Collapsed Structure of Hydrophobically Modified Polyacrylamide Adsorbed at the Air-Water Interface: The Polymer Surface Excess and the Gibbs Equation.

Neutron reflectometry has been used to measure the surface excesses and structures of hydrophobically modified polyacrylamide polymers (HMPAMs) at the air-water (A-W) interface. The HMPAMs were based on a range of commercially available PAM, which were modified by the N-alkylation of the amide group to give an N-CnD2n+1 hydrophobic group with n = 8, 12, and 16 at levels of 0.5, 1, 2, 4, and 6 mol %. A further HMPAM was synthesized in two isotopic forms with either N-CnD2n+1 or N-CnH2n+1 as hydrophobes. For moderate- and high MW species the near surface structure at the A-W interface consists of two layers. All the hydrophobic units are in these two layers as well as a large fraction of backbone units, often amounting to a total volume comparable to that of the hydrophobes. The outer layer next to air contains no water, but the residual volume in the inner layer is filled with water. A further large fraction of the backbone units also form a diffuse third layer extending a substantial distance into the solution. In a low MW HMPAMs there was preferential adsorption of species with higher mol % of hydrophobe and a tendency to form apparently nonequilibrium structures, which in some cases resulted in more complex structures than the simple one characteristic of the large MW polymers. With the exception of this polymer, the variation of the patterns of surface excess and structure with solution concentration suggested that systems containing hydrophobic units at a level of 0.5, 1, and 2 mol % formed equilibrium or near-equilibrium surface layers at bulk concentrations of 0.01-0.35 wt % for C8 to C16 units. However, higher levels of 4 and 6 mol % of the C12 hydrophobe led to much less regular patterns of adsorption, indicating that equilibration is more difficult once the molar fraction of hydrophobe exceeds 2 mol %. The behavior of the surface tension (ST) over the same concentration range as the NR experiments could be accounted for by the Gibbs equation using the directly measured surface excesses and the incorporation of a low charge on the polymers (about 1 charge per 100 backbone units). The presence of such a charge in PAM can arise from hydrolysis of some amide to carboxylate and was known to be present for one of the polymers. The extra structural information obtained by NR on these HMPAMs combined with more recent measurements of the state of ionization in polyacrylates (PAA) allowed us to reinterpret earlier ST and X-ray reflection results on hydrophobically modified HMPAANa containing a similar level of 1 and 2 mol % C12H25 hydrophobes. The Gibbs equation again accounted quantitatively for the ST behavior by using the correct state of ionization of the polymer. Although the adsorption of hydrophobic groups in HMPAANa is about one-tenth of that for the corresponding HMPAM, the ST drops more quickly to lower values for HMPAANa because of its higher level of dissociation, which increases the magnitude of the slope in the Gibbs plot.