Class name to Datatable field

Sand

I have following code for datatable:

Obj.DataTable = $('#mobile-datatable').DataTable( {
        "ajax": {
                "url" : '/Mobile/list/' + Obj.name,
                "dataSrc": function(chek) {
                        return chek.data;
                },
            "responsive": true,
            "order": [],
            "columns": [
                { "data": "mobile_num"},
                { "data": "mobile_model" },
                { "data": "mobile_price" },
                { "data": "mobile_feature" },
            ]
        } );

This is working fine, But I need to add class name for 2nd field - mobile model. I tried

"columnDefs": [
                null,
                { className: "mobile_model", "targets": [ 0 ] },
                null,
                null,
            ]

Above code gives me error "TypeError: def is null". Anyone please help me to assign class name to the datatable fields?