Hydroxyl Dissociation And Surfactant Adsorption At Solid Nanoparticles/water Interfaces

The solid/liquid interface phenomena,such as surface charging,adsorption,and wetting,have been widely studied owing to their fundamental and practical importance.There are many factors affecting the interface properties,such as the composition,surface curvature,and surface microstructure,among which the effect of surface curvature（or particle size）is a basic problem in the colloid chemistry.The size-dependence of the interfacial reactions（e.g.,hydroxyl dissociation）and adsorption at solid/liquid interfaces have received extensive attention,and many theoretical models have been proposed.However,the existing models are lack of universality,indicating that some model assumptions deviate from the actual states.In addition,the structure of adsorption layers（including their composition and thickness）at solid/liquid interfaces were an important topic in adsorption research,but there was no thermodynamic model to predict the structure of adsorption layers.In this work,based on the principle of thermodynamic equilibrium and the interfacecurvature efect,the correlation between the hydroxyl acid-base reaction/adsorption equilibrium and the particle size and that between the structure of adsorption layers and the equilibrium adsorption amount at solid/liquid interfaces were theoretical analyzed,and the"size-effect model" and "adsorption layer model" were developed.Taking SiO₂ and TiO₂ nanoparticles as models,relevant experimental studies were carried out,and the test data,along with literature-reported data,were used to verify the rationality of the so-developed models.It is expected to deepen the understanding of the solid/liquid interface phenomena.Main research contents and conclusions:（1）Size-dependence of hydroxyl dissociation at solid/liquid interfacesBased on the thermodynamics of Gibbs,the principle of chemical reaction equilibrium,and the interface-curvature effect,a size-effect model for interfacial acid-base reactions was developed,to relate the effective surface hydroxyl density(N_s^eff)and the dissociation equilibrium constants（K_a）separately to particle radius（r）.The model predicts that both lnN_s^eff and pK_a decrease linearly with an increase in 1/r.Nine amorphous silica samples with different r（ca.6-196 nm）were chosen as model particles,and their N_s^eff and K_a were determined using acid-base titration.With the decrease of r,the N_s^eff（mol/m²）,apparent pK_a,and intrinsic pK_a all decrease.The test data can be well described using the model equations,indicating that the size-effect model is reasonable.The characteristic parameters N_s^eff,apparent pK_a,and intrinsic pK_a corresponding to flat surfaces（r→∞）for silica particles were estimated to be 9.12 μmol/m²（5.49 sites/nm²）,9.85,and 8.99,respectively.The size-effect on N_s^eff and K_a becomes obvious when r<～40 nm.（2）Size-dependence of adsorption at solid/liquid interfacesA size-effect adsorption model was developed based on the thermodynamics of Gibbs,the principle of adsorption equilibrium,and the curvature-effect.It relates the adsorption equilibrium constant（K_ad）,the saturation adsorption amount（Γm,mol/m²）,and the change in the standard Gibbs free energy of adsorption per mole of adsorbate(ΔG_ad⁰)separately to particle size.A parameter,ΔQ_ad,was introduced,defined as the change of the product of the solid/liquid interfacial tension and the molar volume of solid surface components caused by the adsorption.The model predicts that,with an increase in 1/r,lnΓm decreases linearly,while the lnK_ad and the absolute values of ΔG_ad⁰(|ΔG_ad⁰|)are determined by the AQ_ad,namely,they linearly decrease if ΔQ_ad>0 but increase if ΔQ_ad<0.There exists a critical r（rc）at which the saturation adsorption amount per unit mass（Γmg,mol/g）attains a maximum.The adsorption of cetylpyridinium chloride（CPyCl）,a cationic surfactant,on silica nanoparticles with different r（ca.6-61 nm）was determined at 298 K and pH 9.The csmc and n showed an obvious size-dependence.With a decrease in r,K_ad,Γm,and |ΔG_ad⁰| decrease,indicating a decrease in the affinity of silica particles toward CPyCl.The size-dependent adsorption data can be well described using the model equations,indicating that our size-effect model is acceptable.In addition,our model can describe the size-dependence of the literaturereported data for different adsorption systems,indicating its good universality.The ΔQ_ad for the CPyCl adsorption on SiO₂ was estimated to be ca.1.35 kJ nm/mol.The positive ΔQ_ad value can be attributed to the increase of the molar volume of the silica surface components caused by the CPyCl adsorption.Adsorption can affect the molar volume of solid surface components,which plays a critical role in the size-dependence of adsorption.（4）Size-dependence of surface aggregation of surfactants at solid/liquid interfacesA size-effect model for the surface aggregation of surfactants at solid/liquid interfaces was developed based on the thermodynamics of Gibbs,the principle of adsorption equilibrium,the curvature-effect,and the surface micellization models.It relates the adsorption（aggregation）parameters including adsorption amounts,equilibrium constants,the critical surface micelle concentration（csmc）,and the average aggregation number of surface micelles（n）to particle size.The model predicts the size-dependence of surface aggregation of surfactants,which is determined by the changes in the interfacial tension and the molar volume of surface components caused by adsorption.The adsorption of CPyCl on silica nanoparticles was determined at 298 K and pH 4,showing an obvious size-dependence.The experimental data can be well described using the model equations,indicating that the model developed here is acceptable.Especially,the model can extract information on the interfacial tensions from adsorption data,which is important for the study of adsorption behaviors.（3）A model for the structure of adsorption layers at solid/liquid interfacesAn adsorption layer model,called the "dynamic bonding equilibrium"（DBE）model,was developed based on the adsorption equilibrium thermodynamics with an assumption that,at adsorption equilibrium,the sorbates adsorbed may exist in two limiting states,completely"bonded" and "unbonded" to surface sites.The DBE model relates the mole fraction of sorbate（xA）and thickness（dal）of adsorbed layers.to the equilibrium adsorption amount（Γe）,thus can predict or describe the change of the structure（xA and dal）of adsorbed layers with Γe.The adsorption of surfactants,sodium dodecylbenzenesulfonate（SDBS）and dodecylbenzyltrimethylammonium chloride（DBTMA）,at TiO₂/H2O interfaces was determined using the "sorbent mass variation"（SMV）method.The rationality of the DBE model was confirmed by the test data and the literature-reported adsorption data of small molecules（1propanol）,surfactants,proteins,and polymers on hydrophilic and hydrophobic surfaces in water.This work reveals the intrinsic relationship between the adsorption amount and the structure of adsorbed layers and the equilibrium adsorption amount,and provides a feasible approach to obtain information on the structure of adsorbed layers.