Influence of the surface charge on the homogeneity of colloidal crystals

Five groups of suspensions composed of polystyrene particles, having similar size but different effective surface charge, were adopted to investigate the effects of surface charge and volume fraction on the homogeneity of colloidal crystals through checking the difference between D(exp) and D(uni) by reflection spectroscopy method (D(exp), D(uni) are the experimental and the expected value of the average nearest neighbor interparticle distance by assuming a uniform structure, respectively). We found volume fractions (ranging from 0.006 to 0.02) and structure types basically have no influence on the values of D(exp)/D(uni). Moreover, for crystals formed by lowly charged particles, D(exp)/D(uni) is approximately equal to 1, implying the crystals are homogeneous. With the increase of effective surface charge, D(exp) gradually deviates from D(uni) and the formed crystals become inhomogeneous. Our experimental observations are in accordance with the previous simulation results. Additionally, we also found D(exp)/D(uni) initially drops quickly with increasing effective surface charge and then it tends to an asymptotic value (~0.85), it is supposedly due to the saturation of effective charge. Our relevant computer simulations confirmed that the study scheme that using D(exp)/D(uni) as an indicator to assess the homogeneity of crystal structure is tenable and the simulation results are consistent with experiments.