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Interfacial Adsorption Of Lipopeptide Surfactants At The Solid/Liquid Interface

Posted on:2012-06-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:D H JiaFull Text:PDF
GTID:1101330338993181Subject:Chemical Engineering and Technology
Abstract/Summary:PDF Full Text Request
Lipopeptides are an important group of biosurfactants expressed by microorganisms. Because they are well known for being biocompatible, biodegradable, highly surface active and anti-bacterial active, they are attractive for a wide range of attentions. Natural lipopeptide surfactants are however impure, it is hence difficult to use them for exploring structure-function relation. In this work, a series of cationic lipopeptide surfactants C14Kn (n = 1-4), where C denotes myristic acyl chain and K denotes lysine (Lys), has been synthesized and their interfacial behavior was characterized by studying their adsorption at the hydrophilic silica/water interface and hydrophobic C8/water interface using spectroscopic ellipsometry (SE) and neutron reflection (NR).Surface tension of lipopeptide surfactants C14Kn showed that they reduced water surface tension to 30-40 mN·m-1 and their CMCs were very low, for example CMCC14K1=0.55 mmol·L-1 and CMCs increased with the number of lysine head group (n).SE revealed that the dynamic adsorption process of C14Kn at the hydrophilic silica/water interface is characterized by two steps: a fast first step associated with the sharp increase in the adsorbed amount (Г) within the first 2-3 min and the subsequent slow step leading to an adsorption plateau. Majority of the lipopeptide was found to adsorb with the fast initial step but the subsequent structural relaxation and adjustment led to the adsorption plateau. It took about 15-20 min for C14K1 to reach equilibration whilst it took some 30 min for C14K2-4 to reach the same state. The initial rate of time dependent adsorption and the equilibrated adsorbed amount showed steady decrease with increasing n, indicating the impact of molecular size, structure and charge.It was found that pH, temperature and salt had a strong effect on C14Kn adsorption at the silica/water interface. The surface adsorbed amount showed a steady increase with pH and temperature. The adsorbed amount from C14K1-3with salt dropped below critical salt concentration, but rose above critical salt concentration. However, the adsorbed amount of C14K4 also decreased below critical salt concentration, but remained constant above critical salt concentration. The critical salt concentration increased with n. NR, in conjunction with deuterium labeling, revealed the formation of sandwiched bilayers from C14Kn, similar to conventional surfactants such as nonionic C12E6 and cationic C16TAB. In this model, the inner layer of K groups interacted with the weak negatively charged oxide surface to initiate the adsorption; the hydrophobic affinity between the C14 acyl chains drove the formation of the back-to-back hydrophobic region, with the outer K groups pointing to the bulk solution. The thickness of head group regions was almost constant ranging from 8 to 11 (?), but the thickness of hydrophobic regions decreased from 26 (?) of C14K1 to 21 (?) of C14K2, 14 (?) of C14K3 and 14 (?) of C14K4. The highest volume fraction of C14K2 in the bilayer matched the lowest water association, but the highest water volume fraction in the bilayer was observed from C14K3-4 with the highest n.NR studies examined that whilst all lipopeptide surfactants formed distinct bilayers, the adsorbed amount, the total bilayer thickness, the extent of water incorporation and the extent of back-to-back chain-chain and chain-head intermixing were highly dependent on the size of the head group. Increase in n led to little change in the thickness of the head group, consistent with the assumption of the anchoring of R groups to the oxide surface via electrostatic attraction. Increase in the cross-sectional area with n was responsible for all changes observed, in particular, the chain-chain, chain-head intermixing and acyl chain tilting across the interface, leading to the thickness of hydrophobic regions decreased.The dynamic adsorption process of C14Kn at the hydrophobic C8/water interface is similar to at the hydrophilic silica/water interface and The initial adsorption rate and the equilibrated adsorbed amount also showed decrease with increasing n.C14Kn adsorption on the hydrophobic C8 interface formed a monolayer structure consisted of two sublayers different from the bilayer on the hydrophilic silica surface. The first sublayer next to the C8 layer contains the hydrophobic acyl chains and the second one positively charged lysine head groups projecting into the bulk solution. The thickness of head group regions is 10 (?) for C14K1, 12 (?) for C14K2, 13 (?) for C14K3 and 14 (?) for C14K4, indicating head groups of C14K1-2 were almost perpendicular to the surface and those of C14K3-4 tilting or coiling. The thickness of hydrophobic acyl chains is 17 (?) for C14K1, 13 (?) for C14K2, 10 (?) for C14K3 and 9(?) for C14K4 that decreased with n resulting from tilting tail groups. Increase in the cross-sectional area with n created increased space in the tail region, resulting in increased acyl chain tilting across the interface, leading to the thickness of hydrophobic regions decreased.
Keywords/Search Tags:lipopeptide surfactant, hydrophilic silica/water interface, hydrophobic solid/water interface, dynamic adsorption, interfacial adsorption structure
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