution6, three hand-shakes a t intervals of 30 sec. being found necessary at 25°C. to form the 0 in W type6, which creamed normally upwards4. The experiment was repeated in another cylinder, 3-5 shakes apparently producing a perfect 0 in W emulsion, judging by the film on the cylinder side4; owing however, to a lack of that permanent and abundant foam which characterises complete emulsification of large quantities of oil in an aqueous medium’, the shaking was continued, 11 shakes producing no obvious difference. The emulsion was found to be W in 0 by the drop test, and emulsification must have been complete according to the continuously-draining film on the cylinder side4; the emulsion, however, creamed upwards, as if 0 in W, and not downwards, as would be expected of a W in 0 type when the oil is less dense than wate+, and, therefore, both types of emulsion were suspected as being present at. the same time. On reshaking and keeping the emulsion 42 hr., drop tests showed both types of emulsion present in the cream, the lower Robertson: Kolloid-Z., 7, 7 (1910). J. SOC. Chem. Ind., 36, 1205 (1917);Trans. Faraday SOC.,16, Appendix, 22 (1921). 3 Seifriz: J. Phys. Chem., 29, 834 (1925). 1 Woodman: J. Phys. Chem., 30, 658 (1926). 5 Potrtssium oleate from the sample described by Woodman: J. Agric. Sci., 1 7 , (1927). ~ 6 0 represents the non-aqueous phase in all cases. 7 Woodman. J. Pomol. Hort. Sci., 4,95 (1925).
* Clayton:
NOTES ON DUAL EMULSIONS
89
portions containing more W in 0; after so hr., the emulsion was wholly 0 in W,thus showing inversion.‘ The experiment was repeated twice, in one, vigorous intermittent shaking being employed, and, in the other, gentle partial rotation previous to vigorous shaking.2 The first emulsion was a perfect 0 in Itr type, formed in 3-4 shakes a t intervals of 30 sec., and creaming upwards and normally. The second, by drop tests, was found to be an imperfect ( i e . , incomplete and unstable), JV in 0 type, creaming rapidly to give excess oil continuous phase over W in 0 emulsion cream surmounting unemulsified aqueous phase; on reshaking once and leaving 15 min., the cream was 0 in W,showing inversion1, though this emulsion was unstable and completely cracked in about an hour. It will be noted that the methods of shaking are the reverse of those in the system previously examined2. A third set of experiments was made with the ultimate notion of employing certain hydroxyl compounds as insecticides, ovicides and winter washes for the spraying of trees. Phenol, hexalin (cyclohexanol), cresylic acid and methylhexalin, though good contact poisons, are impracticable when used “naked”, both from the points of view of expense and of harmful effects on vegetation (even when trees are in the so-called “dormant pericjd” in wintel-l). Two methods of dilution are, hoTvever, possible in the special case of t’hese substances, both methods, because of Che presence of soap, yielding washes which will wet easily and spread over the plant surfaces, the skins of insects and eggs, and mosses and lichens on the trunks and branches4. The first is to make solutions in soap solutions5;bhe second is to emulsify them. Emulsions for spraying plants are prepared in two ways5: the toxic oil may be incorporated with the emulsifier to form a clear solution known in practice as a “miscible oil”, which should readily give perfect emulsions on stirring into water.; or a concentrated “stock” or “free” emulsion may be stored and diluted down to the required concentration just previous to spraying. The hydroxyl compounds mentioned need not be made into “miscible oils” as they are soluble in soap solutions, though they are, assisted by soaps, valuable aids in the formation of solutions and “miscible oils” of such toxic hydrocarbons as petroleum fractions6, and of such compounds as tetra- and deca-hydr~naphthalene~.The formation of stock emulsions of these phenols The first emulsion, judged by the abnormal creaming, must have been a mixture of W in 0 and 0 in K;partial mixing in the drop test is apt to escape notice or be neglected if one gets globules denoting non-mixing, L e . , the experimenter tends to judge types by the drop test by non-mixing rather than by mixing. It ,is only fair to record that in some of these examplea of inversion on keeping and re-shaking, the removal of emulsion for drop tests might have altered the phase-volume ratio to such an extent as to cause inversion. I n others, however, no drop tests were done, the type of emulsion being judged entirely by eye, Woodman: J. Phys. Chem., 30, 658 (1926). Hexalin and methyl-hexalin, in virtue of their lack of phenolic properties, might be used in dilute solution or emulsion form as contact insecticides when foliage is on the trees, to replace the more expensive nicotine. 1 Woodman: J. Pomol. Hort. Sci., 4, 38 (1924); #J. SOC.Leather Trades’ Chemists, 8, 517 (1924). 5
e 7
Woodman: J. Agric. Sci., 17, 44 (1927). Woodman: J. Agric. Sci., 17, 44 (1927). rnpublished data.
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ROWLAND MARCUS WOODMAS
and hydrogenated phenols, capable of dilution, will be a matter of some difficulty in view of their solubility in soap solutions; thus to obtain spraying emulsions containing I - I O % of these hydroxyl compounds, the ratio of emulsifiable phase to emulsifier (soap), must be comparatively high to avoid solutions’. I n these circumstances the farmer might as well prepare his own emulsions in the dilute form at the actual place of spraying. The method of experiment was as follows: the emulsifier solutions (from the stock potassium oleate mentioned previously, a stock solution being diluted down as required), were kept at room temperature; as occasion demanded, a given number of C.C. were pipetted into a 50 C.C. cylinder, and a given number of C.C. of the hydroxyl compound then added in such a manner as not to disturb the aqueous medium (if the soap solution were the less dense, the process was reversed). The cylinder was placed in a thermostat a t zs0C. for 30 min., stoppered, rapidly withdrawn, given a vigorous and complete shake (up and down motion of the hand), and then replaced rapidly in the thermostat; the cylinder was withdrawn and shaken every 30 sec. until complete emulsification occurred. Parallel experiments were performed in every case, the method of preparation (at zs°C.), being to give partial gentle rotation or very gentle shaking first2, followed by vigorous and continuous shaking. The emulsions were all examined by the drop test, though it was found possible, after some experience, to tell the type by appearance and by the character of the film on the cylinder’s side. The results are given in Table I : they show that, in the case of 0 in W emulsions, if the phase volume ratio be kept constant, increase in the concentration of emulsifier causes easier emulsification; if the phase volume ratio be altered, preponderance of one phase tends to make that phase, as previously-mentioned work has demonstrated for other systems, the continuous one, espedially when the amount of emulsifier present is low; and, that, in some cases, where inversion occurs with phase-volume ratio change, a critical phase-volume ratio is obtained where both types of emulsions are possible according to variation in the method of shaking, as discussed in a previous paper’, this also occurring mostly when the initial concentration of the emulsifier is small. A spraying emulsion must necessarily be of the 0 in W type: otherwise the object of making the emulsion-to dilute the “naked” and, in large masses, phytocidal oil, with a commonly-occurring, inexpensive and nonphytocidal medium such as water-is defeated. I n view of the fact that both types of emulsions are given, i t will be unsatisfactory to use the systems experimented on here in actual practice; this is because the grower tends to make concentrated emulsions initially; diluting these down to the required concentrations. The wrong type might thus be prepared, with trouble consequent on the inversion to the desired type when diluting to the spraying 1 It is worthy of note in this regard that emulsions are often found, in actual spraying, to be much more toxic to insects and insect eggs, than solutions; cf,, Lees: J. Pomol. Hort. Sci., 4, 104 ( I ~ z s )for , an example of this in the cme of cresyhc acid. *Woodman: J. Phys. Chem., 30, 658 (1926).
SOTES ON DUAL EMULSIONS
ROWLAND MARCUS WOODMAN
92
strength with water or soap solution. The results show, however, that by first making the emulsion so that the phase volume ratio of aqueous to oil media is a t least (2 - 4) : ( I ) , the desirable type of emulsion is obtained, It happens that the stable type of emulsion in the cases discussed is that needed for spraying. This is not so with cresylic acid (or pure cresols), and aqueous gelatine'; here Lhe wrong type (W in 0) is stable, and hence, as the 0 in W type usually cracks t o give free acid quickly, emulsions in these systems will be extremely dangerous to trees. Seifriz2 is inclined to attribute this peculiarity of dual emulsion formation to the wide boiling range and consequent heterogeneity of composition of the oily phase, but this notion is not borne out by the author's results with pure cresols and the present single compounds; Seifriz, however, shows that the existing theories of the mechanism of emulsion type, and also Robertson's explanation for a particular case of phase-volume ratio alteration3, are not feasible. The author has previously tried wetting a finely-divided inert emulsifiercork dust-with either liquid phase-water and toluene-before emulsification, and obtained the same 0 in W type in both cases; this was put down to the preferential wetting of cork by water'. Other experiments were now carried out to see if previous wetting of the containing vessels by one of the phases has any effect on the type subsequently formed. The most favourable conditions for this to occur were chosen, the system investigated being known to yield dual emulsions by different mechanical treatment a t the phase-volume ratio used1. Four 50 C.C. cylinders were drycleaned; into two were pipetted I O C.C. lots of 40 hr. old 0.4% gelatine, and, into the other two, I O cc. lots of cresylic acid; the contents of the cylinders were now swished round so that the insides of tlie vessels were wetted as far as possible, perfect, continuously-draining films being left on the sides. To the cylinders containing aqueous gelatine, I O C.C. lots of cresylic acid were carefully added; to those containing cresylic acid, the same amount of the aqueous gelatine. One of each pair of similarly-treated cylinders was subjected to continuous vigorous shaking, and one to gentle partial rotation previous to vigorous shaking'. If this wetting theory were correct, each pair should, irrespective of mechanical treatment, tend to give the opposite type, the pair of cylinders first wetted by aqueous gelatine giving 0 in W types, and those first wetted by cresylic acid giving W in 0 types. The pairs, however, followed the rule given for this system before', mechanical treatment being found to determine the type formed, and not previous wetting of the cylinder by one of the phases; thus continuous vigorous shaking gave the W in 0 type, and previous gentle rotation the opposite type. Moreover, the 0 in W emulsion formed in the cylinder previously wetted by cresylic acid was much more stable
' Woodman:
J. Phys. Chem., 30, 6 j 8 (1926).
* Seifriz: loc. cit. 8
Robertson: loc. cit.
NOTES ON DUAL EMULSIONS
93
and lasting than that from the cylinder treated oppositely, which, even though cegarded as an accidental occurrence, is, nevertheless, contrary to what one would expect. The emulsions so prepared were of the usual kind described previously'; thus the 0 in W types were unstable, breaking completely in 6-24 hrs., both re-shaking to the 0 in W type. The W in 0 type were stable and lasting, creaming upwards and fully in the normal manner in about two days, and inverting1 to the more unstable 0 in W type on re-shaking*. One hypothesis which has been advanced to some extent previously, and which might explain the mechanism of the formation of emulsions, can be founded on the partition of the emulsifying agent between the two liquid phases'. Such an hypothesis would account for the fact that the liquid phase which tends to dissolve, or, in the limit, wet the emulsifier more easily, becomes the external phase of the emulsion, for then the partition coefficient will be in its favour. The partition coefficient must be largely in favour of the continuous medium where one type of emulsion only is possible in the system. Whilst it may still be largely in favour of the more usual external phase in the case of a system where both types of emulsions are possible by alteration of the phase volume ratio (and may show this by one of the types being more stable), it cannot be so greatly in favour of this phase as is the case with a system yielding one type only in all circumstances. Alteration of the phasevolume ratio, where two types are possible, say by increasing the volume of one phase, the other volume being kept constant, might tend to make that phase the external one, for then a significantly greater amount of emulsifierbut not a greater concentration-would occur in this phase if the partition were anything approaching a finite quantity. A partition theory of formation of types would also account for the apparently inexplicable case quoted by Seifriz3, for the partition coefficient will be different for systems differing in the oil constituent, even though the emulsifier remains the same. It is noteworthy that in many systems where dual emulsions are possible by alteration of the phase-volume ratio, the emulsifiers are appreciably soluble to give some kind of solution-true or colloidal-in both phases; thus soaps are soluble in water, and have been proved soluble t o a great extent in various mineral oils', whilst gelatine is soluble in cresylic acid and the cresols 'Woodman: J. Phys. Chem., 30,658 (1926). * One 0 in W emulsion got by re-shaking an old, creamed W in 0 emulsion seemed perfectly stable; it creamed normally downwards, the supernatant layer being clear aqueous gelatine; no trace of cracking was perceived for 5 days, when a little acid was noted underneath the cream; by the 6th. day there were 8 C.C. of cresylic acid. 3 Seifriz: loc. cit., p. 839. Experience of emulsions teaches that there is a kind of specificity of oils in emulsification, as there is for emulsifiers, ie., that two oils with water and the same emulsifier might give opposite types, just as two emulsifiers with water and an oil can give opposite types; this, then, is the primary explanation of Seifria's case, though the argument as regards the existence of dBerent partition coefficients still holds good. Pickering: J. Chem. SOC.,111, 86 (1917).
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ROWLAND MARCUS WOODMAN
as well as in water’. But, as has been stated, great solubility of emulsifier in both of the phases is not probably necessary for the formation of dual emulsions; in the limit,.wetting of the emulsifier by one phase and solubility to some kind of solution in the other might dispose to dual types on alteration of the phase volume ratio, especially when it is remembered that in some simple one-type systems, wetting of solid particles by one liquid is all that is necessary for emulsification (e.g., use of finely-divided solids as emulsifiers*), and that liquid is invariably the external phase. An explanation of the formation of the two types when the constituents of the system are present in the same proportion, simply by altering the mechanical treatment given during preparation, or by subsequent re-shaking of one type, is much more difficult. It cannot be assumed that there are two partition coefficients under the same conditions of age, etc., for the same emulsifier, for the probability is that only one partition coefficient exists for a substance between two liquid media; but it may be assumed that mechanical conditions interfere with the establishment of this definite partition coefficient, causing a temporary and unstable partition coefficient, or even allowing the definite partition coefficient to obtain only for a portion of the emulsifier. Thus adsorption due to a certain method of shaking has been assumed to have some such effect in preventing establishment of a final and definite partition coefficient in the system gelatine-water-cresylic acid, and to account for the instability of the 0 in W type3; in this case also, ageing of the system has had to be taken as a cause of prevention of the establishment of this definite partition coefficient, for old emulsions of both types give the unstable 0 in W type on reshaking in any manner. Attempts a t determining a partition coefficient in this system are now being made by the author and a colleague. Horticultural Research Station, Cambridge, England. June 1 , 19d8. Stocks: “Brit Ass. Colloid Rpts.,” 1, 74 (1917);Woodman: J. Phys. Chem., 30, 658 (1926);Cooper: Biochem. J., 6, 362(1912). ZPickering: J. Chem. SOC.,91, 2001 (1907). Woodman: J. Phys. Chem., 30, 658 (1926).
