Adsorption of single chain Zwitterionic phosphocholine surfactants: Effects of length of alkyl chain and head group linker

Abstract

The adsorption of a range of single chain zwitterionic phosphocholine surfactants (C nP mC) at the air/liquid interface has been studied by a combination of surface tension and neutron reflectivity. The critical micellar concentration (CMC) for C nPC (or C nP 2C), where n varied from 12, 14 to 16, was found to be 0.91, 0.14, and 1.2 × 10 − 2 mM respectively, and followed the same trend as observed for other zwitterionic and non-ionic surfactants. The area per molecule at the CMC, A cmc, for C nPC was found to remain constant between 50 and 53 Å 2, indicating that the increase in the alkyl chain length had little effect on A cmc at the interface. The neutron reflection measurement also showed an almost constant layer thickness ( τ) of 20 ± 2 Å from all the alkyl chain deuterated PC surfactants (dC nhPC) in null reflecting water (NRW), suggesting that the alkyl chains of the surfactant responded to changes in either chain length or solution concentration by varying their angle of tilt. In contrast, increasing the length of head group linker between P and N atoms in C 12P mC, where m = 2, 4, to 6, resulted in a much slower decrease of CMC from 0.91, 0.7, to 0.5 mM, consistent with a different contribution to the free energy of micellization. A cmc for C 12P mC did not vary when m was increased from 2 to 4, and this observation together with the thickness of the head group region indicated an almost perpendicular projection of the head group in C 12P 2C and C 12P 4C. A further increase in m to 6 resulted in an A cmc of 70 Å 2. This increase in A cmc however did not result in any change in either the total layer thickness or the fraction of the head group region submerged in the aqueous subphase, suggesting that the head group in C 12P 6C was bent away from the surface normal direction. Both increase in temperature from 25 to 40 oC and the addition of 0.1 M NaCl had little effect on the area per molecule or the thickness of C 12P mC surfactant layer, showing that the C 12P mC series behaved like C nP 2C series. The main conclusion from this study is that for all the C nP mC surfactants studied, change in m or n has little effect on the total thickness, the thickness of the alkyl chain or that of the head group region.