Monolayers of ω-Hydroxyalkyldimethyl-octadecylammonium Bromide at Water-Air Interface

M. Hato, H. Minamikawa, K. Okamoto, and M. Iwahashi
[J. Colloid & Interface Science, Vol.161, p.155, 1993]

A series of bipolar surfactants, ω-hydroxyalkyldimethyloctadecylammonium bromide {HADMOA(n); ω-hydroxyalkyl chain length n=10, 12, 14, 16, 18, 20} were synthesized and the monolayer behavior at the air-water interface was studied. The isotherms of the bipolar HADMOA(18) were compared with those for the monopolar counterpart DMDOA (dimethyldioctadecylammonium bromide). The effect of the ω-hydroxyalkyl chain length was also examined. The results were as follows:

• The monopolar DMDOA exhibits a solid condensed region while the bipolar HADMOA (18) exhibits only an expanded region. The monolayer of the HADMOA(18) collapses before a condensed state is achieved.
• The isotherm of the bipolar HADMOA (18) is much less sensitive to temperature and to the ionic strength of the subphase than that of the monopolar DMDOA.
• The general features of the monolayers of the other HADMOA(n) (n=20,16,14,12) are analogous to those of the HADMOA(18); i.e., the monolayers are of the expanded type, and collapse before a condensed state is achieved.
• The above results can be explained by assuming that the HADMOA(n) molecules in the monolayer take a "flat" conformation on the aqueous subphase where both the polar groups, {HOCH_{2}-and N+(CH_{3})_{2}R,R'}, are in contact with water and are never forced out from the interface by such experimental means as lateral compression, lowering the temperature, or increasing the ionic strength of the subphase (Fig.).