STABILITY OF EMULSIONS 221 Table VI Effect of Lmv Concentrations of Dodecylamine on Emulsion Stability Expressed as Per Cent Oil Rdeased at 18,000 rpm for 30 Minutes Concentration X 10-3M pH 0.5 1.0 1.5 2.0 2.7 5.0 5.4 5.8 6.2 9 46 No emulsion No emulsion No emulsion 50 67 69 No emulsion 100 4 54 82 82 96 3 5 26 47 94 Table VII Effect of Mixtures of Hexylamine and Dodecylamine at Total Amine Concentration of 2.7 X 10-aM on Emulsion Stability Expressed as Per Cent Oil Released at 13,000 rpm for 30 Minutes Mole Fraction of Hexylamine pH 1.0 0.75 0.50 0.25 0 5.0 62 ...... 84 Noe•nulsion 5.4 66 69 79 ... 100 5.8 55 79 76 79 96 6.2 23 94 92 91 94 6.6 3 87 82 ... 82 7.0 3 8 11 10 42 7.4 4 3 3 3 7.8 4 4 4 ... 4 8.2 5 4 ..... 4 8.6 5 ... 4 6 6 required to maintain the polymer-amine salt at the oil-water interface (Table V). The n-hexylamine, however, produced "stable" emulsions independent of concentration in the range studied, while the 2-ethylhexylamine showed no definite trend as a function of increasing amine concentration. The reason why these two amincs behaved differently from the longer chain length amincs is not clear from this study. It should be noted also that once this level of stability was reached, further increases in pH by the addition of NaOH had no influence on emulsion stability. Thus, it appears that the amine salt is not dissociated even at pH values of 11.0, nor are the amphiphilic properties of the polymer significantly modified by the addition of the NaOH once this level of stability is achieved. The addition of the butylamincs (n-, ism, see-, and tert-butylamines) does not improve emulsion stability at concentrations up to 1.1 X 10-'-'M. Apparently the hydrophobic l•ropcrtics of these short-chain, water-soluble amincs are not sufficient to influence the polymer-oil interaction.

222 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The effect of the combination of hexyl- and dodecylamine on emulsion stability is shown in Table VII. The stability data of the various mole [ractions of these amines are generally intermediate to the pure amines. However, at pH values of 6.2 and 6.6 the addition of only 0.25 mole fraction of dodecylamine reduces the stability of the emulsion to about that observed with the pure dodecylamine. Thus, impurities of longer chain length amines under certain conditions could markedly reduce emulsion stability when shorter chain length amines are used as the neutralizing amine. CONCLUSIONS From the data, it appears that for a carboxy vinyl polymer to act as a primary emulsifier, it must: (a) assume a conformation that favors adsorp- tion at the mineral oil-water interface (probably extended rather than coiled) and (b) possess enough hydrophobic character so that water solubility will be sufficiently reduced to favor adsorption at the interface. At higher pH values, while the polymer conformation favors adsorption, the hydrophobic properties are so reduced that the polymer favors the aqueous phase over the interface. This is reflected by the instability of emulsions at high pH values in the absence of amine. The addition of an amine of sufficiently long chain length will provide the necessary hydrophobic character to permit adsorption at the droplet-continuous phase interface. Whereas the short-chain butylamines do not provide enough hydrophobic character to stabilize the emulsion, high concentrations of long-chain amines, e.g., dodecylamine, may provide excessive hydrophobic character. This latter effect can also result in self-interaction of the long-chain amine groups on the polymer molecule which results in an unfavorable conformation of the polymer. At lower pH values, even though the conformation of the polymer probably does not favor adsorption at an interface, the hydrophilic-lipophilic balance of the molecule permits some degree of adsorption. Thus, some emulsion stability is observed, in the absence of amine, at pH values below 5.8. The data indicate that the polymer alone, at low pH values, is slightly too hydrophilic to produce very stable emulsions. The addition of hexyl- amine or 2-ethylhexylamine increases the hydrophobic character of the poly- mer so that emulsion stability is increased significantly. However, addition of longer chain length amines, except at high pH values, rendered the polymer too hydrophobic, and thus reduced emulsion stability. But, if the concen- tration of the long chain length amine is reduced sufficiently, stable emulsions can be produced. In the case of dodecylamine, stability was achieved at