Aggregation Behavior Of Benzyldimethyldodecylammonium Bromide At Silica/Water Interfaces And In Aqueous Solutions

The aggregation behavior of surfactants at solid/liquid interfaces and in bulk aqueous solutions has been widely studied because of its fundamental and practical importance.But many fundamental issues are unclear owing to its complexity,such as the influence of adsorbent concentrations(adsorbent concentration effect)and sizes(or curvatures),the microstructure(morphology,composition,thickness,etc.)of adsorbed layers,and the coacervation phenomenon and mechanism in bulk aqueous solutions.Benzyldimethyldodecylammonium bromide(BDDABr)is a widely used cationic surfactant.However,research on its aggregation features at solid-liquid interfaces and those in solutions are scarce.In the current work,the adsorption behavior of BDDABr at silica(SiO2)/water interfaces and the potassium halide(KX)-induced coacervation behavior of BDDABr in aqueous solutions were investigated.In particular,the influence of SiO2 concentration and size on the adsorption and the microstructure of adsorbed layers were determined.It is expected to provide a better understanding for the aggregation behavior of surfactants.The main research contents and conclusions are listed as follows:(1)The adsorption of BDDABr on silica nanoparticles with?12 and 31 nm in size(denoted as S-SiO2 and L-SiO2,respectively)in water was investigated.The influence of solid concentration,particle size,pH,and temperature(T)was determined.No "solid-effect" exists in the adsorption.The adsorption may show the Langmuir type(L-type),S-type,and "double plateau" type(LS-type)isotherms.Especially,the size of silica particles,besides pH and T,has an obvious impact on the adsorption.The S-SiO2 shows a S-type isotherm while the L-SiO2 shows a LS-type isotherm at low pH(?4.0).With the increase of pH from 3.8 to 12,the equilibrium adsorption amount increases and the adsorption isotherm may transform from S-type through LS-type to L-type.With increasing T from 298 to 308 K,the adsorption of the S-SiO2 exhibits an obvious decrease,while that of the L-SiO2 exhibits a very slight decrease.However,the change in T in the studied range has no influence on the isotherm type for each adsorbent.The obtained S-type and LS-type isotherms were analyzed by using the "one-step" and "two-step" surface micellization models,respectively.The results show that the two models can adequately describe the isotherms,suggesting an obvious surface micellization occurs at the interfaces.With increasing pH,the average aggregation number of surface micelles(n)and the critical surface micelle concentration(csmc)all decrease.With increasing T,the n values decrease while the csmc values slightly increase.At given pH and T,the n and csmc values for the S-SiO2 are obviously higher than those for the L-SiO2.The affinity of the S-SiO2 toward BDDABr is lower than that of the L-SiO2,consistent with the dissociation tendency of their surface hydroxyl groups.(2)The sorbent mass variation(SMV)method was used to study the adsorption of BDDABr at SiO2/water interfaces.Special emphasis was placed on the determination of the adsorbed layer microstructure(composition and thickness).The results show that the adsorption isotherms obtained by the SMV method are the same as those obtained by the sorbent mass invariant(SMIV)method.The thicknesses of the adsorbed layers on the S-SiO2 and L-SiO2 are?5.13 and 6.46 nm,respectively.The BDDABr concentrations in the saturated adsorption layers on the S-SiO2 and L-SiO2 are-350 mM and 400 mM,respectively,corresponding to the molar fractions being 6.30×10-3 and 7.20×10-3 or the volume fractions being-0.14 and 0.16,respectively.This work demonstrates that the information about the microstructures of adsorption layers can be obtained by simple adsorption experiments.(3)The effect of potassium halides(KF,KCl,KBr,and KI)on the aggregation of BDDABr in aqueous solutions was investigated.The results show that KX can induce the coacervation of BDDABr in solutions,forming nanoscale coacervates or microemulsion-like structures before the macroscopic phase separation occuring.The induced coacervation abilities of KI,KBr,KCl,and KF reduce in turn.At given T,with increasing BDDABr concentration in the studied range(3-50 mM),the critical phase-separation salt concentration(CPS SC)of KI initially sharply decreases and then increases,that of KBr also initially sharply decreases but then reaches equilibrium,while those of KCl and KF gradually decrease.At a given BDDABr concentration(such as 40 mM),the critical phase-separation temperature(Tps)increases with an increase in the KX concentration.